Cosserat Elasticity

Cosserat (1909) enriched continuum mechanics by giving each material point both displacement u and an independent microrotation φ. The coupling number N² = 2κ/(2μ+κ) controls how strongly spin and strain couple. A new intrinsic length scale l = √(γ/4μ) emerges — solids with microstructure exhibit size effects: thin specimens are stiffer than thick ones. Applications: foams, bone, granular media, crystal dislocations.