الفهرس 
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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 .