Deformation occurs mainly through tension (as opposed to compression which occurs in many forming processes).
Many sheet metal parts are large and highly visible and surface appearance is important.
Some defects that can occur with Sheet Products:
Straining the material rotates slip planes and produces noticeable alignment (preferred orientation) of crystals - texture. As sheet metal will have been subject to a rolling process, the material will have some directionality or anisotropy.
For the volume of a specimen to remain constant, the sum of the 3 principal strains must equal zero.
For a tensile test, length, width and thickness strains occur and the strains in the width and thickness directions need not be equal.
strainw = straint .... and r = 1 ... r0 = r45 = r90 = 1
2 Planar anisotropy - the r values vary at different directions to the rolling direction:
r0 not equal to r45 not equal to r90
This can lea to 'earing' in deep drawing.
3 Normal anisotropy
r0 = r45 = r90 but not equal to 1.
4 Commonly normal and planar anisotropy are both present:
r0 not equal to r45 not equal to r90 not equal to 1
A measure of normal anisotropy is a mean r denoted rm
rm = (r0 + r90 + 2r45)/4
A measure of planar anisotropy is Dr
Dr = (r0 + r90 - 2r45)/4
Typical normal anisotropy values:
22.214.171.124 Suitability of Metals for Sheet Metal Processes
Virtually all wrought metals are to some extent suitable for sheet metal working.
Some steels have been specially developed for sheet metal work.
Low carbon steels - up to about 0.15% C - are most suitable.
Killed steels - producing a fine grain - using aluminium additions produces high n and r values which are suitable for severe drawing operations.
High - strength low - alloy steels are becoming more widely used. However these tend to have a high yield strength/Young's modulus ratio which gives large 'springback'. Tool design must take account of this effect. Grain refinement is carried out by small additions of Ti, V or Nb. These form carbonitride precipitates which inhibit grain growth in the austenite condition and thus refine the ferrite formed on cooling from the controlled rolling temperature. Combination of grain refinement and precipitation hardening gives yield strengths of 350 to 550 MPa.
126.96.36.199 Pre Coated Strip
188.8.131.52 Forming Limit Diagram (FLD)
184.108.40.206 Deep Drawing
Clearance between the punch and the die are generally 7 - 14% of the sheet thickness.
Drawbeads are often used to control the flow of the blank into the cavity.
Limiting Draw Ration The maximum diameter of circle that can be completely drawn into a given cup size under ideal conditions is termed the LDR:
The LDR is not simply a material constant, but depends on all the variables that affect the draw force and strength of the cup wall.
220.127.116.11 Shearing Process Details
Compound or progressive dies are used for mass production of simple sheet parts from strip.
The shearing force is approximately: Ps = C1(Ts)hl where l is the length of cut and C1 = 0.85 for ductile materials and 0.65 for less ductile materials, about 0.7 on average.
To reduce the maximum value of the force needed, the shear blade can be inclined to the horizontal by a few degrees. The length of the cut then becomes l = h/tan(angle)
18.104.22.168 Bending Process Details
The neutral line maintains its original length. When bending around a small radii the neutral line moves towards the compression side, the centreline is elongated and constant volume is achieved by thinning of the sheet. Increased length of the centreline is usually considered when Rb is less than 2h, by assuming that the neutral line is located at one third of the sheet thickness. When the sheet is relatively narrow (w/h less than 8) there is also a contraction in the width w.
Problems that occur in bending include 'orange peel' which may be remedied by using finer grain material. If the ratio of the minimum bend radius to the thickness is not observed, then localised necking or fracture may occur.
Springback occurs and increases the angle of the bend radius. Springback may be eliminated by overbending, applying plastic deformation at the end of the stroke or subjecting the bend zone to compression.
The elastic zone is larger for a relatively gentle bend and for material with a high yield stress to elastic modulus ratio.
The force for bending to 90o: Pb = w h2(TS)/Wb where w is the width of the strip (the length of the bend line) TS is the tensile stress and Wb is the width of the die opening.
The minimum bend radius for various materials at room temperature is shown below:
David J Grieve, modified: 21st November 2006, 18th October 2004, original: 11th October 2002.