Crack closure stress estimation by decodable marker banding techniques and comparison of results with electron fractographic technique

Abstract

A specially programmed marker load sequence was designed that permits unambiguous measurement of crack opening and crack closure stress from marker band spacing using both optical and scanning electron fractography. Crack growth tests using programmed marker loading sequence were carried out on single edge notch tension specimens made out of Al-Cu alloy 2014-T3 (BS: L73 equivalent) from crack initiation to failure and the fracture surface examined under optical and electron microscope. Reproducible marker band patterns on the fatigue fracture surface could be obtained using this method. Measured crack closure/opening stress values are observed to be consistent with known material behaviour. The marker banding technique can be applied over wider crack growth rate ranges compared to striation measurement technique, which is restricted to a small range of crack growth rate and in select materials. Quantitative fractography of the marker bands reveals a degree of load interaction under variable amplitude loading that requires further examination.

Marker Loads for Quantitative Fractography of Fatigue Cracks in Aerospace Alloys

“…Note that the marker bands are evident, as intended, but that the band can disappear at some locations owing to changes in grain orientation with respect to the crack growth direction. Combinations of CA bands with counters The idea of including "counters" in CA marker bands has been proposed and used by several investigators [52], [53], [54]. Example 14 began as a development of such a strategy.…”