ABSTRACT
This chapter describes the basic concepts of stress, strain, strength, deformation and failure as applied to biomaterials. Materials that behave identically, irrespective of the direction of the applied force, such as most orthopaedic metals, alloys, polymers and woven bone, are termed isotropic. Fatigue failure refers to failure of a material with repetitive loading at stress levels below the ultimate tensile strength. Shear stresses occur when two forces are directed parallel to each other but not along the same line and are either pushing towards each other or pulling away. Equal and opposite shear forces, as applied to a cylinder constrained in space, cause the cylinder to twist. This is known as a torsional or torque force. The rigidity of materials to torsion is calculated in a similar way to that of bending rigidity. Torsional rigidity is a measure of the resistance of a material in a particular size and shape to torsional forces.
