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Causes of Fatigue and Cracks in Fasteners

May 24, 2018

Fatigue-vexed Bolts fail violently when this metal debilitating affliction takes hold. A structure, deprived of parts-anchoring strength, literally comes apart as the fasteners crack. If we're to stop fatigue in its tracks, we need to know why the metal alloy is breaking down. To put it another way, to know your enemy approach is called for, one that'll defeat the metal cracking influence.

The Fastener Antagonist

The fatigue is invisible at first, then it shows up as a barely discernible hairline crack. Inevitably, the tiny crack spreads across the bolt, then the fastener experiences an abrupt breakdown. It appears like the material failure occurred suddenly and without warning, when in fact it had been developing for some time. Hard to detect, the only solution here is to identify a fatigue source before it develops that initial crack.

Fluctuating Vibrations

A hardened bolt pulls the flanged edges of a ducting channel together. The joints are airtight, the threads have been preloaded, and the job is considered completed. Yet the bolts fail. Looking back at the first few weeks of operation, we see that the equipment was vibrating. Hard metals don't do well when strong vibrations and dynamic forces are continually cycling a load. Likewise, the contraction and expansion effect introduced by a cycling heat source will inevitably cause cracks to spread through hardened bolts. Fastener ductility is key on this occasion.

Environmental Factors

Improperly conscripted fasteners fail when they're installed in difficult conditions. The metal, lacking a protective coating, corrodes and is eaten away. Corrosion-related fatigue quickly combines with stress corrosion cracking to undermine the required clamping force. Furthermore, corrosive effects worsen when fasteners are placed in places where harsh atmospheres prevail. Lastly, dissimilar materials are known to create galvanic current flows, with the fastener acting as the sacrificial material. Again, the enfeebled bolts crack because they can no longer support their load.

Installation and Manufacturing Errors

Seasoned contractors select bolt types that are rated to handle predetermined loads. They assess the material to ensure it won't corrode, choose wide diameters, and match those bolts with load spreading flat washers. It's the same in the fabrication factory; every fastener line is subjected to a quality assurance test and heat treatment. Thanks to this approach, poorly coated metal finishes are avoided, as are certain undesirable material-compromising effects, such as hydrogen embrittlement.

Back at the factory, the material and post-processing operations are continually checked. Out in the field, preloading actions combine with selection procedures and supplementary washers. Finally, should the worst happen, the cross-sectional area of the failed shank is placed under a microscope so that the wear pattern can be examined.

We specialize in metric fastener bolts, nuts, washers, screws, rivets, construction fasteners, allthread, purlin bolts and purlin brackets, chemical anchor studs, driver bits, hex bolts, hex set screw, hex nutssocket head cap screws, hex head coach screw, coach bolts and nuts, sleeve anchors,  wedge anchors, true bolts, hex flange nuts, hex dome nuts, nyloc nuts, flanged nyloc nutjam nuts,  spring washer, and many other items including zinc nut, stainless nut, galvanised nut, screw bolt, stainless bolt and all types of screws and bolts.

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TCI Fasteners - Topcope
13 Slater Parade, Keilor East VIC 3033 Australia

Telephone: (03) 9336 0155