Youngs modulus vs Ultimate tensile strength

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Discussion Overview

The discussion revolves around the relationship between Young's modulus and ultimate tensile strength (UTS) of materials, specifically focusing on why UTS values are generally lower than those of Young's modulus. The conversation includes theoretical aspects, material properties, and examples, particularly with copper.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Amlesh questions why the ultimate tensile strength of copper (245 MPa) is significantly lower than its Young's modulus (131,000 MPa).
  • One participant clarifies that UTS is the stress at which a material fractures, while Young's modulus is the ratio of stress to strain in the elastic region of the stress-strain curve.
  • Another participant mentions obtaining the copper values from a vendor, indicating reliance on external sources for data.
  • A participant notes that compressive strength and Young's modulus measure different properties, suggesting that comparing their values directly may not be meaningful.
  • It is proposed that there is a general tendency for UTS to be less than Young's modulus, with a reference to theoretical strength values being between E/20 and E/5, where E is Young's modulus.
  • Another participant reiterates that Young's modulus can be viewed as the elastic stress at a strain of 1, suggesting that most materials cannot double in length while remaining perfectly elastic, leading to UTS being numerically less than E.

Areas of Agreement / Disagreement

Participants express varying views on the relationship between UTS and Young's modulus, with some agreeing on the general tendency for UTS to be lower, while others emphasize the distinct nature of the two properties. The discussion remains unresolved regarding the implications of these differences.

Contextual Notes

Participants reference specific values for copper without providing sources for verification, and there is mention of theoretical predictions that may not align with measured values due to structural defects.

Amlesh
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In case FEA we consider ultimate tensile strength to decide whether component will fail OR pass why ultimate tensile strength is lesser than youngs modulus? Example for copper UTS is 245 MPa n youngs moduls 131000MPa. Why it is like this?

Regards,
Amlesh
 
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The UTS is the stress at which the component/material will fracture.

The Modulus of Elasticity or Young's modulus is the ratio of the stress to strain for the elastic region of the stress-strain curve of the material.

Where did you get those values for copper?
 
Rock,

Many thanks for quick reply.

By the way, I got this coper value from one of our vendor.
 
The compressive strength and young modulus are different quantities, measuring different properties. They do have the same unit but they are still different. So comparing their values is a little meaningless.
Like saying that the torque of a given force (in Nm) is less (or more) than the work done by the force over a given distance (also in Nm).

I am not saying that is not interesting to look at the values, as long as you know their meaning.
Actually the so called theoretical strength of many metals was found to be between E/20 and E/5, (if I remember correctly) where E is Young's modulus. These are calculated from crystal structure and bond strength, for a perfect crystal. So it seems that there is a general tendency to have UTS "less" than E, in value.
The measured UTS values are a lot less than predicted by this rule of thumb, due to defects in structure.
 
Last edited:
nasu said:
So it seems that there is a general tendency to have UTS "less" than E, in value.
You can think of Young's Modulus as the elastic stress when the strain is 1 (i.e. the length of the object has doubled). Not many materials can double in length and remain perfectly elastic, so the UTS of most materials is numerically less than E.
 
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Dear All,

Thanks for quick reply.
 

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