Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

14.5 Crack Propagation 415

Fig.14.4

Basic modes of crack growth.

Fig.14.5

Stress field in the vicinity of a crack.

Equation(14.29)appliestoallmodesofcrackopening,withKhavingdifferentvaluesdepending
onthegeometryofthestructure,thenatureoftheappliedloads,andthetypeofcrack.
ExperimentaldatashowthatcrackgrowthandresidualstrengthdataarebettercorrelatedusingK
than any other parameter.Kmay be expressed as a function of the nominal applied stressSand the
cracklengthintheform

K=S(πa)

(^12)
α (14.30)
inwhichαisanondimensionalcoefficientusuallyexpressedastheratioofcracklengthtoanyconvenient
localdimensionintheplaneofthecomponent;foracrackinaninfiniteplateunderanapplieduniform
stresslevelSremotefromthecrack,α=1.0.Alternatively,incaseswhereopposingloadsPareapplied