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ASTM D638:Tensile Properties of Plastics

The tensile test to ASTM D638 yields essential mechanical properties, including tensile stress, strain, tensile modulus, yield point, point of break and Poisson’s ratio. It however, differs from the tensile test to ISO 527-2 in many technical aspects, and therefore there is only limited comparability between the two standards.

The standards ISO 527-1 (general principles) and ISO 527-2 (test conditions for molding and extrusion materials) also describe tensile testing on plastics.The guiding principle of the ISO 527 standard is the high reproducibility for test results across laboratories, companies and national borders.

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Aging tests

The tensile test provides a good basis for demonstrating the change in the mechanical characteristic values of a polymer following aging, heat or medium aging, or weathering.For this, the characteristic values of the tensile test are determined in the newly molded state, as well as after defined aging or weathering periods.

Specimens

  • The preferred test specimen is the dumbbell Type I, which with a thickness of 3.2 mm (1/8 inch) and a gauge length of 50 mm (2 inches) is still relatively similar to common component material thicknesses and at the same time allows for good strain measurement accuracy.
  • Provided that by using the test specimen Type I a fracture cannot be produced in the narrow parallel part of the specimen, ASTM D638 recommends the use of a test specimen Type II, in which the width of the narrow parallel section is significantly reduced.
  • If only a small amount of material is available, or if removal from a component does not allow for a larger specimen, then a specimen Type V is used, which is reduced in every dimension relative to Type I.
  • If specimens with material thicknesses of more than 7 mm can be obtained through mechanical processing, then specimen Type III is used.Here, the width of the narrow parallel section, as well as the shoulder width and the overall length are increased so that the specimen thickness remains less than the width.For sheet thickness of more than 14 mm, the thickness is adjusted through mechanical processing.
  • The test specimen Type IV is ideal for testing of very soft polymers (e.g., rubber) and is used when comparing soft and more stiff polymers.

Testing machine requirements

Force and extension are the two fundamental values measured by a testing machine.As part of periodic calibration when compared to a measuring instrument based on national standards, evidence has shown that these measured values achieve a level of accuracy defined in the test standard across defined measuring ranges.

Force measurement (ASTM E4)
ASTM D638 requires a measurement accuracy of 1% for the measured value.This requirement is verified with a calibration to ASTM E4 for the entire measurement range of the load cell.The quality of a force measuring system is therefore particularly defined by the width of the measuring range over which the accuracy can be achieved.ZwickRoell testing machines can achieve the measurement accuracy required in ASTM E4 starting at 1/1000 of their measurement range.This means you can measure the modulus values and tensile stress of many materials with the same test arrangement and without having to reconfigure the arrangement.

Strain measurement (ASTM E83)
Values to be measured include the crosshead travel, or the change in clamping distance and the directly measured specimen elongation.

For measurement of the crosshead travel, ASTM D638 requires an accuracy of Β± 10% relative to the respective measured value.The crosshead travel is especially used for determination of the nominal strain at break, so that the distances to be measured are relatively large.

The tensile modulus is determined with directly measuring extensometers.The permitted error in strain determination is 0.0002 [mm/mm] specified.This corresponds with a strain error of 0.02% orβ€”at a gauge length of 50 mmβ€”a permitted displacement error of 0.01 mm.

For the measurement of yield points and low elongations up to approximately 20%, ASTM requires compliance with Class C according to the calibration standard ASTM E83.For elongations up to 10% this means a set maximum permitted error value in the strain determination of 0.001 [mm/mm], which corresponds to 0.1% elongation.For the measurement of higher elongations, the relative error of the respective measured value may not exceed Β± 1%.

For the measurement of elongations higher than 20%, ASTM D638 permits a relative measurement error of Β± 20%, which can be achieved even by relatively simple extensometers.

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