Abstract: We analyze unphysical grid distortion caused by one-point quadrature in 4-node isoparametric finite element computer program.It is concluded that the rank deficiency of stiffness matrix results in hourglass distortion.A method that controlls hourglass distortion by eliminating rank deficiency is developed and validated by patch tests.

Key words: isoparametric finite element, hourglass viscosity, patch test