B Question about stress - strain graph and definition of hardness

[songoku]

TL;DR Summary

Please see the graph below

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

 

[Lnewqban]

Please, see:
https://en.m.wikipedia.org/wiki/Strength_of_materials

https://en.m.wikipedia.org/wiki/Hardness

https://en.m.wikipedia.org/wiki/Toughness

https://en.m.wikipedia.org/wiki/Ultimate_tensile_strength

 

[songoku]

Please, see:
https://en.m.wikipedia.org/wiki/Strength_of_materials

https://en.m.wikipedia.org/wiki/Hardness

https://en.m.wikipedia.org/wiki/Toughness

https://en.m.wikipedia.org/wiki/Ultimate_tensile_strength

I think I get the answer to my question but I just want to confirm my understanding, is it correct to say that sponge is wrong example of material that is hard (having high hardness)?

Thanks

 

[Lnewqban]

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”.

 

[songoku]

Thank you very much Lnewqban