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Difference between strength and toughness
- Thread starter wdriver
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Comeuppance
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I think those are both terms for the same characteristic - impact resistance / lateral stress tolerance. Typically, blade steels are compared by their ability to sustain impact and their wear resistance (edge retention). Of course, there's also corrosion resistance and machinability, but you're not likely to hear "toughness" or "strength" used to refer to those.
Comeuppance
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Ah, I hadn't considered hardness - but I've never seen any other term but hardness used to refer to it.
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are you sure? I've never heard of the hardness described as strength.
any ways, within the realm of "toughness" there can actually be several different properties. I read a really good little article on it, linked from here on bf, recently. things like lateral strength, edge stability, impact resistance, etc... "toughness" is usually referred to encompassing all of these traits, but some steels can really vary at which traits they excel at. I will leave the specifics for those more knowledgeable than I.
For me edge stability is more important in a mid sized folder than all out toughness like a chopper needs. for instance most people will tell you 1095 is tougher than your average stainless, or even "super steel". This may or may not be true, I do not beat on my knives in ways to break them, so far they have all been strong enough for me. However I have noticed with cpm m4, the edge is MUCH less likely to deform in any way compared to other steels I have used, including 1095. accidentally striking metal, ceramic, marble, stones, etc... this steel remains mostly unaffected when lesser steels would roll the edge or worse, chip.
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My betters have instructed me that:
"Toughness" is resistance to being fractured on impact, and "Strength" is resistance to deformation under load.
Ed: But what would the ASM know?
"Toughness" is resistance to being fractured on impact, and "Strength" is resistance to deformation under load.
Ed: But what would the ASM know?
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When reading about the elements in different kinds of steel, toughness and strength are often mentioned. What is the difference between them?
Technically; Strength is how much force you need to break something (or to bend it to the point you consider the thing useless). Toughness is how much energy it takes to do so. In the case of steel, strength and hardness are very much proportional. Tough is the oposite of fragile. Ideally you want a knife to be both hard and tough, but it is impossible to get a lot of both things at the same time. If you are reading charts describing materials, strength and toughness are given by unit area. Therefore strength is force/cross section area (pounds/sq feet), called stress, for short. A similar thing is done for toughness.
Consider the extreme examples: glass is very hard (it will scratch most pieces of steel). It is also very strong, thats why fiberglass is used in composite materials. However, glass cracks easily on impact, therefore is not tough at all. The other side of the spectrum is mild steel. You can beat the sh*t out of it and will not crack, although it may change shape a little bit. It is a relatively soft and weak material so it will made a very bad knife (the edge would roll to the side too easily).
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There are very technical meanings assigned to those words, however for the laymen, strength is the amount of force is takes to bend something, toughness is how hard you have to hit it before it chips or cracks. Something like 6x6x10 inch brick of glass would be very strong, but not particularly tough. A paper thin piece of ductile iron would be very tough, but have no strength worth speaking of.
Of course you need to compare apples to apples. So as the strength of 1095 steel goes down the toughness goes up. There are steels that will be much tougher than 1095 at the same hardness. And there are some steels that will loose both toughness and strength if tempered at the wrong temperature. Google Verhoeven, Metallurgy of Steel for bladesmiths & Others who Heat Treat and Forge Steel. Page 41.
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The most basic of laymen terms, Strength, how much can you bench, Tough, can you take a punch. Two totally and completely separate qualities or characteristics. Steel, how many pounds usually in psi is the shear strength the force or load it takes to shear the material basically rip the material into two separate pieces usually sliding along a plane, then for toughness how hard of a blow before the material shows signs of deformation, how hard do you have to swing a hammer that strikes a nail resting on the blades surface to make a dent in it.
Tensile strength is the basic property of the metal. It is generally a measure of how much stress it takes to make the metal permanently deform. For instance if you pick up a piece of steel wire and bend it in your hands, the tensile strength is a measure of how hard you have to push on the wire before it bends and doesn't spring back. Hardness is measured by how much force it takes to put a dent in the surface of the metal, which is a form of permanent deformation of the metal. So yes strength and hardness are 2 ways to say the same thing. Whether you are deforming the metal by bending it or putting a dent in it, this is influenced by the tensile strength of the metal.
Toughness comes into play after you start deforming the metal. A piece of normal steel wire can be bent quite a bit in your hands and will not break. You can bend a wire coathanger into lots of crazy shapes and it won't break, so it has a high ductility and the higher the ductility, the higher the toughness. A higher strength steel that will bend just as much without breaking has an even greater toughness. Toughness of steel samples is tested with an impact machine (Charpy V-notch test) and thus is the amount of energy that the piece of steel absorbs before it fractures, but toughness properties of steel are evidenced in static tests as well. Clamp a knife in a vise like you see in some youtube videos and start pulling on the knife until it breaks or deforms. The amount of force you pull on the handle with before the blade breaks or deforms is a function of the strength. If the blade suddenly breaks without a permanent bend first, it is not very tough. If the blade bends a little bit and won't spring back to the original shape, it has some toughness. The farther it will bend and deform without breaking, the tougher it is. In using a knife for where you are putting a lot of force into the blade, whether you are using a steady push or you are hitting the blade, the tougher metal will let the blade bend and deform, the less tough blade will chip or break.
Tougness comes into play even if you don't hit it, if you just push on the blade hard. And a blade that has higher toughness would be less likely to chip or break.