Damp heat tensile force determination measures the tensile strength, breaking force, and elongation of rubber materials after exposure to high-temperature, high-humidity environments that simulate humid service conditions. This test is essential for evaluating the hydrothermal aging resistance of rubber seals, hoses, cable sheaths, and elastomeric components in automotive, electrical, and construction applications. This article explains the technical principles, sample requirements, test items, measurement methods, and typical applications of rubber damp heat tensile force testing.
Technical Overview
Rubber damp heat tensile force determination is a combined environmental-and-mechanical test in which rubber test pieces are first conditioned in a controlled humid-thermal (damp heat) atmosphere and subsequently tested in uniaxial tension to quantify how moisture and heat affect load-bearing capability. Elevated temperature accelerates thermal aging reactions such as chain scission, crosslink network changes, reversion, and post-curing, while absorbed water can plasticize the elastomer, swell the polymer matrix, weaken filler–rubber interfaces, and promote hydrolysis of susceptible linkages. The net effect is a measurable change in tensile force at break, tensile strength, and elongation properties.
The tension measurement itself is normally performed in accordance with ISO 37, ASTM D412, GB/T 528, or JIS K 6251, while the humid-thermal preconditioning is carried out according to damp heat environmental test procedures such as IEC 60068-2-78 (steady-state damp heat), IEC 60068-2-30 (cyclic damp heat), GB/T 2423.3, and GB/T 2423.4. General requirements for the conditioning and preparation of test pieces follow ISO 23529 or GB/T 2941. By comparing tensile results before and after exposure, engineers and accredited third-party laboratories can express performance retention as a percentage change, providing a quantitative index of hydrothermal aging resistance.
Testing Samples
Standard Dumbbell Specimens
Dumbbell-shaped test pieces are die-cut from vulcanized or thermoplastic rubber sheets of standard thickness, typically 2.0 mm ± 0.2 mm. ISO 37 and GB/T 528 define dumbbell Types 1 through 4, each with specified width of the narrow parallel section, gauge length, and overall dimensions. Thickness is measured at three points within the effective test section using a calibrated thickness gauge, and the median value is used to compute the cross-sectional area. Edges must be smooth and free of nicks or cracks, since stress concentrations cause premature failure and scatter in tensile force readings.
Ring Specimens
Ring test pieces, including actual O-rings, are tested in accordance with ASTM D412 or the ring methods referenced in ISO 37. Ring specimens are particularly relevant for sealing products because they are loaded in a geometry that reproduces service conditions. The initial internal circumference and cross-sectional diameter are measured so that tensile force and elongation can be calculated correctly.
Specimens from Finished Products
Where practicable, test pieces are cut directly from finished goods such as door seals, hose linings, cable jackets, weather strips, and molded gaskets. Product-derived specimens must be prepared with the loading direction aligned to the principal stress direction of the product, and any deviation from standard dimensions must be documented because non-standard geometry limits comparability with data from standard dumbbells.
Testing Items
The principal quantities determined in a damp heat tensile force test program are:
- Tensile strength (MPa), calculated from maximum force divided by original cross-sectional area
- Maximum tensile force and force at break (N)
- Elongation at break (%)
- Tensile stress at specified elongation, commonly 100% and 300% (secant modulus)
- Percentage change in tensile force and tensile strength after damp heat exposure
- Performance retention rate and pass/fail classification against product specification limits
Testing Methods
Specimen Preparation and Standard Conditioning
Test pieces are die-cut with a sharp, lubricated cutting die, inspected for defects, and conditioned in the standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 16 hours, or as specified in ISO 23529 / GB/T 2941. A set of unexposed control specimens is retained for baseline tensile measurement.
Damp Heat Exposure
The exposure specimens are placed in a temperature-humidity chamber set to a steady-state damp heat condition, most commonly 40 °C ± 2 °C with 93% ± 3% relative humidity per IEC 60068-2-78 / GB/T 2423.3. More severe conditions, such as 85 °C and 85% relative humidity, are used for accelerated evaluation of components for tropical or automotive-underhood service. Cyclic damp heat testing, in which temperature cycles between 25 °C and 55 °C at high humidity with condensation phases, follows IEC 60068-2-30 / GB/T 2423.4. Common exposure durations are 24 h, 48 h, 96 h, 168 h, 240 h, 500 h, and 1000 h, selected according to the product specification.
Tensile Force Measurement
After exposure, specimens are allowed to stabilize in the standard atmosphere for a defined recovery period, then tested on a calibrated universal testing machine equipped with appropriate grips and a load cell matched to the expected force range. Dumbbell Type 1 specimens are tested at a crosshead speed of 500 mm/min in accordance with ISO 37 / GB/T 528 / ASTM D412, unless a different speed is specified. The complete force-versus-elongation curve is recorded to capture maximum force, force at break, and elongation at break; non-contact or contact extensometers may be used to improve elongation accuracy.
Result Evaluation
The tensile force and strength of the exposed specimens are compared with the unexposed control values, and the percentage change is calculated as the difference between the post-exposure value and the initial value divided by the initial value, multiplied by 100. Retention is reported as the ratio of post-exposure to initial performance. Results are assessed against the acceptance criteria of the relevant product standard, and any cracking, sticky surfaces, blistering, or discoloration observed after exposure is documented as supplementary evidence of hydrothermal degradation.
Applications
Damp heat tensile force determination is widely applied across industries where rubber components must retain mechanical integrity in hot, humid environments:
- Automotive engineering: door and window seals, coolant and radiator hoses, engine mounts, bushings, and underbody components exposed to tropical climates
- Wire and cable: insulation and sheathing compounds qualified for long-term humid service and stability testing referenced in electrical product standards
- Electronics and electrical equipment: enclosure gaskets, sealing rings, and connector seals for IP-rated and outdoor installations
- Construction: window and door profiles, waterproofing membranes, expansion joint fillers, and bridge bearing pads
- Consumer, medical, and industrial goods: gloves, elastic bands, tubing, diaphragms, and molded seals subjected to sterilization or tropical storage
- Rail transit, marine, aerospace, and defense programs requiring humid-thermal environmental qualification of elastomeric parts
FAQ
What does damp heat tensile force testing evaluate?
It evaluates the ability of a rubber material to retain its tensile force capacity, tensile strength, and elongation after prolonged exposure to combined heat and humidity, which is a direct indicator of hydrothermal aging resistance in service.
Which standards govern the test?
Tension testing is performed to ISO 37, ASTM D412, GB/T 528, or JIS K 6251, while the damp heat preconditioning follows IEC 60068-2-78, IEC 60068-2-30, GB/T 2423.3, or GB/T 2423.4. Specimen conditioning follows ISO 23529 or GB/T 2941.
What are the most common exposure conditions?
The standard steady-state condition is 40 °C and 93% relative humidity; 85 °C and 85% relative humidity is a widely used accelerated condition, and cyclic damp heat between 25 °C and 55 °C with condensation is used when condensation cycles are relevant to the application.
How are the results reported?
Results are reported as tensile strength, force at break, and elongation at break of exposed and control specimens, together with the percentage change or retention rate relative to the unexposed baseline, evaluated against the specification limits of the product.
Can finished products be tested instead of molded sheets?
Yes. Specimens cut from finished products are accepted when the geometry permits, provided the cutting direction, dimensions, and origin are documented; however, data from non-standard product specimens should be interpreted with care when compared with results from standard dumbbell test pieces.