ASTM D638 Test Method – Tensile Properties and Poisson’s Ratio of Plastics
Scope of ASTM D638 Test Method:
ASTM D638 and ISO 527 are technically equivalent test methods. Poisson’s Ratio is a measure of transverse strain against axial strain when uniaxial stress is applied. The uniaxial force is used to deform the sample in both axial and lateral directions. A high tensile modulus means that the test material is rigid and more amount of stress is required to produce a given strain. The tensile properties of plastic materials are determined by ASTM D638 test process, which can be used to test components up to 14 mm thick (0.55 inches).
ASTM D6790/D6790M Determining Poisson’s Ratio of Honeycomb CoresASTM D6790/D6790M Determining Poisson’s Ratio of Honeycomb Cores
Test Procedure of ASTM D638 Test Method:
For the ASTM D638 test, a universal tester is used. The sample is clamped into the grips of the tester. Next, the sample is attached with two strain measuring instruments, one axially and the other laterally.Finally,a uniaxial force is applied at a fixed speed.
The purpose of this ASTM D638 test method is to generate tensile property data for the specification and control of plastic materials. Additionally helpful for qualitative characterization, these data are also beneficial for research and development.
This test technique is required by some material specifications, but there are some procedural changes that take precedence when conforming to the specification. So, before applying this test method, it is advised to consult that material specification.
Sample size:
Usually, a standard tensile bar is used in the ASTM D638 test method. This test procedure can be used to evaluate materials with thickness up to 14 mm (0.55 inches). However, ASTM D882 is the ideal test technique for testing specimens that are in the form of thin sheeting. This includes film less than 1.0 mm (0.04 inches) in thickness. Materials that are thicker than 14 mm (0.55 inches) must be machined down.
Tensile characteristics are known to change depending on specimen preparation, testing speed, and testing environment. Therefore, these aspects need to be carefully controlled when precise comparative results are desired.
Data:
In ASTM D638 test method, a stress-strain graph is produced and Poisson’s ratio is calculated.
Difference between ASTM D638 and ISO 527
| | ASTM D638 | ISO 527 |
| Test Specimen | The standard does not account for the pre-stress experienced by the specimen when gripped by the tensile testing machine. | The pre-stress needs to be quantified before testing. |
| Test Speed | Depends on the material specification. | The test speed is 1mm/min to measure modulus and 5 or 50m/min to measure elongation and strength. |
| Range | Has no fixed strain limits for the modulus measurement. | Has a set range on the stress-strain curve over which the modulus is measured. |
| Geography | Preferred by manufacturers in the US. | Predominantly used in Asia and Europe. |
Conclusion
ASTM D638 test method requires a universal testing machine (tensile testing machine). The most common sized universal or tensile testing machine for moulded plastics is 5 or 10 kN (1,125 or 2,250 lbf). As reinforced polymers and composites get stronger, a greater capacity 50 kN (11,250 lbf) unit may be required.
