Numerical Analysis of Fibre-Matrix Interface for Accurate Debonding in Microbond Tests

Abstract

The performance of fibre-reinforced composites is influenced by the interaction between the reinforcement material, the matrix material, and the interface between them. This combination is crucial for achieving excellent mechanical properties under both static and dynamic conditions. The accurate characterisation of these interfaces is crucial, particularly because interfacial damage often serves as the primary initiator of failure in composites. This dissertation investigates advancements in the characterisation of this interface through the microbond (MB) test, a widely used technique for characterizing interfacial properties. The research integrates experimental MB testing with finite element (FE) modelling to offer a thorough understanding of the fibre-matrix interface. The research presented in this dissertation consists of five publications and two data articles, each focusing on specific aspects of the MB test and interface characterisation.

Traditional MB tests, which have been used for over three decades, involve applying pico-litre sized droplets to single fibres and shearing them off with parallel blades. This method provides indirect estimates of interface parameters. The unitary output of conventional MB tests limits the development of accurate numerical models. The research addresses these critical challenges and provides a solution in calibrating interface parameters, with a specific emphasis on a numerical concept namely Cohesive Zone Model (CZM) parameters.

This dissertation employs a methodological framework that begins with extensive numerical simulations to examine the interdependence of MB test parameters and material behavior. These simulations highlight the significant errors that can arise from traditional models with many engineering presumptions, with discrepancies compared to experimental force measurements reach 80%. Building on these insights, comprehensive FE models were developed to simulate MB tests. Experimental MB tests were conducted by incorporating optical fibres equipped with Fibre Bragg Grating (FBG) sensors. These sensors allowed for the acquisition of localized strain measurements during the experiments. This advancement allowed for improved fitting of interface parameters through CZM, utilizing detailed data on fibre strain and blade reaction forces.

In addition, this thesis explores the broader applicability of these findings by extending them to a wider range of fibres, leading to the development of a novel sample holder structure with a compliant geometry. This structure is employed in micro-fatigue tests, comparing sized and washed glass fibres, and is supported by thermomechanical FE simulations. Further investigations focused on the effect of thin film formation due to Rayleigh-Plateau Instability during droplet preparation, providing additional insights into its impact on MB test results.

The key findings of this dissertation are: i) traditional simplified models in MB tests can lead to substantial errors of interface strength, highlighting the need for more accurate numerical models; ii) FBG sensors significantly enhance MB tests by capturing local fibre strain and enable precise determination of CZM parameters for the interface model, thereby providing a detailed understanding of the interfacial damage process; iii) micro-fatigue tests reveal that fibre-matrix interface friction is a crucial parameter in analysis of interfacial fatigue; iv) the formation of a thin film during droplet preparation increases blade reaction force but does not significantly affect the properly determined CZM parameters, i.e. parameters of interfacial adhesion. In conclusion, this dissertation makes significant advancements in the numerical calibration of interface parameters for accurate debonding analysis with the support of data from MB tests.