Springback

Shape change after unloading a formed part: angular recovery, sidewall curl, twist, compensation and the limits of bend estimates.

Tags: production, forming, bending, springback

Reviewed 2026-11-02Read Markdown

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In a hypothetical coupon trial using included opening angles, the bend measures 89° under load and 92° after release, so recovery is +3°. If recovery stayed unchanged, a 90° released target would suggest 87° under load. Changing stroke can change recovery. Verify the result with the actual material and tooling, and measure other shape changes separately.

Meaning and scope

Springback is elastic recovery on release of forming load. WorldAutoSteel links recovery in AHSS stampings to as-formed flow stress. WorldAutoSteel, springback overview

Check tooling deflection and setup before attributing the entire final angle error to recovery.

Form	Physical distinction
Angular change	Recovery around a bend changes the relative orientation of adjoining regions.
Sidewall curl	A drawn wall develops curvature after bending and unbending; a single angle does not describe the whole wall.
Twist	Cross sections rotate differently along a part under unbalanced residual stresses and torsional response.

WorldAutoSteel describes these three modes in channels and underbody parts, relating them to stress gradients, deformation history and component stiffness. Its explanation also covers global shape changes. A simple angular correction cannot be assumed to remove curl or twist. WorldAutoSteel, types of springback

Material and tooling act together

SSAB's press-brake guidance relates increasing springback to steel strength and die-width-to-thickness ratio, W/t. Compensation requires tooling that permits overbending: in its high-strength-steel example, a 90-degree die opening cannot produce a released 90-degree bend when additional overbend is needed. The guidance is explicitly for SSAB products, not every alloy or forming method. SSAB, springback

The Bending Hardox brochure further relates behavior to yield strength, punch radius, work hardening and the stress distribution across the section. Its simple estimate using tensile strength applies only to its stated thin-plate and die-width conditions; its tabulated strengths are typical values, not acceptance or design properties. These restrictions prevent a convenient grade number from becoming a universal overbend allowance. SSAB, printed V3-2023 brochure, springback discussion

Crowning is a separate compensation for elastic deflection of the bending machine under load. SSAB discusses it independently from material springback. A correction for variation along the bend length and a correction for the part's recovery therefore address different effects, even when both influence the measured angle. SSAB, crowning and springback

Angle conventions and prediction limits

Nippon Steel's ABREX Guidelines for Bending, section 2, defines its springback angle as the angle after unloading minus the angle under load. Its worked examples concern ABREX400 at a stated bend-radius/thickness ratio, and its formula is explicitly approximate. It distinguishes bend radius from punch radius under changing friction. Neither that numerical example nor its force estimate establishes a machine's capability for another material or setup. Nippon Steel, ABREX bending guide

A reported angle can instead mean included opening angle, rotation through the bend or deviation from nominal. Those conventions have different signs and complements. Explanatory synthesis: an equation using opening angles cannot be combined unchanged with measurements expressed as rotation from a flat blank. A released-angle result also does not establish the angle at maximum stroke.

SSAB's BendCalc description bases predictions on steel characteristics, tool/die symmetry, final shape and friction. It lists springback, maximum force, punch depth, angle at maximum stroke and minimum flange height as separate outputs. Its stated product scope is SSAB steel. A predicted angle is not independent proof of crack-free bending, adequate flange support or acceptable force. SSAB, BendCalc scope

Original bounded angle example

Assume an invented trial uses included opening angles. A straight two-flange coupon measures 89° under load and 92° after release. With the stated convention, observed angular springback is 92 − 89 = +3°.

If recovery were unchanged, a target released opening of 90° would imply a trial loaded angle of 90 − 3 = 87°. That is conditional arithmetic, not a qualified machine setting: changing stroke changes the deformation, and recovery need not remain 3°. The example supplies no bend radius, capacity, crack-resistance or repeatability guarantee.

For a separate geometric illustration, assume a perfectly straight 100 mm flange rotates 2° away from its intended orientation about a fixed bend line. Its tip displacement perpendicular to the intended flange direction is 100 × sin(2°) = 3.49 mm. A 25 mm flange with the same idealized rotation gives 0.872 mm. The model omits bend-zone motion, curl and twist; it shows why an angular deviation and a linear positional deviation are not interchangeable tolerances.

Compensation and finished geometry

WorldAutoSteel distinguishes changing the elastic-strain distribution, reducing its creation and constraining its release. Examples include post-stretching to reduce sidewall curl, over-forming and changes to part or tooling geometry. The suitability of a measure depends on the mode and forming route. Its simulation discussion emphasizes material characterization and comparison with physical measurements; a computed compensation does not establish the resulting manufactured geometry. WorldAutoSteel, correcting springback

Explanatory synthesis: a successful angle correction is narrower evidence than conformity of the full component. Final shape depends on the measurement condition, datum definition, local curvature and any later trimming or assembly constraint. A nominally correct included angle can coexist with wall curl or displaced features. A result from one tool/material configuration does not establish the same response after a change in thickness, strength, lubrication or support.

Source versions

Reviewed 2026-11-02 PST. The linked Bending Hardox PDF prints 105 En, V3-2023 and copyright 2023; its URL contains v3-2024, which is not the printed edition. Nippon Steel’s cover prints A111en_02_201908f and ©2019 NIPPON STEEL CORPORATION. These identify the retrieved guide; no exact issue day is inferred from the code or hosting directory. The cited live WorldAutoSteel and SSAB web guidance has no separately verified issue date. The calculations above are original teaching examples.