What controls modulus in an engineering material?

When you say ';Modulus';, I need you to be more specific. You probably mean Modulus of Elasticity or Young's Modulus.





This is the ratio of stress to strain. Stress is force per unit area. For example, if you had a hard steel rod with a load of 10,000 pounds of tension and it had a 1'; diameter, the stress is





F/A = 10/(pi*r*r) or 10 / (3.14 * .25) = 12.73 ksi (kips per square inch) or 12,730 psi (pounds per square inch)





If you think about it, putting a specified force a specified size of material, you will always get the same change in length (unless you actually deform the stuff, in which case all bets are off).





The modulus of elasticity (E) for hard steel is 30 x 10^6 psi





E = stress / strain





strain is delta L / L (change in length divided by original length)





If that steel bar was 1 foot (12 inches) long, you can use E to find the ';stretch';





delta L / L = 12730 psi / 30,000,000 psi = .0004243 (unitless)





delta L = .0004243 (12 inches) = .005'; change in length.





This is important because you also have to remember that metals expand and contract when the temperature changes which inherently changes the stresses and strains on important things suich as bridges and buildings.





Long winded but hope it helps!





Civilsid.