The problem explained in the title is formulated generally and given an explicit solution for tensile loadings opening a half-plane crack in an infinite body. For the half-plane crack, changes in the opening displacement between the crack surfaces and in the stress-intensity factor distribution along the crack front are calculated to first order in an arbitrary deviation of the crack-front position from a reference straight line. The deviations considered lie in the original crack plane. The results suggest that in the presence of loadings that would induce uniform conditions along the crack front, if it were straignt, small initial deviations from straightness should reduce in size during quasistatic crack growth if of small enough spatial wavelength but possibly enlarge in size if of longer wavelength. The solution methods rely on elastic reciprocity, in terms of a three-dimensional version of weight function theory for tensile cracks, and on direct solution of elastic crack problems. The weight function is derived for the half-plane crack by solving for the first-order variation in the elastic displacement field associated with arbitrary variations of the crack front from a straight reference line. Also, a new three-dimensional weight function theory is developed for planar cracks under general mixed-mode loading involving tension and shears relative to the crack, the connection between weight functions and the Green’s function for crack problems is shown, and some results are given for the half-plane crack on the variations of elastic fields for variation of crack-front location in the presence of general loadings including shear.

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