Toughness is how much energy something can take without breaking, e.g. the more weight you can put on a tabletop before it breaks in half, the tougher it is. The opposite of tough is brittle, if that makes more sense. Hardness is how much energy something can take without deforming, e.g. playdoh is not hard.
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.
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.
Strength, hardness, Flexibility all mean different things when engineers talk about them.
Glass is a brittle material which means it will not deform (change dimensions i.e. stretch) when you apply an external force. The opposite of brittle is a ductile material, like steel, which yields and begins to deform once you apply a load above the material's yield threshold. Depending on the material's application ductility/brittleness can be a desired property.
Ductility, strength, and toughness are each separate things.
In many cases, when metal gets stronger it becomes more brittle and less ductile, but this is not a fundamental relationship, in most cases it is an unwanted by-product.
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.
Hardness only determines how difficult something is to scratch. Resistance to cracking is much more complex. For example, I could easily smash a diamond with a steel hammer. But I could also use that diamond to slowly grind the hammer to dust.
> temper (1) to soften; mollify (2) to strengthen (e.g. a metal)
Not contradictory because hardness and toughness are 2 distinct (though correlated) properties. Tempering [1] is basically a process that trades some hardness (i.e. soften) for a lot of toughness (i.e. strengthen).
Work is force times distance. How tough your car (or anything else) is depends on:
1) How much force it takes to flex it -- the spring constant
2) How far it will flex before it breaks
Multiply the two together (or specifically, 1/2 kd^2 where d is how far it will bend before it breaks, k is the spring constant, and 1/2 kd is the average force you're applying), and you get how much energy it takes to break it.
*Everything* flexes. Rigid things just flex less for the same force.
Things which break when flexed just a little bit are called brittle.
Strength and toughness are different properties in materials science. A ceramic mug is strong enough to support great amounts of weight, but it's not very tough. I would think toughness (and elastic modulus) plays more of a safety role in a roof than strength, because strength comes from the pillars.
Less hard, to be more precise [1]. “Strong” and “weak” are way imprecise terms in engineering.
The simplest example is whether it’s strong under pressure (a brick is, a rope isn’t) or under tension (a rope is, a brick isn’t), but there are tons of other properties that are an indicator of strength of a material (https://en.wikipedia.org/wiki/Strength_of_materials), and this may beat diamond in some of them.
[1] reading Wikipedia, even that isn’t simple: “There are three main types of hardness measurements: scratch, indentation, and rebound. Within each of these classes of measurement there are individual measurement scales. For practical reasons conversion tables are used to convert between one scale and another.”
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