Previous analytical treatments to the crack problems in functionally graded materials (FGMs) were mostly based on the assumption that only the shear modulus is variable according to special functions ...while the Poisson’s ratio is constant. However, the material inhomogeneity in real FGMs is more complex than that can be well approximated using the assumption. This paper examines the axisymmetric problem of a penny-shaped crack near and parallel to a graded interface with arbitrarily variable shear modulus and Poisson’ s ratio. To consider the general material inhomogeneity, a new efficient solution of general axisymmetric edge dislocation loop in multilayered elastic medium is derived first using the General Kelvin’s Solution (GKS) based method. This method allows the approximation of general material inhomogeneity using large number of homogenous sublayers without the loss of computational efficiency, accuracy and stability. The challenging issue in multilayered elasticity associated with the calculation of near-source elastic field is also tackled, which enables the high precision evaluation of crack tip field. Exact solutions are given for the full elastic field. Numerical studies are conducted to explore the fracture mechanics of FGMs. The results show that the gradation pattern can have certain effect on the fracture response if the crack is very closed to the graded interface.
Steel wires are used as a bridge construction material and as pre-stressing strands or tendons in pre-stressed structural units among other applications in civil engineering. To date, the estimation ...of the load carrying capacity of a cracked wire has been based on purely experimental classical fracture mechanics work conducted with non-standardised classical fracture mechanics specimens as standard test specimens could not be manufactured from the wire owing to their size. In this work, experimental mechanical tests and finite element simulation with the phenomenological shear fracture model has been conducted to investigate the effect of miniature cracks with dimensions less than or equal to 0.2 mm (which is the limit of the current non-destructive detection technology) on the tensile and fracture properties of flat carbon steel wire. The investigation revealed that the reduction in the displacement at fracture of the wire due to the presence of cracks shallower than 0.2 mm is significantly higher than the reduction in the fracture load of the wire. Consequently, the displacement at fracture and by extension the fracture strain capacity of the wire could serve as a more appropriate parameter to assess the quality and the structural integrity of cracked wires.
La fatigue représente la deuxième cause d’endommagement des structures métalliques anciennes après la corrosion. Celle-ci se manifeste par l’apparition de fissures, dans les zones fortement ...sollicitées présentant des concentrations de contraintes importantes, et peut mener à terme à la ruine de l’ouvrage. Dans le cas des structures métalliques anciennes, les fissures de fatigue s’amorcent généralement au droit des trous de rivets rendant difficile leur détection par les techniques de contrôle non destructif conventionnelles. De même, les matériaux métalliques anciens de la construction, et notamment le fer puddlé, limitent l’utilisation de certaines techniques. Dans le cadre de ce travail de thèse, l’objectif principal est d’étudier l’efficacité d’un renforcement par composites collés dans le cas de fissures de fatigue émanant de trous de rivets. Pour cela, des éprouvettes de petites dimensions, présentant un perçage central et une unique fissure de fatigue en bord de perçage, ont été considérées. De même, deux matériaux métalliques, acier doux et fer puddlé, ainsi que deux procédés de renforcement PRFC (polymères renforcés de fibres de carbone), à Module Normal et à Ultra Haut Module, sont étudiés. Pour chaque procédé de renforcement, différentes configurations de renforcement ont été testées et notamment la mise en précontrainte du plat composite MN avant collage. Cela a permis de mettre en évidence l’efficacité de la technique de collage de plats PRFC pour un renforcement à la fatigue de matériaux métalliques anciens. Par ailleurs, les expressions analytiques simples proposées pour le facteur d’intensité de contraintes pourront par la suite être utilisés pour une application aux structures rivetées en tenant compte de la présence du rivet ainsi que celle des plaques assemblées.
After corrosion, fatigue phenomenon is the main cause of damage in old metallic structures. Fatigue cracks appear in stress concentration area subject to high stresses, and can lead to the ruin of the bridge. In old metallic structures, fatigue cracks mainly occur at the edge of the rivet hole and are thus difficult to detect with the common non-destructive inspection technique (NDI). Moreover, due to the poor quality of the old metallic materials, particularly puddled iron, some of the NDI techniques cannot be used. The main objective of the present work is to study the effectiveness of carbon fibre reinforced polymer (CFRP) laminates in reinforcing fatigue crack emanating from the rivet hole. Thus, investigations on small-scale specimens were done. These specimens consist of metallic plates with center hole from which one single crack emanates. Two metallic materials, puddled iron and mild steel, and two reinforcement processes were used. These reinforcement processes consist of Normal Modulus (NM) and Ultra High Modulus (HHM) CFRP laminates. Furthermore, symmetrical and un-symmetrical reinforcement configurations are considered as well as pre-stressing NM laminates before application. The experimental results showed firstly the efficiency of the different studied reinforcement configurations in slowing down crack propagation. In conclusion of this work, the achieved results, particularly those in terms of Stress Intensity Factor, could be used for reinforcement of riveted structures by CFRP bonding, taking into account the presence of rivets as well as the presence of the others elements of the assembly.
In The Japan Welding Engineering Society standard WES 2805-1997, a fracture mechanics parameter, crack tip opening displacement (CTOD), relative to the flaw located in highly stressed region ...surrounded by clastic stress field is estimated using the CTOD design curve from the flaw size and the applied local strain considering strain concentration in the components. In this report, finite element analyses using three-dimensional solid elements and tensile tests were performed to evaluate an accuracy of CTOD estimated by the design curve for a continuous circumferential surface crack which was present at the corner boxing toe. It was found that a CTOD estimated by the design curve in WES 2805 had to allow for a great margin of safety. It was possible to estimate a CTOD accurately using te applied local strain calculated from the correction function of the stress intensity factor Mk for the design curve.
In practice, cracks of any awkward shape initiated accidentally, e.g. fabrication errors, exist in structures such as offshore steel structure. Existing of these flaws could lead to disastrous ...accidence due to fatigue effect. Therefore, a safe interval of maintenances is substantial in order to keep a reliable residual life and strength of the structure. To assess the fatigue effect, one approach is to make use of the stress intensity factor(SIF) of the cracks in facture mechanics. These flaws are usually treated as semi-elliptical surface cracks(with a and c as the minor and major axis) on the member of the structure e.g. BS7910. For plates with semi-elliptical surface crack, Newman, Raju and Kou have reported the stress intensity factor. However, for plates with partly through-wall cracks, namely an semi-elliptical crack with an imaginary crack depth a which is greater than the thickness (T) of the plate, the stress intensity factor was not reported. In the current work, semi-elliptical partly through-wall cracks (of sizes 1.0<a/t<1.3, 0.01<a/2c>0.2) on a finite plate subjected to remote tension were analysed based on linear elastic fracture mechanics. This work finally produced a comprehensive equation of SIF as a function of a/t and a/2c which was obtained by doing a curve fitting on the SIF which is determined in this work and was obtained by some previous workers.
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