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Acting bending forces cause stresses in the material affecting a remodelling of the workpiece.
Tensile stresses occur at external radii of bendings due to stretching the material, while compressive stresses occur at internal radii of bendings due to upsetting the material.
Between those areas where tensile and compressive stresses act, there is a transition zone where no stresses act It is denominated as neutral axis or neutral layer.
Figure 14 Stresses in the bent
workpiece 1 tensile stresses, 2 neutral axis, 3 compressive stresses
The neutral axis length is needed to calculate the stretched length of the workpiece to be bent
What does the term "neutral axis"
mean?
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3.1. Influence of Material Properties
Plasticity:
Only can such materials be bent that allow a change of shape. Hardened and brittle materials cannot be bent - they break when strong bending forces act
Spring-tempered materials cannot be bent either - they completely spring back to their initial position after bending forces have acted.
Figure 15 Bending property of various
materials 1 bending force acts on brittle material, 2 workpiece fracture, 3
bending force acts on spring-tempered workpiece, 4 workpiece spring-back
Elasticity:
Elastic materials spring back after application of force by a certain measure - what is called spring-back. This measure must always be taken into consideration when bending.
Hard metals spring more back than soft ones.
Figure 16 Workpiece spring-back after
each force reaction
Strength:
When sheet metals are rolled, a fibre structure comes into being (similar to streaks in wood) which can be seen on the surface of clean sheet metals. To avoid streaks at the external edge of bendings, the bending edge must not be in accord with the streak flow.
Figure 17 Rolling direction to be
considered during bending 1 rolling direction
Strain-hardening:
When tensile and compressive stresses change for several times during the bending process (to-and-from bending), the material structure is more and more deformed. The increasing internal stresses lead to a hardening at the bending point The more often the change of stresses takes place, the more brittle the material becomes. If deformation continues, it may result in a fracture.
Figure 18 Crack formation at
strain-hardened bending points 1 zone of strain-hardening
Which material properties are not allowed for a piece to be
bent?
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What does the term "spring-back"
mean?
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What will happen when a sheet metal is bent around a bending
edge being in accordance with streak flow of the rolling
direction?
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What does the term "strain-hardening"
mean?
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3.2. Influence of the Bending Radius
To avoid cracks due to bending, the bending radius has to be selected by a size sufficiently large.
The bigger the bending radius, the smaller the risk of cracks.
The thicker the material is, the bigger the bending radius must be.
The bending radius depends on both the shape and thickness of the workpiece as well as the temperature during bending and the kind of material.
Hence, there are fixed minimum radii for all metals and many section forms.
The following minimum bending radii can be applied to as empirical values:
|
Material |
Radius |
|
Copper |
0.8 up to 1.2 × thickness |
|
Brass |
1 up to 1.8 × thickness |
|
Zinc |
1 up to 2 × thickness |
|
Steel |
1 up to 3 × thickness |
Which influence has the bending radius on the formation of
cracks in the
workpiece?
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Which influence has the workpiece thickness on the bending
radius?
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3.3. Heat Influence
The more the workpiece is being remodelled, the bigger are the stresses inside the material. Particularly, in bending thicker workpieces with small bending radius, there is such a strong stress on the material that it may crack at the bends' external side.
To avoid this formation of cracks, such workpieces must be heated red-hot. The resistance inside the material decreases with growing heat so that remodelling can be effected without great expenditure of force and without the risk of crack formation.
Figure 19 Bending of thick workpieces
1 cold bending leads to crack formation, 2 hot bending makes exact bending
procedure possible
Which influence has the supply of heat on the bending process
with thick
workpieces?
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