ASTM D638 Specimen Types I–V: Dimensions, Test Speeds, Grips and Extensometers
ASTM D638 is the most widely used tensile test for plastics in North America, and most bad D638 data comes from three things: the wrong specimen type, the wrong grips, or the wrong strain measurement. This guide covers all five D638 specimen types (Type I, II, III, IV and V), their dimensions, test speeds, which tensile grips, extensometers and cutting dies you need for each, and how D638 compares with ISO 527. For the full standard overview and equipment list, see our ASTM D638 testing page.
What ASTM D638 measures
ASTM D638, Standard Test Method for Tensile Properties of Plastics, pulls a dumbbell ("dogbone") specimen to failure on a universal testing machine and reports:
Tensile strength at yield and at break
Tensile modulus (modulus of elasticity) from the linear part of the stress-strain curve
Elongation at yield and at break
Poisson's ratio (optional, needs axial and transverse strain)
It applies to unreinforced and reinforced plastics from about 1 mm up to 14 mm thick. Thinner sheet and film (under 1 mm) is tested to ASTM D882 instead, and vulcanized rubber and thermoplastic elastomers are usually tested to ASTM D412.
Which ASTM D638 specimen type should I use?
Type I: the preferred specimen for rigid and semi-rigid plastics 7 mm (0.28 in) thick or less, when you have enough material. Required for reinforced composites.
Type II: use it when a rigid material does not break in the narrow section of a Type I specimen.
Type III: required for material thicker than 7 mm and up to 14 mm (0.55 in).
Type IV: for non-rigid plastics 4 mm or thinner, and when you need to compare materials of different rigidity (for example rigid vs. non-rigid).
Type V: for material 4 mm or thinner when material is limited or expensive, or when specimens must fit in a small space such as an environmental chamber. Common for 3D-printed and medical polymers.
Note: rigid and semi-rigid specimens are normally 3.2 ± 0.4 mm (0.13 in) thick when molded. Always confirm the specimen type called out by your material specification or customer, because it overrides the general guidance above.
ASTM D638 specimen dimensions (Types I–V)
Nominal dimensions in millimeters. W = width of narrow section, L = length of narrow section, WO = overall width, LO = overall length, G = gage length, D = distance between grips. Tolerances and radii are in the standard.
Type I: W 13, L 57, WO 19, LO 165, G 50, D 115, thickness ≤ 7
Type II: W 6, L 57, WO 19, LO 183, G 50, D 135, thickness ≤ 7
Type III: W 19, L 57, WO 29, LO 246, G 50, D 115, thickness 7–14
Type IV: W 6, L 33, WO 19, LO 115, G 25, D 65, thickness ≤ 4
Type V: W 3.18, L 9.53, WO 9.53, LO 63.5, G 7.62, D 25.4, thickness ≤ 4
Two practical takeaways: the 19 mm (0.75 in) overall width of Types I, II and IV fits a standard 1 in wide jaw face, and the tiny 7.62 mm gage length of Type V is very hard to instrument with a clip-on extensometer, which is a big reason labs move to a video extensometer (more below).
How to make D638 specimens: molding, machining or die cutting
D638 specimens can be injection or compression molded, machined from sheet or parts, or die cut from thinner sheet. Die cutting is the fastest and most repeatable option for softer and thinner plastics, provided the die is sharp and the edges come out free of nicks, which act as crack starters and cause early breaks.
We stock ASTM D638 cutting dies for Types I through V in two styles:
Ribbon steel dies: economical, for thinner and softer materials.
Hardened steel dies: for thicker, tougher materials and high-volume labs; they hold an edge longer and can be resharpened with our die sharpening tool.
Pair the die with an arbor press or a sample die press so every specimen is cut with the same straight, even stroke:
ASTM D638 test speeds
D638 sets the speed by specimen type and rigidity. The rule of thumb: use the lowest speed in the standard's speed table that produces a break in 0.5 to 5 minutes. Typical values:
Types I, II, III (rigid and semi-rigid): 5 mm/min (0.2 in/min)
Type IV: 5 mm/min for rigid and semi-rigid, 50 mm/min for non-rigid
Type V: 1 mm/min for rigid and semi-rigid, 10 mm/min for non-rigid
Faster speeds (up to 500 mm/min) are allowed where the material specification calls for them
For modulus, run at the speed the standard specifies for modulus and keep the extensometer on through the linear region. Speed matters more for plastics than for metals because plastics are viscoelastic: faster pulls give higher strength and modulus.
Best grips for ASTM D638
Plastics thin out (neck) as they yield, so the grip has to keep holding as the specimen gets thinner, without crushing the tabs or causing a break at the jaw line. Our recommendations:
Mechanical wedge grips: the workhorse for D638. Wedge action tightens as load rises, so the specimen cannot slip out as it necks. Our 10 kN (2,000 lb) wedge grips are the most common choice for D638, with 5 kN through 50 kN+ wedge grips available for lighter or tougher materials.
Pneumatic grips: best for high-volume labs and softer plastics. Clamping force stays constant as the specimen thins, every operator clamps the same way, and a foot pedal frees both hands to center the specimen. See our pneumatic tensile grips.
Jaw faces: serrated faces for rigid plastics, finer serrations or rubber-coated faces for soft or notch-sensitive materials that break at the jaws.
Alignment: use grips with a specimen stop or centering guide. Off-center specimens bend as they load, which lowers measured strength and adds scatter.
How much capacity do you need? A Type I specimen is 13 × 3.2 mm, about 42 mm². At 100 MPa (a strong unfilled engineering plastic) that is about 4.2 kN; a glass-filled nylon at around 200 MPa is about 8.3 kN. A 10 kN grip and a 5 kN or 10 kN benchtop UTM cover most D638 work; thick Type III specimens of reinforced material can need 50 kN. For more on choosing grips, read How to choose the right grips for tensile testing.
Extensometers for ASTM D638: do you need one?
Yes, if you report modulus. Crosshead displacement includes slack, grip seating and machine compliance, so it overstates strain and understates modulus. D638 requires an extensometer that meets ASTM E83 Class B-2 for modulus. Strength and elongation at break can be reported from crosshead (nominal) strain when the standard allows it.
Clip-on vs. video extensometer for D638
Clip-on extensometers: accurate and affordable for Type I, II and III specimens with a 50 mm gage length. The catch: most must be removed after yield or before break so they are not damaged, and they are difficult to attach to tiny Type V specimens. See our clip-on extensometers, including the 3800 high-elongation model for stretchy plastics.
Video extensometers: non-contact, so they never touch, load or notch the specimen and stay on from the first newton to final break. They handle the 7.62 mm Type V gage length, soft and high-elongation plastics, and testing inside an environmental chamber. Our video extensometers work with most existing UTMs (Instron, MTS, Zwick/Roell, Shimadzu, Tinius Olsen, Galdabini); the ONE M2 is our general-purpose model for plastics and composites, and the Vector B80 measures axial and transverse strain for Poisson's ratio.
Video extensometers track gage marks on the specimen. Our marking tools put repeatable marks at the exact gage length in seconds:
Common ASTM D638 mistakes (and how to fix them)
Breaks at the jaws or outside the gage section: usually tab damage from over-tightening, sharp serrations or nicked die-cut edges. Try finer jaw faces, pneumatic grips with set pressure, or Type II specimens.
Using crosshead strain for modulus: always use an extensometer for modulus.
Leaving a clip-on on past yield: remove it per your procedure, or use a video extensometer so you don't have to.
Zeroing load after clamping: clamping can preload the specimen; balance the load cell before you close the second grip.
Ignoring the toe region: apply toe compensation for slack and seating at the start of the curve.
Skipping conditioning: condition specimens at 23 ± 2 °C and 50 % RH for at least 40 hours (ASTM D618) unless your specification says otherwise; moisture has a big effect on nylons.
Too few specimens: test at least five specimens per sample for isotropic materials (five in each direction for anisotropic).
ASTM D638 vs. ISO 527-2
Many labs run both, especially suppliers to European customers. They give similar but not interchangeable results:
Specimens: ISO 527-2 Type 1A is 170 mm long with a 10 mm wide, 4 mm thick narrow section; D638 Type I is 165 mm long, 13 mm wide and typically 3.2 mm thick.
Modulus: ISO 527 calculates modulus as a chord between 0.05 % and 0.25 % strain at 1 mm/min; D638 uses the linear part of the curve.
Speeds: ISO 527 lets you change speed after modulus is measured, which shortens tests on high-elongation plastics.
Terminology: D638 reports "elongation," ISO 527 reports "strain."
Equipment: the same UTM, grips and extensometer usually cover both, with the right jaw faces and gage length set-up.
ASTM D638 FAQ
What is the most common ASTM D638 specimen? Type I, 165 mm long with a 13 mm wide, 57 mm long narrow section and a 50 mm gage length.
What is the ASTM D638 Type V specimen used for? Small or limited material such as 3D-printed parts, medical polymers and chamber testing. It is 63.5 mm long with a 7.62 mm gage length.
What speed is ASTM D638 run at? Usually 5 mm/min for rigid Type I–IV specimens and 1 mm/min for rigid Type V, choosing the lowest listed speed that breaks the specimen in 0.5 to 5 minutes.
Is ASTM D638 the same as ISO 527? No. They measure the same properties but use different specimens, modulus methods and speeds, so results are not directly interchangeable.
What load cell do I need for D638? Most unreinforced plastics test well under 5 kN; a 10 kN system covers most reinforced Type I work.
Get a complete ASTM D638 setup
Universal Grip supplies everything for ASTM D638: wedge grips and pneumatic grips sized for plastics, clip-on and video extensometers, Type I–V cutting dies, environmental chambers and complete universal testing machines. Our grips and extensometers also fit most existing Instron, MTS and Zwick frames with the right adapters. Download the ASTM D638 datasheet or contact us for a quote and tell us your material, thickness and specimen type.
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