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I'm reading an introduction to structural engineering book and there's a fundamental concept I'm trying to relate to real life:

In construction, we regularly bend iron rebar into a variety of shapes and rebar stays locked in whatever bendy shape we give it.

As I understand it, if the rebar holds its deformed position that means it's behaving plastically and not elastically. When a material transitions from elastic behavior to plastic behavior, doesn't that mean it has yielded?

If it has indeed yielded, shouldn't bent rebar exhibit vastly reduced bearing capacity, particularly about the areas it has been bent?

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  • $\begingroup$ I would have thought the structure overall design would not put any load on the bend point itself. The bent rebar should simply keep two chunks of "something" in the right place. In fact, the loss of 'strength' at the bend is (if I recall correctly) not along the axes of interest. $\endgroup$ – Carl Witthoft Dec 21 '15 at 14:56
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    $\begingroup$ Just a small correction - modern rebar is made of steel, not iron. Large difference in terms of strength and material properties, but I expect this is just a semantical oversight. $\endgroup$ – grfrazee Dec 22 '15 at 17:38
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It depends what you mean by bearing capacity. Plastic deformation increases yield strength at the expense of ductility. Also possibly toughness and fatigue durability if strained too much.

Metals virtually always exhibit increased yield strength when strained plastically below their strain recovery temperatures. The phenomenon is referred to as strain hardening, and is important in a number of product and processing applications as another tool for tailoring material properties. The increase yield strength comes with a number of drawbacks, the most readily apparent of which is reduced elongation at failure. There is a tradeoff between yield strength amd ductility. Additionally, too much plastic strain with no recovery or annealing can result in the formation of voids, microcracks, and other microscopic, volumetric defects. Volumetric defects provide crack nucleation sites when dynamically loaded and accelerate failure in dynamic fatigue, as well as creating stress concentration sites which can reduce durability and toughness eve after annealing.

The image below shows the effects of cold work on strength, hardness, and ductility. Note that as cold work increases to the right, strength and hardness increase, while ductility decreases. While the plot is for a specific metal, the curve shapes apply broadly.

enter image description here

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Bringing rebar to full plastic behaviour and then releasing doesn't make it any less strong. It may make it stronger through work hardening. If you don't repeat this too often, you won't introduce fatigue. You might recognise this or similar diagrams from your book.

The allowable stresses for rebar allow for them to be fully bent to various predetermined or custom shapes. They're made and bent to quite rigorous standards. Just don't re-bend one from shape code 00 to code 75 too often. Har har.

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