trying to reproduce the results of a test
trying to reproduce the results of a test
(OP)
I am testing a new version of an old handle for refrigerators. The handle is basically an aluminium pole with two brackets at each end holding it to the door. Changes have been made to both the handle and the brackets, so I try to test them separately, sort of. I've tested the new handle with the old brackets and am now testing the old handle with the new brackets.
The opening force, applied at the middle of the handle, is about 60 Newtons, possibly even lower due to the nature of the test.
After about 80,000 openings, the old bracket failed. I'm thinking this is either a matter of fatigue or something was a bit off with the test equipment.
With the FEA software we have, I can only look at separate parts, so when calculation the forces acting on the bracket I have considered the handle as being solidly mounted at both ends, resulting in a force of F/2 and a moment of F*L/8 acting on each bracket. The resulting Von Mises stress, as well as max principal stress, is above Rm for the zinc alloy used, which is about 330 MPa.
obviously, something is wrong with my analysis. I'm quite certain that it's the fact that I cannot take into consideration the flex of the door and handle which should be making life easier for the bracket. So what do I do? Reduce the applied forces and moments in the analysis until I get the desired max principal stress?
Also, does anyone know where I can find fatigue diagrams for zinc alloys? aluminum alloys?
The opening force, applied at the middle of the handle, is about 60 Newtons, possibly even lower due to the nature of the test.
After about 80,000 openings, the old bracket failed. I'm thinking this is either a matter of fatigue or something was a bit off with the test equipment.
With the FEA software we have, I can only look at separate parts, so when calculation the forces acting on the bracket I have considered the handle as being solidly mounted at both ends, resulting in a force of F/2 and a moment of F*L/8 acting on each bracket. The resulting Von Mises stress, as well as max principal stress, is above Rm for the zinc alloy used, which is about 330 MPa.
obviously, something is wrong with my analysis. I'm quite certain that it's the fact that I cannot take into consideration the flex of the door and handle which should be making life easier for the bracket. So what do I do? Reduce the applied forces and moments in the analysis until I get the desired max principal stress?
Also, does anyone know where I can find fatigue diagrams for zinc alloys? aluminum alloys?





RE: trying to reproduce the results of a test
Zinc info:
http://www.eazall.com/
Mike Halloran
Pembroke Pines, FL, USA
RE: trying to reproduce the results of a test
RE: trying to reproduce the results of a test
i'm not sure what "Rm" is as a strength.
personally i find the the force i apply to a fridge handle varies significantly, depending on the seal.
why can't you model a handle complete with it's brackets, and check that the loads the FEA is putting across the handle/bracket interface can actually be applied ?
RE: trying to reproduce the results of a test
All the other suggestions are also well stated.
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community
RE: trying to reproduce the results of a test
rb1957: "Rm" translates to "ultimate strength" (thought it was "yield strength" first, but apparently not. glad I looked that up). If I happen to mention "ReL", that would translate to "yield strength".
I could try to model the handle and brackets as one part. didn't think of that. thanks. we only have the catia generative part structural analysis 2 module, so I was stuck thinking that I can't do assemblies.
The door opening force will depend on the seal, the type of magnet used and the opening speed. I've actually measured the impact of the opening speed on the opening force and for a slow opening (10mm/minute), I would expect an opening force as low as 25N. Higer speeds (1000mm/minute) would result in an opening force of at least 50N for a warm cabinet with a hole to reduce any vacuum. I've measured 150N on a normal opening and 78N for a careful first opening of a running cabinet. Always with a good seal of course. The cabinets placed in the machine won't have time to achieve a perfect seal, so maybe I should reduce the load to 50N for my analysis. wish I had equipment to measure the real world values. Maybe I should try to make something up for this.
GBor: not quite sure how I would model the bracket and door as one part. I've used a "contact virtual part" to simulate the door and no clamps (except for keeping the virtual parts in position) but maybe I could use forces instead.
Thanks for your helpful posts guys :)
RE: trying to reproduce the results of a test
Are you approximating a dynamic load with a peak static load?
www.probasci.com -
Implantable FEA for medical device manufacturers
RE: trying to reproduce the results of a test
We haven't had any FE software up until just recently and the designers generally don't seem interested in performing any calculations or analysis, which becomes really frustrating when they want me to test things which are almost certain to fail.
I am trying to determine the likelyhood of fatigue failure using a static load, so yes, I am trying to approximate a dynamic load with a peak static load.
RE: trying to reproduce the results of a test
RE: trying to reproduce the results of a test
RE: trying to reproduce the results of a test
when you restrain a model it is very likely to have high peaks of stress in the vicinity of the constrained points. What I would do as a first step is check where the experimental failure occurs and what stress is calculated in that location using your model, and then make some considerations about fatigue life.
Regards.
SD
'Ability is 10% inspiration and 90% perspiration.'
RE: trying to reproduce the results of a test
"when you restrain a model it is very likely to have high peaks of stress in the vicinity of the constrained points"
Yes, that is why GBor suggested applying a balanced set of loads with minimal supports purely to prevent rigid body motions. Doing this correctly eliminates all spurious stress peaks.
RE: trying to reproduce the results of a test
RE: trying to reproduce the results of a test
I hope that you are not using all the default settings of Catia GPS, it is tuned for speed and not accuracy. The element sizes it suggests produce a highly facetted model and have to be reduced considerably. It is possible to apply balanced loading and minimal supports using GPS, I have seen it done.
RE: trying to reproduce the results of a test
RE: trying to reproduce the results of a test
Since your analysis of the previous design places peak stresses at the observed failure locations, that suggests your load model is good at least in terms of directions and locations of constraints. How does the applied load in your previous design compare with observed breaking loads (static if you have it or can test it?). This will help you determine if the stress magnitudes from your model are also correct.
Once the static model is working, then go after the fatigue condition.
www.probasci.com -
Implantable FEA for medical device manufacturers
RE: trying to reproduce the results of a test
What did I learn? well, for one thing, it seems that the screws loosened somewhat, resulting in virtually no movement at all of the bracket. This should mean that no moment is transferred to the bracket. All I have left then is a force which is equal to and opposite to the force pulling on the handle, divided between the two brackets. This should also mean that if I apply the force further towards either end of the handle, the bracket which is the closest should break sooner
another thing it seems I've been wrong about: I should be recieving the max moment acting on a bracket by applying the force at 0.35*L from that bracket, not by applying it at 0.5*L. or maybe I'm just tired and need to go home :)
RE: trying to reproduce the results of a test
Having tried a number of different load cases and restraints, I have a few conclusions:
1) The bracket can not be considered to be rigidly fixed to both the door and the handle for the analysis. If this was the case, I would be breaking brackets all day long.
2) We have both a force and a moment from the handle (as the stress concentrations show when comparing the analysis to broken brackets).
3) The effect of the moment is lessened by the method of connecting the bracket to the door and to the handle. I don't know by how much though...
4) "contact virtual part" seems to be the best solution for simulating the screws and other parts keeping the bracket in position on the door (other alternatives I've tried tend to result in reaction forces in the opposite direction of what I expected)
5) I have found pores in our zinc brackets and considering the real-world experiences and test results so far (the aluminum brackets seem to be ok) something is not right about them. If it was simply a design error (I'm not saying the design couldn't be better) I would expect more brackets to fail, but it seems very random.
6) I have resorted to using linear elements as opposed to parabolic elements as my computer apparently isn't optimal for this.
RE: trying to reproduce the results of a test
On your point 6), linear four node tetrahedral elements should NEVER be used ! They are simply not good enough, any results you get will be seriously inaccurate. You have no choice but to acquire a better computer.
RE: trying to reproduce the results of a test
RE: trying to reproduce the results of a test
I know where the brackets break, so I don't need to model the whole thing. I will cut the bracket a distance away from where I have the highest stress and then fix the resulting surface (I know, you guys really disapprove of this approach). I will apply a force of 35N at the other end. I will then add a moment which gives me the proper stress (well, at least in the neighbourhood of it).
wish I had the fatigue diagrams for my materials, but lacking that and knowing the ultimate strength and fatigue strengths, could I simply draw a line between these two values for each material and get a reasonable comparison?