Tensile Properties of Semi-Rigid and Rigid Plastics

All manufactured plastics are exposed to mechanical loading conditions throughout their operating lifetime. Mechanical failure can be prevented by determining and routinely verifying the mechanical properties to confirm whether the material is appropriate for the intended application. One such method that is usually used to determine the mechanical properties of plastics is tensile testing. Some examples of groups who perform tensile tests are manufacturers, researchers, consumers, and governing agencies. These tests are performed to verify quality, ensure consistency, and develop standards. A universal test frame, an extensometer, and grips are commonly used to perform the tensile tests.

Shimadzu Scientific Instruments provides a complete range of extensometers, grips, and test frames that meet or exceed the requirements described in ISO 527 and ASTM D638 testing standards.

Experiment

A Shimadzu AG-X Plus universal test frame was used to measure the mechanical properties of Polyoxymethylene (POM), acrylonitrile butadiene styrene (ABS), polystyrene (PS), and Polyethylene terephthalate (PET). The frame was fitted with a Shimadzu TRViewX class B-1 video extensometer, a 10 kN class 1 load cell, and 10 kN pneumatic flat grips.

ASTM D638 Type I samples having a thickness of 3.45 mm were prepared through injection molding. Five samples of each material type were tested at a speed of 5 mm/minutes. Then, using the data processing functions within the Trapezium X testing software, the ultimate tensile strength, yield strength, tensile strength at break, percent elongation, elongation at yield, and elastic modulus were easily established. In order to fit a linear trendline to the elastic region of the stress strain curve, the elastic modulus was measured by using the least squares method. A 0.2% offset was used to determine the yield point and the break point was defined as the point at which a drop in force of 100 N/sec took place.

Before testing, the samples were marked with two adhesive flags to highlight the boundaries for the gage length (Figure 1). The video extensometer identifies the distance between the flags before and during the test so as to precisely determine the initial gage length and displacement with an accuracy of 0.5%. The auto gage length detection feature of the TRViewX video extensometer allows rapid sample preparation without comprising the accuracy of strain calculations. The distance between the grips was adjusted to 115 mm, followed by zeroing the crosshead.

Figure 1. Image of sample with flags attached to mark the boundaries of the gage length.

Using the guidance marks established by the TRViewX video extensometer, the samples were loaded and aligned. Once the samples were aligned, the test was initiated and continued until the criteria for break was fulfilled. The displacement and force measurements were directly determined and used to measure stress and strain. For each sample, the test was conducted a total of five times. The average values for elongation, strength, and elastic modulus were reported together with averaged stress strain curves.

Results

Table 1 shows the average values and standard deviation of five samples, and Figure 2 shows the stress strain curves. Except PS, all the samples demonstrated yielding prior to failure. PS is a brittle material and hence this is to be expected. The results for the elongation at yield, and tensile strength at break produced less than 5% variation, while the results for yield strength, elastic modulus, and ultimate tensile strength produced less than 3% variation. The difference in elongation at break ranges from 2% to 30%. Even though 30% variation is quite large, there are several factors that can contribute to this, such as variations in composition, molecular structure, and processing. For the purpose of this article, attempt is not made to explain the cause of these results through additional analysis.

Table 1. Results from testing tensile tests reporting the mean value and standard deviation of five samples for each type of plastic tested.

Figure 2. Averaged stress strain curves for ABS 3501, ABS ESCR 3501, POM, PET and PS

Conclusion

Combined with the TRViewX video extensometer, the Shimadzu AG-X Plus meets or surpasses the needs of the testing standards described in ISO 527 and ASTM D638 standards. The mechanical testing configuration of Shimadzu's system, in tandem with the data processing capabilities of Trapezium X software, offers a solution for operators to easily and accurately carry out tensile tests on plastic dog bone samples that comply with ISO 527 and ASTM D638 standards.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.