7. Mechanical System Failures

This chapter will discuss the effect of structural material that is subjected to (random) loads. Fatigue fracture catastrophically occurs in product lifetime when there are stress raisers such as holes, notches, or fillets in design. To understand the structural damage, engineer might understand the basic concepts of mechanics of materials—stress, mechanics of material, deformation, slip, fracture, and fatigue. To withstand their own loads, mechanical structures are designed to have proper stiffness and strength. Requirements on stiffness, being the resistance against reversible deformation, may depend on their applications. Strength, the resistance against irreversible deformation, is always required to be high. If improperly designed, small portions of product will suddenly fracture in its lifetime. To understand the design of product—automobile, bridge, skyscrapers, and the others, engineer should figure out why the mechanical system failures will happen. Through the current reliability methodology engineer still doesn’t know whether product design overcomes (random) loads in its lifetime. For example, the failure of mechanical system like aircraft wing during a long flight can occur in short time or tens of thousands of vibration load cycles. To assess the product damage, we need new reliability methodology like parametric accelerated life testing in the design process. And we also will deal with the mechanical corrosion in the later section.