Determining the Elastic Constants and Thickness of the Interphase in Fiberglass Plastic Composites from Micromechanical and Macromechanical Tests

The aim of this paper is to describe a methodology for determining the elastic constants and thickness of the interphase between matrix and fiber in fiberglass plastic composites from macro- and micromechanical testing. Macromechanical testing is tension of unidirectional fiberglass plastics along and across the fiber direction. Micromechanical testing is tension of glass fibers and instrumented microindentation into the matrix and the fiberglass. The interphase thickness is determined by dynamic force microscopy on thin sections without a height difference. The measured interphase thickness is 621 nm. Based on the interphase thickness, a mesomechanical finite element model of a fiberglass monolayer is constructed. As a result, it is found that the elastic modulus and Poisson’s ratio are 12.7 GPa, 0.07. It is established that the elastic properties of the interphase differ significantly from those of the matrix. The paper also explores the possibility of determining the interphase thickness through computational experiments. It turns out that by knowing the actual elastic properties of the matrix and the fiber, as well as the fiberglass monolayer, it is feasible to calculate the interphase and its elastic properties with acceptable engineering accuracy. The deviation of the calculated interphase thickness from the experimentally measured one is 6%.