Chord Modulus

Chord modulus is a key parameter in material testing, providing engineers and quality control teams with a precise measure of a material’s stiffness under defined loading conditions. It is calculated as the slope of a stress-strain curve between two specific points rather than from the very start of deformation. This approach is particularly valuable for materials where the initial portion of the curve is non-linear or where a defined stiffness value is required within a certain strain range.

Alongside measurements such as flexural strength, elastic modulus and tensile strength, chord modulus is a critical performance indicator. It informs design decisions, validates product specifications and supports quality assurance by quantifying how a material resists deformation under load and maintains dimensional stability over time.

Definition and importance

Chord modulus is determined by drawing a straight line, known as the chord, between two points on the stress-strain curve. These points are selected to avoid non-linear behaviour at the start of loading and the onset of yielding later in the curve. The resulting slope gives the average stiffness across that strain range.

This method is particularly useful for plastics, composites and elastomers, which often lack a perfectly linear elastic region. By focusing on a controlled range, engineers can compare materials on a like-for-like basis under consistent testing conditions.

Accurate chord modulus measurements are essential for:

• Design validation - confirming that materials will perform under service loads.
• Material selection - enabling direct comparisons between candidate materials.
• Quality control - ensuring batch consistency in manufacturing.

Applications across industries

• Automotive - assessing stiffness in composite panels and polymer components to ensure crash performance, reduce vibration and maintain dimensional integrity.
• Aerospace - measuring modulus in carbon-fibre laminates where a high stiffness-to-weight ratio is critical.
• Packaging - testing the stiffness of films and boards to ensure packaging retains shape during stacking and transport.
• Electronics - validating that housings, connectors and other parts resist deformation under operational loads.

In all these sectors, chord modulus data supports specification compliance, durability assurance and product optimisation.

Key standards

• ISO 178 - determination of flexural properties for rigid plastics using the three-point bending method.
• ASTM D790 - widely used for flexural modulus testing of reinforced and unreinforced plastics and electrical insulating materials.
• ISO 4624 - pull-off testing for coatings and bonded materials, relevant where modulus is calculated for adhesive layers or coatings.

These standards specify test geometry, loading rates, environmental conditions and calculation methods to ensure results are accurate and comparable across laboratories.

Test types

• Three-point bending - commonly used for plastics, composites and boards, offering a straightforward way to assess flexural stiffness.
• Tensile testing - ideal for metals, polymers and films where direct tensile loading gives a clear modulus value.
• Shear testing - applied to bonded joints and layered composites where interlaminar stiffness is key.

Selecting the correct method depends on the material form, intended application and the requirements of the governing standard.

Carbon-fibre composite panels - in aerospace, three-point bending tests identified the optimal fibre lay-up to maximise stiffness while minimising weight. This improved fuel efficiency without compromising safety margins.

Polymer packaging films - tensile modulus testing across different humidity levels ensured that packaging films retained stiffness during transport and storage, preventing product damage.

Recent innovations include:

• High-resolution digital strain gauges capable of detecting sub-micron displacement changes.
• Custom fixtures designed for irregular specimen shapes or to simulate end-use loading conditions.
• Real-time analysis through VectorPro software, enabling automated modulus calculations to ISO and ASTM standards with full traceability.

These developments improve repeatability, reduce operator influence and speed up data interpretation.

Precision equipment

Mecmesin’s MultiTest-dV and OmniTest ranges offer fine control of test speed, load and displacement. When paired with VectorPro software, they deliver accurate chord modulus values in compliance with ISO and ASTM requirements.

Trusted across industries

Used daily in automotive, aerospace, packaging and electronics sectors, Mecmesin systems are relied on for their accuracy, repeatability and robust construction.

Flexible and adaptable solutions

From standard three-point bending rigs to fully customised fixtures for complex geometries, Mecmesin’s modular systems can be adapted to changing materials, updated standards or new test requirements.

Ongoing support

With accredited calibration, operator training and technical assistance, Mecmesin ensures test systems remain accurate, compliant and productive throughout their service life.

Chord modulus testing is a core technique for evaluating stiffness in a wide range of materials, from advanced composites to everyday packaging films. Whether you are validating a new design, comparing materials or maintaining production quality, accurate testing is essential.

For expert advice on selecting the right chord modulus test method, standards and equipment, contact a Mecmesin specialist to ensure your results are reliable, compliant and fit for purpose.