How to Test Lap-Shear Specimens

Manufacturing processes are moving away from using traditional bolts and rivets to using new, stronger adhesives to hold together materials such as composites and aluminum. With this increase in bonded manufacturing, it is more important than ever to accurately test the adhesive strength of bonds to prevent catastrophic failures.   

There are several tests which are important for adhesives, but the most difficult to perform well is the lap-shear test. In this test, two pieces of material are bonded together and then pulled apart to generate a shear force on the adhesive with no peeling force.

This test can be performed in various ways, but it’s important to understand each one because some may produce results which are drastically lower than expected due to peeling forces.

Adhesively Bonded Rigid Plastic Lap-Shear Joints

The Wrong Way

1.)  No tabs in grips without offsets

a.)  This method is not preferred because the bond line is not in the center of the test axis. It will produce lower max loads than other methods and may result in more rejected materials. Some people may use this method because it requires simple equipment and minimal specimen prep time.

The Right Ways

2.)  Tabs on specimens in grips without offsets

a.)  To perform this test, a user will bond the two pieces together, and then on each end, bond a small additional piece as a tab. With this method, the center of the test axis is aligned with the center of the adhesive, but it requires expertise and extra specimen prep.

b.)  If the tabs are not the right thickness or if the adhesive bonds are too thick or thin, the values produced may still be lower than expected.

3.)  Placing spacers in grips without offsets

a.)  This method requires the user to place pieces of material in the grips with the specimen to keep the bond line in the center of the grip. This can be difficult because the spacer must be identical to the thickness of the specimen and the bond line. If the thickness is not right, then the results produced will be lower than expected.

4.)  Double lap-shear in grips without offsets

a.)  The typical lap-shear specimen is bonded with two additional pieces of material to produce accurate results. This requires extra time, material, and expertise to prepare the specimens properly.

The Best Way

5.)  Untabbed specimens in grips with offsets

a.)  To perform this test, the jaws of the grip are offset to hold the specimen while keeping the adhesive bond line in the test axis. There is no extra specimen prep required for this type of test allowing it to be performed quickly and produce accurate results. Instron offers several grips that can perform this test including the precision mechanical wedge grips.