Stress intensity factor extraction

An interaction integral method is used to extract the individual stress intensity factors for a crack under mixed-mode loading.

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The stress intensity factors KI, KII, and KIII play an important role in linear elastic fracture mechanics. They characterize the influence of load or deformation on the magnitude of the crack-tip stress and strain fields and measure the propensity for crack propagation or the crack driving forces. Furthermore, the stress intensity can be related to the energy release rate (the J-integral) for a linear elastic material through

J=18πKTB-1K,

where K=KI,KII,KIIIT and B is called the pre-logarithmic energy factor matrix (Shih and Asaro, 1988; Barnett and Asaro, 1972; Gao, Abbudi, and Barnett, 1991; Suo, 1990). For homogeneous, isotropic materials B is diagonal and the above equation simplifies to

J=1E¯(KI2+KII2)+12GKIII2,

where E¯=E for plane stress and E¯=E/(1-ν2) for plane strain, axisymmetry, and three dimensions. For an interfacial crack between two dissimilar isotropic materials with Young's moduli E1 and E2, Poisson's ratios ν1 and ν2, and shear moduli G1=E1/2(1+ν1) and G2=E2/2(1+ν2),

J=1-β2E*(KI2+KII2)+12G*KIII2,

where

1E*=12(1E¯1+1E¯2),        1G*=12(1G1+1G2)
β=G1(κ2-1)-G2(κ1-1)G1(κ2+1)+G2(κ1+1),

and κ=3-4ν for plane strain, axisymmetry, and three dimensions; and κ=(3-ν)/(1+ν) for plane stress. Unlike their analogues in a homogeneous material, KI and KII are no longer the pure Mode I and Mode II stress intensity factors for an interfacial crack. They are simply the real and imaginary parts of a complex stress intensity factor, whose physical meaning can be understood from the interface traction expressions:

(σ22+iσ12)θ=0=(KI+iKII)riε2πr,        (σ23)θ=0=KIII2πr,

where r and θ are polar coordinates centered at the crack tip. The bimaterial constant ε is defined as

ε=12πln1-β1+β.

In this section we describe an interaction integral method (Shih and Asaro, 1988) to extract the individual stress intensity factors for a crack under mixed-mode loading. The method is applicable to cracks in isotropic and anisotropic linear materials.