E.J.W. van der Stok
Papers # 2018 Las-Vegas
The effect of eleven different test parameters on the strain hardening modulus was investigated using the Design of Experiments statistical method. The changes in strain hardening modulus due to temperature, tensile drawing speed and the cooling rate after annealing were assessed. The importance of taking account of proper preparation of the test setup was also demonstrated.
The Strain Hardening Test (SHT) is a simple tensile test carried out at 80°C (176°F) in accordance with ISO 18488. The effect of eleven different test parameters on the strain hardening modulus was investigated using Design of Experiments (DoE). This is a statistical method used to quickly determine and quantify the effect of multiple test parameters, including their interactions. It requires only a limited number of tests.
The statistical analysis demonstrated that the locations of the markers, the clamping area and the alignment of the test specimen have a statistically significant effect on the strain hardening modulus. This shows the importance of properly preparing the test setup. The test temperature and the cooling rate after annealing are also important. A change in test temperature of 10°C can result in a change of up to 19 MPa in the strain hardening modulus. This means that the 2°C temperature variation allowed by the ISO 18488 standard may lead to a variation of about 3.8 MPa in the strain hardening modulus.
The DoE statistical analysis also assessed any interactions between the test parameters. For example, it was found that the effect of temperature on the strain hardening modulus for firstgeneration PE differs to that for PE 100RC.
Finally, although the tensile speed on its own is not statistically significant, the tensile speed combined with the test temperature does have a statistically significant effect on the strain hardening modulus.