Closure Measurements on Short Fatigue Cracks

Single-edged notched specimens, 2.3 mm thick, of 2024-T3 aluminum were cyclicly loaded at R ratios of 0.5, 0.0, −1.0, and −2.0. The notch roots were periodically inspected with an optical microscope and with acetate replicas in order to locate the initiation of very short cracks. The loads were selected to produce fatigue lives of 500 000 cycles or less. The specimens and the load schedules were part of an AGARD-sponsored, round-robin test program designed to study the growth of short cracks.

As an addition to the AGARD program, crack opening displacements were measured at single positions across cracks as short as 0.035 mm and as long as the full thickness of the specimen. Two small reflective indentations were placed across the short crack and illuminated with a 15-mW He-Ne laser. This formed interference fringe patterns that could be monitored to measure the relative displacement between the two indentations. Fringe motion was monitored with a minicomputer-controlled optical scanning system to produce real-time crack opening displacements.

The opening load ratios for the short cracks are somewhat smaller than those for long cracks at positive R ratios but are considerably smaller for negative R ratios. The measured compliances of the very short cracks increase linearly with increasing surface crack length and agree quite well with the predictions of linear elastic fracture mechanics.