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How can a metal be more ductile and also perform as well in crash tests?

foota 10y ago
I think the better that you deform under pressure the longer you have to resist the force against you. Imagine a car where the crumple zone instantly shatters upon impact, it would barely even be worth having, whereas one that deforms will resist the force while crumpling. (I'm a cs person, so excuse me if that's totally inaccurate)
hanniabu 10y ago
It all depends if the force is along or against the bend.
csours 10y ago
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.
cjslep 10y ago
One view can be taken from applying conservation of energy. As a material deforms, it is absorbing energy and not transferring it to the vehicle nor the passengers. A brittle material will absorb less energy before shattering, letting the rest of the energy then be transferred to where you don't want it to be.
- maxerickson 10y ago
  It's the energy in the passenger's body that is usually the issue. For instance, seat belts absorb the energy instead of the body flying through a window, crumple zones increase the time it takes to push the energy into a seatbelt (which decreases the force and thus the damage).
- Gibbon1 10y ago
  Yeah this is correct. Less often talked about but actually very important, the amount of energy required to break something[1]. Also as ductile structures deform under load they tend to redistribute the load more evenly. [1] Hammer a piece of glass vs a piece of rubber.
ars 10y ago
I had the same question. I think they mean it can bend (without breaking), but it still takes a huge force to do so - more than the same metal with conventional treatment.
- honkhonkpants 10y ago
  Yes, that's exactly right. Ductility is measured by how far a metal deforms before breaking entirely. Ultimate strength is measured in terms of how much pressure can be applied per unit of cross sectional area. A metal that can be elongated 50% while under 1000 MPa stress absorbs lots of energy. If a metal is less ductile, it will break after little deformation and therefore stop absorbing energy. Note that the metals under discussion here have very high strength (~1000-2000 MPa) and relatively little ductility (~10%), but are more ductile than other materials with this strength. On the other hand there are very ductile alloys that can tolerate 50% elongation and have strengths around 1000 MPa. These are achieved by adding basically a shitload (20% or more by mass) of manganese.
eridius 10y ago
I suspect that what you're imagining by "more ductile" is "it can be crumpled more easily", but I think what it really means is "it loses less strength when crumpled".
sp332 10y ago
As stated in the article, it means that it can bend further without cracking. When metal has cracked apart, it isn't resisting force any more. If it is only bent, then it can still absorb more energy.

foota 10y ago

I think the better that you deform under pressure the longer you have to resist the force against you. Imagine a car where the crumple zone instantly shatters upon impact, it would barely even be worth having, whereas one that deforms will resist the force while crumpling. (I'm a cs person, so excuse me if that's totally inaccurate)

hanniabu 10y ago

It all depends if the force is along or against the bend.

csours 10y ago

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.

cjslep 10y ago

One view can be taken from applying conservation of energy. As a material deforms, it is absorbing energy and not transferring it to the vehicle nor the passengers. A brittle material will absorb less energy before shattering, letting the rest of the energy then be transferred to where you don't want it to be.

maxerickson 10y ago
It's the energy in the passenger's body that is usually the issue. For instance, seat belts absorb the energy instead of the body flying through a window, crumple zones increase the time it takes to push the energy into a seatbelt (which decreases the force and thus the damage).
Gibbon1 10y ago
Yeah this is correct. Less often talked about but actually very important, the amount of energy required to break something[1]. Also as ductile structures deform under load they tend to redistribute the load more evenly. [1] Hammer a piece of glass vs a piece of rubber.

ars 10y ago

I had the same question. I think they mean it can bend (without breaking), but it still takes a huge force to do so - more than the same metal with conventional treatment.

honkhonkpants 10y ago
Yes, that's exactly right. Ductility is measured by how far a metal deforms before breaking entirely. Ultimate strength is measured in terms of how much pressure can be applied per unit of cross sectional area. A metal that can be elongated 50% while under 1000 MPa stress absorbs lots of energy. If a metal is less ductile, it will break after little deformation and therefore stop absorbing energy. Note that the metals under discussion here have very high strength (~1000-2000 MPa) and relatively little ductility (~10%), but are more ductile than other materials with this strength. On the other hand there are very ductile alloys that can tolerate 50% elongation and have strengths around 1000 MPa. These are achieved by adding basically a shitload (20% or more by mass) of manganese.

eridius 10y ago

I suspect that what you're imagining by "more ductile" is "it can be crumpled more easily", but I think what it really means is "it loses less strength when crumpled".

sp332 10y ago

As stated in the article, it means that it can bend further without cracking. When metal has cracked apart, it isn't resisting force any more. If it is only bent, then it can still absorb more energy.