TWI Industrial Member Report Summary 5/1976
By C F Boulton
The phenomenon of fatigue crack retardation due to stress interaction effects in variable amplitude loading spectra was studied in three structural steels. Centre-notched plate specimens were subjected to single applications of intermittent tensile and compressive peak overload stresses in otherwise tensile constant amplitude stress cycling. Crack progress was monitored using the electrical potential difference technique.
The results of these preliminary investigations into the effect of stress interaction indicate that:
i. The application of single tensile peak stresses leads to crack retardation, and hence increases fatigue life.
ii. Crack retardation increases if two tensile peaks are applied consecutively.
iii. Retardation is minimised if compression is applied immediately after the tensile peak stress.
iv. Crack retardation increases with increasing material tensile strength, for specimens subjected to the same stress spectrum.
The results suggest that under complex spectra where stress interaction effects predominate, high yield steels could be used to advantage in fatigue design.