Studies on the Fracture of Glass Fiber-Reinforced Polymers with Nonlinear Material Behavior

Due to the inherent brittleness of the commonly used thermoset polymer matrices and glass or carbon fibers in fiber-reinforced polymers (FRPs), their fracture resistance is typically assessed by means of linear-elastic fracture mechanics (LEFM) methods. Such approaches are based on the small-scale yielding assumption, which is met when the material predominantly presents linear- elastic behavior. The latest advances in material developments for FRPs aim at endowing the laminates with the ability to undergo plastic deformation through the use of elasto-plastic polymer matrices. Although this leads to more damage-tolerant parts, noticeable plasticity in the composites’ response leads to violation of the small-scale yielding assumption, making LEFM methods no longer suitable to quantify their fracture toughness. The goal of this doctoral thesis is to develop thorough understanding of the fundamental fracture and crack-growth mechanisms of glass fiber-reinforced polymers with elasto-plastic matrices. Based on a combined experimental/numerical approach, crack propagation under mode I loading conditions is investigated for laminates containing either a commercial epoxy (EP) or a prototype polyurethane (PU) matrix of similar glass transition temperature. While the epoxy resin undergoes plasticity only to a small degree, two thirds of the strain at break of 12% during tensile loading of the polyurethane is non-recoverable. irst, the validity of the J-integral approach to compute the energy dissipation during propagation of a transverse intralaminar crack in the PU laminates is assessed. To that end, novel experimental techniques are employed to calculate the J-integral by definition using strain fields obtained around the crack tip. This direct approach is then compared with two established data reduction methods and data obtained from virtual tests to determine the suitability of standardized procedures to quantify the fracture energy of such heterogeneous materials. The first of the two established methods is the load-displacement of multiple specimens and the second is the basic procedure described in the standard test ASTM E1820, which was first developed for metallic materials. The results suggest that the procedure described in the aforementioned ASTM standard is accurate enough to quantify the fracture energy of the composites in the range of displacements investigated. Furthermore, error estimations reveal that the straightforwardness of the multiple- specimen method comes at the expense of accuracy of the measurement. Next, the crack-growth toughness (R-curves) of EP and PU laminates are determined and the sources of toughening in the composites are ascertained by means of in situ micromechanical testing in a scanning electron microscope. From these investigations it is concluded that the 6-fold higher fracture energies shown by the PU laminates stem from a different failure mechanism at the fiber/matrix interfaces, which likely originates from stronger bonds. Later, the interlaminar fracture of unidirectional and cross-ply EP and PU composites is investigated. Special attention is dedicated to the quantification of the crack-closing tractions in the fiber bridging zone, a mechanism responsible for the rising R-curve behavior of FRPs. Both the direct and the indirect methods are used in this work. While the former relies on differentiation of the J-integral with respect to the crack-tip opening displacement, the latter is a semi-experimental technique based on quasi-continuous strain measurements using fiber Bragg grating sensors. The degree of nonlinearity in the fracture response of the laminates is assessed by quantifying the fracture energy with linear-elastic (G) or elasto-plastic (J-integral) fracture mechanics methods. Cohesive zone models are developed for the prediction of the delamination response of the laminates. The results show that the PU laminates are 4 times tougher than their EP counterparts regarding the propagation of an interlaminar crack both for unidirectional and cross-ply layups. In addition, the novel experimental techniques used here allowed for the direct derivation of the bridging law for the first time. Finally it is shown that the fracture response of GFRPs with PU matrix can only be accurately predicted when the nonlinear phenomena are properly taken into account