Federal laws such as the Fastener Quality Act seek to ensure standardized, rigorously tested industrial fasteners, and responsible fastener manufacturers, likewise, work continuously to identify vulnerabilities in designs. However, even vigilance and strict specifications cannot prevent all fastener failures. That makes it critical for users of fasteners to understand the their causes.
Fatigue contributes to some cases of delayed failure. Like all mechanical elements, perpetual, cyclic pressure on a stud weld, rivet, or other fastener, will eventually result in normal wear and tear. Because this is an anticipated failure, it is less common than stress cracking. Manufacturers are able to identify variables that may contribute to the part’s weaknesses and provide a measure of a particular series of fasteners relative strength.
Stress cracking is the more common and less predictable of the potential failures. The factors that contribute to stress cracking are not as measurable and fluctuate more than the variable that contribute to fatigue failure. While two factors remain consistent with fatigue failure – the strength of the design and materials as well as the tensile stress they are subject to, other rogue elements add to stress, including:
The most commonly cited cause of stress cracking is hydrogen embrittlement, which occurs when atomic hydrogen is absorbed by a fastener. Many circumstances can result in this vulnerability, including the absorption of hydrogen into a fastener’s surface during a faulty electroplating process. Coupling aluminum alloys to steel creates hydrogen at the steel electrode, which creating hydrogen embrittlement. Incidentally, a more stressed area will absorb hydrogen at a more rapid rate, compounding existing stress.
Caustic cracking becomes a problem when metals and alloys are corroded by sodium or potassium hydroxide. Caustic embrittlement results in cracks in boilers where high concentrations of sodium carbonate undergoes hydrolysis to form sodium hydroxide, making the water alkaline. As alkaline water enters into miniscule cracks existing within the metal, hydroxide levels increase and exacerbate corrosion.
Sulfide Stress Cracking (SSC)
Marine environments are particularly vulnerable to sulfide stress cracking, created when tensile stress is combined with the corrosive presence of water and hydrogen sulfide. The aggressive environment found on oil rigs and other offshore environments may contribute to localized corrosion around bolts, rivets, and welds even in high-strength steels.
While neither stress nor corrosion can be entirely eliminated, starting any joining project with high quality, thoroughly tested fasteners will offer a stronger foundation and a longer lifespan for structures. To learn about the superior quality metal fasteners and fastening systems offered by Production Fastening Systems, contact our specialists today.