Why Threaded Fasteners Fail and Methodology for Failure Analysis

Why Threaded Fasteners Fail and Methodology for Failure Analysis

Problems related to improper manufacturing, selection and installation are just as likely to cause fastener failure as hydrogen embrittlement
and loosening.
  • During the head-forming process, it’s important that the grain flow lines of the metal form in the proper direction. The image at left indicates acceptable grain flow that meets the SAE USCAR8 specification. Photos courtesy SAE International Hydrogen embrittlement is a common cause of fastener failure.
  • The arrows in the photo indicate areas of load stress. Photo courtesy GT Technical Consultants Ltd.
This fastener has an underhead quench crack, although lab engineers initially thought the head failed from hydrogen embrittlement. Photo courtesy GT Technical Consultants Ltd. There are many ways to perform destructive testing. The most helpful, and preferred by manufacturers, is to intentionally destroy an assembly in a controlled environment to determine the performance capability of one or more of its components. Another way, done unintentionally and all too often by consumers, is to simply misuse a product. Consider, for example, the person who spends more time plopping his heavy briefcase on a low-cost, particle-board desk than sitting at it doing work.
Before too long, the fasteners that hold up the desk will loosen or crack and fail. Although this instance of fastener failure could result in bodily harm, it’s unlikely the manufacturer will recall the product because improper customer use caused the problem. However, many products have been recalled because known fastener failure during proper use would likely result in minor or serious user injuries. Case in point: Volvo Cars’ recall of three types of vehicles in March 2017 due to faulty bolts that secure side-curtain air bags. Overall, the company recalled more than 5,500 S90 sedans and V90 cross country and XC90 luxury sports utility vehicles made between November 2016 and January 2017.
The bolts that secured the air bags were improperly manufactured and likely to quickly break due to internal hydrogen embrittlement. Fixtures that effectively secured the air bag assemblies for at least 48 hours were deemed structurally sound by Volvo engineers, while those that failed or faltered had to be replaced. Luckily, no injuries have been connected to this defect. Loose fasteners can also eventually fail. Last year, off-road-vehicle manufacturer Polaris had to recall all of its General and General 4 models due to input shaft fasteners coming loose and allowing the shaft to move along the length of the power steering spine. In extreme circumstances, this movement could have resulted in the shaft completely separating from the spline. General Motors engineers discovered loose fasteners in 2014 on several models, resulting in the company recalling more than 500,000 vehicles that year.
The Chevy Impala had a “superhold” joint fastener that was not torqued to specification. And, a loose bolt in the Chevrolet Camaro and Equinox; GMC Terrain; Buick Regal and LaCrosse; and Cadillac SRX allowed the front and passenger seats to move up and down too freely. The latter condition led to one crash and three injuries. “In 95 percent of failures, something goes wrong with the fastener during installation or maintenance,” claims Guy Avellon, founder and owner of GT Technical Consultants Ltd., and a 40-year veteran of the fastener industry.
“The other five percent is due to the wrong fastener being used for the application.” Strength of materials point of view Failure is the inability to perform the assigned task, failure does not necessarily mean the fracture of the fasteners, there are various causes of failures, some significant causes are discussed below. Failed pieces have shown vibration in tensile strength, Lab analysis shows that 97% fasteners failed due to Hydrogen Embrittlement, 2% because of Tensile Failure due to over tightening and 1% due to metal corrosion. The root cause of failure is because of Hydrogen Embrittlement. TYPES OF FAILURES OF FASTENERS: • Failure because of Overload Many accidents can be characterized as an impact with a non-compliant object such as a truck impacting a concrete bridge support. The fine, grey appearance of the fracture surface is consistent with a sudden overload failure.
Failure from lack of Locking Mechanism To prevent bolts from loosening over time, various locking mechanisms are employed. They include lock washers, locking nuts, jam nuts, mechanical deformations, wire wrap, cotter pins, metal locks, expansion anchors, helical coils and polymer locking compounds. Machinery that is subject to vibratory environments usually is equipped with some sort of locking mechanism. If the locking mechanism is not applied to the machinery during manufacture, a catastrophic event may result.
Metal Fatigue Metal fatigue is the phenomenon characterized by progressive crack growth during cyclic loading. A crack is often initiated at a flaw or stress riser (sharp notch) in a part. Cyclic forces such as vibrations or repeated impact cause the crack to increase in size until the part can no longer sustain the load, and a final fracture occurs.
Failure from Improper Torque When threaded fasteners are utilized, the amount of tightening or bolt torque is often important. Motor vehicle wheel studs require torques ranging from about 100 ft-lbs for smaller vehicles to over 400 ft-lbs for large trucks. The appropriate torque is required in order to prevent relative flexing of the two parts being fastened and to assure an acceptable mechanical connection. Bolt failures as a result of improper torque have occurred in automobile applications.
Corrosion Failure Corrosion of metals can be disastrous to threaded fasteners. Surface and pitting corrosion attacks threaded fasteners as a result of contact with moisture or other corroding media. Since bolts often carry high loads, stress corrosion cracking is another corrosion related failure mode. Corrosion, coupled with forces in a bolt, tends to accelerate cracking.
Hydrogen Embrittlement (HE) A permanent loss of ductility in a metal or alloy caused by hydrogen in combination with stress, either externally applied or internal residual stress.
Galling If you`ve ever had the pleasure of installing or removing stainless steel fasteners, you`ve more than likely experienced galling. Galling is a cold-welding process that results when the threads are in contact under heavy pressure and friction. Or in other words, when fasteners are assembled or disassembled. Methodology for Failure Analysis According to the problem identified and the objective of study. The proposed methodology includes the step by step procedure to obtain the solution. This includes understanding process flow of metal plating, sample collection for testing.
  • Step 1: Understanding field conditions, loads experienced by the fastener, and other factors
  • Step 2: Collecting sample of both the failed piece(s) and unused parts from the same lot.
  • Step 3: Lab analysis - Laboratory testing will confirm whether there are any issues with that lot.
  • Step 4: Understanding & monitoring processes done on fastener – Plating/Galvanizing
  • Step 5: Finding root cause of failure in process/product selection.
  • Step 6: Suggesting best possible alternative.
  • Step 7: Comparing the results obtained with respect to the prior failure rate.
  • Step 8: Application of best suited process.
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