Added some more of the documentation. Need to add references. not sure if the tables and code snippets look how I intend them to

Author: divyaadil23

documentation/multi_physics/user-guide/material_models/material_models_list/readme.md

@@ -90,3 +90,83 @@ $$ \Psi_{vol} = K_p G(J) $$
 where $$ K_p$$ can be interpreted as the bulk modulus. $$G(J)$$ is the penalty function and takes different forms depending on the type of model. Two parameters are p and pl are defined internally to add to the stresses and elasticity tensors. “Struct” , the displacement based formulation calculates these as:
 $$ p = \frac{\partial \Psi_{vol}}{\partial J}$$
 $$ pl = p + J\frac{dp}{dJ}$$
+
+The mixed displacement-pressure formulation does not calculate for p and pl this way. Instead, they are solved along with the displacements.
+
+
+**Quadratic Model:** 
+$$ G(J) = \frac{1}{2} (J-1)^2 $$
+$$p = K_p (J -1) $$
+$$ pl = K_p (2J - 1) $$
+
+**Simo-Taylor91 Model:**
+$$ G(J) = \frac{1}{4}(J^2 - 2 ln(J))  $$
+$$p = \frac{1}{2} K_p (J -\frac{1}{J}) $$
+$$ pl = K_p J $$
+
+**Miehe94 Model:**
+$$ G(J) = J - ln(J) $$
+$$p = K_p (1 - \frac{1}{J}) $$
+$$ pl = K_p$$
+
+So, if using “struct”, this is how you would input the volumetric model:
+
+<div class="struct_vol">
+&lt;<strong>Dilational_penalty_model&gt;</strong> <i>ST91</i> &lt;/<strong>Dilational_penalty_model</strong>&gt;
+&lt;<strong>Penalty_parameter&gt;</strong> <i>4.0E9</i> &lt;/<strong>Penalty_parameter</strong>&gt;
+</div>
+
+For “ustruct”:
+<div class="ustruct_vol">
+&lt;<strong>Dilational_penalty_model&gt;</strong> <i>ST91</i> &lt;/<strong>Dilational_penalty_model</strong>&gt;
+</div>
+
+Isochoric Models:
+
+**Saint Venant-Kirchhoff**
+
+This model is an extension of the linear elastic model with the strain energy postulated as a quadratic function of the Green-Lagrange strain tensor. It is an isotropic material model.
+$$\Psi_{iso} = \frac{\lambda}{2} tr(\mathbf{E})^2 + \mu tr(\mathbf{E}^2) $$
+
+where $$\lambda$$ and $$\mu$$ are Lamé constants. 
+
+In the code (see file set_material_props.h), 
+$$ C_{10} = \lambda $$
+$$ C_{01} = \mu $$
+
+Since these parameters are set automatically, we only need to specify the constitutive model type.
+
+<div class="stvk">
+&lt;<strong>Constitutive_model</strong> <i>type="stVK"</i> &gt; &lt;/<strong>Constitutive_model</strong>&gt;
+</div>
+
+The 2nd Piola-Kirchoff stress is given by
+
+$$ \mathbf{S} = \lambda tr(\mathbf{E}) \mathbf{I} + 2\mu \mathbf{E}$$
+
+**NOTE:** To modify the Lamé constants for any model that uses default parameters, we do it through specifying the elasticity modulus $$E$$ and poisson’s ratio $$\nu$$.
+$$ \mu = \frac{E}{2(1+\nu)} $$
+$$ \lambda =  \frac{E \nu}{(1+\nu)(1-2\nu)} $$
+The bulk modulus $$\kappa$$ is given by
+$$ \kappa = \frac{E}{3(1-2\nu)} $$
+
+$$\lambda$$ and $$\kappa$$ are set to zero if the material is incompressible, i.e. $$\nu=0.5$$.
+
+<div class="stvk">
+&lt;<strong>Elasticity_modulus&gt;</strong> <i>240.56596e6</i> &lt;/<strong>Elasticity_modulus</strong>&gt;
+&lt;<strong>Poisson_ratio&gt;</strong> <i>0.4999999</i> &lt;/<strong>Poisson_ratio</strong>&gt;
+</div>
+
+**Neo-Hookean model**
+$$ \Psi_{iso} = C_{10} (\bar{I}_1 - 3) $$
+
+<div class="stvk">
+&lt;<strong>Constitutive_model</strong> <i>type="neoHookean"</i> &gt; &lt;/<strong>Constitutive_model</strong>&gt;
+</div>
+
+The parameter $$ C_{10}$$ is automatically set (see file set_material_props.h):
+	
+$$ C_{10} = \frac{\mu}{2} $$
+
+**Holzapfel-Gasser-Ogden model**
+$$ \Psi_{aniso} = \frac{a_4}{b_4} \[ exp\{ b_4( \kappa \bar{I}_1 + (1-3\kappa)\bar{I}_{4} - 1)^2 \} - 1\] + \frac{a_6}{b_6} \[ exp\{ b_6( \kappa \bar{I}_1 + (1-3\kappa)\bar{I}_{6} - 1)^2 \} - 1\] $$