ASTM vs ISO test methods: why the numbers differ

Look at a few datasheets and you will see two families of test codes. ASTM standards come from ASTM International, based in the United States. ISO standards come from the International Organization for Standardization. Many tests exist in both families and measure the same idea. They do not produce the same number. This article explains the common pairs and why the difference matters.

Why the results differ

A test result depends on more than the material. Three things change between standards:

• Specimen shape. The size and profile of the bar or dog-bone change how stress is distributed.
• Test speed. Plastics are viscoelastic: they respond differently when loaded quickly or slowly. Faster tests generally give higher strength and lower elongation.
• Setup and calculation. Support spans, clamping, conditioning and how the final number is worked out all vary.

The common pairs

Tensile: ASTM D638 vs ISO 527

Both pull a dog-bone-shaped specimen until it breaks. ASTM D638 defines several specimen types; resin makers often use the smaller Type IV or V bars. ISO 527 uses its own shapes and often measures modulus at a slow speed before speeding up for strength. Results are usually in the same ballpark, but not identical.

Flexural: ASTM D790 vs ISO 178

Both bend a rectangular bar supported at two points and loaded in the middle (three-point bending). Bar sizes, span-to-thickness ratios and speeds differ. Flexural strength is usually higher than tensile strength for the same material, so never compare a flexural value with a tensile one.

Impact: ASTM D256 Izod vs ISO 179 Charpy

Both swing a pendulum into a notched bar and measure the energy absorbed. In Izod, the bar is clamped upright at one end and struck near the top. In Charpy, the bar lies across two supports and is struck in the middle. The units are different too: Izod is usually reported as energy per unit width (J/m), while Charpy is energy per unit area (kJ/m²). There is no conversion between them. Also check whether the specimen was notched or unnotched: unnotched results are far higher.

1. PLA 202.4–38.9
2. PETG 212.6–23.3
3. ABS 106.1–38.6
4. PP 94–44
5. PC 63–21.28
6. PEEK 77–30
7. Photopolymer 180.6–9

色条 = 最低至最高，圆点 = 中位数。共 91 种材料，数据实时来自我们的数据库。

Heat deflection: ASTM D648 vs ISO 75

Both heat a loaded bar and record the temperature at which it bends by a set amount. Specimen orientation and the deflection limit differ slightly. The bigger factor is the load: 0.45 MPa or 1.8 MPa. Always compare like with like. See HDT at 0.45 MPa vs 1.8 MPa.

Hardness: ASTM D2240 vs ISO 7619 / ISO 868

All use a spring-loaded durometer and the Shore A or D scales. They agree reasonably well, but the reading time differs: an instant reading is higher than one taken after a few seconds, because soft materials creep under the indenter. See Shore A vs Shore D.

Density: ASTM D792 vs ISO 1183

Both weigh a sample in air and in a liquid. Density is a physical quantity, so results agree closely. For printed parts, remember that voids and infill make the part lighter than the sheet suggests.

Flammability: UL 94

UL 94 classifies how a specimen burns and self-extinguishes: HB for slow horizontal burning, then V-2, V-1 and V-0 for vertical tests, with V-0 the most demanding of the three. A rating is tied to a specific thickness, and often to a specific colour. A V-0 rating at 3 mm does not mean V-0 at 1 mm. A rating on raw material is also not a certification of your printed part.

Conditioning and test environment

Both families of standards condition specimens before testing, typically at room temperature and moderate humidity for a set time. This matters most for materials that absorb water, such as nylons. A nylon tested straight from the dryer will look stiffer and stronger than one that has been allowed to take up moisture. Some sheets report both, labelled “dry” and “conditioned”. Others report only one and do not say which. If two values for a moisture-sensitive material disagree, check the conditioning first.

Revision years

Standards are revised from time to time, so you may see a year after the code, such as D638-14 or D648-18. Revisions usually refine details rather than change the method, and values from different revisions of the same standard are generally comparable. A missing year is common and is not a cause for concern on its own.

Other standards you may see

Some sheets cite national equivalents, such as Chinese GB/T or German DIN methods, or composites standards like ASTM D3039 for fibre-reinforced materials. Elastomers often use ASTM D412 for tensile properties, which uses its own specimen and much larger strains. The same rule applies: compare within one method.

What to do in practice

1. When comparing two materials, check that the test codes match.
2. If they do not, treat small differences as noise.
3. Check orientation and condition too. A matching standard with different print orientations is still not a fair comparison.
4. When the decision matters, test both materials yourself in the same way.

The hub shows the test method beside every value, exactly as the manufacturer printed it. For the full picture, see how to read a TDS and comparing materials across brands.

Explore engineering materials and their test methods 229 种材料