Sorry Barry, that’s not correct.
So long as the applied load remains below the design yield stress limit, it shall return to its original condition. Only if it is stressed beyond its yield point, will it remain bent.
My response
[The yield stress is the stress limit that when reached the material will plastically deform. Splitting hairs here as if you are under the yield stress by a minute amount the material will not permanently deform]
That is slightly misleading. Work hardening is the localised plastic deformation of atoms along slip planes. These then form dislocations that hinder further movement and thus increase in strength - hence the work hardening. However general over loading of a material, plastic deformation, is also using the same slip planes of the atoms moving relative to each other. The main difference here is that the atoms are moving further apart along the slip planes, whereas in work hardening of an alloy plate to increase its strength the volume is being mechanically reduced which increases the dislocations and hence increases its strength as the atoms have moved closer together. Thus, over loading a material when it gets into the plastic region, the atoms are moving further apart as there is nothing restraining them, such as rollers, unlike a plate that is work hardened in a mill.
My response
[The dislocation will not slip along planes between atoms but rather molecules or grain boundaries.
You cannot increase the volume of atoms without the application of heat.
Solids are incompressible.]
So the mechanism is principally the same, but subtly different result. Because upon further loading the material will begin to “neck” as the volume of material left to plastically deform becomes less, and the atoms shear further apart too.
And it is also not necessarily the case it takes greater load either. Since the work hardening rate will decrease with increasing strain, thus the incremental increase in the yield stress (moving) becomes less. So it may appear that it can take a greater load but as it permanently deforms even further upon reloading the amount of applied load to failure actually decreases – which is why I never advise using highly strain hardened alloys, despite their attractive yield strength.
My response
[ I understand the necking process, but, the material in the neck will have work hardened and have a higher yield stress than the un- necked area. Certainly as the necking continues the load capacity decreases due to the diminishing crass sectional area.]
The UTS is a measure of the maximum stress, it is not the ‘break’ point That comes much later at pure fracture.
My response
[ Yes I did not re-read my post and the UTS is not the break point for aluminum
It does, all materials do (just different degrees of it), it just yields differently because it is an orthotropic material. The individual fibres begin to tear under tension, not dissimilar to a composite. Just in a different manner to isotropic metals like a metal.
