~ Article courtesy of the Fastener Training Institute
If you spent time in the fastener industry, you’re aware of the complex technology and engineering behind seemingly basic nuts, bolts, screws, and washers. Mechanical fasteners have been around for centuries, but that does not mean they’re fail-proof.
Fortunately, that’s where training comes in. The experts in the field guide us to better products, installation techniques, and overall application success.
One such expert was the late Joe Greenslade, an icon in the fastener industry. He taught mechanical fastener technology for 45 years, publishing more than 300 informational articles. He also served eight years as the director of Engineering & Technology at the Industrial Fasteners Institute, a trade organization for North American mechanical fastener manufacturers.
In 2015, Greenslade compiled a list of his most frequently asked fastener questions with answers — including a section dedicated to fastener failure analysis. As he once pointed out, determining the reason for the failure is critical: “It is worthless to propose failure remedies before a definitive ‘root cause’ of the failure is determined. A root cause needs to be discovered before an effective remedy can be proposed.”
Here are a few of the questions he received with his answers to common fastener failures…
When a customer reports a fastener failure, what information should be gathered before trying to remedy the problem?
When a customer reports fastener failures, the information that should be gathered immediately before starting to try to remedy the problem includes:
• The exact part number
• The exact lot number
• The precise description of the failure and/or pictures of the failed parts
• Where the parts are used
• How the parts are driven and how tightening is controlled
• A sample of the broken and unused parts for analysis
When a customer complains about bolts and/or nuts vibrating loose, what is the likely cause?
The most likely cause of bolts and/or nuts vibrating loose is insufficient tightening. The most effective remedy is to use correctly calculated higher tightening torque or better determine the correct tightening value experimentally using extra components.
When a bolt failure has a “necked down” area in the threads, what is the most likely cause of the failure…and the most likely remedy?
When a bolt failure has a “necked down” area in the threads, the most likely cause of the failure is tensile overload due to over-tightening, or the bolt has insufficient strength for the load requirements of the design. The most likely remedy is to reduce torque if the failure occurs during installation or increase bolt strength if failure is during product use. Check the tightening calculation and/or do a tightening experiment.
When the fracture surface of a failed bolt exhibits a “shoreline” pattern, what is the most likely cause?
When the fracture surface of a failed bolt exhibits a “shoreline” pattern, the most likely cause of the failure is fatigue. The most likely remedy is to increase the tightening value.
When a bolt or screw has an intergranular fracture surface immediately under the head or at the first unengaged thread of the bolt or screw, what is the reason?
When a bolt or screw has an intergranular fracture surface within 48 hours of installation, the most likely cause is hydrogen embrittlement (see FTI note below). This can be remedied by baking the parts at 400° F for 14 or more hours. Or better yet, change to a finish not subject to hydrogen embrittlement.
What if this same failure occurs weeks or months after installation?
When a bolt or screw has an intergranular fracture surface immediately under the head or at the first unengaged thread of the bolt or screw weeks or months after installation, the most likely cause is stress corrosion — also called environmental hydrogen embrittlement failures.
This can be remedied by using fasteners with a core hardness of less than HRC 39. Otherwise, consider paint, coating, or a method that protects fasteners from the moist environment. For example, redesign the joint to prevent fasteners from being in moisture or avoid the use of dissimilar materials in the joint construction.
FTI note… Hydrogen embrittlement (HE), is a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with stress — either applied externally or from internal residual stress. Generally, HE is classified under two broad categories based on the source of hydrogen: internal hydrogen embrittlement (IHE) and environmental hydrogen embrittlement (EHE), according to the “Fundamentals of Hydrogen Embrittlement in Steel Fasteners,” by Salim Brahimi.
What is the critical hardness above which hydrogen-induced failures can occur?
HRC 39 is the critical hardness above which hydrogen-induced failures can occur.
Why should designers avoid applications where dissimilar materials come in contact with one another? And what are the two approaches that can be taken to avoid failures when dissimilar materials must be mated with one another?
Designers should avoid applications where dissimilar materials come in contact with one another to avoid potential galvanic corrosion.
The two approaches that can be taken to avoid failures when dissimilar materials must be mated with one another is to select fastener materials that are closer to the component materials on the “galvanic scale,” and/or coat/paint the joint to keep moisture away from the joint.
FTI note… Galvanic corrosion refers to corrosion damage induced when two (or more) dissimilar materials are brought into electrical contact under water.
For additional FAQs from Greenslade, please click here.