The EVA material compression resilience rate test measures the ability of ethylene-vinyl acetate foam to recover its original thickness after being compressed to a defined deflection, making it one of the most important quality-control indicators for cushioning materials. This guide covers the testing principle, applicable international standards, specimen preparation, step-by-step procedures, calculation formulas, and typical applications across the footwear, packaging, and sports protective equipment industries.
Technical Overview
EVA (ethylene-vinyl acetate) is a closed-cell, cross-linked polyolefin copolymer foam widely used in footwear midsoles, insoles, protective padding, and packaging because of its low density, excellent shock absorption, and cost efficiency. In actual service, EVA components are subjected to repeated compression cycles. If the material fails to recover its thickness after each cycle, cushioning performance degrades rapidly, leading to product failure. The compression resilience rate therefore serves as a key durability and performance index.
The compression resilience rate is defined as the percentage of the imposed deformation that the specimen regains after the compressive load is removed. The governing formula is expressed as:
R = [(T1 − T2) ÷ (T0 − T2)] × 100%
- T0 = initial thickness of the specimen before compression
- T2 = thickness of the specimen under maximum compression (for a 50% deflection test, T2 = 0.5 × T0)
- T1 = recovered thickness measured after the load is released and the recovery period has elapsed
A complementary parameter is the compression set (permanent deformation), calculated as CS = [(T0 − T1) ÷ T0] × 100%. The two indicators are inversely related: a lower compression set corresponds to a higher resilience rate and better long-term cushioning stability.
Testing Samples
Standard Laboratory Specimens
Specimens are die-cut or machined from representative sheet stock. Typical dimensions are 50 mm × 50 mm in cross-section with a thickness of 10 mm to 25 mm. The top and bottom faces must be parallel and free of cuts, voids, or collapsed cells. Any molded skin should remain intact unless the test specification requires its removal, since the skin significantly influences recovery behavior.
Finished and Molded Product Samples
Complete products such as footwear midsoles, insoles, and protective pads may be tested either as whole parts between flat platens or as coupons cut from defined locations. Because molded parts exhibit density gradients and skin formation, sampling position must be recorded and kept consistent between comparative tests.
Conditioning Requirements
- Condition specimens at 23 ± 2°C and 50 ± 5% relative humidity for at least 24 hours before testing
- Allow molded parts to outgas and stabilize for at least 24 hours (72 hours preferred) after demolding
- Measure thickness with a gauge applying a standardized foot pressure, and record initial thickness T0 to the nearest 0.1 mm
Testing Items
A complete EVA compression recovery evaluation typically includes the following test items:
- Apparent density, tested per ISO 845, as a reference parameter
- Compression hardness, typically measured with an Asker C durometer
- Static compression set at 50% deflection for 22 hours at 23°C, per ISO 1856 Method A and GB/T 6669
- Static compression set at 50% deflection for 22 hours at 70°C, to simulate accelerated thermal aging
- Compression resilience rate at 50% deflection, derived from the thickness recovery measurement
- Ball rebound resilience, per ASTM D3574 Test H or ASTM D2632
- Dynamic compression fatigue and resilience retention, per ASTM D3574 Tests I and J, for durability qualification
Testing Methods
Static Compression Set and Recovery Test
- Step 1: Measure and record the initial specimen thickness T0 at three points and use the average value
- Step 2: Place the specimen between two flat parallel plates and compress it to 50% of T0 using gauge stops or spacers
- Step 3: Lock the assembly and hold it for 22 ± 1 hours at either 23 ± 2°C or 70 ± 2°C in a forced-air oven
- Step 4: Release the compression quickly and completely, and allow the specimen to recover for 30 minutes (ASTM practice) or 24 ± 1 hours (ISO and GB practice)
- Step 5: Measure the recovered thickness T1 and calculate the compression set and the compression resilience rate using the formulas above
Ball Rebound Resilience Test
A polished steel ball with a mass of 16 g is dropped from a fixed height of 500 mm onto the conditioned specimen. The maximum rebound height is captured photoelectrically or by an equivalent sensor, and the rebound resilience is calculated as R = (h ÷ H) × 100%, where h is the rebound height and H is the drop height. The specimen should measure at least 100 mm × 100 mm with a thickness of 50 mm, or be built up from stacked layers. Three to five impacts are performed at different points and the results are averaged.
Dynamic Compression Fatigue Test
For durability qualification, specimens are subjected to cyclic loading using a constant-force pounding or roller-shear fixture, commonly 75,000 to 80,000 cycles. The residual height loss and the decay of rebound resilience after fatigue are reported, reflecting how the material performs under long-term repeated compression in real use.
Equipment Requirements
- Universal testing machine or dedicated compression set apparatus with parallel platens and accuracy within ±1%
- Forced-air oven with temperature control of ±2°C for elevated-temperature tests
- Digital or dial thickness gauge with a standardized pressure foot
- Ball rebound tester with calibrated drop mechanism and height sensor
- Calibrated spacer blocks, timer, and environmental conditioning chamber
Result Evaluation
Report the mean or median value of at least three specimens. For footwear-grade EVA foam, typical ball rebound resilience values range from 40% to 55%, while premium midsole materials show a compression set below 10% at 50% deflection, which is equivalent to a static compression resilience rate above 80%. Elevated-temperature results at 70°C that differ substantially from room-temperature results indicate insufficient cross-linking or poor cell structure.
Applications
- Footwear industry: quality control of midsoles, insoles, and wedged components where energy return and shape retention directly affect comfort and product lifespan
- Sports protective equipment: helmet liners, shin guards, and knee pads that must maintain impact attenuation after repeated compression
- Protective packaging: reusable cushioning inserts for electronics and precision instruments that must recover after each shipping cycle
- Fitness products: yoga mats, gym flooring tiles, and exercise pads requiring repeated compression recovery
- Automotive and industrial uses: NVH padding, gaskets, expansion joint filler, and flotation components where compression recovery governs sealing and durability
FAQ
What is a good compression resilience rate for EVA foam?
For high-grade footwear midsole foam, a compression set below 10% at 50% deflection, corresponding to a resilience rate above 80% after 24 hours of recovery, is generally regarded as excellent. General-purpose packaging foam commonly accepts a compression set of up to 20%.
Which standards govern this test?
The most frequently applied standards are ASTM D3574 (Test D, E, and H), ISO 1856, ISO 7214 for polyolefin foams, GB/T 6669 for flexible cellular polymeric materials, and ASTM D2632 for vertical rebound resilience.
Why is the test also performed at 70°C?
Elevated-temperature testing accelerates aging and simulates hot storage or transport conditions. Poorly cross-linked or under-blown foam shows a sharply higher compression set at 70°C, revealing weaknesses that room-temperature testing may miss.
How does specimen thickness affect the results?
Thicker specimens contain proportionally more closed cells in the core and generally show slightly better recovery than thin die-cut pieces. Comparisons are only valid when specimen dimensions, skin condition, and density are matched.
How many specimens should be tested?
A minimum of three specimens per condition is required by most standards, and five specimens are recommended to improve statistical reliability. Outliers caused by visible cell damage should be discarded and re-sampled.
Can finished shoes be tested directly?
Yes. Whole midsoles or coupons cut from finished footwear can be tested, but molded skin layers, density gradients, and cemented interfaces must be considered when interpreting the results, as they typically reduce measured recovery compared with flat sheet specimens.
