Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

10.7 Properties of Materials 343

Fig.10.12

“Barrelling” of a mild steel test piece in compression.

Fig.10.13

Stress–strain curve for aluminum.

Fig.10.14

“Double-cup” failure of an aluminum alloy test piece.

curve at the 0.1 percent proof stress. For elastic design, this, or the 0.2 percent proof stress, is taken
astheworkingstress.Beyondthelimitofproportionality,thematerialextendsplastically,reachingits
ultimatestress,σult,at“d”beforefinallyfracturingunderareducednominalstressat“f.”
Afeatureofthefractureofaluminumalloytestpiecesistheformationofa“doublecup”asshown
inFig.10.14,implyingthatfailurewasinitiatedinthecentralportionofthetestpiece,whiletheouter
surfacesremainedintact.Again,considerable“necking”occurs.