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Bending theory

  1. Mar 30, 2009 #1
    Hi all, i am doing a mechnaical HNC and have some questions i need to expand on.

    I am asked to "Define the following in terms of bending theory" and i am strugling to find information in the depth i need.

    Neutral axis: all i know about this is it is a cross section of a beam perpendicular ot its longitudianl axis, and that on one side of the axis the fibres are in a state of tention and on the other a state of compression. Any ideas on how i can extend this or were i can find information?

    Radius of curvature: All i know about this is normally the beam does not bend into circular arc, but whatever shape the beam takes under the sideways loads, it will basically form a curve on an x –y graph.

    Moments of resistance:The fibres above the neutral surface are in compression and those below the neutral surface are in tension. (any ideas on how i can expand this?)

    Section modulus: Section modulus of a beam is the ratio of a cross section's second moment of area to its greatest distance from the neutral axis.

    Am i on the right lines with these and does anybody know how i can expand on them.
  2. jcsd
  3. Apr 1, 2009 #2


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    If you're looking at a beam in the state of pure bending, the longitudinal fibers on the convex side are elongated, and on the concave side they contract. There exists a layer whose fibers bend but don't change their length. This layer is called the neutral layer, and the intersection of a specific cross section with this layer forms the neutral axis for this cross section. Further on, it is derived from the equilibrium equations that, in a cross section, there exist normal stresses of different sign, and since they're continuous along the cross section, there must exist points at which they are equal to zero. These points divide the cross section into two parts - one in the state of compression, and the other in the state of tension. When bending occurs, the beam's cross sections rotate around their neutral axis.

    For the radius of curvature R, this relation holds for bending:

    1/R = M/(EI)
  4. Apr 1, 2009 #3
    Your mechanics of materials book has these terms and a lot more defined in some detail. I strongly recommend studying that as opposed to asking questions here. In that book, you will find the definitions all worked out in a carefully coordinated and complete form, as opposed to the catch as catch can form you will get here. Use the book. These are very important concepts, and without them you are dead in the water.
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