Damage tolerance predictions for spar web cracking in a diminishing stress field

Current United States Air Force (USAF) Damage Tolerance Analysis (DTA) methodology yields short crack growth lives for webs in built-up beams as used in the construction of wing spars. This results in very short inspection intervals for these types of structures. Inspection data from fleet usage and fatigue tests do not support the analytically-based assertion that spar webs are as damage tolerant critical as typically predicted. Current analytical methods involve short edge distances predicting failure when the crack reaches the edge-of-part. In reality, there is remaining residual strength in the spar web.;Current continuing damage methods allow for cracks that grow up into the part after the lower ligament fails. The assumption for this secondary cracking has been to use the same driving force as in the initial crack. However, the high driving force from bending found near the extreme fiber diminishes as the crack grows toward the neutral axis.;This paper utilizes the test results from two specimens to help validate an analytical approach toward crack growth predictions in spar webs. The goal of this approach is to extend short inspection intervals currently required on some USAF aircraft.