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-μm-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 (∼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.