There are several material property terms that get mixed around when converting to layman's. Toughness and hardness are two of them. Another set is Plasticity and Elasticity which measure tension properties.

Elasticity is how much you can deform (stretch) an object, before the deformation is no longer reversible. Think of a rubber band, you can stretch it a little, and it returns to its original shape. Stretch it a lot, and it breaks down and does not return to its original shape.

Plasticity is how much you can deform an object before it breaks. This is the snapping point in the rubber band, where it completely breaks.

These two properties are different, and have to do with how the material is deforming on an atomic level. During elastic deformation, the individual bonds stretch. During plastic deformation, regions, layers, or fibers of the material slip. The relationship is best described by the stress-strain curve [1]. This plots deformation versus the pressure required.

Hardness and Toughness are the compression version of Elasticity and Plasticity.

These properties are often found experimentally, because they are a summation of many different atomic level interactions. Infact most materials are anisotropic, meaning if I measured the toughness of a material along two axises, I would get two different values.

[1] https://en.wikipedia.org/wiki/Stress%E2%80%93strain_curve

> Elasticity is how much you can deform (stretch) an object, before the deformation is no longer reversible. Think of a rubber band, you can stretch it a little, and it returns to its original shape. Stretch it a lot, and it breaks down and does not return to its original shape. > Plasticity is how much you can deform an object before it breaks. This is the snapping point in the rubber band, where it completely breaks.

These both are completely false.

Elasticity is the idealistic property of a solid material to get back to it's original shape after a deformation force is retracted. (property of most solid metals where stress is much below the yield point)

Plasticity is the idealistic property of a solid material to stay in the deformed shape after a deformation force is retracted. (Plasticine)

You are correct, but the statement you're quoting can be easily corrected: replace "elasticity" with "The elastic limit of a material". The elastic limit is also conventionally called the "yield strength".

"Plastic limit" is not a real term, but if it were, it might be a synonym for "ultimate tensile strength". There's also compressive strength and shear strength.

Thank you for a terrific description! I learned something today.

A relevant test for toughness is called the Charpy impact test. It uses a fixed size piece of material and a fixed weight pendulum to snap the material. The amount of force needed to snap the material becomes a score for toughness.

According to the ASM manual in front of me, toughness is the ability of metal to absorb energy and deform plastic ally before fracturing. The Charpy test is a test of absorbed energy, not force.

I misspoke earlier when I said force. I was inadvertently sloppy in my writing.

Does this mean that plastic is very tough and hard?

Edit: Sorry, not plastic. I meant rubber.

No, plastic can be very tough, but is not generally very hard (although there are exceptions to both).

Since plastic can stretch a large amount before breaking it absorbs more energy than a (inverse of tough) object, so it is tough.

Since it is easy to stretch inelastically, it is not hard.

Plastics are... plastic. Uggghhhhh.... https://en.wikipedia.org/wiki/Plasticity_(physics)

Usually anyway. IIRC, Acrylic is a plastic that shatters instead of deforming. But most plastics (ABS (aka Legos), polypropylene (PP, #5 Plastic), Pete (most bottles), HDPE (aka cutting boards)) all are "plastic", as in having high plasticity.

IIRC, metals are actually more "plastic" (deform more before breaking) than erm... plastics. See iPhone aluminum bending issue for details.

I recall that "plastic" was so named because of the material's formability. Thermoplastics soften or liquefy when heated, becoming malleable and can be molded or extruded, retaining shape when cooled. Thermosetting types are products that are cured by heat, UV or chemical reaction, having high plasticity before curing but irreversible rigidity after.

"Plastics" are mostly petrochemical derived organic polymers. Properties of some, like acrylic glazing are relatively hard and brittle, others quite elastic and soft, e.g., foam rubber. Obviously modern life is utterly dependent on these materials which are employed in virtually everything we use from food to space probes.

There are good reasons to believe that production of "plastics" is among the highest and best use of petroleum resources, it's a shame we continue burning it up at a prodigious rate. Trends to find alternative energy sources would have benefit in this respect too.

The history of plastics discovery is pretty interesting too.

The first were based on coal tar, starting around 1870-1880 IIRC. Then came bakelite, still used in cookware handles -- it has exceptionally good thermal resistance properties.

Discovery accelerated through the 1900s, 1910s, and 1920s, and exploded in the 1930s, with the number of materials about doubling IIRC.

Names started changing too -- from chemical (polypropelene, polyester, polystyrene) to brand (nylon, teflon, orlon).

I've also noted that with both coal and oil, there seems to have been an adoption and technological development cycle in which the first use was to burn the materials, and only later, by some decades, did the chemistry develop for materials based on the substrates. There's probably a lesson here.

Plastics aren't hard, as are ceramics, metals and other cristalling structure, because you can scratch/penetrate it.

Check this out for a quick comparaison about toughness: elongation of polymers is pretty easy. http://images.slideplayer.com/32/9819762/slides/slide_7.jpg

For more info: https://en.wikipedia.org/wiki/List_of_materials_properties#M...

Ugh, sorry, I meant to type "rubber" and typed "plastic" instead. Rubber both resists scratches and permanent deformation, no?

You're asking if rubber is considered hard? What type of rubber do you have in mind?

Any, I guess. Tyre rubber is not easily deformed permanently, although I guess it is easy to scratch.

Since the elastic modulus (slope/derivative of deformation vs stress graph) the energy absorbed is not necessarily very high even though you can get high deformations (multiple 100%, compared to 30% or so for soft steels).