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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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