(Unidirectional composites only, not available for woven composites)

The HYDRO field activates or deactivates the hydrostatic strengthening feature. A value of ON activates the hydrostatic strengthening feature, while the default value of OFF deactivates the hydrostatic strengthening feature.

If the hydrostatic strengthening feature is activated, Helius PFA explicitly accounts for the experimentally observed strengthening of the composite in the presence of a hydrostatic compressive stress in the matrix constituent. If the hydrostatic compressive stress in the matrix constituent exceeds a threshold value, the strength of both the matrix and fiber constituents are scaled upwards commensurate with the level of hydrostatic compressive stress level in the matrix constituent.

The threshold value of the matrix average hydrostatic compressive stress is an experimentally determined quantity denoted in index notation by , where < 0. Provided that the matrix average hydrostatic compressive stress ( ) is negative and < , the longitudinal shear strength of the matrix constituent is increased to,

where is the new matrix average longitudinal shear strength and is the original matrix average longitudinal shear strength. A hydrostatic strengthening ratio Sratio is computed as Sratio = ()2/()2. This ratio is then used to increase all of the coefficients of the matrix and fiber constituent failure criteria via

and

where and denote the original strength coefficients that appear in the failure criteria for the matrix and fiber constituents, respectively.

For further information on the hydrostatic strengthening feature, refer to the Theory Manual.