The unique viscoelastic nature of thermoplastics can cause peculiar changes over time. A plastic supporting a fixed load can stretch and deform – a condition known as “creep.” Alternatively, a plastic under a fixed deformation will experience diminished resistance to that deformation over time. For example, a press fit will become looser as polymer chains reorient to a more relaxed state. Likewise, the force exerted by a plastic spring will slowly diminish. This is due to stress relaxation, which is the topic of this article.

Stress relaxation is of particular concern in medical applications in which a change in tightness could lead to leaks of stopcocks or fluid connections. Since plastics differ greatly in how much and how quickly they stress relax, it is important to understand how candidate materials perform under applied stress and how stress relaxation changes tightness over time.

The short-term properties listed on material datasheets do not provide the information needed to predict how a press fit loosens over time. For this, you need long-term data, such as the isochronous stress strain curves generated by Covestro for many of our thermoplastic grades. Below is an example of an isochronous stress-strain curve for a medium viscosity Makrolon® polycarbonate.

Reading horizontally through the various time curves shows the predicted change in strain (elongation) due to creep for a given applied stress. Reading downward through the time curves reflects the change in stress over time due to stress relaxation for a given applied strain. For example, the isochronous stress-strain curve shows that the material will be under about 5,200 psi (36MPa) after one hour at 1.9 percent strain. After 1,000 hours, the stress drops to 4350 psi (30MPa). Dividing this stress by the 1.9 percent strain yields an apparent modulus or creep modulus for use in calculations of stress relaxation after 1,000 hours. The Covestro Part and Mold Design Manual (available for download here) shows examples of how to use isochronous stress-strain curves for predicting changes due to creep and stress relaxation. Note that the plastic does not become less stiff over time. The molecular chains simply reorient into a more relaxed state.

The type of isochronous creep data published by Covestro can involve five or more years of testing. To quickly evaluate the stress relaxation behavior of grades lacking published data, we developed a simple test that directly measures changes in the tightness of a press fit after relatively short time intervals. The test uses a set of stainless steel pins that are press fit into bosses molded out of the test materials. The pins are machined to achieve different amounts of interference and are given shoulders to ensure the same insertion depth for all tests. The torque required to slowly rotate the press fit pins is measured at increasing time intervals after insertion, thus revealing how the tightness of the fit changes over time due to stress relaxation. Note that fresh pin/boss combinations are used for each time interval test.

The following photo shows the machined pins and bosses, and an assembly mounted in the holding fixture on the torque tester. The boss inside diameter measures 0.220 inches (5.59 mm). The insertion pins were made with diameters ranging from 0.002-0.006 inches (0.05-0.15 mm) larger than the boss hole. Assemblies were tested after dwell times up to 1,000 hours.

The torque tester plots torque (N·m) against rotation angle in degrees. The following graph shows room temperature results for a 0.004 inches (0.051 mm) interference fit for three transparent plastics: polycarbonate (PC), acrylic and copolyester. The solid lines show the torque resistance one hour after pin insertion. The dashed lines show torque values for separate samples tested 100 hours after insertion.

The results show dramatic differences in material performance. The stiffer PC squeezes with the most force and requires the highest torque to rotate the pin in the boss. Between one hour and 100 hours, the torque drops by just 6 percent at half a turn. The 100-hour acrylic torque value dropped by 15 percent, in part due to the formation of radial cracks. The copolyester started at a low initial torque and then decreased 65 percent to a value less than one-fifth the 100-hour PC value. Other differences were noticed during testing. While the PC and acrylic materials tended to rotate smoothly, the copolyester exhibited stick/slip behavior that resulted in chatter in the torque readings. Curve fitting techniques were used to calculate and plot the average of the highs and lows over a large number of data points. The averaged results appear to correctly reflect the magnitude of stress relaxation as confirmed by creep testing. In devices with sliding or rotating elements, the frictional stick behavior could give the false impression of a tight fit, when in reality compression is very low. The copolyester also left a deposit on the pins that had to be removed with solvents after each test.

Rigid materials, such as PC, that retain the tightness of fit very well can stress crack over time if the applied strain is too high. The long-term strain needs to be appropriate for the time range and temperature required by the application. Isochronous stress-strain curves that include onset-of-crazing limits, like the one shown earlier, can be helpful in this regard. The 0.004 inches (0.05 mm) interference fit used in the torque study imposes a 1.8 percent long-term strain on the 0.220 inches (5.59 mm) inside diameter of the plastic boss. Reading downwards along the 1.8 percent strain line on the isochronous stress-strain curve shows that crazing/stress cracking is unlikely until well over 60,000 hours at room temperature. This assumes no exposure to aggressive chemicals or other forms of environmental attack.

The design of tightly fitting assemblies needs to take into account differences in the short- and long-term properties of the candidate materials. In the torque test example, the PC exhibits the one-hour tightness of the copolyester but with only about 55 percent as much interference. Adjusting the strain to equalize the starting tightness improves the chemical resistance and long-term performance of the PC. Likewise, increasing the interference in the copolyester to match the one-hour tightness of the PC reduces chemical resistance and accelerates stress relaxation. The copolyester is severely limited in its ability to retain tight fits over time, especially at elevated temperatures. The acrylic is limited by its ability to withstand even modest strain levels without cracking.

Many assemblies require mating parts to maintain a tight fit or leak-proof seal for extended amounts of time. In such applications, consider the following:

• Viscoelasticity can cause assemblies to loosen over time due to stress relaxation
• Isochronous stress-strain curves enable predictions of creep and stress relaxation
• Plastics differ greatly in their ability to resist stress relaxation
• Stick-grip friction should not be mistaken for compressive tightness
• Stay within appropriate strain limits for the time and temperature needed
• Adjust strain levels as needed to achieve the desired tightness for each candidate material

Failure to account for stress relaxation in your design and material selection can lead to unexpected problems during the life of an assembly.

For more information, email mark.yeager@covestro.com.
or connect on LinkedIn: https://www.linkedin.com/in/mark-yeager-68b5424/

Makrolon® is a registered trademark of the Covestro group.

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