Failure of glass-microballoons/thermoset-matrix syntactic foams subject to hydrostatic loading
Lorenzo Bardella, Giovanni Perini, Andrea Panteghini
DICATAM, Faculty of Engineering, University of Brescia, Via Branze 43, 25123 Brescia, Italy
CMT Materials, 107 Frank Mossberg Dr, Attleboro, MA 02703, USA
Nikhil Gupta, Maurizio Porfiri
Department of Mechanical and Aerospace Engineering, Tandon School of Engineering Polytechnic Institute of New York University, Six MetroTech Center, Brooklyn, NY 11201, USA
This study focuses on a lightweight syntactic foam constituted by an epoxy matrix filled with polydispersed Glass Microballoons (GMs) up to 0.75 volume fraction. We present experimental results on hydrostatic loading which demonstrate the possibility of different failure modes depending on whether the surface of the composite is painted/coated or not. In order to explain this surprising behaviour, we propose a three-dimensional Finite Element (FE) micromechanical model. First, we develop a cubic MultiParticle Unit Cell (MPUC) which includes 100 randomly placed GMs and accounting for their polydispersion, in terms of both size and radius ratio. This model is validated by subjecting it to effective uniaxial stress and comparing its predictions of the elastic moduli with experimental findings and an analytical homogenisation technique. Second, towards modelling failure, we implement a structural criterion proposed by our group, which posits that any GM undergoes brittle failure when its average elastic energy density reaches a critical value. We then utilise our measurements of the effective strength under uniaxial compressive stress to identify different critical values for selected types of GMs. Third, on the basis of the MPUC, we reach our goal by developing a larger FE model, including 300 GMs, which enables the study of the stress diffusion from the external surface through an appropriately thick layer of composite, under macroscopic uniform pressure. This micromechanical model allows us to demonstrate the influence of the paint/coating on the syntactic foam failure mode through a detailed analysis of the collapsed GMs and the matrix stress state.
Author Keywords: Syntactic foam; Micromechanics; Glass microballoons; Stress diffusion; Surface coating; Failure mode; Finite Element method.