Fatigue endurance limit for standard fasteners
Fatigue endurance limit for standard fasteners
(OP)
Is there a rough general guideline to the endurance limit for fasteners, e.g. grade 8 bolts, as a percentage of proof load or ultimate tensile strength? Thanks!





RE: Fatigue endurance limit for standard fasteners
longer answer ... endurance limit would be similar to the bulk material, factoring in Kt (at the thread, think of a large number) and preload (and this is were it gets complicated).
RE: Fatigue endurance limit for standard fasteners
RE: Fatigue endurance limit for standard fasteners
RE: Fatigue endurance limit for standard fasteners
When I asked a bolt manufacturer about this I was told that bolts are not subject to fatigue if they are tightened down properly.
If you are unable or unwilling to tighten your bolts down hard, or your joint is too compressible, you will have to look into bolts designed to survive fatigue. There is an example of this shown in my machine design textbook (V.M Faires). Perhaps there is something in yours!
There is always Google.
RE: Fatigue endurance limit for standard fasteners
thread725-178349: Bolt Fatigue
RE: Fatigue endurance limit for standard fasteners
http://www.unbrako.com/dnloadengg.htm
RE: Fatigue endurance limit for standard fasteners
You may want to consider tightening your bolt more if it's experiencing significant cyclic loading. In most bolted joints, the larger cyclic load is experienced by the flange.
RE: Fatigue endurance limit for standard fasteners
Whoever said that didn't know much about the behavior of bolted joints. External loads applied to the joint will cause an alternating stress in the bolt but the amount of load going into the bolt is depends on the interaction between the bolt and the clamped material. For our cases, we have loads due to installation torque and temperature (both internal loads) and pressure and vibration. The load in the bolt will be
Ptotal = Ptorque + P(Delta T) + 1/(1+R)*[P(pressure) +/- P(vibration)] where R is the ratio - KJ/KB. Using +/- for vibe loads is conservative but we make airplane parts so conservative is good.
I recommend that you get a copy of just about any book on bolted joints written or edited by John Bickford. Google "joint stiffness ratio" and one of his books is the third link. I think his latest was called Handbook of Bolted Joints which includes a chapter on methods developed in Germany. VDI 2230 may be the best approach to computing bolt loads available today.
Doug
RE: Fatigue endurance limit for standard fasteners
Every car driven past 50,000 miles has probably accumulated over 100 million "cycles" on the con rod bolts. Likely not very much time spent at max revs and highest inertia loads, but I'd still give most of the credit to "proper" installation torque reducing the range stress to a tiny fraction of the applied alternating stress, not the rolled threads or nice parabolic under head fillets. I think it is no coincidence I have not been able to convince any Non-believers that if their convictions are indeed courageous they should have no qualms whatsoever loosening all their short's (*) con rod and main bearing bolts/nuts, re-snugging to 3.5 lb-ft, then hopping on US I-90 and heading for the opposite coast with a shoebox half full of gas money, but without credit card, cell phone or AAA card.
* http:/
Dan T
RE: Fatigue endurance limit for standard fasteners
You correctly point out that overall joint design is important for maximizing the fatigue life of a fastened joint. But I would disagree with your assertion that the underhead and thread root fillets are not of much benefit. With your example of conrod bolts, the underhead and thread root fillets represent points of stress concentration, commonly referred to as Kt, which can be as high as a factor of 3 or 4 in an analysis.
The conrod bolts, preloaded or not, only experience varying tensile loads for each fatigue cycle. The tension loads due to piston inertias simply relieve the bolt preload strain and don't add to it. This equates to an R value between 1 and 0. The R value is the ratio between the maximum and average fatigue load. A properly preloaded conrod bolt would have not experience an R value above about 0.5. Much less of an impact than a typical Kt for a bolt or screw thread.
So the most beneficial approach to optimizing your conrod bolt fatigue life is to minimize Kt (stress concentrations), by using large bolt underhead and thread root fillets, by using mechanically rolled underhead fillets and threads, and by using conrod bolts with a long and reduced diameter shank body.
The improvements in fatigue life provided by mechanically cold-worked surfaces (thread rolling, shot peening, fillet rolling, etc.) has been shown to provide an increase in fatigue life of at least 300 or 400 percent. Just take a look at the specs for any high performance aircraft bolt.
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Regards,
Terry
RE: Fatigue endurance limit for standard fasteners
Proper joint design is critical. I'm not sure what you consider a "tiny fraction" but if you look at the joint equation:
Ptotal = Ptorque + P(Delta T) + 1/(1+R)*[P(pressure) +/- P(vibration)]
the first two are "internal" loads and the other two are "external," what I think you called "applied." I can't recall ever seeing an R > 4 and I wouldn't categorize 20% as being a tiny fraction particularly when the mean stress is held at such a high level; installation torques often bring you up to a high fraction of Fty. A small alternating load with a high Kt is a killer.
The stress concentrations mentioned by Terry are critical. In many of the bolted joints we analyze, the largest applied load in the whole pile is due to thermal expansion differences between the bolt (usually 17-4 or A286) and the aluminum lug. 100% of that applied load goes straight to the bolt. The head and thread forming processes are critical.
Doug
RE: Fatigue endurance limit for standard fasteners
RE: Fatigue endurance limit for standard fasteners
A local one time yielding in the threads will redistribute the load to more of the threads and thus reduce the overall thread load.
RE: Fatigue endurance limit for standard fasteners
Doug
RE: Fatigue endurance limit for standard fasteners
if you control with a torque wrench the torque is +-33%, this limits the maximum torque you can apply (considering 133% preload) and affects the fatigue life (assuming 67% preload).
if you control with PLI washers your tolerance comes down to +-10%; clearly you can specify a higher torque and you'll get a better fatigue life (as the minimum torque is so much higher).