In this state, the crystal will represent the metal concerned in its lowest energy condition.In theory, this stress can be applied and released continuously with no effect on the metals physical properties or the crystal structure.
In theory this slippage could still result in a perfect crystal of a different shape. In practice, however, this process almost always results in the generation of smaller crystals. ![]() The greater the quantity of foreign atoms introduced, the smaller the individual crystals and the more brittle the material becomes. ![]() In that they obey Hookes law up to an elastic limit ( y ) and above which the material will deform plastically (permanently) but will relax according to its Youngs modulus relationship leaving a residual stress (and strain) in the material. Once this stress is released the metal will relax elastically to the condition shown at point r in Fig 2 but with a residual stress () and strain (e). In practice, however, the process of work hardening, which reduces grain size, also makes the material more brittle reducing its ability to neck before failure ( Fig 2; u-b ) thereby reducing the available failure safety margin beyond ultimate tensile stress. As deformation increases, the outer layer of a deformed beam will become plastic whilst the innermost section remains elastic. Fig 3 shows the sequence of events from a fully elastic bending condition to a plastic hinge, which is when the two outer layers of plastically deformed material converge. If the beam continues to bend after achieving a plastic hinge it will very soon afterwards tear in the region of tensile deformation (the outside of the bend; Fig 3 ). It is inadvisable to allow a beam or plate to bend beyond its plastic hinge condition even in ultimate Limit-State Design conditions. The outer layer of plastic deformation, which is under tension, will stretch and flatten reducing the cross-sectional area, whereas the inner layer of plastic deformation will become larger as it compresses pushing the elastic layer and the neutral axis towards the outside of the beam bend radius. Whilst it is acceptable to design a structure that will deform plastically under ultimate limit state conditions, it is only acceptable if the integrity of the structure remains intact. Deformation in any member that generates a plastic hinge (or worse) cannot be considered to be intact. This will result in an optimistic bending moment prediction. Actual I-Beams and H-Beams are slightly different to the theoretical shapes in the plastic stress calculator (e.g. If you wish to offset this inaccuracy you could alter the flange thickness to generate equivalent cross-sectional areas exactly the same as the actual beam you wish to consider (see CalQlatas Steel Beams ). CalQlatas assessment of possible variations between actual and entered properties (areas and area moments) of I and H beams should be less than 2. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |