Fatigue Crack Growth Threshold Stress Intensity Determination via Surface Flaw (Kb Bar) Specimen Geometry

Abstract

Many aircraft component failures originate from small-surface flaw cracks in highly stressed locations. It is therefore desirable to measure the threshold stress intensity factor that simulates the key conditions of high-stress and small semicircular surface crack shape. Such a technique has been developed and demonstrated on multiple titanium and nickel-base alloys. This technique is effective at both room and elevated temperatures and at both low and high mean stress. The test method is described in detail, but in brief a small EDM flaw is made on the surface of a rectangular specimen. The crack length is monitored via electrical potential drop technique. The crack length and aspect ratio versus the change in electrical potential across the crack tip has been measured and is used to control the load shed during the experiment. Load sheds up to C = - 1181 m - 1 have been demonstrated to give comparable thresholds to those measured using single edge notch or compact tension geometries with lower shed rates. Typical experimental results on both Ti-6Al-4V and an advanced nickel-base superalloy (KM4) are shown at both room temperature and elevated temperatures. This technique allows a rapid determination of threshold stress intensity factor for a material in a manner simulating the types of constraint experienced in components.