Hydrostatic Strengthening

Fiber-reinforced polymer-matrix composites typically exhibit shear strengthening in the presence of a hydrostatic compressive stress.

The hydrostatic strengthening feature in Helius PFA accounts for this effect.

If the hydrostatic strengthening feature is activated, Helius PFA continually monitors the quantity which is indicative of the matrix average hydrostatic stress state. If exceeds an experimentally determined compressive threshold value (denoted < 0), the strengths of both the matrix and fiber constituents are scaled upwards commensurate with the level of hydrostatic compressive stress in the matrix constituent. Provided that < < 0, 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.