Merge pull request #12476 from bwspenc/volumetric_strain

Fix minor issues with documentation of volumetric strain calc

modules/solid_mechanics/src/utils/MaterialTensorCalculatorTools.C

@@ -71,7 +71,8 @@ volumetricStrain(const SymmTensor & symm_strain)
 {
   // Since the strains are logarithmic strains, which are by definition log(L/L0),
   // exp(log_strain) = L/L0
-  // The ratio of the volume of a strained cube to the original volume is thus:
+  // The ratio of the volume change of a strained cube to the original volume
+  // (delta V / V) is thus:
   // exp(log_strain_11) * exp(log_strain_22) * exp(log_strain_33) - 1
   //
   // Since eng_strain = exp(log_strain) - 1, the equivalent calculation using

modules/tensor_mechanics/doc/content/source/auxkernels/RankTwoScalarAux.md

@@ -290,10 +290,10 @@ where $\Delta V$ is the change in volume and $V$ is the original volume.
 This calculation assumes that the strains supplied as input ($T$) are logarithmic strains,
 which are by definition $log(L/L_0)$, where $L$ is the current length and $L_0$
 is the original length of a line segment in a given direction. 
-The ratio of the volume of a strained cube to the original volume is thus:
+The ratio of the volume change of a strained cube to the original volume is thus:
 \begin{equation}
 \label{eq:volumetric_strain_from_tensor}
-s = \exp(T_{11}) * \exp(T_{22}) * \exp(T_{33}) - 1
+s = \frac{\Delta V}{V} = \exp(T_{11}) * \exp(T_{22}) * \exp(T_{33}) - 1
 \end{equation}
 This is the value computed as the volumetric strain.
 

modules/tensor_mechanics/src/utils/RankTwoScalarTools.C

@@ -146,7 +146,8 @@ volumetricStrain(const RankTwoTensor & strain)
 {
   // Since the strains are logarithmic strains, which are by definition log(L/L0),
   // exp(log_strain) = L/L0
-  // The ratio of the volume of a strained cube to the original volume is thus:
+  // The ratio of the volume change of a strained cube to the original volume
+  // (delta V / V) is thus:
   // exp(log_strain_11) * exp(log_strain_22) * exp(log_strain_33) - 1
   //
   // Since eng_strain = exp(log_strain) - 1, the equivalent calculation using