Effect of static pre-loading on fracture toughness of Nicalon® fibre glass matrix composite

Static fatigue of commercially available unidirectional SiC-fibre reinforced glass matrix composite has been followed by measuring the fracture toughness of samples statically pre-loaded at different stresses and times. Pre-load levels were just below the loads corresponding to fracture strength, the (bending) pre-stress being on level 300, 310 and 315 MPa. Three hold times, 25, 50 and 75 h, were applied at each of the stresses. For fracture toughness (KIc) determination, a chevron-notch (CN) three-point bend test was applied. The method has been found to be useful to accurately detect and quantify very subtle microdamage on fibre/matrix interfaces caused by static pre-stressing. Despite of the high scatter of the measured KIc values, a decrease in fracture toughness with increasing pre-stress and hold time has been found. A parameter is proposed that may be used to quantify the cumulative and combined effect of pre-stress and hold time on fracture toughness of composites.