The shock absorbing ability of steel

[ballballisicious]

can anyone explain the shock absorbing ability of steel? thank you

 

[Astronuc]

Please elaborate - as in do you mean a steel spring (e.g. in a shock absorber) or do you mean something like a steel (or any strong metal) plate that absorbs/dissipates/deflects the shock wave of an explosive or impact of a missile?

 

[ballballisicious]

um.. actually, i want to talk about mild steel used in car bodies, because it has to be crash worthiness, so i think i better explain the shock absorbing property of that. is that related to the internal structure (the arrangment of the atoms) or the compositions that makes up mild steel?

 

[Loren Booda]

For protection of a car's occupants, my guess is that an exponentially collapsible structure with the greatest Young's Modulus for the least density would be optimal. Consider a honeycomb design with increasing wall thickness away from the impact.

You may wish to compare steel to carbon, in the form either of graphite or nanotubes. Another approach involves a bumper filled with water which, when compressed, jets out of apertures while approaching closely the critical pressure of solidification, thus using the near phase change to absorb the mechanical impact.

 

[PerennialII]

A bit of a general topic, in general we're talking about the ability of mild steels (and most steels overall as long as we don't go to really high strength ones etc.) to be able to plastically deform without fracturing. Under loading (static and dynamic, dynamic changes the response (a rate sensitive material) but not the principles that much) steels exhibit linear-elastic Hookean response followed by an elastic-plastic response, the linear region typically being defined up to strains of 0.2%, whilst the elastic plastic region can carry to several tens of percents (fracture strains are typically in the envelope of 10-70%) ... so, the elastic-plastic region is really extensive in terms of deformation, and I believe is what you call the ability to absord the shock. More than anything it is related to the mechanisms of plasticity in mild steels (i.e. what controls and affects the movement of dislocations, so to answer your question when we're comparing steels its a composion and treatment thing), and the configuration in mild steels is typically such that there ain't that many mechanisms impeding plastic deformation ... and since the amount of plastic deformation can be easily correlated to the energy the structure made of such steel can absorb, use of relatively low strength, "standard", steels becomes feasible (and due to the same elements they also strain harden quite a bit, another property which is quite essential in this application). I can probably find you a couple of links for further details ... ?

 

[Chronos]

Nice elaboration, perennialII. Toughness [an engineering thing] is the big issue in design. Toughness relates to plastic deformation and tensile strength. Steel is still very popular because it is very tough and cheap. Composite materials are, however, gaining ground. They are getting pretty tough and weigh less.

 

[ballballisicious]

PerennialII.. your explanation is good, but can u give me suggest some link related to that? coz i have to quote the website in my project.. thanks

 

[Astronuc]

Just adding to what PerennialII already mentioned,

when I looked at stress-strain curves for mild steel, I found Young's (Elastic) Modulus of about 30 E6 psi, proportional limit of 0.00125, and strain at yield of about 0.0013-0.0014 (1.3-1.4%).

The plastic range varies depending on composition, annealing/working, tempering, etc.

The ideal material has strength, ductility (large deformation/strain between yield and failure), and toughness (resistance to fracture) under high loads and high strain rates - 1-10%/sec. Obviously the stresses are going to be above yield and perhaps locally above UTS.

In addition to material properties (like composition, heat treatment), the designer will employ geometrical effects (tube wall/strip/shell) thickness and cross-section. Forward most the strip/tube wall thickness is reduced and thickness/cross section increases toward passenger compartment. The thinner sections collapse first.

The passenger compartment has to be the strongest to resist deformation.

This maybe helpful - http://www.a-sp.org/database/custom/bulletins/default.asp?bid=18

A more general resource:
http://www.autosteel.org/ulsab/index.htm

http://www.autosteel.org/technical_info/cdradio_2004may_index.htm

I was thinking that Society of Automotive Engineers (www.sae.org[/url]) would be helpful, but I could not readily find any basic info on automotive steels or mild steels. If you have a particular composition or grade, then try [url]http://www.matweb.com/search/SearchSubcat.asp[/URL],

or try a manufacturer/supplier website.

Best wishes for success on your project.

 

[PerennialII]

Thanks ! I think Astronuc's text & links do a good job in explaining the case, especially the structural design details which are at least as important as the material when it comes to performance of the car body itself. And I'd say the point risen by Chronos goes to the heart of the matter (ductility, toughess, plastic design etc.). Couple of links on the somewhat theoretical side of things which hopefully will state what I did in a bit broader and general form :

http://www.engin.brown.edu/courses/En222/notes.htm
http://jamaica.u.arizona.edu/ic/mse257/class_notes/disclocation.html
http://www.mae.cornell.edu/zabaras/Courses/MAE612/mae612.html
http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-002Spring2004/8513DFBA-EB19-4AA4-91FE-D997187ABB24/0/lec10.pdf
http://hotrodders.com/kb/body-exterior/articles/sheet_metal.html
(the last one ... well, its a link at least).

 

[ballballisicious]

whao... thanks a lot for the links, they'r brilliant!
but, i have a little question
mild steel is one of low carbon steel which means not so strong when compared to high carbon one, why is mild steel used instead of high carbon steel?
well, i was thinking whether the mild steel is heat treated first. but how can annealing change the crystal structure of mild steel? is it something related to the body centered cubic and face centered cubic thing?
and from a steel website, they said heat treating of mild steel is process annealing, but they also said the process will soften the steel and improve machinability, it really confuses me, i thought annealing is going to increase the hardness and strength of mild steel, but its seems to be differnet from what i thought its going to be, can anyone clarify that, coz my teahcer will probably ask me to explain it.
any help will be appreciated.

 

[PerennialII]

mild steel is one of low carbon steel which means not so strong when compared to high carbon one, why is mild steel used instead of high carbon steel?

When you want a structure to be able to absorb as much energy as possible, you're essentially trying to maximize the area under the stress-strain curve (which turns out to be the strain energy density, which when integrated over volume is strain energy). So you would want to have a material with extreme strength (=high level of stress in the curve) and extreme failure strain (=the curve continuing really far). In addition, if you have high strain hardening, the curve has for higher strains a positive slope, increasing the energy "intake". Its a characteristic of steels that if we increase its strength, we adversely affect its ductility, which means for one that the failure strain decreases (and also strain hardening). In practise, the decrease of ductility associated with strength increase is such that the total energy the material can absord decreases, and as such it becomes feasible to use lower strength ones. [this has a cost effectiveness factor built in, there are ductile higher strength steels which would do a better job, but make the component more expensive one and have typically some manufacturing issues were mild steels behave better, so its an optimization thing between different factors. Also, the "softness" of mild steel gives you some feasible properties e.g. in a car body in which the deformations can be concentrated (has some fine structural design included) where you want them (one typically used design feature)]

well, i was thinking whether the mild steel is heat treated first. but how can annealing change the crystal structure of mild steel? is it something related to the body centered cubic and face centered cubic thing?

Heat treatment of steel is a topic worth many credits (=has quite a bit of stuff in it). Annealing itself is a heat treatment which aims in recrystallization of the microstructure. Typically it reduces internal stresses and "homogenizes" the microstructure by reducing the number of defects and microstructural irregularities & features (which might have been "put" there to increase strength and hardness). So its a treatment reducing hardness, increasing ductility etc (at a constant temperature followed by a relatively slow cooling).

it really confuses me, i thought annealing is going to increase the hardness and strength of mild steel, but its seems to be differnet from what i thought its going to be, can anyone clarify that, coz my teahcer will probably ask me to explain it.

As above, its vice versa. Before annealing the microstructure typically has elements making it a bit too hard & high strength for applications typically suited for "mild steel" (before annealing it ain't quite a mild steel). Those feasible ductility properties of mild steel are attained to great extent (depending on steel again) as a result of annealing (if you need more on the process & details of what is going on in the steel itself "holler").

 

[Gokul43201]

PerenailII has covered the matter very nicely, but I'd like to add just one little thing here for ballball...

I think you may be confusing annealing with quenching. These two types of heat treatment usually result in diametrically opposite effects, the former raising ductility and toughness, while the latter raises strength and hardness (and of course, brittleness).

Anyway, the heat treatment of steels is a huge topic by itself, and trying to make too many generalizations is ill-advised.

 

[Gokul43201]

On a side note : strange as it may seem, some cast irons have very good vibration damping properties. I think they are called SG irons (because of spheroidal graphite nucleii), and get used in automobile cylinders, pump rotors and other places that can be a source of vibration.

 

[PerennialII]

On a side note : strange as it may seem, some cast irons have very good vibration damping properties. I think they are called SG irons (because of spheroidal graphite nucleii), and get used in automobile cylinders, pump rotors and other places that can be a source of vibration.

Think I've heard about the same thing ... their better than steel thermal conductivity is also a property utilized in many applications.

 

[Chronos]

What Gokul said is entirely appropriate. In stress relief you trade off hardness for ductility. Hardness increases resistance to catastrophic failure. Ductility trades off that resistance for yield strength. This is a very tricky thing in design engineering. Mechanical strength depends on geometry. Yield strength conserves geometry while permitting a bit of real world 'give' in design. For this reason, the more important factor in most design schemes is usually yield strength.

Footnote: Vibration is the curse of design considerations. Rigidity, while a good thing in principle, is a very bad thing when it comes to vibration. Fatigue is very strongly associated with rigidity and wear is very strongly associated with the lack of rigidity [hardness]. Optimizing the two is difficult.

 

[Astronuc]

A really good reference on carbon steels is ASM International's Specialty Handbook, "Carbon and Alloy Steels" http://www.asminternational.org/template.cfm/Template.cfm?Section=Bookstore&Template=/Ecommerce/ProductDisplay.cfm&ProductID=10520

I have it and two companions "Stainless Steels" and "Heat Resistant Materials".

But they are a bit pricey. Each of these costs $225 for non-members or $180 for members.

The American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) have some good references as well.

 

[hartmann]

Hello ¡¡¡
This is my very first post in this very great forum. I like to ask you If someone has idea of the high strenght high quality hardened and tempered steel response under the shock of high velocity projectiles with high hardness ogive (regarding striking velocities of around 500-1000 m/s and tip hardness of around 650-680 Brinell fardness)
Thanks in advance to all and best regards