Appendix A.7 - Argument #8: Hydrostatic Strengthening

(Unidirectional composites only, not available for woven composites)

The 8th argument activates or deactivates the hydrostatic strengthening feature. A value of 1 activates the hydrostatic strengthening feature, while a value of 0 deactivates the hydrostatic strengthening feature.

Experimentally observed strengthening of the composite in the presence of a hydrostatic compressive stress is explicitly accounted for if the hydrostatic strengthening feature is activated. 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 in the matrix constituent.

The threshold value of the matrix average hydrostatic compressive stress is an experimentally determined quantity denoted 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 used to increase all of the coefficients of the matrix and fiber constituent failure criteria via the following equations

i = 1,2,3,4,5

i = 1,2

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