ultimate shear strain

[crisb]

Can anyone help with the relationship between the ultimate shear strain for a plastic material in pure shear, to the ultimate uniaxial strain for the same material in pure tension? Is it 1.73, 1.5 or some other ratio?

 

[gwolf]

1) You're using strain - why? Ultimate material values for metals are always quoted as stress because they work off nominal tensile test sections and are effectively an elastic limit (even though at failure there is lots of local plasticity and necking).

2)When I worked on helicopter gearboxes made of aluminium alloy we either had shear limit allowables or used Shear stress limit = approx 0.6xtensile stress limit. We applied the same 0.6 factor to get ultimate shear stress allowable from ultimate tensile allowable.

I could ramble on but I suspect you need to give a bit more detail.

 

[crisb]

I am trying to interpret and report some test results on girders which were strain gauged, had tensile material tests undertaken, and were also subject to FEA. There are areas of pure shear that had trixial strain gauges and I would like to make an assesssment of how close those areas were to material failure.

 

[GBor]

crisb,

Look at the online calculators developed by Materials Sciences Corporation (

http://www.materials-sciences.com).

One of these calculators is for stress and strain. If you know the angle of rotation to go from pure shear to pure strain, it should calculate the strain for you.

 

[gwolf]

Right, now I understand.

For a ductile material the VonMises criterion is the best way of assessing strength against material limits. What you need to do is this:

Use a textbook or online calculator as GBor suggests to determine the complete stress state on the surface of the component (Sxx,Syy,Txy). You are lucky because you have a 3 gauge rosette which completely defines the surface stress state. Then calculate principal stresses S1,S2 and set S3 to 0.0, then VonMises and compare this to your material limit and ultimate stress. Don't forget to use appropriate ultimate factors - Aerospace is 1.5, not sure what you must work to.

Compare this also with the VonMises stress in your FE model.

Then go home for a well deserved beer.