Double Lap Shear

Double lap shear testing is a mechanical method used to measure the shear strength of adhesive bonds. The test uses a layered joint configuration where adhesive is placed between two interfaces. Unlike a single lap shear test, the double lap arrangement reduces bending stresses by positioning the adhesive layer symmetrically between two outer substrates. This results in a more uniform shear stress across the adhesive layer and produces more reliable data.

In a standard double lap shear specimen, three rectangular strips of material are bonded together. The centre strip is pulled in tension, while the two outer strips balance the loading, so the adhesive bond is stressed in shear rather than peel or tension.

The double lap shear test is widely used to determine the performance of adhesives in structural and semi-structural applications. It allows engineers to:

• Quantify the shear strength of adhesive joints with precision.
• Compare adhesives under different curing conditions, substrates, and surface treatments.
• Understand how bonded joints will behave under operational loads.
• Identify likely failure modes such as cohesive failure, adhesive failure, or substrate fracture.

The test is valuable across multiple industries. In aerospace it is used to qualify adhesives for bonding composites and lightweight alloys. In automotive it helps validate adhesives for structural joints in lightweight vehicles. In packaging it ensures laminate bonds withstand operational stresses, and in medical devices it confirms the reliability of bonded components used in safety critical products.

Specimen preparation

Specimens are prepared according to standardised dimensions. ASTM D3528 specifies strips typically 25 mm wide and 100 mm long, bonded with an overlap length of 12.5 mm. ISO 4587 describes similar specimen preparation for rigid-to-rigid assemblies. Consistency in overlap, thickness, and curing conditions is essential for accurate results.

Surface preparation is also critical. Substrates may require cleaning, abrasion, or priming before adhesive application to ensure repeatable bond strength. After adhesive application, specimens are cured under controlled conditions to achieve consistent performance.

Test procedure

The specimen is mounted in a universal testing machine. Grips hold the central strip on one side and the two outer strips on the other. A tensile load is applied at a constant speed until failure occurs. The test machine records load and extension throughout the procedure. Shear strength is calculated by dividing the maximum load by the bonded area.

Failure modes and interpretation

After testing, both the load-extension curve and the fracture surface are examined. Failure may occur in one of three main ways:

• Cohesive failure, where the adhesive itself fractures.
• Adhesive failure, where separation occurs at the adhesive-substrate interface.
• Substrate failure, where the bonded material breaks before the adhesive does.

Each mode provides useful insights into joint performance and adhesive suitability.

Mecmesin offers a range of universal testing machines and fixtures for adhesive testing. The MultiTest-dV system is a versatile bench-top tester with capacities up to 2.5 kN, suitable for routine quality control and research. The OmniTest range provides higher capacities up to 50 kN, making it ideal for testing stronger structural adhesives and composite materials.

Dedicated fixtures are available to ensure correct alignment and minimise bending during shear tests. VectorPro software is used to capture and analyse test data, providing automatic calculations of shear strength and clear graphical representation of joint behaviour.

Double lap shear testing is carried out to international standards that ensure consistency and comparability of results. The most widely used are:

• ASTM D3528 - Standard test method for lap shear strength of adhesives using double lap joint specimens.
• ISO 4587 - Adhesives, determination of tensile lap shear strength of rigid-to-rigid bonded assemblies.

For comparison, related single lap shear methods include ASTM D3163 and ASTM D1002, which specify single lap test arrangements. These are widely used for adhesive testing but may be more influenced by bending effects compared with the double lap method.

Mecmesin has extensive experience in designing equipment and fixtures for adhesive bond strength testing. Our engineers can recommend the most suitable testing machine, grips, and software configuration for your application. Whether you are developing new adhesives, validating materials to international standards, or running routine quality control, Mecmesin can provide a reliable and compliant testing solution tailored to your needs.