# Material Selection, material indexes

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1. Jul 4, 2017

### Thom_Silva

Hi,

I'm working in a design team for the first time and I'm in charge of choosing a material for an application. I have the constraints, support a tensile load of 50 kg , and the objective which is minimize the weight.
I'm using the concept of material indexes to find the best material for the application:

The equation that I want to minimize, regarding that the following constraint is maintained:

If we substitute A in the first equation we get:

The problem i have now understanding is that the method tells me that i should only carry in minimizing (density/ failure strength), if i find the material that in fact minimizes this, how do i now the first constraint:

is maintained, in literature i'm recurring to they never check this, should i check this for the material i end up choosing , or is this step unnecessary ?

Thank you very much !!

2. Jul 4, 2017

### CWatters

The substitution you made eliminates the cross section area A. That clearly doesn't mean the part can have an arbitrary cross section area (eg A=0).

It's like solving two simultaneous equations involving x and y. You usually make a substitution that eliminates one variable and allows the other to be calculated. You then have to go back and find the other one.

3. Jul 5, 2017

### Baluncore

The fatigue and environmental specifications are usually most important. First identify suitable materials that are available, then design solutions that can be attached to the structure without attachment, fastener, fatigue or corrosion problems.