AN APPROACH TO MEASURE THE SHAPES OF 3-DIMENSIONAL SURFACE CRACKS DURING FATIGUE-CRACK GROWTH

This study presents a technique for measuring shapes of three dimensio nal surface cracks continuously during fatigue crack growth. The techn ique uses a laser interferometric system to measure crack compliance a nd a photomicroscopic system to measure surface crack length. Using a compliance expression for surface cracks, valid for the range of crack aspect ratios (alc) of 0.2 to 2.0, aspect ratio calculations employin g compliance and surface crack length measurements are demonstrated fo r cracks growing from EDM notches of different geometries (shallow or deep). The experimentally determined aspect ratio variations during cy clic crack growth are shown to be in good agreement with the expected variations in aspect ratio, predicted using the stress intensity facto r equations for surface cracks. The effects of deviations in the compl iance measurement location from the center of the surface crack due to assymetric crack growth are also accounted for through corrections of compliance measurements for crack-asymmetry. The fatigue crack growth rates of surface cracks, after incorporating the variations in aspect ratio in the calculations, agreed with the large-crack growth data fo r all crack geometries.