Plastic Tensile Testing Method Standards: A Complete Technical Guide to ISO 527 and ASTM D638

Plastic tensile testing is the most fundamental method for evaluating the mechanical performance of plastic materials, and it is governed by several internationally recognized standards, including ISO 527, ASTM D638, and GB/T 1040. This technical guide explains the principles, specimen requirements, test procedures, measured parameters, and application scenarios of plastic tensile testing standards, helping engineers and quality professionals select the correct method and interpret results accurately.

Technical Overview

Tensile testing is the most widely performed mechanical test for plastics. In this test, a dumbbell-shaped specimen is gripped at both ends and pulled in uniaxial tension at a constant crosshead speed until yielding or fracture occurs. During the test, the applied force and resulting extension are continuously recorded, allowing key mechanical properties such as tensile strength, yield stress, elongation at break, and tensile modulus to be determined.

Several major standards organizations have published plastic tensile testing standards, and the most commonly applied ones include:

• ISO 527-1 and ISO 527-2: the international standard for determining tensile properties of plastics, covering general principles and test conditions for moulding and extrusion plastics.
• ASTM D638: the American standard for tensile properties of plastics, widely used in North America and in global supply chains.
• GB/T 1040.1 and GB/T 1040.2: the Chinese national standards, which are technically aligned with ISO 527.
• JIS K 7161 and JIS K 7162: the Japanese national standards, adopted from ISO 527-1 and ISO 527-2.
• ISO 527-3 and ASTM D882: supplementary standards covering tensile testing of plastic films and sheets.

Although these standards share the same testing principle, they differ in specimen geometry, test speed, conditioning requirements, and result calculation. Therefore, data obtained under different standards are not directly comparable, and the applicable standard must always be specified in test reports.

Testing Samples

ISO 527 Specimens (Type 1A and Type 1B)

ISO 527-2 specifies two preferred dumbbell specimen types. Type 1A is intended for directly moulded specimens, while Type 1B is used for specimens machined from sheets, plates, or finished products. Both types have a preferred thickness of 4 mm, a gauge length of 50 mm, and a width of 10 mm in the parallel section. The main difference is the length of the parallel section: 80 mm for Type 1A and 60 mm for Type 1B. Smaller specimen types, such as Type 5A and Type 5B, are defined in ISO 527-2 for cases where limited material is available.

ASTM D638 Specimens (Type I to Type V)

ASTM D638 defines five specimen geometries. Type I is the preferred specimen for rigid and semi-rigid plastics, with an overall length of 165 mm, a width of 13 mm in the narrow section, a preferred thickness of 3.2 mm, and a gauge length of 50 mm. Type II and Type III are used for specific material categories and thicker sections, Type IV is suitable for non-rigid plastics or where machining is not possible, and Type V is a reduced-size specimen for material screening or when only small samples are available. ASTM D638 applies to specimens with thicknesses up to 14 mm; for films thinner than 1 mm, ASTM D882 should be used instead.

Specimen Preparation and Conditioning

Specimens may be injection moulded, compression moulded, or machined from finished parts, but the preparation method must be recorded because processing conditions strongly influence tensile results. All specimens must be free of visible defects, such as voids, notches, flash, or machining marks, in the gauge section. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C and 50 % relative humidity in accordance with ISO 291 or ASTM D618. A typical conditioning period is 40 hours or longer, depending on the material and its moisture sensitivity.

Testing Items

Depending on the material behavior, a tensile test conducted according to ISO 527 or ASTM D638 can determine the following parameters:

• Tensile strength: the maximum tensile stress sustained by the specimen before rupture or yielding.
• Tensile stress at yield: the stress at the first point on the stress-strain curve where strain increases without an increase in stress.
• Tensile stress at break: the stress recorded at the moment of specimen rupture.
• Tensile strain at yield: the strain corresponding to the yield point, expressed as a percentage.
• Tensile strain at break (elongation at break): the strain at which the specimen ruptures, expressed as a percentage of the initial gauge length.
• Tensile modulus (Young's modulus): the ratio of stress to strain within the initial linear region of the stress-strain curve, typically determined as a chord modulus between strains of 0.05 % and 0.25 %.
• Poisson's ratio: the ratio of transverse strain to axial strain, which can be determined by ISO 527-1 using transverse deformation measurement.
• Nominal strain and strain at maximum force, where required by the applicable standard.

Brittle materials, such as unreinforced polystyrene or highly filled compounds, usually fail without yielding, while ductile materials, such as polyethylene and polypropylene, exhibit a distinct yield point followed by cold drawing and significant elongation before break.

Testing Methods

Equipment Requirements

The test is performed on a universal testing machine equipped with suitable grips, typically wedge-action or pneumatic grips designed to prevent slippage without inducing premature failure at the grip faces. The force measurement system should comply with ISO 7500-1 or ASTM E4, and the machine class should meet the accuracy requirements of ISO 5893. Strain measurement may be performed by the crosshead displacement, but for accurate modulus determination, a contact extensometer or video extensometer complying with ISO 9513 or ASTM E83 is strongly recommended. The testing machine should also offer an environmental chamber when properties must be measured at elevated or sub-zero temperatures.

Test Procedure

The general test procedure under ISO 527 and ASTM D638 consists of the following steps:

• Measure the width and thickness of the narrow section of each specimen at several points, and record the minimum cross-sectional area for stress calculation.
• Set the initial grip separation according to the specimen type, for example 115 mm for ISO Type 1A and ASTM Type I specimens.
• Mount the specimen symmetrically in the grips, ensuring axial alignment to avoid bending stresses.
• Attach the extensometer to the gauge section if accurate strain measurement is required.
• Select the test speed specified for the material: ISO 527 commonly uses 1 mm/min for modulus determination followed by a material-specific speed such as 50 mm/min or 200 mm/min, while ASTM D638 commonly uses 5 mm/min for modulus and 50 mm/min for strength of rigid plastics.
• Start the test and record the force-extension curve until the specimen yields, breaks, or reaches the specified end condition.
• Repeat the test on at least five valid specimens, and report the arithmetic mean and standard deviation for each parameter.

Data Evaluation

Tensile stress is calculated by dividing the measured force by the original minimum cross-sectional area of the specimen. The yield point is identified as the first point on the curve where the slope becomes zero, and the modulus is calculated as the slope of the chord connecting the stress-strain curve at 0.05 % and 0.25 % strain. If a specimen fails outside the gauge section, the result should be discarded and a retest performed unless the failure is representative of the material behavior. All results must be reported together with the specimen type, preparation method, test speed, conditioning atmosphere, and the specific standard edition used.

Applications

Plastic tensile testing in accordance with ISO 527, ASTM D638, and equivalent national standards is applied throughout the plastics industry value chain:

• Incoming material inspection and quality control of resins, compounds, and masterbatches.
• Material datasheet generation and comparison of candidate materials during design and material selection.
• Research and development of new polymers, blends, composites, and recycled materials.
• Verification of the effect of processing parameters, such as moulding temperature, on mechanical performance.
• Failure analysis and comparison of failed parts against reference material properties.
• Regulatory compliance demonstration and certification testing by accredited third-party laboratories.
• Aging studies, including heat aging, UV weathering, and chemical exposure, where tensile properties are monitored as degradation indicators.

Automotive, electronics, packaging, medical device, construction, and consumer goods industries all rely on standardized tensile data to specify, approve, and qualify plastic materials.

FAQ

What is the difference between ISO 527 and ASTM D638?

Both standards measure the same fundamental properties, but they use different specimen geometries, thicknesses, and test speeds. For example, the preferred ISO 527-2 Type 1A specimen is 4 mm thick with a 10 mm wide parallel section, while the preferred ASTM D638 Type I specimen is 3.2 mm thick with a 13 mm wide narrow section. Because of these differences, tensile results from the two standards are not directly interchangeable, and the required standard should be agreed upon before testing.

Is GB/T 1040 equivalent to ISO 527?

Yes. GB/T 1040.1 and GB/T 1040.2 are the Chinese national standards adopted from ISO 527-1 and ISO 527-2. They specify essentially the same specimen types, test speeds, and calculation methods, so test results obtained under GB/T 1040 are generally consistent with ISO 527 results.

How many specimens are required for a valid tensile test?

Both ISO 527 and ASTM D638 require a minimum of five specimens per material condition. For materials with high variability, or when results are used for certification, more specimens should be tested to improve statistical reliability.

Why is the modulus measured at a very low speed?

The tensile modulus is determined within the initial linear elastic region, at strains between 0.05 % and 0.25 %. A low speed, typically 1 mm/min, is used for this segment to ensure accurate, quasi-static strain measurement before the test speed is increased to determine yield and break properties.

Can films and thin sheets be tested with these standards?

Rigid sheets thicker than approximately 1 mm can be tested with ASTM D638 or ISO 527-2, while plastic films should be tested according to ISO 527-3 or ASTM D882, which define strip specimens and speeds appropriate for very thin materials.

Why do tensile results sometimes scatter even within the same batch?

Scatter can arise from specimen preparation defects, internal voids, molecular orientation from processing, humidity-sensitive materials that have not been fully conditioned, grip slippage, or misalignment. Careful specimen preparation, proper conditioning, and a well-aligned testing setup are essential for reproducible results.