Abstract

The post-impact strength of composite materials is one of the main design parameters of aeronautical structures in terms of damage tolerance. During the low-velocity impact test, from a threshold energy, the laminate only partially returns the energy received during the impact to the indenter (elastic recovery). The remaining energy is absorbed by the laminate and dissipated in the form of damage (interlaminar and intralaminar), plastic deformation of the polymer matrix and breakage of the carbon fibers. To date, few authors have attempted to quantify the participation of each of the damage mechanisms in the overall energy absorption process of the laminate due to their experimental difficulty. In this work, a methodology has been developed capable of performing damage of similar extent and location to that produced in a low-velocity impact, but without damaging the fibers, through the application of local induction heating. For this purpose, the residual strength and stiffness of AS4/PEEK laminates, subjected to impacts over a wide range of energies (30-70J), have been compared with those obtained in laminates damaged by electromagnetic currents, for equivalent damage extensions. The results reveal that the breakage of carbon fibers has a great influence on the loss of stiffness of the laminate, but not on its strength, confirming the role of delamination as the main responsible for the loss of the strength capacity of the damaged material.

Abstract

In this work, the Mode I and Mode II interlaminar fracture toughness of a hybrid laminate composite consisting of carbon fiber-reinforced layers and glass fiber-reinforced layers was characterized. Unidirectional laminates were used for the tests, and the stacking sequence was chosen with the aim of achieving pure fracture modes in the tests. Mechanical characterization was carried out using three-point bending tests with different spans, taking into account the effects of indentation, shear, and support rotation. Mode I and Mode II interlaminar fracture tests were performed using the ADCB (Asymmetric Double Cantilever Beam) and AENF (Asymmetric End Notched Flexure) tests, respectively, also considering the effects of shear, local deformation, and rotations due to bending. The data were obtained using a recently published analytical model that allows the resistance curve to be found for each load-displacement data obtained from the testing machine.

Abstract

Delamination is common in composite layered materials. These out-of-plane cracks are conventionally modelled through a layerwise strategy where each composite layer is represented with finite elements. The interface between layers is modelled with cohesive elements. This modelling strategy involves a fine discretisation through the thickness of the laminate. Additionally, cohesive elements need small in-plane elements (typically less than 1 mm) to accurately represent the interface fracture process zone. As a result, this conventional approach leads to prohibitive computational costs for large assemblies. The present work develops a strategy that is orders of magnitude more efficient for modelling delamination in large structures. The model initiates with a single solid-shell element representing the laminate thickness. The out-of-plane stresses are accurately recovered through the thickness of the laminate. The model is enriched with additional nodes at interfaces, where delamination is detected, to kinematically describe the cracks. Then, a novel energy-based cohesive method is used to model the crack propagation using large elements (e.g. 5 mm). The presented examples show that the novel modelling strategy is orders of magnitude faster than the conventional layerwise method while achieving comparable accuracy.