Determination of Yield Stress in Plastics: Test Methods, Standards, and Applications

Yield stress determination is one of the most fundamental mechanical tests for plastics, defining the stress level at which a material begins to deform permanently under tensile loading. This article explains how plastic yield stress is measured in accordance with ISO 527, ASTM D638, and GB/T 1040 standards, covering specimen preparation, test equipment, calculation methods, and practical applications in quality control and product design.

Technical Overview

Yield stress (tensile stress at yield) is the stress at which a plastic material first departs from elastic behavior and begins to deform plastically. On a stress-strain curve, the yield point is defined as the first local stress maximum, where the strain continues to increase without any further increase in stress. Ductile thermoplastics such as polypropylene, polyethylene, and ABS typically display a distinct yield peak followed by cold drawing, while brittle materials such as general-purpose polystyrene or unfilled thermosets may fracture before any measurable yielding occurs.

The determination of yield stress is performed by uniaxial tensile testing using a universal testing machine. The principal international and national standards governing this test include ISO 527-1 and ISO 527-2, ASTM D638, and the Chinese national standards GB/T 1040.1 and GB/T 1040.2. The measured yield stress is a key input for material datasheets, design allowables, quality control, and material comparison, and it is sensitive to temperature, humidity, strain rate, specimen preparation method, and molecular orientation. Accredited third-party laboratories routinely perform this test as part of material qualification and lot-release programs.

Testing Samples

Thermoplastic Materials

Ductile and semi-rigid thermoplastics are the primary candidates for yield stress determination, including polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA), polycarbonate (PC), polyamide (PA), polyoxymethylene (POM), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT). Both unfilled resins and glass- or mineral-filled compounds can be evaluated, although filled systems may exhibit reduced ductility or no visible yield point.

Thermosetting and Reinforced Compounds

Thermosets such as phenolics, epoxies, and unsaturated polyesters, as well as fiber-reinforced molding compounds, may also be tested. Because these materials are frequently brittle, the reported result is often the stress at break rather than the stress at yield, and this distinction must be clearly documented in the test report.

Specimen Geometry and Preparation

Specimens are typically injection-molded or machined dumbbell-shaped (dogbone) test pieces. Under ISO 527-2, the preferred specimen types are 1A and 1B, both derived from the multipurpose test specimen specified in ISO 3167. Under ASTM D638, Type I specimens are the preferred geometry for rigid plastics, with Type II through Type V used for specific thickness ranges. For sheet and film products, rectangular strip specimens per ISO 527-1 or the applicable ASTM clauses may be used. ISO 527-1 recommends testing at least five specimens for each condition, and specimens must be free of voids, notches, flash, and machining marks that could induce premature failure.

Testing Items

A standard tensile test campaign for yield behavior typically reports the following parameters:

• Tensile stress at yield (MPa), calculated at the first stress maximum on the stress-strain curve
• Tensile strain at yield (%), the strain corresponding to the yield point, normally measured with an extensometer
• Tensile stress at break (MPa), reported when the specimen fractures before yielding
• Tensile strain at break (%), indicating overall ductility
• Tensile modulus (MPa), measured at a separate low crosshead speed of 1 mm/min as specified in ISO 527-1
• Statistical evaluation of results, including median, mean, and standard deviation across all valid specimens

Testing Methods

Conditioning and Test Atmosphere

Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618, typically for a minimum of 88 hours for hygroscopic materials such as polyamide. Hygroscopic resins may additionally be tested in the dry-as-molded state if required by the material specification, since absorbed moisture can significantly lower the measured yield stress.

Equipment and Calibration

Testing is conducted on a calibrated universal testing machine equipped with suitable grips that prevent slippage and stress concentration at the jaws. The force measurement system must comply with ISO 7500-1 or ASTM E4, and strain measurement is performed either by a contact or optical extensometer meeting the accuracy classes of ISO 9513 or ASTM E83. Crosshead speed must be controlled within the tolerance defined by the governing standard.

Test Procedure

The width and thickness of the narrow parallel section of each specimen are measured at a minimum of three locations, and the minimum cross-sectional area is used for stress calculation. The specimen is mounted in the grips with the gauge length set as specified, a small preload is applied to remove slack, and the crosshead is driven at the speed specified by the relevant material standard. Common speeds for yield stress determination are 5 mm/min, 50 mm/min, or 200 mm/min depending on the material class, while the modulus is determined at 1 mm/min. Force and extension data are recorded continuously to generate the stress-strain curve.

Determination and Calculation of Yield Stress

Yield stress is defined as the first stress maximum on the stress-strain curve at which an increase in strain occurs without an increase in stress. It is calculated by dividing the force at the yield point by the original minimum cross-sectional area of the specimen. If the curve exhibits a plateau rather than a distinct peak, the stress at the midpoint of the plateau may be reported. If the specimen breaks with a strain of less than approximately 10 % before yielding, the material is classified as brittle and the tensile stress at break is reported instead. All invalid results, such as failures within the grips, are discarded and retested.

Applications

Yield stress data are used throughout the plastics value chain for a wide range of engineering and commercial purposes:

• Incoming inspection and lot release of raw resin and compounded materials against supplier datasheets
• Generation and verification of material property data for design calculations and finite element analysis
• Research and development, including evaluation of new formulations, additives, fillers, and processing parameters
• Assessment of aging, annealing, sterilization, and environmental exposure effects on mechanical performance
• Failure analysis of cracked or deformed plastic components in service
• Regulatory compliance and specification conformance for automotive, packaging, medical device, electronics, and construction applications

FAQ

What is the difference between yield stress and tensile strength?

Yield stress is the stress at the first plastic deformation point, while tensile strength is the maximum stress sustained during the entire test. For ductile plastics, the yield stress and tensile strength often coincide; for materials that strain-harden or draw, the maximum stress may occur after yield, and for brittle materials the stress at break is the only meaningful value.

Should I choose ISO 527 or ASTM D638?

Both standards measure tensile properties using comparable principles, but they differ in specimen geometry, preferred crosshead speeds, and conditioning details, so results are not directly interchangeable. The choice is usually dictated by the specification, the end market, or customer requirements; European and international clients typically reference ISO 527, while North American specifications commonly cite ASTM D638. The Chinese standard GB/T 1040 is technically aligned with ISO 527.

How many specimens are required?

ISO 527-1 requires a minimum of five valid specimens per condition, and ASTM D638 similarly recommends testing at least five specimens for isotropic materials. Additional specimens may be needed for anisotropic materials or when testing in both flow and transverse directions.

Why do yield stress results vary between laboratories?

Variations typically arise from differences in specimen preparation (injection molding versus machining), conditioning state, crosshead speed, grip alignment, and strain measurement technique. Using an accredited third-party laboratory with calibrated equipment and standardized specimen production minimizes inter-laboratory scatter.

Can yield stress be determined at other temperatures?

Yes. Both ISO 527 and ASTM D638 permit testing at alternative temperatures using an environmental chamber, which is essential for applications involving cryogenic, elevated-temperature, or thermal-cycling service conditions. Because the yield stress of thermoplastics decreases markedly with temperature, the test temperature must always be reported with the result.