To determine certain properties of polymers, it is necessary to do a Dynamic Modulus Analyzer, or DMA. This is important because polymers behave very differently from metals when under a load. In this blog post, we are going to provide a beginners guide to why DMA is necessary and how it affects our choice of polymers.
Polymer Behavior versus Metal Behavior
Plastics do not behave as perfectly elastic materials when they are initially loaded, which means that their modulus of elasticity is not as easy to determine. The stress-strain curve for a metal exhibits distinct straight-line portion; the slope of that straight-line portion represents the modulus of elasticity of the metal.
Plastics, on the other hand, dont exhibit that straight-line portion because they do not behave the same as metals. Instead of reporting the slope of a straight-line portion, the slope of a tangent line is reported. The behavior of plastics is too complex to be accurately represented using standard elastic material equations.
Another major difference between the behavior of polymers and metals involves the viscoelastic, or time-dependent, behavior of polymers under stress. When polymers are subject to stresses for an extended period of time, they exhibit a behavior known as creep. Their stiffness is in part a function of how long the load has been applied; the shorter the load time, the stiffer the material behaves.
This can be a major factor in some applications, and means that the typical modulus of elasticity values wont necessarily reflect the true behavior of the polymer, especially when elevated temperatures are involved.
Dynamic Modulus Analyzer
Modulus data that is time and/or temperature dependent is typically developed using a Dynamic Modulus Analyzer or Dynamic Mechanical Analyzer, abbreviated DMA. The DMA technique allows for parameters such as time, temperature, frequency, and stress levels to be accounted for when determining the properties of a material.
DMA curves are used to represent how the stiffness of a material varies across a wide temperature range. This are very useful in exploring the behavior of thermoplastics at different temperatures. For example, if you know the expected operating temperature you should use a DMA curve to determine the expected stiffness at those temperatures for different thermoplastics.
DMA curves provide a much better idea of how certain polymers will behave when compared to the simple values provided in data sheets. This more detailed information could be the key to selecting the appropriate material for an application, especially if elevated temperatures are involved.
If you are going to use a thermoplastic material in an application, keep in mind the key differences in the response of metals and polymers. Properties such as stiffness are far more sensitive to temperature and duration of load in polymers, and polymers require a different approach to estimating these values.
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