Limit between flexibile and solid!!

[pompelmo]

Hello, I have a question we can argue about for a long time..
..every man who knows a little about phisycs knows that avery material has it's own flexibility..one have more (ruuber) other have less (rock)..
..but I think no one knows where is the limit (F=-kx...k is the factor of flexibility) when we can talk about flexibile material and when we can talk abou solid material..Do you know the answer???

if there is no limit than can rubber be solid and a rock flexibile material...

[DaveKillens]

Wow, that's a broad topic. Although I'm not trained in structural enginering and don't know the math, about all materials posess properties that can be cosidered as "flexibility", and "rigidity" Of course, there's a lot more thna just that.
Take for example the coil spring. It has a certain strength, and springs back to it's original position when released. Almost all materials (especially in racing applications) are flexed to some degree, and how much they resist, and how they return to their original position is what give smany engineers headaches.
For example, I read that when Ford first instrumented the GT-40, they discovered that the rear driveshafts flexed as much as 270 degrees when under full acceleration. I wonder how many consider the driveshafts as part of the driveline damping system? Or that under maximum load and RPM, piston rods can suddenly flex and bend? That what appears strong and inflexible under first impression can flex like licorice when subjected to loads?
And to take the complete opposite, the B-2 stealth bomber of the US air force is constructed of mainly carbon fiber. Not only to assist in reducing radar reflectivity, but to give the airframe a very inflexible structure, with as little flex as possible? You have to remember, to control and reduce radar reflections, you have to control the angles of each part of the airframe, to allow the wingtips to flex up and down, and change their relative angle is unwanted. I hear that at low altitude, it's a very rough ride.

[ReubenG]

The departure of materials from non-linear behaviour is a hugely complicated field- whether you are considering material non-linearity or geometric non-linearity.

In a nutshell, the stress/strain point at which a material is no longer elastic (linear in response and capable of full recovery) is referred to as the yield point. This varies as a function of temperatutre, strain rate etc from material to material.

Will have look at some of my under grad texts to see which give good simple explanations.

[West]

Are u talking about elastic and plastic deformation?

Basically, a material can handle enough stress/strain up to its yield point, as Reuben said. This zone, before the yield point, is the elastic zone. Beyond this point, the material will not recover its properties. We call this the plastic deformation zone.

I forgot this stuff too. But this isn't what ur looking for, I bet.

[ReubenG]

I might have misread Pompelmo a little - I think he was referring to problems where some parts are treated as flexible, whether purely elastic or including plasticity, and some parts are "analytically rigid" i.e. they are assumed not to distort in any way.

All material can flex or distort - ceteris parabis, a diamond requires more force to distort a given amount than steel, steel more than wood, etc.

Determining whether to treat something as rigid in a problem is extremely difficult - one has to consider loading, geometry and material properties (which may vary during the problem). Many strucutural analysts will start a problem by assuming certain parts are rigid, just to get a first order solution. Depending on the resources available, one can then start treating other parts as flexible - this normally comlpicates matters exponentially. Really good, experienced structural engineers and analysts are able to estimate which parts may be assumed to be rigid without compromising the solution's accuracy.

If you want a look at a good material property database, check out matweb.com.
http://www.efunda.com/formulae/solid_me ... stress.cfm has a decent explanation of stress/strain etc.