Volume 25, Issue 1 (January 1997)
High Resolution R-Curve Characterization of the Fracture Toughness of Thin Sheet Aluminum Alloys
The plane-strain initiation fracture toughness and plane-stress stable crack growth resistance were determined with a single small compact tension (C(T)) specimen for each of three precipitation hardened aluminum alloy sheets (AA2024-T3, AA2519-T87 (+Mg+Ag), and AA2650-T6). Crack length was monitored precisely with direct current potential difference (DCPD) measurements, and specimen plasticity was accounted for with the J-integral. The DCPD technique resolves a small amount of crack-tip process-zone damage (≈20 µm) that constitutes crack initiation under plane-strain constraint. Two measures of initiation toughness are calculated: the elastic-plastic fracture toughness detected by DCPD (JICi, KJICi) and the toughness based on ASTM Standard E 813 () and the toughness based on ASTM Standard E 813 (JIC, KJIC). High resolution of fracture initiation is necessary to obtain a lower bound initiation toughness,). High resolution of fracture initiation is necessary to obtain a lower bound initiation toughness, KJICi, because plane-strain constraint is present ahead of the fatigue precrack but is rapidly lost with crack extension in thin sheet. KJIC overestimates toughness due to constraint loss coupled with the offset blunting line definition of fracture initiation. The J-integral/DCPD method provides a reproducible measure of the plane-stress linear-elastic resistance curve (KJ − Δa) that compares reasonably to R-curves determined for large middle-cracked tension specimens. The small specimen method is effective for studies pertaining to alloy development, environmental effects, and fracture mechanisms.