Transcription of 8. TIME DEPENDENT BEHAVIOUR: CREEP
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8. TIME DEPENDENT BEHAVIOUR: CREEPIn general, the mechanical properties and performance of materials change withincreasing temperatures. Some properties and performance, such as elastic modulusand strength decrease with increasing temperature. Others, such as ductility, increasewith increasing is important to note that atomic mobility is related to diffusion which can bedescribed using Ficks Law:D=DOexp QRT ( )where D is the diffusion rate, Do is a constant, Q is the activation energy for atomic motion,R is the universal gas constant ( K) and T is the absolute temperature. Thus,diffusion-controlled mechanisms will have significant effect on high temperaturemechanical properties and performances.
Primary I Secondary II Tertiary III Constant Load Constant Stress t f Figure 8.3 Strain time curve for a creep test In principle, the creep deformation should be linked to an applied stress. Thus, as the specimen elongates the cross sectional area decreases and the load needs to be decreased to maintain a constant stress.
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