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.
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