The BBC article heavily confuses hardness and toughness. The researchers seem to care about hardness here.
The actual paper (http://advances.sciencemag.org/content/2/7/e1600319.full) is much better and is free.
The actual paper (http://advances.sciencemag.org/content/2/7/e1600319.full) is much better and is free.
Hardness is a characteristic of a solid material expressing its resistance to permanent deformation. The Rockwell or Vickers hardness scales are most commonly used in the industrial blade industry.
Toughness on the other hand is the maximum amount of energy a material can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.
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
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)
"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.
I misspoke earlier when I said force. I was inadvertently sloppy in my writing.
Edit: Sorry, not plastic. I meant rubber.
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.
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.
"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 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.
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...