Tensile testing is one of the fundamental mechanical characterization techniques for identifying the response of plastic to uniaxial tensile stress. ASTM D638 gives a guideline to conduct tensile tests on rigid and semi-rigid plastic products in the form of moulded, extruded, or machined samples. The viscoelastic nature and strain rate sensitivity of plastics dictate that the test needs to be conducted under standard conditions to get valuable information. ASTM D638 is therefore very commonly used in the study of polymers, quality control in the production of products, and the regulations.
Overview of ASTM D638 Standard
ASTM D638 provides guidelines for testing the tensile values of plastics at controlled temperature and humidity. The standard proposes different kinds of specimens (Type I to Type V) to accommodate different material thicknesses and manufacturing operations. It gives specifications for the test equipment, gripping procedures, the strain gauge, and the display of the presentation data. The test may be applied to a tremendous proportion of thermoplastics and thermosetting plastics, exceptions being those with excessive creep or special testing needs.
Specimen Preparation
The ASTM D638 test is highly sensitive to specimen preparation. It is typically assumed that a dog-bone geometry of a test specimen causes the distributions of stress to be uniform throughout the gauge length. The standard determines the correct sizes of every kind of specimen, like the total length, gauge length, width, and thickness. Their production may be done by injection molding, compression molding, extrusion, or machining, but the surface defects and stresses need to be minimized. The specimens are conditioned in normal laboratory conditions before testing with the aim of ensuring moisture and temperature balance.
