modulus young?

Question

why is it necessary to keep deflection as small as posible?

Answer 1

This is associated with the rigidity and stability. Lesser is the deflection (for high Modulus of Elasticity) in more rigid and stable structures. This is very much desired. Suppose, a beam used in a bridge is very much unstable so excessive vibration causes more discomfort for passengers in the cars. Further, if the beam is not rigid then imagine that the middle portion of the bridge is sagging like a rope hung on two posts. This is also not desired. So it is always desired to keep the deflection small (sagging occurs but invisible to the resolution of naked eye.

Answer 2

Deflection is generally considered a bad as related to construction, however it is not necessarly bad. You need to put it in context.

For instance you may want something to deflect, in a an earthquake for instance or inside a plane's wing. When you are learning about deflection or anything they generally teach using the most generic and common type of problems. Just remember that these types of problems are common as you grow and learn you will find the the specifics that you learn in school may not necessarily apply to ever situation.

Answer 3

If the deflection becomes large, then the material changes shape (the Poisson effect), this shape change also changes the effective stress through the cross section. As the stress changes (it always increases), the apparent stress strain relationship can be non-linear. However, if actual strain (not engineering strain) is used, this problem can be avoided up to the point of the elastic limit of the material.

All this applies to testing materials and using these test results for design of stiffness critical structure.

Answer 4

..,when measuring Young's modulus? Because you don't want to drive the deflection into the region of plastic deformation. Also, if you measured the test sample before doing the test, the process of stretching it reduces its cross section which reduces the accuracy of the test.