Why Fasteners Fail After Passing the Stress Test
By Dr Lou Raymond (Ph.D., P.E. (CA), FASTM, FIAE)
When a fastener fails because of hydrogen embrittlement, the blame often tends to be unfairly pointed at the supplier. TR Fastenings, a leading global manufacturer and distributor of industrial fastenings, has experienced various cases in the past where large bolted joints have passed the stress test, yet unexpectedly failed in service; however, this has been shown to be due to the way the have been used and not due to any manufacturing errors. If a bolt is designed to hold 100 kg and 200 kg is applied, it will break! Once installed and in service, environmental exposure can cause analogous situations when stressed above a threshold stress that causes time delayed failure.
"We are a manufacturer and supplier of components and we don't always get told where our fasteners end up," says Geoff Budd, managing director of TR Fastenings. "And that is why this is such an important topic. People think when they have this sort of failure it is caused by problems during manufacture, but actually it's often the specific way that they are used that has brought on the phenomenon. That is why we want to educate customers, so we are hugely grateful for the research undertaken by Dr. Louis Raymond into this subject."
One of the world’s leading experts on hydrogen embrittlement, US-based Dr. Raymond is a renowned materials scientist with over 30 years’ experience as consultant, educator, diagnostician, and forensic scientist. In the following, Dr. Raymond answers frequently asked questions related to the cause of fastener failures.
Q – How can a fastener experience a hydrogen embrittlement service failure after is has passed all of the standard ASTM or ISO stress testing requirements?
A – Post mortem analysis of broken fasteners that attribute the cause of the failure to hydrogen embrittlement, is often quickly and erroneously, put the blame on the fastener manufacturer. Failure analysts that find intergranular cracking (IG) on the fracture surface are quick to attribute it as a hydrogen embrittlement failure to improper plating procedures, such as omitting post plating baking treatments; - but these requirements are not enough to prevent failure by hydrogen embrittlement, once the fastener is put into service. In fact, the same conditions that allow the hydrogen to be removed by baking after plating, allow the hydrogen to re-enter even more aggressively once the fastener is installed, after it is under an applied service stress.
The mere presence of water, often due to condensation, will dissociate to generate the hydrogen in the presence of plating such as cadmium on steel. The potential for generating conditions for failure by hydrogen embrittlement after manufacture under installed conditions are not adequately addressed by the designer. Environmental effects due to galvanic coupling in the presence to dissimilar metals present in the entire assembly, or even galvanic coupling with the plating alone, must be evaluated to obtain the desired service life. A variety of failure analysis techniques must be applied to identify how the hydrogen is produced. Therefore, a fastener can be manufactured to satisfy all standard test requirements that are used to establish the fastener as being free of hydrogen embrittlement from the plating process, but the fastener must still be found to not fail from hydrogen embrittlement in service.
Q – When a fastener breaks in-service, how is hydrogen embrittlement identified as the cause?
A – The characteristics of hydrogen embrittlement in steel are identified by fractographic analysis using the scanning electron microscope. Its features are described as intergranular, with secondary cracks and tear ridges on the grain facets. The origin is below the service of a notch or thread root. The failure is described as a time-delay fracture or the fracture occurs over a period of time once the fastener is installed.
Q - When a fastener fails by hydrogen embrittlement due to processing or plating, what are the characteristics of fracture?
A – The photographic fingerprint of a hydrogen embrittlement failure is called “Intergranular” (IG) fracture.
Q – What features separate cause of hydrogen embrittlement due to plating and processing from that due to a service environment?
A – None, the fracture face features are Identical to a fastener that fails in an environment that produces hydrogen.
Q – Doesn’t application of Post Plating Baking Standards prevent hydrogen embrittlement failure in service?
A – No, the standards only assure that the manufacturing and plating process do not result in hydrogen embrittlement of the fasteners.
Q – Can a part that has successfully passed the Post Plating Baking Standards eventually fail in service because of hydrogen embrittlement?
A – Yes, inherent in a plated fastener is the potential of a galvanic couple, which is produced by a combination of the plating and the steel of the fastener. Under these conditions, only moisture or condensate from the atmosphere is required to activate the galvanic couple, which then will produce hydrogen on the surface of the fastener during service usage.
Q – Will this hydrogen cause embrittlement?
A – Yes, hydrogen produced on the surface will diffuse into the steel just as during the plating process. In fact, because the fastener is now under stress, it acts more like a sponge and absorbs a lot more hydrogen than it would while it is being plated; since during plating, the fastener is not under a state of stress except for residual fabrication stresses.
Q – If a fastener were manufactured to satisfy all of the Post Plating Baking Standards designed to prevent hydrogen embrittlement, is it possible for a service failure to occur by hydrogen embrittlement?
A – Yes, under certain conditions, a fastener that is plated and meets all of the Post Plating Baking Standard requirements to prevent hydrogen embrittlement during manufacturing and processing could fail if the parts were exposed to a moist environment. In fact, the results of sustained load, notched round tensile tests per ASTM Standard F519 on hydrogen embrittlement that are used to certify manufacturing and plating processes, have been found to be dependent on the humidity of the room during test.
Q – If a fastener were manufactured without a Post Plating Baking treatment designed to prevent hydrogen embrittlement, would a hydrogen embrittlement failure occur in a dry environment?
A – Yes, If the fastener were embrittled due to processing and plating and did not meet the requirements of the Post Plating Baking Stress Test Standards, it would also fail over a period of time once put into service or being stored, even if it were placed in a desiccant or in an atmosphere free of moisture.
Q – How does one protect against hydrogen embrittlement failures during service once the part had been manufactured and plated free of hydrogen?
A – Only by taking the correct preventative action during designs of the fastener systems such as the use of a dichromate or a non-conductive, rust-prevention grease during storage. Other measures should be directed at reducing galvanic couples in the fastener system. The most severe test would be to assemble the fastener with washers as an entire unit with all dissimilar metals present and expose it to 96-hour salt fog test per ASTM G46 or an alternate immersion test per ASTM G44.
Q – After the fastener is found to fail in service, how can one identify the exact cause of failure?
A – Only with a very thorough failure analysis that takes into account each step of the process and uses advanced analytical techniques. A thorough post-failure analysis must take into account the degradation and strength of the fastener due to the environment in order to determine if the service conditions were the cause of failure. After the part breaks in service, it is too late for any economic solution to the problem. It is then the responsibility of the designer to select the appropriate plating/fastener combination for a given application. Currently, this area is severely neglected throughout the industry and needs immediate attention.
For further information on Dr Lou Raymond visit: www.louraymond.com/bio.html
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