Creep Testing of Polymers at Elevated Temperatures

Technical Challenge

Polymers are prone to deform slowly over long periods of time when subjected to applied load, a phenomenon known as creep.  Over time, the deformation can grow so large that the part no longer functions as intended.  This deformation is more pronounced at higher temperatures. 

Our client wanted to compare the performance of two polymers utilizing a creep test, to select the material that would be least prone to deformation during high temperature processing conditions. 

 

Veryst Solution

Veryst performed a series of creep tests to compare the creep performance of the two polymers from which the client’s part was made.  Veryst developed a creep test fixture (shown in Figure 1) that can be placed in an oven for testing at elevated temperatures.  The fixture uses a pneumatic lift to begin the test, so that the temperature can be precisely controlled during testing. 

We measure strain during the test with digital image correlation (DIC) through the viewing window of the oven (as shown in Figures 2a and 2b).  Digital image correlation is a non-contact full-field strain measurement technique that can accurately measure large deformations at elevated temperatures, even for very soft materials such as plastics. 

This fixture can also be used to measure the creep response of materials over longer time frames, from days to weeks.  The design allows for multiple specimens to be tested at the same time to reduce total test time. 

Fixture used to conduct creep tests
Figure 1. Fixture used to conduct creep tests

 

Figure 2a. The creep fixtures and pneumatic lift Veryst developed.
Figure 2b. Strain contours were computed using DIC for Material A over 10 minutes, compared at two different temperatures.

 

Creep strains from tests at 90°C show the performance of the client’s two candidate materials in comparison to each other (see Figure 3). 

Creep strains for Material A at a wider range of temperatures show that at 90°C, the creep strain increases rapidly (shown in Figure 4 and Figure 5). 

Our client used this data to select the appropriate polymer for their device.  Additionally, this data was used to set an upper temperature limit specification for the manufacturing process to avoid creep failure during high temperature processing conditions, to ensure stable product performance during manufacturing and produce high-quality reliable products to their customers.

Results for the creep tests at 90°C and stress for Materials A and B
Figure 3. Results for the creep tests at 90°C and stress for Materials A and B show that Material A has significantly less deformation than Material B and is therefore more suitable for the client’s application.

 

Creep strain curves for Material A from 40°C to 110°C in 10°C increments at 1 MPa.
Figure 4. Creep strain curves for Material A from 40°C to 110°C in 10°C increments at 1 MPa. At temperatures above 90°C, the creep rate starts to increase significantly for this material. The material failed at 110°C, as indicated by the ‘x’ in the plot.
Creep data for Material A
Figure 5. The creep data for Material A demonstrates that the creep strain increases rapidly at temperatures above 90°C; setting an upper temperature limit specification would prevent product failure.
Tags
Bioengineering, Medical Devices
Manufacturing
Materials Engineering
Materials Selection
Polymers
Testing

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