Meaning
Deformation analysis of load-bearing polymer components requires a time-dependent stiffness value to account for continuous viscoelastic creep under sustained stress. Engineers use the apparent modulus to replace the instantaneous elastic modulus in standard structural formulas. This value decreases over time as the material continues to deform under a constant load.
The calculation stops being accurate once the material exits the linear viscoelastic region or approaches its yield point.
Stiffness Reduction
Polymer chains slowly disentangle and slide past one another when subjected to a prolonged force. This structural relaxation causes the apparent modulus to drift downward from the initial datasheet value. High operating temperatures accelerate this molecular movement and further depress the strength of the part.
Moulded parts in hot environments require a lower design limit to prevent structural failure.
Calculation Method
Determining the time-dependent value involves dividing the constant applied stress by the total strain measured at a specific duration. The resulting apparent modulus provides a straightforward way to calculate long-term deflection without running complex non-linear finite element simulations. Designers typically extract these stress and strain coordinates from isochronous curves supplied by resin manufacturers.
If the duration of the load exceeds the tested time, extrapolation becomes necessary.
Production Impact
Structural parts that experience continuous assembly loads, such as snap-fits or threaded bosses, must be designed using these reduced stiffness values. Moulding conditions that induce high residual stresses can accelerate the apparent decay of stiffness during service. Regrind usage also lowers the molecular weight distribution, which speeds up the creep rate and reduces the effective lifetime of the product.
