Figure 3 below shows the stress-strain results from a typical tensile test on a polymer material, as can be seen the test plot is made up of four different regimes. The macro-mechanical response of the material comprises of 4 distinct deformation characteristics.
Figure 3: Uniaxial Tension Test Results for a Viscoelastic Rate Dependent Material
The test results show that the slope of the line is not constant throughout the 4 regimes and the material is thus said to exhibit non-linear elasticity. The elastic region is defined in the small initial portion of the results where the slope is constant. On the molecular level the linear elastic phase is caused by the Van der Waal forces acting between the polymer chains. These forces resist the deformation, however once the strain in the material reaches a critical level, the polymer chains begin to slide with respect to one another. The response is non-linear deformation once the Van der Waal forces are overcome.
The yield point shows the local maximum stress value of the material after which the polymer chains show large scale sliding. Subsequently, the response shows a relative softening and later hardening of the material. The strain hardening phase is a result of the randomly oriented polymer chains re-aligning themselves in such a way that requires a higher force application for continued deformation.
Figure 4 shows the test results from testing Polyethylene material as per ASTM D638 at three different speeds under isothermal conditions. At the slowest crosshead speed of 5mm/minute, the yield strength and the modulus of the material are at their lowest value. As the test speed increases, the yield strength and modulus also increase. The material stiffness increases with the increase in strain rate. The material appears to be getting stronger and tougher under high strain rate conditions. The same effect can also be carried out by keeping the strain rate constant but by decreasing the temperature progressively.
Figure 4: Test Results for PE Material under Variable Strain Rate/Speed
At our laboratory we have studied the mechanical behaviour of High Density PolyEthylene (HDPE) polymer under the effect of various temperatures and strain rates. Uniaxial tensile tests were performed to determine the dynamic response of HDPEs at strain rates varying from 0.0001 sec-1 to 10 sec-1. Dynamic tests were performed at seven different strain rates, and the results in terms of true stress-strain curves are shown in Figure5. The results show that yield stress increases with the increase in strain rate.
The experimental results reveal that the stress-strain behaviour of HDPEs is much different at lower and higher strain rates. At higher strain rate, the HDPEs yield at higher stress compared to that at low strain rate. At lower strain rate, yield stress increases with the increase in strain rate while it decreases significantly with the increase in temperature. Likewise, initial elastic modulus increases with the increase in strain rate. Yield stress increases significantly at higher strain rates in the material. The stress-strain curves show almost similar mechanical response in which initial nonlinear elastic behaviour was observed followed by subsequent yielding, strain softening and hardening. Yield stress changes significantly with the increase in strain rate. An increase of 20.6 % in yield stress was calculated with strain rate increase from 0.0001 sec-1 to 100 sec-1 At all strain rates, ductile behaviour of HDPEs was observed. Strain-rate dependency of the stress-strain behaviour of polymer materials has now been well documented. This feature of mechanical behaviour is important in engineering applications for automotive and aerospace crashworthiness where the design of a polymer component is required to resist shock and impact loading and other strength stiffening effects.
Figure 5: Test Results for HDPE Material under Variable Strain Rate/Speed
Figure 6: AdvanSES Non-contact Measurement and DIC Setup
Some materials have higher strain rate sensitivity as compared to other materials. This is more dependent on the micro structural makeup and deformation physics. It is advisable to test the materials over a range of strain rates and use the data in FEA modelling and simulation.
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