N. J. Goldfine, D. Grundy, J. Cammett, Jentek Sensors, Inc., Waltham, MA; V. Zilberstein, JENTEK Sensors, Inc., Waltham, MA
This presentation discusses the results of MWM-Array monitoring during fatigue tests of aluminum 7075-T651 and Ti-6Al-4V coupons. The goal is detection of early-stage cracks in aluminum and titanium alloys. Results are presented for two different coupon geometries. The aluminum coupons are flat plates with electro-polished and as-machined holes. The titanium coupons have exposed, curved surfaces that are shot peened. MWM‑Arrays are used in both scanning and surface-mounted configurations. Acetate replicas provide the actual crack lengths at multiple intervals throughout the fatigue tests of the titanium coupons.
The MWM-Array, in both scanning and surface-mounted modes, offers a convenient and effective way to monitor such coupon tests to study early-stage fatigue behavior. One observation from these tests, that is generally well known, was that at early stages, multiple small cracks (<0.005 in. long at the surface) formed and then coalesced into larger cracks. This is true for both shot-peened Ti-6Al-4V and polished or as-machined aluminum 7075. This behavior is observed, for example, in actual engine components, where multiple shallow cracks initiate and grow in regions with fretting damage. For aluminum 7075, multiple small cracks generally initiate at inclusions before coalescing into longer cracks. Although, a single large crack eventually dominates the NDT signature, this appears to occur when the crack is relatively long; for example, when a dominant crack in titanium alloy components is greater than 0.015 in. (length at the surface). This is a concern for both prognostics (life prediction) and for qualification of Nondestructive Testing (NDT) methods.
Summary: This presentation discusses the results of MWM-Array monitoring during fatigue tests of aluminum 7075-T651 and Ti-6Al-4V coupons. The goal is detection of early-stage cracks in aluminum and titanium alloys. Results are presented for two different coupon geometries. The MWM-Array, in both scanning and surface-mounted modes, offers a convenient and effective way to monitor such coupon tests to study early-stage fatigue behavior.
