~ Article courtesy of Fastener Training Institute (FTI)
It’s important to know your fastener facts — and failures! Knowledge is power and in the fastener industry, proper training ensures the highest level of safety and accuracy when assembling any application.
Mechanical fasteners are far more complex and varied than most people understand. This was according to the late (and great) Joe Greenslade, an icon in the fastener industry. This understanding quickly changes, however, for those who make a career specifying, supplying, or using fasteners.
To support those working with or in the fastener industry, Greenslade taught mechanical fastener technology for 45 years, publishing more than 300 informational articles. He also served as the director of Engineering & Technology at the Industrial Fasteners Institute, a trade organization for North American mechanical fastener manufacturers.
Additionally, he compiled a list of his most frequently asked questions with answers that cover the specifying, inspecting, and installing of mechanical fasteners — and more.
Here are a few of his answers related to fastener failure analysis and what to do when things go wrong.
How effective is it usually to propose failure remedies before a definitive “root cause” of the failure is determined?
It’s 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.
When a customer reports fastener failures, what information should be gathered immediately before starting to try to remedy the problem?
- Exact part number
- Exact lot number
- Precise description of the failure and/or pictures of failed parts
- Where the parts are used
- How the parts are driven and how is tightening controlled
- A sample of broken and unused parts for analysis
When a bolt failure has a “necked down” area in the threads, what is the most likely cause of the failure? What is 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. Also, check the tightening calculation and/or do a tightening experiment.
When the fracture surface of a failed bolt exhibits a “shoreline” pattern, what’s the most likely cause of the failure? What is the most like remedy?
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.
Meeting standards is critical to ensure a safe and reliable assembly. Organizations, such as the American Society of Mechanical Engineers (ASME) and ASTM International, support the industry with the latest codes and standards for testing, materials, and engineering.
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:
1. If the failure occurred within 48 hours of installation, what’s the most likely cause?
When a bolt or screw has an intergranular fracture surface immediately under the head or at the first unengaged thread of the bolt or screws within 48 hours of installation, the most likely cause is hydrogen embrittlement.
Note from the FTI: 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.
2. What is a likely remedy?
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.
3. If the failure occurs weeks or months after installation what’s the most likely cause?
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 (EHE) failures.
4. What is the most likely remedy?
This can be remedied by using fasteners with a core hardness of less than HRC 39 — or, else paint, coat, or otherwise protect fasteners from the moist environment, redesign the joint to prevent fasteners in standing moisture, or avoid use of dissimilar materials in joint construction.
When a customer complains about bolts and/or nuts vibrating loose, what is the most likely cause and remedy?
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.
The late (and great) Joe Greenslade.
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?
Designers should avoid applications where dissimilar materials come in contact with one another to avoid potential galvanic corrosion.
Note from the FTI: Galvanic corrosion is an electrochemical process that occurs when two different metals or alloys come into direct electrical contact in the presence of an electrolyte.
What are the two approaches that can be taken to avoid failures when dissimilar materials must be mated with one another?
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.
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