Forming Metals
In well-designed products, the actual stresses never reach the yield stress. Therefore, products are not permanently deformed under the maximum load. Only accidental overloads such as crashing a car or overloading a structure will create permanent deformation.
All metals, however, must be roll formed with stresses above the yield stress. Material formed with a stress below yield will spring back to its original shape. However, if the forming stress exceeds the maximum tensile stress, the product will crack and tear during forming.
The stress/strain diagram in Figure 3 reveals that the larger the difference is between the yield and the maximum tensile stresses, and the larger the elongation, the better the chance of successful forming. It is difficult, if not impossible, to form metals with extremely high yield and tensile stresses and with near zero elongation. These materials will crack at the bend lines because the elongation created by the bending is larger than the maximum elongation of the material.
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| Figure 3: This stress/strain diagram reveals that the larger the difference is between the yield and the maximum tensile stresses, and the larger the elongation, the better the chance of successful roll forming. |
