That is a pretty broad category. Some parts associated with said item can.
Do engines get fatigue wear?
- Thread starter Threeofnine
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dnewton3
Staff member
I had a friend who studied metalurgey in school, and he'd say this:
All metals have a fatigue point (max force value) and a fatigue life (max cyclic exposure). Both can lead to failure if operated past those limits.
The question becomes one of "if" you'll ever operate an engine in a condition that it would see one or the other actually happen.
We have no way of knowing if any of this one particular engine's components have a manufacturing defect that would be exposed in stressful operation. We don't know how much it's been babyed in the past, or run too hard. Etc .... Even healthy components have a finite life, if that life is lived hard enough for long enough.
The reality is that "Yes", absolutely there is a potential to "fatigue" this engine to failure. The only way he can tilt the odds in his favor is to reduce the operational forces (don't rev it hard or load it hard). If he continues to drive it, the cyclic count goes up. No other way around these facts.
All metals have a fatigue point (max force value) and a fatigue life (max cyclic exposure). Both can lead to failure if operated past those limits.
The question becomes one of "if" you'll ever operate an engine in a condition that it would see one or the other actually happen.
We have no way of knowing if any of this one particular engine's components have a manufacturing defect that would be exposed in stressful operation. We don't know how much it's been babyed in the past, or run too hard. Etc .... Even healthy components have a finite life, if that life is lived hard enough for long enough.
The reality is that "Yes", absolutely there is a potential to "fatigue" this engine to failure. The only way he can tilt the odds in his favor is to reduce the operational forces (don't rev it hard or load it hard). If he continues to drive it, the cyclic count goes up. No other way around these facts.
Every metal has a fatigue life. The thing with some metals (ferrous metals and Titanium) is that they have an "endurance limit", which is a stress level below which the fatigue life is infinite. (over 10 million cycles).
All other metals have a finite fatigue life, regardless of stress level. There is no stress level so low that aluminum or magnesium won't eventually crack with repeated cycles. But through careful engineering, that "eventual" date can be made to be decades or centuries, so it's not much of a practical difference vs the actual infinite life point of ferrous alloys.
Aluminum fatigue is why aircraft have regular in-depth inspections at regular intervals. Key structural elements are often replaced on regular schedules to reset the fatigue calendar and forestall cracking.
Indeed.
This is why stiffness is often as important or more important than strength. You might have two crankshafts, for example, that have identical strength in terms of being made from the same material (say, AISI 4340), but they will have wildly different real world fatigue life if one is much stiffer than the other.
You also get to the point with premium materials where the quality of the alloy (cleanliness, freedom from inclusions) is far more significant than the nominal mechanical attributes of tensile strength and yield strength.
Inclusions will initiate fatigue failures and make even those most "expensive" alloys perform like something much lower grade.
This is why sometimes, instead of upgrading the material in terms of going to a different steel, for example, it's quite often a smarter upgrade to upgrade the same material with more cleanliness or better heat treat/refinement.
For example, you can produce some amazing performance with lowly ductile iron with sophisticated austempering heat treatment, performance that will rival and exceed some steel grades in lab testing. But it's still a cast part, and that's all for naught if you have inclusions or casting defects in stressed locations.
Fatigue isn't really a wear element. Fatigue is a degradation of bonds within the crystal lattice of the alloy.
Wear is a surface phenomenon. Fatigue can start deep below the surface.
I've seen gear failures where the crack initiated deep in the material at the base of a gear tooth, then propagated to the surface. It creates something called a TIFF (Tooth Internal Fatigue Fracture).
This is a purely academic answer back when I was in school: yes for aluminum block / head and no for steel. Most likely you will wear out the rest of your engine or your car before your block's metal got fatique. You are more likely to get a warp head or head gasket leak, scraping the pistons out of spec, etc than your block gets fatique.
All of this summarized, @Threeofnine, I think, is, “let ‘er rip!!”
pistons?
They are aluminum, I corrected myself in post #10.
Originally I was just thinking heads/block but obviously there are many more components to consider.
So then which one are you saying is the better choice? Some times things are counter intuitive. The less stiffer maybe the better choice??
I suppose that really factors in with something like cast iron crankshafts.
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When you say 80% of redline do you mean running it in a lower gear for an extended amount of time?
I would think that the valve stem seals would go before any of the metal parts.
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That is a good point. A lot of times an engine burning oil could just be the valve guides and not the rest of the engine.
The bearings/rubbing/sliding parts will wear out long before any actual fatigue happens.
Not near the issue it once was....The material used in modern positive seal valve guide seals is quite durable.
Preferably driving up a mountain at 80% load.
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When you say modern, how far back?
Last 20-25 years.
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