brittle or ductile?

[PerKr]

I'm looking at a particular component which will quite often break during testing. I have this component in two versions, one made from Zn 5, one made from EN AC-46100. Trying to get an understanding of why they break (or why one would break and not the other), I am now wondering if the components should be considered as brittle or ductile for the calculations and analysis. How do you know?

 

[EdStainless]

Look at the fracture and see. Any signs of deformation and stretching? If not then consider it brittle.

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[CRMagruder]

Are there mill test reports or property data from texts or codes for the materials you are looking working with? I'm not familiar with the alloys you listed.

The tensile test data should have value for elongation, listed as a percentage. Ductile materials have elongation values around 20 to 30%, whereas alloy steels in the hardened condition will have values less than 10%. I am sure there is a cutoff value that structural and pressure vessel codes have for the ductile to brittle classification. This property is also dependent on temperature. At lower temperatures some metals become dangerously brittle.

 

[metengr]

PerKr;
What type of test did you perform on these materials???? Did the failures of both materials occur during some type of product or proto-type testing, impact, bend or standard tensile test?

Brittle or ductile fracture appearances alone mean nothing unless you know what conditions caused the fracture. For example, if ductile or brittle fracture were caused by proto-type testing, a brittle fracture appearance could be the result of fatigue failure or lack of notch toughness or lack of ductility.

Standard tensile testing will provide information on the strength properties and ductility of the two materials. Examination of the fracture appearance from a standard tensile test can be used to evaluate if the materials in question exhibit ductile or brittle (as mentioned above) fracture behavior.

 

[PerKr]

metengr: the test performed was a standard door opening test as described in the refrigeration standards. The failures will typically occur just about anywhere between 50,000 and 200,000 cycles so I'm assuming it's a fatigue failure.

looking at the fracture, most of it seems to have been brittle. I have a few small areas where I have had either a ductile failure or possibly some sort of wear from the machine (it doesn't stop when the handle breaks, so in this case these areas could result from the parts hitting eachother). I've sent a request to receive all samples which have broken in the field to get a better idea of what has happened.

 

[metengr]

Send them to a reputable metallurgical lab for detailed analysis of the failures.

 

[EdStainless]

The fatigue analysis will take some SEM time.
My hunch is that it is fatigue and that it starts from a defect that is put either by (poor) design or manufacturing.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[TVP]

FYI, these are high pressure die casting alloys: one is zinc-based, the other aluminum. Fatigue will almost always initiate from porosity in these alloys, either microporosity due to solidification shrinkage, or macroporosity due to entrapped gas. Brittle or ductile fracture are not really appropriate descriptions for fatigue failure-- they are used to describe overload failures.

PerKr,

You need to have the parts evaluated by a competent metallurgical lab. Quantify the defect size, etc. Do you know the stresses on the part? If pore-free parts are not overstressed, then you need to work with the casting vendors to improve their processing of the parts. If pore-free parts are overstressed, then you need better alloys, reduced stresses, etc.