Introduction To Brittle Failure- How Brittle Failure Occurs
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. Main types of Failures in materials are , brittle failure , ductile failure , fatigue and creep fracture or failure.
Reasons Of Material Failure :
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 Failure/ductile Failure, fatigue or progressive fracture, delayed fracture. Ductile/brittle Failure occurs over short period of time, and distinguishable.
Fracture is a form of failure, and is defined as the separation or fragmentation of a solid body into two or more parts under the action of stress. Fracture that occurs over a very short time period and under simple loading conditions (static i.e. constant or slowly changing) is considered here. Fracture under complex condition, for example alternating stress, is considered in later sections.
Brittle Failure /Fracture:
Brittle Failure / fracture that takes place with little or no preceding plastic deformation. It occurs, often at unpredictable levels of stress, by rapid crack propagation. The direction of crack propagation is very nearly perpendicular to the direction of applied tensile stress. This crack propagation corresponds to successive and repeated breaking to atomic bonds along specific crystallographic planes, and hence called cleavage fracture. This fracture is also said to be transgranular because crack propagates through grains. Thus it has a grainy or faceted texture. Most brittle fractures occur in a transgranular manner. However, brittle fracture can occur in intergranular manner i.e. crack propagates along grain boundaries. This happens only if grain boundaries contain a brittle film or if the grain-boundary region has been embrittled by the segregation of detrimental elements.
brittle fracture in metals is believed to take place in three stages –
(1) plastic deformation that causes dislocation pile-ups at obstacles,
(2) micro-crack nucleation as a result of build-up of shear stresses,
(3) eventual crack propagation under applied stress aided by stored elastic energy.
As mentioned earlier, brittle fracture occurs without any warning sign, thus it needs to be avoided. Hence brittle fracture and its mechanism have been analyzed to a great extent compared to ductile fracture. Brittle fracture usually occurs at stress levels well below those predicted theoretically from the inherent strength due to atomic or molecular bonds. This situation in some respects is analogous to the discrepancy between the theoretical strength shear strength of perfect crystals and their observed lower yield strength values.
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