ISO 37 — Tensile Testing of Rubber

Overview

ISO 37 specifies the determination of tensile stress–strain properties of rubber and thermoplastic elastomers. Key outputs include tensile strength, elongation at break, and stress at defined elongations (e.g., M100, M200). Reliable results depend on correct die type, grip choice, strain measurement, and control of test rate and environment.

Tip: For highly extensible materials, prefer video extensometry to avoid contact with the gauge section. Ensure clear marker tracking and adequate lighting.

Industries & material

ISO 37 underpins tensile characterization across a wide range of rubber and elastomer applications. Below are common sectors and typical materials you may encounter, with notes relevant to testing.

Common pitfalls

• Grip breaks and notch effects: Ensure smooth dumbbell edges and correct clamping pressure; reject breaks at or near the grip per ISO rules.
• Slippage at high strains: Use pneumatic grips with compliant faces and verify with ink marks; increase pressure in small steps to avoid cutting.
• Incorrect gauge length: Set and verify initial marker spacing per die type; re-check after mounting as rubber can creep under clamp load.
• Rate mismatches: Program speeds to achieve the specified strain rate range; consider segmented ramps for very extensible materials.
• Extensometer out-of-range: Choose a device with sufficient travel; switch to video extensometry beyond clip-on limits to avoid loss of strain data.
• Environmental drift: Temperature and humidity shift modulus; document conditions and stabilize specimens before testing.

Quick win: Standardize a short pre-test checklist (die type, gauge length, pressure, extensometer range, speed) — it eliminates most re-tests.

Specimens & die types

• Die types: Select per ISO 37 Type 1/2/3/4 based on thickness and application; maintain dimensional tolerances meticulously.
• Conditioning: Stabilize specimens at specified temperature/humidity; record dwell time and actual conditions.
• Surface quality: Smooth edges to avoid notch sensitivity; avoid talc or lubricants in the gauge section.
• Replicates: Test ≥5; more for materials with high variability.

Grip selection & jaw faces

• Pneumatic grips with pressure control help maintain constant clamping during large elongations.
• Jaw faces: rubber-coated or serrated as appropriate; avoid cutting into the specimen ends.
• Use anti-slip markers to detect movement and adjust pressure accordingly.

Instrumentation & extensometry

• Load cell sized for expected forces (operate in 10–90% range).
• Extensometry: video extensometer recommended for high elongations; clip-on options for moderate strains.
• Gauge length: set per die type; verify marker spacing pre-test.

Rates, speeds & gauge length

Program crosshead speeds to achieve the required strain rates. For very extensible rubbers, use segmented speeds or ramps to capture the full curve without losing resolution at low strains.

Quick setup checklist

Procedure

1. Measure specimen dimensions and set gauge length markers.
2. Install grips and set clamping pressure; perform a trial pull to verify no slip.
3. Zero load and strain; start test and record force–elongation continuously.
4. Stop at break; record break location (gauge vs. grip) and necking observations.

Calculations

Reporting

• Die type, dimensions, conditioning, gauge length.
• Grip type, jaw faces, and pressure settings.
• Extensometer type, strain range, sampling rate.
• Results: strength, elongation, modulus points; curves; n, mean, SD; outlier handling.

QA, verification & uncertainty

Verify load cell, extensometer calibration, and rate accuracy at planned intervals. Maintain uncertainty budgets and method locks. Run periodic proficiency checks.

Recommended Testometric setup

Model suggestions for ISO 37

Configurations vary by compound and die type; FITCO can supply a validated ISO 37 kit with grips and extensometry.

FAQs