In Understanding How Materials Fail: Stress v. Fatigue, forensic engineering expert witness Clyde C. Richard, Ph.D., P.E., writes on material failure:
This overload of the material can contribute to an immediate accident or accelerate the breakdown of the materials. This is why most tow hitches are supplied with warnings, ratings and consumer education material. The second failure mechanism is fatigue. Fatigue is a phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow with the application of cyclic stress.
During the visual inspection, the expert views the material under increasing magnifications to look for stress or fatigued related evidence such as plastic deformation, tearing, brittleness and necking. Necking is found in ductile metals or flexible metals where an extreme stress has been applied. Engineering experts can also perform a series of tests using an Emission Spectroscope, a Fourier Transform Infrared (FT-IR) spectrometer and/or Tensile Testing Machines to determine a material’s composition and strength.