Post-Aging Tensile Strength Testing of Materials: Standards, Methods, and Retention Evaluation

Post-aging tensile strength testing determines how thermal, UV, ozone, and chemical exposure affect the mechanical performance of plastics, rubbers, and other polymeric materials. By comparing tensile strength and elongation at break before and after accelerated aging, manufacturers can quantify retention rates, validate long-term durability, and demonstrate compliance with ISO, ASTM, IEC, GB, and UL standards. This article explains the sample types, test items, methods, and typical applications of post-aging tensile strength evaluation performed by accredited third-party laboratories.

Technical Overview

Post-aging tensile strength testing is a two-stage destructive evaluation in which a material is first subjected to controlled aging conditions and then tested in tension to determine how its mechanical properties have changed. Polymeric materials such as thermoplastics, thermosets, elastomers, adhesives, and cable compounds degrade progressively in service due to heat, oxygen, ultraviolet radiation, ozone, humidity, and contact with fluids. The dominant degradation mechanisms include oxidative chain scission, additional crosslinking, loss of plasticizers and antioxidants, and photodegradation of the polymer backbone.

Because these processes develop slowly at room temperature, laboratory evaluation relies on accelerated aging in air ovens, xenon arc or fluorescent UV weathering chambers, ozone cabinets, and immersion tanks. Tensile testing performed before aging (the unaged control set) and after aging (the exposed specimen set) yields the tensile strength retention rate and the percentage change in elongation at break, which are the most widely used indicators of aging resistance. A retention rate close to 100 percent indicates that the formulation, stabilization package, and processing conditions provide adequate durability for the intended service environment.

The evaluation is governed by an interlocking set of international and national standards, including ISO 527 and ASTM D638 for plastic tensile properties, ISO 37 and ASTM D412 for rubber tensile properties, ISO 188, ASTM D573, and GB/T 3512 for heat aging, ISO 4892, ASTM G154, and ASTM G155 for artificial weathering, ISO 1431 and ASTM D1149 for ozone exposure, ASTM D471 for fluid immersion, IEC 60811-501 for cable insulation and sheaths, IEC 60216 for thermal endurance of insulating materials, and UL 746B for long-term properties of polymeric materials.

Testing Samples

Rigid and Semi-Rigid Plastics

Injection-molded or machined multipurpose specimens conforming to ISO 3167, ISO 527-2 specimen types 1A and 1B, or ASTM D638 Type I dumbbells, with a preferred thickness of 4 mm for ISO specimens or 3.2 mm for ASTM specimens. Typical materials include PP, PE, ABS, PA, PC, POM, PET, PBT, PVC compounds, and glass-fiber-reinforced grades.

Rubbers and Thermoplastic Elastomers

Die-cut dumbbell specimens with a nominal thickness of 2 mm, cut from cured sheets or finished products in accordance with ISO 37 (types 1 and 2) or ASTM D412 (dies A to F, commonly die C). Materials include NR, SBR, EPDM, NBR, CR, FKM, silicone rubber, and TPE or TPV compounds used for seals, hoses, and gaskets.

Cable Insulation and Sheathing Compounds

Dumbbell or tubular specimens prepared from the insulation or outer sheath of finished wires and cables, tested per IEC 60811-501 and the corresponding product standards for PVC, XLPE, and halogen-free compounds.

Adhesives, Sealants, Films, and Coated Fabrics

Free films or bulk-cast specimens of structural adhesives and sealants, thin plastic films tested per ISO 527-3, and strip specimens of coated or laminated textiles, geomembranes, and photovoltaic encapsulant films.

Testing Items

Depending on the product specification, a post-aging tensile evaluation program may include the following measured and calculated quantities:

• Tensile strength before aging (unaged control values)
• Tensile strength after aging under each exposure condition
• Tensile strength retention rate (aged value divided by unaged value, expressed as a percentage)
• Percentage change in tensile strength relative to the unaged control
• Elongation at break before and after aging, together with its retention rate or variation
• Tensile stress at defined strain, such as 100 percent elongation, and modulus changes
• Visual assessment of aging effects such as cracking, chalking, blistering, or discoloration

Elongation at break is generally the most sensitive early indicator of degradation, because both chain scission and excessive crosslinking sharply reduce the ability of a material to stretch before failure.

Testing Methods

Stage 1: Accelerated Aging Conditioning

The specimens are exposed to one or more controlled aging environments selected to simulate or accelerate the intended service conditions:

• Thermal aging in a forced-ventilation air oven per ISO 188, ASTM D573, ASTM D865, ASTM D3045, or GB/T 3512, using common conditions such as 70, 85, 100, 125, or 150 degrees Celsius for durations from 24 hours to 1000 hours.
• Artificial weathering in xenon arc or fluorescent UV chambers per ISO 4892-2 and ISO 4892-3, ASTM G155, ASTM G154, or GB/T 16422, with controlled irradiance, black panel temperature, and water spray cycles.
• Ozone aging in an ozone cabinet per ISO 1431-1, ASTM D1149, or GB/T 7762, typically at 50 pphm ozone concentration, 40 degrees Celsius, and 20 percent static elongation.
• Fluid or chemical immersion aging per ASTM D471, using oils, fuels, water, or process chemicals at defined temperatures and durations.
• Humidity or water immersion aging per ISO 62 or ASTM D570 where moisture-driven plasticization is a concern.

Stage 2: Tensile Testing After Aging

After removal from the aging environment, specimens are allowed to rest and are reconditioned at 23 plus or minus 2 degrees Celsius and 50 plus or minus 10 percent relative humidity per ISO 291, ASTM D618, or ISO 23529, typically for 16 hours to 96 hours before testing. Tensile testing is then performed on a calibrated universal testing machine:

• Plastics: ISO 527-1 and ISO 527-2, ASTM D638, or GB/T 1040, with test speeds from 1 mm/min for modulus determination up to 500 mm/min depending on specimen type and material rigidity, and a 50 mm extensometer gauge length.
• Rubbers: ISO 37, ASTM D412, or GB/T 528, typically at a crosshead speed of 500 mm/min, reporting median values of tensile strength and elongation at break.
• Cable compounds: IEC 60811-501, comparing aged and unaged median tensile strength and elongation at break against the variation limits set by the relevant cable product standard.

Stage 3: Data Evaluation and Retention Calculation

The tensile strength retention rate is calculated as the aged tensile strength divided by the unaged tensile strength multiplied by 100 percent, while the percentage variation is the aged value minus the unaged value divided by the unaged value multiplied by 100 percent. Typical acceptance criteria found in material and product specifications include retention rates of at least 75 to 80 percent for tensile strength and elongation at break, or maximum permitted variations of plus or minus 25 to plus or minus 30 percent for cable compounds. For thermal endurance programs per IEC 60216, a criterion such as 50 percent elongation retention is used to derive temperature indices and extrapolated service life through Arrhenius analysis.

Applications

Post-aging tensile strength testing supports material qualification, formulation development, incoming material control, and regulatory compliance across many industries:

• Automotive and mobility: underhood rubber hoses, seals, gaskets, and plastic components exposed to heat, oils, and fuels.
• Wire and cable: qualification of PVC, XLPE, and low-smoke halogen-free insulation and sheathing against IEC, EN, GB, and UL cable standards.
• Construction and infrastructure: PVC profiles, waterproofing membranes, geomembranes, structural sealants, and window gaskets.
• Aerospace and rail: elastomeric seals, vibration isolators, and interior materials requiring documented long-term thermal endurance data.
• Medical devices and packaging: sterilization aging and shelf-life validation of polymers, films, and closures.
• Energy and electronics: photovoltaic backsheets and encapsulants, connector insulators, and plastic housings subjected to UV radiation and heat.

FAQ

Why is tensile strength tested after aging instead of only in the unaged state?

Unaged tensile values only describe the initial quality of a material, while aged values reveal how antioxidants, UV stabilizers, and crosslink systems perform under sustained exposure. Many specifications are written specifically in terms of retention rate or maximum permissible variation, so aging data is essential for qualification.

How are aging temperature and duration selected?

They are chosen from the governing product standard or the material datasheet. Where no specific standard exists, a common practice is to test at several temperatures around the intended service temperature and apply time-temperature superposition or Arrhenius extrapolation per IEC 60216 to estimate service life.

How many specimens are required?

At least five valid dumbbell specimens per condition for plastics per ISO 527 and ASTM D638, and typically three or more per condition for rubber per ISO 37 and ASTM D412, together with an equivalent unaged control set tested at the same time on the same machine.

Why must specimens rest after aging before tensile testing?

A rest and reconditioning period of 16 hours to 96 hours at standard laboratory atmosphere allows residual heat, volatiles, and moisture gradients to equilibrate, so that the tensile results reflect the true aged state of the polymer rather than transient physical effects.

What does a low retention rate indicate?

A tensile strength or elongation retention rate well below the specification limit usually indicates insufficient antioxidant or UV stabilization, excessive crosslinking, oxidative chain scission, or processing defects such as residual stress and poor dispersion of additives.

Can post-aging tensile testing be outsourced to accredited third-party laboratories?

Yes. Accredited third-party laboratories routinely perform aging and tensile evaluation to ISO, IEC, ASTM, EN, GB, and UL standards and issue test reports that are accepted for product certification, export documentation, supplier qualification, and design verification.