Standard Test Method for Tensile Properties of Plastics

Theory:

ASTM D638 is the standard specification for plastics tensile testing. The plastics industry is the third largest in the manufacturing sector and accounts for about $450 billion in US GDP. There are over 15,000 plants spread out over all 50 states. Many offshored plastics manufacturing operations are returning with the energy and oil revolution. The recent downdraft in oil prices has affected the whole sector, however the industry is still achieving productivity growth north of 2% a year. The US plastics sector employs 1.5 million people.

Geometry:

This test uses a standard "dogbone" style sample. There are a few different types of geometries, however the cross-sectional area is the only important factor. Generally dogbones that are less than 14mm thick are tested. Please see the official specification for more information on the size and shape of the samples.

Solution:

There are several types of grips that can work for this application and selecting the right one largely depends on the exact material and sample geometry,  UGC reccommends the roller grips for tests below about 500 lbs. The hook attachment adds a nice self-centering feature which is also cost effective. Vise Grips can also be used but wedge grips are necessary for higher strength samples. Serrated grip jaws are able to bite into the sample and secure it properly. 

Results:

There are a myriad of different calculations that can be derived from these relatively simple tensile tests. Ultimate Tensile Strength and Elongation at Break are the most common. Large compounders and resin suppliers such as BASF and DOW will use advanced analysis to derive data like the Modulus of Elasticity or Secant Modulus. Poisson's Ratio can also be calculated however it requires a transverse (sideways) extensometer.

Eccentric Roller Grip

ASTM D638

Plastics Tensile Strength

Plastics Testing Grip
Plastics Wedge Grip