An investigation of the effects of layer thickness on the fracture behavior of layered NiAl/V composites

The effects of vanadium layer thickness (100, 200 and 400 mu m) On the resi stance-curve behavior of NiAl/V microlaminates are examined in this paper. The fracture resistance of the NiAl microlaminates reinforced with 20 vol.% of vanadium layers is shown to increase with increasing vanadium layer thi ckness. The improved fracture toughness (from an NiAl matrix toughness of s imilar to 6.6 MPa root m to a steady-state toughness of similar to 15 MPa r oot m obtained from finite element analysis) is associated with crack bridg ing and the interactions of cracks with vanadium layers. The reinitiation o f cracks in adjacent NiAl layers is modeled using finite element methods an d the reinitiation is shown to occur as a result of strain concentrations a t the interface between the adjacent NiAl layers and vanadium layers. The d eviation of the reinitiated cracks from the pure mode I direction is shown to occur in the direction of maximum shear strain. Toughening due to crack bridging is also modeled using large-scale bridging models. The intrinsic t oughness levels of the microlaminates are also inferred by extrapolating th e large scale bridging models to arbitrarily large specimen widths. The ext rapolations also show that the small-scale bridging intrinsic toughness inc reases with increasing vanadium layer thickness.