ASTM D638 tensile test plastic data appears on almost every datasheet a buyer opens, yet the number means little without knowing how it was produced. The method pulls a moulded or machined dog-bone specimen in a universal testing machine until it yields or breaks, recording force and extension. This test method guide explains the setup, the variables that move results, and how to read the reported numbers like a buyer.
What the ASTM D638 tensile test plastic method measures
D638, current edition D638-22, covers rigid, semi-rigid and non-rigid plastics up to 14 mm thick, reinforced or unfilled, and reports the tensile properties of plastics buyers actually design with: tensile strength at yield or break, elongation at yield or break, and modulus of elasticity. Films and sheets under 1.0 mm belong to ASTM D882 instead, and rubbery materials to D412. The test is destructive and statistical, so reports cite an average with a standard deviation across at least five specimens.
Specimens: why the type i specimen dominates
Five dumbbell geometries are defined, and the choice is mostly a thickness decision:
- Type I: 165 mm long, 50 mm gauge length, 13 mm narrow section, recommended 3.2 mm thick; the default for rigid and semi-rigid plastics up to 7 mm.
- Type II: used when material does not break in the narrow section of a Type I.
- Type III: for material from 7 mm up to 14 mm.
- Type IV: a 6 mm wide die for non-rigid plastics up to 4 mm, shared with ASTM D412.
- Type V: a small 63.5 mm specimen where material is limited.
Width and thickness are measured to the nearest 0.025 mm because tensile stress divides by the original minimum cross-section: a 13 by 3.2 mm Type I section is 41.6 mm2, and a 2,080 N peak force reads 50 MPa.
Speed, conditioning and strain measurement
Plastics are strain-rate sensitive, so speed is a result, not a detail. For Types I to IV the standard speeds are 5, 50 and 500 mm/min, and the rule is to pick the lowest speed that ruptures the specimen within 0.5 to 5 minutes; Type V uses 1, 10 and 100 mm/min. Specimens are conditioned per ASTM D618 Procedure A, nominally 40 hours at 23 degrees C and 50% relative humidity, and tested in that atmosphere. Modulus requires an extensometer, Class B-2, typically a 50 mm gauge on a Type I, because crosshead displacement alone includes machine and grip compliance and distorts the number. A specimen that breaks in the grip or at the tab radius is discarded, not averaged in.
Reading datasheet numbers like a buyer
Three habits keep the numbers honest. First, never compare across standards without conversion: ISO 527 uses 1A and 1B specimens and different speeds, so an ISO 527 strength and a D638 strength are not the same measurement. Second, check which property the datasheet quotes, because tensile strength at yield and at break differ for ductile materials and a grade can look strong on one line and ordinary on the next. Third, treat elongation at break as a laboratory value rather than a design limit: it is measured on an unnotched, perfect dog-bone at 23 degrees C, which no real part ever is. Our datasheet reading guide covers the remaining fields, and grade-level examples sit in the industrial polypropylene specifications.
Where buyers use D638 in practice
Incoming QC compares a lot's tensile results against the agreed specification window, failure analysis benchmarks a cracked part against retained resin data, and material selection between families leans on D638 numbers heavily, for instance the modulus and elongation split in our PA6 versus PA66 comparison. If a supplier cannot state specimen type, speed and conditioning alongside the value, the number is not yet comparable.
Specifying D638 in an RFQ
- Name the standard and edition: ASTM D638, with ISO 527 accepted as an alternative where the market requires it.
- Fix specimen type and thickness: usually a Type I machined or moulded at 3.2 mm.
- Fix the speed and atmosphere: 5 or 50 mm/min as the property dictates, 23 degrees C and 50% RH.
- Require statistics: mean and standard deviation across five specimens minimum, with the test report attached.
Related certification and test references are collected under certification guides.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
