ASTM vs ISO TEST STANDARDS
The Case For Astm In North America!
One constant source of confusion in evaluating plastic materials is finding a comparable data set to assist in deciding what material to use for an application. One common source of ambiguity is the test methodology used to create the standard datasheet. The most common two methods are ASTM (American Society for Testing and Materials) and ISO (International Organization for Standardization).
As the name suggests, ASTM is the preferred method of testing for North American OEMs. On the other hand, ISO is dominant in Europe. Asia is a mixed bag of ISO, ASTM, and JIS (Japanese Industrial Standards) for materials that are manufactured by Japanese OEMs. For the sake of this short article, we will focus on a comparison of ASTM and ISO test standards.
The Most Common Question Is?
Can one simply compare the same test results in iso to astm for the same test? The below table displays the results of three common mechanical properties and 2 common electrical properties for the same material.
If we look at Tensile Modulus, which measures how much a material can deform (stretch) in response to stress before it yields. Comparing ISO 527-1-2 test results to ASTM D638 test results for the same material yields a 10% difference in actual results.
What's The Difference?
So, what attributes to this difference? The ISO test methods call out a dog bone-shaped test specimen 1B. Test specimen 1B has a length of 75mm, a thickness of 2mm, and a gauge length of 25mm. ASTM D 638 for the same test uses a Type 1 specimen with a 165mm length, 3.2mm thickness, and a gauge length of 50mm. Also, the ISO test method calls out a test speed of 1 mm/min. compared to 5 mm/min for ASTM.
Clearly, the differences in such parameters will yield different and incomparable results. Because the rheology of each material differs, there is no factor that can be applied to compare results.
The same applies to ISO test methods for electrical properties. Though one tests for the same property, the inconsistencies in methodology yield significantly different results as shown in the table.
It is highly recommended that manufacturers who market plastic resin and shape products in North America provide data in ASTM format. Too often materials are discounted from use due to insufficient relevant data.
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