When bending spring steel does the compressed portion of the metal become denser or does it distort or is it a combination of both?
It is a combination of both, but the change in density is negligible.
Is there any term that you are aware of that would express how much of the compression forces are due to distorsion or actual compression.
The reason I am asking is that I am looking for a value to attach to the way compression forces are actually working as say in wood with a lot of air inside and metal with almost no air.
Ductility is the area you are interested. The great value of steel is it’s high level of ductility. I don’t believe the same mechanics apply to wood because it wouldn’t be considered a solid material in the same sense.
would strain cover it?
I think ductility may cover it. Not sure if it covers while working in the elastic range instead of the plastic range though.
I’m not sure if there is any distinction to be made. When you bend something, you’re compressing one side of it and stretching the other side.
Wood is definitely a solid material, however, at a micromechanics level it is best treated as a composite of cellulose fiber bonded together with a lignin matrix. Because wood has a grain direction the properties are anisentropic (vary with orientation) but this is true of many other solid materials such as aluminum.
The deformation of an element within a solid body under pure compression and tension is called Poisson’s Effect, and is determined by an innate material property called the Poisson’s ratio (ν), which for steel is between 0.28 and 0.31 depending on alloy. The volumetric change per unit volume ΔV/V = (1+ΔL/L)[SUP]1-2ν[/SUP] - 1. The change is small but not insignificant; under thermal stresses a compression due to chilling or heating can cause a small volume change but result in a large change in long dimension, which can pull struts away from a structure. In pressurized systems, the expansion of a pipe can result in high cyclic tensile stresses that can cause unreinforced bends to fatigue or pull fittings apart. (This is why b-nuts that are used to connect threaded fittings on pipes should always be mechanically locked rather than relying on friction to retain them.)
Ductility refers to the post-elastic behavior of a material. Ductile materials deform gracefully without brittle fracture out to a significant elongation, providing visual cues of incipient failure and work hardening may even protect against initial failure so the tolerance for statistical variance in loads is high; for some steels that deformation can exceed 20%. Low ductility materials like tool steels and high number alloys of aluminum tend to fracture with little deformation, which means that the tolerance for variance in loads is low, and if used in a highly loaded or cyclically loaded state additional margin to failure must be provided and/or a greater degree of non-destructive inspection is performed on critical elements.
Compression in the loaded area and resulting volumetric change are part and parcel of the same phenomenon; there is no separation or sharing of load. For the most part, wood does not have “a lot of air inside”; it is less dense because the cellulose and lignin are inherently less dense than the crystalline matrix of iron and interstitial carbon (and other alloying elements) that are found in steel. In some hardwoods there is considerable porosity, but while the tree is alive this is filled with water and resin; only after it is cut down or bored into by insects do these volumes evacuate and contain air.