Effect of Sonic Thermographic Inspection on Fatigue Crack Growth in an Al Alloy

摘要

Sonic thermography has recently emerged as an important inspection technique capable of resolving inspection problems that contemporary methods have struggled with, such as tightly closed cracks in metallic structures and kissing bonds in composite repairs. The technique uses elastic waves injected by an acoustic horn resonating at typically either 20 or 40 kHz, which often excites lateral motion at the surfaces of a defect. This motion induces frictional heating and in turn a thermal indication, which can often be detected with sensitive thermal imaging equipment. Although shown to be quite useful in detecting many types of structural defects, several issues require close investigation and perhaps most critical of these is whether the technique is indeed nondestructive. No published work could be found that examines the impact of high intensity insonification, in the context of nondestructive inspection, on the fatigue crack growth characteristics of structural metals. This is despite an accepted understanding that thermal signatures produced during the intense acoustic excitation can evolve from a mechanically irreversible change of state. An experimental study was undertaken to examine the impact of repeated high intensity insonification on the rate of crack growth in Al7075 coupon specimens subject to mechanical tensile testing. Under a representative inspection regime no evidence was found of an accelerated or otherwise altered rate of crack growth compared to that measured for a benchmark group. It tentatively suggests that the technique is structurally benign when applied to cracked Al7075 components. (1 table, 7 figures, 11 refs.).