Ballistic damages of GFRP and their repairs using ultrasound matrix impregnation

The research of type of damages was carried out using the GFRP plates made of STEF (matrix - epoxy resin, filler - plain weave glass fibre fabric, six layers, 2 mm of thickness), which were fired at the special stand. Experiments were conducted with steel spherical impactors, 6.35 mm diameter (weight of 1.05 g) with terminal velocity 50-900 m/s. Experimental data of ballistic curves was fitted by least-square regression according to the classical Lambert-Jonas equation. According to this data, the perforation begins with velocity ~180 m/s (ballistic limit V50). The dependence of delamination area vs. initial velocity V0 was obtained as results of experimental data optical processing. The maximum delamination area is formed when the velocity is close to the ballistic limit. The main reason of it is that the specimens absorb all the kinetic energy of the impactor at this velocity. It was found that at the impact point the maximum of interlayer crack gaps (delamination thickness) reaches ~50 microns. The tests provided on tensile specimens with ballistic damages were performed at INSTRON testing machine to determine the residual strength with such defects. On the basis of the experiments we propose to replace the damaged area by an open hole with the equivalent diameter. For an arbitrary loading the evaluation of residual strength was made by the express-method based on the energy approach. Thus, determining the size of the damaged area and the equivalent diameter holes, we can predict the residual strength of the structural element with a defect. Due to the fact that small damages significantly reduce the fracture load of specimens, an effective method of repairingdelamination was developed. The repair occurs by filling the cracks between the layers using the epoxy resin compound. Epoxy resin links the layers together that provides their further joint work. The factors influencing the rheological properties of the matrix (epoxy compound viscosity and surface tension in terms of capillary effects) were studied. It was shown that a matrix ultrasound heating up to the temperature of 60 °C allows a complete filling of the above-mentioned crack of delamination at the length up to 20 mm for 90 seconds. Further, for complete matrix curing we used the local infrared heater. As a result of the repair, GFRP strength properties run up to 80- 90 % of the initial undamaged one.