Effect of the intermediate principal stress on 3-D crack growth

An experimental and numerical investigation of 3-D crack growth in biaxial compression is presented. We tested a series of transparent casting resin samples, each with a single initial penny-shape crack at the sample centre. We applied biaxial compression with different ratios (σ_x/σ_y) between the lateral (the intermediate principal stress) and the axial (the major principal stress) loads. The initial penny-shape cracks were inclined at 30° to the major principal stress but parallel to the intermediate principal stress. The experimental results revealed the qualitative influence of the intermediate principal stress on shape of 3-D crack growth: in uniaxial compression, the initial penny-shape crack produces wings wrapping around it. The wrapping hampers the ability of the wings to grow resulting in the appearance of the maximum wing length of the order of the size of the initial crack. In biaxial compression with high intermediate principal stress the wings straighten and grow to an extent sufficient to split the sample. The threshold for the intermediate principal stress separating these two regimes of wing growth is surprisingly low: 5.7% of the major principal stress. The XFEM (extended finite element method) modelling showed that this threshold corresponds to the transition in the pattern of direction of the secondary principal tensile stress near the initial crack from directions roughly perpendicular to the intermediate principal compressive stress direction to the radial directions with respect to the initial crack; the latter pattern causes wing wrapping.