Cracks are Janus-faced, which cause material failure on one hand and serve as a powerful approach for material processing on the other. To predict cracks and control them in a desired manner, foundational fracture mechanisms are continuously being pursued, based on simplified modes of planar cracks. In reality, cracks usually occur in a three-dimensional (3D) irregular manner. Prediction of 3D fractures is of particular significance for understanding the fundamental fracture mechanisms. However, controlling cracks in typical 3D modes is rare. Here we report that controllable 3D helical cracks on heterogeneous spindle knots are induced by biaxial thermal stresses. The thermal expansion mismatch between the tough core and brittle shell during the heating process generates biaxial stresses in the axial and circumferential directions. Surface cleavage and interface delamination driven by the release rate of elastic strain energy are harmonized due to the unique spindle geometry and cooperate to produce a 3D helical crack. This finding is helpful for understanding complex cracking mechanisms and provides a promising prospect for controlling or eliminating 3D cracks.
- Biaxial stress