Fracture toughness and R-curve behavior of laminated brittle-matrix composites

The fracture toughness and resistance-curve behavior of relatively coarse-scale, niobium/niobium aluminide (Nb/Nb3Al) laminated composites have been examined and compared to other Nb/Nb3Al composites with (in situ) Nb particulate or microlaminate reinforcements. The addition of high aspect-ratio Nb reinforcements, in the form of 20 vol. pet of 50- to 250-mu m-thick layers, was seen to improve the toughness of the Nb3Al intermetallic matrix by well over an order of magnitude, with the toughness increasing with Nb layer thickness. The orientation of the laminate had a small effect on crack-growth resistance with optimal properties being found in the crack arrester, as compared to the crack divider, orientation. The high fracture toughness of these laminates was primarily attributed to large (similar to 1- to 6-mm) crack-bridging zones formed by intact Nb layers in the crack wake; these zones were of sufficient size that large-scale bridging (LSB) conditions generally prevailed in the samples tested. Resistance-curve modeling using weight function methods permitted the determination of simple approximations for the bridging tractions, which were then used to make small-scale bridging (SSB) predictions for the steady-state toughness of each laminate.