# Is assigning density value in static analysis necessary?

1. Jul 3, 2010

### balajiperumal

Hi all,

My job is to simulate the practical testing procedures into software and check the results of the model before prototype. To do this we assign the physical properties of the material say YOUNG'S MODULUS, POISSON'S RATIO and DENSITY. Through these properties the software recognises the material and does the analysis as per boundary conditions given and loads applied.

The above shown physical properties are assigned to model for different analysis.

So, now my issue is: Is assigning DENSITY value to the model really necessay, when it is a STATIC ANALYSIS?

2. Jul 3, 2010

### Astronuc

Staff Emeritus
If one is simply determining stress-strain, then no, density is not a factor. One only needs elastic (Young's) modulus and Poisson's ratio, and perhaps YS and UTS, if one wishes to determine plastic strain vs elastic strain.

3. Jul 3, 2010

### Claws

Doesn't that entirely depend on the part and whether mass/gravity should be taken into account?
I wouldn't want to neglect weight if doing a static analysis on a bridge :)

4. Jul 3, 2010

### balajiperumal

Yes,

But, Why should I neglect only for static analysis and why not another analysis?

5. Jul 3, 2010

### Astronuc

Staff Emeritus
It depends on how load/force/weight/pressure needs to be specified. If one can simply input a force/pressure, then there is no need to input density, although at some point, one would need volume and density to calculate a mass with which to determine weight for certain analyses.

6. Jul 3, 2010

### Astronuc

Staff Emeritus
It depends the analysis, the response of the material, and the sensitivity to mass distribution. Where local accelerations would be important, e.g., in ballistics, knowing density would be necessary. In other words, if resistance to acceleration is important then knowing the mass (density * volume) would be important.

Density would be important for mass continuity, especially where a fluid is being compressed or allowed to expand. But, I think the OP was referring to solids.

7. Oct 24, 2011

### hosseinnet

Hi
But I'll appreciate that if you mention some reference (book) about that. in which formulas we have density and we omit or annoy it?
And i have one more question what's the deference between Deformable and Ductile?

8. Oct 24, 2011

### AlephZero

There are several good reasons for including density in the model defintion right from the start. Some of them are:

1. You can do an automatic check on the mass and CG position of the model, and compare it with measured properties of the real component, or at least with the output from your CAD system. That checks for mistakes like the wrong thickness of plates, the wrong cross section of rods and beams, etc.

2. Static analysis often includes the self-weight of the component as a load case, or the model may be used for a simple approximate analysis of a shock loading, etc.

3. The same model may be used for dynamic analysis later on, possibly by a different group of engineers from those who originally created it. In the long run it saves time to define all the properties when the model is first created, instead of repeatedly changing the model and having to check what has already been defined and what has not.