Constraint Fracture in Simulated Weldments

Simulated under-matched butt weldments have been fabricated using commercially pure titanium (CP Ti) as an interleaf material diffusion bonded to a higher strength 6A1-4V titanium (6A1-4V Ti). Four-point bend bars were constructed where-by the only changing geometric feature is the thickness of the CP Ti interleaf which varied from 12 to 1.7 mm. The CP Ti and 6A1-4V Ti elastic moduli are 112.3 and 115.1 MPa while the yield stresses are 366 MPa and 896 MPa respectively. A through-crack was inserted in the center of the CP Ti interleaf. Due to the close moduli, the elastic T constraint terms do not change as the CP Ti interleaf thickness is varied. However, under plastic loading, the thickness of the interleaf has a large effect on the local plastic fields surrounding the crack tip as well as the initiation J values. Using a hybrid approach with moire interferometry and finite element methods, it has been found that the higher strength 6A1-4V Ti acts to constrain the crack tip plastic zone. The net result of this constraint is a reorientation of plastic flow in the CP Ti interleaf from 90 to 45 and 135 degrees and a mean stress 2.8 times larger that the flow stress. To understand the effect of the hydrostatic constraint on the initiation process in the CP Ti interleaf, a series of double interleaf tensile bars have been developed to obtain stress-strain curves of the CP Ti interleaf under varied constraint states. These tensile specimens indicate that for the highly constrained CP Ti interleafs, that the mode of fracture initiation is due to a form of cleavage initiating at the titanium carbide stringers.