ISO 3385 Soft foam impact fatigue test by constant-load pounding

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ISO 3385: Flexible cellular polymeric materials - Determination of fatigue by constant-load pounding.

ISO 3385 specifies a constant-load pounding fatigue test to evaluate thickness loss and hardness loss of flexible cellular materials (primarily polyurethane and rubber latex foams) used in load-bearing upholstery applications. It simulates long-term cyclic loading to assess material durability and service performance.

ISO 3385 Test Principle:

Principle: the repeated indentation of a test piece by an indentor. The area of the indentor is smaller than the area of the test piece. A key requirement is that the maximum load reached during each compression cycle is maintained within specified limits (750 N ± 20 N). This simulates constant-load pounding fatigue.

ISO 3385 Soft foam impact fatigue test by constant-load pounding

The test subjects a foam specimen to repeated, controlled constant-load indentation to simulate real-world dynamic loading (e.g., repeated sitting on furniture/vehicle seats). Key metrics—thickness loss and indentation hardness loss—are measured pre- and post-fatigue cycling to quantify material fatigue resistance. The core principle is that cyclic compression causes cell structure degradation, leading to permanent deformation (thickness loss) and reduced load-bearing capacity (hardness loss).

Testing Method & Apparatus of the ISO 3385:

The standard specifies the use of a pounding test machine. Two types are described:

Machine Type A	A simpler, manually adjusted device. It typically uses a crank drive and a suspended weight mechanism. The operator must continuously adjust the indentor position during the test to maintain the specified 750 N ± 20 N load as the foam softens.
Machine Type B	Fully automated, controlled device. It is driven by electro-mechanical, pneumatic, or hydraulic mechanisms and is equipped with a load cell. It automatically controls and maintains the peak force (750 N ± 20 N) and the loading cycle parameters. Cycle Rate: (70 ± 5) strokes per minute.
Indenter	Diameter of 250 mm ± 1 mm, with a 25 mm radius at the lower edge.
Support Platen	Perforated (6 mm holes, 20 mm pitch) to allow air escape; fully supports the specimen
Loading Cycle	The maximum force (750 N) must be applied for no more than 10% of each cycle's duration. The indentor must be completely unloaded in each cycle.

Test Samples information:

Standard size: Right parallelepiped, 380 ± 20 mm × 380 ± 20 mm × 50 ± 2 mm (slabstock foam), and a thickness of 50 mm ± 2 mm. The standard also allows testing of shaped components if agreed upon by interested parties.

Orientation: Testing should be conducted in the direction the product will be stressed in service. For materials with cell orientation, the direction may be agreed upon by parties.

Conditioning: Material shall not be tested less than 72 hours after manufacture, unless equivalent results can be proven at 16 or 48 hours.

Minimum 16 hours at 23 ± 2 °C and 50 ± 5% RH before testing.

Quantity: Typically 3 specimens per test for statistical validity.

ISO 3385 Soft foam repeat compression by constant-load pounding Test Procedure

1, Initial Measurement: Conduct all testing in a controlled climate. Measure the original thickness (d₁) and the original hardness index (H₁) of the test piece on the same surface, according to ISO 2439, Method A.

2, Machine Setup:

Place the test piece concentrically under the indentor.

Type A: Adjust the stroke to match the test piece thickness (±10%) and the machine to apply the 750 N ± 20 N load. Continuous manual adjustment is needed during the test to maintain this load.

Type B: Set the machine to achieve (70 ± 5) strokes/min. Establish control of the peak load (750 N ± 20 N) and dwell time (≤10% of cycle) within the first 10 cycles. The machine automatically maintains tolerance.

3, Fatigue Cycling: Run the machine for 80,000 continuous load cycles.

4, Recovery: After cycling, remove the test piece and let it rest in an unstrained state for 10 min ± 0.5 min.

5, Final Measurement: Re-measure the thickness (d₂) on the fatigued area. Then, measure the final hardness index (H₂) at (40 ± 1)% indentation. Importantly, this 40% indentation level is calculated using the originalthickness d₁, not the fatigued thickness d₂.

6, Replication: Repeat the entire procedure for the remaining two test pieces.

7. Expression of Results:

Results are calculated for each test piece and the median of the three values is reported.

Loss in thickness, Δd (%): 100 × (d₁ - d₂) / d₁

Loss in hardness, ΔH (N): H₁ - H₂

Percentage hardness loss, ΔH (%): 100 × (H₁ - H₂) / H₁

8, Test Report: Must include reference to the standard, material description, conditioning, sample details, machine type (A or B), and the three median results mentioned above.

Test standard ISO 3385 Applications:

Furniture Industry: Evaluate foam durability for sofas, chairs, and mattresses.

Automotive Sector: Test seat foams for passenger comfort and long-term performance.

Bedding & Packaging: Assess fatigue resistance of flexible foam in load-bearing applications.

Material R&D: Compare fatigue performance of different foam formulations (polyurethane, latex).

Quality Control: Ensure foam products meet durability specifications for load-bearing use.

Related Test Standard:

ISO 1856	Flexible cellular polymeric materials - Determination of compression set
ASTM D3574	Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams. ASTM D3574 I3 is equivalent fatigue testing as ISO 3385.
ISO 3386-1	Polymeric materials, cellular flexible -- Determination of stress-strain characteristics in compression -- Part 1: Low-density materials
ISO 2439	Flexible cellular polymeric materials - Determination of hardness (indentation technique).

Q1: What is the core purpose of ISO 3385?

A: ISO 3385 standardizes a constant-load pounding fatigue test to measure thickness loss and indentation hardness loss of flexible foams after cyclic loading, evaluating their long-term durability under repeated compressive stress.

Q2: Which materials does ISO 3385 apply to?

A: It mainly applies to flexible cellular polymeric materials, especially polyurethane foams and latex foams used in load-bearing applications like furniture, automotive seats, and mattresses.

Q3: Why is ISO 3385 test important for foam materials?

A:Predicts the service life and anti-sagging performance of foams in actual use.

Provides a unified global benchmark for material comparison and quality control.

Helps manufacturers optimize foam formulations and avoid premature product failure.

Meets regulatory and customer requirements for seating and bedding durability.

Q4: Can ISO 3385 replace compression set tests (ISO 3386‑1)?

A: No. ISO 3385 tests dynamic fatigue under repeated loading, while ISO 3386‑1 measures static compression set. They reflect different deformation behaviors and are complementary.

Q5: What happens if a foam fails the ISO 3385 test?

A: High thickness/hardness loss indicates poor fatigue resistance; the foam will sag quickly in seating/bedding, leading to poor comfort, shorter product life, and rejection by buyers or standards.

Q6: How do the results relate to real-world performance?

A: The standard is explicit (Clause 1, Scope): The measured losses are related to, but are not necessarily the same as, the losses likely to occur in actual service. It is a comparative laboratory assessment, not a direct prediction of a product's exact lifespan. A foam with lower thickness and hardness loss in this test is generally expected to be more durable in real-world cushioning applications.

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