Stress concentrators 5

[cntrlnr]

Having an opportunity to design a cross member from which a load will be suspended I find no examples for my configuration in SHIGLEY which provides stress concentration factor charts by Peterson. I have reviewed Peterson directly and found no reference to my configuration either.

What I anticipate must not be that novel. A pair of channels, web to web with an inverted "U" hoop sandwiched in between, with equally thick uprights at each end also sandwiched between the channels. A single pin will be inserted through both channels at each end of the inverted "U" hoop on the neutral axis of the channels equidistant from the trasverse centerline of the composite cross beam. A load will be suspended from, and be shared equally from each end of the cross beam assembly.

The channels comprising the cross beam will be in 4 point bending.

What model should I use to determine a k-sub-e stress concentration factor?

 

[zcp]

I would recommend a simple FEA (finite element analysis) of your lifting arrangement to determine max. stress and thus stress concentration factor.

Or your could get a consultant to do a quick analysis of the arrangement and even provide calcs for you to review.

Will this be an infinite life scenario?

ZCP

http://www.phoenix-engineer.com

 

[cntrlnr]

ZCP; Thanks for the response.

FEA is an interesting suggestion. We have the capability to model it with Unigraphics in-house but I will most likely not be able to corral the resources.

Consultants cost money, don't they?

Infinte life is another interesting point. In reality we have fairly rigorous periodic NDT inspection requirements for identifying crack initiation and propogation, but I have no way of knowing how many fatigue cycles the members will be subjected to. We can calc the number of picks but I expect that each lift will represent and indeterminate number of fatigue loading cycles. Certainly designing for infinite life is conservative but the structural dimensions may become a challenge for the lifting eye relative to its' crane hook interface geometry. And we still need to include a stress concentration factor as well as a reliability factor and using .5Sult for the rotating beam endurance limit factor.

 

[Kwan]

Can you use Peterson's lug stress concentration factors? This is in chapter 5. I can't quite picture your configuration (my fault without a doubt); however, I think you have a pin at the end of a beam? Hence, you have just pin loading and can use a lug solution. If in the middle you would have pin loading and a stress field (mc/I). If this is the case you can combine the pin effects with the open hole effects. Peterson probably shows how to do this (I know it has a solutin for pin loading and stress field in the same direction). If not I can look up the formula for you.

 

[zcp]

If you are lifting overhead, it seams like your 4 to 1 FS may handle your fatigue issue for you. If you have periodic inspection, you could shorten the interval at first and would have a 2nd layer of protection.

And consultants do cost money, but not as much as you might think for something like this. [Shameless plug for me.]

ZCP

http://www.phoenix-engineer.com

 

[israelkk]

cntrlnr

Visiting the doctor when you are ill cost money too. The outcome of a failure of a design sometimes can be too costly to imagine. The time you invested reviewing Peterson book and Shigley costs money too. Everything in life costs money but there are cases when you have to rely on the help of those who are more knowledgeable or have the authorization from the state to produce such designs.

 

[rb1957]

personally i doubt the usefullness of FEA in determining a stress concentration factor. as i understand it, you've got 4 pins in shear, Michael Niu "Airframe Structural Design" has a nice methodology for a Kt due to shear loads. i'd have thought that if your bearing stress was reasonable, say a FS between 4 and 6 which should be easily achieved, then there's no fatigue issue. youo're also (probably) far enough away from the peak bending stress in the channel (since you're some ways away from the remote fiber)that this isn't an issue either ... and these two concentraions occur at differnt locations on the hole, and typically relieve one another.

 

[cntrlnr]

Thank you all. My fecitious humor regarding consultant fees was either not recognized as such, or not well received. So much for dispelling the dry engineer stereotype. My apology if I offended any readers.

Think of a 60" wide cross beam with two holes on the neutral axis, 12" apart. The neam is loaded at the ends with an inverted hoop lifting eye pinned through the two neutral axis holes.

CFertainly I can just use a large safety factor and not do the math for fatigue loading material strength modifiers but I wanted to follow the book and account for heat, size, reliability, stress concentration, etc.

With the holes being on the neutral axis and the beam in bending it may not be necessary to consider stress concentration for this arrangement.

cntrlnr

 

[zcp]

cntrlnr, I got the little wry smile at the end of your 'consultants cost money?' line. I hear it all the time.....

Regarding your by the book analysis, you may want to look at your stress concentrations as load increaseing factors (kf) for that load case vs strength reduction factors (ke). You may not want to apply kf to all your loads and having your endurance limit unfactored by stress concentration can prove helpful. Just my two bits, good luck with your analysis.

Don't forget twist for a swinging load.....

ZCP

http://www.phoenix-engineer.com

 

[unclesyd]

Here is a site posted by metengr that should provide some assistence.

http://www.fatiguecalculator.com/index.htm