The elastic energy stored in the long length of the fiber dissipates when the fiber breaks ( Fig. 2C). The long-length transmission of tension alleviates the stress concentration at the tip of flaws, which is called stress deconcentration. When the membrane is stretched, the soft matrix enables the tension in an individual collagen fiber to transmit over a long length ( Fig. 2B). A BP membrane consists of a network of strong collagen fibers embedded in a soft matrix ( Fig. 2A). These experiments interpret the extraordinary flaw-insensitive fatigue resistance of BP. Optical microscopy shows that rupture of BP pulls out long lengths of collagen fibers. Digital image correlation (DIC) shows that a crack tip does not concentrate strain in BP but does concentrate strain in TPU. By contrast, a synthetic material, thermoplastic polyurethane (TPU), is insensitive to flaws of size ~100 μm.
We show that BP is insensitive to flaws of size ~1 cm. In particular, the BP is chemically fixed by glutaraldehyde following the common practice in bioprosthetic heart valves. This paper studies how BP resists crack growth under cyclic stretch.
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