Fracture Toughness of Duplex Structures: Part II — Laminates in the Divider Orientation

Laminates containing 25 percent of tough (unaged) maraging steel plates and 75 percent of brittle (aged) maraging steel plates were prepared by a special diffusion bonding and heat treating technique so that the thickness of the tough constituent was either 0.02, 0.04, 0.06, or 0.08 in. Compact tension fracture toughness specimens were machined so that the laminates were parallel to the sides of the specimens. They were then fractured according to the recommendations of the ASTM and three indices of toughness were measured: K_Ic, as recommended by the ASTM, K^o_max based on the maximum load and original crack length and Kⁱ_max, based on the maximum load and instantaneous crack length. Four tests were performed at each thickness of the tough constituent; two each at crack lengths of approximately 0.30 and 0.55 in. The plane-strain fracture toughness of the laminates was slightly higher than K_Ic of the monolithic brittle constituent and was independent of either crack length or the dimensions of the tough constituent. Regardless of the thickness of the tough phase the composite toughness was highest for specimens containing cracks 0.30 in. long when the throughness was measured in terms of K^o_max or Kⁱ_max. The thoughnessas a function of thickness exhibited a relative maximum when the tough phase was 0.04 in. thick and a relative minimum when the tough phase was 0.06 in. thick for both the 0-30 and 0.55-in. crack lengths. The experimental results are in broad agreement with a proposed model for crack extension in the divider orientation.