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Bovine pericardium (BP) has been used as leaflets of prosthetic heart valves. The leaflets are sutured on metallic stents and can survive 400 million flaps (~10-year life span), unaffected by the suture holes. This flaw-insensitive fatigue resistance is unmatched by synthetic leaflets. We show that the endurance strength of BP under cyclic stretch is insensitive to cuts as long as 1 centimeter, about two orders of magnitude longer than that of a thermoplastic polyurethane (TPU). The flaw-insensitive fatigue resistance of BP results from the high strength of collagen fibers and soft matrix between them. When BP is stretched, the soft matrix enables a collagen fiber to transmit tension over a long length. The energy in the long length dissipates when the fiber breaks. We demonstrate that a BP leaflet greatly outperforms a TPU leaflet. It is hoped that these findings will aid the development of soft materials for flaw-insensitive fatigue resistance. 

In many applications, glass fabrics are subject to cyclic forces. Here we show that a glass fabric can tear under a much lower cyclic force than monotonic force. For samples of a given width, a threshold force exists below which the fabric does not tear under cyclic load, and a critical force exists at which the fabric tears under monotonic load. For example, for 80 mm wide sample, the threshold force is 12.78 N, and the critical force is 344.73 N. Under cyclic force of amplitude between the threshold force and critical force, tear initiates after some number of cycles. Under either cyclic or monotonic force, the fabric tears in three modes: pullout of transverse yarns, pullout of transverse yarns and break of longitudinal yarns, and break of transverse and longitudinal yarns. We summarize the observed tear modes on the plane of two axes: the amplitude of force and the width of sample. It is hoped that this study will aid the development of fatigue-resistant fabrics and fabric reinforced composites.