High-Temperature Tensile Testing of Plastics: Standards, Methods, and Applications

High-temperature tensile testing of plastics measures tensile strength, modulus, and elongation behavior under controlled elevated-temperature conditions that simulate real service environments. Performed in accordance with ISO 527 and ASTM D638 using universal testing machines equipped with environmental chambers, this test is critical for automotive, aerospace, electronics, and appliance applications where plastic components must retain mechanical integrity above ambient temperature. This article explains the test principle, specimen requirements, measured parameters, test procedure, and typical applications of elevated-temperature tensile testing.

Technical Overview

High-temperature tensile testing is a fundamental mechanical test used to determine how plastic materials behave when subjected to uniaxial tensile loading at elevated temperatures. Because polymeric materials are highly sensitive to temperature—exhibiting reduced strength, lower modulus, and increased ductility as temperature rises—room-temperature data alone cannot reliably predict performance in thermal service environments such as engine compartments, electrical enclosures, or industrial processing equipment.

The test is typically performed to ISO 527-1 (general principles), ISO 527-2 (test conditions for moulding and extrusion plastics), and ASTM D638 (Standard Test Method for Tensile Properties of Plastics), with the Chinese national standard GB/T 1040 series being technically equivalent to ISO 527. A universal testing machine (UTM) is fitted with an environmental chamber capable of maintaining a stable elevated temperature, and a calibrated non-contact extensometer (such as a video extensometer) is used to measure strain accurately inside the chamber.

Typical temperature capability ranges from ambient up to 250°C to 300°C, covering the service conditions of engineering thermoplastics such as PA, PBT, PC, POM, PPS, PEEK, and high-temperature thermosets. Common test temperatures include 40°C, 60°C, 80°C, 100°C, 120°C, 150°C, 180°C, 200°C, and 250°C, selected according to the intended application or product specification.

Testing Samples

Rigid and Semi-Rigid Moulding and Extrusion Plastics

For ISO 527-2, the preferred specimens are the 1A and 1B multipurpose dumbbell specimens, with a preferred thickness of 4 mm. The 1A specimen is normally injection moulded directly, while the 1B specimen may be machined from sheets, laminates, or finished parts. For ASTM D638, rigid and semi-rigid plastics are tested using Type I, II, III, IV, or V dumbbell specimens, with Type I being the most common for materials with a thickness of 7 mm or less and Type V used for limited-material situations or small parts.

Films and Sheets

Thin plastic films are tested in accordance with ISO 527-3 or ASTM D882 using rectangular strip specimens cut from the film web. High-temperature testing of films is particularly relevant for packaging, capacitor films, insulation films, and flexible printed circuit substrates that experience thermal exposure during processing or service.

Specimens Machined from Finished Products

When it is not possible to produce moulded specimens, dumbbell specimens may be machined from finished components such as housings, brackets, connectors, or pipes. The machining must be performed carefully to avoid introducing residual stresses, notches, or heat damage that could distort the test results. Specimen orientation relative to the moulding flow direction must be recorded, since anisotropy significantly affects tensile properties.

Specimen Conditioning and Number of Specimens

Prior to testing, specimens are conditioned in accordance with ISO 291 or ASTM D618 (typically 23°C and 50% relative humidity for standard atmospheres) unless otherwise specified. At minimum, five valid results are required to characterize a single material at a single temperature, and a greater number (for example, ten) is recommended when the scatter of the data is high. For hygroscopic materials such as polyamides, the moisture state (dry as moulded or conditioned to equilibrium) must be controlled and reported, because moisture content strongly interacts with temperature effects.

Testing Items

High-temperature tensile testing at a specified elevated temperature typically yields the following measurable parameters:

• Tensile strength at break (MPa) at the specified test temperature
• Tensile strength at yield (MPa) at the specified test temperature
• Tensile stress at 50% strain or at other specified strain values
• Tensile strain at yield (%)
• Nominal tensile strain at break (%)
• Tensile modulus, also known as Young's modulus or elastic modulus (MPa), measured between strain limits of 0.0005 and 0.0025 as defined in ISO 527-1
• Secant modulus and chord modulus as required by ASTM D638
• Complete stress-strain curve and failure mode description (brittle fracture, ductile fracture, necking)
• Poisson's ratio, when required and when suitable biaxial strain measurement is available

By repeating the test at several temperatures, a strength-versus-temperature curve can be constructed, providing engineers with design data across the full thermal operating window of the material.

Testing Methods

Equipment and Setup

The core instrument is a screw-driven or servo-hydraulic universal testing machine with a force capacity appropriate to the material (typically 1 kN to 50 kN), equipped with a temperature-controlled environmental chamber mounted around the grips and gauge length region. Forced-air circulation inside the chamber ensures temperature uniformity, and the chamber temperature is verified with a calibrated reference sensor positioned near the specimen. Strain measurement inside the chamber is best performed with a non-contact video extensometer viewing the specimen through an inspection window, thereby avoiding thermal expansion errors and mechanical interference associated with clip-on extensometers.

Temperature Conditioning and Soak Time

Once the chamber reaches the set point, the specimen is held at the test temperature for a sufficient soak period so that its entire cross section reaches thermal equilibrium. Common practice, consistent with the guidance of ISO 527 and related procedures, is a soak time of at least 5 minutes per millimetre of thickness, with a minimum hold of approximately 10 minutes and a typical maximum exposure time of 30 minutes to avoid thermal ageing effects during conditioning. For materials that absorb moisture or oxidize at elevated temperature, exposure time before loading must be minimized and documented.

Test Speed and Loading Procedure

Test speeds follow ISO 527 or ASTM D638 requirements. For modulus determination, a low speed of 1 mm/min is prescribed. For strength and elongation measurement of rigid moulding plastics, speeds of 5 mm/min, 50 mm/min, or 200 mm/min are commonly applied depending on the specimen type and standard clause selected. The specimen is loaded in tension until rupture, and force and extension data are continuously recorded at a sampling rate sufficient to capture yield and strain-softening behavior. Testing is repeated for each specimen at the same temperature, and invalid results (for example, failure within the grips) are discarded and replaced.

Data Reduction and Reporting

The measured force-extension data are converted into engineering stress and strain using the original specimen cross-sectional dimensions. The test report includes the material identification, specimen type and preparation method, conditioning history, test temperature and soak time, test speed, individual and mean values of all measured tensile properties, the stress-strain curves, the failure mode, and the standard references (ISO 527-1/-2/-3, ASTM D638, or GB/T 1040). It is recommended that testing be carried out in accredited third-party laboratories with traceably calibrated force, extension, and temperature measurement systems to ensure data validity and acceptance by downstream customers.

Applications

Elevated-temperature tensile data for plastics are used across many industries in which thermal exposure governs component design and safety:

• Automotive: under-hood components, air intake manifolds, coolant system parts, sensor housings, connectors, battery module components, and interior parts near heat sources
• Electrical and electronics: connectors, coil bobbins, switch housings, lamp holders, LED lighting supports, and insulation parts operating above ambient temperature
• Home appliances and HVAC: kettle bodies, microwave components, hair dryer housings, fan impellers, and blower parts
• Aerospace and rail: interior panels, ducting, brackets, and structural clips exposed to hot ambient conditions
• Industrial equipment: gears, bearings, seals, pump components, and pipeline linings in hot fluid or gas service
• Materials research and development: comparison of resin grades, filler and reinforcement effects, thermal ageing studies, heat-deflection correlation, and generation of material database values for finite element simulation
• Quality control: incoming resin inspection, lot release testing, and validation of process changes such as moulding condition shifts

FAQ

What temperature range can be covered in high-temperature tensile testing?

Standard environmental chambers used with universal testing machines typically operate from approximately ambient up to 250°C or 300°C. Common engineering thermoplastics such as PA, PBT, PC, PPO, PPS, and PEEK can be evaluated across this range. For temperatures approaching or exceeding 300°C, specialized furnaces and high-temperature extensometry are required.

How long must the specimen be held at the test temperature before loading?

A widely applied practice is a soak time of at least 5 minutes per millimetre of specimen thickness, with a minimum of about 10 minutes, ensuring that the gauge region reaches thermal equilibrium without undergoing significant thermal ageing. The actual hold time used should always be stated in the test report.

What is the difference between ISO 527 and ASTM D638 specimens?

ISO 527-2 uses the 1A/1B multipurpose dumbbell specimens with a preferred 4 mm thickness, while ASTM D638 defines Type I through Type V specimens with different geometries and preferred thicknesses (Type I is 3.2 mm nominal). Because gauge lengths and cross-sections differ, tensile results from the two standards are not directly interchangeable, and the specification used must be selected according to the customer or product requirement. GB/T 1040 is the Chinese national standard aligned with ISO 527.

Why is a non-contact extensometer preferred inside the chamber?

Clip-on mechanical extensometers conduct heat away from the specimen, are affected by their own thermal expansion, and can be damaged at high temperatures. A video extensometer measures strain optically through the chamber window without contacting the specimen, providing accurate modulus and strain-at-break data while eliminating thermal errors.

Can high-temperature tensile testing be combined with other thermal evaluations?

Yes. Elevated-temperature tensile testing is frequently combined with heat deflection temperature (HDT) testing per ISO 75 or ASTM D648, Vicat softening point testing per ISO 306 or ASTM D1525, and thermal ageing studies, giving a complete picture of a plastic material's short-term and long-term mechanical performance under heat.