In the temperature range from R. T. to 500°C, the characteristic curves of high temperature fatigue strength of mild steel SM41A showed the maximum stress at 400°C (so called the“peaking temperature”) and the minimum ones at 150°C and 500°C.
Based on these results, the following four model conditions were adopted as the temperature variations. In model A, the specimens were held
Nh cycles at 500°C and then dropped to 150°C, while in model B
Nh cycles at 150°C and then raised to 500°C. Model C was a convex type such as the specimens were held at 150°C in the first 30 minutes, then raised to 500°C, held there for
Nh cycles and downed to 150°C again. Model D was the same type as model A except the initial temperature was held at the peaking temperature instead of 500°C.
The rotary bending fatigue tests were made under these temperature conditions at three stress levels. By taking the cumulative cycle ratio as the abscissa and log
Nh/log
N500 as the corresponding ordinate, we constructed log
Nh/log
N500 versus Σ
n/N diagrams expressing the relation among repeated stress, hold-time and fatigue life simultaneously. The main results obtained from these diagrams were as follows:
(1) The fatigue life at the second temperature was higher than those at the constant temperature both 150°C and 500°C for model A, B and C.
(2) The fatigue life at the second temperature in model A was appreciably higher than that in model B.
(3) For model D, with increasing hold-cycles
Nh the fatigue strength reached the maximum value at the peaking temperature.
(4) According to the microphotographs of specimens, a great deal of etch pits and slip bands were found in model D, while in the case of model A and C, they were not so much. They were hardly observed in model B.
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