A380 - Ductile or Brittle 1

bjpil · Sep 30, 2003

Hello,
I have a question as to what dominant characteristic does A380 cast aluminum has, is it a brittle or ductile material? The reason I ask is to determine which failure theory to use, distortion energy theory or maximum normal stress theory. Is there a rule of thumb that would help to define a brittle vs. ductile material?

Thanks for your help,

BJP

unclesyd · Sep 30, 2003

I would consider A380 to be brittle alloy due to the very low elogation 2.5%. We always look at the 5% elongation as cut off in Al alloys. This puts most of the more common casting alloys in what would be cosidered brittle. We used a lot of 356 components and all failures were of a brittle nature. It has an elongation of 3%-3.5%.

CoryPad · Sep 30, 2003

Assuming the porosity is low (which isn't always a good assumption), then A380 definitely is ductile and should obey the distortion energy theory.

As for a rule of thumb, look for fracture strain of 1% as a line - below 1%, the material is brittle; above 1%, the material is ductile.

Regards,

Cory

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unclesyd · Sep 30, 2003

I didn't read your question correctly so "CoryPad" had to straightened me out. I was thinking and typing at the same time, two things at once. Our design of cast Al components was based on the end use. If this component was going to have personnel exposure we limited it’s use based on the expected mode of failure. Injection or permanent molded Al alloys were all limited as to speed. as most of these components were rotating. We still had some failures. Of all the failure analyzed macroscopically we found none that failed in what would be classified a ductile failure.

Sorry for the misinformation.

bjpil · Oct 2, 2003

Cory,

Thanks for the info. I need a little bit of a clarification with respect to your rule of thumb. I am unfamiliar with the term fracture strain. I am assuming fracture strain is the maximum strain value that the specimen reaches before it breaks. Therefore, if the specimen has a strain value greater 1% (A380 is 3.5% in 2 inch gage length) then it is a ductile material. Is this correct?

Thanks again,
BJP

CoryPad · Oct 2, 2003

You have defined correctly the term fracture strain. I use it (instead of elongation, for example) because the name is the definition, so it is clear, descriptive, etc. As you have shown, A380 will be above 1%, so it should behave according to the distortion energy theory. Just be aware that the behavior may be different than say commercially pure wrought aluminium with its unrestricted FCC structure.

Regards,

Cory

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A380 - Ductile or Brittle 1

bjpil

Mechanical

unclesyd

Materials

CoryPad

Materials

unclesyd

Materials

bjpil

Mechanical

CoryPad

Materials

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