Дисертації з теми "Stresses and deformations condition"

The stresses and deformations in cross-ply composite tubes subjected in circumferential temperature gradients are studied. The motivation behind the study is the anticipated use of composite tubes in space structures where the tube is exposed to the heat of the sun on one side and the cryogenic temperatures of space on the other. Experiments were performed to measure the functional form of the temperature gradient and the displacements. It was found that the form of the temperature gradient, T(Ɵ), can accurately be represented by T(Ɵ) = A + BcosƟ¸ and that the displacement of the tube is parabolic in the axial coordinate. Two types of analytical solutions were developed: an exact elasticity U solution and an approximate solution. The approximate solution includes a linear variation of the material properties with temperature and uses the principle of complementary virtual work in conjunction with a Ritz approximation on the stress field. The elasticity solution predicts that high tensile stresses could crack the matrix. The effect of including temperature-dependent material properties is to reduce the circumferential dependency of the stresses.

Joints are critical parts of timber structures, transmitting static and dynamic forces between structural members. The ultimate behavior of a loaded building depends strongly on the structural configuration and the capacity of the joints. The collapse of a whole building or less extensive accidents that may occur is usually starting as a local failure inside or in the vicinity of a joint. Such serious failures have recently occurred in our Nordic countries. Especially the collapse of two large glued laminated timber structures clearly indicates the need of an improved joint design. The trend toward larger and more complex structures even further increases the importance of a safer design of the joints.

An aim of this partly experimental and partly numerically based thesis has been to investigate if steel-to-timber dowel joints are affected by moisture-induced stresses. The experimental results showed that the load-bearing capacity of the joints is reduced by such a moisture influence. Most of the decrease in load-bearing capacity observed was found in joints initially exposed to restrained shrinkage deformations caused by the presence of dowel fasteners in the joint area. The load-bearing capacity was, however, also found to decrease in joints exposed to an initial decrease in moisture without any fasteners present in the specimens during storage before loading. An explanation of this unexpected behavior is that moisture gradients cause tensile stresses. It is shown by numerical simulations that the moisture-induced stresses are so large that they may have a considerable influence on the joint behavior.

Use of contact-free measurement methods, used in some of the experimental tests, was in many ways found to be superior to traditional measurement techniques, but was also found to be a valuable complement to the numerical analysis performed. From numerical results obtained in combination with results from contact-free measurements several observations of considerable interest were made. For dowel-type joints loaded in tension parallel to the grain a strongly non-uniform strain distribution was found in the joint area. It was further observed that the shear and tensile strains were concentrated close to the fasteners in the joint area. These concentrations will influence the failure mode of the joint. A general observation was that the larger sized joints failed in a brittle manner.

Keywords: constraint stresses, contact-free measurement, dowel-type joints, humidity variations, moisture-induced deformations, timber structures

1. Lors du suivi du retrait restreint à l’aide de l’essai à l’anneau en condition de dessiccation, le béton formulé à base de ciment au laitier de haut-fourneau a fissuré bien avant le béton formulé à base de ciment Portland.

2. Le retrait total en condition libre du béton formulé à base de ciment au laitier de haut-fourneau est nettement supérieur à celui du béton formulé à base de ciment Portland. Cette différence de comportement est principalement due à l’accroissement rapide et plus élevé du retrait endogène des bétons formulés à base de ciment au laitier de haut-fourneau.

Au vu de ces résultats expérimentaux, il a semblé intéressant de déterminer quel était l’impact de la déformation endogène des bétons formulés à base de ciments au laitier de haut-fourneau (CEM III) sur leur sensibilité à la fissuration. Afin de répondre à cette question, les déformations différées (retrait endogène, fluage propre en compression et en traction) au jeune âge de trois compositions de béton avec différentes teneurs en laitier (0, 42 et 71%) ont été étudiées expérimentalement en conditions libre et restreinte. Cependant, le suivi du retrait endogène libre et restreint a nécessité le développement de plusieurs dispositifs expérimentaux limitant au maximum les artefacts de mesure, tels que la TSTM (Temperature Stress Testing Machine). De plus, l’interprétation de ces résultats expérimentaux a également nécessité une caractérisation du comportement de ces matériaux à l’échelle macro- et microscopique.

Finalement, cette étude a montré que malgré une déformation endogène plus élevée, les bétons formulés à base de ciment au laitier de haut-fourneau fissurent après le béton formulé à base de ciment Portland. Ce comportement est dû à :

-l’impact du laitier sur la réaction d’hydratation du matériau cimentaire ;

-la présence d’une expansion de la matrice cimentaire des bétons formulés à base de ciment au laitier de haut-fourneau au jeune âge qui retarde l’apparition des contraintes de traction au sein du matériau ;

-la plus grande capacité de ces matériaux cimentaires à relaxer les contraintes de traction/

Today, the use of concretes with mineral additions (fly ash, slag) for civil engineering structures is spreading worldwide. Indeed, the production of blended cements is more respectful of the environment than the production of Portland cement, because it allows reducing greenhouse gas emissions and using industrial wastes. Slag cement concretes are also largely used for their good resistance to alkali-silica reactions, sulphate attacks and chloride diffusion. However, some of constructions built with slag cement concretes have exhibited cracking at early age due to their restrained deformations, such as thermal, autogenous and drying shrinkage. Following these observations, a preliminary experimental study was realized in the laboratory of BATir Department at ULB. It revealed several characteristics of the behaviour of slag cement concretes:

1. The study of restrained deformations under drying conditions by means of ring tests showed that the slag cement concretes seem more prone to crack than the Portland cement concretes;

2. The total free shrinkage for slag cement concrete is clearly larger than for Portland cement concrete. This difference of behaviour is mainly due to the fast and large increase in the autogenous deformation of the slag cement concrete.

Following these experimental results, the effect of the autogenous deformation on the cracking sensibility of slag cement concretes seemed interesting to investigate. Their deformations (autogenous deformation, compressive and tensile basic creep) have been studied at early age for three concretes characterized by different slag contents (0, 42 and 71%) under free and restrained conditions. For monitoring free and restrained autogenous deformations, several test rigs aimed at limiting artefacts were designed, like the TSTM (Temperature Stress Testing Machine). Moreover, the behaviour of these concretes was also characterized by a study at a macro- and microstructure scale.

Finally, this study shows that the slag cement concretes under sealed and fully restrained conditions crack later than the Portland cement concrete, despite the fact that they are characterized by the largest autogenous deformation. This behaviour is due to:

- the slag effect on the hydration reaction of cementitious material;

- the cement matrix expansion of the slag cement concretes at early age which delays the occurrence of tensile stresses inside the material;

- the largest capacity of this concrete to relax tensile stresses.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

monitoring of civil engineering structures in order to locate small damages automatically. A

review of the very wide literature on Structural Health Monitoring (SHM) points first out that

the methods can be grouped in four categories based on their need or not of a numerical model,

as well as their need or not of information of the damaged structure to be applied. This state

of the art of the SHM methods highlights the requirement to reach each levels of SHM, which

is in particular for the localization of small damages in civil engineering structures the needs

for a non-model based output-only damage sensitive feature extraction technique. The origin of

the local sensitivity of strains to damages is also analyzed, which justifies their use for damage

localization.

A new method based on the modal filtering technique which consists in combining linearly

the sensor responses in a specific way to mimic a single degree of freedom system and which

was previously developed for damage detection is proposed. A very large network of dynamic

strain sensors is deployed on the structure and split into several independent local sensor networks.

Low computational cost and fast signal processing techniques are coupled to statistical

control charts for robust and fully automated damage localization.

The efficiency of the method is demonstrated using time-domain simulated data on a simply

supported beam and a three-dimensional bridge structure. The method is able to detect and

locate very small damages even in the presence of noise on the measurements and variability

of the baseline structure if strain sensors are used. The difficulty to locate damages from acceleration

sensors is also clearly illustrated. The most common classical methods for damage

localization are applied on the simply supported beam and the results show that the modal filtering

technique presents much better performances for an accurate localization of small damages

and is easier to automate.

An improvement of the modal filters method referred to as adaptive modal filters is next

proposed in order to enhance the ability to localize small damages, as well as to follow their

evolution through modal filters updating. Based on this study, a new damage sensitive feature

is proposed and is compared with other damage sensitive features to detect the damages with

modal filters to demonstrate its interest. These expectations are verified numerically with the

three-dimensional bridge structure, and the results show that the adaptation of the modal filters

increases the sensitivity of local filters to damages.

Experimental tests have been led first to check the feasibility of modal filters to detect damages

when they are used with accelerometers. Two case studies are considered. The first work

investigates the experimental damage detection of a small aircraft wing equipped with a network

of 15 accelerometers, one force transducer and excited with an electro-dynamic shaker. A

damage is introduced by replacing inspection panels with damaged panels. A modified version

of the modal filtering technique is applied and compared with the damage detection based principal

component analysis of FRFs as well as of transmissibilities. The three approaches succeed

in the damage detection but we illustrate the advantage of using the modal filtering algorithm as

well as of the new damage sensitive feature. The second experimental application aims at detecting

both linear and nonlinear damage scenarios using the responses of four accelerometers

installed on the three-storey frame structure previously developed and studied at Los Alamos

National Labs. In particular, modal filters are shown to be sensitive to both types of damages,

but cannot make the distinction between linear and nonlinear damages.

Finally, the new method is tested experimentally to locate damages by considering cheap

piezoelectric patches (PVDF) for dynamic strain measurements. Again, two case studies are investigated.

The first work investigates a small clamped-free steel plate equipped with 8 PVDFs sensors, and excited with a PZT patch. A small damage is introduced at different locations by

fixing a stiffener. The modal filters are applied on three local filters in order to locate damage.

Univariate control charts allow to locate automatically all the damage positions correctly.

The last experimental investigation is devoted to a 3.78m long I-steel beam equipped with 20

PVDFs sensors and excited with an electro-dynamic shaker. Again, a small stiffener is added to

mimic the effect of a small damage and five local filters are defined to locate the damage. The

damage is correctly located for several positions, and the interest of including measurements

under different environmental conditions for the baseline as well as overlapping the local filters

is illustrated.

The very nice results obtained with these first experimental applications of modal filters

based on strains show the real interest of this very low computational cost method for outputonly

non-model based automated damage localization of real structures.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished