I remember in Year 2 or 3, one of the modules was talking about materials and the strength of materials. And I remember there was something called cyclic loading, which is especially common in testing components used in aeroplanes. The component is subjected to a load, say tensile stress, that increased from zero to a maximum value within a certain time, and then is decreased to zero again within a certain time. So the cycle is repeated many many times to investigate when failure will take place.
Some materials don't withstand cyclic loading too well. They fail quite soon into the cycles. Other materials are a lot better, either because of their natural properties or that they have been reinforced (by material or by structure)
I also remember that there is a method of strengthening some materials which is quite interesting. The material is subject to a load (i.e. stress or heat) and then after that relaxed. And because the load induced more stresses and stress lines within the materials, they eventually prevent further cracks or stress lines from propagating throughout the material. So by subjecting the material to an initial load and initial stresses, the material becomes stronger and can take even more load eventually. So it doesn't mean that initial stress and cracks will always lead to failure of the material; sometimes it makes the material a lot stronger.
I also remember that in crack propagation, while many cracks and stress lines in the materials can strengthen it, sometimes all it takes is just one crack in the material for absolute failure. One crack to start snacking its way through the internal structure, moving quickly through, inflicting simple and direct damage.
I never really liked the materials part of the engineering courses (I didn't take chemistry in Junior College) but one can't deny how relevant it is in the world today...
