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Anonymous Poster #1

Yield Stress for Plastic

02/21/2013 11:48 PM

Hi All,

I need some clarification on Yield Stress for plastic material.

For example PBT - Yield stress is given as 33MPA and Ultimate stress is given as 31MPA.i took this data from Matweb.Could any one tell me whether this value is correct.Also why Yield is more than Ultimate Stress.

Thanks In advance for your Support.
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SolarEagle
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#1

Re: Yield Stress for Plastic

02/22/2013 3:01 AM

Ultimate stress is the maximum stress that a given unit area of a certain material is expected to bear without failing.... Yield stress is level at which a metal or other material ceases to behave elastically.....
http://en.wikipedia.org/wiki/Ultimate_tensile_strength
http://www.thefreedictionary.com/yield+stress

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Codemaster
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#2

Re: Yield Stress for Plastic

02/22/2013 4:39 AM

It depends on the plastic, there are lots of them out there. Those figures are about right for common plastics. Pipes in uPVC, PP, PE etc have a design stress about 10 MPa, and a factor ~ 3 below yield is reasonable.

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passingtongreen
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#3
In reply to #2

Re: Yield Stress for Plastic

02/22/2013 9:51 AM

I think that OP is pointing to the Yield being higher than the Ultimate, inferring that it fails before it reaches Yield.

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Ried
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#4

Re: Yield Stress for Plastic

02/23/2013 3:26 PM

I would guess this is somewhat normal for plastics. Most materials will exhibit a stress strain curve where at yield the stress drops, and then rebounds to increase again until ultimate strength. I am not surprised that a plastic will might not have the same regain in stress before the ultimate breaking stress.

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