No abstract

The concept that alternating shear stress is the primary cause of fatigue with the normal stress on the critical shear plane as an influencing factor has been developed for the case of mean (or static) stresses superimposed on combinations of torsion and axial load or bending. The influence of the maximum stress of the cycle of stress on the allowable alternating stress for a given number of cycles and on the orientation of the critical shear plane is explored. The predictions of the theory are consistent with the known trends of fatigue data both for ductile metals and cast irons. The theory explains the fact that the influence of mean stress is weak for torsion and stronger for bending of ductile metals, but strong for both torsion and bending of cast irons. As far as is known this is the first rational theory for the influence of mean stress.

No abstract

Similar thermal expansion instabilities, consisting of isothermal, time‐dependent changes in thermal expansion after a rapid change in temperature, were observed in epoxy resins with different degrees of cross‐linking. Creep experiments performed at different stages of expansion show decreases in tensile creep rate with decreased expansion. The role of changes in moisture content as a possible cause of the dimensional instability is examined for epoxy resins and a highly cross‐linked polyurethane. Results indicate that the state of expansion is the primary cause of changes in creep rate rather than temperature or moisture content.