Sheet Metal, Forming & Fabricated Structures

A Formed Sheet Component Is Not a Flat Sheet Moved Into Shape

Sheet metal forming is the manufacturing process in which a flat sheet is shaped into a three-dimensional component by bending, stretching, drawing or pressing it into a die or between dies. The process families include brake forming (bending in a press brake), stamping (pressing a sheet into a die with a punch), deep drawing (drawing a sheet into a deep cavity), roll forming (progressive bending through a series of rolls), hydroforming (forming against a fluid pressure), and stretch forming (stretching a sheet over a form). Each family produces a different geometry and a different material condition, but they share a common structural characteristic: the formed component is not simply a flat sheet that has been moved into shape. The forming process changes the thickness (thinning at the bend outer fibre, thickening at the inner fibre), the material response (work hardening in the deformed region), the residual stress state (locked-in stresses from the non-uniform deformation), and the surface condition (stretching may roughen the surface, tool contact may mark it). A structural model that uses the nominal sheet thickness, the nominal material properties and the nominal geometry for a severely formed component is not representing the actual hardware — it is representing a flat sheet that happens to have the nominal shape, which is not what the component is.

A FORMED SHEET COMPONENT IS NOT SIMPLY A FLAT SHEET THAT HAS BEEN MOVED INTO SHAPE.

Forming Processes and Their Structural Significance

The forming processes differ in how they deform the sheet, and each has a different structural signature. Brake forming bends the sheet along a line, producing a bend with a defined radius and a localised change in thickness and properties; it is used for brackets, angles, channels and simple structural sections. Stamping presses the sheet into a die, producing a three-dimensional shape with complex deformation; it is used for panels, brackets and structural skins. Deep drawing produces a cup or box shape by drawing the sheet into a cavity; the deformation is concentrated at the flange and the corner radii, where thinning is most severe. Roll forming progressively bends the sheet through a series of rolls, producing long constant-section profiles; the deformation is distributed and relatively gentle. Hydroforming forms the sheet against fluid pressure, producing complex shapes with more uniform deformation and less thinning than stamping. Stretch forming stretches the sheet over a form, producing large-curvature skins with controlled elongation. Each process has a domain where it is appropriate, and the structural engineer should know which process produced the component, what deformation it imposed, and where the thickness, properties and residual stress are changed by the forming.

• Bending (brake forming) — the sheet is bent along a line; the bend outer fibre thins, the inner fibre thickens; the bend region work hardens and retains residual stress.
• Stamping — the sheet is pressed into a die; complex three-dimensional deformation; thinning at corners and deep regions; springback after forming.
• Deep drawing — the sheet is drawn into a cavity; deformation concentrated at the flange and corners; thinning at the corner radii; risk of tearing at the draw ratio limit.
• Roll forming — the sheet is progressively bent through rolls; long constant sections; distributed deformation; relatively uniform properties.
• Hydroforming — the sheet is formed against fluid pressure; more uniform deformation; less thinning than stamping; complex shapes producible.
• Stretch forming — the sheet is stretched over a form; large-curvature skins; controlled elongation; uniform thinning over the formed region.

Bend Radius, Springback, Thinning and Work Hardening

The structural features that forming introduces are interconnected. The bend radius is the primary geometric parameter: a smaller bend radius produces more severe deformation, more thinning at the outer fibre, more work hardening and more springback. The minimum bend radius is limited by the material ductility — too small a radius causes splitting — and by the sheet thickness. Springback is the elastic recovery of the sheet after forming: the formed shape is slightly less bent than the die, because the elastic component of the deformation is released. Springback must be compensated in the tooling or in a secondary operation, and it is a source of dimensional variation. Thinning at the bend outer fibre reduces the local section: a sheet that is 2 mm thick in the flat region may be significantly thinner at the outer fibre of a tight bend, and the structural model that uses 2 mm everywhere overestimates the strength and buckling resistance at the bend. Work hardening in the deformed region increases the yield strength and the hardness but reduces the ductility: the formed region is stronger but less tolerant of further deformation. The residual stress from forming is locked into the component: the outer fibre is typically in residual tension (it was stretched), and the inner fibre is in residual compression (it was compressed). These residual stresses affect the fatigue performance and the buckling behaviour, and they can cause distortion when the component is machined or cut after forming.

• Bend radius — the radius of the bend; smaller radius gives more thinning, more work hardening, more springback; limited by material ductility and thickness.
• Springback — the elastic recovery after forming; the formed shape is less bent than the die; must be compensated and is a source of variation.
• Thinning — the reduction in thickness at the bend outer fibre; proportional to the bend severity; reduces local strength and buckling resistance.
• Work hardening — the increase in yield strength and hardness in the deformed region; increases strength but reduces ductility.
• Anisotropy — the rolling direction of the sheet affects the bend performance; bending transverse to the rolling direction may behave differently from bending along it.
• Residual stress — the locked-in stress from forming; outer fibre in tension, inner fibre in compression; affects fatigue and buckling.
• Local wrinkles — compressive buckling of the sheet during forming, typically at the inner fibre or at deep-draw corners; a geometric and structural defect.
• Geometric tolerances — the dimensional variation of the formed shape; springback, thinning and tool wear all contribute; wider than machining.

Sheet Before and After Forming

The diagram below shows a sheet before and after forming, with the original thickness, the local thinning at the bend, the springback, the residual stress and the formed geometry annotated. The diagram makes the point that the nominal thickness is not preserved everywhere in severe forming: the bend outer fibre is thinner, the inner fibre is thicker (or wrinkled), and the formed region has different properties and residual stress from the flat region. A structural model that uses the nominal thickness everywhere does not represent the actual formed component.

[DIAGRAM: Two views of a sheet metal component, side by side. LEFT — SHEET BEFORE FORMING: a flat sheet of uniform thickness, with the rolling direction arrow and the original thickness labelled. A bend line is indicated. RIGHT — SHEET AFTER FORMING: the same sheet bent into a channel or bracket shape. Annotations mark: the original thickness in the flat region (unchanged), the local thinning at the bend outer fibre (reduced), the thickening or wrinkle at the bend inner fibre, the springback angle (the formed angle is slightly less than the die angle), the residual stress distribution (tension at the outer fibre, compression at the inner fibre, shown as a colour band or arrow), and the work-hardened region (higher yield strength, lower ductility). A callout notes: "Nominal thickness is not preserved everywhere in severe forming — the bend region must be verified and represented in the analysis where it is critical."]

Forming Effects and Structural Implications

The table below maps the principal forming effects to their structural consequences and to the analysis or modelling consideration that should be applied. The table is not exhaustive, but it covers the effects that most frequently govern the structural behaviour of formed sheet components: bend radius, springback, thinning, work hardening, anisotropy, residual stress, wrinkles and geometric tolerance.

Thermal Strain in Forming and Fabrication

Forming and fabricated structures frequently involve thermal processes — hot forming, welding, heat treatment, thermal stress relief — that introduce thermal strain. The thermal strain is the deformation that occurs when a material is heated or cooled and is constrained or non-uniformly heated. In forming, hot forming at elevated temperature reduces the flow stress and the springback but introduces thermal contraction on cooling; in welded fabrication, the thermal cycle of the weld introduces residual stress and distortion. The thermal strain is the product of the coefficient of thermal expansion and the temperature change. While the formula is simple, the structural consequence is not: the thermal strain, if constrained, produces thermal stress, and the thermal stress, if it exceeds yield, produces permanent distortion and residual stress. The engineer should understand that any thermal process in the manufacture of a sheet or fabricated structure introduces a thermal strain that may affect the as-built geometry and the residual stress state.

Thermal strain:

  ε_thermal = α · ΔT

where:
  ε_thermal = thermal strain (dimensionless)
  α         = coefficient of thermal expansion of the material (per °C or per K)
  ΔT        = temperature change (°C or K)

Note: If the thermal strain is constrained (the component is not free to expand or contract), it produces thermal stress: σ_thermal = E · α · ΔT. If the thermal stress exceeds the yield strength, permanent distortion and residual stress result. This is the mechanism by which welding, hot forming and heat treatment introduce residual stress and distortion into fabricated structures.

Sheet Structures Derive Stiffness From Geometry

Sheet structures — aircraft skins, panels, ducts, brackets, marine superstructures — are efficient because they derive their stiffness from geometry, not from material thickness. A thin curved skin carries load primarily in membrane (in-plane tension and compression), and the curvature provides the stiffness that a flat sheet of the same thickness would not have. A formed channel or hat section has a second moment of area that is determined by the shape of the section, not by the thickness of the sheet; the forming process has created the structural efficiency by moving the material away from the neutral axis. This is why forming is a structural process, not just a shaping process: the geometry that the forming creates is the geometry that carries the load. However, the efficiency depends on the stability of the thin sheet: a formed section that is efficient in bending may buckle at a load well below the material strength, and the buckling behaviour depends on the thickness, the curvature, the residual stress and the imperfections — all of which are affected by the forming process. The structural analysis of a sheet structure must consider not only the nominal geometry and the nominal thickness but also the as-formed thickness, the residual stress and the geometric imperfections, because these are the conditions that determine the buckling behaviour.

SHEET STRUCTURES DERIVE STIFFNESS FROM GEOMETRY — AND THE GEOMETRY IS CREATED BY THE FORMING PROCESS. The as-formed condition, not the nominal flat-sheet condition, is what carries the load.

Nominal Thickness Is Not Preserved in Severe Forming

The most common forming-related structural error is to use the nominal sheet thickness in a structural model when severe forming has materially thinned a bend or corner region. The nominal thickness is the thickness of the flat sheet as supplied; it is not the thickness of the formed component at every point. At the outer fibre of a tight bend, the sheet thins as it is stretched around the radius; the reduction can be significant, and it reduces the local section, the local strength and the local buckling resistance. A structural model that uses the nominal thickness at the bend overestimates the strength and the buckling resistance, and the margin is not real. The formed thickness must be verified where forming is severe — by measurement of the as-formed component, by forming simulation, or by a conservative assumption based on the bend severity — and the verified thickness must be used in the analysis of the critical region. For a component where the bend is not structurally critical (the load is in the flat region, the bend is away from the high stress), the nominal thickness may be adequate. For a component where the bend is critical (the load transfers through the bend, the bend is at a support, the bend is in a buckling-critical region), the formed thickness must be in the model. The engineering judgement is in deciding which case applies.

USING NOMINAL SHEET THICKNESS IN A STRUCTURAL MODEL WHEN SEVERE FORMING HAS MATERIALLY THINNED A BEND REGION CAN OVERESTIMATE LOCAL STRENGTH AND BUCKLING RESISTANCE. The formed thickness must be verified where forming is severe.