Determining the Yield strength and Ultimate tensile strength of HDPE

[Anmol Dubey]

TL;DR Summary

Apparently yield strength cannot be greater than ultimate tensile strength but in my graph the yield strength is greater
Trying to determine the yield and ultimate tensile strength

Help would very appreciated. Also I am new to this website so if something seems wrong please tell me :)

 

[jrmichler]

Tell us something about your test setup, the material under test, and what you observed during the test. Also, it helps to post the full image - save as a JPG, click Attach Files >> Insert >> Full Image.

 

[Anmol Dubey]

Tell us something about your test setup, the material under test, and what you observed during the test. Also, it helps to post the full image - save as a JPG, click Attach Files >> Insert >> Full Image.

The substance being tested is Polyethylene. I have uploaded a picture of the fracture and the graph as jpg

 

[Anmol Dubey]

Summary:: Apparently yield strength cannot be greater than ultimate tensile strength but in my graph the yield strength is greater
Trying to determine the yield and ultimate tensile strength

Help would very appreciated. Also I am new to this website so if something seems wrong please tell me :)

 

[jrmichler]

I did a quick search using terms stress strain curve polyethylene. The first hits discussed strain softening, strain rate, and the effect of temperature. The plot below shows the effect of strain softening. It's the portion of the stress strain curve from a to c.

Also, polyethylene comes in many forms - some of which are LDPE, medium density, HDPE, UHMWPE. They have large differences in material properties. I suggest that you do some searching of the material properties of your type of polyethylene and also polymers in general. Pay special attention to strain softening.

 

[Anmol Dubey]

I did a quick search using terms stress strain curve polyethylene. The first hits discussed strain softening, strain rate, and the effect of temperature. The plot below shows the effect of strain softening. It's the portion of the stress strain curve from a to c.
View attachment 298696
Also, polyethylene comes in many forms - some of which are LDPE, medium density, HDPE, UHMWPE. They have large differences in material properties. I suggest that you do some searching of the material properties of your type of polyethylene and also polymers in general. Pay special attention to strain softening.

Could you please tell me where the UTS and Yield stress would be on my graph?

 

[Jodo]

The first peak on the graph is the yield point. UTS is normally higher then the yield stress. Is it possible the sample slipped in the "grips" during the tensile test?

 

[ahmasrieng]

I have the same problem exactly, can anyone advise me?

 

[berkeman]

I have the same problem exactly, can anyone advise me?

Welcome to PF.

After reading the responses above, what questions do you still have? Can you upload your test results (use "Attach files" below the Edit Window) and tell us what you think your problem might be? Thanks.