Stretchable composites with high initiation fatigue threshold comparable to soft tissues

Fiber-reinforced soft composites have emerged as promising materials for applications in artificial tissues and flexible electronics, where long-term durability is essential. However, these soft composites often show limited resistance to crack initiation under cyclic loading, and improving the fatigue threshold remains challenging. Here, a stretchable composite consisting of knitted fabric and a soft matrix with long polymer chains has been developed to resist fatigue crack initiation. The long polymer chains toughen the matrix and suppress matrix damage induced by yarn movement. Three modes of fatigue crack growth are observed, and two fatigue thresholds are identified: initiation threshold G _a and steady-state threshold G _b. It is found that G _a is governed by the synergy of matrix toughness, interfacial strength, and fabric architecture. A low G _a is caused by either a brittle matrix or a weak interface, due to premature matrix damage and yarn pullout. G _a can also be reduced by finer yarns and lower fabric density. By tailoring the synthesis parameters, the optimized soft composite achieves an enhanced G _a of 3224 J/m², comparable to soft tissues and an order of magnitude higher than the neat matrix. The reinforcing mechanism is further verified by the finite element simulation, showing that fibers can suppress the driving force for crack propagation and reduce stress concentration at the crack tip. This study may provide guidance for designing fatigue-resistant soft composites.