Determination of the interface failure mechanism during transverse loading of single fibre SiC/Ti-6Al-4V composites from torsion tests

Abstract

During transverse loading of continuous fibre-reinforced composites, large tangential shear stresses develop at the interface at about 45° to the loading axis. These shear stresses may initiate interface failure and frictional sliding, increasing the load in the matrix and lowering the applied stress to cause normal fibre-matrix separation. In the present study, an attempt has been made to measure the tangential shear strength of the interface for single fibre SiC/Ti-6Al-4V composites with varying interfaces using torsion testing and finite element stress analysis. An energy based fracture mechanics approach was developed to predict the micro-mechanical response of the composite to torsional loading following interface crack initiation. The fracture mechanics analysis suggests that a non-linearity would appear in the strain versus angle of twist curve once debonding initiates. Based on the onset of non-linearity and finite element stress analysis, the tangential shear strength for SCS-6/Ti-6Al-4V interface (carbon-rich coated) was estimated at 50 MPa; that for the SCS-0/Ti-6Al-4V interface (uncoated) was greater than 80 MPa, and that for the AC1/Ti-6Al-4V interface (carbon coated) was about 40 MPa. Comparison of these results with experimental observations and finite element analysis of transverse tension tests indicates that failure in SCS-6/Ti-6Al-4V and AC1/Ti-6Al-4V composites is initiated by tangential shear failure of the interface. In SCS-0/Ti-6Al-4V composites, the results are not conclusive.