The partial recovery of shape by elastic recovery when the load is removed after bending or other working. It is larger for materials with a higher yield stress and a lower Young's modulus.
Japanese name: スプリングバック
Springback appears in 1 practice question across 1 topic, including Materials processing and joining.
When a sheet is bent, the outside is stretched and the inside compressed, and most of the cross-section deforms plastically. Even so, stresses of about the yield stress remain in the cross-section, and when the part is taken out of the die the shape recovers by the elastic strain corresponding to these stresses. This is springback. Its size is roughly set by the ratio of yield stress to Young's modulus, Y/E, and it is larger for materials with a higher yield stress and a lower Young's modulus. It occurs not only in bending but also in press drawing and stretch forming, and because it distorts the dimensions and shape of parts, it is one of the most troublesome problems in designing press forming.
Ignoring work hardening and assuming the whole cross-section has yielded, the bending moment per unit width is Yt²/4 and the bending stiffness is Et³/12, so the recovery of curvature on unloading is Δ(1/R) = 3Y/(Et), where t is the sheet thickness. For example, bending a sheet with Y = 300 MPa and t = 1 mm to a radius of 10 mm gives a radius after springback of about 10.45 mm for steel (E = 210 GPa) and about 11.5 mm for an aluminum alloy (E = 70 GPa). With one-third the Young's modulus, the recovery is about three times larger. Because the recovery of curvature is fixed, the gentler the bend, the larger the recovery relative to the original shape.
Car bodies use high-strength steel sheets to save weight, but their Young's modulus is almost the same as mild steel while only the yield stress is higher, so springback becomes large. When hat-shaped sections are pressed, not only do the corner angles open, but shape distortions such as curling of the side walls (wall curl) and twisting of the whole part also occur. In V-bending, depending on conditions, negative springback (spring-go), in which the part closes further than the bent angle, can also occur. Because simple bending formulas cannot fully predict this, forming simulation is used in practice.
The basic countermeasure is to bend extra by the amount that will spring back (overbending). Other effective methods press the bend hard with the die at the end of bending to compress it and make the whole cross-section yield (bottoming, coining), or bend while applying tension so that the stress distribution across the section becomes closer to uniform. Today it is common to compensate the die shape in advance so as to cancel the springback predicted by forming simulation. On the material side, warm forming, which lowers the yield stress by heating, and hot stamping (press hardening), which forms at high temperature and quenches in the die, also suppress springback in high-strength parts.
Answer: C. A material with a high yield stress and a low Young's modulus
Even after plastic deformation, stresses of about the yield stress remain across the section of a bent sheet, and when the load is removed it springs back by the corresponding elastic strain (roughly σ/E). Springback is therefore larger the higher the yield stress and the lower the Young's modulus. High-strength steel sheet, with the same Young's modulus but higher strength, and aluminum alloy sheet, with a Young's modulus about one-third that of steel, show large springback and are harder to bring to shape. The option of a soft and stiff material is the reverse, and the option saying it is set only by the working speed is also wrong. Countermeasures include overbending by the amount of recovery and pressing the bend firmly in the die so that the bend region deforms plastically.
Mainly the ratio of yield stress to Young's modulus Y/E, the sheet thickness and the bend radius. The larger Y/E, the thinner the sheet and the larger the bend radius, the larger the relative recovery. It also changes with the amount of work hardening, friction with the die and the bending method.
For the same bend radius, the thicker the sheet, the smaller the recovery of curvature, because the recovery Δ(1/R) ≈ 3Y/(Et) is inversely proportional to thickness. Bending thick sheet needs large forces, however, and cracking on the outside or wrinkling on the inside becomes more likely, so other problems need attention.
They tend to. Titanium alloys, with about half the Young's modulus of steel, and aluminum alloys, with about one-third, spring back about two to three times as much as steel at the same yield stress. Titanium alloys also have a high yield stress, so their springback is especially large, and forming them hot is common.
Springback is therefore larger the higher the yield stress and the lower the Young's modulus.
When a sheet is bent and removed from the die, its bend ang…Recovery Young's modulus Carbon equivalent Extrusion Heat-affected zone Hot isostatic pressing Investment casting Sensitization Shrinkage cavity