Plant MaterialsDOE-HDBK-1017/2-93PLANT MATERIAL PROBLEMSPLANT MATERIAL PROBLEMSMaterial problems in a nuclear reactor plant can be grouped into two categories,one concerning the nuclear reactor core and one that will apply to all plantmaterials. This chapter discusses specific material problems associated withfatigue failure, work hardening, mechanical forces applied to materials, stress,and strain.EO 1.14DEFINE the following terms:a.Fatigue failureb.Work hardeningc.CreepEO 1.15STATE measures taken to counteract or minimize the effects of thefollowing:a.Fatigue failureb.Work hardeningc.CreepFatigueFailureThe majority of engineering failures are caused by fatigue. Fatigue failure is defined as thetendency of a material to fracture by means of progressive brittle cracking under repeatedalternating or cyclic stresses of an intensity considerably below the normal strength. Althoughthe fracture is of a brittle type, it may take some time to propagate, depending on both theintensity and frequency of the stress cycles. Nevertheless, there is very little, if any, warningbefore failure if the crack is not noticed. The number of cycles required to cause fatigue failureat a particular peak stress is generally quite large, but it decreases as the stress is increased. Forsome mild steels, cyclical stresses can be continued indefinitely provided the peak stress(sometimes called fatigue strength) is below the endurance limit value.A good example of fatigue failure is breaking a thin steel rod or wire with your hands afterbending it back and forth several times in the same place. Another example is an unbalancedpump impeller resulting in vibrations that can cause fatigue failure.The type of fatigue of most concern in nuclear power plants is thermal fatigue. Thermal fatiguecan arise from thermal stresses produced by cyclic changes in temperature. Large componentslike the pressurizer, reactor vessel, and reactor system piping are subject to cyclic stresses causedby temperature variations during reactor startup, change in power level, and shutdown. Rev. 0Page 27MS-05
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