Tuesday 25 February 2020

Creep Strength and Toughness of High Performance Plastics

In this blog post, we discuss the creep and toughness behavior of high performance plastics.

Creep – a general definition

In general, creep is the deformation of a material over a certain time under a constant load and constant environmental conditions. Estimation of creep properties is done by using the ISO standard 899-1 or the ASTM D2990. For testing, a defined force is applied onto a tensile bar over a certain period of time (e.g. 1000 hours) and temperature range. Temperature can vary between 23°C to 150°C.

As a plastics design engineer you are interested in such results when you are dealing with application parts which are under high load for a long period of time. Overall, environmental changes impact the creep behavior too. Especially the increase of the temperature decreases creep performance dramatically.

Creep strength of high performance polymers

In Figure 1, different unfilled high performance polymer grades are compared to each other at different temperatures. PAI and PBI show both excellent resistance toward creep. PEEK has good creep resistance at room temperature. However, the creep resistance at temperatures around 150°C decreases down to 8 MPa, since the glass transition temperature of PEEK is reached.
Figure 1: Tensile creep strength of different high performance plastics.
Adding fibers will increase the creep resistance. Carbon fiber filled PEEK will still not outperform unfilled PAI at 150°C in terms of creep resistance. For example, PEEK with 30% carbon fiber shows at 100°C a better creep performance as unfilled PAI. At 150°C, this turns around and PAI has the better creep resistance.

Toughness of high performance polymers

In general, toughness is the combination of polymer strength and ductility. Obtaining the toughness can be done over impact testing. A though polymeric material has a good ability to absorb energy during plastic deformation. There is a direct relation between the brittleness of a polymer and energy absorption.

There are two major tests for estimating impact values: Izod and Charpy test. Both can be done with notched and un-notched specimen. The load is applied at high rate and the absorbed energy is measured (Joule).

Figure 2 shows the maximal and minimal toughness values of high performance polymers. Among the amorphous high performance polymers, PolyPhenylenSulfone (PPSU) shows the highest energy absorption, whereelse PolyPhthalAmides (PPA) show high toughness values among the semi-crystalline high performance polymers.
Figure 2: Toughness values of different high performance polymers.
I hope you found this information useful and can support you in your next design decision with high performance plastics.
Thank you for reading!
Best regards, Herwig

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[1] Laurence W. McKeen: The effect of temperature and other factors on plastics and elastomers, 2008
[2] Omnexus.com

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