Cracks Interaction due to Sequential Indentation in Curved Brittle Coating: Effect of Relative Orientation

This study investigates the influences of pre-existing cracks to simulate defects in structures using curved bi-layer system consisting of glass coating on a polymeric support layer, to illustrate the role of defects on the evolution of failure of hard coating bi-layer system under loading. The densities of pre-existing cracks were initiated near the margins and the relative orientations were varied. The pre-existing cracks were induced by using hard spherical indenter. The failure of these samples was investigated under single-cycle axial loading tests using soft indenter of low elastic modulus. The onset of fracture was observed in situ using a video camera. Margin cracks were observed to propagate from pre-existing cracks near the margins, while the pre-existing cracks in the compression zone under the soft indenter were arrested. Experimental trends suggested that critical loads were increased with specimens having no defects (pre-existing cracks). Finite element modeling was used to evaluate the stress distribution in the brittle coating and compared with the experimental trend. It is concluded that radial cracks can initiate and propagate anywhere in the tensile region. The larger the flaws the smaller the loads required for defects pre-existing cracks to propagate radials to failure.