Volume 2, Issue 9 (October 2005)
Load History Effects Resulting from Compression Precracking
Compression precracking (CPC) has seen renewed interest as a possible alternative procedure for generating fatigue crack growth threshold data with minimal load history effects, but recent testing confirms results from the literature that compression precracking does induce load history effects through residual stresses that influence subsequent fatigue crack growth test data. Using the CPC method, specimens are precracked with both maximum and minimum compressive loads. Compressive yielding occurs at the crack-starter notch, resulting in a local tensile residual stress field through which the fatigue crack must propagate. Although the tensile residual stress field contributes to the driving force for precracking, it also introduces the possibility of history effects that may affect subsequent fatigue crack growth. The tensile residual stress field elevates the local driving force at the crack tip, promoting higher crack growth rates than would be expected from the applied loading. This paper presents three-dimensional finite element results and experimental data for compact tension specimens that characterize the load history effects induced by compression precracking. The analysis results indicate that for low tensile loading levels near the threshold region, the residual stresses cause the calculated crack tip driving force to increase from the applied driving force by 25% or more. In addition, significant crack growth of about two times the estimated plastic zone size is needed to grow away from the residual stress field and reduce the calculated crack tip driving force to within 5% of the applied driving force. Experimental results show that growth of about two to three times the estimated plastic zone size is necessary to establish steady growth rates under constant ΔK loading for the materials and loading levels evaluated. Constant ΔK testing following compression precracking will demonstrate when residual stress effects are no longer significant and will ensure consistent growth rates.