metal+fatigue

=metal fatigue=
 * [[image:652px-Pedalarm_Bruch.jpg width="388" height="359" caption="Fracture of an aluminium crank arm. Dark area of striations: slow crack growth. Bright granular area: sudden fracture."]] || [[image:220px-Ewing_and_Humfrey_fatigue_cracks.jpg width="284" height="324" caption="Micrographs showing how surface fatigue cracks grow as material is further cycled. From Ewing & Humfrey (1903)"]] ||

Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure. Watch the following video on fatigue analysis overview using SolidWorks software [|http://youtu.be/D1IUwBVSHX8] (just watch the first 2 1 / 2 minutes) Further information: The design, incorporating fatigue restraints, of a bicycle crank using SolidWorks click on [| http://youtu.be/E_u0J_wyvtY] Examples of fatigue failure and their failure mode [] Fatigue testing example []
 * Fatigue** is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.

=creep= Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near melting point. Creep always increases with temperature

Stages of creep
Strain as a function of time due to constant stress over an extended period for a viscoelastic material.

Stages of Creep
In the initial stage, or primary creep, the strain rate is relatively high, but slows with increasing strain. This is due to work hardening. The strain rate eventually reaches a minimum and becomes near constant. This is due to the balance between work hardening and annealing (thermal softening). This stage is known as secondary or steady-state creep. This stage is the most understood. The characterised "creep strain rate" typically refers to the rate in this secondary stage. Stress dependence of this rate depends on the creep mechanism. In tertiary creep, the strain rate exponentially increases with stress because of necking phenomena.

Creep test
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=visual inspection= This is also an important part of the materials selection and manufacturing/fabrication process. [] This is now available as an automated system []

How much do you remember?

Complete the following quiz =quiz - fatigue and creep=

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