The major difference between plastics and the more traditional materials is the time-dependent viscoelastic behavior of polymers. Plastic parts under load relax with time if they are maintained at a controlled deformation (stress relaxation), or they continue to deform if they are held under a constant load (creep).
Creep is the continued extension or deformation of a plastic part under continuous load. It results from the viscoelastic flow of the polymer with time.
Creep is probably the most widely studied long-term property. As a result there is an abundance of data available in the literature and from resin manufacturers. Creep data is usually expressed as ôapparent creep modulusö as a function of the logarithm of time under constant load (assumed to be constant stress). Remember that modulus is the ratio of stress over strain; therefore, apparent creep modulus is the constant stress divided by the actual measured strain (the deformation which changes with time).
Creep measurements are probably the easiest long-term tests to perform --- one simply sets up the specimen, hangs a weight on it, and periodically measures and records the change in deflection. Tensile creep is probably the ôpurestö data, but it isnÆt the most common creep data available, most likely due to gripping and slippage difficulties. Compressive creep is reserved primarily for rubbers and elastomers where stress relaxation and compressive flow are important performance parameters for long-term service.
Flexural creep taken in a 3-point bend arrangement is most widely performed, generally because it is the easiest to set up and monitor. A rectangular plastic specimen is supported horizontally by two steel pins and a weight is placed on the specimen at the midpoint of the two supports. A dial indicator at the location of the weight monitors the deflection with time.
One limitation with flex creep is that it is not a ôpureö stressed state. The constant stress is calculated as the maximum ôfiberö stress that occurs directly under the load on the underside surface of the bar --- this is the only point at which that maximum fiber stress exists. Actually, the stress distribution through the bar varies from tensile on the underside surface of the bar to compressive stress on the topside surface. The compressive stress tends to inhibit the overall deflection of the specimen.
Also, as the specimen deflects, the bar must move along the supports to accommodate the deflection. If the calculated fiber strain on the underside surface exceeds about 5%, a significant portion of the constant load is consumed as the driving force to ôpullö the specimen through the supports --- rather than merely bend the specimen. Therefore, the ôconstantö stress begins to decrease as the experiment continues.
In addition, at some strain level, probably around 5%, the actual strain on the bar at the point of loading stops increasing --- no further curvature occurs in the central area of the specimen --- but the specimen continues bend toward the ends of the bar as it slides through the supports. Inasmuch as the apparent fiber strain is calculated based on the amount of deflection from the original horizontal position, the method begins to yield erroneous data. Therefore, flexural creep experiments, although informative and easy to perform, can lead to somewhat conservative (optimistic) or even erroneous results. Care should be taken on the interpretation of these data.
Creep occurs in all plastic parts which are under stress. The higher the stress and the longer the part is under the stress dictates whether or not creep may be a significant factor in the performance of a part.