ASTM D638 Tensile Testing for Plastics

Information on the most widely used ASTM standards within the materials testing industry

Descriptions of the most prominent lSO standards within the materials testing industry

Standards including EN, DIN, JlS, and more

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ASTM D638 : Standard Test Method for Tensile Properties of Plastics

ASTM D638 determining the tensile properties of unreinforced and reinforced plastics using dumbbell-shaped (dogbone) specimens tested under closely controlled conditions of conditioning, temperature, humidity, and crosshead speed. For measuring the tensile mechanical properties of unreinforced and reinforced plastics, including thermoplastics, thermosets, molded plastics, and plastic composites.

It is the go-to tensile test for rigid and semi-rigid plastics in the form of molded, machined, or die-cut flat specimens, covering thicknesses up to 14 mm (0.55 in.). For films thinner than 1.0 mm, ASTM D882 is preferred instead.

Test Principle and test methods:

This test applies uniaxial tensile force to a standard dumbbell-shaped plastic specimen at a constant crosshead speed under controlled temperature and humidity. As the tensile load increases, the specimen undergoes elastic deformation first, then plastic deformation, and finally fractures. During the test, the system records real-time load and elongation data to plot a stress-strain curve. Key tensile indicators are calculated based on this curve and the specimen’s original cross-sectional area. Additionally, the standard provides an optional procedure to measure Poisson’s ratio at room temperature, defined as the absolute value of the ratio of transverse strain to axial strain within the material’s linear elastic range under uniaxial tension.

A critical note: Plastics are highly sensitive to strain rate, temperature and specimen preparation. The constant crosshead movement may create a deviation between crosshead speed and actual strain rate on the specimen’s gauge section, which is acknowledged in the standard and must be considered for data application.

The load and extension (strain) are recorded continuously to produce a stress–strain curve.

From this curve, key tensile properties are extracted:

Tensile strength (at yield and/or at break)

Tensile stress at yield / break

Elongation at yield / break (or nominal strain at break)

Modulus of elasticity (Young's modulus) from the initial linear region

Optionally Poisson's ratio.

Test Specimens (Critical Detail)

Type	When Used	Key Dimensions (reduced section)
Type I (Preferred)	Material ≤ 7 mm thick; sufficient material available	W (narrow width) = 13 mm; L (narrow length) = 57 mm; gauge length = 50 mm; overall length ≥ 165 mm
Type II	Material won't break in narrow section with Type I	W = 6 mm; same L = 57 mm
Type III	Materials > 7 mm up to 14 mm thick	W = 19 mm; overall length ≥ 246 mm
Type IV	Non-rigid plastics ≤ 4 mm; or to compare rigid ↔ non-rigid	W = 6 mm; L = 33 mm; gauge = 25 mm; overall ≥ 115 mm
Type V	Limited material ≤ 4 mm; or many specimens in limited space (environmental exposure racks)	W = 3.18 mm; L = 9.53 mm; gauge = 7.62 mm; very small!
Key specimen rules	Thickness tolerance for molded specimens: nominally 3.2 ± 0.4 mm where possible; machined-from-sheet keeps the sheet's native thickness (up to 14 mm max). Materials > 14 mm must be machined down to 14 mm before cutting Type III specimens. Surfaces must be free of visible flaws, scratches, or machining marks — finish sanding parallel to the long axis. Gauge marks: only wax crayon or India ink — never scratch, punch, or impress marks. Minimum 5 specimens per sample for isotropic materials; for anisotropic materials, 5 parallel + 5 perpendicular to the anisotropy direction.

ASTM D638 Plastic tensile test equipment required:

Universal Tensile Testing Machine	Constant-rate-of-crosshead-movement tensile tester, composed of a fixed member, a movable crosshead, drive mechanism and load indicator. The total elastic longitudinal strain of the machine’s fixed/movable parts shall not exceed 1% of the specimen’s gauge-length strain under full load.
Grips	Two types are allowed: fixed grips and self-aligning grips, both designed to prevent specimen slippage and ensure the specimen’s long axis aligns with the tensile direction. Serrated grip surfaces (similar to coarse files) are recommended for most thermoplastics; finer serrations apply to rigid thermosets. Anti-slip accessories (abrasive cloth, 80# double-sided abrasive paper) can be used if slippage or grip-area fracture occurs.
Extensometer (Critical for Strain & Modulus Testing)	Elastic Modulus Test: B-2 class extensometer, maximum strain error ≤ 0.0002 mm/mm, with automatic continuous recording. Low Elongation (≤20%) & Yield Elongation: At least Class C extensometer (fixed strain error ≥ 0.001 strain or ±1.0% of indicated strain, whichever is larger). High Elongation (>20%): Measurement error shall not exceed ±10% of the measured value. Poisson’s Ratio Test: Dual-axis (biaxial) extensometer or paired axial + transverse extensometers, capable of synchronous strain recording with ≤1% measurement error.
Dimensional Measuring Tools	Micrometers complying with ASTM D5947 (Physical Dimensions of Solid Plastics Specimens) to measure specimen width and thickness to the nearest 0.025 mm.

Test Parameters & Speeds

Specimen Category	Speeds Available	Notes
Type I, II, III (rigid/semirigid)	5 mm/min (±25%) → nominal strain rate ≈ 0.1/min	Default low-speed option
	50 mm/min (±10%) → ≈ 1/min	Most common
	500 mm/min (±10%) → ≈ 10/min	High-speed screening
Type IV (nonrigid)	5, 50, 500 mm/min	Same pattern
Type V (tiny specimen)	1, 10, 100 mm/min	Scaled for miniature geometry

Selection rule: If the material spec doesn't dictate speed, choose the lowest speed that still produces rupture within 0.5–5 minutes.

For Poisson’s Ratio: Apply a preload less than 5 N at 0.1 mm/min to eliminate specimen bending, then reset extensometers to zero before formal testing.

Detailed Test Procedures of ASTM D638 Tensile Testing for Plastics:

1, Dimensional Measurement: Measure the width and thickness of each specimen at the gauge section center and 5 mm from both gauge ends to the nearest 0.025 mm. For tubes/rods, measure outer/inner diameters at two positions 90° apart.

2, Specimen Mounting: Clamp the specimen into grips, ensuring perfect alignment between the specimen’s long axis and tensile direction. Tighten grips firmly to avoid slippage but not to crush the specimen.

3, Extensometer Installation: Attach the extensometer to the specimen’s full gauge length. For modulus and Poisson’s ratio tests, install qualified high-precision extensometers.

4, Parameter Setup: Set the crosshead speed per requirements, start the testing machine.

5, Data Recording: Continuously record the load-elongation curve, and capture load/elongation values at yield point (if exists) and break point.

6, Post-test Handling: Remove fractured specimens. For highly extensible materials, separate tests for modulus and break properties may be needed to prevent extensometer damage.

7, Retest: Perform retests for invalid specimens (fractured outside gauge section, slippage, etc.).

8. Data Calculation Indexes:

Tensile Strength at Yield / Break: Maximum tensile load divided by the original average cross-sectional area of the gauge section (unit: Pa or psi).

Percent Elongation at Yield / Break: (Gauge length elongation ÷ original gauge length) × 100%.

Nominal Strain: Calculated via crosshead displacement (only for qualitative reference when necking occurs).

Elastic Modulus (Young’s Modulus): Slope of the linear segment of the stress-strain curve (stress difference ÷ corresponding strain difference).

Secant Modulus: Used for materials without an obvious linear elastic region (stress at specified strain ÷ specified strain).

Poisson’s Ratio: Absolute value of (transverse strain change ÷ axial strain change) within the linear range (per Annex A3).

Statistics: Calculate arithmetic mean and estimated standard deviation for each group of test data.

Industry / Application Fields

Sector	Typical Use
Plastic resin suppliers / compounders	Material certification, datasheet generation, R&D formulation
Automotive	Interior/exterior trim plastics (PP, ABS, PC/ABS, nylon), under-hood semi-rigid components — tensile strength & modulus go into FEA models
Consumer goods / appliances	Housing plastics — QA/QC against material specs
Medical devices	Polymers (polycarbonate, acrylic, PE, PP) — biocompatibility-qualified resins need mechanical verification
Electronics & enclosures	UL/corporate material specs often reference ASTM D638 for modulus & strength
Pipe / profile / construction	Reinforced plastics, fittings, panels (though pipe itself may use other ASTM tubular specs)
Aerospace (non-primary structure)	Thermoplastics & composites characterization (though primary composites ASTM D3039)
Packaging (rigid)	Cups, caps, closures — the rigidside; films go to ASTM D882

Related Standard:

ISO 527	Plastics — Determination of tensile properties
JIS K 7161	Plastics -- Determination of tensile properties
GOST 11262	Plastics. Tensile test method
DIN 53455	Testing of plastics; Tensile test
KS M 3006	Determination of tensile properties of plastics
GBT 1040.1	Plastics—Determination of tensile properties—Part 1: General principles
ASTM D412	Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension
ASTM D882	Standard Test Method for Tensile Properties of Thin Plastic Sheeting
ASTM D3039	Tensile of polymer-matrix composites with high-modulus continuous/discontinuous fibers (>20 GPa) — the composite
ASTM D651	STANDARD TEST METHOD FOR TENSILE STRENGTH OF MOLDED ELECTRICAL INSULATING MATERIALS
ASTM E132	Poisson's ratio at room temperature (parallel methodology)

A single column tensile tester is a sophisticated material testing instrument designed for measuring mechanical properties of various materials under tension, compression, bending, shear, and other loading conditions. As the name suggests, it features a single vertical column supporting a moving crosshead that applies force to test specimens.

Tensile test fixture for plastics is based on the test standatd ASTM D638, determination of the tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature, speed.

High Elongation Extensometer is mainly used together with the tensile test machine for the rubber tension testing. It can measure the elongation of the product with high precision and accurately capture the displacement data. With a deformation measurement range typically from 10mm to 800mm.

Video extensometer, is a non-contact real-time high-precision strain measurement system, which is based on a separate camera and real-time image processing algorithm, by taking images of the experimental process, analyzing image characteristic changes, dynamically measuring the strain changes.

Q1: What exactly does ASTM D638 measure?

A: It determines the tensile properties of plastics—tensile strength (at yield and/or break), elongation, and modulus of elasticity—using a constant-rate-of-crosshead-movement tensile test on dumbbell-shaped specimens.

Q2: Why is ASTM D638 so important in plastics engineering?

A: It provides the foundational mechanical data used in:

Material datasheets (resin suppliers)

Engineering design and FEA modeling

Quality control and incoming inspection

Regulatory and procurement specifications (including U.S. DoD approval)

Without D638, “tensile strength” numbers from different labs would not be comparable.

Q3: What main tensile properties can be obtained from ASTM D638 tests?

A: The standard covers both mandatory and optional test indicators:

Mandatory: Tensile strength at yield, tensile strength at break, percent elongation at yield/break, nominal strain at break, elastic modulus (Young’s modulus) and secant modulus (for non-linear materials).

Optional: Poisson’s ratio at room temperature (tested via biaxial extensometer).

All results need to be reported with average values and standard deviations.

Q4: What is "toe compensation"? Why is it mandatory in ASTM D638?

A: The "toe region" refers to the initial curved segment of the stress-strain curve, caused by test system slack and specimen seating, rather than the material’s true performance. Toe compensation extrapolates the linear elastic segment to correct the zero-strain point on the curve. It is mandatory for calculating accurate elastic modulus, strain and offset yield strength; ignoring this step will result in large calculation errors.

Q5: Can I machine specimens from thick sheets?

A: Yes. Materials > 14 mm must be machined to 14 ± 0.4 mm for Type III specimens. Machine equally from both surfaces if the original thickness is between 14–51 mm.

Q6: Does specimen preparation affect results?

A: Yes—significantly. Injection-molded specimens can show different properties than machined ones due to molecular orientation. For comparative tests, all specimens must be prepared identically.

Q7: Can I use crosshead movement to calculate elongation?

A: No. The standard explicitly forbids this for modulus or elongation calculations. You must use a calibrated extensometer on the gauge length. Crosshead movement is only acceptable for nominal strain(qualitative, e.g., at break when necking occurs).

Q8: Can I use D638 data for long-term creep or impact design?

A: No. The standard warns that data from this short-term test cannot be extrapolated to very slow (creep) or very fast (impact) loading conditions without additional testing.

Q9: Why does my polypropylene show huge variation in elongation at break?

A: This is normal. Unreinforced polyolefins (PP, PE) show high variability due to inconsistent necking/drawing. The standard recommends using yield properties (strength, elongation at yield) for specifications instead of break properties.

Q10: How does ASTM D638 relate to ISO 527?

A: Both test tensile properties of plastics, but they differ in technical content (specimen geometries, speeds, calculations). Data are not directly interchangeable.

Q11: What is the difference between percent elongation and nominal strain? When to use each?

A: 1. Percent elongation: Calculated from extensometer data (gauge length deformation), valid only when the specimen deforms uniformly (no necking). It is the preferred index for engineering design. 2. Nominal strain: Calculated from crosshead displacement. When obvious necking occurs on the specimen (most ductile plastics), percent elongation loses reference value, and nominal strain is used for qualitative description only.

Q12: Why do tensile test results of the same plastic vary greatly in different laboratories?

A: Plastics are highly sensitive to multiple factors: 1. Inconsistent specimen preparation (machining, molding, cutting direction). 2. Different test speeds, temperature and humidity conditions. 3. Poor equipment calibration or specimen misalignment. 4. Different grip types and clamping methods. For comparable results, all test conditions and specimen preparation processes must be strictly unified.

Q13: When do we need to use secant modulus instead of elastic modulus?

A: Elastic modulus is calculated from the linear segment of the stress-strain curve. For plastics with no obvious linear elastic region (many flexible or modified plastics), secant modulus (stress at a specified strain divided by the corresponding strain) shall be adopted for stiffness characterization as required by the standard.

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