Five-Point Bend

The five-point bend test is a specialised method for assessing the mechanical performance of structural panels, particularly their resistance to planar shear forces. This method evaluates a panel’s rolling shear properties, which are critical for understanding how it will perform under real-world loads.

In the five-point configuration, the test specimen is supported at three points from beneath, with two load points applied from above. This setup creates a defined shear stress zone within the core of the panel, allowing engineers to measure how materials behave when subjected to bending and shear simultaneously.

The method is widely used in industries where sandwich panels, laminated composites, and bonded structures are common, including aerospace, construction, automotive manufacturing, and marine engineering. By understanding a material’s performance under these loading conditions, engineers can design products with appropriate safety factors and durability.

The ASTM D2718 standard sets out the procedures for conducting planar shear tests on structural panels using the five point bending method. It provides guidance on specimen dimensions, loading rates, support span configurations, and how to interpret data for accuracy and repeatability.

Key areas covered include:

• Specimen preparation – Panel thickness, cutting accuracy, and moisture conditioning requirements.
• Test configuration – Spacing of supports, position of loading points, and fixture alignment.
• Load application – Loading speed, preloading, and continuous data capture requirements.
• Data reporting – Calculating shear stress and modulus, and presenting rolling shear properties in a consistent format.

Compliance with ASTM D2718 ensures that results are consistent, comparable, and suitable for both engineering and regulatory purposes. It is widely recognised for benchmarking planar shear performance in engineered wood products, composite laminates, and sandwich panels.

Specimen preparation

• Cut specimens to the standardised dimensions specified in ASTM D2718.
• Ensure surfaces are smooth and free from defects that could affect results.
• Condition specimens in a controlled environment to stabilise temperature and moisture content.

Fixture setup

• Arrange three lower supports with equal spacing.
• Position two upper loading points midway between the lower supports.
• Check alignment to ensure the load is distributed evenly across the specimen width.

Test execution

• Mount the specimen without inducing pre-stress.
• Apply the load at the specified rate to the two upper points.
• Record force and deflection continuously until failure or the required load limit is reached.

Data capture and analysis

Use high-precision force measurement equipment for accurate readings.

Calculate shear strength and shear modulus using the formulas in ASTM D2718.

Typical test loads range from a few hundred newtons for thin composite laminates to several kilonewtons for thicker structural panels. Span lengths and roller diameters are selected according to specimen thickness and stiffness.

The five-point bend test is an essential tool for quality control and product development in several sectors:

• Construction – Testing planar shear performance of structural insulated panels (SIPs), plywood, and oriented strand board (OSB).
• Aerospace – Measuring core shear properties in honeycomb sandwich panels for fuselage and wing structures.
• Automotive – Assessing bonded composite panels in chassis assemblies and interior components.
• Marine – Ensuring fibre-reinforced polymer panels maintain structural integrity under shear loading.

By verifying shear resistance before deployment, manufacturers reduce the risk of in-service failure and can optimise material selection for both performance and cost.

While the five-point bend test is designed to evaluate planar shear, other bending methods address different mechanical behaviours:

• Three-point bend test – Applies a single central point load, ideal for flexural strength but less suited for isolating shear effects.
• Four-point bend test – Uses two loading points without the central three-support base, offering uniform bending but not optimised for planar shear measurement.

Advantages of five-point bend testing:

• Isolates shear properties within the core of sandwich or laminated panels.
• Reduces concentrated bending stresses compared to three-point bending.
• Provides more representative results for materials subject to combined bending and shear in service.

Accurate results rely on correct setup and consistent procedures:

• Load-deflection analysis – Identify the initial shear failure point and determine modulus values.
• Shear strength – Calculate from peak load values adjusted for specimen geometry.
• Shear modulus – Determine from the slope of the initial linear region of the load-deflection curve.

Best practices for accuracy:

• Use rigid fixtures with precise roller spacing to minimise test variability.
• Calibrate load cells and displacement transducers regularly.
• Maintain consistent environmental conditioning for all specimens.
• Avoid premature failures caused by misalignment or surface flaws.

Mecmesin provides precision testing systems fully compatible with ASTM D2718, offering accurate and repeatable results for planar shear evaluation.

Standards-ready systems

OmniTest and MultiTest-dV frames, combined with high-resolution load cells, ensure full compliance with ASTM requirements.

Customisable fixtures

From small laboratory samples to full-scale production panels, Mecmesin can supply configurable jigs and roller supports tailored to specimen dimensions and load requirements.

Integrated software

VectorPro software allows engineers to:

• Capture force-deflection data in real time.
• Perform automated calculations for shear strength and modulus.
• Generate professional reports for quality control and compliance documentation.

By combining precision mechanics, advanced software, and adaptable fixture design, Mecmesin delivers reliable data for informed engineering decisions.

For advice on implementing five-point bend testing in your laboratory or production process, contact Mecmesin’s technical team. Our specialists can recommend the right equipment, fixtures, and procedures to meet your testing requirements and ensure compliance with ASTM D2718.