> The > Modulus of Elasticity tells you how much the material will squish and then > recover fully (elastically). > Well, technically, the elastic limit (compression) tells you how much it will squish
and then recover fully. The modulus of elasticity tells you how much it will squish under a given load. Usually the modulus of elasticity is only defined up to the elastic limit. After permanent deformation starts, it gets kind of hard to define the modulus. But the force does not go to zero. For structures such as energy absorbing bumpers, the force required to continue deforming after reaching the elastic limit is a pretty important variable. Modulus of elasticity is not always a single number. (Another way of saying the stress strain curve is not always a straight line.) But, for many materials, below their elastic limit, the curve is close enough to a straight line that a single number works to characterize it. In geometric terms, the modulus is just the slope of the stress strain curve in the portion of the curve below the elastic limit. The elastic limit is usually given in %. That is, how many percent of the original thickness will a material squish (or stretch) before it becomes permanently deformed. The modulus is usually given in the same units as strength, e.g. PSI, Pascals, etc. This can be a bit confusing. You have to work out the geometry to see why the units are the same for modulus and strength. Most often, the actual value for modulus, when given in the same units as the strength, will be a much higher number. Another point to keep in mind is that compressive strength and modulus, and tensile strength and modulus are often rather different. For small distances, compressive and tensile modulus are often similar, but compressive and tensile strength can be quite a bit different as for instance with most non-reinforced masonry. The effective compressive modulus of any flexible material is usually quite low unless it is quite thick (cross ways). This is because it buckles or bends under much less force than it would support if it did not bend. Think Kevlar string for a good example. Technically, an engineer would probably not consider the bending force to be the same as compressive modulus. Mark Holm [EMAIL PROTECTED] RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
