Search

A technique is presented which can be used to measure the interfacial normal strength in transversely loaded composites. The technique combines the measurement of local deformations during tests in a Scanning Electron Microscope and numerical simulations, in which the measurements are used as boundary conditions. Deformations are measured by placing a grid of markers on the surface of single fibre specimens. The displacements of these markers are measured during loading of the specimen. Numerical simulations show that the radial and tangential interface stresses increase towards the specimen surface, causing the initiation of debonding at the surface. This effect is supported by tests under an optical microscope. The interfacial normal strength is defined to be the maximum radial interface stress just before the initiation of debonding. Based on the presented results, it can be concluded that the presented technique is a promising tool for the measurement of the interfacial normal stress.

A mixed experimental—numerical method is presented which can be used to characterize interphases in composite materials. This method is marked by using measured field quantities, such as displacements and strains, and finite element simulations of the experiments to fit interphase parameters using an iterative minimum variance estimation algorithm. Field quantities are measured using marker fields in the vicinity of the interphase. The markers are applied using the electron beam of a scanning electron microscope (SEM). A marker grid, consisting of dots with a diameter of 0.2 µm, is placed on the surface of unidirectional glass fibre-reinforced epoxies. The displacement field is determined by calculating the relative displacements of the markers during transverse tensile experiments in a SEM. Subsequently, the experiment is modelled using the finite element method. The positions of the border markers of the marker grid define the geometry of the model, and the displacements of these markers the boundary conditions. A mathematical interphase model is included. The parameters, quantifying the mechanical behaviour of the interphase, are fitted using the iterative parameter estimation procedure. Based on the presented results, it can be concluded that the method is a promising tool for the characterization of interphases.