Fracture Toughness of Duplex Structures: Part I — Tough Fibers in a Brittle Matrix

Abstract

Tough maraging steel fibers were embedded in a relatively brittle maraging steel matrix by a special diffusion bonding and heat treating technique. Three indices of fracture toughness were computed. KIc as recommended by the ASTM, Komax, the fracture toughness based on the maximum load and initial crack length and Kimax, the fracture toughness based on the maximum load and instantaneous crack length. While KIc for the fibrous composite was no higher than KIc for the matrix material, both Komax and Kimax were significantly higher for the composite. Since fracture of the fibrous composites occurred in a stable manner (fast fracture was not observed) the load displacement records were converted into crack growth resistance curves. The R curves exhibited a serration at GIc, but continued to increase significantly above GIc and multiple serrations, apparently as a result of the fibers, were present. An approximate calculation shows that the increase in crack growth resistance cannot be attributed solely to plastic deformation of the fibers. Scanning electron microscopy revealed local decohesion between the fibers and the matrix and the character of the R curve was attributed to local crack front pinning by blunting or reduction in constraint. As a model of delamination in a fibrous structure, holes were drilled perpendicular to the parting plane in 7075-T6 aluminum specimens. The load versus displacement and R curves were similar to those observed in the fibrous composites. In addition, pinning of the fatigue crack was observed in the vicinity of the holes.