Explain why strain rate (test speed) is an important consideration when performi

Question

Explain why strain rate (test speed) is an important consideration when performing and interpreting tensile tests.

Given tensile bars made of each of three grades of polyethylene, linear low density (LLDPE), high density (HDPE), and cross-linked (XLPE, made from a resin having a density between that of the LLDPE and HDPE). Predict the relative values of tensile strength, elongation at break and modulus of each. That is, for each property place the three materials in order from largest to smallest.

Explanation / Answer

Strain rate is the speed at which a material is deformed and different strain rates can have a big effect on the tensile properties of some materials, overall in plastic materials and polymers. If you slowly pull on a piece of this kind of material, it will stretch an enormous amount before it breaks. But if you pull on it quickly, it breaks almost immediately. So it’s important to remember that stress/strain data captured at lower strain rates may not produce accurate predictions for the properties of that material at high strain rates. Using that data to analyze and design parts and structures can result in those parts and structures being perfectly able to withstand predicted day-to-day forces. However, when subject to sudden high strain rates such as those found in a collision, those parts and structures could shatter rather than absorb the energy of that collision.

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