A method for generating fatigue marker bands using a constant Kmax test procedure

Marker bands were introduced onto fatigue fracture surfaces by a block loading technique wherein Kmax was held constant while Kmin was varied. Materials examined included Astroloy nickelbase superalloy, 1020 hot-rolled steel. AC112-TL aluminum, and 2024-T3 aluminum. The use of a constant Kmax test procedure allowed ΔK to be varied without affecting crack growth rates, thereby providing a convenient method for documenting the shape of a progressing flaw. The nature of contrast differences between marker bands generated at different ΔK levels were examined in terms of changes in fracture micromechanisms as well as changes in fracture surface roughness. Results indicate that a micromechanism transition from cleavage-like faceted growth at low ΔK to striated growth at intermediate ΔK was responsible for the major difference in fracture surface appearance.