ABAQUS Coupled Displacement Analysis of Rotors 1

[blueranger]

I would be interested in the opinions of some of the resident experts regarding this issue.
I have carried out Coupled Displacement analysis of a Brake Disc Rotor (material is Grey Cast Iron) in ABAQUS using a solid 3D model of the Brake Rotor in isolation.This means that i have used coupled temp/displ. elements to calculate
metal temperatures and subsequent stresses.
I have applied appropriate minimum structural restraints and the loading has been applied in the form of Heat Flux
applied to the swept surfaces (brake pad contact surfaces )
of the rotor.
I have calculated Heat Fluxes from a fairly complex pc based
application and it is not necessary to expand on this at the present time.
The subsequent calculated temperatures are very close to what i calculated from a Temperature only analysis in ABAQUS
and more importantly also correlate very well with both Dynomometer and Vehicle measured values for the same brake
schedule.
The problem is the calculated stresses from the ABAQUS analysis which reach values over double the UTS of the material.This is hardly believeable as we have not experienced any failures and little signs of yield during test.
I feel the reason for the high values of stress is that i have only input one value of Youngs Modulus in the analysis.
I think i might have to try and input temperature dependant stress strain for Modulus for Grey Cast Iron.
Any opinions on this,know where i could get this data and finally any guidance on application of this data in my ABAQUS analysis.Thanks in advance.

 

[tld23]

Hi BlueRanger,
This is a well explained post, I wish more were like this. You do not say if you are doing a psudo-steady state model or a transient model. I have the following comments/advice:

0) I have used the fullu coupled temperature displacement elements extensively and found them to be reliable.
1) you do not mention the other properties, in particular check the thermal expansion coefficient which should be about 11ppm/degC, the conductivity at 75W/m.degC, the heat capacity at 500J/kg.degC and the density 7150kg/m^3. All values are approximate.
2) I have not looked any values up, but would not expect the modulus to to change a great deal upto say 400C or even greater.
3) Steel is a poor conductor. Cast Iron is very strong in comperssion, much stronger than in tension. I would expect the the brake would heat rapidly on the rubbing surface but that the heat would be slow to conduct away. This means that a thin layer in the brake will try to expand but will be constrained by the bulk of the material. For a 300degC change I would expect a stress of upto 700MPa - well in excess of the strength of the material (assumed to be about 200MPa at room temp). But the important thing is that the high stress region will be a THIN layer in COMPRESSION. This means the ballancing thermal stresses will be in tension but LOW. Hence you may have to find compression strength data for fuller assessment of the results.

I hope that the above is of help - there again I could be barking up the wrong tree!



TERRY

 

[blueranger]

Terry

Thanks for the reply to my post.This is a simulation of a single Vmax Stop (160km/hr) at 1G and the calculated stop time used in the analysis is 4.53secs.
Compressive stresses on the rotor swept surfaces are not a concern.The stresses of interest are tensile bending stresses at the 5 teeth (on rotor inner diameter) where the rotor locates on to a hub.
These bending stresses at the tooth fillet radii are the ones of concern which i believe are overestimated because i think Grey Cast Irons show considerable variations in Youngs Modulus between room temperature and the maximum operating temperature of this Single Stop simulation which is about 600 Deg C.
Properties used in the temperature calculation such as Density,Thermal Conductivity and Specific Heat have all been validated and units checked.
Simulation of a Vmax single stop is the most severe requirement from a stress point of view because it is a short event resulting in high thermal gradient between the rotor inner and outer diameters.
Hope this clarifies the type of simulation and any comments regarding the sensitivity of peak stresses to the modelling of variable Youngs Modulus would be appreciated.Thanks.

 

[brad]

Great post by Terry.
Blue ranger, it sounds like you've done your homework. I'm a bit puzzled.
My only comment (which is a repeat by Terry): even though you've checked your units, check them one more time. I'm always amazed how easy it is to goof up units (especially doing thermal analyses if you are using mm length basis).
Sorry I can't add anything better than that.
Brad

 

[tld23]

Blueranger,
I looked up the modulus-temperature data in the ASM Handbook No.1, chapter 2 on "Grey Cast Iron", Figure 12. The handbook suggests figures of 130GPa at room temperature and 100GPa at 600degC.

TERRY
TERRY