Finite element modeling of the uniaxial compression behavior of carbon microballoons

Recent interest in syntactic foams has led to a multitude of new research efforts into these materials and their constituents. Our contribution includes a uniaxial compression technique to obtain mechanical properties of single microballoons (MBs). We have used finite element modeling, with ANSYS 8.0, to simulate the uniaxial compression of individual carbon microballoons (CMBs), obtaining deformed shapes and stress states under experimentally determined CMB failure conditions. Previously published data on CMBs, namely failure load, displacement at failure, wall thickness and Young’s modulus, were used for linear elastic analysis. Boundary conditions were chosen to achieve axisymmetry and compression was displacement controlled. When modeling the average conditions for three CMB populations, predicted maximum first principal stresses at failure ranged from 637 to 835 MPa. Compression of CMBs with various radius-to-thickness ratios (R/t) was also simulated and a transition of failure mode, from fracture in flexure to buckling, was observed at critical R/t values. Given that real CMBs have imperfections, two common experimentally observed defects, non-concentricity of the internal cavity and through-thickness holes, have also been considered. The two flaws were modeled in different locations relative to the location of load application and their effects on the stress state in CMBs were evaluated. Through-thickness holes in CMB walls were more detrimental to the compressive properties of the CMBs, despite the fact that thin regions in a CMB’s wall could cause buckling in a situation where the CMB’s average radius and thickness would otherwise predict failure to occur in flexure.