Plastic tensile yield strength testing determines the stress at which a plastic material begins to deform plastically, making it one of the most fundamental indicators of polymer mechanical performance. Performed in accordance with internationally recognized standards such as ISO 527 and ASTM D638, this test supports material qualification, quality control, and structural design across a wide range of industries. This article explains the technical principles, specimen requirements, test procedures, calculation methods, and typical applications of tensile yield strength testing for plastics.
Technical Overview
Tensile yield strength (σy) is defined as the first stress on the stress–strain curve at which an increase in strain occurs without an increase in stress. In practical terms, it marks the transition from elastic (recoverable) deformation to plastic (permanent) deformation. For most ductile thermoplastics, the yield point appears as a distinct peak or plateau on the force–extension curve, often accompanied by visible necking of the specimen.
Yield strength is a critical design parameter because plastic components loaded beyond this point will not return to their original shape, which can lead to dimensional instability, loss of preload in fastened assemblies, or functional failure. Ductile materials such as polypropylene (PP), polyethylene (PE), polyamide (PA), polycarbonate (PC), ABS, and rigid PVC typically exhibit a well-defined yield point. Brittle materials such as polystyrene (PS), PMMA, and many unfilled thermosets often fracture before yielding, in which case tensile strength at break is reported instead.
The test is conducted using a universal testing machine in accordance with recognized standards including ISO 527-1 and ISO 527-2, ASTM D638, GB/T 1040, and JIS K 7161/7162. Accredited third-party laboratories operating under ISO/IEC 17025 management systems commonly perform this test to generate reportable, legally and commercially recognized data for suppliers, buyers, and regulatory bodies.
Testing Samples
Moulding and Extrusion Plastics (ISO 527-2 Type 1A/1B; ASTM D638 Type I)
The standard specimen for rigid thermoplastics is a dumbbell-shaped (dog-bone) test piece. The ISO 527-2 Type 1A specimen measures 170 mm in overall length with a 10 mm wide, 4 mm thick gauge section, a 50 mm gauge length, and 115 mm grip separation; the Type 1B specimen has the same overall dimensions and is preferred for machined samples. The ASTM D638 Type I specimen measures 165 mm in overall length with a 13 mm narrow section, a preferred thickness of 3.2 mm, a 50 mm gauge length, and 115 mm grip separation.
Small and Machined Specimens
When the available material is limited or specimens must be machined from finished parts, scaled-down geometries such as ISO 527-2 Types 1BA and 1BB or ASTM D638 Type V may be used. Because results depend on specimen geometry, data obtained from small specimens must be clearly identified as such in test reports.
Thin Films
Plastic films with a thickness typically below 1 mm are tested as rectangular strip specimens in accordance with ISO 527-3 or ASTM D882, which define specimen widths, gauge lengths, and test speeds appropriate to flexible film materials.
Fibre-Reinforced Plastics
For isotropic fibre-reinforced compounds, ISO 527-4 applies, while unidirectional fibre-reinforced materials are tested under ISO 527-5. Reinforced specimens are frequently tabbed or machined with special care to avoid premature failure at the grips.
Testing Items
Depending on the material behavior and the client's requirements, the following tensile properties are typically determined and reported:
- Tensile stress at yield (σy), expressed in MPa
- Tensile strain at yield (εy), expressed as a percentage
- Tensile modulus (Et), the elastic stiffness in the initial region of the curve
- Tensile strength at break (σB) and tensile strain at break (εB)
- Nominal tensile strain, based on crosshead displacement or grip separation
- Upper and lower yield stress for materials exhibiting a pronounced yield drop
Testing Methods
Specimen Conditioning
Before testing, specimens are conditioned in a standard atmosphere, typically 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291, or per ASTM D618 Procedure A (commonly 40 hours) unless a material standard specifies otherwise. Conditioning duration is extended for hygroscopic materials such as polyamides, for which moisture content strongly affects yield behavior.
Apparatus and Calibration
Testing is performed on a calibrated universal testing machine equipped with a suitable load cell (verified per ISO 7500-1 or ASTM E4), appropriate wedge-action or pneumatic grips, and an extensometer for direct strain measurement. For modulus determination, ISO 527-1 requires an extensometer of at least class 1 accuracy per ISO 9513; non-contact video extensometers are widely accepted and help eliminate influence on thin or soft specimens.
Test Procedure
- Measure the width and thickness of the gauge section of each specimen with calibrated instruments and calculate the initial cross-sectional area.
- Set the grip separation and gauge length according to the specimen type and standard.
- Mount the specimen axially in the grips, applying a small preload to remove slack without inducing strain.
- Apply the specified constant crosshead speed and continuously record the force–extension curve to fracture or to the point where the required properties have been determined.
- Identify the yield point as the first peak or plateau where strain increases without an increase in stress, and calculate the corresponding stress and strain.
Test Speeds
Typical test speeds are 1 mm/min for modulus determination under ISO 527 and 5 mm/min for yield strength determination of Type 1A/1B specimens; ASTM D638 Type I specimens are commonly tested at 5 mm/min (0.2 in/min). Because plastics are viscoelastic, the specified speed must be strictly controlled and reported, since higher strain rates generally produce higher measured yield stress.
Calculation and Result Evaluation
Yield strength is calculated as σy = Fy / A0, where Fy is the force at the yield point and A0 is the original cross-sectional area of the gauge section. When a material does not exhibit a distinct yield point, an offset yield strength may be determined, for example at 0.2 % offset under ASTM D638 or as a proof stress at a specified strain under ISO 527. Both ISO 527 and ASTM D638 require a minimum of five valid specimens; breaks occurring outside the gauge section or at grips are normally rejected and retested, and the mean, standard deviation, and individual values are reported.
Applications
Tensile yield strength data are used throughout the plastics value chain, from raw material development to end-product verification. Typical applications include:
- Incoming resin inspection and lot-to-lot quality control for compounders and moulders
- Material datasheet generation and grade comparison during material selection
- Automotive interior and exterior components, where permanent deformation must be avoided
- Packaging materials, including films, sheets, and PET container preforms
- Pipes, fittings, and pressure-rated thermoplastic components for water and gas infrastructure
- Consumer electronics housings, appliances, and structural brackets
- Medical device components requiring documented mechanical compliance
- Verification of recycled and reprocessed plastics against virgin-material specifications
- Failure analysis, where comparing the yield strength of a failed part with reference data helps identify degradation, processing defects, or material substitution
FAQ
What is the difference between tensile yield strength and tensile strength at break?
Yield strength is the stress at which plastic deformation begins, while tensile strength at break is the stress at the moment of fracture. In ductile plastics the yield stress is usually the highest point of the curve and governs practical design limits; in brittle plastics that fracture without yielding, only the strength at break is reported.
What happens if the specimen breaks before yielding?
If a material fractures in the elastic region, no yield strength can be determined. The test report should state that the material failed in a brittle manner and report the tensile stress and strain at break together with a description of the fracture behavior.
Are ISO 527 and ASTM D638 results directly comparable?
No. The two standards use different specimen geometries, thicknesses, and in some cases speeds and modulus definitions, so results are not interchangeable. When reporting or comparing data, the specific standard and specimen type must always be stated.
How many specimens are required for a valid test?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per condition. Additional specimens are recommended when the material is anisotropic, when testing in different directions, or when statistical confidence intervals are needed.
Which factors most influence the measured yield strength?
Temperature, humidity, conditioning state (especially for polyamides), test speed, specimen preparation method (moulded versus machined), molecular orientation, and internal stresses all measurably affect results. Standardized conditioning and strict adherence to the specified parameters are therefore essential for reproducible data.
Can the test be performed at elevated or sub-ambient temperatures?
Yes. ISO 527-1 and ASTM D638 both permit testing at other temperatures when an environmental chamber is used, provided the specimen temperature is verified and stabilized before loading. Since yield strength of thermoplastics decreases significantly with increasing temperature, the test temperature must always accompany the reported result.