Question about stress - strain graph and definition of hardness

Click For Summary

Discussion Overview

The discussion revolves around the interpretation of a stress-strain graph, specifically the definitions of ultimate tensile strength and fracture stress, as well as the concept of hardness in materials. Participants explore the implications of these definitions and their application to materials like sponge.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether point D, defined as ultimate tensile strength, should be the highest point on the stress-strain graph, noting a discrepancy where point D appears not to be at the maximum.
  • The same participant raises a concern about the relationship between ultimate tensile strength and fracture stress, suggesting that it seems counterintuitive for a material to withstand a higher stress than what is required to fracture it.
  • Another participant provides links to Wikipedia articles on strength of materials, hardness, toughness, and ultimate tensile strength, indicating a search for further information.
  • A different participant seeks confirmation on whether sponge is an appropriate example of a hard material, questioning its classification in terms of hardness.
  • One participant suggests that the teacher's use of sponge as an example may have a rationale, but expresses doubt about the hardness of sponge, implying it is generally not considered hard.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the definitions and applications of hardness and tensile strength, with no clear consensus on whether sponge is a suitable example of a hard material.

Contextual Notes

Participants are grappling with the definitions and implications of stress-strain relationships, particularly the conditions under which materials exhibit different properties. There is an unresolved discussion about the specific characteristics of hardness and how they apply to various materials.

Who May Find This Useful

Individuals interested in material science, engineering, and the mechanical properties of materials may find this discussion relevant.

songoku
Messages
2,509
Reaction score
393
TL;DR
Please see the graph below
1629351979180.png


This is from my notes:

Point D is called ultimate tensile strength and defined as highest possible within this material.
So it means that point D should be at the highest point of the graph (more like absolute maximum in math)? Because it seems that from the graph point D is not at maximum point, or maybe point D is a little bit misplaced?Point E is fracture stress, defined as value of stress in the material when sample breaks.
From the graph, point E is below point D so the amount of stress needed to break the material is less than the highest stress (ultimate tensile strength) the material can withstand? Does it mean that the fracture point actually depends more on the strain rather than stress? Because I find it a bit weird that the material can withstand, let say 106 Pa, of stress but fracture by 105 Pa of stress.Last question is about definition of hardness (not related to the graph). Definition of hardness in my note is the resistance to plastic deformation of the surface or resistance to scratching.
What does it mean by "resistance to plastic deformation of the surface"? Does it mean that we only consider the surface of the material, whether it can undergo plastic deformation or not, or maybe the surface is resistance to scratching or not?
One example of the material given by the teacher is sponge. I can imagine sponge is resistance to plastic deformation but I don't understand the "of the surface" part because if I squeeze a sponge, all parts of the material deform, not only the surface. I am also not sure about the resistance of scratching of sponge. I think maybe I can leave some scratch on its surface by using knife.

Thanks
 
Reply
  • Like
Likes   Reactions: Delta2
Physics news on Phys.org
I am sure your teacher had a good reason to use that example.
Unless a specific natural sponge has some hard horns or shell, it seems to me that it is mostly “not hard”.
 
Reply
  • Like
Likes   Reactions: songoku
Thank you very much Lnewqban
 
Reply
  • Like
Likes   Reactions: Lnewqban

Similar threads

High School Creep and its effects on a material
  • · Replies 1 ·
Replies
1
Views
1K
Stress-Strain Graphs of different materials
  • · Replies 9 ·
Replies
9
Views
7K
Understanding Yield Strength & Stress Strain Curve
  • · Replies 2 ·
Replies
2
Views
6K
Confusion Calculating Young's Modulus
  • · Replies 5 ·
Replies
5
Views
2K
Shear stress damage due to thermal gradient
  • · Replies 6 ·
Replies
6
Views
3K
Interpreting Stress-Strain Graph: Which Material Has the Highest Ductility?
  • · Replies 1 ·
Replies
1
Views
3K
High School A statistical definition of Young's Modulus?
  • · Replies 8 ·
Replies
8
Views
3K
Graduate Measuring Tensile Strength in an alternative way
  • · Replies 2 ·
Replies
2
Views
3K
Graduate Stress and Strain Tensor Basics: Definition, Equations & Moduli
  • · Replies 1 ·
Replies
1
Views
3K
Ductile material engineering stress-strain curve
  • · Replies 3 ·
Replies
3
Views
5K