Tech Talk Blog

However, because the substrates are deformed in tension, the shear stresses are concentrated at the ends of the overlap in a lag effect. When a joint is loaded the adhesive stresses beyond its elasticity point and on further loading the adhesive is stressed beyond its yield point in shear and regions of uniform stress develop at the edges of the joint. As the load is increased, these uniform shear regions will spread through the whole of the overlap and a limit will be reached when the joint can carry no further load. An upper limit on strength can therefore be derived as

Pmax = tyLb

Where:

1. Pmax     is the maximum joint strength possible
2. ty            is the adhesive yield stress in shear

This maximum strength is not achieved because the shear strains exceed the limits for the adhesive, the effect of peels stresses and failure of the substrate.

Lap shear bond stresses without load

Lap shear bond stresses without a load

Lap shear bond under load

Lap shear bond under load

Experimental data for the shear stress distribution indicate that:

Where:

1. tmax     is the maximum adhesive shear stress
2. Ga          is the shear modulus of the adhesive
3. Es           is the Young’s modulus of the substrate
4. ta            is the thickness of the adhesive layer
5. ts            is the thickness of the substrate

Thus, the bigger the ratio between the stiffness of the substrate and the adhesive layer, the more uniform the shear stress distribution.

The next post will illustrate examples of bonding joints with qualitative comments as to their performance.

As always, you can check out our Shooting Star Archives to read about real-world applications or visit the TriStar site.

The design of an adhesive bond may be simple or complicated depending on the adhesives function. For optimum performance of the materials and adhesive, some general principles should be considered:

1. Stress in the direction of maximum strength
2. Maximize bonding surface
3. Adhesive applied uniformly
4. Adhesive is thin and continuous
5. Minimize stressed areas

It is important to know the duration, direction and load of the forces being applied to designed joint. Most adhesives used for structural purposes are relatively strong in shear. Conversely, these same adhesives have relatively low adhesive strength in tensile or peel.

The design of the bond line should take into consideration the forces that occur when the device is under load. A simple lap shear bond has peel forces at the ends while the center observes little stress.

For lap joints loaded in tension, the load is transferred predominantly by shear stresses in the adhesive layer. If the adhesive was loaded uniformly then

ta = [ P / (Lb) ]

Where:

1. ta         is the adhesive shear stress
2. P          is the load
3. L          is the overlap length
4. b          is the width

The next post will expand on this discussion and provide more details about shear bonds under stress…

But if you can’t wait – look to our Video Learning Center for more information right now!