Mixed-mode crack opening displacement measurements for small fatigue cracks growing in Astroloy at 20°C

Crack opening displacements were measured for small fatigue cracks in Astroloy being grown with uniaxial stress application under high-cycle fatigue conditions. Four cracks were investigated including one that grew from 27 to 74 μm in three increments. Most of the cracks grew at an angle to the loading axis and all opened bimodally. Crack opening scaled with distance from the crack tip similar to an elastic crack, which allowed the calculation of a local stress intensity factor for both mode I and mode II. The proportion of mode II stress intensity factor was relatively large, varying as 0.06 < ΔK II /ΔK I <0.42, with an average of ∼0.3. Thus, uniaxial loading remote to the cracks resulted in a bimodal opening response on the scale of the cracks.