From: Brita 19/04/99 16:44:49 Subject: Hooke's Law post id: 7328 Could someone please explain to me the basic principles of elasticity, plasticity, malliablity etc. And a really basic explanation of Hooke's Law. Ta From: Chris (Avatar) 19/04/99 17:26:57 Subject: re: Hooke's Law post id: 7338 You need two definitions: stress which is a force per unit area (or pressure) and strain which is deformation. If you plot the stress (y-axis) versus strain (x-axis) you will get a linear graph for a while (depending on the material). In other words a uniform increase in stress will result in a uniform strain or deformation. This part of the graph is the elastic region, where deformations are not usually permanent. At a breaking point the material suddenly enters a plastic region where the amount of strain increases greatly with very little stress until the material shears or breaks. Plastic strain is not usually reversable. As an example of the two, imagine stretching a piece of wire. Initially the wire will stretch fairly well, the more you pull, the more it stretches. You don't see to obvious a change in the wire other than its lengthening, and if you stop it will eventually return to its former shape. This is elastic extension, the ability of an object to deform like this is its elasticity. At some point your wire will suddenly begin to change colour (probably go white) around a stress point. That point will rapidly get more stretched than the rest of the wire and then it will break. This deformation, from the moment the whiteness and stress point appeared, will not reverse itself. This is plastic strain, and the ability of an object to deform in this way is its plasticity. The Hooke relation lets you work out how much force or pressure you need to deform a material by a certain amount. Malleability measures how far a material will compress into a flat sheet - eg a lump of plasticene is more malleable than you are. Hope this helps! Chris From: Cam (Avatar) 19/04/99 17:36:54 Subject: re: Hooke's Law post id: 7339 Imagine pulling something stretchy, like a spring. The harder you pull it, the further it stretches. Hooke's Law simply states that if the stretch is directly in proportion with the pull, and reversible (ie if you let go, it will return to its original length) then the spring is "elastic". If you stretch the spring too far, it may not go back to its original length. In that case, you'll have exceeded its "elastic limit". Interestingly, as you exceed the elastic limit, the stretch is no longer proportional to the pull. When you are operating beyond the elastic limit of a material, it's called "plastic" deformation. Nearly all materials have a zone of elastic behaviour before they start to deform plastically. Some materials (like glass) never reach the elastic limit; they simply break before they get there. Other materials have a negligible elastic zone, like butter. You don't see butter springing about all over the kitcken! Malleability is a measure of how easily a material's elastic limit is exceeded, and it is permanently deformed. Butter is very malleable because it has a low elastic limit - a small deflection will not spring back. A clock spring is not very malleable - you can twist and stretch it out of shape, and it will go "sproing" back again. Malleability is an important property of a material if you want to make something with it by pushing and shoving it into the desired shape, eg. pottery clay. Steel gets more malleable when it's hot, just like butter! That's why blacksmiths don't try to belt a horseshoe out of a piece of cold steel. They heat it up to red-hot first; it makes life a lot easier! From: Alan 19/04/99 22:40:19 Subject: re: Hooke's Law post id: 7374 Cam, malleability is the permanent deformation under compression without breaking, so effectively it's the opposite of plasticity so Chris was right. And Chris the point where the material begin to deform permanently is the yield point, not hte breaking point. The Guy have explained hookes law really well. Brita read from here To work through each stage. There is a region where for a given stress, you will get a given strain. The material is elastic and the relationship between the two is a constant E (known as the modulus of elasticity). This relationship is hooke's law. E = stress / strain = constant (Hookes Law) Stress by the way is the force per unit area. Strain is the change in area / the area Elasticity is the ability for a material to behave in an elastic way. Elastic behaviour occurs when something is stretched it returns to the same size. Plasticity is the ability to ungo plastic behaviour without breaking. Plastic or inelastic behaviour occurs when something is stretched it results in permanent deformation (change in dimensions) in other words it permanently gets longer. The point of transition between the elastic and inelastic regions is known as the yield point. Beyond this point stress is not proportional strain. So an incremental increase in stress will result in a larger than incremental strain. At a certain point known as the Ultimate Tensile Strength, (UTS), the material will undergo localised stretching. At this point the material will rapidly reduce in cross section at one location and will proceed to the breaking point with decreasing stress, even though the force or load is still constant. Malleability is sort of the opposite. Instead of the material being stretched it becomes compressed. Technically it is the ability for a material to be compressed without breaking. Ductility is the percentage change in length / original length And its the property which permits plasticity to occur. With some materials, they break before the UTS is reached, ie before locallised stretching. These materials are called Brittle. Ideal brittle materials break in the elastic region (glass). While some materials break in the plastic region, with a small amount of diension change. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.