The tensile shear strength of the adhesive is the key index to evaluate its mechanical properties, reflecting the ability of the adhesive joint to withstand shear loads in the direction parallel to the adhesive layer. This performance directly affects the reliability and durability of the bonded structure in practical applications. As the core testing equipment, the tensile testing machine can accurately determine the strength value by simulating a specific stress state. This method is widely used in quality control and research and development in many industrial fields such as aerospace, automobile manufacturing, electronic packaging, building materials and composite bonding.
Test Principle:
The basic principle of the test is to glue two test pieces of the adhesive material in a specific lap form to form a standard tensile shear specimen. The specimen is installed in the upper and lower fixtures of the tensile testing machine, and the crossbeam of the testing machine moves at a uniform speed, applying a tensile load parallel to the adhesive joint surface on the specimen until the joint is damaged. The maximum load during the process is recorded and the tensile shear strength is calculated based on the lap area. The main technical standards at home and abroad clearly stipulate the specimen size, preparation process, test conditions and result processing to ensure the comparability and repeatability of the test results.
Commonly used standards include: national standard GB/T 7124, international standard ISO 4587, American Society for Testing and Materials standard ASTM D1002, etc. These standards may have slight differences in specimen size (e.g., lap length, width) and must be selected according to the applicable area or protocol of the product before testing.
System composition
The complete test system is mainly composed of the main engine of the tensile testing machine, special fixtures, force value sensors, displacement measurement devices and control systems. The testing machine should have good neutrality and load coaxiality to reduce the impact of additional bending moment on the test results. For adhesive testing, self-tightening wedge clamps or hydraulic clamps are often used to ensure that the specimen does not slip and is subjected to uniform force during the testing process. The range and accuracy of the force sensor need to match the expected adhesive strength. An ambient temperature and humidity control box is available as an option to study the bonding properties under different environmental conditions.
Sample preparation
Specimen preparation is the primary factor affecting the accuracy of test results. The material to be bonded is usually metal (such as aluminum alloy, steel) or composite material, and its surface needs to be cleaned, polished or chemically treated according to standards to ensure the bonding effect. The amount of glue, curing temperature, pressure and time should strictly follow the process specifications of the adhesive manufacturer. The prepared specimen needs to be adjusted for a specified time under standard temperature and humidity conditions.
The testing process mainly includes the following steps: First, the exact size of the lap area of the specimen is measured and the shear area is calculated. Subsequently, the specimen is symmetrically clamped in the testing machine fixture to ensure that the stress axis is consistent with the center line of the adhesive layer. Set test parameters such as the beam movement speed (usually 1-50 mm/min). Start the test and record the load-displacement curve continuously. Stop the test when the load drops to a certain percentage of the maximum value, or when the specimen is completely separated. Record maximum failure load and failure mode.
Data analysis
The tensile shear strength (τ) of an adhesive is calculated using the following formula:
τ = Fmax / (b × l)
Among them, Fmaxis the maximum failure load (unit: N), b is the lap width (unit: mm), and l is the lap length (unit: mm). Intensity results are usually expressed in megapascals (MPa).
Failure mode analysis is crucial for assessing the quality of glue joints and is categorized into the following types:
| Cohesive destruction | The destruction occurs inside the adhesive layer |
| Adhesion breakdown | Failure occurs at the interface between the adhesive and the adhesive |
| Material destruction | The destruction occurs inside the material being glued |
| Mixed destruction | Both or more of the above modes exist at the same time |
Generally, cohesive failure indicates good interfacial bonding, and the strength results can truly reflect the adhesive properties. The test report should contain information such as the arithmetic mean of strength, standard deviation, failure mode and test conditions.
Influencing factors
Test results are influenced by multiple factors. In terms of sample preparation, errors will be introduced due to the size deviation of lap joints, uneven thickness of the adhesive layer, and incomplete curing. In terms of equipment, the neutrality of the fixture and the accuracy of loading rate control are the key. In terms of environment, changes in temperature and humidity can alter material properties. In terms of operation, the deviation of the clamping angle of the specimen will lead to stress concentration. Therefore, it is necessary to strictly control the process parameters, calibrate the equipment regularly, and document in detail all test conditions that may affect the results in the report.
Conclusion
Testing the tensile shear strength of adhesives using a tensile testing machine is a standardized, repeatable evaluation method. By following rigorous testing standards and controlling specimen preparation, equipment operation, and environmental conditions, reliable data can be obtained to provide a quantitative basis for adhesive selection, process optimization, and structural design. Combined with the failure mode analysis, the method can deeply reveal the failure mechanism of adhesive bonding, which has a supporting effect on improving the bonding reliability of products.
References
GB/T 7124-2008, Adhesives - Determination of tensile shear strength (rigid materials vs. rigid materials)
ISO 4587:2003， Adhesives — Determination of tensile lap-shear strength of rigid-to-rigid bonded assemblies
ASTM D1002-10， Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal)