Semi-analytical mass matrix for 8-node brick element
  Nowadays integration of mass matrix components in the element domain is performed using various numerical integration schemes, each one possess different level of accuracy, alters in number of integration (Gauss) points and requires different amount of computations. Herein semi-analytical approach is suggested. Metric (Jacobian determinant) is approximately modeled using its evaluations in certain points. Analytical integration is performed to derive simple explicit closed-form expressions for each term of the mass matrix. Two schemes are discussed: the first assumes constant metric (CM) in the initial domain, using evaluation at the centroid. The second allows for linear variation of the metric (LM linear metric) in the domain using 3 additional evaluation points. Both schemes are exact for rectangular and non-rectangular parallelepiped mesh. Careful symbolic manipulations and convenient choice of evaluation points allow us to avoid unnecessary operations. The accuracy of both schemes is studied numerically using randomly generated coarse mesh. Significant superiority in accuracy over equivalent schemes is reported. An important implication of this study is that it can replace currently used schemes. 
Eli Hanukah
math.NA