Failure of glass-microballoons/thermoset-matrix syntactic foams subject to hydrostatic loading

• Published in: European journal of mechanics, A, Solids Vol. 70; pp. 58 - 74
• Main Authors: Bardella, Lorenzo, Perini, Giovanni, Panteghini, Andrea, Tessier, Noel, Gupta, Nikhil, Porfiri, Maurizio
• Format: Journal Article
• Language: English
• Published: Berlin Elsevier Masson SAS 01.07.2018; Elsevier BV
• Subjects: Composite materials; Compressive properties; Compressive strength; Diffusion layers; Epoxy resins; Failure analysis; Failure mode; Failure modes; Finite element method; Flux density; Glass microballoons; Mechanical properties; Microballoons; Micromechanics; Modulus of elasticity; Paints; Plastic foam; Plastic foams; Polymers; Protective coatings; Stress diffusion; Stresses; Surface coating; Syntactic foam; Syntactic foams; Thermosetting resins; Three dimensional models; Unit cell; Micromechanics; Finite element method; Failure mode; Stress diffusion; Syntactic foam; Surface coating; Glass microballoons
• ISSN: 0997-7538; 1873-7285
• DOI: 10.1016/j.euromechsol.2018.01.007

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. •Different failure modes of syntactic foams under pressure depending on surface paint.•Three dimensional micromechanical modelling accounting for filler polydispersion.•Micromechanical models with 0.69 volume fraction of brittle glass microballoons.•Models for the stress diffusion from the external surface to the inner composite.•Micromechanical explanation of failure modes dependent on surface treatment.