Geometry and Size Effects on J-R and δ-R Curves under Plane Stress Conditions

Abstract

J-R curves and δ-R curves were measured on 5-mm-thick compact, three-point bend, and center cracked tension specimens with different widths and different precrack lengths. The crack tip opening displacement (δ) was measured at the fatigue crack tip. The materials tested were an age-hardened aluminum alloy; the same alloy in a furnace cooled, very soft condition; and an alloy steel. It was found that the J-integral correlates three to five times as much crack growth in the tension specimens as in the compact and bend specimens, or, in other words, a valid R-curve detemined on a center cracked tension specimen covers at least three times more crack growth than a valid R-curve determined on a compact or bend specimen of the same width and crack length. Furthermore, δ correlates even more crack growth: a valid δ-R curve determined on a compact or bend specimen covers about five times more crack growth than a valid J-R curve. For center cracked tension specimens the amount of crack growth measured was not sufficient to establish limits for a valid δ-R curve; however, at least 60% of the original ligament width can be expended by crack growth for a valid δ-R curve. It is worth noting that no geometry effect on the J-R curve and δ-R curve was found as long as the R-curve data could be considered “valid.”