Meaning
Visualizing polymer deformation under long-term loads requires a set of curves that show the relationship between stress and strain at specific exposure times. This graphical representation is known as an isochronous stress strain plot and it is constructed from multiple creep test curves. The curves allow designers to extract the stress-strain behavior of a material for a fixed service life.
They do not represent dynamic loading where the force oscillates or changes rapidly.
Data Generation
Constant-stress creep tests are run at various loads to measure strain over thousands of hours. Technicians then slice across these different creep curves at a specific time, such as one hundred hours, to gather data points. Plotting these data points on a new graph yields the isochronous stress strain curve.
Design Utility
Engineers use these charts to determine the long-term stiffness of molded parts without performing complex non-linear mathematical modelling. By selecting the curve that corresponds to the target service life of the part, the designer can read the allowable stress for a given strain limit. This method is particularly useful for designing structural housing parts that must not sag or deform under their own weight over several years.
It prevents the overestimation of part strength that occurs when relying on standard datasheet modulus values.
Processing Variable
Injection moulding conditions influence the accuracy of these long-term design curves. Parts moulded with low holding pressure often contain microvoids that accelerate creep and cause the material to exceed the strain limits predicted by the isochronous stress strain curves. High levels of molecular orientation can also make the deformation behavior highly anisotropic.
