Meaning
Calculation of directional yield points in anisotropic materials is achieved through a specific mathematical expansion of traditional stress theories. Hill48 yield criterion defines the start of plastic deformation in materials with different strengths along three orthogonal axes. This model uses six independent parameters to describe the shape of the yield surface in stress space.
It is particularly relevant for simulating the thermoforming or stamping of thermoplastic composites.
Orientation Influence
Molecular alignment during the melt phase creates a material that is stronger in the flow direction than in the cross-flow direction. The criterion allows engineers to input different tensile and shear yield strengths for each principal direction. Without this adjustment, a simulation would likely underestimate the risk of tearing or thinning in the weaker zones of a part.
Symmetry Requirement
Orthotropic symmetry is a fundamental assumption of this mathematical approach. It requires the material to have three mutually perpendicular planes of symmetry, which usually align with the machine and transverse directions of a sheet. If the polymer morphology is more complex or random, more advanced non-quadratic criteria may be required.
Parameter Identification
Mechanical testing in multiple directions is necessary to find the constants for the yield equation.
