ASTM D638 – Tensile Properties of Plastics

Overview & scope

ASTM D638 is the primary American standard for determining the tensile properties of unreinforced and reinforced plastics using standard dumbbell ("dogbone") specimens. It is maintained by ASTM Committee D20 on Plastics and is the default tensile reference for compounders, moulders, masterbatch producers, and NABL/ISO 17025-accredited laboratories across India and worldwide.

The standard reports tensile strength, tensile modulus, elongation at yield and break, and yield strength for rigid and semi-rigid plastics tested under controlled speed and environment.

Tip: ASTM D638 is intended for rigid and semi-rigid plastics. For thin films and sheeting under 1 mm, use ASTM D882 instead — the specimen geometry and gripping approach differ significantly.

What does it measure?

Tensile strength at yield and at break (MPa), tensile modulus of elasticity (MPa or GPa), percent elongation at yield and at break, and Poisson's ratio when biaxial extensometry is used. It does not measure flexural or impact behaviour — see ASTM D790 and ASTM D256 respectively.

Sample materials

Polypropylene (PP), polyethylene (HDPE/LDPE), ABS, polycarbonate (PC), nylon (PA6, PA66), POM, PET, PBT, glass-fibre reinforced compounds, and engineering thermoplastics — including recycled and compounded grades for QC.

Latest edition

Refer to the current ASTM D638 revision on astm.org and confirm the edition cited in your customer specification, as conditioning and speed references have been updated across revisions.

Industries & applications

Specimen types & geometry

ASTM D638 defines five specimen types (I–V). Type I is the default for rigid plastics ≥ 7 mm wide and ≤ 14 mm thick and should be used unless material thickness or availability requires another type.

Tip: Always state the specimen type in the report — modulus and elongation values from Type I and Type IV specimens of the same material are not directly comparable.

Specimen preparation

1. Prepare specimens by injection moulding to the cavity dimensions of the chosen type, or machine them from sheet with smooth, defect-free edges.
2. Inspect for sink marks, voids, flash, and weld lines in the gauge section — discard any specimen with visible defects.
3. Condition at 23°C ± 2°C and 50% ± 5% RH for a minimum of 40 hours per ASTM D618 unless the material specification states otherwise.
4. Measure width and thickness of the narrow section at three points with a calibrated micrometer (≤ 0.01 mm); use the minimum cross-sectional area for stress.
5. Mark the gauge length for extensometer placement, avoiding scoring the specimen surface.

Grip selection

• Wedge action or pneumatic grips with serrated or rubber-faced jaws suit most rigid plastics; match jaw inserts to specimen thickness.
• Align the specimen on the load axis to avoid bending stresses that cause premature shoulder failures.
• For reinforced or high-strength compounds, use higher-capacity grips and confirm no slippage with pre-marked tabs.
• Discard results where the specimen breaks inside the grip or at the tab radius rather than the gauge section.

Instrumentation & extensometry

• Extensometer is mandatory for modulus. Crosshead displacement alone is not acceptable for tensile modulus reporting — machine and grip compliance distort low-strain data.
• Use a clip-on contact extensometer (e.g. 50 mm gauge for Type I) or a non-contact video extensometer for delicate or notch-sensitive materials.
• Select a load cell so peak force falls between 10% and 90% of rated capacity for best resolution.
• Sample at ≥ 20 Hz; increase for brittle materials that fail abruptly.

Test procedure

1. Record temperature, humidity, and specimen identification for the batch.
2. Verify current load cell and extensometer calibration; zero both channels.
3. Mount the specimen squarely in both grips, aligned to the load axis without pre-stress.
4. Attach or enable the extensometer at the marked gauge length.
5. Run at the specified crosshead speed and acquire the full force–extension curve to break.
6. Reject any specimen failing in the grip or tab radius and repeat on a fresh specimen.
7. Export tensile strength, modulus, yield, and elongation with the curve for traceability.

Calculations & outputs

Example: a Type I specimen with width 13.0 mm and thickness 3.2 mm has A₀ = 41.6 mm². At a peak force of 2,080 N, tensile strength = 2080 / 41.6 = 50 MPa.

Crosshead speed guidance

• 5 mm/min — standard speed for rigid Type I and Type II specimens.
• 50 mm/min and 500 mm/min — used for more ductile materials or where the specification requires faster rates.
• As specified — always follow the governing material or OEM specification and document the speed in every report.

Tip: Plastics are strain-rate sensitive — reporting a result without the test speed makes it impossible to compare across labs.

Reporting requirements

• Material identification, grade, lot/batch number, and moulding or machining method.
• Specimen type, individual dimensions, cross-sectional area, and number tested (with any rejected specimens and reasons).
• Conditioning, test temperature and humidity, crosshead speed, grip type, and extensometer type/gauge length.
• Results with statistics: tensile strength, modulus, yield strength, elongation (mean, SD, CV%).

ASTM D638 vs ISO 527 — key differences

Both standards measure tensile properties of plastics but use different specimen geometries and speeds, so results are not directly numerically comparable. Select the standard cited by your customer or target market.

Tip: If a specification simply says "tensile test," confirm ASTM D638 or ISO 527 before moulding specimens — the dogbone geometries differ and archived data will not be comparable.

Recommended Testometric setup

Model suggestions for plastics

Specifications vary by configuration; contact FITCO India for a tuned method and accessories aligned to ASTM D638.

FAQs