Stress-free edge influence on thermal fatigue damage in an SCS-6/Ti-24A1-11Nb composite

Unidirectionally reinforced [90]₈ specimens of an SCS-6/Ti-24A1-11Nb (at.%) composite (35 vol.% fiber) in three different gage-section widths were thermally cycled in air between 150 and 815°C for 500 cycles. During thermal cycling, matrix cracks initiated at the composite surface and propagated into the composite normal to the fiber direction. However, near the stress-free edges of all specimens, a region void of cracks existed which extended an average of 5.4 fiber diameters into the width of the composite. This cracking pattern was attributed to the presence of a thermally induced cyclic tensile residual stress which dissipates to zero near the stress-free edge of the composite. The finite element method was employed to determine how the fiber-matrix interface shear resistance influences the development of these residual stresses. Using coulomb friction as a measure of shear resistance, the matrix residual stresses in the fiber direction had a peak value of 500 MPa. A frictional coefficient range of 0.18–0.22 was found to give between 95% and 99% of this peak value within 5.4 fiber diameters from the edge. Thermal cycling of the model between 150 and 815°C provided evidence that the resultant cyclic stresses were tensile in nature and were suggested as the probable cause of the periodic surface cracks. The reduction in post-cycling transverse strength with increasing gage-section width indicated that the smaller-width specimens exhibited less damage per cross-sectional area than the wider specimens.