Deformation defined
Deformation is defined as the change in the shape of a body caused by the application of a force (stress). It is proportional to the stress applied within the elastic limits of the material.
Creepy things can happen with long term loading of thermoplastics
Designers and users new to thermoplastics often get caught out by creep, the quaintly named but descriptive term for long term deformation. When a load is applied to a thermoplastic there is an immediate deformation due to stretching of polymer chains, which is largely reversible, but some are unaware that, over a long period of time under load, there is a slower increase in deformation due to unentanglement and slippage of polymer chains, most of which is reversible. The irreversible part is the viscous component of viscoelastic materials.
Creep deformation (creep strain), as well as increasing with time and with increasing load, is also sensitive to temperature, particularly above the glass transition point. After one year at room temperature under a modest load of 5 MPa, polypropylene exhibits creep deformation of 1.5 %. Under the same conditions polycarbonate would deform by only 0.2 %. At a service temperature of 60oC, polypropylene this deformation increases to 2.5% for the same loading.
Reducing deformation
Deformation under creep conditions, as well as increasing with time, load (stress) and temperature, is reduced by increasing the level of crystallinity, by the addition of fillers and by introducing crosslinking. Because of its higher crystallinity, polyacetal (POM) is 5 times more resistant to creep than polypropylene.
In designing components which will be subjected to long term loading, designers should use creep modulus values instead of the normal short term modulus in mechanical design equations and finite element analysis. Because comprehensive creep data is expensive to generate, the data will not be readily available for all grades. Designers may have to use data for similar grades. Components should be designed to avoid long term loading as much as possible, for example in snap-fit assembly.
Extrapolating creep data to longer times can be risky. There is the possibility of material failure (rupture) lurking just around the corner.
(Picture courtesy of Exponent Inc.)