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The effects of mixed mode fracture and fatigue crack path have been investigated in tensile-shear spot weld specimens having different gaps between their sheets. Non-linear analysis has been performed to obtain the stress distributions along the line perpendicular to the fatigue crack path. The amounts of effective stress and notch strength reduction factors have been obtained using the volumetric method. Fatigue crack growth approach has been applied to obtain stress intensity factors in mode I and mode II of fracture and to estimate fatigue crack propagation of spot welds. The results obtained from numerical predictions such as the volumetric method and the fatigue crack growth approach have been compared with the available fatigue test data. The results obtained from the fatigue crack growth approach show that spot weld specimens with bigger nugget diameter have the smaller values of stress intensity factors compared with those spot welds with smaller nugget diameters. However, with due attention to the fact that fatigue cracks propagate in the mixed mode condition, the ratio of mode I stress intensity factor to the mode II become more important parameter while predicting the fatigue life of specimens.

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In this paper, the effect of bandwidth and loading level parameters changes of random load histories on the fatigue crack growth is studied by the central limit theory. In this method, the life probabilistic distribution and probability of failure function and reliability function are derived by the central limit theory. The walker equation is used to account the stress ratio effect in the fatigue crack growth rate equations. Then, the theoretical results are verified by the test results in the three random loading conditions with the various bandwidths and loading levels. A good agreement among the theory and test results was observed. The probability of failure and reliability diagrams via number of cycles is presented too.

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Experimental and numerical analysis of a single overload on the fatigue life of AISI 4140 CT specimens was studied. Fatigue tests were conducted on base line CT specimen under single overload ratios of 1.5 and 1.75. Numerical analyses were performed on 2D incorporating the stress intensity factor and the J-integral as driving force. Furthermore, ABAQUS commercial software was used to simulate elastic-plastic crack growth and crack closure. Overload-induced retardation effects on the crack growth rate are considered based on the crack closure concept. A novel model for considering pre-overload and post overload effect on the crack growth is introduced. This model is based on the effective stress intensity factor and the effective J value. Overload increases fatigue lives by factors of 1.24 and 1.5 for overload ratios of 1.5 and 1.75, respectively. Two dimensional FEM results are in good agreement with the experiment with a maximum error of 10% using stress the intensity factor based method and -6% using the J integral based method. Comparing present paper method with Harmain model indicates that this model requires fewer experiments and Harmain model requires less calculation.

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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 half-elliptical, 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.

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In this paper, effects of severe plastic deformation (SPD) on the fatigue crack growth, mechanical properties, texture, roughness and fracture toughness of Al-6063 were studied. The Al-6063 alloy was deformed by ECAP process. The average grain size refined to less than 100nm. The textural study conducted before and after ECAP process. The fatigue crack growth tests were performed for different load range at same load ratio. The yield and ultimate stresses increased about 230% and 79% after ECAP process, respectively. The elongation reduced from 16.6% to 7% after four passes of ECAP process. The fatigue crack growth rate increased after first pass of ECAP process. The Paris equation parameters changed before and after ECAP but there is no significant change for different load ranges. The fracture toughness decreased after first pass of ECAP process. The atomic force microscopy (AFM) were used for measuring roughness. The scanning electron microscope (SEM) pictures were made for fracture surface study. The ductile and fissured fracture with large dimples were seen before ECAP process. The fracture surface with refined dimples observed after ECAP process.

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ABSTRACT
Equal Channel Angular Pressing (ECAP) is one of the methods of refining and fine-graining metal materials. In this research, ECAP operation was performed on samples of 5182 alloy in 1 to 4 passes at ambient temperature. After implementation of the specimens through ECAP, prepared to obtain mechanical properties such as hardness, tensile and metallography. The results of these experiments showed that the mechanical properties of the packed materials through ECAP have improved compared to the normal state. Using a scanning microscope, it was observed that the average grain size decreased from 131 μm in the initial state to 745 nm after the ECAP process after the fourth pass. The results of hardness test also showed a 213% increase compared to normal.  The increase in yield stress after 4 passes is about 3 times. Finally, the crack growth of these materials under fatigue loading was compared with the non-ECAP mode by creating a suitable pre-crack. It was observed that crack growth is faster in ECAP materials and the failure surface is smoother compared to normal. Also, the deviation of the crack from its path in microstructure materials is less than normal. Finally, by comparing the Experimental results of crack growth with the results of numerical analysis, the accuracy of the numerical results is validated and confirmed.