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At distorted, bent or deformed metal sheets, pipes and sections axial elongations and linear compressions are to be noticed in the material:
At the external radii of bendings, tensile stresses are caused by axial elongation of the material, at the internal radii of bendings, compressive stresses are caused by linear compression of the material.
Figure 11 Axial elongation and
compression when bending 1 before bending, 2 after bending
Materials put up resistance to any sort of deformation.
If they are exposed to little force only, the resistance of the material is not overcome. The material springs back to its original position - the strain was elastic. This process is called "elastic recovery".
Figure 12 Elastic recovery after too
little action of force
If the force affecting the material is stronger than the resistance which the material offers, it comes to a plastic deformation with the material springing back by the size of its elastic strain afterwards. Therefore the measure of the elastic recovery must always be taken into consideration during bending and twisting.
Figure 13 Elastic recovery with
action of more force
In case or distorted or bent workpieces, tensile and compressive stresses must be eliminated by systematic action of force. This happens, if - by appropriate tools and devices - another force is exerted opposite to the original one which can be recognized from the direction of the distortion or bending. Here again, the elastic recovery of the workpiece must be considered.
Figure 14 Elastic recovery after
action of counterforce
In case of deformed or dented metal sheets, the compressive and tensile stresses mostly occur in a few places only, where the original action of force cannot always be seen.
Such tensions can only be eliminated by countertensions the effect of which blots out the original tension.
The more the workpiece has been deformed, the greater are the internal tensions in the material.
Internal tensions can also be eliminated by local heating up or soft annealing of the entire workpiece:
- Steel parts must slowly cool down in the air after heating.
- Copper parts are water-quenched immediately after having been heated up.
Often, locally heated sectional steels or metal sheets return to their right initial positions themselves when cooling. If this does not happen, they are deformed in the desired way in red-hot condition.
Generally, thick sections and pipes with thick walls should be straightened in red-hot condition, because the resistance of the material decreases with increasing temperature. Therefore, the force required for straightening is less, the brittleness of the already deformed material is reduced, so that fissures of fractures during the renewed deformation are avoided.
Figure 15 Straightening of heated
material
What tensions occur at the bending radii of deformed
materials?
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What happens if a too little force acts on the
material?
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How are tensile and compressive stresses eliminated at bent
workpieces?
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Why is it better to straighten thick sections and pipes with
thick walls in red-hot
conditions?
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