346 CHAPTER 10 Materials
Fig.10.17
Strain hardening of a material.
Fig.10.18
Typical creep curve.
10.7.4 Creep and Relaxation
WehaveseeninChapter1thatagivenloadproducesacalculablevalueofstressinastructuralmember
andhenceacorrespondingvalueofstrainoncethefullvalueoftheloadistransferredtothemember.
However, after this initial or “instantaneous” stress and its corresponding value of strain have been
attained,agreatnumberofstructuralmaterialscontinuetodeformslowlyandprogressivelyunderload
overaperiodoftime.Thisbehaviorisknownascreep.AtypicalcreepcurveisshowninFig.10.18.
Somematerials,suchasplasticsandrubber,showcreepatroomtemperatures,butmoststructural
materialsrequirehightemperaturesorlong-durationloadingatmoderatetemperatures.Insome“soft”