Two-dimensional crack growth in FRP laminates and sandwich panels

Fiber-reinforced polymer (FRP) composite materials are currently being selected for the design of lightweight and efficient structural members in a wide number of engineering applications. Sandwich panels with glass fiber-reinforced polymer (GFRP) face sheets and low-density cores are notably one of the most common applications of FRP materials in the civil engineering field. The load-bearing capacity of FRP structures can be significantly reduced by delamination and debonding damage which, in actual structural members, may extend all around its perimeter, thus constantly changing the size of the crack front. However, most research efforts concerning the fracture characterization of delamination and debonding damage have focused on one dimensional (1D) fracture specimens where cracks propagate longitudinally with an approximately constant crack width, thus resulting in fracture properties that may lead to inaccurate predictions of fracture behavior in real structures. With the aim of establishing a more realistic fracture approach to delamination and debonding damage in FRP structures and thus improving their fracture characterization, the two dimensional (2D) crack growth in GFRP laminates and GFRP/balsa sandwich panels is investigated in this research. The 2D quasi-static delamination behavior in GFRP plates with embedded defects subjected to out-of-plane tensile loads was experimentally investigated. Increasing loads were obtained due to the disproportionate increase of the crack area throughout the propagation of the 2D crack. Analysis of the load-bearing and compliance responses indicated that a stiffening of the opened part of the plates occurred as a result of in plane stretching stresses that developed due to the boundary conditions associated with an embedded 2D crack. To investigate the effect of the stress stiffening and associated fracture mechanisms, the 2D delamination behavior was numerically studied. Results confirmed a 50% increase, compared to 1D fracture, in the total strain energy release rate (SERR) required to propagate the crack. This was correlated to the increase of the fiber-bridging area as a result of the increase in the flexural stiffness (from beam to plate) and the stress-stiffening effect. A numerically-based method, suitable for determining the mean total SERR involved in the Mode I-dominated 2D delamination of FRP laminates was further developed and validated. This method permits a reduction of experimental and computational efforts. The 2D fracture investigation was extended to GFRP/balsa sandwich panels whose 2D debonding behavior was experimentally studied under quasi-static and fatigue out of-plane tensile loads. Circular embedded disbonds were introduced at the face sheet/core interface. Stretching strains again developed, thereby activating shear fracture modes which, depending on the face sheet layup, triggered different crack propagation behaviors. The fatigue load-displacement hysteresis loops exhibited an increase in the cyclic stiffness due to the stiffening caused by the stretching stresses. The introduction of plies prone to develop fiber-bridging at the face sheet/core interface resulted in enhanced quasi-static debonding resistance, while under fatigue their fracture efficiency was highly dependent on the fatigue amplitude. High R-ratios improved fatigue performance due to a reduction in fiber-bridging crushing.