Numerical modelling of through-thickness reinforced structural joints

Abstract

The main objective of this research study was to develop numerical models to analyse the mechanical and fracture properties of through-thickness reinforced (TTR) structural joints. The development of numerical tools was mainly based on the finite element (FE) method. A multi-scale approach was used: the bridging characteristics of a single reinforcement was studied at micromechanical level by simulating the single-pin response loaded either in mode-I or in mode-II. The force-displacement curve (bridging law) of the pin was used to define the constitutive law of cohesive elements to be used in a FE analysis of the global structure. This thesis is divided into three main parts: (I) Background, context and methodology, (II) Development for composite joints, and (III) Development for hybrid metal-composite joints. In the first part the objectives of the thesis are identified and a comprehensive literature review of state-of-art throughthickness reinforcement methods and relative modelling techniques has been undertaken to provide a solid background to the reader. The second part of the thesis deals with TTR composite/composite joints. The multi-scale modelling technique was firstly applied to predict delamination behaviour of mode-I and in mode-II test coupons. The bridging mechanisms of reinforcements and the way these increase the delamination resistance of bonded interfaces was deeply analysed, showing how the bridging characteristics of the reinforcement features affected the delamination behaviour. The modelling technique was then applied to a z-pin reinforced composite T-joint structure. The joint presented a complicated failure mode which involved multiple crack path and mixed-mode delamination, demonstrating the capability of the model of predicting delamination propagation under complex loading states. The third part of the thesis is focused on hybrid metal/composite joints. Mode- I and mode-II single-pin tests of metal pin reinforcements embedded into a carbon/epoxy laminate were simulated. The model was validated by comparing with experimental tests. Then the effects of the pin geometry on the pin bridging characteristics were analysed. The model revealed that both in mode-I and mode-II small pins perform better than large pins and also that the pin shape plays an important role in the pin failure behaviour. The modelling technique was then applied to simulate a metal-composite double-lap joint loaded in traction. The model showed that to obtain the best performance of the joint an accurate selection of pin geometry, pin arrangement and thickness of the two adherends should be done.