Tensile Testing for the Break Strain of Plastic Materials at Different Gauge Lengths

The physical properties of plastic materials are a vital part of product design and quality control. As a result of this, numerous materials tests are carried out. The most rudimentary of these tests is tensile testing, which is referenced to in numerous product specifications. Strength, elastic modulus, and breaking strain are some of the tensile characteristics of plastic materials which are evaluated during tensile testing.

Previously, the tensile testing of plastics was carried out in accordance with the customary test procedures outlined in ISO 527-1 (JIS K 7161). However, in 2012, ISO 527-1 was subject to major revisions, which were followed by updates to JIS K 7161 in 2014. A number of key changes took place as a result of these amendments, of which the most significant are reviewed below.

To begin with, the favored gauge length for type 1A geometry specimens was increased to 75 mm. It is still acceptable to use the earlier recommended gauge length of 50 mm, but as the longer 75 mm length has become an option, it is more commonly chosen. This is largely because its increased size makes it easier to create a break inside the gauge length.

A 75 mm gauge length is also advisable on account of the correlation between gauge length and extensometer accuracy, as highlighted below. ISO 527-1 (JIS K 7161) stated that in order to calculate elastic modulus with an acceptable degree of precision, tests must be carried out within a standard error of 1 %.

Figure 1 indicates absolute values for the extensometer accuracy needed when employing gauges of 75 mm and 50 mm lengths. It indicates that a 75 mm gauge length offers absolute accuracy of ±1.5 μm, while a 50 mm gauge length provides an accuracy of ±1.0 μm. Put simply, the permitted range of absolute accuracy can be increased by using a 75 mm gauge length over a 50 mm gauge length, within the same test.

Comparison of Absolute Accuracy between 75 mm and 50 mm Gauge Lengths.

Figure 1. Comparison of Absolute Accuracy between 75 mm and 50 mm Gauge Lengths.

This allows for a broader range of options when selecting an extensometer, and enables the use of the TRViewX non-contact digital video extensometer, with absolute accuracy of ±1.5 μm, for standard testing. The amendment also added a process for determining strain at yield point.

Previously, strain was calculated through Method A using equation (1). Method B was a recently added method, and determines strain using equation (2).

For materials that display yielding and necking, Method B is favored. An illustrated image of a strain calculation carried out using Method B can be seen in Figure 2. Methods A and B are both supported by Shimadzu's TRAPEZIUM X software. In this article, PP (polypropylene), PVC (polyvinyl chloride) and PC (polycarbonate) specimens are tested according to contemporary standards.

Strain Calculated by Method B.

Figure 2. Strain Calculated by Method B.

Measurement System

An AGS-X table-top precision universal testing instrument, a contact extensometer, and a non-contact digital video extensometer were used to take measurements, with both extensometers employing a gauge length of 75 mm. Full details of the equipment used can be found in Table 1.

Table 1. Experimental Equipment

. .
Testing Machine : AGS-X
Load Cell : 5 kN
Gripping Device : Pneumatic parallel gripping device
Gripping Teeth : Single-cut file teeth
Software : TRAPEZIUM X (Single)
Displacement Gauge : SG75-10, TRViewX 240S

Measured Results

The starting test speed of 1 mm per minute was changed to 50 mm per minute at 1 mm displacement, with the contact extensometer detached at 1 mm displacement. Figure 3 demonstrates testing with the SG75-10, while Figure 4 details testing with the TRViewX. The results of these tests can be found in Tables 2 and 3 respectively. Figure 5 shows the stress-strain curve obtained using the SG75-10.

Testing with SG75-10.

Figure 3. Testing with SG75-10.

Testing with TRViewX.

Figure 4. Testing with TRViewX.

Test Results Using SG75-10

Figure 5. Test Results Using SG75-10.

Table 2. Test Results Using SG75-10

Specimen Strength
[MPa]
Elastic Modulus
[GPa]
PP 32.5 1.66
PC 63.3 2.30
PVC 73.4 3.23

Table 3. Test Results Using TRViewX

Specimen Strength
[MPa]
Elastic Modulus
[GPa]
PP 32.5 1.63
PC 63.7 2.26
PVC 73.4 3.12

As the results shown in Table 2 and Table 3 demonstrate almost no difference, they offer confirmation that the tests were therefore carried out effectively.

Conclusion

Significant amendments have been brought into the testing standard for the tensile testing of plastic materials. The addition of a 75 mm gauge length has offered many advantages, including a greater choice of extensometers for elastic modulus measurement, as well as making it easier to form a break within the gauge length.

The introduction of Method B for determining strain also offers greater precision when calculating strain in materials, particularly when using materials that display yielding and necking.

The equipment detailed in the piece can be used to carry out successful tests, adhering to the up-to-date ISO 527-1: 2012 (JIS K 7161-1: 2014) standard.

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.

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