HIGH-RESOLUTION R-CURVE CHARACTERIZATION OF THE FRACTURE-TOUGHNESS OFTHIN SHEET ALUMINUM-ALLOYS

The plane-strain initiation fracture toughness and plane-stress stable crack growth resistance were determined with a single small compact t ension (C(T)) specimen for each of three precipitation hardened alumin um alloy sheets (AA2024-T3, AA2519-T87 (+Mg+Ag), and AA2650-T6). Crack length was monitored precisely with direct current potential differen ce (DCPD) measurements, and specimen plasticity was accounted for with the J-integral. The DCPD technique resolves a small amount of crack-t ip process-zone damage (approximate to 20 mu m) that constitutes crack initiation under plane-strain constraint. Two measures of initiation toughness are calculated: the elastic-plastic fracture toughness detec ted by DCPD (J(ICi), K-JICi) and the toughness based on ASTM Standard E 813 (J(IC), K-JIC). High resolution of fracture initiation is necess ary to obtain a lower bound initiation toughness, K-JICi, because plan e-strain constraint is present ahead of the fatigue precrack but is ra pidly lost with crack extension in thin sheet. K-JIC overestimates tou ghness due to constraint loss coupled with the offset blunting line de finition of fracture initiation. The J-integral/DCPD method provides a reproducible measure of the plane-stress linear-elastic resistance cu rve (K-J - Delta alpha) that compares reasonably to R-curves determine d for large middle-cracked tension specimens. The small specimen metho d is effective for studies pertaining to alloy development, environmen tal effects, and fracture mechanisms.