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

Fibre breakeage in composite materials is usually a determining damage mechanism for its structural integrity due to the high energy associated, in comparison with matrix cracking. For this reason, the assessment of the translaminar fracture toughness is relevant for accurate numerical predictions of composite structures. However, there are scarce investigations related to this topic for additive manufactured composites reinforced with continuous fibres.  In this investigation, the translaminar fracture toughness of 3D-printed continuous fibre reinforced polymer (c-CFRP) composites was characterised using double-tapered compact tension (2TCT) specimens. The 2TCT geometric dimensions were obtained through a parametric study to prevent undesired failure modes. The results show a translaminar fracture toughness of 17.4 N/mm for the tested 0/90 laminates. The fracture toughness corresponding to the tensile failure of the 0° ply was derived using a rule-of-mixtures approach. Post-mortem micrographic and X-ray analysis indicated the presence of fibre pull-outs in the crack surface and confirmed the absence of any additional damage, validating the use of 2TCT geometry for the determination of the translaminar fracture toughness in additively manufactured c-CFRP composites.