We continue our discussion on some of the common problems that can occur with some common industry standards, and how these factors can be minimized and eliminated. Today we will discuss the ASTM D1002 Standard Test Method for Apparent Shear Strength of Single Lap Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal to Metal). This test method is mainly used for comparative purposes. Two metal plates are bonded together with adhesive. These samples are then pulled along the long axis of the sample until failure occurs.
The main challenge when testing to this method is sample preparation. The accuracy of test results is dependent on the preparation of the test samples.
The first step to ensure proper sample preparation involves the test panels. ASTM D1002 recommends using one of six different metals for testing. It is important to choose athe panel material with a yield strength that exceeds the expected shear strength of the adhesive under test. If the wrong panel material is chosen, the results may be influenced by the tensile yielding of the panel versus the shearing of the adhesive.
Prior to adhesive application to the test panels, the panels must be cleaned and dry dried the plates per the procedure prescribed by the manufacturer. Clean test panels are imperative to achieving accurate test results. If there is excessive dirt or oils from handling on the bonding surface this can cause incomplete bonding of the adhesive to the test panel.
Once the test panels are cleaned, the adhesive can be mixed for application. It is necessary to follow the manufacturer’s instructions for mixing. The adhesive must have complete mixing prior to application to ensure the results from the test can be repeated and compared from batch to batch.
The adhesive must now be carefully applied to the test panels. If the sample is not prepared correctly, this is another area that can cause issues. One must use caution in order to prevent bubble formation when applying the adhesive to the test panel. Bubbles can lead to incomplete bonding due to air pockets and cause lower apparent shear strength values. It is also important to ensure the adhesive is applied with a consistent thickness in the bonding area. By applying the adhesive with a uniform thickness, the aids in proper sample alignment can be achieved when loading the sample into the universal tester thus achieving a true shear. Sample alignment in all axes is critical for this test procedure. .
Once the adhesive has been applied to the test panel, the sample is ready to cure. It is important to follow the curing instructions provided by the manufacturer. Variations in cure time and conditions may cause undesirable effects on the adhesive. Short-term changes in the curing temperature or humidity can lead to chemical changes or internal stresses in the adhesive that can have an effect on the shear strength.
When loading the sample into the universal tester, it must be mounted such that the sample is only loaded in shear. Self-aligning grips can be helpful in achieving this condition. Sample alignment in all axes is critical for this test procedure.
This blog post was adapted from a recent article by Amy Peterson, DDL Lab Manager, Products & Materials Division that appeared in the publication Adhesive & Sealants Industry (ASI).