Meaning
Mathematical frameworks for predicting the onset of plastic deformation in materials with directionally dependent mechanical properties define the stress states required to initiate permanent flow. Plastic moulders use anisotropic yield criteria to calculate how oriented polymer chains or fiber reinforcements alter the pressure needed to deform a part. Traditional isotropic models fail here.
These specific criteria map the yield strength across different directions, establishing limits beyond which the material deforms permanently under complex load conditions.
Directional Variation
Molecular alignment during polymer extrusion or injection moulding creates a highly oriented structure that resists load better along the flow path than across it. Practical application of anisotropic yield criteria allows tool designers to estimate where early deformation will occur under load. Adjusting the injection gate location changes the fiber orientation distribution.
This change modifies the directional yield strengths and prevents premature part failure.
Mathematical Formulation
Stress tensors are modified with orientation-dependent coefficients to scale the contribution of each shear and normal component. Calculating these coefficients requires testing specimens cut at multiple angles from a molded sheet. The resulting equations predict yielding under complex multiaxial loads.
Incorrect coefficients lead to either overbuilt parts or mechanical failures.
Processing Consequence
Moulding conditions directly dictate the directional properties through shear rates and melt temperatures. High shear increases orientation and heightens the directional discrepancy in yield limits. Utilizing anisotropic yield criteria helps determine if regrind material, which has shorter fibers and lower orientation, will meet the structural requirements of the final product.
Choosing a high-melt-strength resin reduces orientation and produces a more uniform yield limit.
