Experimental Determination of Strength Distribution of S2-glass Fibers Across a Wide Range of Gage Lengths

Abstract

In a unidirectional polymer-matrix composite under axial tensile loading, the failure process is governed by the dynamic localization and clustering of multiple fiber breaks. Experimental evidence shows that it takes only a cluster of 15-30 fiber breaks to cause the composite to fail catastrophically. Thus it is extremely important to develop a better understanding of this micromechanical damage localization mechanism. The brittle fracture and clustering of fiber breaks is a locally dynamic process which is governed by the size (which, in turn, determines the strength) and spatial distribution of defects within the fiber. Single fiber tensile tests are performed on specimens in the gage length on the order of 4-30 mm and fitted to a Weibull distribution to describe the statistical and gage-length dependent nature of strength in these fibers. However, since the micromechanical damage localization zone is on the order of a few fiber diameters (10 um diameter for S-glass fiber), Weibull paramaters were also determined using the single fiber fragmentation test, which correspond to a gage length of 365 micrometers. The Weibull modulus obtained from the 2 methods are significantly different, indicating that a different flaw population dominates the fiber failure behavior at these smaller length scales. The results indicate that a bi-modal strength distribution similar to the ones developed for Carbon fibers might be more appropriate to describe the statistical variation in the strength of S2-glass fibers. This paper ends with a description of a new experimental technique that is being developed where these fibers are continuously subjected to bending over small radii of curvature on the order of 10-100 microns, triggering the critical surface defects on the tension side and causing the fibers to fragment, at characteristic distances that are significantly smaller than the saturation length in the fragmentation experiments.