3 point Flexure/bending test for ductile materials question

[Marchese_alex]

I was looking up info on this topic and came up with this article (http://eng.sut.ac.th/metal/images/stories/pdf/Lab_7Bend_Eng.pdf)... what I don't get is why it says that bending test for ductile materials isn't good.


"The stress is essentially zero at the neutral axis N-N. Stresses in the y-axis in the positive direction represent tensile stresses whereas stresses in the negative direction represent compressive stresses. Within the elastic range, brittle materials show a linear relationship of load and deflection where yielding occurs on a thin layer of the specimen surface at the midspan. This in turn leads to crack initiation which finally proceeds to specimen failure. Ductile materials however provide load-deflection curves which deviate from a linear relationship before failure takes place as opposed to those of brittle materials previously mentioned. Furthermore, it is also difficult to determine the beginning of yielding in this case. The stress distribution of a ductile material after yielding is given in figure 3 b). Therefore, it can be seen that bend testing is not suitable for ductile materials due to difficulties in determining the yield point of the materials under bending and the obtained stress-strain curve in the elastic region may not be linear. The results obtained might not be validated. As a result, the bend test is therefore more appropriate for testing of brittle materials whose stress-strain curves show its linear elastic behaviour just before the materials fail."

Does the stress strain curve(http://dolbow.cee.duke.edu/TENSILE/tutorial/node4.html ) only applies when is in tension? <--------- is this correct?

can anyone explain it to me in more simple way

 

[pongo38]

You can do a stress/strain or load/displacement graph for tension, compression or torsion. For a number of reasons, compression can be difficult to measure. Fig 3b is not clear or incomplete. Try this link http://www.bestinnovativesource.com/2012/12/29/plastic-bending-of-rectangular-sectioned-beams/ for an explanation of the gradual development of a plastic hinge as load increases in the yield zone. An 'ideal' elastic plastic stress strain curve might be bi-linear looking like a knee when sitting down, but in bending, because of the stress redistribution described in the above link (and poorly represented in fig 3b), the stress strain curve in bending will look like a real knee, rounded and short-cutting the sharp change from linear elastic to linear plastic.