STRETCH FORMING

JOMY JOSEPH

STRETCH FORMING

Stretch forming consists in wrapping a sheet or a profile around a shape. Two jaws hold the edges of the part and put the material totally in plastic mode for forming.

STRETCH FORMING

KEY FEATURES

Application of tensile forces.

Large radius of curvature.

Stress gradient is relatively uniform.

Spring back is eliminated.

Larger deformations for ductile materials.

Failure occurs by localized necking or diffuse necking.

Stretch Forming Equipment

Longitudinal equipment : stretches the work piece along its length.

Transverse equipment : stretches the work piece along its width.

Stretch forming equipment can be interfaced to a host computer and may include features such as computer numeric control (CNC) and an integral front panel or console.

Specifications

Jaw specifications : number of jaws, jaw width, distance between jaws, stroke per jaw, tonnage per jaw, jaw swing, and gripping pressure.

Die table specifications : size and weight, minimum and maximum tonnage, work piece length and width, tilt angle, forming speed, and stroke.

The hydraulic system specifications: Power unit parameters: Motor speed, pressure,

flow, and fluid type .Ram parameters: Force, stroke, open/close rate

and pressing rate . Minimum and maximum tension cylinder pull.

Force required in stretch forming

APPLICATIONS

Aircraft industries

Fuselage, wing, flap, tail, etc.

Automotive stamping

Automotive body panels.

Roofs curves, structures and uphill doors, bay window framing for train cars.

Wheel passages, cabins of agricultural materials and civil works equipment.

ADVANTAGES

Precision and good repeatability because of spring back control and fewer residual stresses.

Complex form capability.

No deformation after welding and machining.

Elimination of handwork operations.

PROCESS CONSTRAINTS

Sheet tearing or fracture

Appearance of surface defects due to plastic material instabilities. In particular, the PLC (Portevin-Le Châtelier effect which materializes as bands developing on the surface of the sheet

Elastic discharging, resulting in a spring-back at the end of the process and during the post-process operations (cutting, chemical machining, etc.)

STRETCHABILITY

The most appropriate measure of formability for stretch forming is the strain hardening exponent, or n value.

σ = kεn

where k is a constant. ε is strain

A high value of n is desired if the strip is to show good stretch formability.

The n-value is the key parameter in determining the maximum allowable stretch as determined bythe Forming Limit Curve (FLC).

The height of the FLC is directly proportional to the terminal n value.

The n-value also contributes to the ability of steel to distribute the strain more uniformly in the presence of a stress gradient. The higher the n-value, the flatter the strain gradient.

A higher n-value (solid lines in Figure) compared to a lower n-value (dashed lines) means a deeper part can be stretched for equal safety margins or a larger safety margin for equal depth parts.

Consider a section of a sheet loaded in tension.

σr = constant

Strain gradient is reduced by greater strain hardening (larger n).

NECKING IN SHEET

In biaxial tension, the necking which occurs in uniaxial tension is inhibited if σ2/ σ1 > ½ , and instead the material develops diffuse necking which is not highly localized or readily visible to the eye.

Eventually in the stretching of a thin sheet, plastic instability will occur in the form of a narrow localized neck.

Necking as forming limit

Diffuse necking usually will not constitute a limit to forming because the thinning is spread out over a fairly wide area of the sheet.

Local necking is readily detected on exposed surfaces so that it represents a forming limit. Formation of a local neck is followed by fracture of the sheet.

Variation of necking limits with strain ratio

ε1* - strain for necking in the direction of largest principal strain.

Εu – strain for diffuse necking in pure tension

σ = ε2/ ε1 – strain ratio where ε1 is algebraically large principal strain.

STRETCHING TEST

Stretchability is measured by the factor H/d

H - maximum height of stretch without fracture.

d - diameter of the punch

STRETCHING TEST FOR DP STEELS

Hemispherical punch with 100 mm diameter is used

Portevin-Le Chatelier Effect

The PLC effect is a plastic instability resulting in unhomogeneous deformation (strain localization).

This phenomenon is known to arise due tomicro structural processes, i.e. the dynamic interaction between mobile solute atoms and gliding dislocations, known as the Dynamic Strain Ageing or DSA).

The PLC effect is clearly visible on the surface of the sheet (groups of parallel lines) and are also observable on the stress-strain curve (serrated curve : sudden drop of stress at almost constant strain)

REFERENCESMechanical metallurgy, George E. Dieter

Metal forming technology, Dr. R. Narayanasamy

www.steeluniversity.org

Dual phase steels for auto body : design, forming and welding aspects, Carlsson, Larsson, Nilsson

Advanced High Strength Steel (AHSS) Application Guidelines , INTERNATIONAL IRON & STEEL INSTITUTE Committee on Automotive Applications

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