How can I determine the life of a weld that is welded in the middle of a deflecting surface? All I have been able to find is cantilever examples. I am needing to test the welds of en eyelet that is welded in the middle of the bottom side of a 20 foot piece of large tubing. The eyelet is going to be lifting 40,000+ lbs by way of slings, and the tubing deflects 11cm or so. I have tried strain gage testing by installing the gage so that it is on the weld and the tubing, but the portion of the gages that are on the deflecting surfaces are causing me to get outrageous stress readings that would tear that lifting eye right off if those stresses were really present. I honestly don't think there is anything wrong with the design, but I can't figure out a way to validate it. An idea I had was installing a gage completely on the weld, parallel to the deflection, and based on those stress readings, use an S-N curve to determine the number of cycles of deflection that would cause the welds to fail. Would this work? There is alot of talk of redesign, but I really don't think it is necessary. I just need a way to test and/or prove that the welds will hold up over time. Any help, or a point in the right direction would be greatly appreciated.
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Life of a weld on a deflecting surface??? 1
- Thread starter icruz
- Start date
MikeHalloran
Mechanical
What makes you think you are qualified to be designing lifting gear?
Pardon the pointy response, but we don't know you, and this is serious shit.
Mike Halloran
Pembroke Pines, FL, USA
Pardon the pointy response, but we don't know you, and this is serious shit.
Mike Halloran
Pembroke Pines, FL, USA
- Thread starter
- #3
Mike,
1st: Where do I claim to have designed anything? I'm pretty sure all I said is that I am testing it.
2nd: I am fresh out of school & 2 months into my job & in my limited experience with weld analysis, I never encountered a scenario like this.
3rd: I thought this was a site where you can seek answers. I had no idea flashing credentials & meeting certain qualifications was necessary to ask for help on here.
4th: I am well aware of what I am & am NOT qualified for at this stage of my career. And I know when I'm in need of some assistance, which is why I posted on this site in the first place. So who exactly do you think YOU are to call out someone asking for help (just curious)? I never claimed to know something that I didn't.....just asked for some guidance. So please excuse your rudeness or excuse yourself from the thread.
1st: Where do I claim to have designed anything? I'm pretty sure all I said is that I am testing it.
2nd: I am fresh out of school & 2 months into my job & in my limited experience with weld analysis, I never encountered a scenario like this.
3rd: I thought this was a site where you can seek answers. I had no idea flashing credentials & meeting certain qualifications was necessary to ask for help on here.
4th: I am well aware of what I am & am NOT qualified for at this stage of my career. And I know when I'm in need of some assistance, which is why I posted on this site in the first place. So who exactly do you think YOU are to call out someone asking for help (just curious)? I never claimed to know something that I didn't.....just asked for some guidance. So please excuse your rudeness or excuse yourself from the thread.
MikeHalloran
Mechanical
Perhaps I read too much into your statement,
"There is alot of talk of redesign, but I really don't think it is necessary."
That's the sort of defensive thing that might be said by someone who designed the extant item.
Could you please provide a drawing or photo? I think I know what you're talking about. It sounds nontraditional, and scary, so I'd like everyone to be talking about the same elephant.
Mike Halloran
Pembroke Pines, FL, USA
"There is alot of talk of redesign, but I really don't think it is necessary."
That's the sort of defensive thing that might be said by someone who designed the extant item.
Could you please provide a drawing or photo? I think I know what you're talking about. It sounds nontraditional, and scary, so I'd like everyone to be talking about the same elephant.
Mike Halloran
Pembroke Pines, FL, USA
"Outrageous readings" sounds worrying coming from something that surely has more or less been placed to work precisely to investigate such stresses. If there is concern about the gauging devices being misused, someone should know well in the process. Respect how to validate, prior to tests, either the design conforms to some proven solution to the problem, as described per some technical procedure for the lifting devices, or at least one should try to use some of the good software there is out there for fatigue. This would be technical validation, if well applied, and then one should only expect failures either from bad construction or misuse, typically overload, far excessive impact or inadequate concomitant devices damaging our part; not design.
- Thread starter
- #6
Mike,
I can see how that statement can come off that way, but I only said that because we ran some test in exaggerated scenarios, and all the strain gages gave us passing numbers except for the 2 gages that had weld-to-tubing installation that was in line with the deflection. Those 2 gages gave outrageously high numbers, leading us to believe that the displacements of the deflecting surface were causing the gages to give exaggerated readings.
I've attached a couple pics in a word doc to see what we are working with. The tubing that the eye and gussets are welded to is 20 ft long and deflects downward 11 cm when the max load is applied. It IS nontraditional, but it's what the customer requested. I just need to determine if the welds will hold up with the deflection as part of the dynamic. I could end up finding out it won't hold. I just need conclusive results.
I can see how that statement can come off that way, but I only said that because we ran some test in exaggerated scenarios, and all the strain gages gave us passing numbers except for the 2 gages that had weld-to-tubing installation that was in line with the deflection. Those 2 gages gave outrageously high numbers, leading us to believe that the displacements of the deflecting surface were causing the gages to give exaggerated readings.
I've attached a couple pics in a word doc to see what we are working with. The tubing that the eye and gussets are welded to is 20 ft long and deflects downward 11 cm when the max load is applied. It IS nontraditional, but it's what the customer requested. I just need to determine if the welds will hold up with the deflection as part of the dynamic. I could end up finding out it won't hold. I just need conclusive results.
This is just a guess:
I remember a seminar were the speaker said that no weld made was incidental. Meaning that no matter how hard you try, load is going to be attracted to stiffness. The eyelet is going to make the beam stronger
If you are looking for some really precise numbers, you can analyze the section as if the eyelet was reinforcing the beam. You could then figure out how much load was being taken by the eyelet through a hand analysis. The force the weld will see will be the load required to drive the force into the "reinforced section" together with the load place on the eyelet during lifting.
Now if I remember correctly, the AISC used to have a chart for allowable stresses for members under fatigue loading (not sure the 13th edition has this because I have not done one in a while). I imagine what ever jurisdiction you are dealing with would have a similar chart. I would then adjust the size of the weld to bring the stress in the weld below the numbers given in the AISC chart.
Of course, this is just a guess.
I remember a seminar were the speaker said that no weld made was incidental. Meaning that no matter how hard you try, load is going to be attracted to stiffness. The eyelet is going to make the beam stronger
If you are looking for some really precise numbers, you can analyze the section as if the eyelet was reinforcing the beam. You could then figure out how much load was being taken by the eyelet through a hand analysis. The force the weld will see will be the load required to drive the force into the "reinforced section" together with the load place on the eyelet during lifting.
Now if I remember correctly, the AISC used to have a chart for allowable stresses for members under fatigue loading (not sure the 13th edition has this because I have not done one in a while). I imagine what ever jurisdiction you are dealing with would have a similar chart. I would then adjust the size of the weld to bring the stress in the weld below the numbers given in the AISC chart.
Of course, this is just a guess.
MikeHalloran
Mechanical
My limited experience says that strain gages actually under-report strains that are outside of their normal linear range.
Did the strain gages' output return to zero when the load was removed?
Mike Halloran
Pembroke Pines, FL, USA
Did the strain gages' output return to zero when the load was removed?
Mike Halloran
Pembroke Pines, FL, USA
- Thread starter
- #10
- Thread starter
- #11
ishvaag,
Something I failed to mention is that a test mechanism was created to remove deflection from the equation, and our numbers were fine, so I need to determine what kind of fatigue life these welds will have when adding the tubing's deflection to the dynamic.
Something I failed to mention is that a test mechanism was created to remove deflection from the equation, and our numbers were fine, so I need to determine what kind of fatigue life these welds will have when adding the tubing's deflection to the dynamic.
MikeH.... play nicely with the other children, or I’m going to take away you mousie and non-musical keyboard, I’m supposed to be the Old Ogre here. 
This, and parts below, were started right after Mike’s first post and then I got busy on something else for a while.
Icruz..... You have to know enough about your problem, have enough engineering knowledge on the subject, to ask a meaningful question, if you want some serious discussion in return. The way you have asked the question prompted Mike’s reply, and suggests that you are in way over your head. The admission that you are fresh out of school goes a long way toward letting us know where to start helping, rather than hassling. But, it never ceases to amaze me that the powers-that-be will give a young engineer this kind of problem, without any guidance or mentoring, ask a stupid question and just assume his answer should be a piece of cake, all because of their own ignorance. A sketch with complete dimensions, types of materials, weld sizes, force directions and values, etc. etc. would be very helpful in making this discussion meaningful. What does this 20' large piece of tubing do, that you can apply a 40kip load to its bottom with the sling? Does the 11cm deflection happen within 1cm of the weldment or over the 20' length? Your photos really don’t tell us much either: I see a force application lug (lifting lug?) with so much garbage welded to it, but not well detailed, that it will have a very low fatigue life. You haven’t really explained that this thing has a fatigue life or is it lifted 10 times over its life, and you are looking for max. stress and the potential of fracture? You have to describe the whole problem with enough info. so someone with experience and some judgement can see what’s going on, and offer some ideas.
I don’t think strain gages are going to help you solve your problem, or answer the wrong question either. How do you know which portion of the strain gage gave you the high strain/stress readings, and do you know how strain gages work and are used? What S-N curve would you compare your stresses to, how do you know it represents your conditions? You can strain gage that mess until you are blue in the face and the results won’t tell you much of anything.
If that’s what the customer speced. he should have designed it and know it will work. And, if your company is helping with the design, then you have to be smart enough to tell the customer that their detail stinks, and is not a good solution, and won’t work very well. Between the force lug and the three gussets perpendicular to the lug, you have an absolutely awful detail welded to a thinner tube, and have to be able to explain why this is so, not try to prove that it might work. Whether you designed it or not, to work on something like this you must understand the design and know how to fix it, or everyone is being ill served.
Show us some meaningful details and a description of the problem which will allow us to understand what you are really trying to do.
This, and parts below, were started right after Mike’s first post and then I got busy on something else for a while.Icruz..... You have to know enough about your problem, have enough engineering knowledge on the subject, to ask a meaningful question, if you want some serious discussion in return. The way you have asked the question prompted Mike’s reply, and suggests that you are in way over your head. The admission that you are fresh out of school goes a long way toward letting us know where to start helping, rather than hassling. But, it never ceases to amaze me that the powers-that-be will give a young engineer this kind of problem, without any guidance or mentoring, ask a stupid question and just assume his answer should be a piece of cake, all because of their own ignorance. A sketch with complete dimensions, types of materials, weld sizes, force directions and values, etc. etc. would be very helpful in making this discussion meaningful. What does this 20' large piece of tubing do, that you can apply a 40kip load to its bottom with the sling? Does the 11cm deflection happen within 1cm of the weldment or over the 20' length? Your photos really don’t tell us much either: I see a force application lug (lifting lug?) with so much garbage welded to it, but not well detailed, that it will have a very low fatigue life. You haven’t really explained that this thing has a fatigue life or is it lifted 10 times over its life, and you are looking for max. stress and the potential of fracture? You have to describe the whole problem with enough info. so someone with experience and some judgement can see what’s going on, and offer some ideas.
I don’t think strain gages are going to help you solve your problem, or answer the wrong question either. How do you know which portion of the strain gage gave you the high strain/stress readings, and do you know how strain gages work and are used? What S-N curve would you compare your stresses to, how do you know it represents your conditions? You can strain gage that mess until you are blue in the face and the results won’t tell you much of anything.
If that’s what the customer speced. he should have designed it and know it will work. And, if your company is helping with the design, then you have to be smart enough to tell the customer that their detail stinks, and is not a good solution, and won’t work very well. Between the force lug and the three gussets perpendicular to the lug, you have an absolutely awful detail welded to a thinner tube, and have to be able to explain why this is so, not try to prove that it might work. Whether you designed it or not, to work on something like this you must understand the design and know how to fix it, or everyone is being ill served.
Show us some meaningful details and a description of the problem which will allow us to understand what you are really trying to do.
The question is that I think that the introduction of the loads must be very much at the tips of the connection device, and so a high concentration of stress there is likely, local deformation included. And then in the HAZ zone, so, in the actualy fused metal to most effects, are likely to be high, and so I don't see necessarily the high numbers just a product of some bad testing. In any case, in our things, it is doing the numbers what reveals what is the technical opinion.
slickdeals
Structural
@SteelPE......I think that speaker might have been Duane Miller or Omer Blodgett.
icruz...your application is dynamic. If you were using a static stain gauging system, that is probably why you were getting inordinately high stresses. Another reason is that your gauges were not properly applied.
Make sure that your frequency of data collection matches the configuration. When you lift with a lifting lug, you will induce cyclic stress in the entire system. The load will bounce, even if it seems like just a little, it will impose a wide range of stress levels and frequencies.
Use a dynamic strain monitoring system. I actually prefer bonded strain gauges rather than welded ones. I've used them hundreds of times with good success for both static and dynamic applications from bridges to very highly stressed and fast amusement rides. Use redundant gauges...they're relatively cheap.
Make sure your strain monitoring/analysis system is calibrated. A simple calibration that I have used is to clamp a flexible, 1-inch wide steel flat plate about 18 inches long to a sturdy table. Leave 12 inches cantilevered off the table. Apply load to the end of the plate using a calibrated scale or dead weight. Measure the load application point to the edge of the table and compute the expected moment and resulting bending stress. Apply a strain gauge centered on the plate, directly on top of the table edge. Repeat the loading. Compare the results. Bend the plate downward with your hand and suddenly release it. Watch the decay curve for the dynamic load/vibration. The decay should be consistent.
Make sure that your frequency of data collection matches the configuration. When you lift with a lifting lug, you will induce cyclic stress in the entire system. The load will bounce, even if it seems like just a little, it will impose a wide range of stress levels and frequencies.
Use a dynamic strain monitoring system. I actually prefer bonded strain gauges rather than welded ones. I've used them hundreds of times with good success for both static and dynamic applications from bridges to very highly stressed and fast amusement rides. Use redundant gauges...they're relatively cheap.
Make sure your strain monitoring/analysis system is calibrated. A simple calibration that I have used is to clamp a flexible, 1-inch wide steel flat plate about 18 inches long to a sturdy table. Leave 12 inches cantilevered off the table. Apply load to the end of the plate using a calibrated scale or dead weight. Measure the load application point to the edge of the table and compute the expected moment and resulting bending stress. Apply a strain gauge centered on the plate, directly on top of the table edge. Repeat the loading. Compare the results. Bend the plate downward with your hand and suddenly release it. Watch the decay curve for the dynamic load/vibration. The decay should be consistent.
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- #16
bradleyelwood
Mechanical
icruz,
Sounds like you are just trying to drain the swamp, not fight with the alligators. If I read the above correctly, you are trying to test/analyse a design by others. It would be helpful if you told us what the tubular beam did________________. From the looks of the photo, my guess is that it is some kind of spreader beam used "below the hook" of a crane. If so, look up ANSI 30.20 and it will tell you the rest of the story about required annual testing, Factor of safety on ultimate, etc. If not, I appologize for a wrong assumption. The usual requirements for "below the hook" apparatus is a Factor of Safety of 5 on ultimate. The F of S of 5 helps with fatigue.
My background is Mechanical, a master's degree specializing in strain gages and also a PE in structural. Somewhere in my career, I found out about practical fatigue loading of tubular members with welded attachments. The detail doesn't look good if it occurs at maximum bending stress in the beam.
The strain you are measuring may indeed be real. As a reality check I would suggest you mount the same type of gage(s) on the tubular beam a slight distance away from the welded lug and compare the strain (and stress) to what you know the stress should be from simple bending stress theory. Then compare this to the stains and stresses in the weld as you load the beam in bending.
In addition to the obvious bending stresses, spreader beams which have Ixx larger than Iyy have real issues with lateral buckling. Let us know the rest of the story and we may be able to assist.
Sounds like you are just trying to drain the swamp, not fight with the alligators. If I read the above correctly, you are trying to test/analyse a design by others. It would be helpful if you told us what the tubular beam did________________. From the looks of the photo, my guess is that it is some kind of spreader beam used "below the hook" of a crane. If so, look up ANSI 30.20 and it will tell you the rest of the story about required annual testing, Factor of safety on ultimate, etc. If not, I appologize for a wrong assumption. The usual requirements for "below the hook" apparatus is a Factor of Safety of 5 on ultimate. The F of S of 5 helps with fatigue.
My background is Mechanical, a master's degree specializing in strain gages and also a PE in structural. Somewhere in my career, I found out about practical fatigue loading of tubular members with welded attachments. The detail doesn't look good if it occurs at maximum bending stress in the beam.
The strain you are measuring may indeed be real. As a reality check I would suggest you mount the same type of gage(s) on the tubular beam a slight distance away from the welded lug and compare the strain (and stress) to what you know the stress should be from simple bending stress theory. Then compare this to the stains and stresses in the weld as you load the beam in bending.
In addition to the obvious bending stresses, spreader beams which have Ixx larger than Iyy have real issues with lateral buckling. Let us know the rest of the story and we may be able to assist.
@slickdeals
It was one of the guys from Lincoln Electric. I think it was Duane Miller. He showed us one particular interesting photo where someone had welded a plate to the side of the arms of a large front end loader to install some non OEM equipment.
After some loading cycles the loader arms sheared right off (this appeared to be a 2" thick plate about 30" long or so). The loader went back to the mfr under warranty. The cause of the crack was due to forces being driven into and out of the plate during loading because of the added stiffness creating stress risers.
Very interesting.
It was one of the guys from Lincoln Electric. I think it was Duane Miller. He showed us one particular interesting photo where someone had welded a plate to the side of the arms of a large front end loader to install some non OEM equipment.
After some loading cycles the loader arms sheared right off (this appeared to be a 2" thick plate about 30" long or so). The loader went back to the mfr under warranty. The cause of the crack was due to forces being driven into and out of the plate during loading because of the added stiffness creating stress risers.
Very interesting.
Gumpmaster
Structural
A couple of questions (mistakes I've made in the past)
1. Are you pumping the right voltages into the gages?
2. What is your sampling rate?
3. Are you using a rosette? You probably have a multiaxial strain.
Maybe an outside the box solution. The detail you have is will create stress concentrations no matter what. Maybe a couple of lifting slings looped over the spreader beam would reduce the stresses.
An 11cm deflection over a 20ft beam is huge! Are you sure you have the right beam for the application? Maybe something with a thicker wall would be better.
1. Are you pumping the right voltages into the gages?
2. What is your sampling rate?
3. Are you using a rosette? You probably have a multiaxial strain.
Maybe an outside the box solution. The detail you have is will create stress concentrations no matter what. Maybe a couple of lifting slings looped over the spreader beam would reduce the stresses.
An 11cm deflection over a 20ft beam is huge! Are you sure you have the right beam for the application? Maybe something with a thicker wall would be better.
- Thread starter
- #19
Sorry guys. I've been out of the office all day. Since this is something still in design phase, I'm afraid of sharing "too much," so I would be unable to upload detailed sketches, measurements, etc. Bu
Gump....No, I am not using a rosette. I will have to get back to you on your other questions when I return to the R&D shop.
Bradley...it is a sling attachment for a reach stacker to lock into so it can be used for crane-like applications. Worth mentioning is that there are also 2 lifting lugs on the ends of the beam, for 2 point slinging.
In my opinion, when slinging 40kip, I don't see why you would want to sling on a single lug. We may end up just de-rating the center lug, but I still need to test it. I need to also do theoretical calculations, for the welds, for reporting purposes. In all my text books and research online, I cannot find analysis done on welds that have deflection perpendicular to the welding plane. Can someone point me in the right direction for that type of analysis?
Gump....No, I am not using a rosette. I will have to get back to you on your other questions when I return to the R&D shop.
Bradley...it is a sling attachment for a reach stacker to lock into so it can be used for crane-like applications. Worth mentioning is that there are also 2 lifting lugs on the ends of the beam, for 2 point slinging.
In my opinion, when slinging 40kip, I don't see why you would want to sling on a single lug. We may end up just de-rating the center lug, but I still need to test it. I need to also do theoretical calculations, for the welds, for reporting purposes. In all my text books and research online, I cannot find analysis done on welds that have deflection perpendicular to the welding plane. Can someone point me in the right direction for that type of analysis?
It seems you will have a SigmaI tensile coming from bending and a SigmaII tensile from direct pullout from the downwards force. Then check the interaction as in Pilkey or other for combined stress against some limiting value that takes into account stress concentration adn fatigue life. Or directly investigate the vonMises stress against such limit from some FEM analysis.
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