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(OP)
Someone floated a project the other day which involved refurbishing a used item of plant for a customer who has no loading history for it. The plant structure can be considered to be fatigue sensitive and some anecdotal evidence suggests that maybe at some point someone was actually concerned about this but there's really nothing to work with.

For the purposes of debate, lets say that the structure all checks out and welds are subjected to NDT and turn out to be in good condition. How do you go about determining the remaining fatigue life? It could be 90% of its way through its design life or 10%; at either point, the structure should still be in good condition. I recognise this is probably a somewhat unusual case and maybe no one has a great answer but even a good text reference would be appreciated.

Side note: I was unsure if this was better here or in structural but figured that maybe the aero engineers might have valuable input and they'd be more likely to monitor this forum.

### RE: Residual fatigue life assessment of structure with unknown loading history

If all the tests indicate that the item is in fairly good shape it could be offered to the customer at a discount price with the provision that there is no warranty of continued performance due to the fact that past history is unknown. If the customer balks that that, quote them the price for a new item with guaranteed zero usage. Give them all the information required to make an informed decision and assume as much risk as they are willing to take. Let them make the choice. If you share everything with them they could not accuse you of hiding any critical information.

### RE: Residual fatigue life assessment of structure with unknown loading history

Hi

If there is absolutely no data and there is no idea of what the loadings are, I don’t know where you would start. That said an old rule of thumb which you may or not be aware of:- Stresses in a structure do not usually cause a fatigue failure provided the working stresses do not exceed 25% UTS of the material. So if by chance, some loads that the plant would be subjected too, or could be reasonably estimated then one could run a check for stress level and check it against the UTS. The downside is the design usually ends up much larger than it really needs to be.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Residual fatigue life assessment of structure with unknown loading history

maybe cut a piece of flange, maybe with a hole in it, maybe "virgin" and drill a hole, and run a fatigue test on it and compare the result to new material (ie book values and/or run a test on new material).

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
Thanks so far.

@Jboggs - not an option. It's registered, has to have a lifespan etc. It's one of the few cases where our workplace safety government bodies do get involved and is a definite killer if someone screws up the life assessment. Once it's classified, it SHOULD be cross-checked on a yearly basis to make sure the utilisation matches the original classification etc. I can understand the end customer's desire. There's a million dollar piece of gear standing there, looking for all the world to be in good shape, getting wasted. And for all I know, it's got years of life in it. I just don't know and I'm not sure how anyone can find out.

@desertfox - I'm aware of the endurance limit. Without undertaking a detailed review of the design, I'm 99.8% sure a fairly significant number of components will be stressed well beyond it. I'm extra confident of this because at some point someone significantly 'downgraded' it, which is almost always done in an attempt to squeeze under that limit.

@rb1957 - that's a great idea and I hadn't thought of it. It may not be practical in this case because a) the bits that are likely to be heavily worked might be rather hard to cut out and b) without good utilisation data, it can be rather difficult to determine what's been worked hard (for example it is possible to heavily work one area or component whilst leaving the remainder in almost 'as new' condition). However, it merits much more thought. Maybe I need to take a bucket load of samples which would correspond to a lot of repairs to the sampled locations thereafter. Maybe I can find a good canary component which is fatigue sensitive and always loaded. Regardless, it's a potential approach which could yield enough data to make a proper scientific call on the issue.

### RE: Residual fatigue life assessment of structure with unknown loading history

yes, I understand no historical spectrum of loads. But if you test a piece and it fails at 50% of the predicted number of cycles, well, you know something ... just with not much confidence !

If this new application is fatigue critical or fatigue sensitive then your situation is very problematic.

Another approach, from my "wheel-house", would be DTA ... develop an inspection program that'd ensure cracks would be found before the piece failed. You can avoid the unknown history by assuming a just-not-detectable crack is present; inspect now, then good for a "repeat" inspection interval. Not at all easy to do, but another approach.

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@MintJulep - believe it or not, even prior to your message, I was doing my level best to short of paying for it but so far I haven't found it included in any of the subscriptions I have access to. Strange. I've read a couple of other thermography/fatigue papers and abstracts and the feel I get is that it may not be practical for larger plant where our loads might be in the realm of 50 tonnes or more; the excitation might well need to be of similar scale to demonstrate detectable results and might be difficult to achieve in the real world. I might have to just cop it on the chin and pay for it; it just stings more when you do and it doesn't help . That said, I'm super interested in whether there's a ND means of assessing such things. If it was relatively reasonable in price, I'd consider investing in adding it to more common place inspection work to build up a personal database of cumulative damage results for similar equipment. Extra useful if I can correlate it to cases where I DO know the history.

@rb1959 - I might not have been clear enough; I wasn't referring in my reply to just the lack of load history but also the specific lack of per component load history. Normally what we assume is that each load cycle loads each component in a worst case kind of fashion. However, when we're working backwards like this, we must almost make the opposite assumption to be conservative. IE - we can't test a convenient sample only from component A and assume it to apply to component B in the case that movable loads (and the idiosyncracies of the previous usage) might have managed to only really load component B leaving A in perfect condition. That being the case, you end up having to test a lot of different areas. Not impossible, just needs to be allowed for. It's a good idea.

The application was and is definitely fatigue sensitive. Always needs to be considered although, quite often, the equipment may be so under utilised that it doesn't actually apply.

I must profess lack of familiarity with the DTA acronym but I get the general picture. I've employed that approach once previously on a monorail in a foundry that couldn't stop working for the period it would require for the very unique replacement to be sourced, fabricated and installed. It was expensive and I didn't sleep much. Definitely not easy to do, I'd definitely employ a fracture mechanics consultant to advise and on a substantial structure the cost of undertaking the NDT on a regular basis will probably mean that it's unreasonable on a longer term basis (ie good to limp it through for a period if there's considerable financial incentive to do so, not so suitable for a 10-25 year lifespan).

### RE: Residual fatigue life assessment of structure with unknown loading history

rather_be_riding,

How catastrophic will it be if this thing fails?

--
JHG

### RE: Residual fatigue life assessment of structure with unknown loading history

if you don't know the fatigue load spectrum, how do you do fatigue analysis ? You could say 100,000 limit load cycles, or you say you've designed for infinite life.

But to understand the current material's fatigue state given an unknown history, yu can tet a piece under standard loads and correlate the result with material standards. This is not very good, in that confidence should be very low (how do you know your sample is representative ?).

Like drawoh, what are the consequences of fatigue failure ? what is this in any case ? (machinery ? structure ??) what material ??

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@drawoh and @rb1957 - loss of life would not be an unreasonable outcome although productivity and damage to plant are substantially more likely. And of course I'd rather avoid the blow to the ego, the reputational damage, the loss of registration and potential jail time . Conservative is the only approach here but I do think there's at least room to consider if there's an alternative option to the super safe 'nah - can't be done, mate' advice. I'd classify this as lifting equipment (which is why I'm involved). I'm expecting the majority of what I'm concerned about to be AS 3678 Grade 250 plate with maybe a 300 or 350 thrown in here or there. There's a possibility that there might be a few pinned joints or the like out of 4140 or the like. Technically that all has to be sampled to be sure though unless someone somewhere miraculously finds some drawings. Welds are highly likely to be high quality sub-arc on longer runs and MIG in the detailed areas; hopefully of high quality too - they should have fabricated in the first place with fatigue in mind.

You have a reasonable point about representative behaviour. The only way past that is multiple samples, from multiple locations and using them all to come to an overall conclusion where the weakest link will define the remaining life. Of course, we're also looking to ensure we have sufficient safety factor so 100k full load cycles to sample failure will end up being something more like 33k design life cycles combined with a more stringent maintenance plan than usual.

Even if this project doesn't go ahead (or I bow out due to excessive risk), I think this is a good discussion/debate to have as such things come up semi regularly in my experience. It's quite common that lifting equipment is massively underutilised but there's just no data to support such an assessment. The easy out is to say it can't be done but it's wasteful and if a solid approach to control the risk can be developed, I think it's almost a little unprofessional.

### RE: Residual fatigue life assessment of structure with unknown loading history

As I tell junior engineers regularly, let the standard process drive engineering and take it one step at a time. Attempting to solve technical problems without first understanding what they are, how many exist, or their importance in the grand scheme is pointless and causes rookie mistakes. Create a DFMEA to identify and rate all possible failure modes, then perform whatever engineering analysis necessary on the highest scores to identify weaknesses, standards and acceptance criteria that must be met, and any functional testing necessary. Follow that up with a manufacturing PFMEA. As you're not starting with certified material, the PFMEA will obviously identify material testing on critical components identified by the DFMEA. The material either meets your requirements or it doesnt for any given component, but a lifecycle analysis completed XX years ago is irrelevant.

Engineering at a high level is all the same. Follow the process, not only does it CYA legally but also makes the myriad of project planning simple while minimizing risk. There may be a lot of expensive work to do on this project or very little, but you won't know until you get through the planning.

### RE: Residual fatigue life assessment of structure with unknown loading history

Is there some failure mode where fatigue causes a structure to crumble as dust at N+1 cycles or explosively ruptures without showing any external cracks? For fatigue intolerant structures the typical approach is to make a decent guess about the smallest non-detectable crack and the pace at which that crack might progress under the known loading to set the period of inspection.

You don't need the drawings - they are a hindrance. You need to accurately reverse engineer the as-built item and have an FEA done to see which areas are subject to the highest stress and supply an estimate of what the critical crack length will be. The toughest part is identifying the material properties in the heat affected zones, particularly the alloy, but I suspect it can be done.

Strip it, dye penetrant inspect the whole thing for external cracks, maybe Magnaflux it; then do X-rays for the places where the FEA suggests internal cracks might originate or just to find where voids might exist.

There doesn't seem like there's a lot of risk - there may be a lot of cost.

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@3DDave - nothing so dramatic. Inspection is difficult/time consuming/expensive during service. Catastrophic failure of such things in my experience is usually due to poor maintenance. The rub is that poor maintenance is more the norm than the exception.

At a guess, it might take a day and a half or two days to crawl over the structure to NDT it each time. That's not including any of the time required to strip it and recoat. A somewhat similar item of plant I was involved with briefly a few years ago had crack growth periods in the realm of a fortnight. The inspection costs were phenomenal but at least it didn't fall down.

I think you're right. Entirely possible to reduce the risk substantially with that approach. It just might be that the client doesn't like the price, particularly that of ongoing inspections.

### RE: Residual fatigue life assessment of structure with unknown loading history

#### Quote (rather be riding)

There's a million dollar piece of gear standing there, looking for all the world to be in good shape, getting wasted.

This is your problem - not the fatigue life issue (IMHO).

What there is, ia a pile of scrap metal. If, as you state, this needs to be licenced/registered for a set period to be of value to anyone then without the past data you can't realistically provide the data to make an assessment, other than maybe 6 months or 1 year.

The same thing crops up on boilers and pressure vessels when someone comes on and says I have this PV, but the nameplate is missing and I don't have any paperwork to go with it. It's a crying shame, but the same answer, what you have is an expensive paperweight. It's the fault of the previous owner not to do the testing or provide the data, or maybe they never had it but weren't as carful as you are.

Do you have ANY history of this thing, even when it was made, where it was installed, was it inspected in the past - anything? If not or very little then I think it's time to walk away.

I used to visit Baku occasionally years ago and on every journey down the coast you went past a huge jacket on its side, ready built and just abandoned by the Russians when they bugged out. It sat there for years whilst everyone tried to find a use for it and if it could support anything worthwhile without collapsing. No real data or design or material certs worth talking about. All sorts of samples were taken and the design reverse engineered and in the end I think they scrapped it or had to accept it could only hold up a fraction of what it should. Cut your losses early is what I'm hearing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Residual fatigue life assessment of structure with unknown loading history

I think I am with LittleInch on this one in that its time to walk away, I remember I job where we were putting new equipment on old foundations and the client wanted where possible to use the existing foundation bolts even though they were 40 plus years old, hence to say all the foundation bolts were renewed, no way were we risking using foundation bolts we knew nothing about. The more (rather be Riding) tells us the more I am inclined to walk away, seems to me that someone wants a cheap job without carrying the responsibility

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Residual fatigue life assessment of structure with unknown loading history

It seems as though a risk assessment is in order. This piece of equipment is not really different from a new machine which fails due to a hidden flaw. Define the protocol and procedures that will be used to monitor and ensure safety, and get the owners to commit to the safeguards and sign off on it. If you can't afford the price of the consequences, then move on.

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@LittleInch, desertfox, dvd - fair and reasonable I reckon. That's probably where it will end up. I actually suspect that if I give them an estimate on addressing it the 'right' way, they'll chase around getting three quotes (which is reasonable) and one of those three will be a service provider that doesn't use an consulting engineer and just doesn't care. Lifting/handling equipment is full of such cowboys unfortunately. In not so distant times, there was a crane supplier who used to buy old equipment out of factories, cut and shut them to new dimensions, paint them and install them as 'new'. I suppose the quotes didn't explicitly state that the crane was new! Every now and then, I get someone with one of these nightmares referred to me and it would be great to be able to help them out in a manner that doesn't saddle them with excessive maintenance costs thereafter. Nothing to do with this project at all but similar kind of problem and application.

Re: history. I know where it stands is where it was installed. Way too big to move without someone definitely knowing about it. Sometimes, that means you can have a really solid educated guess at the use something has seen but it appears in this case noone can/will. I don't have a date of manufacture but would be surprised if we can't track that down somehow. There should be an original registration with the state body. The hard part there is usually just getting their assistance. It will have been inspected and maintained; to what degree is anyone's guess and the documentation is in the wind. So sweet FA on the whole. Without the loading history, all the rest is pointless at this stage anyway.

### RE: Residual fatigue life assessment of structure with unknown loading history

Well you can only hope the regulator is wise to these cowboys...


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Residual fatigue life assessment of structure with unknown loading history

Refurbishing a crane is to a large degree an economic question. From your statements above, the costs of investigating this crane to the point that the risks of future service are acceptable (at least as far as your firms's involvement is concerned) are to high to be practical.

From a strictly engineering standpoint, if the crane is designed to FEM or something similar, you are more likely to find fatigue life limiting situations in the machinery, and in structural locations with stress concentrations. Developing an inspection plan is important, the plan will help to identify the risks and costs. If the inspection plan is too expensive to be worth your effort you have your answer.

### RE: Residual fatigue life assessment of structure with unknown loading history

#### Quote (rather_be_riding ..... lets say that the structure all checks out and welds are subjected to NDT and turn out to be in good condition. How do you go about determining the remaining fatigue life? It could be 90% of its way through its design life or 10%; at either point, the structure should still be in good condition. I recognise this is probably a somewhat unusual case and maybe no one has a great answer but even a good text reference would be appreciated.)

I just screened your posts and still i do not see any clue for the type of ( a used item of plant ). The loading history is essential data . If not available, you may look for experience with comparable equipment operating under similar conditions,speak with operator for the loading history. You need to conduct fatigue assessment study to determine the remaining fatigue life.

I will suggest you to look to the Part 14 – assessment of fatigue damage of ASME FFS-1 (Fitness-For-Service )

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@All - having had an opportunity to discuss the issue with the end client, it turns out 3 of the 4 engineers involved had a VERY different idea to the one who approached my client with the project in the first place so this is all academic. However, I threw this out there for the purpose of the discussion just as much as for this particular case.

@LittleInch - the regulator appears to be too worried about whether people are working in high vis and whether trestles are load rated to concern themselves with actually understanding lifting equipment. Such is life. Luckily there will always be a few companies out there who want to do the right thing for some reason.

@FacEngrPE - this applies to almost all lifting equipment in my opinion.

@HTURKAK - lifting equipment was noted but the specifics don't matter - I was intending to float a generalised question to best 'fish' out suggestions from a broad range of experiences. Which as it turns out, has yielded your reference which I'll chew through at my leisure down the track. All approaches do appear to rely on the existence of a load history at first glance however. It was as very quick glance though so don't judge too harshly if you're aware of a specific area which is particularly pertinent.

@rb1957 - I'll answer this one more generally. These cases may come up in or out of service. A common one I've come across involves cranes which have been taken from service and resold without history or even knowledge of their used status to a third party. Sometimes the unsuspecting third party finds out they've been screwed and then they go looking for solutions. The load history in that case is truly completely unknown and could range from unused through to absolutely shagged. Unused mystery lifting equipment is surprisingly common in my experience as well. Sometimes it's left around a site from the previous users and the new user/owner/occupant gets a grand idea to use it. Sometimes, it was a headache that was too difficult to deal with at the time; production was ramping down maybe and the associated handling equipment could be substituted or eliminated rather than maintained or repaired. Then at some point a rejig in lines or processes requires new means of handling something in the area the old equipment is in so they look at repurposing. A very common example would be crane runways as well. They're often reused for multiple crane installations. Sometimes they're checked for alignment if you're lucky. Rarely are they reviewed for fatigue and standard inspections don't look at runways in any particular detail if at all because everyone is too busy getting excited about the mechanical aspect. If you're lucky, they might get some love every 25 years or so. Thanks for the acronym. The DT should have been obvious.

@CWB1 - I understand your train of thought; however lets assume there's some reasonable level of confidence in the history for the sake of arguing over it. At the end of the day, if I sign off on it, I HAVE to rely on that history if I'm going to offer a non-restrictive use of the equipment which is what we really expect to see happen in the vast majority of cases. If I don't trust the history, it's either out of service or subjected to crack growth analysis and much more expensive inspections designated in line with what that yields. The third option might be rb1957's lovely suggestion of testing components which could be very useful and isn't an approach I've previously tried. Technically, if we limit our discussion briefly to cranes specifically, they're required to be assessed for remaining working life on an annual basis by comparison of the loading history with the design classification. At some point, it's essential to rely upon the history, even if you've looked over the crane and detected no signs of fatigue damage because we're talking about the future condition of the item not just how it was at the time of inspection.

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@rb1957 - jinx on that. It's either rely on it or go nuclear on cost. Sorry about the slow reply - it's rather early here ;)

### RE: Residual fatigue life assessment of structure with unknown loading history

ok, so there isn't a piece of equipment sitting around, this is more a general query about fatigue design and maybe some horrible practices ?

It is an unfortunate outcome of fatigue design that having defined a safe life, then the structure is unsafe to operate beyond that life without further investigation. Yes, the structures look sound (or rather "look" sound) but fatigue defects (undetectable fatigue defects) may exist, so continued operation is at peril.

How a used crane can be sold without knowledge of it's previous life, with the intention of being used as a crane, is quite beyond me.

Although, I would have thought that cranes would've been designed for low stress levels, and therefore low/no fatigue effects.

Two approaches to extend the fatigue life, without going "all in" on a through DTA ...
1) repeated "limit" loads which would validate some period of operations (like an annual check-up) ... would require some analysis to validate the approach.
2) design the crane structure to be fail-safe, and for the failures to the clear and obvious. The crane "could" operate with a failed member for a limited period of time, whilst the replacement part was being brought on-site.

My industry has by and large left fatigue behind, but now safety depends on various inspectors and maintenance techs ... but it seems to work ...

### RE: Residual fatigue life assessment of structure with unknown loading history

This is a bit off topic - a robust inspection program with knowable inspectors is an essential part of reducing the risks of operating cranes. Well trained operators and riggers are also essential. Companies that chose to ignore this will eventually incur the wrath of their OHS regulators.
However if your company is a supplier of an organization that has decided safe crane operation is really important , and has made it a contract requirement as the US Navy does, you will get in trouble with them first. The NAFAC Navy Crane Center site is worth a read.

The NCC site has several useful safety videos, which are generally applicable.

### RE: Residual fatigue life assessment of structure with unknown loading history

(OP)
@rb1957 - Originally the question matched a specific piece of equipment I received an enquiry about but after I managed to speak to someone who actually knew what was going on, it didn't fit the profile. As you can see it's pretty much either a case of it's too hard/expensive or figure out your damage tolerance and inspect to suit. By now it's definitely more a general query to see if anyone approaches things differently to me. Your sample approach is the outstanding option and one day the right job will come along and I might be able to give it a whirl.

RE: used lifting equipment. It's not overly hard really when you think about it. Buy cheap crane, cut and shut, sell aforesaid crane to unsuspecting buyer who makes widgets and knows precisely nothing whatsoever about cranes bar the fact that they need one and it needs to lift a certain load. Get money, try to be far away before anything happens. A lot of the time I imagine no one ever knows anything more about it and everyone continues on merrily. It's not even all that hard to do it if you're honest about it being second hand really. Most people approach stuff like this on the basis that if it's not cracked now, it's OK for as long as they want to use it. I regularly have to argue with people who think a test load proves the competence of a structure beyond all doubt.

Completely wrong on the stress level/fatigue front in the majority of cases. Of course where people are aiming for the higher classifications, you're right but those are the exception rather than the rule. Once again this is a function of cost too. You can sell a C3 crane to someone and conscientiously avoid asking whether there's any possibility that they might really need a C8 crane for the rather continuous duty role they have in mind. You'll be cheaper than the supplier who quotes a C8 crane. Over time, this seems to have the effect of causing everyone to avoid up-speccing rather than resulting in issues for the disingenuous. When cracking starts appearing after 5 years, you might be outside warranty and you can assist the client with great sincerity explaining how they have used it far in excess of its design intent and no one can do anything about that now but you'll do them a great price on a higher classification crane because you can see they're in a tough spot.

Side note - it wasn't my intention to whine about less than professional practises. It just tends to come up when you have to consider what's likely to happen in the real world and how much you can trust anything at face value.

@FacEngrPE - no argument here. Just wasn't the point of the post. I spend a fair amount of time trying to advocate for better quality inspections. It's difficult on the main because of the cost drivers. No-one loves paying for PM. Fixing stuff that ain't broke (yet) isn't very sexy.

### RE: Residual fatigue life assessment of structure with unknown loading history

#### Quote:

lets assume there's some reasonable level of confidence in the history for the sake of arguing over it. At the end of the day, if I sign off on it, I HAVE to rely on that history if I'm going to offer a non-restrictive use of the equipment which is what we really expect to see happen in the vast majority of cases. If I don't trust the history, it's either out of service or subjected to crack growth analysis and much more expensive inspections designated in line with what that yields.

Depending on the work being quoted, I dont see how using that info is possible. If you're simply recertifying a crane that has been continuously operated and maintained under the original O&M program then sure, history is useful and important. Removing and rebuilding (OP said refurbishing) the structure OTOH is a different circumstance entirely bc its outside the original engineer's scope of work - new install + O&M. In that instance you will be 100% design responsible and cannot claim flaws in the original design or O&M records caused failure. Even if there are no failures, if you are discovered to have taken significant shortcuts by relying on the original design (without being the original designer) or O&M history then they are well within their rights to sue or get regulators involved bc that's a pretty flagrant violation of the standard of care.

As to how shops refit cranes, its not a particularly expensive or lengthy process for simple traveling bridges bc realistically - theyre simple machines.

### RE: Residual fatigue life assessment of structure with unknown loading history

buying a used crane without history ... caveat emptor. Someone buying a crane should know what they have to do to recertify the crane, if not refer PT Barnum.

I assume cranes have some design life (I suspect to be based on a number of max load lifts). History would only help if thoroughly approved/stamped or some such. I've recently done a project where I showed the remaining life in a plane based on the airplane operation records. Airplane records (logbook) are taken as fact. If the crane's records are not held in the same regard as an aircraft's logbook, then they are of little value other than to say number of lifts completed.

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