Closure measurements on short fatigue cracks

Single-edged notched specimens, 2.3 mm thick, of 2024-T3 aluminum were cyclically loaded at R-ratios of 0.5, 0.0, -1.0, and -2.0. The notch roots were periodically inspected with an optical microscope to locate the initiation of very short cracks. Acetate replicas were also taken. The loads were selected to produce fatigue lives of 500,000 cycles or less. 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. The opening load ratios for the short cracks are somewhat smaller than for long cracks at positive R-ratios, but are considerably smaller for negative R-ratios.