MODES OF STRUCTURAL FAILURE

Structural failure refers to defects in which a load bearing component of the building is unable to support and transfer loads to another element. Just a note before starting with failure modes, it is a LARGE topic with an awful amount of crazy knowledge to understand. That’s why I’ll only be mentioning the most obvious things… because my purpose is only to make you AWARE that those problems even exist…Hoping that if you are concerned by one of these failure modes, you will do your own research to understand. Some common modes of failure are as follow:

•         Fracture:  I guess that everyone has seen some cracks in a bridge or in a wall… well, that’s a sign that the structure has a problem. When a new crack appears or existing crack propagates, you are basically looking at the fracture mechanism. Ductile structures fracture slowly and deform plastically before seeing a sign of a crack while brittle structures are sneakier as they tend to fail suddenly and abruptly without showing any sign of plastic deformation.
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•      Yielding: It occurs in a body which experiences stress in excess of the yield stress. Yielding is only called yielding when it actually compromises the integrity or function of the part that yields. Sometimes you may observe a localized yielding that causes strains near stresses concentrations in your part, but then… the stress is redistributed and the yielding actually stops.
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•      Buckling: Buckling is the sudden change in shape of a structural component under load such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a gradually increasing load, when the load reaches a critical level, a member may suddenly change shape and the structure and component is said to have buckled. It is very dangerous because buckling failure can happen much before the material failure (Yielding). It occurs to long and slender members subjected to axial compressive stress. For a member to withstand loads against buckling failure it should be designed to reduce its slenderness by increasing its cross-sectional dimension, bracing a member at its intermediate points along its length or height. For economic reasons people consider bracing a member as cheaper and easier than increasing its cross sectional dimensions.
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•     Fatigue: When you use a system, even if it is designed to be resistant enough to the loads applied to it… after some cycles of loading, the parts start to lose strength and become weaker. Fatigue is the weakening of a material caused by cyclic loading that results in progressive and localized structural damage and the growth of cracks. Once a fatigue crack has initiated, each loading cycle will grow the crack by a small amount, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
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•     Creep: Creep may be defined as a time-dependent deformation at elevated temperature and constant stress. It follows, then, that a failure from such a condition is referred to as a creep failure or, occasionally, a stress rupture. The temperature at which creep begins depends on the alloy composition. Creep is simply the natural tendency of some solid materials to move slowly or deform permanently under stress. It is increased when the materials are heated (the extreme example is when steel is melt under high heat to make it change shape)