Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal)

 

Theory:

ASTM D1002 is the most popular test for lap shear. There are over 20 more ASTM specific tests for testing the shear strength of an adhesively bonded material. This outline focuses on metal to metal bonds. Several types of laps exist including bevelled, double, double butt, joggle, and single. There are several failure modes for the lap shear test. The adhesive can detach from the metal substrate, the adhesive can seperate from itself, or the adhesive could be so strong that it actually shears the metal and tears a chunk of it away.

 

The use of adhesives and epoxies in manufacturing has increased rapidly over the last few years for several reasons. Lightweighting, especially in the automotive and aerospace sectors, has led manufacturers to replace heavier screws and bolts with adhesives. This also simplifies the manufacturability of these items because it is much easier for a robotic arm to apply a layer of epoxy instead of inserting a nut and bolt and then tightening it. Unfortunately this makes the disassembly and repair of these units more difficult.

 

The Construction Industry is also a large consumer of these types of sticky materials. $7.5B of adhesives will be used in the global construction industry as the demand in China continues and the domestic housing sector also improves. There are a few different types of adhesives including those made from acrylics, polyvinyl acetates, polyurethanes, silicones, and epoxies. Since many adhesives are really just oil product derivatives, the recent downdraft in oil prices has put pressure on the industry overall. The good news is that over time the industry will rebalance which will benefit the US economy because of its newfound prominence in the oil industry.

Geometry:

The accompanying picture shows the various ways in which two pieces of metal can be bonded together with an adhesive or epoxy.

Test samples are typically 1 inch wide and 1 inch of sample height is secured in the grip. Usually a 1x1 inch square is bonded together to form the joint.

Solution:

Wedge grips are the defacto selection because of the high amount of force that is usually required to successfuly perform this procedure. Other solutions such as hydraulic grips are also acceptable. 

Analysis:

The values reported for the lap shear test are relatively straight forward. The maximum force, also commonly referred to as the shear strength, is really just the peak load. The maximum shear stress is slightly different and requires the user to divide the maximum force by the area. The units are then reported as PSI or similar. The failure mode should also be recorded.