Designing for Assembly with Adhesives

Based on an original publication by CircuitMedic.

A modern manufacturing staple, industrial adhesives are essential to a host of design applications including structural bonding, cylindrical assembly, threadlocking, flange and thread sealing, thermal management, wire bonding, harnessing and more. Today's adhesives make it easy to bond dissimilar materials and simplify the production of designs that would have once been impossible.

Engineered to distribute load stress over an entire area as opposed to a single fastening point, adhesives offer several advantages over traditional fastening methods - including more even distribution. With a seal as strong as a bond, adhesives resist flex and vibration stresses better than a riveted joint and eliminate corrosion often found in mechanical fastening.

Downsides of using adhesives alone may include increased setting and curing time, as well as surface preparation and planning for potential joint disassembly. Often two methods are better than one - and mechanical fasters can work together with adhesives to form an even stronger bond.

Fundamentals of Adhesive Joint Design

While stress has substantial bearing on the success of an adhesive joint assembly, the engineer must also be knowledgeable of stress distribution types in order to form a successful joint:

  • Tensile Stress: Pulls apart and elongates an assembly.
  • Compressive Stress: Squeezes an assembly together.
  • Shear Stress: Pulls parallel objects apart in a lengthwise motion, resulting in a sliding effect.
  • Peel Stress: Lifts or peels a flexible substrate away from the bonded surface.
  • Cleavage Stress: Occurs with inflexible substrates when the joint is forced open at one end.

Most industrial adhesives are engineered to handle tensile, compressive and shear stress with ease. They are not, however, designed to accommodate peel and cleavage stress, and engineers should seek to eliminate these from the joint design. The best options ensure maximum bond area while incorporating both mechanical and adhesive bonding methods together.

Five types of joints are generally used for adhesive-bonded assemblies:

  • Lap Joint: Sometimes referred to as an "overlap joint", this is created by placing one substrate on top of another.
  • Joggle Lap Joint: Formed when one substrate molds or wraps around another substrate.
  • Butt Joint: Create when two objects are bonded end-to-end; also called a "cylindrical joint" when cylinders are bonded.
  • Scarf Joint: Created when substrates are bonded at an angle to increase bonding area.
  • Strap Joint: Combines a lap joint with a butt joint.

With an increasing variety of industrial adhesives designed for a range of application challenges, selection should revolve around the parts to be bonded and the final product being created as well as multiple environmental considerations. The proper adhesive will not only resist initial stresses but will also stand up to a potentially severe environment.

The most common cause of joint failure does not surround adhesive strength, but is rather the result of poor design and lack of insight during the selection process. Keep in mind that assemblies should always be tested to ensure a successful, secure bond - during both the manufacturing process and throughout the lifetime of the device.

 

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