A Study on Effect of Residual Stress on Crack Growth of Aluminum Plate
One technique used to enhance the fatigue strength of a fastener hole is to introduce a compressive residual stress field around the hole. An applied load must overcome this residual stress before the crack can grow, thus leading to a longer fatigue life. The cold expansion of the holes is largely employed to obtain improvement in fatigue performance of structures with initially uncracked holes. In this research, considered is a fatigue crack growth from a hole under the influence of residual stress. The entire cold working processing- including hole expansion, elastic recovery, and finish reaming of two aluminum alloys 2024-T351 and 7075-T6 is simulated. This method of fatigue life prediction is compared with a superposition technique and experimental data. The crack closure level is computed from a finite element simulation and a fatigue life is predicted. The conclusions obtained here are beneficial to the understanding of the phenomenon of fatigue crack initiation and growth at the perimeter of cold worked holes.
Fatigue crack behavior of stainless steel 304 by the addition of carbon nanotubes
Fatigue is the main source of almost half of whole mechanical failures. This research investigated the effect on cyclic fatigue behavior of stainless steel 304 (SS304) when including carbon nanotubes (CNTs) at the crack tip. The cyclic fatigue tests were conducted on compact tension (CT) specimens to establish the relationship between crack growth and the number of cycles (a-N). It is found that the incorporation of a small amount of CNTs increased the fatigue life of the SS304/metal. Micrographs showed that the enhancement in fatigue life is caused by CNTs dense arrangement around the crack tip, entangled with each other, and finer grain size. Smooth bonding at the interface of the CNTs and SS304 grains is also observed.
The Bauschinger Effect on Stress Intensity Factors for a Radially Cracked Gun Tube.
Abstract : The theoretical predicted fatigue life of a high strength steel tube which has undergone an autofrettage procedure is significantly higher than the experimental prediction. To account for the discrepancy, attention is now turned to developing better elastic-plastic models for a high strength steel. An improved material model shows that reverse yielding may occur in the inner portion of the tube. This reverse yielding reduces the residual compressive hoop stress considerably which has an adverse effect on bore crack propagation. This study considers the stress intensity factors due to a radial crack taking the Bauschinger effect into consideration. The elastic-plastic interfaces during loading and unloading in the autofrettage process divide the tube into three-ring regions. The residual stress distribution in each region is quite different. When a crack grows from one region into another, the previous method using functional stress intensity fails. A new method is used to obtain stress intensity factors for a radial crack growing out of the reverse yielding zone. This approach is based on crack face weight functions obtained by Sha using stiffness derivative finite element techniques coupled with singular crack-tip elements. (Author)