Characterizing crack behaviour in nacre-like alumina/epoxy lamellar composites: Microstructure and crack tortuosity

Abstract

Nacre type lamellar microstructure offers a promising prospect to achieve high strength and high toughness in artificial composite materials. Understanding the underlying failure mechanisms and crack behaviour is a prerequisite for efficient designing of these composites. To this end, we have fabricated lamellar alumina/epoxy composites with varying lamellae thickness using bi-directional freeze casting method. The fracture experiments are performed to record the crack evolution as a function of lamellae thickness and crack orientation. These tests reveal the rising crack resistance curve with complex fracture pattern and high degree of crack tortuosity. Moreover, the peak force and crack deflection seem to depend on the lamellae thickness and crack orientation. The mode-mixity $K_{\text{II}}^k/K_I^k$ at the deflected crack tip is evaluated analytically for all cases. To experimentally characterize the anisotropy in crack propagation, a non-dimensional parameter ${\delta }^{\ast }$ = $\max \left\{\left|{\delta }^{+}\right|,\left|{\delta }^{-}\right|\right\}$/ $\min \left\{\left|{\delta }^{+}\right|,\left|{\delta }^{-}\right|\right\}$ is proposed using crack mouth opening displacement (CMOD) measurements. The variables ${\delta }^{+}$, and ${\delta }^{-}$ represent CMOD on the either side of the current location of the crack tip. It is shown that δ∗ follows the variation of $K_{\text{II}}^k/K_I^k$ and thus can be used to predict the degree of mode-mixity in crack propagation. Extended finite element simulations (XFEM) are carried out to compliment the experimental understanding of crack behaviour in lamellar composites. Finally, our study reveals new insights on crack growth and proposes an experimentally measurable parameter to quantify the mode mixity in lamellar anisotropic composites.