Material Science

Failure can be defined, in general, as an event that does not accomplish its intended purpose. Failure of a material component is the loss of ability to function normally. Components of a system can fail one of many ways, for example excessive deformation, fracture, corrosion, burning-out, degradation of specific properties (thermal, electrical, or magnetic), etc. Failure of components, especially, structural members and machine elements can lead to heavy loss of lives, wealth and even may jeopardize the society! This chapter deals with the study of failures by mechanical means i.e. application stresses.

Even though the causes of failure are known, prevention of failure is difficult to guarantee. Causes for failure include: improper materials selection, improper processing, inadequate design, misuse of a component, and improper maintenance. It’s the engineer’s responsibility to anticipate and prepare for possible failure; and in the event of failure, to assess its cause and then take preventive measures.

Structural elements and machine elements can fail to perform their intended functions in three general ways: excessive elastic deformation, excessive plastic deformation or yielding, and fracture. Under the category of failure due to excessive elastic deformation, for example: too flexible machine shaft can cause rapid wear of bearing. On the other hand sudden buckling type of failure may occur. Failures due to excessive elastic deformation are controlled by the modulus of elasticity, not by the strength of the material. The most effective way to increase stiffness of a component is by tailoring the shape or dimensions. Yielding or plastic deformation may render a component useless after a certain limit. This failure is controlled by the yield strength of the material. At room temperature, continued loading over the yielding point may lead to strain hardening followed by fracture. However at elevated temperatures, failure occurs in form of time-dependent yielding known as creep. Fracture involves complete disruption of continuity of a component. It starts with initiation of a crack, followed by crack propagation. Fracture of materials may occur in three ways – brittle/ductile fracture, fatigue or progressive fracture, delayed fracture. Ductile/brittle fracture occurs over short period of time, and distinguishable. Fatigue failure is the mode in which most machine parts fail. Fatigue, which is caused by a critical localized tensile stress, occurs in parts which are

subjected to alternating or fluctuating stress. Stress-rupture occurs when a metal has been statically loaded at an elevated temperature for a long time, and is best example for delayed fracture.

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