Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 134

The resistance of a material to fatigue, a process of slow crack growth under repeated stress cycles, is important in several biomedical and engineering applications. Implants used in total joint replacement or in repairing large defects in the long bones of the limbs, for example, are subjected not just to a constant or slowly varying stress but to repeated cyclic stresses as well during normal activities of walking, running, and jumping. Stents used to keep coronary arteries open have to withstand the pressure pulsations of blood flow through the arterial vessels.

Fatigue occurs by a process of slow crack growth under cyclic stresses that are often well below the strength of the material. Cracks formed during cyclic loading or accidentally present in the original material can grow slowly and, when they have propagated sufficiently far, failure occurs, often in a brittle manner, at stresses well below the strength of the material (Figure 4.11).

Figure 4.11 Schematic representation of the fracture surface of a ductile metal after fatigue failure in one‐way bending (a) and two‐way bending (b).

Fatigue behavior is commonly studied by subjecting specimens to cyclic loads often sinusoidal in nature, in the requisite loading mode such as tension, compression or bending. Specimens, commonly of a geometry similar to those used to measure the strength of the material (Section 4.2.1), are loaded for the requisite number of cycles of until they fail.