Buckling of elastomer-embedded thermoplastic microspheres under uniaxial compression revealed by in-situ X-ray micro-CT.

Understanding and predicting the morphological changes of filler particles within composites under load is critical to design improved materials. Syntactic foams, consisting of microspheres embedded in a solid matrix, have recently emerged as an ultra-lightweight metamaterial with highly tuneable mechanical properties. Glass microspheres are commonly used in syntactic foams, but can fracture, leading to a loss in the bulk mechanical properties. It is thought that hollow thermoplastic microspheres instead buckle, inducing strong, reversible, constitutive nonlinearity in the syntactic foam. Here we demonstrate, in-situ, the buckling of embedded thermoplastic shells under uniaxial compression. The results validate several important predictions from theory; regardless of sphere size, equatorial buckling prevails whenever the shell walls are thin relative to their diameter. Furthermore, the buckling wavelength is shown to be tuneable via different mechanical and geometrical parameters for the microspheres and/or matrix. Now better understood, we anticipate that tuneable, controlled and coordinated microsphere buckling can be exploited in diverse applications including strain sensing.