E-glass/DGEBA/m-PDA single fiber composites: The effect of strain rate on interfacial shear strength measurements

Precise measurements of fiber break regions have been made during the singl e fiber fragmentation test (SFFT) procedure on E-glass/diglycidyl ether of bisphenol-A (DGEBA) /meta-phenylenediamine (m-PDA) test specimens. From the se measurements, the location and size of each fiber fragment was determine d, and the resulting information was used to construct fragmentation maps o f the tested fiber. By comparing these maps, the fragmentation process supp orts random fragmentation along the length of the fiber. Since the interfac ial shear strength (IFSS) or the interfacial shear stress transfer coeffici ent (I-STC) is obtained from the fragment length data at the end of the tes t (saturation), frequency histograms of the fragment length data were const ructed to determine the repeatability of the fragmentation process. Since t he SFFT is performed by sequential step-strains of the test specimen, test protocols were developed by controlling the step size of each strain increm ent and the time between each step-strain (dwell time). For the testing pro tocols used in this research. the E-glass/DGEBA/m-PDA frequency histograms of the fragment lengths were found to be generally repeatable. However, whe n the effective strain rate of the test was altered by changing the dwell t ime between strain increments, the fragment distribution at saturation of t he E-glass/DGEBA/m-PDA SFFT specimens changed. The direction of the change was found to be inconsistent with the effect one might expect when only the nonlinear viscoelastic behavior of the matrix is considered. However, the magnitude of the change observed in the E-glass/DGEBA/m-PDA SFFT specimens is not universal. Fragmentation data obtained on E-glass/polyisocyanurate S FFT specimens revealed a much smaller change in fragment length distributio ns with the same change in testing protocols. Consistent with the results o btained on the E-glass/DGEBA/m-PDA, fiber fragmentation occurs when the pol yisocyanurate matrix exhibits nonlinear viscoelastic behavior. The implicat ion of these results for interfacial shear strength measurements is discuss ed.