Dissertations / Theses on the topic 'Small strain dynamic properties'
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1
Beyerlein, Kenneth Roy. "Simulation and Modeling of the Powder Diffraction Pattern from Nanoparticles: Studying the Effects of Faulting in Small Crystallites." Doctoral thesis, Università degli studi di Trento, 2011. https://hdl.handle.net/11572/368693.
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Accurate statistical characterization of nanomaterials is crucial for their use in emerging technologies. This work investigates how different structural characteristics of metal nanoparticles influence the line profiles of the corresponding powder diffraction pattern. The effects of crystallite size, shape, lattice dynamics, and faulting are all systematically studied in terms of their impact on the line profiles.
The studied patterns are simulated from atomistic models of nanoparticles via the Debye function. This approach allows for the existing theories of diffraction to be tested, and extended, in an effort to improve the characterization of small crystallites. It also begins to allow for the incorporation of atomistic simulations into the field of diffraction. Molecular dynamics simulations are shown to be effective in generating realistic structural models and dynamics of an atomic system, and are then used to study the observed features in the powder diffraction pattern.
Furthermore, the characterization of a sample of shape controlled Pt nanoparticles is carried out through the use of a developed Debye function analysis routine in an effort to determine the predominant particle shape. The results of this modeling are shown to be in good agreement with complementary characterization methods, like transmission electron microscopy and cyclic voltammetry.
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Venables, R. "Dynamic strain ageing and the fatigue behaviour of nimonic 901." Thesis, University of Nottingham, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376646.
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Voorhies, Katherine Desiree. "Static and Dynamic Stress/Strain Properties for Human and Porcine Eyes." Thesis, Virginia Tech, 2003. http://hdl.handle.net/10919/31867.
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Every year, more than 2.4 million eye injuries occur in the United States, with over 30,000 of those injured left blind in at least one eye as a result. Computer modeling is one of the most versatile ways to study ocular trauma, however, existing models lack accurate stress and strain properties for ocular globe rupture. A pressure system was built to examine static and dynamic globe rupture pressures for healthy postmortem human and porcine (pig) eyes. Maximum rupture stress for the quasi-static tests was found to be 11.17MPa for human tissue and 12.08MPa for porcine tissue, whereas stress for the dynamic tests was found to be 30.18MPa for human tissue and 26.01MPa for porcine tissue. Maximum rupture stress results correlate well with static material properties used in published research (9.4MPa), and dynamic properties of 23MPa found in published research. Healthy postmortem human eyes were ruptured statically and dynamically to determine the relationship between stress and strain for the ocular globe under intraocular pressure loading. Stress-strain relationships were investigated and values for the elastic modulus were found to be slightly lower than that previously published. This research shows that it is important to differentiate between tissue type, and static versus dynamic failure properties before drawing conclusions from computer models and other published research. Now that rupture can be accurately determined, safety systems designed to protect eyesight in automotive, sports, and military applications can also be applied to protect the quality of life for humans in these applications.Master of Science
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Wang, J., S. Dong, Ashraf F. Ashour, X. Wang, and B. Han. "Dynamic mechanical properties of cementitious composites with carbon nanotubes." Elsevier, 2019. http://hdl.handle.net/10454/17465.
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YesThis paper studied the effect of different types of multi-walled carbon nanotubes (MWCNTs) on the dynamic mechanical properties of cementitious composites. Impact compression test was conducted on various specimens to obtain the dynamic stress-strain curves and dynamic compressive strength as well as deformation of cementitious composites. The dynamic impact toughness and impact dissipation energy were, then, estimated. Furthermore, the microscopic morphology of cementitious composites was identified by using the scanning electron microscope to show the reinforcing mechanisms of MWCNTs on cementitious composites. Experimental results show that all types of MWCNTs can increase the dynamic compressive strength and ultimate strain of the composite, but the dynamic peak strain of the composite presents deviations with the MWCNT incorporation. The composite with thick-short MWCNTs has a 100.8% increase in the impact toughness, and the composite with thin-long MWCNTs presents an increased dissipation energy up to 93.8%. MWCNTs with special structure or coating treatment have higher reinforcing effect to strength of the composite against untreated MWCNTs. The modifying mechanisms of MWCNTs on cementitious composite are mainly attributed to their nucleation and bridging effects, which prevent the micro-crack generation and delay the macro-crack propagation through increasing the energy consumption.
5
Lo, Kai Fung. "Small-strain shear modulus and damping ratio determination by bender element /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?CIVL%202005%20LOK.
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Kemper, Andrew Robb. "Material Properties of Human Rib Cortical Bone from Dynamic Tension Coupon Testing." Thesis, Virginia Tech, 2005. http://hdl.handle.net/10919/43709.
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The purpose of this study was to develop material properties of human rib cortical bone using dynamic tension coupon testing. This study presents 117 human rib cortical bone coupon tests from six cadavers, three male and three female, ranging in age from 18 to 67 years old. The rib sections were taken from the anterior, lateral, and posterior regions on ribs 1 through 12 of each cadaver's rib cage. The cortical bone was isolated from each rib section with a low speed diamond saw, and milled into dog bone shaped tension coupons using a small computer numerical control machine. A high-rate servo-hydraulic Material Testing System equipped with a custom slack adaptor, to provide constant strain rates, was used to apply tension loads to failure at an average rate of 0.5 strains/sec. The elastic modulus, yield stress, yield strain, ultimate stress, ultimate strain, and strain energy density were determined from the resulting stress versus strain curves. The overall average of all cadaver data gives an elastic modulus of 13.9 GPa, a yield stress of 93.9 MPa, a yield strain of 0.883 %, an ultimate stress of 124.2 MPa, an ultimate strain of 2.7 %, and a strain energy density of 250.1 MPa-strain. For all cadavers, the plastic region of the stress versus strain curves was substantial and contributed approximately 60 strain % to the overall response and over 80 strain % in the tests with the 18 year old cadaver. The rib cortical bone becomes more brittle with increasing age, shown by an increase in the modulus (p < 0.01) and a decrease in peak strain (p < 0.01). In contrast to previous three-bending tests on whole rib and rib cortical bone coupons, there were no significant differences in material properties with respect to rib region or rib level. When these results are considered in conjunction with the previous three-point bending tests, there is regional variation in the structural response of the human rib cage, but this variation appears to be primarily a result of changes in the local geometry of each rib while the material properties remain nearly constant within an individual.Master of Science
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Kates, Gina L. "Development and implementation of a seismic flat dilatometer test for small-and high-strain soil properties." Thesis, Georgia Institute of Technology, 1996. http://hdl.handle.net/1853/20234.
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Bisplinghoff, Jill Aliza. "Biomechanical Response of the Human Eye to Dynamic Loading." Thesis, Virginia Tech, 2009. http://hdl.handle.net/10919/31880.
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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
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Almaari, Firas, and Essam Aljbban. "Strain Rate Effect on Fracture Mechanical Properties of Ferritic-Pearlitic Ductile Iron." Thesis, Linnéuniversitetet, Institutionen för byggteknik (BY), 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-78858.
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This study investigates the effect of strain rate on fracture properties of Ferritic-Pearlitic Ductile Iron. A series of dynamic three point bending tests, with various load application rates, are conducted on Charpy V-notch specimens, in room temperature and approximately -18 °C. The tests are performed in a custom-made fixture and during the tests, force and displacement data are recorded. A XFEM (Extended Finite Element Method) model of the test setup has been established and material data from the tests are used as input to the model. The test results show a strong dependency of the strain rate regarding the force needed for crack initiation. Moreover, it can be concluded that low temperature makes the material very brittle, even at low load application rates.
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Sears, Nicholas C. "Investigations into the Quasi-Static and Dynamic Properties of Flexible Hybrid Electronic Material Systems." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1525278328687427.
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Akutagawa, Keizo. "The effect of incorporation of low molar mass liquids on the dynamic mechanical properties of elastomers under strain." Thesis, Queen Mary, University of London, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294965.
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Pokharel, Janak. "CYCLIC LOAD RESISTANCE AND DYNAMIC PROPERTIES OF SELECTED SOIL FROM SOUTHERN ILLINOIS USING UNDISTURBED AND REMOLDED SAMPLES." OpenSIUC, 2014. https://opensiuc.lib.siu.edu/theses/1545.
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The liquefaction resistance of undisturbed soil samples collected from a selected location in Carbondale, Southern Illinois was evaluated by conducting cyclic triaxial tests. Index property tests were carried out on the sample for identification and classification of the soil. Cyclic triaxial tests were conducted on undisturbed sample after saturation, undisturbed sample at natural water content and remolded samples prepared by compaction in the lab. The results were used to evaluate the effect of saturation and remolding on liquefaction resistance of the local soil. Effect of effective confining pressure on dynamic properties of soil (Young's Modulus and Damping ratio) was also studied. Forty five stress controlled cyclic triaxial tests were performed. Three different values of initial effective confining pressure (5 psi, 10 psi and 15 psi) were used and cyclic stress ratio was varied from 0.1 to 0.5 in order to apply different cyclic shear stresses. The results show that the cyclic load resistance of soil decreases as a result of remolding. Saturated undisturbed samples show increase in resistance to liquefaction with increase in initial confining pressure. Remolded samples were prepared by compaction in the lab keeping unit weight and water content equal to that of undisturbed samples. Remolded samples show increase in liquefaction resistance with increase in confining pressure. Undisturbed samples at natural water content show increase in resistance to develop axial strain with increase in confining pressure. Both the rate of excess pressure development and axial strain development increase significantly as a result of remolding. While investigating the effect of saturation of undisturbed samples on liquefaction resistance of soil, interesting observations were made. The excess pressure buildup rate was faster in case of saturated undisturbed samples compared to that in samples with natural water content. On the other hand, rate of strain development was significantly high in case of sample with natural water content compared to that in saturated sample. Also, results obtained from cyclic triaxial tests on saturated undisturbed samples were compared with results obtained from similar tests on Ottawa Sand (Lama 2014) sample. The comparison shows that the saturated undisturbed soil samples of the selected local soil have very high resistance to liquefaction both in terms of initial liquefaction and development of 2.5% and 5% axial strain. Modulus of Elasticity and damping ratio were studied as important dynamic properties of soil. Young's Modulus was observed to decrease significantly at higher strain levels for all three types of samples. Young's modulus increased with increase in effective confining pressure, the effect of confining pressure being large at low strain level and almost insignificant at higher strain level. Damping ratio was highest in undisturbed sample at natural water content and smallest in remolded sample and damping ratio for saturated undisturbed sample falls in between. The damping ratio did not show any definite correlation with strain and confining pressure at lower strain level. But, for strain higher than 1% double amplitude axial strain, damping ratio significantly decreases with increase in strain. Damping ratio increases with increase in confining pressure as observed at high strain for all samples.
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Seo, Youngmi. "Structure and Dynamic Properties of Interfacially Modified Block Copolymers from Molecular Dynamics Simulations." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492628195548591.
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Deshpande, Yogesh. "Quasi-static and Dynamic Mechanical Response of T800/F3900 Composite in Tension and Shear." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1525276843674052.
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Dongfang, Shiping [Verfasser], Markus [Akademischer Betreuer] Hecht, Markus [Gutachter] Hecht, and Bartosz [Gutachter] Firlik. "Running dynamic properties of small-radius railway wheels / Shiping Dongfang ; Gutachter: Markus Hecht, Bartosz Firlik ; Betreuer: Markus Hecht." Berlin : Technische Universität Berlin, 2021. http://d-nb.info/1241183198/34.
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Goudarzy, Meisam [Verfasser], Tom [Akademischer Betreuer] Schanz, and J. Carlos [Akademischer Betreuer] Santamarina. "Micro and macro mechanical assessment of small and intermediate strain properties of granular material / Meisam Goudarzy. Gutachter: Tom Schanz ; J. Carlos Santamarina." Bochum : Ruhr-Universität Bochum, 2016. http://d-nb.info/1089006837/34.
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Goudarzy, Meisam Verfasser], Tom [Akademischer Betreuer] Schanz, and J. Carlos [Akademischer Betreuer] [Santamarina. "Micro and macro mechanical assessment of small and intermediate strain properties of granular material / Meisam Goudarzy. Gutachter: Tom Schanz ; J. Carlos Santamarina." Bochum : Ruhr-Universität Bochum, 2016. http://d-nb.info/1089006837/34.
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Martin, Morgana. "Dynamic mechanical behavior and high pressure phase stability of a zirconium-based bulk metallic glass and its composite with tungsten." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22693.
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Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008.Committee Chair: Thadhani, Naresh; Committee Member: Doyoyo, Mulalo; Committee Member: Kecskes, Laszlo; Committee Member: Li, Mo; Committee Member: Sanders, Thomas; Committee Member: Zhou, Min.
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Kvedaras, Rokas. "Automatizuotų skaitmeninių sistemų mažiems pokyčiams įvertinti tyrimas." Doctoral thesis, Lithuanian Academic Libraries Network (LABT), 2006. http://vddb.library.lt/obj/LT-eLABa-0001:E.02~2005~D_20060212_175704-20968.
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1. The method for automatic digital balancing of Wheatstone resistance bridge was developed and investigated using DAC R-2R matrix for evaluation of resistance small changes. Balancing method enables reducing of external influence impact to evaluation results and avoids most disadvantages that are common to classic systems based on unbalanced Wheatstone bridge. The parameters of connecting wires and channel switches are not impacting evaluation results in the system developed. Advantages of the system developed are proven by experiments. System developed allows low cost implementation of systems for evaluation of resistance small changes.
2. Possibilities for simplification of circuit for resistance small changes evaluation by using digital signal processing means are proven. It is established that by using known and the proposed methods for improvement of reliability of evaluation results resolution of the evaluation is 12–14 bits (0,024 % - 0,006 % accuracy). It is established that it is necessary to use digital signal processing methods for achieving such resolution.
3. New structures of resistance small change evaluation systems ensuring resolutions of 212 and 28 intended for laboratory investigations and monitoring of constructions are proposed. Experimental model of system with resolution of 28 was made and investigated. Methods of reducing noises in long cables were established during experiment. In general it was proven that the model is suitable for monitoring tasks... [to full text]
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PILLAI, Vinoshene. "Intravital two photon clcium imaging of glioblastoma mouse models." Doctoral thesis, Scuola Normale Superiore, 2021. http://hdl.handle.net/11384/109211.
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Yun-HsiangChung and 鍾允翔. "Study of small strain dynamic properties of soil in offshore wind farm near Changhua area." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/369md4.
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Kacar, Onur. "Building a framework for predicting the settlements of shallow foundations on granular soils using dynamically measured soil properties." Thesis, 2014. http://hdl.handle.net/2152/24886.
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In this dissertation, the framework is being developed for a new method to predict the settlements of shallow foundations on granular soil based on field seismic and laboratory dynamic tests. The new method combines small-strain seismic measurements in the field with nonlinear measurements in the field and/or in the laboratory. The small-strain shear modulus (Gmax ) of granular soil and the stress dependency of Gmax is determined from the shear wave velocity measurements in the field. Normalized shear modulus (G/Gmax ) versus log shear strain(log [gamma]) curves are determined from field or laboratory measurements or from empirical relationships. The G/Gmax -- log [gamma] curves and Gmax values are combined to determine the shear stress-shear strain response of granular soil starting from strains of 0.0001% up to 0.2-0.5%. The shear stress-shear strain responses at strains beyond 1.0-2.0 % are evaluated by adjusting the normalized shear modulus curves to larger-strain triaxial test data. A user defined soil model (MoDaMP) combines these relationships and incorporates the effect of increasing confining pressure during foundation loading. The MoDaMP is implemented in a finite element program, PLAXIS, via a subroutine. Measured settlements from load-settlement tests at three different sites where field seismic and laboratory dynamic measurements are available, are compared with the predicted settlements using MoDaMP. Predictions with MoDaMP are also compared with predictions with two commonly used methods based on Standard Penetration and Cone Penetration tests. The comparison of the predicted settlements with the measured settlements show that the new method developed in this research works well in working stress ranges. The capability of the new method has significant benefits in hard-to-sample soils such as in large-grained soils with cobbles and cemented soils where conventional penetration test methods fail to capture the behavior of the soil. The new method is an effective-stress analysis which has applicability to slower-draining soils such as plastic silts and clays.text
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Sanjeev, Shinde Ninad. "An Application of Resonant Column along with Bender Elements Tests for Assessing Dynamic Properties and Liquefaction Potential of Sand." Thesis, 2021. https://etd.iisc.ac.in/handle/2005/5654.
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For determining the response of foundation soils subjected to different kinds of vibrations emerging from (i) an occurrence of an earthquake, (ii) movements of trains over railways tracks, (iii) running of vehicles over roads, and (iv) continuous running of different types of the vibratory machines such as turbines, generators, and reciprocating engines, it is essential to know the dynamic properties of soils at different levels of strains and stresses. The resonant column and bender element tests have been extensively used over past few decades by numerous researchers for evaluating the dynamic properties of soils in a strain range of 0.0001 % to 0.1 %. In the present thesis, the resonant column and the bender elements tests have been employed to determine the dynamic properties (namely, elastic moduli and damping ratio) of not only soils (sand) but even for metal specimens as well to check the applicability of the tests for very stiff specimens. With a provision of continuous excitations, it was also attempted to extend the usage of the resonant column apparatus to examine the liquefaction potential of a fully saturated sand specimen. In addition, an attempt has also been made to introduce an automated predominant frequency-based approach to predict very accurately the arrival times of the waves while interpreting the bender elements tests results.
From the study of the received bender elements signals, it has been noted that, due to the presence of near field effects, it becomes a little difficult task to measure accurately the arrival point of the shear wave. The different available procedures in literature to mark the arrival times of the shear wave often require manual intervention and are often prone to errors. Therefore, it was aimed to develop an automated method to identify the time of the arrival of the shear wave. In the proposed method, the predominant frequency of the received signal was first evaluated and then, with the help of the sliding Fourier transform approach, the arrival time of the shear wave was identified. The main advantage of the proposed approach is that the method can be fully automated by following the algorithm which can be easily coded into a computer program, and it eliminates any manual judgement while determining the travel time. The proposed automated approach was applied to the bender element tests performed on dry and saturated sands by varying the input frequency of the signal, confining pressure, and void ratio. The results obtained from the proposed method were found to be very promising and were noted to be consistent with the data obtained on the basis of the resonant column tests
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Zheng, Yi Mei, and 鄭宜玫. "Dynamic Behavior of Geogrids in a Local Small Strain Range." Thesis, 2010. http://ndltd.ncl.edu.tw/handle/94664824850653239327.
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碩士建國科技大學
土木與防災研究所
98
In order to use working stress analysis reasonably, the suitable material parameters of geogrid is required. Due to nonlinearity of the load-strain curve, it is necessary to establish the local load-strain curve at the desired strain level to determine its material properties. In this research, the tensile loading system is equipped with stepper motor, non-contact proximitors, and data acquisition device with 22 bits resolution. As the measurable minimum delta-strain is close to 0.00125%, the local load-strain curve is readily obtained. A series of strain-controlled cyclic tensile loading tests was performed on 4 woven types of PET geogrids. The local strain curves are constructed at around three strain levels (ε=1, 3, and 5%). The peak-to-peak strains (p-p) of cyclic tensile tests were performed from 0.02% to 1% at each strain level to demonstrate the variation of stiffness (J) and damping ratio (h). It is shown a general trend of that the stiffness increased while the damping ratio decreased with increasing cyclic strain (p-p) at each strain level. For MD40, the J value is obviously affected by the cyclic frequency (f). As higher strain level, a high cyclic frequency gets high J value, but it is not obvious on damping ratio. In comparison between these 4 woven types of PET geogrids, the stiffness obtained at different strain level is affected by the manufacture process. This paper describes the tensile loading system, presents test results, and discusses the behavior of geogrids in a small strain range at different strain level.
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Sebastian, Resmi. "Elastic Wave Propagation and Evaluation of Low Strain Dynamic Properties in Jointed Rocks." Thesis, 2015. http://etd.iisc.ac.in/handle/2005/3653.
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When the point under consideration is not near to the source of vibration, the strains developed in the rock mass due to the passage of waves are usually of small magnitude, and within the elastic range. However, the rock mass may be subjected to a wide range of strain levels depending on the source of vibration and the wave frequency, even within the elastic limit. The present study is based on the two general conditions existing at field, long wave length propagation of waves and intermediate wavelength propagation of waves. When the wavelength of propagating wave is much longer than the joint spacing, it is referred to as long wavelength condition and is associated with propagation of low frequency waves across closely spaced joints. When wavelength of propagating wave is nearly equal to joint spacing, it is known as intermediate wavelength condition and is associated with propagation of high frequency waves. Long wave length propagation of waves has been studied by conducting laboratory experiments using Resonant Column Apparatus on developed plaster gypsum samples. The influence of joint types, joint spacing and joint orientation on wave propagation has been analyzed at three confining stresses under various strain levels. The wave velocities and damping ratios at various strain levels have been obtained and presented. Shear wave velocities are more dependent on confining stress than compression wave velocities across frictional joints whereas, compression wave velocities are more dependent on confining stress than shear wave velocities across filled joints. Wave velocities are at minimum and wave damping is at maximum across horizontal joints whereas wave velocities are at maximum and wave damping is at minimum across vertical joints. Shear wave velocity and shear wave damping are more dependent on joint orientations than compression wave velocity and compression wave damping. As Resonant Column Apparatus has some limitations in testing stiff samples, a validated numerical model has been developed using Discrete Element Method (DEM) that can provide resonant frequencies under torsional and flexural vibrations. It has been found from numerical simulations, that reduction of normal and shear stiffness of joint with increasing strain levels leads to wave velocity reduction in jointed rock mass. Intermediate wave length propagation of waves has been studied by conducting tests using Bender/ extender elements and the numerical simulations developed using 3DEC (Three Dimensional Distinct Element Code).Parametric study on energy transmission, wave velocities and wave amplitudes of shear and compression waves, has been carried out using the validated numerical model. The propagation of waves across multiple parallel joints was simulated and the phenomenon of multiple reflections of waves between joints could be observed. The transformations of obliquely incident waves on the joint have been successfully modeled by separating the transmitted transformed P and S waves. The frequency dependent behavior of jointed rocks has been studied by developing a numerical model and by applying a wide range of wave frequencies. It has been found that low frequency shear waves may involve slips of rock blocks depending on the strength of rock joint, leading to less transmission of energy; while low frequency compression waves are well transmitted across the joints. High frequency shear and compression waves experience multiple reflections and absorptions at joints.
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Sebastian, Resmi. "Elastic Wave Propagation and Evaluation of Low Strain Dynamic Properties in Jointed Rocks." Thesis, 2015. http://etd.iisc.ernet.in/2005/3653.
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When the point under consideration is not near to the source of vibration, the strains developed in the rock mass due to the passage of waves are usually of small magnitude, and within the elastic range. However, the rock mass may be subjected to a wide range of strain levels depending on the source of vibration and the wave frequency, even within the elastic limit. The present study is based on the two general conditions existing at field, long wave length propagation of waves and intermediate wavelength propagation of waves. When the wavelength of propagating wave is much longer than the joint spacing, it is referred to as long wavelength condition and is associated with propagation of low frequency waves across closely spaced joints. When wavelength of propagating wave is nearly equal to joint spacing, it is known as intermediate wavelength condition and is associated with propagation of high frequency waves. Long wave length propagation of waves has been studied by conducting laboratory experiments using Resonant Column Apparatus on developed plaster gypsum samples. The influence of joint types, joint spacing and joint orientation on wave propagation has been analyzed at three confining stresses under various strain levels. The wave velocities and damping ratios at various strain levels have been obtained and presented. Shear wave velocities are more dependent on confining stress than compression wave velocities across frictional joints whereas, compression wave velocities are more dependent on confining stress than shear wave velocities across filled joints. Wave velocities are at minimum and wave damping is at maximum across horizontal joints whereas wave velocities are at maximum and wave damping is at minimum across vertical joints. Shear wave velocity and shear wave damping are more dependent on joint orientations than compression wave velocity and compression wave damping. As Resonant Column Apparatus has some limitations in testing stiff samples, a validated numerical model has been developed using Discrete Element Method (DEM) that can provide resonant frequencies under torsional and flexural vibrations. It has been found from numerical simulations, that reduction of normal and shear stiffness of joint with increasing strain levels leads to wave velocity reduction in jointed rock mass. Intermediate wave length propagation of waves has been studied by conducting tests using Bender/ extender elements and the numerical simulations developed using 3DEC (Three Dimensional Distinct Element Code).Parametric study on energy transmission, wave velocities and wave amplitudes of shear and compression waves, has been carried out using the validated numerical model. The propagation of waves across multiple parallel joints was simulated and the phenomenon of multiple reflections of waves between joints could be observed. The transformations of obliquely incident waves on the joint have been successfully modeled by separating the transmitted transformed P and S waves. The frequency dependent behavior of jointed rocks has been studied by developing a numerical model and by applying a wide range of wave frequencies. It has been found that low frequency shear waves may involve slips of rock blocks depending on the strength of rock joint, leading to less transmission of energy; while low frequency compression waves are well transmitted across the joints. High frequency shear and compression waves experience multiple reflections and absorptions at joints.
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Lin, Yung-Ching, and 林永慶. "The Strain Rate and Temperature Dependence of the Dynamic Shear Properties of Al-Sc Alloy." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/86563883479343051561.
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碩士國立成功大學
機械工程學系碩博士班
93
The dynamic shear deformation behavior and fracture characteristics of Al-Sc alloy are studied using a split-Hopkinson torsional bar at shear strain rates of 800 s-1, 1800 s-1 and 2800 s-1 and temperatures of -150℃, 25℃ and 300℃. The experimental results indicate that shear strain, shear strain rate and temperature all have a significant influence on the mechanical properties of Al-Sc alloy. At constant temperature, the flow shear stress, fracture shear strain, work hardening rate, yielding shear strength, work hardening coefficient, strain rate sensitivity and temperature sensitivity increase with increasing strain rate, while the activation energy decreases. Under a constant strain rate, the flow shear stress, work hardening rate, yielding shear strength, work hardening coefficient, strain rate sensitivity and temperature sensitivity decrease with increasing temperature, while the fracture shear strain and activation energy increase. The fracture surfaces are characterized by dimple features, which are indicative of ductile fracture. The appearance and density of these dimples are significantly dependent on the strain rate and temperature. The fracture surfaces on the gauge length are twisted into band-like features as a result of large localized shear deformation. This phenomenon provides clear evidence of adiabatic shear bands. Only a deformed shear band is found. The width of the shear band is determined by the strain rate and temperature. It is found that microvoid nucleation and growth play a significant role in shear band formation. The presence of precipitates accelerates the adiabatic shear fracture formation via void nucleation and growth. Finally, it is shown that the Kobayashi & Dodd constitutive equation accurately describes the high-strain-rate shear plastic behavior of Al-Sc alloy under the current test conditions.
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Umberg, David 1987. "Dynamic properties of soils with non-plastic fines." Thesis, 2012. http://hdl.handle.net/2152/ETD-UT-2012-05-5854.
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The results from an experimental study on the dynamic properties of sand with nonplastic silt are presented. Combined resonant column and torsional shear equipment is used to evaluate the effects of confining pressure, shearing strain, frequency, and number of cycles of loading on the dynamic properties of silty sand. The goal of this study is to determine if relationships in the literature for sands and gravels are accurate for predicting the shear modulus and material damping characteristics of soil with nonplastic fines or if the incorporation of a fines content parameter improves predictions. This goal was primarily accomplished by reconstituting and testing samples of an alluvial deposit from Dillon Dam, Dillon, Colorado according to predetermined gradation curves with variable amounts of non-plastic fines. Among the findings of this investigation are: (1) soil parameters such as Cu and D50 can be related to dynamic properties of soils with up to 25% fines, (2) the effects of non-plastic fines on the small-strain dynamic properties of soils are not very pronounced for soils with less than 25% fines, and (3) an increase in the amount of non-plastic fines in uniform soils or soils with more than 25% fines generally results in lower values of small-strain shear modulus, higher values of small-strain material damping, and more linear G/Gmax - log([gamma]) and D - log([gamma]) curves. The effect of non-contacting, larger granular particles in a finer soil matrix is also investigated along with the impact of removing larger particles from laboratory samples.text
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Li, Yi-Chun, and 李翊群. "Determination of Small-strain Dynamic Shear Modulus and Damping Ratio of Calcareous Sand by Resonant Column Test." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/59301385428896988240.
Full textAbstract:
碩士國立臺灣大學
土木工程學研究所
104
Calcareous sands are composed of calcium carbonate, which is weak structurally compared to quartz sand. Calcareous sand has high void ratio and angular particle shape. They are mostly distributed in tropical and subtropical shallow waters where earthquakes occur frequently. It has been observed in past earthquakes that sites that have high content of calcareous sands experienced strong shaking and liquefaction. Hence, it is important to study the dynamic behavior of calcareous sands as their behavior may have an adverse effect on coastal and offshore structures during earthquakes. In this study, resonant column tests were performed to evaluate the small-strain dynamic properties of calcareous sands. Sands with different contents of calcium carbonate are considered. Specimens are prepared by pluviation method. Effect of confining pressure on small-strain shear modulus and damping ratio is also investigated. Laboratory test results indicate that small-strain shear modulus increases and damping ratio decreases with increasing confining pressure. Shear modulus decreases with increasing calcium carbonate content at low confining pressure. However, the influence of calcium carbonate content on shear modulus becomes smaller with increasing confining pressure. On the other hand, effect of calcium carbonate content on damping ratio and modulus reduction curves is insignificant at all levels of confining pressure. Predictions of maximum shear modulus and damping ratio from empirical models by Hardin & Richart’s equation and Menq are compared to the test data. The comparison indicates that there would be bias in the prediction if the effect of calcium carbonate content is not considered. Hence new coefficients for those empirical models, which are functions of calcium carbonate content, are proposed.
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CHEN, JIN-XIU, and 陳錦修. "The effect of dynamic strain aging and dynamic precipitation on the high-temperature tensile properties of austenitic Fe-Mn-Al allyos." Thesis, 1990. http://ndltd.ncl.edu.tw/handle/47552808157593741011.
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Mu-ShiunHsieh and 謝牧勳. "Effects of Temperature and Strain-Rate on the Dynamic Shear Properties of AISI 4140 Alloy Steel." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/57976366054383787394.
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Gong, Qian-Ji, and 龔虔稽. "Influences of Temperature and Strain-Rate on the Dynamic Shear Properties of Unweldable Al-Sc Alloy." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/11298228855535849909.
Full textAbstract:
碩士國立成功大學
機械工程學系碩博士班
94
This study uses the torsional split-Hopkinson bar to investigate the shear response and fracture characteristics of unweldable Al-Sc alloy during mechanical testing at shear strain rates of 800 s-1, 1500 s-1, 2200 s-1 and 2800 s-1 and temperatures of -150 , 25 and 300 . The experimental results show that both the shear strain rate and the temperature have a significant effect on the shear properties of the Al-Sc alloy. At a constant temperature, the shear stress, fracture shear strain, work hardening rate, yielding shear strength, work hardening coefficient, strain rate sensitivity and temperature sensitivity all increase with increasing strain rate. However, the inverse tendency is observed with increasing temperature at a constant strain rate. It is found that the Kobayashi and Dodd constitutive equation provides accurate predictions of the high strain rate shear plastic behaviour of unweldable Al-Sc alloy. SEM fractographic observations reveal that the fracture surfaces are characterized by a dimple-like structure. The density of the dimples increase with increasing strain rate at a constant temperature, or with an increasing temperature at a constant strain rate. The presence of precipitates in the fracture surface indicates that the fracture initiates at the interface of the matrix and the precipitates. Finally, twisted shear bands are observed on the equatorial plane of the gauge length section of the deformed specimens. The microhardness of these shear bands increases with the strain rate, but decreases with the temperature as a result of different work hardening effects.
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Xu, Shu-Lin, and 許樹林. "Determining the dynamic properties of composite materials at higher strain rates using the split Hopkinson pressure bar." Thesis, 1992. http://ndltd.ncl.edu.tw/handle/76445719935340280853.
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Pang, Chan Hoi. "The mechanical properties of raw potato tissues as measured by uniaxial compression and small strain oscillatory shear tests." 1995. http://hdl.handle.net/1993/18959.
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Liu, Chia-Hao, and 劉佳濠. "Effect of Strain Rate on Dynamic and Static Mechanical Properties of Rocks by Synchronizing Acousto-optic Nondestructive Technique." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/32256500318744377352.
Full textAbstract:
碩士國立臺灣大學
土木工程學研究所
101
As examining the application of rock mechanics and engineering association, the behavior of quasi-static is used to study because of simplifying assumptions. Except for actual cases, such as sliding slope and falling rock, etc. are often required considering the higher strain rate even the impact of the mechanical behavior. The researches of strain rate in Taiwan are mainly in the macroscopic scale. Therefore, this study compares the mechanical behaviors of macro to the behaviors of micro by using nondestructive techniques in order to seek quasi-brittle rocks at different strain rates of failure features. This study uses AE (Acoustic Emission) and synchronizes ESPI (Electronic Speckle Pattern Interferometry) to locate micro-seismic activities and to measure the deformation of the surfaces of specimens. Several key factors are also considered: rock types – artificial (cement mortar) rocks and natural (Mushan sandstone) rocks, strain rates for quasi-static (10-5 - 10-1 s-1) by MTS loading system and high strain rate (102 - 250 s-1) by SHPB (Split-Hopkinson Pressure Bar). Meanwhile, in conjunction of nondestructive techniques at quasi-static strain rate. This research is about to explore: macroscopic – the loading curve until pre-peak, max. stress, Young’s modulus, failure strain and failure type; Microscopic – localization and initial crack. From the results, the maximum stress, Young’s modulus and failure strain of two kinds of quasi-brittle materials are rising with strain rate at the quasi-static. And the failure types are changed from shear failure to splitting failure. The high strain rate, the max. stress and Young’s modulus are extremely increasing, but the failure strain is decreasing. In this study, nondestructive techniques show that strain rate from 10-5 to 10-3 s-1 is appropriate for observation. When the strain rate increased from 10-5 to 10-3 s-1, the load level of localization also happened earlier from 40-67% to 19-30%, and the load level of initial crack occurred from 83-93% to 35-56%. It means the failure features of microscopic and macroscopic could both take place ahead of schedule with the increasing strain rate. To build a database of different strain rates for engineering to consult is advised.
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Hsu, Chia-Hsiang, and 徐嘉祥. "Influence of Strain Rate and Relative Density on the Dynamic Impact Properties and Microstructure in Fe-2Ni Sintered Alloy." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/47861922274734675289.
Full textAbstract:
碩士國立成功大學
機械工程學系碩博士班
93
This study uses the split-Hopkinson bar to investigate the plastic deformation behaviour of Fe-2Ni sintered alloys under high strain rate loading. The Taguchi method is used to design the sintering process factors such that the Fe-2Ni alloy specimens have different densities. The loading tests are performed at room temperature under strain rates ranging from 2.5x103s-1 to 8.1x103s-1. OM and SEM techniques are used to analyze the microstructureal characteristics of the deformed specimens in order to identify the correlating between the mechanical and microstructural properties of Fe-2Ni sintered alloys with different sintering densities. The experimental results indicate that the mechanical properties of the Fe-2Ni sintered alloys depend significantly on the strain rate, strain and sintering densities. Under constant strain, the flow stress, strain rate sensitivity and theoretical raised temperature increase with increasing strain rate, while the work hardening rate and activation volume decrease. At a constant strain rate, the flow stress, strain rate sensitivity and theoretical raised temperature increase with increasing relative density, while the work hardening rate and activation volume decrease. Microstructural obsernations reveal that the sintering process factors influence the effect of dynamic loading on the microstructures of the sintered alloys. Microstructural analysis reveals that adiabatic shear bands are formed when the strain rate exceeds 4.0x103s-1. The formation of these adiabatic shear bands suggests that localized regions of extremely high temperature are generated. It is observed that the local high temperature causes a dynamic recrystallization effect. Finally, applying the Khan-Huang-Liang constitutive equation with the experimentally determined specific material parameters provides accurate predictions of the plastic flow behaviour of Fe-2Ni sintered alloys with relative densities ranging from 85% to 95% under the current test conditions.
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Hung, Ching-Hua, and 洪敬華. "Determination of Small-Strain Dynamic Shear Modulus and Damping Ratio of Partially Saturated Laterite from Linkou Tableland by Resonant Column Test." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/4wxsth.
Full textAbstract:
碩士國立臺灣大學
土木工程學研究所
105
Lateritic soil usually locates in the tropical and subtropical regions. Lateritization is one of the principal pedogenic processes acting on soils. Under high temperature and heavy precipitations, there would be heavy weathering of rocks and minerals. Leaching of soil would occur as water runs through the soil. As a result, base ions, except iron and aluminum, would move out of soil. Many past studies had been performed to investigate the characteristics of saturated lateritic soil. Only few researches focus on the characteristics of unsaturated lateritic soil. In this study, lateritic soils from three different areas of Linkou Tableland were collected. Resonant column tests were performed to evaluate the small-strain dynamic properties of unsaturated lateritic soil, including the shear modulus and damping ratio. Impact of iron oxide content, degrees of saturation, and confining pressure on these small strain properties were examined. Finally, empirical model for estimating maximum shear modulus and minimum damping ratio was developed.
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Chen, Shan-Lung, and 陳善隆. "Effect of adding a small amount of micro-size powder on the dynamic properties in a rotating drum." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/n735w8.
Full textAbstract:
碩士國立高雄應用科技大學
模具工程系
105
Effects of the adding a small amount of micro-size powder and the rotating speed on the dynamic properties of the granular materials in the rotating drum was studied in this thesis The average velocity, fluctuation velocity, granular temperature and dynamic angle of repose were obtained by the particle tracking method and the image analysis technology. The results indicate that the amount of micro-size powder and rotating speed have a significant effect on the dynamic properties and dynamic angle of repose in the rotating drum. The dynamic properties is strengthened with the increase of the addition mount of micro-size powder because of the bearing effect and reduces the friction between particles. Additionally, the dynamic properties is enhanced with the increasing rotational speed.
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Chen, Chin-Wei, and 陳志維. "The Effect of Strain Rate and Welding Current Mode on the Dynamic Impact Properties and Microstructre of Plasma Arc Welded 304L Stainless Steel." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/76415677677521900127.
Full textAbstract:
碩士國立成功大學
機械工程學系碩博士班
90
A split Hopkinson bar is used to investigate the effects of strain rate and welding current mode on the dynamic impact properties of plasma arc welded 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is welded by plasma arc welding(PAW) process variations, namely continuous current and pulse current, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 1200 to 7700 at room temperature. Results indicate that the mechanical properties and the microstructure largely depend on impact loading. Increasing the strain rate of impact loading increases both flow stress and strain rate sensitivity. However, the inverse tendency is observed for the activation volume. The results also show the greater flow stress, work hardening rate and strain rate senstivity of pulse current welds compared to those of continuous current welds. By using the proposed constitutive equation proposed by Zerrilli-Armstrong with the experimentally determined specific material parameters, the flow behaviour of pulse current welds and continuous current welds can be described successfully for the range of test conditions. The effect of loading rate on mechanical response and impacted substructure of 304L stainless steel PAW welds are found directly related to dislocation density and the amount of martensite. OM and SEM fracture feature observations reveal that adiabatic shear band formation is the dominant fracture mechanism of both continuous and pulse current PAW welds of 304L stainless steel. Adiabatic shear band is initially formed near the fusion line and then crack occurs along the direction of maximum shear stress and induces specimen fracture. Microstructural observations reveal that the morphologies and characteristics of both dislocation substructure, mechanical twins, micro-shear bands and martensite formation, are strongly influenced by welding current mode and strain rate. At higher strain rate, greater dislocation density and more martensite transformation are observed, but a decay of twin density is showed. The microstructrues of continuous and pulse current welds are compared and their dynamic properties correlated. There are greater dislocation density and more martensite transformation in the pulse current welds and correspondingly they have higher flow stress under the dynamic deformation. Significant strengthening is found to result from dislocation multiplication and martensite transformation.
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Zhuang, Zhi-Hao, and 莊志皓. "Effects of adding small amount of fine powder on the size-induced granular segregation and the dynamic properties in the rotating drum." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/fwr9pg.
Full textAbstract:
碩士國立高雄應用科技大學
模具工程系
106
Granular material mixing and segregating process are very common in the industries (e.g. Powder Metallurgy, Additive Manufacturing, Metal Injection Molding). It is well known that more than two kinds of particulate material in the mixing process will be segregated because of the size difference between the particles. The small particles will concentrate in the core regime of the rotating drum due to the percolation effect. In this study, we carried out the experiments to investigate the effects of the addition of small amounts of fine powders on the size-induced granular segregation and dynamic properties in a rotating drum. The intensity of segregation, segregation rate, repose angle, the velocities and granular temperature are successfully measured by employing the image processing technology and the particle tracking method. The results of the experiments indicate that the addition of the small amount of fine powder has a significant influence on the size-induced segregation behavior and dynamic properties. The flow behavior and size-segregation are enhanced because of the bearing-like effect when the small amount of fine powder is added to granular materials.
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Narayan, Shashi Prakash. "The Effect Of Strain Rate And Temperature On The Development Of Magnetic Properties In Nano Crystalline Nd-Fe-B Alloy." Thesis, 1997. https://etd.iisc.ac.in/handle/2005/2137.
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Narayan, Shashi Prakash. "The Effect Of Strain Rate And Temperature On The Development Of Magnetic Properties In Nano Crystalline Nd-Fe-B Alloy." Thesis, 1997. http://etd.iisc.ernet.in/handle/2005/2137.
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Gopinath, K. "Tensile And Low Cycle Fatigue Behavior Of A Ni-Base Superalloy." Thesis, 2009. https://etd.iisc.ac.in/handle/2005/631.
Full textAbstract:
Background and Objective: Nickel-base superalloys, strengthened by a high volume fraction of Ni3Al precipitates, have been the undisputed choice for turbine discs in gas turbines as they exhibit the best available combination of elevated temperature tensile strength and resistance to low cycle fatigue (LCF), which are essential for a disc alloy. Alloy 720LI is a wrought nickel-base superalloy developed for disc application and exhibit superior elevated temperature tensile strength and LCF properties. It is distinct from contemporary disc alloys because of its chemistry, (especially Ti, Al and interstitial (C and B) contents), processing and heat treatment. However, literature available in open domain to develop an understanding of these properties in alloy 720LI is rather limited. This study was taken up in this background with an objective of assessing the tensile and LCF properties exhibited by alloy 720LI within a temperature regime of interest and understand the structure-property correlations behind it.
Tensile Behavior: The effect of temperature and strain rate on monotonic tensile properties were assessed at different temperature in the range of 25 – 750°C (0.67 Tm) at a strain rate of 10-4 s-1 and strain rate effects were explored in detail at 25, 400, 650 and 750°C at different strain rates between 10-5 s-1 and 10-1 s-1. Yield and ultimate tensile strength of the alloy remains unaffected by temperature till about 600°C (0.58Tm) and 500°C (0.51Tm), respectively, beyond which both decreased drastically. Negligible strain rate sensitivity exhibited by the alloy at 25 and 400°C indicated that flow stress is a strong function of strain hardening rather than strain rate hardening. However at 650 and 750°C, especially at low strain rates, strain rate sensitivity is relatively high. TEM studies revealed that heterogeneous planar slip involving shearing of precipitates by dislocation pairs was prevalent under strain rate insensitive conditions and more homogeneous slip was evident when flow stresses were strain rate sensitive. The planarity of slip is also considered responsible for the deviation in experimental data from the Ludwick–Hollomon power-law at low plastic strains in regimes insensitive to strain rate. Irrespective of strain rate sensitivity and degree of homogeneity of slip, fracture mode remained ductile at almost all the conditions studied.
Dynamic Strain Ageing: Alloy 720LI exhibits jerky flow in monotonic tension at intermediate temperatures ranging from 250-475°C. After considering all known causes for serrated flow in materials, the instability in flow (Portevin-LeChatelier (PLC) effect) is considered attributable to dynamic strain ageing (DSA), arising from interactions between diffusing solute atoms and mobile dislocations during plastic flow. As the temperature range of DSA coincided with typical bore and web temperatures of turbine discs, its possible influence on tensile properties is considered in detail. No significant change in tensile strength, ductility, or work hardening is observed, due to DSA, with increase in temperature from smooth to serrated flow regime. However strain rate sensitivity, which is positive in smooth flow regime turned negative in the serrated flow regime. Analysis of serrated flow on the basis of critical plastic strain for onset of serrations revealed that in most of the temperature-strain rate regimes studied, alloy 720LI exhibits ‘inverse’ PLC effect which is a phenomenon that has not been fully understood in contrast to ‘normal’ PLC effect observed widely in dilute solid solutions. Other characteristics of serrated flow viz., stress decrement and strain increment between serrations are also analyzed to understand the mechanism of DSA. Though the activation energy determined using stress decrements suggest that carbon atoms could be responsible for locking of dislocations, based on its influence on mechanical properties and also on its temperature regime of existence, weak pinning of dislocations by substitutional solute atoms are considered responsible for DSA in alloy 720LI.
LCF Behavior: LCF studies were carried out under fully reversed constant strain amplitude conditions at 25, 400 and 650°C with strain amplitudes ranging from 0.4-1.2%. Different cyclic stress responses observed depending on the imposed conditions are correlated to the substructures that evolved. Low level of dislocation activity and interactions observed in TEM is considered the reason behind stable cyclic stress response at low strain amplitudes at all temperatures. TEM studies also show that secondary γ’ precipitates that are degraded through repeated shearing are responsible for the continuous softening, observed after a short initial hardening phase, at higher strain amplitudes. Studies at 400°C show manifestation of DSA on LCF behavior at 400°C in the form increased cyclic hardening which tends to offset softening effects at higher strain amplitudes. Plastic strain dependence of fatigue lives exhibited bilinearity in Coffin-Manson plots at all temperatures. TEM substructures revealed that planar slip with deformation concentrated on slip bands is the major deformation mode under all the conditions examined. However, homogeneity of deformation increases with increase in strain and temperature. At 25°C, with increasing strain, increased homogeneity manifested in the form of increased number of slip bands. At 650°C, with increase in strain, increased dislocation activity in the inter-slip band regions lead to increased homogeneity. It is also seen that fine deformation twins that form at 650°C and low strain amplitudes play a role in aiding homogenization of deformation. Unlike other alloy systems where an environmental effect or a change in deformation mechanism leads to bilinearity in Coffin – Manson (CM) plots, our study shows that differences in distribution of slip is the reason behind bilinear CM plots.
While the properties and behavior of alloy 720LI under monotonic and cyclic loading conditions over a range of temperatures could be rationalized on the basis of deformation substructures, the thesis opens up the door for further in-depth studies on deformation mechanisms in 720LI as well as other disc alloys of similar microstructure.
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Gopinath, K. "Tensile And Low Cycle Fatigue Behavior Of A Ni-Base Superalloy." Thesis, 2009. http://hdl.handle.net/2005/631.
Full textAbstract:
Background and Objective: Nickel-base superalloys, strengthened by a high volume fraction of Ni3Al precipitates, have been the undisputed choice for turbine discs in gas turbines as they exhibit the best available combination of elevated temperature tensile strength and resistance to low cycle fatigue (LCF), which are essential for a disc alloy. Alloy 720LI is a wrought nickel-base superalloy developed for disc application and exhibit superior elevated temperature tensile strength and LCF properties. It is distinct from contemporary disc alloys because of its chemistry, (especially Ti, Al and interstitial (C and B) contents), processing and heat treatment. However, literature available in open domain to develop an understanding of these properties in alloy 720LI is rather limited. This study was taken up in this background with an objective of assessing the tensile and LCF properties exhibited by alloy 720LI within a temperature regime of interest and understand the structure-property correlations behind it.
Tensile Behavior: The effect of temperature and strain rate on monotonic tensile properties were assessed at different temperature in the range of 25 – 750°C (0.67 Tm) at a strain rate of 10-4 s-1 and strain rate effects were explored in detail at 25, 400, 650 and 750°C at different strain rates between 10-5 s-1 and 10-1 s-1. Yield and ultimate tensile strength of the alloy remains unaffected by temperature till about 600°C (0.58Tm) and 500°C (0.51Tm), respectively, beyond which both decreased drastically. Negligible strain rate sensitivity exhibited by the alloy at 25 and 400°C indicated that flow stress is a strong function of strain hardening rather than strain rate hardening. However at 650 and 750°C, especially at low strain rates, strain rate sensitivity is relatively high. TEM studies revealed that heterogeneous planar slip involving shearing of precipitates by dislocation pairs was prevalent under strain rate insensitive conditions and more homogeneous slip was evident when flow stresses were strain rate sensitive. The planarity of slip is also considered responsible for the deviation in experimental data from the Ludwick–Hollomon power-law at low plastic strains in regimes insensitive to strain rate. Irrespective of strain rate sensitivity and degree of homogeneity of slip, fracture mode remained ductile at almost all the conditions studied.
Dynamic Strain Ageing: Alloy 720LI exhibits jerky flow in monotonic tension at intermediate temperatures ranging from 250-475°C. After considering all known causes for serrated flow in materials, the instability in flow (Portevin-LeChatelier (PLC) effect) is considered attributable to dynamic strain ageing (DSA), arising from interactions between diffusing solute atoms and mobile dislocations during plastic flow. As the temperature range of DSA coincided with typical bore and web temperatures of turbine discs, its possible influence on tensile properties is considered in detail. No significant change in tensile strength, ductility, or work hardening is observed, due to DSA, with increase in temperature from smooth to serrated flow regime. However strain rate sensitivity, which is positive in smooth flow regime turned negative in the serrated flow regime. Analysis of serrated flow on the basis of critical plastic strain for onset of serrations revealed that in most of the temperature-strain rate regimes studied, alloy 720LI exhibits ‘inverse’ PLC effect which is a phenomenon that has not been fully understood in contrast to ‘normal’ PLC effect observed widely in dilute solid solutions. Other characteristics of serrated flow viz., stress decrement and strain increment between serrations are also analyzed to understand the mechanism of DSA. Though the activation energy determined using stress decrements suggest that carbon atoms could be responsible for locking of dislocations, based on its influence on mechanical properties and also on its temperature regime of existence, weak pinning of dislocations by substitutional solute atoms are considered responsible for DSA in alloy 720LI.
LCF Behavior: LCF studies were carried out under fully reversed constant strain amplitude conditions at 25, 400 and 650°C with strain amplitudes ranging from 0.4-1.2%. Different cyclic stress responses observed depending on the imposed conditions are correlated to the substructures that evolved. Low level of dislocation activity and interactions observed in TEM is considered the reason behind stable cyclic stress response at low strain amplitudes at all temperatures. TEM studies also show that secondary γ’ precipitates that are degraded through repeated shearing are responsible for the continuous softening, observed after a short initial hardening phase, at higher strain amplitudes. Studies at 400°C show manifestation of DSA on LCF behavior at 400°C in the form increased cyclic hardening which tends to offset softening effects at higher strain amplitudes. Plastic strain dependence of fatigue lives exhibited bilinearity in Coffin-Manson plots at all temperatures. TEM substructures revealed that planar slip with deformation concentrated on slip bands is the major deformation mode under all the conditions examined. However, homogeneity of deformation increases with increase in strain and temperature. At 25°C, with increasing strain, increased homogeneity manifested in the form of increased number of slip bands. At 650°C, with increase in strain, increased dislocation activity in the inter-slip band regions lead to increased homogeneity. It is also seen that fine deformation twins that form at 650°C and low strain amplitudes play a role in aiding homogenization of deformation. Unlike other alloy systems where an environmental effect or a change in deformation mechanism leads to bilinearity in Coffin – Manson (CM) plots, our study shows that differences in distribution of slip is the reason behind bilinear CM plots.
While the properties and behavior of alloy 720LI under monotonic and cyclic loading conditions over a range of temperatures could be rationalized on the basis of deformation substructures, the thesis opens up the door for further in-depth studies on deformation mechanisms in 720LI as well as other disc alloys of similar microstructure.