Volume 5, Issue 7 (July 2008)
Laser Generated Crack-Like Features Developed for Assessment of Fatigue Threshold Behavior
Developing methods to evaluate damage tolerance in high cycle fatigue is an important issue for the propeller industry. Traditional methods to establish threshold stress intensity factors for aluminum and martensitic steel have relied mainly on load shedding through-crack test specimens resulting in a pre-cracking history and the associated closure effects. For threshold determination of real life defects, a surface-crack test method, devoid of pre-cracking and closure issues, is needed. To this end, laser generated crack-like features (LGCLFs) were created and have been used successfully to evaluate the threshold crack growth properties of cracks in 7075-T651 with and without shot peening in axial R = 0.1 loading. For this work, a laser part marking system was used to generate a LGCLF semi-circular in shape, and measuring ∼0.015 in. (0.381 mm) deep by ∼0.030 in. (0.762 mm) long, with a notch height of 0.0007–0.001 in. (17.8–25.4 μm) with a tip radius of 0.000 14 in. (3.5 μm). When evaluated metallographically, these laser notches exhibited a very thin recast layer and a negligible heat affected zone. Specimens produced with these flaws were tested in R=0.1 tension-tension fatigue to produce a stress versus life plot of the data. A NASGRO crack growth model was used to generate stress versus cycle data points for the same geometry over the same stress range with a 7075-T651 material model and a crack growth threshold of 1.35 ksi ⋅(in.)0.5 resulting in very good agreement with the test data. This threshold compares favorably with the small crack threshold of 1.27–1.33 ksi⋅(in.)0.5 produced by da/dN testing performed by NASA at R=0.1 using a compressive pre-cracking method. Testing with samples shot peened prior to generating a LGCLF exhibited significantly higher endurance stresses and failure initiations were not from the LGCLF.