NBR Cork Rubber Tensile Stress at Given Elongation Testing: Complete Technical Guide

NBR cork rubber tensile stress at given elongation testing measures the modulus characteristics of nitrile cork rubber composites at a specified extension, typically 100% elongation. This technical guide explains the material, applicable standards such as ISO 37 and ASTM D412, specimen preparation, test procedures, calculation methods, and typical property values for quality control and certification purposes.

Technical Overview

NBR cork rubber is an engineered elastomeric composite in which granulated cork particles are dispersed and chemically bonded within a nitrile butadiene rubber (NBR) matrix. The material combines the excellent oil and fuel resistance of nitrile rubber with the compressibility, resilience, and sealing conformability of cork, making it a preferred material for gaskets, seals, and vibration-control components.

Tensile stress at given elongation, commonly designated as M100, M200, or M300 depending on the specified extension, is the stress required to stretch a test piece to that elongation, calculated on the original cross-sectional area of the specimen. It is a fundamental modulus parameter that reflects the stiffness of the vulcanizate under tensile loading.

For NBR cork rubber, this parameter is particularly meaningful for several reasons:

• The cork phase physically restricts the extensibility of the rubber network, so the stress response at a defined extension is highly sensitive to cork loading and dispersion quality.
• The measured value directly reflects the state of cure, indicating whether the material is under-cured, properly vulcanized, or over-cured.
• It provides a reliable quality-control index for batch-to-batch consistency in compounding and molding processes.
• The relatively low ultimate elongation of cork-filled compounds makes the 100% elongation point the most practical and reproducible reference for specification purposes.

Testing Samples

Standard Laboratory Dumbbell Specimens

Dumbbell specimens are the standard test pieces for tensile stress at given elongation testing. They are die-cut from flat vulcanized sheets with a nominal thickness of 2.0 mm ± 0.2 mm. Common specimen configurations include ISO 37 Type 1 and Type 2 dumbbells and ASTM D412 Die C specimens, all of which feature a narrow central section with a benchmark gauge length of 20 mm to 25 mm. Type 2 dumbbells with a 4 mm wide narrow section are frequently selected when the available sheet material is limited.

Ring Specimens

Small ring test pieces may be used for materials supplied in tubular or ring form. Ring specimens are stretched between two mandrels or spindles on the testing machine, and the tensile stress at given elongation is calculated using the original cross-sectional area of the ring. Results from ring specimens should not be directly compared with dumbbell results, as stress concentration at the contact points can produce different measured values.

Finished Product Specimens

When testing is performed for acceptance or failure analysis, specimens may be die-cut directly from finished gaskets, sheets, or molded parts. The sampling location and orientation relative to calendering or grain direction must be recorded, since cork rubber can exhibit slight anisotropy. Specimens containing surface texture, fabric imprint, or obvious voids should be excluded or documented separately.

Specimen Conditioning

All test pieces shall be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity prior to testing. A minimum interval of 16 hours between vulcanization and testing is required to allow the material to stabilize. Specimens must be free from mechanical damage, and any test piece showing die tears or edge defects at the narrow section shall be discarded.

Testing Items

The core testing program for NBR cork rubber tensile stress evaluation typically includes the following items:

• Tensile stress at 100% elongation (M100), the primary specification index for cork rubber compounds.
• Tensile stress at 200% elongation (M200), where applicable for high-rubber-content grades.
• Tensile stress at 300% elongation (M300), only when the elongation at break of the compound permits.
• Tensile strength (TS), the maximum stress recorded during the test.
• Elongation at break (Eb), the extension at the moment of rupture.
• Optional supporting tests such as hardness, compression set, and density to provide a complete material characterization.

Tensile Stress at 100% Elongation

M100 is the most widely specified parameter for NBR cork rubber because many cork-filled grades exhibit an elongation at break in the range of 100% to 250%. The 100% elongation point therefore provides a stable, reproducible measurement that lies comfortably within the usable deformation range of the material while remaining sensitive to cure state and formulation changes.

Tensile Stress at Higher Elongations

M200 and M300 determinations are appropriate for NBR-rich formulations or low cork-loading grades with sufficient extensibility. If the specimen breaks before reaching the specified elongation, the tensile stress at that elongation shall be reported as not achievable, and the test report shall state the actual elongation at break.

Testing Methods

Applicable Standards

Tensile stress at given elongation testing of NBR cork rubber is performed in accordance with the following internationally recognized standards:

• ISO 37:2017, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties.
• ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension.
• GB/T 528, the Chinese national standard, technically aligned with ISO 37.
• JIS K 6251, the Japanese industrial standard for tensile testing of rubber.
• ISO 23529, covering general requirements for sample preparation and conditioning.
• ISO 5893, specifying force and elongation measurement accuracy classes for the testing machine.

Specimen Preparation

Specimens are cut using a sharp, properly maintained dumbbell die in a clicker press or with a rotary cutting device. Dull dies introduce micro-tears at the specimen edges and produce artificially low and scattered results, which is especially critical for cork composites where cork particles already act as stress concentrators. The width of the narrow section is measured with a traveling microscope or calibrated width gauge, and thickness is measured at three points along the narrow section using a dial thickness gauge with a flat foot and approximately 1% accuracy. The median of the thickness readings is used in the cross-sectional area calculation.

Equipment Requirements

Testing is conducted on a calibrated universal tensile testing machine meeting ISO 5893 Class 1 force accuracy, equipped with pneumatic or self-tightening grips that prevent slippage without crushing the specimen. Elongation is measured using a non-contact video extensometer, an automatic mark-tracking system, or a contact extensometer attached to the gauge marks. The standard crosshead speed for ISO 37 and ASTM D412 standard dumbbells is 500 mm/min ± 50 mm/min, and the same speed must be used for all comparative testing.

Test Procedure

The typical test sequence is as follows:

• Measure and record the width and thickness of the specimen narrow section, and calculate the original cross-sectional area.
• Apply gauge marks at the specified benchmark distance on the narrow section of the dumbbell.
• Mount the specimen symmetrically in the grips with the gauge marks centered between them.
• Apply a slight initial preload just sufficient to remove slack, then zero the extension measurement.
• Start the test at 500 mm/min and continuously record the force-extension curve.
• Capture the force value at the specified elongation, for example the force at 100% extension of the original gauge length.
• Continue extension until rupture and record the maximum force and elongation at break.
• Repeat the test on a minimum of three specimens, preferably five, and discard any result from a specimen that breaks outside the narrow section.

Calculation and Reporting

The tensile stress at given elongation is calculated as the recorded force at the specified extension divided by the original cross-sectional area of the specimen: S = F / (w × t), where F is the force in newtons, w is the width, and t is the thickness in millimeters, giving the result in megapascals. In accordance with ISO 37, the median value of the individual results is reported, and individual values are also listed when required by the specification. The test report shall identify the standard used, specimen type, thickness, test speed, temperature and humidity, the specified elongation basis, and any deviations or anomalies observed during testing.

Applications

Tensile stress at given elongation testing of NBR cork rubber supports a wide range of industrial and engineering applications:

• Transformer and electrical equipment gaskets, where controlled stiffness ensures long-term sealing under bolt compression.
• Automotive gaskets for oil pans, valve covers, timing covers, and fuel systems that require combined oil resistance and conformability.
• Industrial flange and pipe sealing materials for oil, fuel, and coolant service.
• Anti-vibration pads, machine mounts, and damping sheets for shock and noise attenuation.
• Friction and clutch facing components that use cork rubber as a compliant functional layer.
• Incoming material inspection and supplier qualification for gasket fabricators and OEM supply chains.
• Compound development and cure-system optimization during formulation research.
• Type approval, PPAP documentation, and third-party certification testing performed by ISO/IEC 17025 accredited laboratories.

FAQ

What is tensile stress at given elongation?

It is the stress, calculated on the original cross-sectional area, that is required to stretch a rubber test piece to a specified elongation such as 100%, 200%, or 300%. It is a modulus-type property that characterizes the stiffness of the vulcanized compound at a defined strain level.

Why is M100 the primary index for NBR cork rubber?

Because the cork particles embedded in the NBR matrix limit chain extensibility, many cork rubber grades break at elongations between roughly 100% and 250%. The 100% elongation point therefore falls reliably within the deformation range of nearly all grades, providing a reproducible and specification-friendly index of compound stiffness and cure state.

What are typical values for NBR cork rubber?

Indicative ranges for commercial NBR cork rubber sheets are a tensile strength of approximately 2.5 to 7 MPa, an elongation at break of approximately 100% to 250%, and an M100 commonly between 1.5 and 4.5 MPa. Actual values depend strongly on cork loading, particle size, NBR acrylonitrile content, and the cure system, so the certified values for the specific grade should always be confirmed with the manufacturer.

Which standards govern the test?

The principal standards are ISO 37, ASTM D412, GB/T 528, and JIS K 6251, with conditioning and machine requirements covered by ISO 23529 and ISO 5893. These standards are broadly technically equivalent, but specimen geometry and reporting conventions differ slightly, so the specific standard used must always be stated in the test report.

What factors can affect the test results?

Key influencing factors include cork particle size and loading level, dispersion uniformity, cure state, specimen die sharpness, internal voids and porosity, thickness variation, calendering direction, test temperature, and crosshead speed. Consistent preparation and strictly controlled test conditions are essential for meaningful comparisons between batches or suppliers.

How many specimens should be tested?

A minimum of three valid specimens is required, and testing five specimens is recommended for better statistical reliability. Results from specimens breaking outside the narrow parallel section are invalid, and the median value is reported in accordance with ISO 37.