Mechanical failure is the term used to describe an element or component’s failure to continue functioning due to one or more of several modes of mechanical failure. These modes include buckling, corrosion, creep, fatigue, fouling, fracture, impact, thermal shock, wear, and yielding. Many of these don’t immediately result in catastrophic or ultimate failure, but are considered mechanical failures nonetheless due to their irreversible nature.
Modes of Mechanical Failure
Buckling – Just as the knees may buckle, if a structural column were to do the same, this would be nothing less than a mechanical failure of that column. Buckling is caused by high compressive stresses “sandwiching” the vertical column between its 2 ends, thereby giving it no other alternative than to buckle outward at its weakest point.
Corrosion – Commonly known as rust in materials such as iron and steel, corrosion is the often unwanted and detrimental chemical reaction engineered materials have with their surroundings, leading to a reduction in material cross section and eventually its load bearing capacity.
Creep – Being a “time-dependent” deformation, creep occurs very slowly over a long period of time and may or may not lead to an ultimate failure. It can occur due to high stress levels that are lower than the yield strength of the material but are high enough to cause slow deformation. Creep will always increase with temperature, making it a bigger concern for components that will be subject to high temperatures. The behavior of ice when subject to higher temperatures is an extreme example of creep.
Fatigue – This is the cumulative result of repeated cyclic loading. Once a certain threshold is crossed, small cracks begin forming on the surface of the material, which due to repeated loading cycles, eventually leads to failure. Shapes such as squares and those with sharp corners contain higher stress concentrations, thereby aiding in fatigue. A well-used lever arm such as a lug wrench that finally breaks after many years of hard use is an example of fatigue.
Fouling – This is the impediment or obstruction of a mechanical component’s functions due to a significant amount of growth or collection of undesired material on its surface. The buildup of plaque on ones teeth and marine life on ship hulls are examples of fouling. Increased drag and the consequent increase in fuel usage is a direct result of fouling on ship hulls.
Fracture – Many of us may know all too well this mode of failure via a broken arm or leg. There are 2 types of fracture: Brittle and Ductile. Brittle fracture is a more or less sudden and complete breakage of a brittle material, whereas ductile fracture – also known as rupture – displays considerable plastic deformation before fracture.
Impact – Just imagine a sledge hammer coming down on a ceramic tile and you’ll get the idea of what kind of failure mode this is. Impact is a relatively high force delivered to a localized area over a short time period. Obviously, such a blow is considerably more devastating than a smaller force delivered over a longer period of time.
Thermal shock – I’m sure we’ve seen this one in action before. Thermal shock is the expansion or contraction of a material due to a sudden and extreme temperature change, and can be illustrated well by pouring boiling water into a cold glass. This is also the cause of extensive road damage due to repeated freeze/thaw cycles of trapped water.
Wear – Components that are in direct contact with moving parts are subject to wear. This is the gradual removal or erosion of surface material and consequently, cross sectional area, leading to eventual failure. Worn carbon brushes are a good example of wear-induced failure.
Yielding – Yielding occurs when the imposed load exceeds the load bearing capacity of the structural element/s. It is when structural deformation goes beyond elastic and into plastic deformation – meaning that the deformation or damage is irreversible. Yielding may or may not lead to catastrophic failure but is always irreversible.
Some, such as corrosion and fouling, may not be considered mechanical failure technically, but are included because they often lead directly to it. Engineers must take all these various modes of mechanical failure into consideration when designing structures, as well as an appropriate factor of safety. Factors of safety must be balanced so as to provide safe and durable structures but also to keep them as cost-efficient as possible.