Searching for simplification of material properties
Searching for simplification of material properties
(OP)
Hi All
I am doing some metal forming (cold) analysis, for that , my material properties can only be inserted in a simplified way. that is : sigmaf = C*(fi)^N
After tensile test, I noticed differences between the real material and the one in the FEM model ,since I can not model the real one, I would like to ask you , do you think there is too much difference between the two ?
the upper photo attached is the one in the software, and the lower is the one from the tensile test
I am doing some metal forming (cold) analysis, for that , my material properties can only be inserted in a simplified way. that is : sigmaf = C*(fi)^N
After tensile test, I noticed differences between the real material and the one in the FEM model ,since I can not model the real one, I would like to ask you , do you think there is too much difference between the two ?
the upper photo attached is the one in the software, and the lower is the one from the tensile test





RE: Searching for simplification of material properties
http://www.eng-tips.com/viewthread.cfm?qid=401047
Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer
RE: Searching for simplification of material properties
Thank you Aero, but not exactly , maybe fatigue will be my next concern , so I will study this post well , here the material is steel, but you see, this kind of material representation, makes it behave like aluminum ,no ? for me, this problem pops out for example in extrusions , I get a bit longer lengths of deformed material than expected. like 1 mm in every 10 or 20 mm length
RE: Searching for simplification of material properties
RE: Searching for simplification of material properties
Probably the easiest is to digitize the tensile test figure. A download
program "engauge" works well though on some versions one has to change the
colors to black.
In a spread sheet then compute the Emodulus= Stress/Strain in elastic region.
Then compute the plastic strain = total Strain - Stress/Emod
Then plot it on the same figure that shows your stress vs plastic strain.
Just for fun I did the digitizing:
Using E=21000 (?) Strain Sts CompE PlasticStr 0 0 0 0.0 0.00066 25.8 39017 -0.00057 (make -ve Plas Strs to zero) 0.00258 50.0 19342 0.00020 0.00325 80.3 24709 -0.00057 0.00517 96.9 18767 0.00055 0.00583 122.7 21062 -0.00002 0.00712 150.0 21062 0.00000 0.00843 189.4 22468 -0.00059 0.00971 206.0 21214 -0.00010 0.01036 225.7 21776 -0.00038 0.01164 237.8 20425 0.00032 0.01229 251.5 20459 0.00032 0.01357 262.1 0.00109 0.01421 266.6 0.00151 0.01421 272.7 0.00123 0.01612 280.2 0.00277 0.01739 284.8 0.00383 0.02056 287.8 0.00685 0.02372 290.8 0.00988 0.02690 296.8 0.01276 0.03196 301.4 0.01761 0.03640 305.9 0.02183 0.04083 308.9 0.02612 0.04400 311.9 0.02915 0.04717 313.4 0.03224 0.05287 319.4 0.03766 0.06047 323.9 0.04504 0.06363 322.4 0.04828 0.07694 339.0 0.06080 0.09912 366.2 0.08168 0.11813 385.8 0.09976 0.13903 403.8 0.11980 0.16247 420.4 0.14245 0.18844 436.9 0.16763 0.22137 454.9 0.19970 0.26063 474.4 0.23803 0.27519 480.4 0.25231 0.28025 475.9 0.25759Seems like an odd material. Elastic to 0.01 Strain??You might want to do a units check at the tensile machine. :)
RE: Searching for simplification of material properties
I got the tension results as force displacement, at converted them myself , you can find them attached
RE: Searching for simplification of material properties
well or I have mistaken the digitizing, or it's the problem you mentioned , cause it's not done on a machine with great performance. you can check this one
RE: Searching for simplification of material properties
extensometer or of the Instron(or whatever) ram?
The Eng Strain you have is Column B divided by 230 which would imply a 9 inch
gauge length extensometer, which is rather large.
If 230 mm is the spacing between the grips then you should probably use
the length of the reduced section gauge length instead. - better yet would
be using an extensometer. Ram displacement for measuring deflection won't give
great results, generally. Always check to see if the elastic modulus makes
sense. It is not a "constant", however, from steel to steel it varies maybe
up to 10%.
RE: Searching for simplification of material properties
"If 230 mm is the spacing between the grips then you should probably use
the length of the reduced section gauge length instead."
you mean under the same test conditions ?
RE: Searching for simplification of material properties
and close to an AISI 1006 or 1008, i.e. a low carbon steel more or less.
Thus your modulus should be somewhere around 200 Gpa or 30,000 ksi. Unless I've
messed up, the modulus from your test data is out by a factor of 10 and
a further guess is that something is wrong with the strain calibration.
The test speed doesn't matter for modulus. Ask the test folks if they used
an extensometer and measured the strains in the gauge length or if they relied
on the deflection of the crosshead of the test system to compute "stain"
a 230mm extensometer is unusual. Also maybe someone just dropped a decimal point
in the strain results?
If there was no extensometer then the deflection of the machine is a measure
of crosshead deflection, grip deflection, loadcell deflection etc., all added
together which makes for a poor measure of strain on the specimen, or perhaps
in this case a wire(?). Personally I would just do the tensile test again with
an extensometer on the specimen or wire. Its also a good idea to request the
strain as strain= Delta_L / Lo and not as a "%" strain. In some Euro tests I've
also seen strain expressed as 1/1000 or other odd zero shifting measures, so
just ask for plain ol' strain.
RE: Searching for simplification of material properties
when the stress-strain diaghram starts from plastic strain, should I add the Elastic stress to the final stress results ?
cause in this case the final stress sounds too high for me, I feel a low carbon steel doesn't have a UTS above 600 MPa
RE: Searching for simplification of material properties
Not sure what you mean by adding the stresses together, but total strain = elastic strain + plastic strain For a given stress one can compute the elastic strain = stress/E and then subtract it from the total strain to get the plastic strain.RE: Searching for simplification of material properties
RE: Searching for simplification of material properties
The line goes upwards to the first point, but it is confusing. Probably due
to Ramberg-Osgood equation flaw. There is always a transition point where
one must "jump" from the elastic line to the stress vs total strain line,
as plasticity starts, due to the equation. Its a pain to see it on a curve.
In order to derive a stress-strain diagram from the stress vs plastic strain
diagram start at the point of non-linearity (about 100,000,000 ?) on the graph.
Pick a Stress. Get plastic strain from graph. Compute elastic strain = Stress/Emod
Add plastic and elastic to get total strain.
Result: Stress vs Total Strain.
Pick next stress and repeat.
Check on a graph. Draw the elastic line from zero,zero upwards.
Then draw the computed Stress vs Total Strain from above. You will see
the jump or transition point, probably.