ASTM D638 Tensile Testing of Plastics

0 1
ASTM D638 Tensile Testing of Plastics

Tensile testing plastics is the process of stretching or pulling an object with a constantly changing force. ASTM D638 covers the tensile properties of both reinforced and unreinforced plastics in the form of dumbbell-shaped molded plastic test specimens.

Why ASTM D638?

If you are testing rigid plastic samples between 1 to 14 mm (0.03 to 0.55 in) then ASTM D638 is right for you.

How Do I Perform ASTM D638?

ASTM D638 establishes procedures for collecting data using a universal testing machine (UTM). You must start by measuring the thickness of the sample. The most common specimen is Type I, which has a thickness of 3.2 mm (0.125 in) and a gauge length of 50 mm (2 in). That material thickness reflects common plastic components and allows for good strain measurement accuracy.

Using the universal test machine, the specimen is placed between two clamps or grips with the edges of the specimen parallel to the direction of the load. The specimen grips are tightened to hold the specimen securely within the fixture. The test specimen is then pulled apart at tensile rates ranging from speeds from 1 to 500 mm/min (0.3 to 19.6 in/min) until it breaks.

How Do I Choose the Best Plastic Sample Type For ASTM D638?

  • Flat specimens are typically molded, die-cut, or machined into a dogbone or dumbbell shape, which ensures that the break occurs in the center of the specimen rather than at the clamping areas.
  • Type I specimens have a gauge length of 50 mm (2 in). If a Type I sample break cannot be produced in the narrow parallel portion of the dumbbell specimen, ASTM D638 recommends use of a Type II specimen, where the width of the narrow parallel section is reduced.
    Rigid tubes and rods, both of which must also be machined into a dogbone shape.
  • If specimens are more than 7 mm (0.27 in) thick, then a Type III is used. Multiple dimensions ensure that the specimen thickness remains less than the width. If sheet thickness is more than 14 mm (0.55 in), thickness is adjusted through machining or reducing the sample down to 14 mm (0.55 in) or less.
  • If only a small amount of material is available, then a specimen Type V is used, which is reduced in every dimension versus a Type I. Type V specimens are smallest, with a gauge length of only 7.6 mm (0.3 in).

What Values Do You Need to Report to Conform With ASTM D638?

These are the most common tensile properties calculated using ASTM D638 and can be collected using our controller software:

  • Tensile strength
  • Tensile modulus
  • Elongation
  • Poisson’s Ratio