الفهرس | Only 14 pages are availabe for public view |
Abstract Fatigue behavior of materials is of great interest in many fields of applications of science and engineering, since repeated cycling loading with high stress amplitude cycling stress and strain can severely limit the useful life of many industrial components and units in a variety of branches. In the present work, the influence of stress changes on fatigue life of low carbon steel has been studied . Experiments are carried out under controlled stress fatigue test type. Completely reversed fatigue tests are performed. Stress levels are slected to have a fatigue life greater than 10,000 cycles. Three levels of cycle ratio (R); where R=Ln~\; are used, namely 0.25, 0.50, and 0.75 . Then the stress level has been changed by : -increasing, in case of low then high (L-H )loading program, or -decreasing, in case of high then low (H-L) loading program. This second stress level is kept until fracture has occurred. This case of loading is called two-stage test. Based on the obtained experimental results, a non-linear cumulative damage law has been formulated, which provides a sufficiently accurate estimation of the fatigue life under variable stress levels. The fatigue curves are described, and modified to represent cyclic varying fatigue conditions by considering various values of the cycle ratio (R) . Multi-stage tests are also carried out as three-stage loading program, low, high, then low (L-H-L)and high, low, then high (H-L-H), and its experimental results are plotted in the form of damage curves (D- R curves). Statistical analysis of the two damage stage test results are very essential because of the wide scatter of the fatigue test results. For this reason, a statistical treatment is proceeded and the obtained probability lines of failure for the two used loading programs are plotted on special logarithmic-normal probability papers. Finally, the effect of fatigue residual stress which result due to fatigue prestress on the mechanical properties of test specimen material is studied. The results of these tests show that fatigue residual stress has a damaging effect on mechanical properties of used material at the same cycle ratio (R) and after fatigue prestress . |