Life Prediction for Al in the Microcrack Regime Using SAW NDE

Harr:onic generation of surface acoustic 11aves (SA\~) is a useful tool for stud:·in~ surface microcrack initiation during fatigue of high strength aluminum alloys. We have developed a r:10del \~hie~' relates the length and density of such r . .icrocracks, initiaterl during fatigue, \·lith SAW harmonic generation signals. It was found that the resulting quantitative relationship het\'leen the acoustic data anci the re11ainina fatigue life is accl!r·ate to v1ithi11 5:: over the last 50f of the fatigue life, spent in the microcrack initiation phase. Extensive tests are now underway to study the general aprl icabil ity of these concepts to a wide range of higl:-strength Al alloys. Furthermore, we have started to separate the various contributions to harmonic generation from 1::icrocrac! s that are sr.1aller than the average srain C.iar.:cter, fror.J those that have been able to grow out of the grain of their origin. These considerations are important in that tbey (1) provide rletai lerl insi~ht into the rrocess of fatigue damage and (£:) r:!aj' ir:1prove our remaining life prediction capability, particularly during the first 90% of the total fatigue life. This concept secns to ~ave applicability to the "Retirer.1ent for Cause" prograr:1 of the Air Force.