ASTM D638

What the test does

A flat, dumbbell-shaped plastic specimen is clamped by its wide ends in the grips of a universal testing machine. The moving crosshead pulls the specimen along its axis at a constant speed until it yields and finally breaks in the narrow central section. Force and extension are recorded continuously, so the whole stress–strain curve is captured, not just the breaking point.

What it measures, and why it matters

The method reports tensile strength at yield and at break, elongation, tensile modulus and optionally Poisson's ratio. Yield strength is the load a moulded part can carry before permanent deformation; elongation at break separates ductile grades from brittle ones and flags embrittlement from degraded regrind. Modulus feeds stiffness calculations and finite-element models directly. Because plastics are rate- and temperature-sensitive, the figures rank materials and support lot-release control rather than predicting service life at other rates.

Specimen

Five flat dumbbell geometries are defined, chosen by thickness and rigidity; the commonly preferred type is roughly 165 mm long; material thinner than 1.0 mm is tested to ASTM D882 instead. Exact dimensions and tolerances are tabulated in the standard. Specimens are moulded, machined or die-cut from sheet with smooth, notch-free edges. A valid test needs at least five specimens — five per principal direction for anisotropic materials — and any that breaks at a flaw or outside the narrow section is discarded and replaced. Conditioning follows ASTM D618's standard atmosphere of 23 °C and 50 % relative humidity, typically for at least 40 h, testing in the same atmosphere.

What the machine must be capable of

Most rigid plastics break a standard dumbbell below about 5 kN, so a 5–10 kN frame covers most of this work; soft grades instead demand clean force resolution below 100 N. The method requires load indication accurate to ±1 % of reading, verified to ASTM E4 (ISO 7500-1 Class 1 or better, in ISO terms).

Test speed comes from the governing material specification; failing that, the method directs the lowest tabulated speed (roughly 1 to 500 mm/min by specimen type) that breaks the specimen in half a minute to five minutes. Plastics are strongly rate-sensitive, so the speed used must be reported with the result.

Modulus requires an extensometer classified to ASTM E83 Class B-2 or better; non-contact or long-travel devices suit elongations beyond roughly 20 %. Serrated wedge grips (self-tightening under load) or pneumatic side-action grips (constant force, gentler on soft specimens) are usual — a grip that allows slippage or off-axis loading invalidates the result. Testing away from the standard atmosphere needs a temperature chamber.

What goes wrong in practice

Jaw breaks are the classic failure: over-tightened or misaligned grips crush the tab and the specimen fails at the grip line, invalidating the result. Slippage is the opposite trap, producing a spuriously compliant curve that corrupts modulus. Clip-on extensometer knife edges can nick notch-sensitive materials and trigger premature failure there. Skipped conditioning is invisible in the data but shifts hygroscopic materials such as polyamides dramatically — the numbers look plausible and are simply wrong.

Related and equivalent standards

ISO 527-1 and ISO 527-2 (2019 and 2025 editions at the time of writing) cover the same physical test but are not equivalent: specimen geometries, speeds and modulus evaluation differ, so results do not transfer. ASTM D882 takes over below 1.0 mm thickness; ASTM D3039/D3039M is preferred for continuous-fibre composites, where dumbbells fail at the radius. ASTM D618, E4 and E83 supply conditioning, force verification and extensometer classification respectively.