Plastic yield strength testing measures the stress at which a plastic material transitions from elastic to plastic deformation, providing critical data for material selection, product design, and quality control. This guide covers the governing standards such as ISO 527 and ASTM D638, specimen requirements, test procedures, and typical industrial applications of yield strength evaluation for polymers.
Technical Overview
Yield strength is one of the most important mechanical properties of plastic materials. It is defined as the stress at which a material begins to deform plastically, meaning that the deformation becomes permanent and will not be recovered when the load is removed. Below the yield point, the material behaves elastically and returns to its original shape upon unloading; above the yield point, molecular chains begin to slip, untangle, or form shear bands and crazes, producing irreversible deformation.
In practice, many ductile thermoplastics exhibit a distinct yield peak on the stress-strain curve, followed by necking and cold drawing before final fracture. For such materials, the yield stress is taken directly as the first local maximum of the curve. Brittle polymers and stiff materials without a clear yield point require alternative determination methods, such as the offset method (commonly 0.1% or 0.2% offset strain) or the tangent intersection method.
Accurate yield strength data allows engineers to set safe design limits, compare material grades, verify supplier specifications, and comply with regulatory requirements across industries such as automotive, construction, packaging, medical devices, and consumer electronics. Accredited third-party laboratories routinely perform these tests under controlled conditions in accordance with ISO, ASTM, EN, GB, and JIS standards.
Testing Samples
Commodity Thermoplastics
Common materials include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and acrylonitrile butadiene styrene (ABS). These ductile polymers typically show a well-defined yield point and are tested using standardized dumbbell specimens.
Engineering Thermoplastics
Materials such as polycarbonate (PC), polyamide (PA, nylon), polyoxymethylene (POM), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT) are widely used in load-bearing applications where yield strength values directly influence structural design margins.
High-Performance and Specialty Polymers
Polymers such as PEEK, PPS, PI, and PTFE, as well as glass-fiber or carbon-fiber reinforced compounds, can be evaluated for yield behavior, although reinforced materials often fracture before yielding and are reported by ultimate tensile strength instead.
Specimen Forms
- Injection-molded or compression-molded dumbbell specimens (ISO 527-2 types 1A and 1B; ASTM D638 types I through V)
- Machined strips and panels cut from finished products for verification testing
- Films and sheets tested using specialized specimens per ISO 527-3
- 3D-printed (additive manufactured) coupons for process validation
Testing Items
Yield strength testing is typically performed as part of a full tensile characterization, which may include the following parameters:
- Tensile stress at yield (yield strength), expressed in MPa
- Tensile strain at yield, expressed as a percentage
- Ultimate tensile strength and tensile stress at break
- Tensile modulus (Young's modulus) measured in the initial linear region
- Offset yield strength, where no distinct yield point exists
- Upper and lower yield stress for materials showing a yield drop
- Necking behavior and elongation at break
Testing Methods
Governing Standards
- ISO 527-1 and ISO 527-2: Determination of tensile properties of plastics
- ASTM D638: Standard test method for tensile properties of plastics
- GB/T 1040 series: Chinese national standard, technically aligned with ISO 527
- JIS K 7113 and related JIS standards for Japanese market requirements
Equipment
Testing is conducted on a calibrated universal testing machine (UTM) equipped with appropriate load cells, pneumatic or mechanical grips, and a contact or video extensometer for accurate strain measurement. Environmental chambers allow testing at elevated or sub-ambient temperatures when specified.
Specimen Conditioning
Prior to testing, specimens are conditioned under standard laboratory atmosphere, typically 23°C ± 2°C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618, for a minimum of 40 hours unless otherwise specified. Hygroscopic materials such as polyamide may require additional drying or moisture conditioning because moisture content significantly affects yield behavior.
Test Procedure
- Measure and record the width, thickness, and gauge length of each specimen at multiple points
- Mount the specimen in the grips, ensuring axial alignment to prevent bending stresses
- Apply the specified crosshead speed: 1 mm/min for modulus determination and 5, 50, or 500 mm/min for yield and strength measurement, depending on specimen type and standard
- Record load and extension continuously until yield, necking, or rupture
- Calculate engineering stress as force divided by the original cross-sectional area
- Identify the yield point as the first stress maximum at which strain increases without a stress increase, or apply the offset method when no distinct peak is present
Data Evaluation
A minimum of five valid specimens is tested per direction and condition, and results are reported as mean values with standard deviation. The stress-strain curve is analyzed to extract yield stress, strain at yield, and related parameters, with all data traceable to calibrated equipment and documented test conditions.
Applications
- Automotive: validating interior trim, bumpers, clips, and under-hood components against crash and vibration loads
- Pipes and fittings: pressure-rated water and gas piping systems where yielding defines long-term service limits
- Packaging: optimizing film and container strength while minimizing material usage
- Medical devices: verifying housing and structural components for sterilization and handling loads
- Electronics: confirming connector, insulator, and enclosure integrity during assembly and use
- Construction: quality control of PVC profiles, geomembranes, and cable sheathing
- Material research and development: comparing new polymer formulations, recyclates, and bio-based grades
FAQ
What is the difference between yield strength and tensile strength?
Yield strength is the stress at which permanent plastic deformation begins, while tensile strength is the maximum stress the specimen withstands before necking or fracture. For ductile plastics, tensile strength may coincide with the yield point; for brittle plastics, fracture may occur before any measurable yield.
Which standard should be chosen: ISO 527 or ASTM D638?
The choice depends on the target market and customer specification. ISO 527 is common in Europe and internationally, while ASTM D638 dominates in North America. Specimen geometry, gauge length, and test speeds differ slightly, so results from the two standards are not directly interchangeable.
How many specimens are required?
Both ISO 527 and ASTM D638 require a minimum of five specimens per material and condition. Additional specimens are tested if scatter is high or if anisotropic behavior must be characterized in different directions.
What factors influence yield strength results?
Temperature, humidity, strain rate, specimen preparation quality, injection molding conditions, and material aging all affect yield strength. Glass-fiber content, filler loading, and polymer molecular weight also have significant effects, which is why conditioning and controlled test parameters are essential.
Can yield strength be tested at low or high temperatures?
Yes. Environmental chambers allow testing from approximately -70°C to +300°C, which is valuable for automotive, aerospace, and outdoor applications. Results must always state the test temperature since polymer yield stress decreases markedly with increasing temperature.
How should results be interpreted for design purposes?
Yield strength values from short-term laboratory tests should be combined with safety factors and, where relevant, long-term creep data, because plastics are viscoelastic and their effective strength decreases under sustained loading. Accredited third-party laboratories can provide guidance on applying test data to real-world service conditions.