ISO 527-1 & ISO 527-2 Tensile Test on Plastics

ISO 527-1/-2 describes the tensile test for plastic molding and extrusion materials that are thicker than 1 mm. This includes, in particular, thermoplastic and thermosetting molding materials. The test can be performed on specimens that are injection-molded, pressed, or machined.

For other types of plastics or material groups, additional parts of the ISO 527 standard series apply:

• Plastic films and sheets less than 1 mm thick: ISO 527-3
• Fiber-reinforced plastics composites: ISO 527-4 and ISO 527-5

Specimen conditioning is essential for obtaining comparable and reproducible test results to ISO 527-1/-2. Plastics react to temperature and humidity, which can alter characteristic values such as tensile modulus, yield stress, tensile strength, and elongation.

Therefore, the standard requires that specimens in a test series be prepared, conditioned, and tested under identical conditions. This is the only way to ensure that any differences in the results are truly attributable to the material itself, and not to varying environmental conditions.

Therefore, for meaningful material comparisons, conditioning and test conditions should always be consistently followed and documented.

For many test tasks to ISO 527-1/-2, a clip-on extensometer is an economical solution that complies with the standard. ZwickRoell clip-on extensometers are available with gauge lengths of 50 mm and 75 mm. They are particularly well-suited for routine tests, as well as for determining tensile modulus and strain characteristic values.

Clip-on extensometers are particularly well-suited for:

• Quality control applications
• Rigid and medium-rigid thermoplastics
• Materials with moderate elongation
• Standard measurements of tensile modulus, yield strength, and tensile strength

For high strain, varying material classes, or automated test sequences, both sensor arm extensometers and optical systems offer additional advantages. These systems enable automatic strain measurement across wider measurement ranges and meet the high requirements of ISO 527-1/-2 for modulus determination.

The required materials testing machine capacity depends on the material being tested and the expected test loads. However, for tensile tests to ISO 527-1/-2, the maximum test load is not the only decisive factor. Equally important are the measurement accuracy and the force range over which that accuracy is achieved. ISO 527 requires that force measurements comply with the requirements of ISO 7500-1, typically with an accuracy of ±1% of the measured value (class 1).

For most applications to ISO 527-1/-2, the following applies:

• Systems ranging from 1 kN to 5 kN are suitable for materials with lower strength and small specimens.
• 10 kN systems are commonly used for routine testing of plastics.
• Higher capacities may be needed for highly reinforced materials or unusually large cross-sections.

The most important aspect is not just the maximum force, but also choosing a testing machine and a load cell that provide sufficient measurement resolution and accuracy across the expected force range. A properly sized load cell improves measurement quality, especially when determining modulus and testing materials with lower strength.

ASTM D638 and ISO 527 are the most commonly used standards worldwide for tensile testing of plastics. Both standards are used to determine mechanical characteristic values such as tensile modulus, yield strength, tensile strength, and elongation. However, the test results are not directly reproducible, as the test methods and the evaluations differ in several respects.

The most important differences are:

• Specimen geometries and gauge lengths: ASTM D638 uses five standardized specimen types, with type I being the preferred specimen for rigid plastics. To obtain comparable results, ISO 527 primarily recommends specimen types A1 (injection-molded) and A2 (machined). For A1, a gauge length of 75 mm is preferred, while for A2, a gauge length of 50 mm is used. In addition, ISO defines further reduced-size or application-specific specimens.
• Requirements for strain measurement
• Definition of test speed
• Determination of the results: The standards differ in how they determine individual characteristic values and in how they evaluate elongation after the yield point. As a result, the same materials can yield different results when tested to ASTM D638 and ISO 527.

ASTM D638 is primarily used in North America, whereas ISO 527 is more commonly specified internationally. Companies with global markets therefore often maintain testing capabilities for both standards. Results obtained to ASTM D638 and ISO 527 should only be compared with each other if the methodological differences are considered.

ISO 527-3 is used for plastic films and sheets that are less than 1 mm thick. This part of the ISO 527 series addresses the specific requirements for thin films and sheet materials, defining suitable test specimens and adapted test conditions for them.

Alternatively, ASTM D882 is often used for tensile tests on films, especially in North America. However, the results to ASTM D882 and ISO 527-3 are not directly reproducible due to differences in test conditions and evaluation methods.

The ISO 527 series includes dedicated parts specifically for polymer matrix composites. ISO 527-2 specifies the test conditions for molding and extrusion compounds. ISO 527-4 covers the testing of isotropic and orthotropic fiber-reinforced plastics, while ISO 527-5 addresses the testing of unidirectional fiber-reinforced plastics.

Long-fiber-reinforced materials, such as CFRP or GFRP, exhibit different mechanical properties depending on fiber orientation, laminate structure, and manufacturing process. For this reason, special specimen geometries and test conditions are used for these materials, as defined in the relevant parts of the ISO 527 series.

In North America, ASTM D3039 is also frequently used for tensile tests on composite materials. However, the results according to ASTM D3039 and ISO 527-4/-5 are not directly reproducible due to differences in test conditions.

Determining the tensile modulus is one of the most challenging parts of the tensile test to ISO 527-1/-2. The tensile modulus is determined in the linear portion of the stress–strain curve, between 0.05% and 0.25% elongation. The standard prefers the calculation using a regression line, as this method yields statistically more reliable results than evaluating just two individual measuring points.

For measuring the modulus, ISO 527-2 specifies a test speed of 1 mm/min for the preferred specimen types A1 and A2, which corresponds to a strain rate of about 1% per minute.

The accuracy of the strain measurement is especially important. Since, with a gauge length of 75 mm, the measurement covers only a change in length of about 150 µm, or about 100 µm with a gauge length of 50 mm, ISO 527-1/-2 imposes additional requirements on the extensometer. For modulus determination, the change in length must be measured with an accuracy of ±1% or, alternatively, ±1 µm to ±1.5 µm.

Tensile strength is the highest tensile stress a specimen reaches during a tensile test to ISO 527. It is calculated from the maximum force measured, based on the original specimen cross-section.

For many thermoplastics, the maximum stress occurs at the yield point. In this case, the tensile strength is equal to the stress at the yield point. With other materials, the highest stress is only reached just before fracture occurs. The tensile strength is then determined as the maximum stress reached just before the specimen fails.

Poisson's ratio is typically determined through a tensile test to ISO 527. In this process, the longitudinal and transverse strains of the specimen are measured simultaneously and then compared to each other. In addition to characteristic values such as tensile modulus, tensile strength, and yield strength, Poisson's ratio can also be determined.

The width and thickness of the specimen must be precisely determined before testing, since all stress-related characteristic values—such as tensile modulus, tensile stress, and yield stress—are based on the specimen's original cross-section.

Particular attention should be paid to thickness measurement of injection-molded specimens, as sink marks can lead to measurement errors. Even a deviation of just 0.1 mm, with a specimen thickness of 4 mm, can lead to an error of about 2.5%. For this reason, ISO 527-1/-2 recommends thickness measurement with a micrometer.

The specimen width should also be measured at the positions specified in the standard to minimize the effect of draft angles and to obtain representative cross-sectional values.

Strain measurement is one of the most critical factors in testing to ISO 527-1/-2. Especially when determining the tensile modulus, elongation in the range of 0.05% to 0.25% must be measured with very high precision.

Since the measured changes in extension in this range are only about 100 to 150 µm, ISO 527-1/-2 supplements the general requirements for extensometers with additional specifications for modulus determination. Insufficient accuracy in strain measurement can significantly affect the modulus result.

Therefore, for testing to ISO 527-1/-2, only strain measurement systems that meet the required measurement accuracy are suitable—for example, clip-on extensometer, sensor arm extensometers, or optical measuring systems.