Fatigue Crack Growth Rate and Stress-Intensity Factor Corrections for Out-of-Plane Crack Growth

摘要

Fatigue crack growth rate testing is performed using automated data collection systems that assume straight crack growth in the plane of symmetry and that use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which underestimates the crack growth rates for cracks that propagate out-of-plane. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on two-dimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys. Results indicate the correction process works well for high ΔK levels, but it fails to capture the mixed-mode effects at ΔK levels approaching threshold (da/dN ～ 10{sup}(-10) meter/cycle). Based on the results presented in this paper, the authors propose modifications to ASTM E 647: to be more restrictive on the limits for out-of-plane cracking (15°); to add a requirement for a minimum of two visual measurements (one at test start and one at test completion); and to include a note on crack twisting angles, with a limit of 10° being acceptable.