MEASUREMENT OF STRESS-CONCENTRATION IN A FIBER ADJACENT TO A FIBER BREAK IN A MODEL COMPOSITE

The axial stress in a continuous fiber adjacent to a broken fiber has been measured directly. The model composites used are hybrid and comprise of a single fiber of either Nicalon or carbon flanked by two Kevlar fibers and embedded in a large cross-section of epoxy resin. The center fiber breaks cleanly at small strains, and the axial stress in Kevlar fibers can be measured within the transparent matrix by using a Raman microprobe. The laser probe was focused to about 10 mu m, so this was the spatial resolution of the stress measurement. The stress transfer length of about 200 mu m agrees with theory and photoelastic measurements, but the stress concentration factor does not agree with theoretical predictions. A significant amount of load is transferred to the matrix in the geometry used here. Load transfer models based on the concept of an effective radial interaction distance are proposed which give a better agreement with the measured stress concentration factors.