Measurement of interfacial shear strength by means of micromechanical single-fibre pull-out tests

Interfacial shear strength (IFSS) is one of the key parameters in composite technology. To characterize it, most commonly composites with unidirectionally aligned fibres are manufactured: A transverse tensile test or a short beam shear strength test is then performed to measure the apparent interlaminary shear strength (ILSS). However, the maximum stress found in such macromechanical tests is also governed by various additional factors coming from the preparation of the test samples. Here, the single-fibre pull-out (SFPO) test has several advantages. By involving only single fibres in the test, most dependencies on the manufacturing process of the test specimen are avoided. The single-fibre pull-out test not only determines the apparent, but for instance also the local interfacial shear strength, which is essential for applications with cyclic load. Fast and effective micromechanical testing of the interfacial shear strength has recently been standardized as DIN SPEC 19289. This paper describes the pull-out test in general and shows results for the interfacial shear strength of common thermoset matrix materials to carbon and glass fibres. In addition, the paper gives an example for the application of the SFPO test for the characterization of recycled carbon fibres, coming from a solvolysis process.