Hello and welcome to a new blog post. Today I present to you another important multipoint and long-term data set for polymer material selection and part design: tensile creep modulus.
The creep strength and toughness of High Performance Plastics we discuss here.
Mechanism of creep
Creep, also known as cold flow, is the deformation under a static load over time and helps to gain insights over the product lifetime. Understanding the creep behavior is one puzzle key during polymer material selection. Creep resistance materials are needed for applications such as structural components, joints, fittings and hydrostatic pressure vessels. In general we can distinguish between primary, secondary, and tertiary creep. When you conduct a creep test it is important to keep the applied stress on the material at a constant level. This allows in turn to plot the lifespan of your product.
Primary, Secondary, and Tertiary Creep
Exposure to heat or load will result in a strain reaction of the polymer. It is the stretching and straining of the entanglements with the macro molecular network. The polymer passes relatively rapid through this first phase.
The primary creep rate decreases and we see a steady state phase of the creep. This is also known as strain hardening. Polymers undergo a linear progression during this second phase. The secondary creep phase is much longer than the first and is relevant for estimation of the part failure time.
The last phase is again a rapid phase. Microstructural changes such as internal cracks will lead to a fast failure of the part.
Creep behavior of amorphous and semi-crystalline polymers
Main creep mechanism of amorphous polymers is molecular de-tangling, slipping, and rearrangement. On the other hand, the creep of semi-crystalline polymers is restricted by the crystalline regions.
Table 1 shows the creep behavior of amorphous and semi-crystalline polymers at a stress level of 8 to 9 MPa at 31°C.
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| Table 1: Creep behavior of amorphous and semi-crystalline polymers at a stress level of 8 to 9 MPa at 31°C. |
More multipoint design data you can find here
Thanks for reading and #findoutaboutplastics
Greetings,
Herwig
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Literature:
[1] DuPont - Design Guide
[2] https://www.xcentricmold.com/the-impact-of-creep-in-plastics/
[3] https://www.researchgate.net/publication/317206602_Long-term_Loading_-_Tensile_Creep_Modulus_-_data/download
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