I was recently asked to serve as an expert witness in a lawsuit where catastrophic failure in a bolted joint led to a severe debilitating injury. The discovery phase of a legal proceeding, where information is requested and people are deposed under oath, is a sobering, expensive and lengthy process.
If considered before it becomes a required activity, product liability can serve as a framework to review your product development processes and decision-making. Ask yourself, “How would I fair if all my business practices and engineering decisions came under critical scrutiny?”
Let’s first put joints in perspective relative to product liability. In short, joints are the root of all evil. It has been shown that failure at the part interface (the joint) is the most likely cause of product failure in mechanical systems.
No matter what the product, joints also carry a unique double risk. First, like all aspects of product design, the joint must be engineered to perform the task expected of it. However, because by definition joints are created through assembly, an entirely different set of parameters directly affects the joint’s ability to perform as expected-the assembly process and its potential effect. So, the joint not only needs to be designed correctly; it needs to be assembled correctly.
A good way to approach joint validation is the same way you would balance your family budget. In both cases, one has to balance supply and demand. When an imbalance exists, you can address the situation by altering either the supply side or the demand side. But, if the imbalance isn’t corrected, eventually something bad will happen.
A joint’s “supply” side is defined as its capacity to function against all inputs, while the “demand” on the joint is the magnitude of all those inputs. Unfortunately, this is where this analogy is most applicable to the subject at hand.
I would be willing to wager that the vast majority of joints in the marketplace have never undergone analysis. In many cases, this is an acceptable shortcut because the supply so greatly outstrips the demand or the consequences of failure are minor.
However, in a significant number of cases, the absence of catastrophic failure is as much a function of luck as design, and one incident of “bad luck” can erase decades of “good design.” If this might describe your company and its products, the good news is that you don’t need significant resources to augment your luck.
While gaining accurate and complete loading knowledge is expensive, when starting from zero even basic estimation is a significant step forward. Remember that loads can be derived either from calculation or from testing.
Often, basic information can be gained from simple load cells requiring little money or experience. In fact, a shipping scale is better than nothing. During a deposition, answering the question, “How was the design of the subject joint determined?” with a blank stare and a blank piece of paper is not a position you want to be in.
Because approximately 80 percent to 95 percent of structural failures occur through a fatigue mechanism, it’s important to take oscillating loads into account when they are present. Arguably, the best starting point may be comparing your estimated loads to S-N fatigue curves for your materials. These curves estimate fatigue life in load cycles of a particular material based on stress. Such curves are typically available for most materials.
The steps described above will essentially compare estimated demand to ideal supply. Ideal, because some capacity must be assumed, and if these values aren’t achieved in practice, your joints will end up in debt. Therefore, the second phase of joint analysis is ensuring that the supply that is actually available is as close as possible to what was assumed initially. Using bolted joints as an example, this would involve setting the installation torque to achieve the desired initial clamp load and then estimating the level of relaxation which subsequently occurs.
A fundamental requirement of understanding joint behavior during assembly is to view critical parameters in two dimensions. For bolted joints, the entryway into that understanding is a torque-angle trace. This trace is generated by tightening a bolt, often to joint failure, with a power tool, while incorporating a torque and angle transducer, either internal or external to the tool. A torque-angle trace reveals the joint’s “personality” and greatly aids in optimizing installation torque.
The same equipment can be used to estimate clamp load relaxation by allowing much more accurate torque audits than possible by reading the peak value of a torque wrench. While this entry-level method is not capable of measuring bolt tension directly, with some experience it can be inferred.
Engineering and testing joints will not eliminate the potential for a product liability action, but it sure beats never having considered it, and effectively staking your future on your luck. A