Multiscale analysis of damage-tolerant composite sandwich structures

Abstract

Composite sandwich structures are widely regarded as a cost/weight-effective alternative to conventional composite stiffened panels and are extensively utilized for lightweight applications in various sectors, including the aeronautical, marine and transport industries. Nevertheless, their damage tolerance remains a critical issue. This work aims to develop reliable analytical and numerical tools for the design of damage-tolerant advanced foam-cored composite sandwich structures for aerospace applications. It comprises of original experimental observations together with novel numerical and analytical developments, as detailed below. A novel analytical model for predicting the post-crushing response of crushable sandwich foam cores is presented. The calibration of the model is performed using experimental data obtained exclusively from standard monotonic compressive tests. Hence, the need for performing time-consuming compressive tests including multiple unloading-reloading cycles is avoided. Subsequently, the translaminar initiation fracture toughness of a carbon-epoxy Non-Crimp Fabric (NCF) composite laminate is measured. The translaminar fracture toughness of the UD fibre tows is related to that of the NCF laminate and the concept of an homogenised blanket-level translaminar fracture toughness was introduced. A multiple length/time-scale framework for the virtual testing of large composite structures is presented. Such framework hinges upon a novel Mesh Superposition Technique (MST) and a novel set of Periodic Boundary Conditions named Multiscale Periodic Boundary Conditions (MPBCs). The MST is used for coupling different areas of the composite structure modelled at different length-scales and whose discretizations consist of different element types. Unlike using a sudden discretization-transition approach, the use of the MST eliminates the undesirable stress disturbances at the interface between differently-discretized subdomains and, as a result, it for instance correctly captures impact-induced damage pattern at a lower computational cost. The MPBCs apply to reduced Unit Cells (rUCs) and enable the two-scale (solid-to- shell) numerical homogenization of periodic structures, including their bending and twisting response. The MPBCs allow to correctly simulate the mechanical response of periodic structures using rUCs (same results as if conventional UCs were used), thus enabling a significant reduction of both modelling/meshing and analysis CPU times. The developments detailed above are finally brought together in a realistic engineering application.