= Bulk modulus
{title2=$\kappa$}
The <bulk modulus> relates hydrostatic compressive pressure to fractional volume compression. In <isotropic> three-dimensional <linear elasticity>, the spherical part of the <stress tensor> is $\kappa\operatorname{tr}(\varepsilon)I$, so uniform dilation has <strain energy density> $\kappa(\operatorname{tr}\varepsilon)^2/2$. Positive <bulk modulus> and positive <shear modulus> make the isotropic <elastic stiffness tensor> positive on nonzero symmetric <strains>.
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