Stretchable and fatigue-resistant materials

In developing a material for a load-bearing application, attention inevitably falls on the resistance of the material to the growth of a crack, characterized by toughness under monotonic load, and by threshold under cyclic load. Many methods have been discovered to enhance toughness, but they do not enhance threshold. For example, stretch-induced crystallization and inorganic fillers have made the toughness of natural rubber well above 10000 J/m2, but have left the threshold of natural rubber around 50 J/m2. Here we describe a principle of stretchable and fatigue-resistant materials. To illustrate the principle, we embed unidirectional fibers of a soft and stretchable material in a matrix of a much softer and much more stretchable material, and adhere the fibers and the matrix by sparse and covalent interlinks. When the composite is cut with a crack and subject to a load, the soft matrix shears readily and delocalizes the high stretch of a fiber over a long segment. A threshold of 1290 J/m2 is reached, below which the composite does not suffer any mode of failure (fiber break, kink crack, or matrix fracture). The principle of stretchable and fatigue-resistant materials is applicable to various materials, layouts, and methods of fabrication, opening an enormous design space for general applications.