DETERMINATION OF THE INTERFACE FAILURE-MECHANISM DURING TRANSVERSE LOADING OF SINGLE-FIBER SIC TI-6AL-4V COMPOSITES FROM TORSION TESTS/

During transverse loading of continuous fibre-reinforced composites, l arge tangential shear stresses develop at the interface at about 45 de grees 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 tangenti al shear strength of the interface for single fibre SiC/Ti-6Al-4V comp osites with varying interfaces using torsion testing and finite elemen t stress analysis. An energy based fracture mechanics approach was dev eloped to predict the micro-mechanical response of the composite to to rsional loading following interface crack initiation. The fracture mec hanics analysis suggests that a non-linearity would appear in the stra in versus angle of twist curve once debonding initiates. Based on the onset of non-linearity and finite element stress analysis, the tangent ial shear strength for SCS-6/Ti-6Al-4V interface (carbon-rich coated) was estimated at 50 MPa; that for the SCS-0/Ti-6Al4V interface (uncoat ed) was greater than 80 MPa, and that for the AC1/Ti-6Al4V interface ( carbon coated) was about 40 MPa. Comparison of these results with expe rimental observations and finite element analysis of transverse tensio n tests indicates that failure in SCS-6/Ti-6Al-4V and AC1/Ti-6Al-4V co mposites is initiated by tangential shear failure of the interface. In SCS-0/Ti-6Al-4V composites, the results are not conclusive.