Understanding the fatigue of metals is critical in designing safe, reliable structures. Metal fatigue was first discovered in the 1850s when railroad axles would break for unknown reasons. This was the first time in human history that a mass produced item (train axles) underwent repetitive loading (carrying coal). The first attempt to understand this phenomena used constant amplitude loading to develop the Stress-Life curve. Later, in the 1950s and 1960s, in order to develop lightweight structures for aerospace and NASA moon missions, the concept of constant strain testing was developed to create the more advanced Strain-Life curves for materials.
As part of my research, the Smith-Topper-Watson method for fatigue was studied. This approach combines concepts from the Stress-Life and Strain-Life models. My dissertation recognizes that energy is needed to move grains along grain boundaries, break bonds and open cracks in material. Energy is defined as force times displacement. Strain Energy is defined as stress (force intensity) times strain (displacement intensity or stretch). The Hunter Energy Life Model creates a relationship between strain energy and material life to fully capture the mechanism of failure of materials.