크리프(creep) vs 피로(fatigue)

creep
- 금속이 정하중에서 시간이 경과할수록 변형이 증가하는 현상. 저온조건에서는 거의 볼 수 없고 고온에서 변형이 커지는 현상을 볼 수 있음.
- In materials science, 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.

 

fatigue
- 금속이 정하중에서는 충분한 강도를 가지고 있으나, 반복하중이나 교번하중을 지속적으로 받으면 비록 하중이 작아도 마침내 파괴되는 현상. 
- 탄성한계 이하의 작은 하중에도 그것이 매우 자주 반복적으로 작용하면 재료의 감도는 감소하는데, 이 현상을 재료의 피로라고 한다.
- In materials science, 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.
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.