Abstract
In this experiment, we distribute the bending angle to 60$^{\circ}$, 90$^{\circ}$, 120$^{\circ}$, and we separate it in to two groups. One is straightening right after bending and the other is straightening after one week. The bars we will use are HD13 and HD16. The number of tests will be 60 times. In the case of HD16, the increase of bending angle decreased yield strength and maximum strength. And compared with thinner bars, HD16 showed bigger differences in yield strength and maximum strength when the 'being bent' duration got longer. So it shows that when we bend and after straighten a bar, stress change takes place on the surface of the bar. At the beginning it shows it's ductility after that it deforms while transforming it's character to destroying. And also, if we straighten a bar after bending, it doesn't get straighten like the original form. So it makes torsion and this torsion makes moments in different areas.
