Conservative estimates of the true interfacial shear strength (IFSS) were obtained by applying the Coulomb-Mohr failure theory to microbond test data in this combined experimental and computational study. Experimentally, interfacial strengths of unsaturated polyester on untreated and silane treated optical glass fibers were measured with an axisymmetric microbond test system. Commonly reported average IFSS values were 6.51 MPa for untreated fibers and 8.01 MPa for silane treated fibers with coefficients of variation that ranged from 9.7-22%, which was an improvement from the previous parallel blade system that ranged from 17-66%. Axisymmetric finite element analysis (FEA) was completed with the aid of a Microbond Input Generator program that reduced model development and analysis time by 97% from 1000 minutes to under 30 minutes making this feasible to perform FEA on every experimental microbond sample. FEA models include the complex microbond bead geometry, contact loading conditions, and constituent material properties measured according to ASTM standards. Application of the Coulomb-Mohr failure theory revealed a conservative estimate of the true IFSS in the absence of compressive radial stresses to be 19.2 MPa for untreated fibers and 25.1 MPa for silane treated fibers. Comparisons are also made with the maximum IFSS value.
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