ENH: Add Rice-Thompson criterion for dislocation emission (K1e)

[jameskermode]

Summary

• Adds methods to RectilinearAnisotropicCrack for computing the critical stress intensity factor for dislocation emission (K1e), enabling comparison with the Griffith cleavage K1c (k1g) to evaluate ductile vs brittle behavior via the Rice-Thompson criterion
• Implements both isotropic (k1e_iso, Rice JMPS 1992) and anisotropic (k1e_aniso, Beltz & Rice JMPS 1994) solutions
• Adds critical energy release rate methods g1e (Rice 1992) and g1e_fcc (Andric & Curtin JMPS 2017)

New methods on RectilinearAnisotropicCrack

Method	Description
g1e(max_gamma)	Critical energy release rate for emission (= unstable stacking fault energy)
g1e_fcc(surface_energy, max_gamma)	Energy release rate for FCC emission (Andric & Curtin 2017)
k1e_iso(max_gamma, phi, theta)	K1e using isotropic elasticity
k1e_aniso(max_gamma, phi, theta)	K1e using anisotropic elasticity

References

• J. R. Rice, "Dislocation nucleation from a crack tip: An analysis based on the Peierls concept", JMPS (1992)
• G. E. Beltz and J. R. Rice, "Dislocation nucleation at metal-ceramic interfaces", JMPS (1994)
• P. Andric and W. A. Curtin, "New theory for Mode I crack-tip dislocation emission", JMPS (2017)

Test plan

• Verify K1e values against published results for known materials (e.g. Si, Fe, Cu)
• Compare isotropic vs anisotropic solutions for cubic crystals
• Add unit tests for the new methods

Cherry-picked from the emission branch (commit 5012798 by @leiapple).

🤖 Generated with Claude Code