STRESS-FREE EDGE INFLUENCE ON THERMAL FATIGUE DAMAGE IN AN SCS-G TI-24AL-11NB COMPOSITE/

Unidirectionally reinforced [90](8) specimens of an SCS-6/Ti-24Al-11Nb (at.%) composite (35 vol.% fiber) in three different gage-section wid ths were thermally cycled in air between 150 and 815 degrees C for 500 cycles. During thermal cycling, matrix cracks initiated at the compos ite surface and propagated into the composite normal to the fiber dire ction. However, near the stress-free edges of all specimens, a region void of cracks existed which extended an average of 5.4 fiber diameter s into the width of the composite. This cracking pattern was attribute d to the presence of a thermally induced cyclic tensile residual stres s which dissipates to zero near the stress-free edge of the composite. The finite element method was employed to determine how the fiber-mat rix interface shear resistance influences the development of these res idual stresses. Using coulomb friction as a measure of shear resistanc e, the matrix residual stresses in the fiber direction had a peak valu e 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 diameter s from the edge. Thermal cycling of the model between 150 and 815 degr ees C provided evidence that the resultant cyclic stresses were tensil e in nature and were suggested as the probable cause of the periodic s urface cracks. The reduction in post-cycling transverse strength with increasing gage-section width indicated that the smaller-width specime ns exhibited less damage per cross-sectional area than the wider speci mens.