Mechanical behavior and failure mechanisms in polymeric syntactic foams
Date of Issue2016
School of Mechanical and Aerospace Engineering
Syntactic foams have been increasingly used in load bearing structures in the low to high speed applications due to their excellent specific properties. The aim of this research is thus to fully understand the overall mechanical properties and the associated failure mechanisms under different strain rates. The quasi-static (QS) compression tests and the finite element (FE) modeling of a representative elementary volume syntactic foam were first performed to investigate the elastic behavior and failure mechanism of the glass microballoon epoxy syntactic foams as well as the effect of glass microballoon volume fractions (V) and the radius ratio η. The elastic characteristics of the foam vary with both the V and η. The localized stresses concentrate in various zones within the foam. Dependent on the V, micro-cracks can propagate either in the preferred longitudinal or diagonal directions in the foam. To directly observe the internal microstructural change of the constituents during the failure process, the x-ray micro computed tomography (µXT) with interrupted uniaxial compression was conducted on cenosphere epoxy syntactic foams. Moreover, to further investigate the failure micromechanisms of the foam, FE modelling of the full scale foam specimen was developed and experimentally validated to predict the localized stress, fracture of cenospheres and deformation in the matrix. The FE predictions were related to the µXT observations to analyze the underlying mechanisms of internal 3D failure process in the plateau region of the foam. It was found that the internal compressive failure in microscopic scale consists of (1) the crushing of hollow spheres and (2) the plastic deformation and fracture of the matrix. The failure mechanisms in the two constituents are determined by the localized stress state and the stress transfer between the constituents, and govern the different strain stages of bulk stress–strain behavior of the foam. The failure mode of the individual hollow sphere was vertical splitting. Finally, to investigate the strain rate dependent failure mechanisms of syntactic foam, the QS compressive tests and Split-Hopkinson pressure bar tests were controlled to stop the deformation of the foams at various strains. It was found that the failure mechanism is significantly affected by strain rates. At dynamic rates, macro-cracks form earlier in the matrix and can split hollow spheres. An empirical constitutive equation was used to quantitatively relate the damage to the strain and strain rate.