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.

Modeling of Crack propagation by finite element method under mixed mode conditions is of prime importance in fracture mechanics. This paper describes an application of finite element method to the analysis of mixed mode Crack growth in linear elastic fracture mechanics. Crack growth process is simulated by an incremental Crack-extension analysis based on the maximum principal stress criterion, which is expressed in terms of the stress intensity factor.In this paper, a procedure to correct the direction of Crack propagation in the analysis of finite element is presented to ensure that a unique final Crack path is achieved for different analysis of a problem by using different increments of Crack. For each increment of Crack extension, finite element method is applied to perform a single region stress analysis of the Cracked structure. Results of this incremental Crack extension analysis are presented for several geometries.

Modeling of Crack propagation by a finite element method under mixed mode conditions is of prime importance in the fracture mechanics. This article describes an application of finite element method to the analysis of mixed mode Crack growth in linear elastic fracture mechanics. Crack - growth process is simulated by an incremental Crack-extension analysis based on the maximum principal stress criterion which is expressed in terms of the stress intensity factor. In this paper a procedure is employed to correct direction of Crack propagation to ensure that a unique final Crack path is achieved for different analysis of a problem by using different increments of Crack. For each increment of Crack extension, finite element method is applied to perform a single - region stress analysis of the Cracked structure. Results of this incremental Crack – extension analysis are presented for several geometries.

The construction of Latiyan Buttress Dam was finished in 1968. The result of dam instrumentation indicated that till July of 1970 the dam was in a suitable situation. However at that time vertical Cracks were observed on the walls of buttress 10/11, for the first time. These Cracks were located on both sides of the gate control room, which extend from the crest level down to the floor of control room. The rehabilitation operations were done by injection of epoxy into the Crack openings in winter 1970, but it did not work properly and the Cracks opened again in summer 1971. Measurement of opening and closing of Cracks shows a cyclic loop behavior affected by seasonal variations in temperature. The latter seems to be the major cause of Cracking. In this study finite element analysis of Crack propagation has been done by use of Nonlinear Fracture Mechanics concepts and Smeared Crack Method. In this method its suggested that the Crack will be replaced by a continuum with changeable physical properties. If the failure criterion is met, Cracking takes place in the direction normal to the maximum principal stress. After Cracking, the stiffness of concrete normal to the Crack plane is reduced and concrete model becomes orthotropic. Therefore by propagation of Crack in the structure the finite element mesh will not change while the material behavior relation changes. At first we find the distribution of temperature in the dam body by Heat Transfer Analysis for both summer and winter. Then by applying different static load combinations, the load combination that produce the Crack is found. The result of static analysis demonstrates that applying the following load combination would cause high tensile stresses in the Cracked region: Dam Weight + Hydrostatic Pressure + Summer Temperature Load + Lateral Pressure of Adjacent Blocks By analyzing of the model under cyclic temperature loads it is concluded that after a while the results should become steady and increasing the loading cycles does not make major differences in the results. We can conclude that by applying static loads the Crack pattern wont propagate and the dam wont get in danger. It means that to keep services of this dam it is not necessary to do rehabilitation operations to resist static loads and the dam is safe in the normal loading conditions.

Due to the significant advancement in the mining and tunneling industry, the depth of underground excavation has been considerably increased. As a consequence of increase in depth, the intact rock strength plays a significant role in rock mass failure. Therefore, the study of intact rock failure has become more crucial. Crack damage stress and Crack initiation stress are the two important characteristics of intact rock failure, which have been studied rarely in the shear failure process. This study aims to investigate the Crack damage and Crack initiation stresses in the shear failure by numerical modeling of direct shear test (DST) using the discrete element method. First, the micro-mechanical properties of the numerical model were calibrated against physical experiments in uniaxial tension, uniaxial compression and triaxial compression tests. Then, the numerical models were validated by comparing against physical direct shear tests under different normal stresses. By undertaking the DST under eight different constant normal stresses, these two stress levels have been studied in brittle, brittle-ductile transition and ductile failure modes. It was revealed that the Crack initiation and Crack damage stresses occur in 70-91 and 85-95 percent of the peak shear strength, respectively. Also, it was observed that the ratio Crack initiation and Crack damage stresses to the peak shear strength decrease with an increase in the normal stress. The outcomes would be useful for studying the structures under shear stress like landslides and rock bolts.

In this study, fatigue growth of external surface Cracks on the autofrettaged cylinders under bending is investigated. Autofrettage is a process in which a thick-walled cylinder subjected to internal pressure with known amount, causing some portions on the inner zone of the cylinder deformed plastically. In this case, removing the pressure causes compressive residual stresses on the inner layers and tensile stresses on the outer wall. The goal is increasing the fatigue durability of the product by inducing residual compressive stresses into materials, but along with this, there are adverse tensile stresses which can decrease the life due to the outer defects. In this paper, the external Cracks are in the forms of halfelliptical, semi-elliptical and semi-circle. Samples made by aluminum 2024 alloy. The cylinders were autofrettaged up to 40 and 60 percent. Cracks were located in circumferential direction and normal to cylinder axis. The numerical simulations were performed by finite element method. Experimental data and numerical results were compared. Results show that the number of load cycles to fracture, in the 60% autofrettaged cylinders are smaller than those for 40% and also smaller than the state without autofrettage. Distribution of stress intensity factor along the Crack front is symmetric and Crack grows in its initial plane which indicating the dominance of the first mode of failure during the Crack growth.In all samples, after some steps of the growth, Crack front transforms to the semi-elliptical shape until complete fracture.