Nowadays total joint replacements are widely used in the world, so in average 800,000 joint surgeries are done yearly only in Europe and North America. However implant loosening is and remains as the major issue of all implant failures and therefore causes revision surgery procedures. Studies and experiments have identified poor fixation of implants most likely is the main cause of long term implant failure and in this case the cement-implant INTERFACE cavities are very effective due to resultant stress concentration. In this study the theory of this problem, continuum and mathematical equations for an inhomogeneous material by using Eshelby’s equivalent inclusion method with a spherical void as a special type of inhomogenities is addressed and a new yield criterion with respect to the void’s volume fraction is derived and changes in material elasticity tensor concerning Mori-Tanaka’s theorem also determined, then by using finite element method and remeshing technique a macro scale cement-implant INTERFACE cavity is modeled and concerning the loss of strength due to void existence and the INTERFACE stress concentration, the CRACK initiation and propagation  phenomenon is numerically investigated with respect to different orthopedic cement material properties. The results show that CRACK propagates at the INTERFACE at constant stress and strain by elastoplastic material and it propagates in cement bulk by considering elastic material properties for cement that both could cause implant loosening even in very small void’s volume fractions in which there are no significant changes in cement yield stress and elasticity tensor according to analytical solution. But numerical simulation shows that when a homogenous cement structure is achieved via high vacuum mixing method, there is a uniform stress distribution in the cement structure and no stress concentration zone forms even at high stress levels and also there is no appropriate local site for CRACK initiation.

Hypothesis: Fatigue CRACK growth (FCG) of rubber composites as controlled by the viscoelastic losses, is strongly dependent on the polymer-filler interfacial phenomena. The type of filler-polymer bonding at the INTERFACE and the extent of mobility restriction of rubber chains resulting from the interaction by the filler are of the critical ones. In highly filled rubber compound, the amount of mobility restriction is almost dictated by the filler-filler interaction. Regulating the surface energy of the filler can be an effective method to control the filler-filler interaction, to distinguish the two interfacial phenomena, and to pave the way of studying their significance. Methods: Ultrasil VN3 and solution styrene-butadiene rubber (SSBR) were of the base composite materials. Using two silanes with a short and a long aliphatic chain length, the surface of Ultrasil was modified in our lab to a certain level of grafting density which could bring the required surface energy and the filler-filler interaction. By controlling the surface energy of silica treated in the lab, and by making a systematic comparison of the resulting composites, it was possible to study the role of covalent bonding at the INTERFACE, the role of filler-filler interaction and severity of mobility restriction and finally the role of silane chain length. Findings: Fatigue CRACK growth experiment revealed that the severity of mobility restriction and the filler-filler interaction of the composite have the highest impact on the amount of viscoelastic dissipation and the rate of CRACK growth. The covalent bonding at the INTERFACE can deviate the CRACK from growing in the original direction and thus it may act as a physical barrier to improve CRACK growth resistance. For highly filled compounds where the properties are almost dictated by the filler-filler interaction, the role played by the chain length of silane is minor.

In this paper, the INTERFACE CRACK of two non-homogenous functionally graded materials is studied. Subsequently, with employing the displacement method for fracture of mixed-mode stress intensity factors, the continuous variation of material properties are calculated. In this investigation, the displacements are derived with employing of the functional graded material programming and analysis of isoparametric finite element; then, with using of displacement fields near CRACK tip, the mixed-mode stress intensity factors are defined. In this present study, the problems are divided into homogenous and non-homogenous materials categories; and in order to verify the accuracy of results, the analytical and numerical methods are employed. Moreover, the effect of Poisson's ratio variation on mixed-mode stress intensity factors for INTERFACE CRACK be examined and is shown in this study. Unlike the homogenous material, the effect of Poisson’s ratio variations on mixed-mode stress intensity factors at INTERFACE CRACK between two nonhomogenous is considerable.

The energy release rate for delamination in a laminated composite is supposed to be the material property being considered as independent of non-material property variables. However, Mode I fracture toughness(GI) is found to vary with lamina arrangement, geometrical dimensions, and process-induced stresses.  In this investigation, the influence of lamina stacking arrangement on process-induced stresses and their effects on GI of laminated composites are studied. Unidirectional (UD) ([0]16) and cross-ply ([902/06]s, [904/04]s and [906/02]s) Glass/ epoxy (GE) composites with the delamination plane at 0◦//0◦ were prepared by manual layup method and post-cured at 120 °C for 4 hours. GI of composite laminates were experimentally determined using a double cantilever beam(DCB) specimen as per ASTM D 5528. The slitting method was applied to determine the Process-induced stresses in GE laminates. The stacking sequence of laminas was found to have a noticeable effect on the state of residual stresses and GI of GE laminates. Residual stresses do not have much influence on the GI for delamination initiation, whereas GI  for the CRACK propagation was found to increase with a gradual increase in compressive residual stresses in GE laminates.

Wherever two plates with different materials welded or bonded together there is a risk of bonding detachment near or across the INTERFACE of the plates. In order to model this phenomena the distribution of stress in a plate comprised of two different parts separated along an INTERFACE and having a middle edge CRACK is sought. Depending on the level and conditions of loading, plate materials may have elastic, plastic or creep types of deformation. A numerical meshless technique namely Element Free Galerkin Method (EFGM) is used for this analysis. A multi purpose EFGM computer program which is developed by authors is used to analyze Elasto-plasto-creep stress and deformation fields near a CRACK in a plate made up of two different parts. Prior to introducing the results a concise explanation of EFG Method and nonlinear constitutive equation modeling has been represented. Then by assuming a purely elastic behavior some of the results of the problem are compared with their similar available analytical solutions. Finally typical solutions of the problem in its complete nonlinear form are represented and assessed. j and C* integrals are used to capture the singularity of the CRACK tip stress in static and creep conditions respectively. The obtained results confirm that a CRACK taken place in the INTERFACE of two plates is more critical than a CRACK far from the INTERFACE.

To ensure the rail transportations safety, evaluation of fatigue behavior of the rail steel is necessary. High cycle fatigue behavior of a rail steel was the subject of investigation in this research using fracture mechanics. Finite element method (FEM) was used for analyzing the distribution of the stresses on the rail, exerted by the external load. FEM analysis showed that the maximum longitudinal stresses occurred on the railhead. To find out about the relation of CRACK growth with its critical size, and to estimate its lifetime, the behavior of transverse CRACKs to rail direction was studied using damage tolerance concept. It revealed that transverse CRACK growth initially occurred slowly, but it accelerated once the CRACK size became larger. Residual service life was calculated for defective segments of the rails. In addition, allowable CRACK size for different non-destructive testing intervals was determined; the allowable CRACK size decreased as the NDT intervals increased.