B Question about stress - strain graph and definition of hardness

AI Thread Summary
Ultimate tensile strength, defined as the maximum stress a material can withstand, is represented by point D on the graph, which may be inaccurately placed if it is not at the highest point. Point E indicates fracture stress, the stress level at which the material breaks, and is indeed lower than point D, suggesting that fracture may depend more on strain than stress. The discussion raises questions about the definition of hardness, specifically regarding "resistance to plastic deformation of the surface," which pertains to the material's ability to resist deformation and scratching at its surface level. The example of a sponge is debated, as it may not accurately represent a hard material due to its deformable nature. Overall, the conversation emphasizes the nuances of material strength and hardness definitions.
songoku
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TL;DR Summary
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
 
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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”.
 
Thank you very much Lnewqban
 
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