Determining the Yield strength and Ultimate tensile strength of HDPE

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Discussion Overview

The discussion centers around determining the yield strength and ultimate tensile strength (UTS) of high-density polyethylene (HDPE) through experimental testing. Participants are sharing observations, challenges, and seeking clarification on the interpretation of stress-strain curves related to polyethylene materials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses confusion about their results, noting that their graph shows yield strength greater than ultimate tensile strength, which is typically not expected.
  • Several participants request more information about the test setup and the specific type of polyethylene being tested, suggesting that material properties can vary significantly among different forms of polyethylene.
  • Another participant mentions strain softening and its effects on the stress-strain curve, indicating that this phenomenon could be relevant to the observations made in the tests.
  • A participant suggests that the first peak on the graph corresponds to the yield point and questions whether sample slippage during the test could have affected the results.
  • Multiple participants report having similar issues and seek advice on how to interpret their results and identify potential problems.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the relationship between yield strength and ultimate tensile strength, with some noting that their observations contradict typical expectations. There is no consensus on how to resolve the discrepancies in the reported values or the interpretation of the stress-strain curves.

Contextual Notes

Participants highlight the importance of understanding the specific type of polyethylene being tested, as different forms may exhibit varying mechanical properties. There is also mention of factors such as strain rate and temperature that could influence the results, but these aspects remain unresolved in the discussion.

Who May Find This Useful

This discussion may be useful for individuals conducting tensile tests on polymers, particularly those interested in the mechanical properties of polyethylene and related materials.

Anmol Dubey
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TL;DR
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 :)
 

Attachments

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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.
 
jrmichler said:
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
 
0001.jpg
20220315_161203.jpg

Anmol Dubey said:
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 :)
 
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.
Polymer.jpg

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.
 
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jrmichler said:
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?
 
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?
 
I have the same problem exactly, can anyone advise me?
 
ahmasrieng said:
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.
 

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