Mechanical testing is a process used by engineers to determine the mechanical properties of a material in response to stress.
Many types of mechanical tests are possible. One of the most common is the uniaxial tensile test. In this test, a specimen is placed between two grips attached to a stationary base and a mobile crosshead, and the crosshead is moved upward at a constant rate, stretching the specimen. The specimen is attached to a load cell that measures the force used to stretch the specimen at each moment in time. From these data, a load-deflection curve can be generated. By taking into account the geometry of the specimen, a stress-strain curve can also be created and general material properties obtained. The advantage of using this approach is that these material properties may be used to predict the mechanical behavior of any component of any shape constructed from the same material.
Different materials have different behavior when loaded in tension. Some materials are brittle and fracture suddenly when a critical stress level is attained. Other materials are ductile and stretch significantly before pulling apart. The loading rate can change how the material behaves. In general, materials are more brittle at higher loading rates. Temperature also has an effect, with most materials exhibiting more ductile behavior at higher temperatures. Some materials undergo a ductile-brittle transition–shifting behavior over a small temperature range.
Biological materials that contain a high concentration of fibers exhibit nonlinear behavior. Upon initial loading, the stress-strain curve is shallow as the fibers within the material are straightened. As more and more fibers are straightened, the slope of the stress-strain curve slowly increases. Once the fibers are straight, the slope of the stress-strain curve stabilizes and linear behavior is observed.
Most biological materials are also viscoelastic. This means that they exhibit elastic behavior upon initial loading, but solid-state viscous flow within the material also occurs over time. Some soft tissues within the body may be characterized as elastic porous materials. They consist of highly porous solid bodies with a high fluid content. Behavior of these materials are affected by the loading rate, material porosity, and flow resistance.
Materials may be isotropic, having the same mechanical characteristics is each direction. They may also be anisotropic, having different materials properties in different directions. Transversely isotropic materials are common in biological materials. These materials have certain characteristics along one material direction and different characteristics is any direction perpendicular to the first direction. An example of a transversely isotropic material is a tendon. It has fibers aligned with its long direction that give it increased strength and stiffness in this direction. Perpendicular to the long direction, the material is weaker and less stiff. Testing in this direction pulls the fibers apart rather than stretching them. It is weak in any direction that is not aligned with the fibers and has similar mechanical properties. Thus, the material is isotropic in the directions transverse to the long axis.
Other types of mechanical tests include compression testing, biaxial tensile testing, torsional testing, three-point bending, four-point bending, impact testing, fatigue testing, and hardness testing. Each of these tests evaluates different aspects of material behavior.
