Biaxial stress controlled three-dimensional helical cracks

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. Materials are prone to crack under stress — this can either cause materials failure or, when deliberately induced, offer a useful manufacturing step. In both situations, knowing how to control and predict how materials crack will help in their design and synthesis. Yet exerting control is difficult — for example, we have all seen pottery cracked along random directions. Yong Zhao and co-workers have now prepared core–shell fibres that undergo helical cracking at specific positions. A tough glass fibre was dip-coated with a brittle metal oxide film featuring regular spindle knots. On calcination, the thermal expansion undergone by the tough core does not match that of the brittle shell, creating longitudinal and circumferential stresses. The stress lines in turn cause the knots to crack into helical coils, whose shapes depend on the initial formation process. These findings represent a step forward along the way of controllable fracture. Regular microhelics on a heterogenous spindle knot are obtained by controlled biaxial stresses in three dimensions. The spindle knot has a tough core and a brittle shell, resulting in biaxial stresses that arise from a thermal expansion mismatch during a heating process. Surface cleavage and interface delamination are harmonized due to the special spindle geometry and cooperate to 3D helical crack. This study not only widens our understanding of the cracking phenomena, but also sheds light on the control and design of regular cracks in arbitrary dimensions. It holds promise for applications in eliminating or controlling cracks for manufacturing process, especially at micro/nanosales, which domains are difficult to be generated by routine methods.