A STUDY OF RESIDUAL CAUSTICS GENERATED FROM FATIGUE CRACKS

The method of caustics was used to determine the stress intensity fact or of fatigue cracks in steel compact tension specimens. Under zero lo ad a residual caustic was observed at the rip of a fatigue crack indic ating the presence of a residual stress field. Caustics were generated at increasing static loads and the stress intensity factors were comp ared with those predicted by theory. It was found that the difference between each measured stress intensity factor and its corresponding th eoretical value was a constant for the range of loads. This difference was shown statistically to be equal to the stress intensity factor de termined from the residual caustic. The proposed mechanism for the for mation of this residual caustic was probably due to crack tip plastici ty effects and not due to crack closure. It was concluded that residua l caustics can be measured to quantify crack tip behaviour in fatigue cracks and have been shown to be a useful tool in the measurement of r esidual stress fields.