Last time we introduced a way to look at the factors that cause damage in engineered components and structures.
Figure 1 helps to demonstrate the fact that every part, component or structure is at some temperature. That’s shown as the outer circle. The composition of the environment also surrounds the part. The environment could be anything from the hard vacuum of outer space, with its accompaniment of cosmic rays and solar winds, to something resembling the surface of Mercury, the core of Jupiter or Earth. It could be a forest, a city or a location near the ocean. Pressure, temperature, types of molecules, atoms and subatomic particles all contribute to the environment. In the center of any part that is subject to stress, however, we have the three major types of loading.
- Monotonic: A single load or force inflicted on the part at some strain rate that comes to a fairly rapid end.
- Sustained: A load that is inflicted on the part and sustained for a longer period of time, sometimes years, decades or centuries.
- Fatigue: Cyclic loading, which may be regular and repeating or may have random elements, but is nominally by definition below the yield strength. Thus, it usually takes thousands or hundreds of thousands of repetitions before a crack initiates.
Tensile-test and Charpy coupons are loaded in a monotonic manner. Creep and stress-relaxation tests are loaded in a sustained manner, often at the elevated temperatures used in industrial processes. The base stones of the Pyramid of Giza have been subject to a sustained load for a long time. Of course the environment has changed in that time, but engineering simplifications are often useful in understanding why some structure lasted or did not.
Fatigue tests, reflecting varying usage conditions, are used in the development of many consumer products (e.g., cars, office furniture, theater seats). Trees swaying in the wind are subject to fatigue, but they rarely break for that reason. Bridges and buildings are also subject to fatigue, but it’s not practical to test the whole structure.
Many wear scenarios may be simply visualized as a sub-category of fatigue, where the stresses developed may (at least locally) vastly exceed the yield strength. Corrosion in the absence of stress may be viewed by focusing on the two outer rings of the Wheel of Damage.
Fig. 1. Debbie Aliya's Wheel of Damage