Tesi sul tema "Dynamic loading"

The response of an Echinodome to static and dynamic point loads, and explosive type loadings was examined both theoretically and experimentally. The finite element method of analysis was employed in the theoretical investigation. Semi-loof thin shell elements were used to model a GRP prototype on which the experiments were performed. The stress distribution of the Echinodome under a static symmetric point load were investigated both experimentally and theoretically. Then the Southwell technique was employed in estimating the critical buckling load from deflection measurements. Experimental estimates were then compared with the numerical predictions in the form of non-linear collapse and non-linearbifurcation buckling loads. A free vibration was performed to determine the structural natural frequencies and typify the mode shapes. The shock response spectra of several pulse shapes were determined using the finite element method. The most severe loading function was established to be a step loading with infinite duration and zero ramping time and was then employed as the load-time history in an axisymmetric and symmetric non-linear dynamic buckling analysis. The dynamic collapse buckling loads were found to be smaller in magnitude than their static correspondents. A modal testing was then carried out on the Echinodome prototype to determine the experimental modal parameters (natural frequencies, damping values and mode shapes). Newly developed correlation techniques were adopted in the comparison of the experimentally derived parameters with those predicted and poorly modelled regions were identified. Great improvement was achieved by correcting the experimental data and updating the finite element model's boundary conditions. A set of underwater free field experiments was performed to determine the pulse characteristics for a specific explosive charge, followed by another set while the prototype was in a floating submerged state and acting as the target for the same explosive charge. A theoretical simulation was accomplished by employing a finite element-boundary element approximation for the modelling of the structure and infinite fluid media respectively. Measured responses were compared with the numerical predictions and means of acquiring better theoretical approximations are mentioned. The loading conditions to be experienced by an underwater LNG Echinodome vessel are reviewed with emphasis on accidental dynamic loads (impact and explosion). A state of the art storing configuration is proposed for the Echinodome in order to limit the extent of damage and hence minimise risk during upset conditions. Finally, appropriate design, construction and prestressing procedures were recommended.

The finite element method in conjunction with the theory of viscoplasticity, and to a lesser extent plasticity, have been used for modelling of plasticity base progressive fracturing in transient dynamic problems. The difficulties associated with the computational modelling of wave propagation problems are discussed. The central difference method is utilised for step-by-step direct time integration of equations of motion, which is coupled with the lumped mass and proportional damping matrices. Special attention is devoted to a correct integration of energy states. Computer implementation takes advantage of modern computer languages and programming techniques, which results in a efficient and fully portable computer code. The strain softening phenomenon is explained in some detail. The consequences of the change of type in the governing of classical rate-independent continuum at the onset of strain softening are investigated. As a solution to that problem, the enrichment of the continuum with the higher-order derivatives in form of strain-rate dependent model (viscoplasticity) is explored and unique, stable and mesh insensitive results are observed for mode-I and mode-II failure problems. A constitutive model for quasi-brittle materials is proposed. The progressive damage is assumed to be isotropic and is modelled with the decohesion process. To account for different behaviour of heterogeneous materials under increasing hydrostatic pressure the degree of softening is rendered state-of-stress dependent. The model is based on the Mohr-Coulomb yield criterion and non-associated flow rule is utilised. The problems associated with non-smoothly intersecting yield surfaces for Mohr-Coulomb yield criterion are discussed with emphasis on correct evaluation of the viscoplastic parameters in singular regions.

The Design of structures under dynamic loading is a demanding subject in safety of engineering design since conventional static failure criteria are unable to deal with structures under transient loading. This work is a contribution to this significant phenomenon to investigate the response and failure of structures to pulse loading. An experimental rig has therefore been designed to achieve the target. A series of experiments has then been carried out to investigate the structural failure under pulse loading using a shock tunnel. A non-linear transient analysis of plates and cylindrical structures under pulse loading has also been performed using ANSYS finite element code in order to introduce a failure criterion for these specific conditions. A large-scale heat exchanger under pressure pulse loading was also analysed experimentally and numerically. The impulsive load has been chosen to be above the static design pressure to investigate the effects of impulsive load and its duration on the plate failure. A critical curve is presented to determine the critical pulse loading and its duration for structures. The relations between the transient pressure loading, its duration and the natural frequency of the structure are also explored. It is indicated that the value of the impulsive load on structures may exceed the static design pressure without structural failure. Both experimental work and numerical analyses suggest that the design criteria for structures under dynamic loading are more flexible than those under static loading in which no freedoms in deviation of any simple yield criterion exist. It is concluded that using a proper failure criterion for any specific problem can increase safe working region of the structures which leads to economical and safe dynamic design of structures.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 2009.
Thesis Advisor(s): Kwon, Young. Second Reader: Didoszak, Jarema. "December 2009." Description based on title screen as viewed on January 26, 2010. Author(s) subject terms: Tensile tests, Strain rate effects, Dynamic loading, Failure criterion. Includes bibliographical references (p. 37-38). Also available in print.

This Thesis explores the mechanisms controlling the mobility of large objects embedded in granular materials. The goal is to establish which loading conditions lead to their motion and to identify the underlying physical mechanisms. To this aim, laboratory experiments were conducted using a canonical mobility test, which involves uplifting a plate embedded in a dry granular packing vertically. The experimental conditions were varied in order to evidence the effect of the presence of water in the granular packing, and the effect of dynamic loadings whereby the plate is moved at different velocities or subjected to a cyclic force. Three main discoveries emerged from these experiments. The first discovery is the development of a drag force instability when the plate is driven at a constant and slow velocity, which vanishes at higher velocities. The second discovery is a visco-elastic dynamics developing in immersed packing, which is evidenced by drag force relaxation dynamic and a velocity-driven increase in drag force. The third discovery is a drop in packing resistance when the plate is subjected to a low magnitude cyclic force. The physical origin of these mobility responses is consistently analysed and rationalised, considering elementary mechanical processes. The conclusions of this Thesis form a fundamental basis to advance a variety of engineering discipline pertaining to foundation design, excavation techniques and bulk material handling.

Explosive-generated waves exhibit high-energy loading profiles featured with mechanical characteristics applied over a wide range of strain rates. Recent decades have seen unprecedented occurrence of high-energy trauma associated with blast wave exposure. One such blast-specific pathology is blast-induced heterotopic ossification (bHO), which refers to ectopic bone formation due to inappropriate mesenchymal stromal cell (MSC) osteogenesis in non-skeletal tissues. Significant effort has been made into deciphering the molecular mechanisms that allow the onset of bHO, however little research has been reported on the exact role of the biomechanical processes involved in transducing blast-associated mechanical stimuli into molecular events stimulating osteogenesis in MSCs. The research presented in this thesis investigated the stimulation of osteogenesis in periosteum-derived mesenchymal stromal cells (PO MSCs) in response to mechanical insults simulating blast landmine trauma. This involved the development of experimental biocompatible in vitro platforms and the tailoring of biomechanically-relevant stimuli of varying stress intensities (up to 70 MPa), and strain rates (0.01 to 3000 /s). Subsequently, cell health and the stimulation of osteogenesis were investigated by studying the expression of Runx2 and Osteocalcin (OC) genes. We found that cell health was not affected by single-pulse loadings of wide range of impulse levels (0.20 to 95000 N.s). We showed evidence of mechanically-stimulated osteogenesis in PO MSCs through the upregulation of Runx2 and OC genes in loaded samples. Furthermore, our results highlighted that the stimulation of osteogenesis in MSCs did not result solely from the effect a single mechanical parameter, but rather the combined action of several features. We showed that osteogenesis stimulation in MSCs arised from the complex interplay between the mechanical characteristics of the loading along with the environment used to convey the stress wave. Finally, our research indicated that PO MSCs are finely tuned to respond to mechanical stimuli that fall within defined parameters.

The 2010-2011 Canterbury, New Zealand, Earthquake Sequence (CES) resulted in 185 fatalities and approximately $NZ40 billion in damage, much of which was due to liquefaction and related phenomena. As a result, an extensive soil improvement field testing program was initiated and Rammed Aggregate Piers� (RAP) were shown to be a feasible method to mitigate the risk from liquefaction during future events. To better design and more fully assess the efficacy of reinforcement techniques against liquefaction, pre- and post-treatment in-situ test data are compiled, to include results from cone penetration tests (CPT), direct-push crosshole tests, and vibroseis (T-Rex) shaking tests. The data are used to evaluate the capabilities of numerical tools to predict the liquefaction response of unimproved and improved sites. A finite difference (FD) numerical model is developed in a FLAC platform and a coupled analysis using the Finn model with Byrne (1991) formulation is conducted. The FD model calibrated for top-down shakings similar to the vibroseis tests succeeded in qualitatively reproducing the general observed behavior without quantitatively matching the in-situ values for shear strains and excess pore pressure ratios. The introduction of the RAP elements to the FD model reduced the shear strain, but slightly overestimated that reduction. Considering more advanced constitutive models that better simulate the complexity of the soil behavior under dynamic loading would likely increase the accuracy of the predicted response.
MS

This thesis is devoted to solution of the two-dimensional elastodynamic problem for a cracked bimaterial loaded by harmonic waves. The system of boundary integral equations for displacements and tractions at the interface is obtained from Somigliana identity with the allowance for the contact interaction of the opposite crack faces. Full expressions of the integral kernels derived by the consecutive differentiation of the Green's displacement tensor are given. Due to the contact that takes place between the faces of the crack under the applied external loading, the resulting process is a steady-state periodic, but not a harmonic one. Thus, components of the stress-strain state are expanded into exponential Fourier series. The collocation method with a piecewise constant approximation on each linear continuous boundary element is used for the numerical solution. The problem is solved using the iterative algorithm. The solution is refined during the iteration process until the distribution of physical values satisfies the imposed constraints. The results are obtained for the interface crack subject to normal tension-compression, normal shear, or oblique tension-compression waves with different values of the angle of the wave incidence and the wide range of the dimensionless wave number. The distributions of the normal and tangential components of the contact forces and displacement discontinuities on the surface of the crack are investigated. The stress intensity factors are computed and analyzed for various values of the wave frequency, the friction coefficient, and material properties. The maximal stress intensity factors at the trailing crack tip differ from the SIF values at the leading crack tip showing non-symmetry of solution with respect the space and time variables. It is concluded that the crack closure and friction effect change the solution both qualitatively and quantitatively, as the difference between comparable results can achieve 30-50%.

This thesis presents the kinematics occurring during lab-based dynamic compaction tests using high speed photography and image correlation techniques. High speed photography and X-ray microtomography have been used to analyse the behaviour of sandy soil subjected to dynamic impact. In particular, the densification mechanism of granular soils due to dynamic compaction is the main theme of the thesis. High speed photography and digital image correlation (DIC) techniques have enabled the deformation patterns, soil strains and strain localisations to be observed. Image correlation and X-ray scans revealed the formation, rate and growth of narrow tabular bands of intense deformation and significant volumetric change and provided answers towards a better understanding of the densification mechanism in dry granular soils due to dynamic compaction. As a quantitative tool, high speed photography has allowed the propagation of localised deformation and strain fields to be identified and has suggested that compaction shock bands control the kinematics of dynamic compaction. The displacement and strain results from high speed photography showed that soil deformation in the dynamic tests was dominated by a general bearing capacity mechanism similar to that widely stated in classic soil mechanics texts. Comparative static loading tests have been conducted to enable the dynamic effects to be clearly distinguished. This has enabled the densification process taking place below the soil surface to be investigated and identified. Simulations of the physical models were carried out using LS-DYNA finite element formulations for comparison and verification purposes. The FE simulations verified the general characteristics from the photography findings. However, simulation results were unable to predict the exact details of the strain localisation due to surface impacts during physical model tests.

The breast contains no muscle or bone and its primary supporting structures are reported to be the skin and Cooper’s ligaments. It has been hypothesised that independent breast motion, which is already known to cause pain for many women, may also cause damage to the breast structure leading to breast ptosis (sag). Breast damage could be estimated by applying the published strain failure limits (60%) for human skin to measurements of breast skin strain. However, there have been few attempts to measure breast skin strain as gravitational deformation makes it difficult to identify the neutral breast position in which there is no external strain on the breast skin. A gold-standard method for estimating the neutral breast position based on Archimedes’ principle was developed and implemented within this thesis. Fourteen female participants, with breast sizes 30 to 34 under band and B to E cup size, had semi-permanent henna markers applied to their torso (4 markers) and left breast (an array of 17 markers including the nipple). Participants were immersed up to their neck in two fluids with mass-densities above (water) and below (soybean oil) the reported range of breast mass-densities (919 kg.m^-3 to 986 kg.m^-3). The mid-point between the breast position in water and soybean oil provided the gold-standard neutral position estimate. Participants also performed nine alternative novel or previously published methods for estimating the neutral breast position. Alternative methods were assessed for accuracy and precision when estimating the gold-standard neutral nipple position. To investigate the effects of gravity and dynamic activity on the breast, participants had their breast displacement and breast skin strain assessed in the static gravity-loaded (bare-breasted) position and during an incremental-speed (bare-breasted) treadmill test (4 kph to 14 kph). Breast pain was recorded in each condition using an 11-point numerical rating scale. The gold-standard method was implemented to obtain an accurate (measurement error ≤ 1.4 mm) and precise (TEM ≤ 1.2 mm, SD ≤ 3.7 mm) measurement of the neutral breast position. Evaluation of novel and previously published neutral position methods revealed that the buoyancy in water method achieved the most accurate estimation of the gold-standard neutral nipple position (absolute differences ≤ 5.6 mm, TEM ≤ 1.2 mm, SD ≤ 2.6 mm). Comparison of the gravity-loaded and neutral breast positions demonstrated that gravity caused the nipple to move posteriorly (mean change 15.6 mm), laterally (mean change 7.5 mm) and inferiorly (mean change 25.9 mm) relative to the torso, and induced potentially damaging skin strains up to 75%. During bare-breasted incremental-speed treadmill activity the nipple was displaced furthest from the neutral position in the inferior direction (mean of 45.5 mm); a result which could not be attained using conventional measurements of nipple range of motion (ROM). Strain analysis indicated potentially damaging skin strains during the incremental-speed treadmill activity (up to 114%), particularly in the upper-outer region of the breast. This finding supports the anticipated relationship between breast motion and breast damage. Breast pain was most strongly correlated to superior nipple displacement from the neutral position during treadmill activity (p < 0.001, r = 0.725), and a significant correlation was observed between breast pain and breast skin strain (p < 0.001, r = 0.361). Combination of breast displacement and skin strain data provided a comprehensive analysis of the effect of gravity- and motion-induced breast displacements on the breast skin. Incorporation of the neutral breast position into future biomechanical research may lead to improved assessment of breast motion and breast damage, and a deeper understanding of motion-induced breast pain. Within clinical research, identification of the neutral breast position may enable the development of breast models that are better able to predict the gravitational deformation of the breast during surgery. Consideration of the neutral breast position, and subsequent breast strain, also has applications within the field of breast support design and evaluation. A breast support garment that positions the breast in the neutral position and restricts motion to within the reversible strain limits of the skin may protect the breast from skin damage and the associated breast pain.

The anterior cruciate ligament (ACL) is one of four major intra-articular knee ligaments and plays a key role in knee stability. Rupture of the ACL is one of the most common and debilitating sport-related knee injuries. Most ACL ruptures do not involve direct collisions, but occur during landing, cutting, and pivoting tasks common to sports such as soccer, basketball, and netball. Rates of ACL rupture in young people have increased enormously in Australia over the past two decades. Generally, ACL ruptures are 3.5-4 times more frequent in female compared to male athletes. Among females, those aged 15-19 years are at highest risk of ACL rupture being ~4 times more likely to sustain injury than their pre-pubertal counterparts. Analysis of video footage of ACL injury events, cadaveric experiments, and biomechanical studies has yielded a consensus that external knee loads applied in three planes of motion (i.e., sagittal, frontal, and transverse) contribute to ACL rupture. Laboratory-based biomechanical studies that directly instrumented the ligament have been performed, albeit sparingly for obvious reasons of invasiveness, and show the ACL sustains substantial loading during non-injurious motor tasks. Moreover, studies using external biomechanical measures to examine ACL loading employing computational models have been limited, and have not included, and thus are insensitive to, an individual’s knee muscle activation patterns in muscle force estimates Valid models of the ACL and its loading profile have been challenging to create, and instrumented measures of ACL loading without concurrent modelling of the neuromusculoskeletal dynamics will fail to provide insight into the role of specific muscle and external loads in loading the ACL. Therefore, the mechanisms underlying ACL loading during dynamic motor tasks, through the interaction of muscles, contacting articular bodies, and other soft tissues, remain unclear. This deprives injury prevention and rehabilitation programs of personalized targets that are mechanistically linked to in vivo ACL loading. The purposes of this thesis were to develop and validate a computational model that can accurately estimate ACL force based on outputs of neuromusculoskeletal models in individuals with an intact ACL; determine ACL loading in drop-land-lateral jump task in mature females, therein examining the mechanisms that contribute to ACL loading, and; and determine effects of pubertal maturation on females’ ACL loading during a dynamic motor task considered provocative for the ACL. This thesis involved the development, validation, and application of a computational model to quantify ACL force during dynamic motor tasks. First, an ACL force model was developed using the most relevant, complete, and accessible cadaveric data from the literature. These data comprised measurements of ACL force or strain across various knee flexion angles in response to uni- and multiplanar external knee loads. Using a portion of these data, algebraic equations were fitted to well describe ACL loading in response to both uni- and multiplanar knee loading. The model was then validated using the remaining experimental data not used in model development. The validated ACL force model was then combined with an electromyography (EMG) -informed neuromusculoskeletal model to estimate ACL force developed during a standardised drop-land-lateral jump task performed by healthy females in laboratory conditions. Ninety-three females, aged 8 to 20 years, volunteered to participate in this study. All participants were recreationally active and had no history of lower limb injury or knee pain. Participants were divided into 3 groups: pre-, early/mid-, or late/post-pubertal based on Tanner’s pubertal classification system. Each participant attended a laboratory-based testing session, wherein three repeated trials of a standardized drop-land-lateral jump from a box with box height set to 30% of their lower limb length, while 3D motion capture, ground reaction forces, and surface EMG were acquired. For purposes of calibrating the EMG-informed neuromusculoskeletal model, three trials of running at a natural self-selected style (speed range from 2.8 to 3.2 m.s-1) were performed. These laboratory data were then used in a neuromusculoskeletal model to estimate ACL loading. The OpenSim modelling software was used to scale a generic anatomical model to match each participant’s gross dimensions, mass, and inertia, followed by morphometric scaling to preserve fibre and tendon operating ranges, and last adjust each muscle’s maximum isometric strength based on empirical relationships between mass and height with lower limb muscle volume. Using this scaled model, the external biomechanics (i.e., model motions and joint loading) and muscle tendon unit actuator kinematics (i.e., moment arms, lengths, and lines of action) were determined. The EMG signals were conditioned into normalized linear envelopes, which were combined with the OpenSim external biomechanics and muscle tendon unit actuator kinematics to drive a model in the Calibrated EMG-informed Neuromusculoskeletal Modelling (CEINMS) toolbox. The CEINMS was first calibrated and then run with an EMG-informed neural solution to estimate lower limb muscle and tibiofemoral contact forces. The muscle tendon unit kinematics and forces, along with the joint loads were then incorporated into the validated ACL force model to quantify ACL force. For each participant, the contribution of muscle and intersegmental loads to ACL forces were calculated across the stance phase of the drop-land-lateral jump task. Specific statistical analyses were run to address each of the research questions and encapsulated as a series of research manuscripts in the format of journal articles. The first study developed and validated a computational model that predicted the force applied to ACL in response to multiplanar knee loading that was estimated by a subject-specific neuromusculoskeletal knee model, as described above. The study demonstrated these models’ utility by applying it to a sample of motion capture data. First, a three-dimensional (3D) computational model was developed and validated using available cadaveric experimental data to estimate ACL force. The ACL force model was valid as it well predicted the cadaveric data, showing strong statistical correlation (r2=0.96 and P<0.001), minimal bias, and narrow limits of agreement. Second, by combining a neuromusculoskeletal model with the ACL force model, it was revealed that during a drop-land-lateral jump task the ACL is primarily loaded through the sagittal plane, mainly due to muscular loading. The computational model developed in study one was the first validated accessible tool that could be used to develop and test knee ACL injury prevention programs for people with normal ACL. The method used to develop this model can be extended to study the abnormal ACL upon the availability of relevant experimental data. The paper describing these results was published as Nasseri A., Khataee H., Bryant A.L., Lloyd D.G., Saxby D.J. Modelling the loading mechanics of anterior cruciate ligament, Computer Methods & Programs in Biomedicine, 184 (2020) 105098. doi: 10.1016/j.cmpb.2019.105098. Study two determined ACL force and the key muscular and biomechanical contribution to this ACL loading in a standardized drop-land-lateral jump task performed by sexually mature young females. Three-dimensional whole-body kinematics, ground reaction forces, and muscle activation patterns from eight lower limb muscles were collected during dynamic tasks performed by healthy females (n=24), all who were recreationally active. Collected data were used to model the external biomechanics, muscle-tendon unit kinematics, and muscle activation patterns using established biomechanical modelling software packages (i.e., OpenSim and MotoNMS). These biomechanical and electromyographic data were then used to calculate the lower limb muscle, joint contact and the ACL forces through an EMG-informed neural solution combined with a validated ACL force model. Peak ACL force (2.3 ± 0.5 BW) was observed to occur at 14% of the stance phase during the drop-land-lateral jump task. The ACL force was primarily developed through the sagittal plane, and muscles were the dominant source of ACL loading. The main ACL muscular antagonists were the gastrocnemii and quadriceps, while the hamstrings were the main ACL agonists. Our results highlighted the important role of gastrocnemius in ACL loading, which could be considered more prominently in ACL injury prevention and rehabilitation programmes. The paper describing these results is accepted for publication as Nasseri A., Lloyd D.G., Bryant A.L., Headrick J., Sayer T.A., Saxby D.J. Mechanism of anterior cruciate ligament loading during dynamic motor tasks. Medicine and Science in Sports and Exercise. Study three determined and compared ACL loading during a drop-land-lateral jump task in females across three pubertal stages of maturation. Further, the relative contributions to ACL force from three planes of motion (sagittal, frontal, and transverse) were compared. In this, sixty-two participants were divided into pre-pubertal (n=19), early/mid-pubertal (n=19) or late/post-pubertal (n=24) groups based on Tanner’s pubertal classification system. Each participant completed a biomechanical testing session wherein we collected three-dimensional body motion, ground reaction forces, and EMG during drop-land-lateral jump task. Using these data, the aforementioned ACL force and neuromusculoskeletal knee model was used to assess ACL loading and the key contributions to this loading. To analyse the ACL force in a continuous manner, statistical parametric mapping (SPM) analysis was used. SPM ANOVA and post-hoc t-tests were used to compare total ACL force and contributors to this force over the stance phase of the drop-land-lateral jump task between three groups of females across maturation. Compared to pre- and early/mid-pubertal, females in late/post pubertal group showed significantly higher ACL force during a large percentage of the stance phase, which encompassed the peak ACL forces. The forces developed through sagittal and transverse planes were significantly higher in late/post-pubertal group compared to the two other groups over large percentages of the stance phase. The contribution of the frontal plane mechanisms to ACL force was not significantly different across sexual maturation, while the pre- and early/mid-pubertal groups were not significantly different for any of the outcome measures. The larger ACL forces observed in late/post-puberty group (14-20 years) may partially explain the higher rate of ACL injury in females aged 15-19 years in the last decades. In addition, it has been shown that ACL growth plateaus at the age of 10, prior to full sexual maturation and cessation of growth in stature. Thus, females in late/post pubertal group are potentially heavier, have similar sized ACL, but with greater ACL forces compared to their less sexually mature counterparts. These reasons together could be the foundation, at least in part, for the higher ACL forces observed in this group. The manuscript describing these results is under review as Nasseri A., Lloyd D.G., Minahan C., Sayer T.A., Paterson K., Vertullo C.J., Bryant A.L, Saxby D.J. Effects of pubertal maturation on anterior cruciate ligament forces during a landing task in females. American Journal of Sports Medicine. In conclusion, a computational ACL force model was developed and validated that provided a platform for integration of external biomechanics, muscle and joint contact forces to calculate in vivo ACL force. This ACL force model enabled examination of the ACL loading mechanism by exploring the main muscular and biomechanical contributions to ACL loading; and the effects of pubertal maturation on ACL loading in females. The variability in the magnitude and contributions to ACL force across a wide age range of participants suggest estimation of ACL force is necessary to understand the potential ACL injury mechanisms and design ACL injury prevention programs, rather than relying on external biomechanics that are proposed as surrogates of ACL injury.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School Allied Health Sciences
Griffith Health
Full Text

Advanced development environments are essential for efficient realization of complex industrial products. Powerful equation-based object-oriented (EOO) languages such as Modelica are successfully used for modeling and virtual prototyping complex physical systems and components. The Modelica language enables engineers to build large, sophisticated and complex models. Modelica environments should scale up and be able to handle these large models. This thesis addresses the scalability of Modelica tools by employing incremental compilation and dynamic loading. The design, implementation and evaluation of this approach is presented. OpenModelica is an open-source Modelica environment developed at PELAB in which we have implemented our strategy for incremental compilation and dynamic loading of functions. We have tested the performance of these strategies in a number of different scenarios in order to see how much of an impact they have on the compilation and execution time.

Our solution contains an overhead of one or two hash calls during runtime as it uses dynamic hashes instead of static arrays.

Conducting a case study, this thesis investigates the dynamic behavior of Karun IV arch dam (Iran) under the effect of maximum feasible thermal input. A three-dimensional finite element model is created using ANSYS software. The thesis determines the impact of thermal loadings under two scenarios of normal and minimum water levels. The dam-foundation, dam-water and water-foundation interactions are considered in the modeling to accurately capture its dynamic response. Furthermore, in addition to the water compressibility, appropriate wave absorbing boundaries are used for the reservoir far end and bottom and for the foundation. Stability, static, modal, and thermal analyses are conducted as initial conditions. Using three orthogonal earthquake components, linear dynamic time history analyses with and without thermal loading are performed. The comparison of the results verified that the application of thermal effects on the dynamic analysis increases the maximum tension, changes the maximum pressure, and decreases the displacements in the dam body. This study concludes that thermal loading must be considered in the dynamic analysis of an arch dam as it can worsen the tensile cracks.
Applied Science, Faculty of
Engineering, School of (Okanagan)
Graduate

Abstract Liquid containing tanks (LCTs) are used in water distribution systems and in the industry for storing water, toxic and flammable liquids and are expected to be functional after severe earthquakes. The failure of a large tank during seismic excitation has implications far beyond the economic value of the tanks and their contents. Then seismic design becomes a high necessity for this type of structure. However, tanks differ from buildings in two ways: first, during seismic excitation, the liquid inside the tank exerts a hydrodynamic force on tank walls, base, and roof in addition to the hydrostatic forces. Second, LCTs are generally required to remain watertight. Many current standards and guidelines such as ACI 350.3-06, ACI 371R-08, ASCE7, API650, EUROCODE8 and NZSEE 1986 code, cover seismic designs which are based primarily on theoretical analysis. This analysis is still not enough to fully describe the behavior of this structure under seismic oscillation noting that the theoretical analysis is based on a linear model and two dimensional spaces. So the focus of this study is to measure two important dynamic parameters which are the natural period and the maximum sloshing height of the water under harmonic motion by conducting an experimental investigation and computational fluid dynamic (CFD) simulation. Open-foam is the numerical tool chosen in this study. There is currently no study done with this tool to measure the behavior of the water inside a square tank neither under seismic motion nor harmonic oscillation.Finally, a comparison between the experimental, the analytical and the numerical results will be presented to confirm the level of validity of each method. Then a conclusion is made to summarize this research and to propose future works.

The ability to withstand dynamic loading represents an important design criteria for crucial applications such as those adopted in the automotive and aerospace industries. Numerical simulations can lead to a reduction of time and costs for designing composite structures by replacing testing campaigns that are performed in order to assess whether the design requirements of the structure are met. The present thesis deals with the development of a robust simulation methodology within the FE explicit commercial code PAM-CRASH in order to predict the damage behaviour of Carbon Fiber Reinforced Polymers when loaded dynamically. The strain rate dependence of the carbon/epoxy composite under study is identified and a material-characteristic strain rate model is developed starting from experimental data. A delay damage model based on a Continuum Damage Mechanics approach is used to predict the response of composite laminates under dynamic loading. The simulation methodology is validated against experimental data from a patch to a coupon level by using solid elements to model the plies of the laminate. A dynamic three-point bending simulation is performed at the sub-component level by modelling the composite structure through the use of solid elements for the plies and cohesive elements for the interfaces between them. Rather good agreement is found in terms of stiffness and strength between the results from the numerical simulations and those obtained from the experimental tests. Limitations are identified in the sensitivity of the strain rate model to the damage limits set to stop the scaling of the lamina elastic moduli and in severe dynamic effects, e.g. stress waves, which affected the simulations at high strain rates.

The objective of this research was to analyze the effects of impact loading on electronic component failure. A standard fiberglass composite printed circuit board (PCB) card was used in two impact tests. The first test consisted of a PCB card with four adhered strain gauges, which were mounted inside an aluminum box fabricated for testing. Impact testing was conducted with weights ranging from 0 to 30 lb., and the corresponding strain values were recorded. For the second set of impact tests, a new circuit card was mounted inside the aluminum box. The new circuit card maintained the same dimensions, but no strain gauges were attached. Solder joints were placed at nine different locations on the card, and testing was conducted to determine the impact load at solder joint failure. Both visual and resistance inspections were conducted after each impact. After seven drop tests were conducted, no failure had been detected. This lack of failure was attributed to the rigidity and substantial nature of the aluminum box used in testing. Upon completion of both impact tests, two Finite Element Method (FEM) models were built. The first FEM model represented a scaled version of the PCB card, four solder joints, and a silicon computer chip. Strain data from the PCB card testing was input into the model, and a corresponding solder joint strain was calculated. The second FEM model was a full-scale version of the aluminum box and mounted circuit card. A force was applied to the box, and the various stains were recorded on the PCB card. The collection of this data has helped to establish a valuable relationship between the strains in PCB cards and solder joints, and it will increase the understanding of electronic component failure under impact loading conditions.

Many previous studies have been focused on the behaviour homogeneous granular soils under the quasi-static loading, however, various soil types exist in the field. Therefore, based on the evaluation of these previous studies, an extensive study has been addressed to expose the dynamic behaviour of cohesive-frictional soils associated with the effects of fines content, the effect of moisture content and the type of impact regime. The proposed study mainly investigates the behaviour of sand – clay mixtures to impact loading, both from a loaded plate dropped from different heights and one dropped repeatedly from a fixed height. The Aberdeen beach sand and the Teuchan clay were used for the study and mixed in different proportions to create soils of varying proportions. The six soil samples used have known volumetric proportions of sand : clay and the tests were carried out under the dry condition and two other moisture contents. The results determine the optimal percentage of fines content and its related moisture condition to obtain more stable performance of the granular soils under dynamic compaction. It can be implemented to enhance the quality of ground improvement techniques for the construction. A Soil Model Tester for 2-Dimension program (SM2D) [Chan (1988)] was used to modify the existing material model before being used for Finite Element simulation. The impact test results were used to verify the numerical model developed using an explicit u-w schemebased finite element program, GLADYS2E [Chan et al. (1992, 1994)]. Such use of explicit schemes requires the use of time step lengths which are smaller than a critical value, in order that stability and accuracy of solution are ensured. A semi-empirical formula has been developed for the critical time step determination using MATLAB.

The behaviour of Tidal Stream Turbines (TST) in the dynamic flow field caused by waves and rotor misalignment to the incoming flow (yaw) is currently poorly understood. The dynamic loading applied to the turbine could drive the structural design of the power capture and support subsystems, device size and its proximity to the water surface and sea bed. In addition, the strongly bi directional nature of the flow encountered at many tidal energy sites may lead to devices omitting yaw drives; accepting the additional dynamic loading associated with rotor misalignment and reduced power production in return for a reduction in capital cost. For such a design strategy it is imperative to quantify potential unsteady rotor loads so that the TST device design accommodates the inflow conditions and avoids an unacceptable increase in maintenance action or, more seriously, suffers sudden structural failure. The experiments presented as part of this work were conducted using a 1:20th scale 3-bladed horizontal axis TST at a large towing tank facility. The turbine had the capability to measure rotor thrust and torque, blade root strain, azimuthal position and speed. The maximum outof- plane bending moment was found to be as much as 9.5 times the in-plane bending moment, within the range of experiments conducted. A maximum loading range of 175% of the median out-of-plane bending moment and 100% of the median in-plane bending moment was observed for a turbine test case with zero yaw, scaled wave height of 2m and intrinsic wave period of 12.8s. A Blade Element Momentum (BEM) numerical model has been developed and modified to account for wave motion and yawed flow effects. This model includes a new dynamic inflow correction which is shown to be in close agreement with the measured experimental loads. The gravitational component was significant to the experimental in-plane blade bending moment and was included in the BEM model. Steady yaw loading on an individual blade was found to be negligible in comparison to wave loading (for the range of experiments conducted), but becomes important for the turbine rotor as a whole, reducing power capture and rotor thrust.

Blindness due to ocular trauma is a significant problem in the United States considering that each year approximately 500,000 years of eyesight are lost. The most likely sources of eye injuries include sports related impacts, automobile accidents, consumer products, and military combat. Out of the 1.9 million total eye injuries in the country, more than 600,000 sports injuries occur each year and 40,000 of them require emergency care. In 2007, approximately 66,000 people suffered from vehicle related eye injuries in the United States. Of the vehicle occupants sustaining an eye injury during a crash, as many as 15% to 25% sustained severe eye injuries and it was shown that within these severe eye injuries as many as 45% resulted in globe rupture.

The purpose of this thesis is to characterize the biomechanical response of the human eye to dynamic loading. A number of test series were conducted with different loading conditions to gather data. A drop tower pressurization system was used to dynamically increase intraocular pressure until rupture. Results for rupture pressure, stress and strain were reported. Water streams that varied in diameter and velocity were developed using a customized pressure system to impact eyes. Intraocular pressure, normalized energy and eye injury risk were reported. A Facial and Ocular Countermeasure Safety (FOCUS) headform was used to measure the force applied to a synthetic eye during each hit from projectile shooting toys. The risk of eye injury for each impact was reported. These data provide new and significant research to the field of eye injury biomechanics to further the understanding of eye injury thresholds.
Master of Science

Seabed instability surrounding an immersed tunnel is a vital engineering issue regarding the design and maintenance for submarine tunnel projects. It has been recognised that the pore water pressures and stresses in seabeds are affected by the water pressures generated by the natural dynamic loading. If the pore water pressure reaches the initial mean stress, the liquefaction could occur with the effective stress in seabed vanishing. To avoid seabed instability around the immersed tunnel, the study of seabed dynamic behaviour is necessary under the real hydrodynamic loading. Two mechanisms of wave-induced liquefaction has been reported in the literature, based on a mass of laboratory tests and field exploration, which are transient liquefaction and residual liquefaction. The transient liquefaction is motivated by the oscillatory excess pore water pressures under wave pressure vibration which usually happens with amplitude reduction and phase lag of pore pressure in seabed soil. While the residual liquefaction is on the consequence of the excess pore water pressure build-up under cyclic wave loading. The liquefied seabed soil will behave like a heavy fluid without any shear resistance to supported structures on it, thus leading to catastrophic failure of the immersed tunnel. In the present study, the main objective is to investigate the mechanism of soil response and liquefaction caused by waves and currents in the seabed foundation around the immersed tunnel. An integrated numerical model is established to analysis the seabed behaviour under natural dynamic loading, including ocean waves and currents. In the integrated model, the fluid sub-model is responsible for simulating the two-phase incompressible flow motion inside and outside the porous media, which is governed by the VARANS (Volume-Averages Reynolds Averaged Navier-Stokes) equation, while the seabed model is established adopting the LRBFCM with Biot’s "u− p" approximation which considered the inertial term of soil skeleton. The new conceptual meshfree model for residual mechanism considers the coupling effects between the development of the pore pressure build-up and the evolution of the seabed stresses by adding a source term associated with the shear stresses in the seabed. Good agreements with analytical solution and laboratory experiments validates this newly proposed numerical model. The LRBFCM is examined to be reliable in simulation of wave-induced oscillatory and residual liquefaction behaviour of a seabed. The wave-induced dynamic response of the oscillatory and residual seabed response is investigated adopting the developed integrated model. A series of results, including the seabed stresses, the pore pressure accumulation and the liquefaction potential in the seabed foundation are obtained. The existence of the immersed tunnel affects surrounding seabed dynamic behaviours significantly, including the seabed stresses and the pore water pressures, leading to the local redistribution in the adjacent region of the immersed tunnel. Both the maximum oscillatory and residual liquefied depth on the right-hand side of the tunnel is smaller than that on the left-hand side (the ocean wave is set as propagating along the x-direction from the left-hand side to the right-hand side). From the numerical results, the seabed oscillatory liquefaction is more likely to occur under a shallow water area with the waves of large wave height and long period, moreover, a seabed with lower permeability and degree of saturation is more likely to be liquefied. For the residual seabed response, the residual liquefaction is more likely to occur in the seabed foundations with low relative density and poor drainage condition. The seabed response around the immersed tunnel under combined nonlinear Stocks waves and currents loading is investigated both in oscillatory and residual mechanisms. The simulation results show that the risks of both oscillatory and residual liquefaction are much higher for the seabed under wave combined following currents, and the appearance of opposing current could decreased the probability of the liquefaction occurrence.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Eng & Built Env
Science, Environment, Engineering and Technology
Full Text

The vertical dynamic behaviour of machine foundations subjected to vertical dynamic loading was investigated for surface and embedded foundations. The responses of these machine foundations were determined using analytical and numerical solutions ranging from simple to complex. An accurate prediction of impedance functions for the foundation system is a key step in the design procedures. The prediction accuracy depends on how close the modelling procedures are to reality. The subject of soil dynamics is complex. At times, the choice of the analysis model is based on the experience of the engineer with the model. The chosen model may or may not result in an optimal, efficient, and accurate design. The current advancement in manufacturing technology calls for machine foundation systems with high performance, availability and reliability. The analysis and design of such complex, large and sensitive machine foundations requires good understanding of their dynamic behaviour. The aim of this thesis is to investigate and evaluate the most accurate analytical and numerical models for determining the dynamic behaviour of surface and embedded machine foundations. Surface and embedded footings were cast at the experimental station at the University of Pretoria. The vertical dynamic behaviour of these foundations was determined by vertical harmonic loading. The measured impedance functions were compared with predicted responses obtained from analytical solution of the Winkler model, elastic half-space theory, simplified Lysmer (1965) model, Veletsos and Verbic (1973) models, the Dyna5 program and numerical solution of finite element method (Abaqus). The dynamic responses of the surface foundation predicted by the analytical solution proposed by Veletsos and Verbic (1973) soil with mass, compared reasonably well with the results obtained from field-measured data. The measured impedance functions of the embedded foundation were compared with the predicted results ascertained using the analytical solution proposed by Novak and Beredugo (1972), Dyna5 program and numerical solution of the finite element method (Abaqus). It is shown that embedment increases stiffness, natural frequency, natural frequency ratio, and damping ratio. The embedment reduces resonant amplitude, resonant amplitude ratio and has an insignificant effect on resonant frequency and resonant frequency ratio. The measured dynamic responses compared favourably with the results predicted by the finite element method (Abaqus). The conclusion is that the analytical model proposed by Veletsos and Verbic (1973) soil with mass, and the finite element method (Abaqus) can be used to accurately predict the dynamic response of surface and embedded machine foundations respectively.
Thesis (PhD)--University of Pretoria, 2015.
tm2016
Civil Engineering
PhD
Unrestricted

This thesis analyses the impact of granular aggregates with structures using experiments and numerical simulations. Original contributions include an insight into multiple factors affecting the loading and damage to the structures, along with study of numerical parameters important for realistic prediction of the interaction between the granular media and structures. It extends the current understanding related to such interactions, with an underlying motivation to guide strategies in order to reduce the structural damage. The response of structures impacted by granular media (sand or soil) is of significant research interest for many applications. One of the applications is for landmine explosions which causes ejection of soil from ground and damage to structures impacted by this ejected soil. Experimentation is done in a laboratory setting where the cylindrical sand slugs are generated at high speed using an impulse provided by a piston. This induces a velocity gradient along the slug, because of which the slug expands during the flight before impacting the target. Deformable as well as rigid flat targets are considered in two orientations relative to the incoming slug: perpendicular (i.e. normal orientation) and inclined at an angle of 45°. The targets are supported by force transducers to capture the loading from the slug. Simulations are performed using a combination of discrete particle and finite element schemes, which enables the analysis of the fully coupled interaction between the flowing granular media and the structure. A contact model involving multiple parameters is used for inter-particle and particle-target contact. Firstly, a numerical analysis is performed to characterise the temporal evolution of slugs and their impact on monolithic beams constrained at the ends. Out of all the parameters used for inter-particle contact definition in discrete particle method, only the contact stiffness is found to effect the velocity gradient in the slug before it impacts the target. Other factor influencing the gradient is the acceleration provided by the piston. A strong dependence of beam deflection on the stand-off distance is observed due to the velocity gradient in the slugs. As the second step, the effect of target surface properties on the transmitted momentum is analysed. Experiments are done by applying coatings of different hardness and roughness on the target surface impacted by sand slugs. For normally oriented targets, the transmitted momentum is observed to be insensitive to the change in surface coating. In contrast, for inclined targets, a significant influence of coatings is observed. Additionally, the momentum transmitted to the inclined targets is always less than that for normal targets. Numerical analysis of this surface effect reveals that assuming the slug particles to be spherical shape in simulations does not capture the particle/target interactions accurately and under-predicts the frictional loading on the target. Following this, a detailed numerical study is done to understand the effect of the shape of particles in the slug. Simple shaped non-spherical particles are constructed by combining spherical sub-particles. With increasing angularity of particles in the slug, the frictional loading on the target is shown to increase. This results in an increase of momentum transmitted to inclined targets. For normally oriented targets however, the particle shape does not affect the overall transmitted momentum, which is a behaviour similar to that observed when studying the effect of target surface properties. In addition, effect of fracture of particles in the slug is analysed by using beam connections between sub-particles that break during the impact with the target. If the fracture results in increasing particle angularity, the transmitted momentum increases, whereas the situation reverses if fracture results on more spherical shaped particles. Lastly, a strategy to reduce the loading on the targets is analysed by using sacrificial coating on the target surface. In experiments, this coating is placed on the rigid target surface using a lubricant at their interface. When impacted by the slug, this coating slides on the target surface, resulting in a reduction of frictional loading on the target. If the friction at the coating/target interface vanishes, the transmitted momentum approaches the theoretical minimum value. Simulations are used to first validate the experimental observations and then to extend the concept of sliding coatings using deformable targets. Both the transmitted momentum and deflections depended on the thickness of the target and coating. When a coating is used, the deflections increase due to reduction in target thickness. It is found that the best strategy to reduce the damage to the target is to use least possible thickness of the coating and minimise the friction at the interface between the coating and the target. The presented work examines many of the factors that affect the loading on the target impacted by granular slugs, in addition to characterising the expansion of slugs before the target impact. The analysed factors include those already known such as target stand-off distance, inclination and unveils others such as target surface properties and granular properties. The numerical analysis discloses important parameters and shows the effect of particle shape, highlighting the shortcomings of widely used spherical particle assumption in the numerical studies. A strategy using a sacrificial coating to reduce damage to the target is also analysed.

Les structures aéronautiques peuvent être soumises à des sollicitations sévères telles que les collisions, les impacts de volatiles, etc … Sous l’effet de ces sollicitations rapides, qui du fait de leurs temps caractéristiques très courts limitent les transferts thermiques, le matériau peut dissiper l’énergie dans des zones de déformation localisée qui peuvent conduire à une ruine prématurée de la structure. Le travail de la thèse porte sur la définition d’une méthodologie expérimentale destinée à étudier les conditions de rupture de matériaux à haute résistance à vocation aéronautique consécutives à un endommagement dynamique. Ce travail comprend : •la mise au point d’essais rapides de cisaillement ; •des observations microstructurales des matériaux avant et après sollicitation ; •la simulation numérique des essais
Aeronautical structures may be submitted to severe loading such as collisions, bird strike, etc. Under dynamic loading, involving quasi adiabatic conditions, the material may dissipate energy within zones of localised deformation wich may lead to the premature failure of the structure. The PhD work aims at defining an experimental methodology devoted to study the conditions of fracture of aeronautical, high strength materials intervening after dynamic damage. Tasks include notably: * definition of dynamic shear tests * microstructural observation of the material before and after loading * numerical simulation * Development of fracture criterion

Historic rock-mounted lighthouses play a vital role in the safe navigation around perilous reefs. Their longevity is threatened by the battering of waves which may be set to increase with climate change. The protection of this historic heritage needs the identification of both structural dynamic parameters (natural frequencies and shape modes), and of the worst-cases wave load combination, able to affect that natural frequencies. This dissertation was developed during a period of five months at University of Plymouth, along with the researching team of the project STORMLAMP. The project is divided in three parts; the first involving a meteocean analysis, developed by means of peak over threshold technique, aimed to address realistically a test campaign held afterwards. The second, focused on the dynamic analysis of the Dubh Artach lighthouse and was developed by means of a Matlab toolbox provided to the group by Prof. Brownjohn from Exeter University, partner of the project as well. It is aimed on one hand to detect the dynamic properties of the structure and, on the other hand, to recognize eventual directionality in the structural response. The third phase was held at Plymouth University laboratory “COAST”. During this phase, a laboratory campaign, involving more than 100 tests, allowed to perform a parametric analysis aimed to identify the parameters, of an extreme wave, that influence more the impact force and that the wave exerts on the structure. To extract impact time history, force signals were decomposed by means of Empirical mode decomposition and Duhamel integral algorithms. Image analysis, moreover, allowed to locate run-up caused by those waves upon a steel cylinder and to integrate a study of the run-up as well. The analysis led to several considerations useful on one hand for the prediction of the worst-case loading of the Dubh Artach lighthouse and, on the other hand, for the introduction of the NewWave theory for the design of coastal structures.

The increasing need for regional development has led engineers to find safe ways to construct the infrastructure of transportation on soft soils. Soft soil is not able to sustain external loads without having large deformations. The geotechnical properties of soft soil which is known for its low bearing capacity, high water content, high compressibility and long term settlement as well. In pavement engineering, either highway or runway as an infrastructure, a pavement encompasses three important parts namely traffic load, pavement and subgrade. Traffic load generated from tire pressure of vehicle and/or airplane wheels are usually around 550 kPa even more on the surface of the pavement. Pavement generally comprises granular materials with unbounded or bounded materials located between traffic load and subgrade, distributing the load to surface of subgrade. One of the promising soil improvement techniques is a piled embankment. When geosynthetics layer is unrolled over piles, it is known as geosynthetics supported piled embankment. Particularly in deep soft soil, when piles do not reach a hard stratum due to large thickness of the soft soil, the construction is an embankment on floating piles. Furthermore, because of different stiffness between piles and subsoil, soil arching effect would be developed there. By using Finite Element analysis, some findings resulted from experimental works and several field tests around the world as field case studies are verified. Some important findings are as follows: the stress concentration ratio is not a single value, but it would be changed depending on the height of embankment, consolidation process of subsoil, surcharge of traffic load, and tensile modulus of geosynthetics as well. Ratio height of embankment to clear piles spacing (h/s) around 1.4 can be used as a critical value to distinguish between low embankment and high embankment. When geosynthetics is applied to reinforce a pavement/embankment, the vertical distance of geosynthetics layers and number of geosynthetics layers depend on the quality of pavement material. The lower layer of geosynthetics withstands a tensile stress higher than upper layer. Primary reinforcements for geosynthetics in piled embankments are located at span between piles with maximum strains at zones of adjacent piles. Traffic load that passes through on the surface of the pavement can reduce the soil arching, but it can be restored during the off peak hours. Settlements of embankments on floating piles can accurately be modelled using the consolidation calculation type, whereas the end-bearing piles may be used the plastic calculation type. Longer piles can be effectively applied to reduce a creep. By applying length of floating piles more than 20% of soft soil depth, it would have a significant impact to reduce a creep on a deep soft soil.

Slender precast concrete wall panels are currently in vogue for the construction of tall single storey warehouse type buildings. Often their height to thickness ratio exceed the present New Zealand design code (NZS 3101) limitations of 30:1. Their real performance under earthquake attack is unknown. Therefore, this study seeks to assess the dynamic performance of slender precast concrete wall panels with different base connection details. Three base connections (two fixed base and one rocking) from two wall specimens with height to thickness ratios of 60:1 were tested under dynamic loading. The two fixed based walls had longitudinal steel volumes of 1.27% to 0.54% and were tested on the University of Canterbury shaking table to investigate their proneness to out-of-plane buckling. Based on an EUler-type theoretical formula derived as part of the study, an explanation is made as to why walls with high in-plane capacity are more prone to buckling. The theory was validated against the present and past experimental evidence. The rocking base connection designed and built in accordance with a damage avoidance philosophy was tested on the shaking table in a similar fashion to the fixed base specimens. Results show that in contrast with their fixed base counterparts, rocking walls can indeed fulfil a damage-free design objective while also remaining stable under strong earthquake ground shaking.

This thesis is concerned with defining, theoretically, approximate values of wind loading and predicting dynamic response of air-supported structures subject to suddenly applied loads. Wind loading on air-supported structures is a phenomenon involving significant mutual interaction between inertial, elastic and aerodynamic forces. The aerodynamic forces described by fluid mechanics equations are examined in the first part of the thesis, Chapters 2 to 6. Chapter 2 contains a brief discussion of wind as a flow of air around rigid bodies. This review is followed by an introduction to modern wind engineering, and then by discussion on the theoretical and/or experimental methods to assess wind response of flexible structures. Under the simplifying assumption of three-dimensional potential flow of an incompressible, inviscid, steady air flow, the three-dimensional pressure coefficient distribution on an open-sided paraboloid shallow shell roof is examined in Chapter 5, employing three versions of a vortex-lattice method. The modified Hedman method with horseshoe vortices in the plane z=0, and a boundary condition of tangential flow applied on the body of the shell yielded the best results. In the Appendix to Chapter 5 a real flow solution based on the 'SIMPLE' algorithm is investigated for a numerical example of a thin shell submerged in steady flow - a two-dimensional approximation of the section employed in Chapter 5. For a 3D structure which cannot be adequately represented by 2D model a simple 3D potential flow solution is likely to yield more accurate pressure distributions than a sophisticated 2D real flow analysis. The wind tunnel tests described in Chapter 6 were conducted on a thin, rigid eliptic paraboloid subject to two flow conditions: uniform flow, and in the thick turbulent boundary layer. The theoretical results predicted fairly well the mean pressure distribution on the shell in uniform flow, except on the rearmost part of the model, where separation occurred. In the case of the turbulent boundary layer flow, discrepancies in mean pressure coefficient distributions are of the same order as for uniform flow. However, as the turbulent boundary layer flow is a much more complicated phenomenon than the theoretical description of potential flow, the above conclusion cannot be generalized without further work. The vortex-lattice method, due to its simplicity, can be easily incorporated into any structurefluid interactive scheme accounting for both static and quasi-dynamic behaviour, and an assessment of dynamic response is essential for the design of large air-supported structures. The second part of the thesis, Chapters 7 to 12, is concerned with the structural response of air-supported structures; with special emphasis on the dynamic response following sudden release of a loading system. Chapter 7 gives a review of methods of analysis for pneumatic structures; those experiencing strong geometric nonlinearities are especially focused. The dynamic relaxation method with kinetic damping is discussed in Chapter 8 with respect to the static analysis of pneumatic structures; structural idealization depending on the fabric patterning, type of loading and kind of membrane material being used. Two series of model tests are described; both employing fairly large scale pneumatic domes. The first test model constructed using an orthotropic woven fabric is subject to centrally placed suddenly applied loading. The second test model constructed with very lightweight polythene is subject to suddenly released loading, both central and asymmetric. For this case the internal air and added mass effects become dominant. Explicit dynamic analysis using a centred finite difference scheme is employed in Chapter 9 to analyse the response of pneumatic structures; and in particular to assess the response of the test structures. The influence of surrounding air is included as far as internal air stiffening is considered. For the suddenly unloaded dome, a revised, more efficient numerical scheme is developed, where checking for buckling is carried out at each time step, but creep strains, updated stiffness matrices and unit pressure vectors are calculated at less frequent intervals. In Chapter 10 the tests on the impulsively loaded and unloaded pneumatic domes are described. Dome membrane properties are established from static and dynamic tests on specimens. For dynamic tests a new procedure is devised, to model more closely the state of stresses, by including twodimensional stresses in the testing area of the specimen. Still and movie cameras were used in the static and dynamic tests on the pneumatic domes to record deflection. The results were analysed by means of photogrammetric techniques. The static results compare very well with theoretical predictions. The theoretical dynamic trace for the apex nodal deflection of the impulsively centrally loaded dome differs only slightly from experimental results. The heavy central load influences greatly the response. Discrepancies between theoretical and experimental dynamic responses of the very lightweight and suddenly unloaded dome are however large. The main area of error is caused by improper modelling behaviour of the surrounding air which should be treated as an intrinsic part of the structure. A coupled fluid-structure explicit dynamic analysis, including membrane and air modelling, is presented in Chapter 11. The behaviour of irrotational, inviscid, compressible fluid is described from a Lagrangian point of view. Although only the simplest axisymmetric case is considered, the amount of computing is enormous, hence the procedure cannot, at present, be advocated for use in practice. In Chapter 12 the added mass effect due to vibrating air is discussed. A method to account for virtual mass in shallow pneumatic structures, based on potential incompressible flow and discrete source distributions, is presented and included in the numerical explicit dynamic procedure. The results for the centrally unloaded dome show a great improvement in terms of frequencies, with only a small increase in computing time compared with the numerical scheme of Chapter 9. The discrete source distribution method to calculate added mass effects can be easily extended to any shape of pneumatic structure, and when combined with an explicit dynamic analysis can provide a useful scheme for calculating frequencies and the approximate dynamic response of air-supported structures.

The everyday passage of trains over railway bridges produces fatigue damage at critical bridge locations. The amount of fatigue damage accumulated is very sensitive to the stress ranges producing it. The passage of trains produces dynamic amplification of the internal stresses which depends on the train velocity. Therefore, it is imperative to have a reliable estimation of dynamic effects as these directly affect bridge member stresses. Although this topic is well treated in terms of plate girder bridges and dynamic effects considering the ultimate limit state, less literature is available on the case of tlUSS railway bridges and the fatigue limit state. This thesis addresses this gap of quantifying dynamic effects for everyday train passages and their interaction with the accumulation of fatigue damage in tlUSS railway bridges. Three-dimensional finite element (FE) analyses of a typical metallic tlUSS railway bridge are canied out under the passage of railway freight loading and the effect of different modelling parameters on the intemal forces is qhantified. Subsequently, dynamic amplification factors (DAFs) for all the bridge members are estimated from the FE analyses, under different load models and train velocities, and compared with their bridge code counterparts. Statistical analysis of the estimated DAFs is also employed to propose distributions that capture the variability of the DAF among the bridge members which can then be used for the purposes of probabilistic analysis. Lastly, the effect of dynamic amplification on fatigue damage is explicitly quantified by comparing the damage estimates obtained through the use of codespecific DAFs with the ones obtained in this study.

Bien que le PMMA possède des bonnes propriétés mécanique et optique, sa fragilité devient un des problèmes à prendre en compte quand on l’utilise. Une méthode consistant à mélanger des nanoparticules de caoutchouc au PMMA a été montrée comme améliorant la résistance àla rupture et aux impacts du matériau composite obtenu. Ce mélange est nommé RT-PPMA(pour rubber toughened PMMA). Lors de cette étude, une classe de RT-PMMA commercial,appelée PMMA Resist est considérée. De manière plus spécifique, la réponse de trois nuances de RT-PMMA différant par leur concentration en particules de caoutchouc est étudiée.La caractérisation thermomécanique consistant en des tests de traction, compression et du cisaillement -compression a été menée sur les trois nuances de RT-PMMA à différentes vitesses de déformation et températures. Les vitesses de déformation s’étalaient entre 10-5s-1et 1200s-1, et les températures étaient comprises entre -50°C et 70°C. Comme attendu,la réponse des nuances de RT-PMMA montre la forte dépendance à la vitesse de déformation,la température et la concentration en particule de caoutchouc. En outre, la sensibilité au blanchiment sous contrainte (stress-whitening) induit par micro-craquelure (crazing) vs.décohésion particule/matrice dépend également des trois paramètres cités précédemment. De plus, une structure complexe de bande de cisaillement est observée sur les nuances de RTPMMA lors de la compression dynamique et du chargement en cisaillement-compression.La capacité d’arrêt de fissures de la classe de RT-PMMA à l’étude a été menée en réalisant des essais d'impact de type Kalthoff and Winkler (KW)-. La vitesse de projectile est comprise entre 50 m/s et 100 m/s. Des plaques avec deux entailles qui servent comme pré-fissures ont été utilisés lors des essais de choc. L’interaction du projectile avec l’échantillon a été enregistré avec une caméra ultra rapide de 105 à 106 images par seconde. L'examen postmortem de la microstructure a été observé en utilisant la microscopie électronique à balayage(SEM). La résistance aux chocs de RT-PMMA dépend fortement de la concentration de particules de caoutchouc. En particulier, une concentration plus élevée de particules en caoutchouc aide à ralentir la fissure et ainsi augmenter la capacité d’arrêt de fissures du matériau structural. Les particules de caoutchouc gênent la propagation de fissures et le blanchiment sous contrainte apparaît le long du chemin de propagation de fissures.Une première tentative de modélisation constitutive pour les trois nuances de RT-PMMA a été réalisée en se basant sur le travail fait par Arruda et Boyce (1995). Les modèles dépendant de la vitesse et de la température ont été calibres en considérant les résultats expérimentaux et la dépendance de quelques paramètres a la concentration de particules en caoutchouc et au trajet de chargement est mise en évidence. Les modèles doivent encore être unifies
While PMMA possesses good mechanical and optical properties, its brittleness is one of the issues to be accounted for when using it. An approach consisting in blending small rubber nanoparticles in PMMA has been shown to improve the resistance and impact toughness of the resulting composite material. This mixture is called rubber toughened PMMA or shortly RT-PMMA. In the present study, a class of commercial RT-PMMA, namely PMMA Resist,is considered. More specifically, the response of three grades of RT-PMMA differing by their rubber particle concentration is investigated. A thermomechanical characterization consisting of tension, compression and shearcompression tests has been first carried out on the three grades of RT-PMMA at various strain rates and temperatures. The strain rate range was 10-5s-1 to 1200s-1, and the temperature range was from -50°C to 70°C. As expected, the RT-PMMA grades response exhibits a strong dependence on strain rate, temperature and rubber particle concentration. Moreover,the sensitivity of RT-PMMA to crazing vs particle-matrix debonding induced stress whitening under tension loading also depends on the three above mentioned parameters. Additionally, a complex pattern of shear bands is observed on the RT-PMMA grades under dynamic compression and shear-compression loading. Next, the crack arrest capability of the class of RT-PMMA under consideration has been investigated by carrying out Kalthoff and Winkler (KW)-like impact test. The projectile impact velocity range was 50 m/s to 100 m/s. Double-notched plates representing the precracked structures were used for the impact tests. The interaction between the projectile and the plate was recorded by using a high-speed camera at 105 to 106 frames per second. Post mortem microstructure was observed using scanning electron microscope (SEM). Impact resistance of RT-PMMA is seen to strongly depend on the rubber particle concentration. In particular, a higher rubber particle concentration aids to slow down the crack tip velocity and thus to increase the crack arrest capability of the structural material. Crack propagation is hindered by the rubber particles and particle-matrix debonding induced stress whitening appears at the crack propagation path. A first attempt of constitutive modelling for the three grades of RT-PMMA has been donebased on the work by Arruda and Boyce (1995). The rate and temperature dependent models are calibrated by considering experimental results and the dependence of some parameters on the rubber particle concentration and loading path is evidenced. The models have still to be unified

Plane strain transient finite thermomechanical deformations of heat-conducting particulate composites comprised of circular tungsten particulates in nickel-iron matrix are analyzed using the finite element method to delineate the initiation and propagation of brittle/ductile failures by the nodal release technique. Each constituent and composites are modeled as strain hardening, strain-rate-hardening and thermally softening microporous materials. Values of material parameters of composites are derived by analyzing deformations of a representative volume element whose minimum dimensions are determined through numerical experiments. These values are found to be independent of sizes and random distributions of particulates, and are close to those obtained from either the rule of mixtures or micromechanics models. Brittle and ductile failures of composites are first studied by homogenizing their material properties; subsequently their ductile failure is analyzed by considering the microstructure. It is found that the continuously varying volume fraction of tungsten particulates strongly influences when and where adiabatic shear bands (ASB) initiate and their paths. Furthermore, an ASB initiates sooner in the composite than in either one of its constituents. We have studied the initiation and propagation of a brittle crack in a precracked plate deformed in plane strain tension, and a ductile crack in an infinitely long thin plate with a rather strong defect at its center and deformed in shear. The crack may propagate from the tungsten-rich region to nickel-iron-rich region or vice-a-versa. It is found that at the nominal strain-rate of 2000/s the brittle crack speed approaches Rayleigh's wave speed in the tungsten-plate, the nickel-iron-plate shatters after a small extension of the crack, and the composite plate does not shatter; the minimum nominal strain-rate for the nickel-iron-plate to shatter is 1130/s. The ductile crack speed from tungsten-rich to tungsten-poor regions is nearly one-tenth of that in the two homogeneous plates. The maximum speed of a ductile crack in tungsten and nickel-iron is found to be about 1.5 km/s. Meso and multiscale analyses have revealed that microstructural details strongly influence when and where ASBs initiate and their paths. ASB initiation criteria for particulate composites and their homogenized counterparts are different.
Ph. D.