Let’s Define “Structural Joint” – to Ensure the Integrity of the Joint Design and Bond

Posted on 2/28/2017 2:10:59 PM By Dan Daley

Let me start by introducing myself and my colleague, James Swope, at the ChemQuest Group, Inc., as new contributors to the ASC Blog. In 2017, we will discuss adhesive and sealant technology for use in transportation applications. Jim and I are both excited to be contributing posts on topics that are relevant to adhesives’ end users and tap into our working knowledge – having spent decades in our previous roles – at formulating and end-use companies, respectively.   In our current roles, identifying unmet market needs is central to nearly every ChemQuest project in which we are engaged, especially on behalf of automotive tier suppliers and their end customers.

A buyer or user of adhesives – who is new to adhesives technology – might want to learn all they can about chemistries, resin types, application methods, performance properties, dissimilar substrates, vendors and formulated products on the market.  In our discussion of blog topics, we found that even our most experienced colleagues had different opinions as to the meaning of “structural bonding” or “structural joint”, which can and does lead to intriguing discussions.

While the word “structural” might conjure up images of “strength” for many people, Merriam-Webster Dictionary’s definition of “relating to the way something is built or organized, ”certainly resonates with me as a materials engineer. The durability and integrity of the structural joint is completely dependent on how expertly it is designed and organized:  Materials and their properties are a factor as well as how well the actual product is built and how every stage of building is executed.

Consider the following real world example from my 30-year career managing the Polymer and Paint Labs in a Materials Engineering group for a commercial vehicle OEM. We decided to reorganize some of my responsibilities for adhesives and sealants under my company’s Fastener Engineering team to form our new “Joining and Sealing” Group. I was looking forward to shedding some responsibility but instead assumed the role of managing the newly formed team when the Fastener Engineering manager found new employment.

One challenge that emerged was how to properly classify and specify various classes of joints to correctly communicate the requirements to the plant.  Not just bonded joints but all joints.  From extremely critical brake and steering systems to clips and retainers to all the bonded joints with which I had much experience.  In our organization, the design engineering team had responsibility for the system and joints within it.  We consulted, reviewed and otherwise took the necessary steps to ensure the joint, fastener/adhesive, material, finish and specifications were correct.  We also provided test, analysis and plant support as required. 

At issue was that each of our engineers considered their joint to be “critical” and extremely important to their parts and system.  To a certain extent, they weren’t mistaken, every joint was “structural” and provided a critical function. However, the “everything is critical” mindset manifested itself on the plant floor as “if every joint is critical then every joint should be treated the same.”  The result of this could be that the tooling and process control may not be properly differentiated.  In reality, different joints had specific manufacturing requirements that needed to be clearly defined.  Torque on a wheel nut had to be precisely applied, verified and recorded.  The windshield required specified surface preparation and a controlled, uniformly-shaped, continuous bead of adhesive was applied.  Automated welding with in-process controls produced a high-quality cab, which was periodically audited in full tear downs to check the controls.

In vehicle assembly, the safety of the product is paramount.  A structural joint is what we (engineers) define it to be.  A supplier may promote their adhesive or other materials as “structural” for any number of reasons related to its chemistry and proven performance, but in practice it is structural only if it is built in a designed structural joint.  Moreover, many materials, fasteners and adhesives with a wide array of properties can be used to create a structural joint to achieve the desired impact on the final product. 

The moral of the story is that any joint that is worth the time and money to design and construct is worth doing well – and every joint is not created equal.  While some joints are unequivocally more critical to the assembly and/or end product than others, engineers need to be clear about the requirements for each joint, lest we obscure which joints are truly critical to those downstream of us in the plant.