ASM Metals HandBook Vol. 14 - Forming and Forging
Подождите немного. Документ загружается.
(Ref 73). Springback increased with increases in flow stress as well as die radius and clearance and with a decrease in
material thickness.
For minimum springback, the ratio of the thickness to the die radius should be greater than 0.4, beyond which a further
increase has little effect. The springback developed by the dual-phase steel increased more rapidly with the level of cold
work than that developed by the high-strength steels, as anticipated from its higher strain-hardening exponent.
Springback after the combined effects of bending and stretching has been investigated using a tensile machine and a
three-point bending fixture (Ref 19). Tests were performed on four thin-gage (~0.4 mm, or ~0.016 in.) low-carbon steels
in various tempers with yield strengths ranging from 155 to 670 MPa (22.5 to 97.0 ksi). For applied tensile stresses that
are below the yield strength of the material, the springback decreased linearly with the tensile stress to the same extent
whether the stress was applied during or after bending.
For tensile stresses above the yield strength, tension during bending decreased the springback to a level that was
independent of the initial yield strength and thickness. Stretching after bending, which deforms the entire cross section
plastically in tension, decreases the springback progressively to extremely low levels.
Correlation Between Simulative Tests and Material Properties
Quantitative correlations between the results of simulative tests and select tensile properties have been determined for the
Olsen cup, Swift flat-bottomed cup, and Fukui conical cup tests (Ref 50) and for the Swift round-bottomed cup test (Ref
64). In the first correlation, 48 materials were tested, including aluminum-killed, rimmed, and stainless steels and
aluminum alloys in various tempers (Ref 50). Tensile properties included the directionally averaged percentage of total
elongation,
e
T
, to indicate the stretchability, and the normal anisotropy, r
m
, to indicate drawability. The following
relationships and correlation coefficients were obtained:
Test parameter Relationship Correlation
coefficient
Olsen cup height/punch diameter
0.217 + 0.00474
e
T
+ 0.00392r
m
0.925
Limiting draw ratio (Swift)
1.93 + 0.00216
e
T
+ 0.226r
m
0.835
Formability index (Fukui)
0.525 + 0.0134
e
T
+ 0.207r
m
0.757
The Olsen test involves a much greater ratio of stretching to drawing than the Fukui test, which in turn involves a much
greater ratio than the Swift flat-bottomed cup test.
In the second correlation, 50 different steels were tested (Ref 64), and the results were correlated with the average n and r
values and thickness, t, as follows:
(Eq 31)
References cited in this section
13.
J. Havranek, The Effect of Mechanical Properties of Sheet Steels on the Wrink
ling Behavior During Deep
Drawing of Conical Shells, in Sheet Metal Forming and Energy Conservation,
Proceedings of the 9th
Biennial Congress of the International Deep Drawing Research Group, Ann Arbor, MI, American Society
for Metals, 1976, p 245-263
14.
J.S.H. Lake, The Yoshida Test--A Critical Evaluation and Correlation with Low-
Strain Tensile Parameters,
in Efficiency in Sheet Metal Forming,
Proceedings of the 13th Biennial Congress, Melbourne, Australia,
International Deep Drawing Research Group, Feb 1984, p 555-564
19.
N. Kuhn, On the Springback Behavior of Low-Carbon Steel Sheet After Stretch Bending, J. Austral.
Inst.
Met., Vol 12 (No. 1), Feb 1967, p 71-76
48.
M.Y. Demeri, The Stretch-Bend Forming of Sheet Metal, J. Appl. Metalwork., Vol 2 (No. 1), 1981, p 3-10
49.
O.S. Narayanaswamy and M.Y. Demeri, Analysis of the Angular Stretch Bend Test, in
Novel Techniques in
Metal Deformation Testing, The Metallurgical Society, 1983, p 99-112
50.
A.S. Kasper, "Forming Sheet Metal Parts," Paper MF 69-516, Society of Manufacturing Engineers, 1969
51.
T.R. Thompson, G. Glover, and R. Jackson, "The Formability of Sheet Steels," Technical Bulletin 18, No.
2, Broken Hill Proprietary Company, 1974, p 15-19
52.
S.S. Hecker, A Cup Test for Assessing Stretchability, Met. Eng. Quart., Nov 1974, p 30-36
53.
J.R. Newby, A Practical Look at Biaxial Stretch Cupping Tests, Sheet Met. Ind.,
Vol 54, March 1977, p
240-252
54.
K. Nakazima, T. Kikuma, and K. Hasuka, "Study on the Formability of Sheet Steels," Technical Repo
rt
264, Yawata Iron & Steel Company, Sept 1968, p 141
55.
A.K. Ghosh, How to Rate Stretch Formability of Sheet Metals, Met. Prog., May 1975, p 52-54
56.
A.K. Ghosh, The Effect of Lateral Drawing-In on Stretch Formability, Met. Eng. Quart., Aug 1975, p 53-64
57.
R.A. Ayres, W.G. Brazier, and V.F. Sajewski, Evaluating the Limiting Dome Height Test as a New
Measure of Press Formability, J. Appl. Metalwork., Vol 1 (No. 1), 1979, p 41-49
58.
R. Stevenson, Correlation of Tensile Properties With Plane-Strain Limiting Dome Height,
J. Appl.
Metalwork., Vol 3 (No. 3), July 1984, p 272-280
59.
R.D. Adamczyk, D.W. Dickinson, and R.P. Krupitzer, "The Edge Formability of High-Strength Cold-
Rolled Steel," Paper 830237, Society of Automotive Engineers, 1983
60.
D.F. Eary and E.A. Reed, Techniques of Pressworking Sheet Metal, 2nd ed., Prentice-Hall, 1974, p 136-172
61.
D.V. Wilson, B.J. Sunter, and D.F. Martin, A Single-Blank Test for Drawability, Sheet Met. Ind.,
Vol 43,
June 1966, p 465-476
62.
K.M. Frommann, The Prediction of Metal Stamping Behavior, in
Proceedings of the American Metal
Stamping Association Conference (Detroit), April 1968, p 1-55
63.
A.S. Kasper and P.J. VanderVeen, "A New Method of Predicting the Formability of Materials," Paper
720019, Society of Automotive Engineers, 1972
64.
M.W. Boyles and H.S. Chilcott, Recent Developments in the Use of the Stretch-Draw Test, Sheet Met. Ind.,
Vol 59, Feb 1982, p 149-156
65.
G.M. Goodwin, "Formability Index," Paper MF 71-165, Society of Manufacturing Engineers, 1971
66.
K. Yoshida, H. Hayashi, K. Miyauchi, M. Hirata, T. Hira, and S. Ujihara, Yoshida Buckling Test, in
Proceedings of the International Symposium on New Aspects of Sheet Metal Forming,
Iron and Steel
Institute of Japan, 1981, p 125-148
67.
H. Abe, K. Nakagawa, and S. Sato, Proposal of Conical Cup Buckling Test, in
Proceedings of the Congress
of the International Deep Drawing Research Group (Kyoto, Japan), May 1981, p 1-19
68.
J. Havranek, Wrinkling Limit of Tapered Pressings, J. Austral. Inst. Met., Vol 20, 1975, p 114-119
69.
A.M. Szacinski and P.F. Thompson, The Effect of Mechanical Properties on the Wrinkling Behavior of
Sheet Material in the Yoshida Test and in a Cone Forming Test, in Efficiency in Sheet Metal Forming,
Proceedings of the 13t
h Biennial Congress, Melbourne, Australia, International Deep Drawing Research
Group, Feb 1984, p 532-542
70.
J.E. O'Donnell, E.J. Ripling, and R.M. Brick, A New Tool for Testing Thin Sheet, Met. Prog.,
May 1962, p
67-71
71.
J.E. Bower, Use of Springback Tester With Steels Having Nonlinear Stress-Strain Curves, Mater.
Res.
Stand., Dec 1965, p 607-610
72.
W.G. Granzow, "A Portable Springback Tester for In-
Plant Determination of the Strength of Sheet Steels,"
Paper 830238, Society of Automotive Engineers, 1983
73.
R.G. Davies, Springback in High-Strength Steels, J. Appl. Metalwork., Vol 1 (No. 4), 1981, p 45-52
Formability Testing of Sheet Metals
Brian Taylor, General Motors Corporation
Determination of Forming Limit Diagrams
Forming limit diagrams indicate the limiting strains that sheet metals can sustain over a range of major-to-minor strain
ratios. Two main types of laboratory tests are used to determine these limiting strains. The first type of test involves
stretching test specimens over a punch or by means of hydraulic pressure--for example, the hemispherical punch method.
This produces some out-of-plane deformation and, when a punch is used, surface friction effects. The second test
produces only in-plane deformation and does not involve any contact with the sample within the gage length.
The first type of test has been used much more extensively (Ref 6, 74, 75, 76) than the second and provides slightly
different results (Ref 33, 77). Good correlation has been obtained between forming limit diagrams determined in the
laboratory and production experience.
The hemispherical punch method for determining forming limit diagrams uses circle-gridded strips of the test
material ranging in width from 25.4 to 203 mm (1.0 to 8.0 in.) that are clamped in a die ring and stretched to incipient
fracture by a 102 mm (4.0 in.) diam steel punch (Ref 74, 76). The narrowest strip fractures at a minor-to-major strain ratio
of about -0.5, which is comparable to that obtained in a tensile test. As the strip width is increased, the strain ratio
increases to a slightly positive value for a full-width specimen. Further increases in the ratio to a maximum value of +11.0
(balanced biaxial stretching) are achieved by using progressively improved punch lubrication (oiled polyethylene, oiled
neoprene) and by increasing thicknesses of polyurethane rubber.
The strains are measured in and around regions of visible necking and fracture. The forming limit curve is drawn above
the strains measured outside the necked regions and below those measured in the necked and fractured regions, as shown
in Fig. 31.
Fig. 31 Strain measurements and forming limit diagram for aluminum-killed steel. Source: Ref 76.
In-plane determination of the forming limit diagram can be achieved by using the uniaxial tensile test, rectangular
sheet tension test, or Marciniak biaxial stretching test with elliptical and circular punches, as described earlier in this
article. The forming limit curve can be determined over the full range of strain ratios, without introducing any out-of-
plane deformation. A comparison of the in-plane and punch methods showed close agreement for negative strain ratios
and slightly higher values in the punch test at plane strain and for positive strain ratios (Ref 33).
References cited in this section
6.
S.P. Keeler and W.A. Backofen, Plastic Instability and Fracture in Sheets Stretched Over Rigid Punches,
Trans. ASM, Vol 56 (No. 1), 1963, p 25-48
33.
M.L. Devenpeck and O. Richmond, Limiting Strain Tests for In-Plane Sheet Stretching, in
Novel
Techniques in Metal Deformation Testing, The Metallurgical Society, 1983, p 79-88
74.
C.C. Veerman, L. Hartman, J.J. Peels, and P.F. Neve, Det
ermination of Appearing and Admissible Strain in
Cold-Reduced Sheets, Sheet Met. Ind., Vol 48, Sept 1971, p 678-694
75.
A.B. Haberfield and M.W. Boyles, Laboratory Determined Forming Limit Diagrams, Sheet Met. Ind.,
Vol
50, July 1973, p 400-411
76.
S.S. Hecker, Simple Technique for Determining Forming Limit Curves, Sheet Met. Ind.,
Vol 52, Nov 1975,
p 671-676
77.
A.K. Ghosh and S.S. Hecker, Stretching Limits in Sheet Metals: In-Plane Versus Out-of-
Plane
Deformation, Met. Trans., Vol 5, Oct 1974, p 2161-2164
Formability Testing of Sheet Metals
Brian Taylor, General Motors Corporation
Circle Grid Analysis
Circle grid analysis is a useful technique for ensuring that a die is adequately prepared for production and for diagnosing
the causes of necking and splitting failures in production (Ref 78). The forming limit diagram for the type and gage of
work material selected must first be obtained. Arrays of small diameter (2.5 mm, or 0.1 in.) evenly spaced circles are
printed or etched on several blanks in the critical strain regions, preferably in the same location on each blank. Some of
the blanks are formed into parts, and the major and minor axes of the deformed circles are measured in the critical
locations. The critical strain regions of the part are identified by visual observation of necking or splitting, or by previous
experience with similar parts. The local strains are then calculated from the measured dimensions and plotted on the
forming limit diagram.
If the maximum strains measured are close to or above the forming limit strains, problems with the tooling, lubrication,
blank size or positioning, or press variables are indicated, whether or not necking or splitting actually occurs. Fluctuations
that occur in operating conditions and in the properties of the work material over a production run will eventually cause
failure if the material is strained to its full capacity. In this case, some of these variables should be changed until the circle
grid strain measurements fall below the material limits by a given safety factor, such as 10%.
If the maximum strains measured are significantly below the limit strains and necking or splitting occurs, the batch of
work material is substandard. The material used in die tryout must have typical, or slightly lower, forming properties than
the production material. The use of superior material may indicate an adequate forming safety margin that will disappear
when a more typical or lower formability material is used. It is good practice to form a few gridded blanks of a standard
(nonaging) reference material periodically during a production run to determine the trends in the maximum strains. If the
strains are approaching the maximum limits, corrective measures can be taken before any actual failures occur.
Circle Grids. Many types of circle grid patterns have been used, such as square arrays of contacting or closely spaced
non-contacting circles and arrays of overlapping circles. The contacting and overlapping circles provide improved
coverage, but are more difficult to measure manually and, at this time, cannot be measured automatically.
With small, closely spaced circles, it is possible to determine strain gradients accurately, provided the circles are not too
small for accurate measurement. Circles with 2.5 mm (0.1 in.) diameters have been found to be a good size. Both open
and solid circles have been successfully used, and automatic systems have been developed for measuring both types.
Applying Circle Grids to the Blanks. The circle grids can be applied to the blanks by a printing or photographic
technique or by electrochemical etching. Printed and photographically applied circles are easily damaged and tend to rub
off in areas contacted by the dies. This has led to general acceptance of etched circles.
In the electrochemical etching process, an electric stencil with the required grid pattern is placed on the blank and covered
with a felt pad soaked in an etching solution. An electrode is placed on the pad, and a low-voltage (up to 14 V) current is
passed between the electrode and the blank for a short time, usually less than 1 min. This produces a lightly etched and
oxidized pattern on the surface of the blank. The stencils, etching solutions, and power supplies for this process are
commercially available. Different metals require different solutions, levels and types of voltage, and etching times.
Measuring Strains From Deformed Circles. Deformed circles can be measured manually by means of dividers and
a ruler, graduated transparent tapes, or a low-power microscope with a graduated stage. Automatic systems, known as
grid circle analyzers, have also been developed for measuring the dimensions of the circles and calculating and displaying
the major and minor strains (Ref 79, 80). These systems are now commercially available.
In regions of high curvature, the most accurate method of measurement is use of the transparent tape because it follows
the contour of the part and measures the arc length, while the other methods measure the chord length. The tapes have a
pair of diverging lines graduated to give direct readings of the strain, as shown in Fig. 32.
Fig. 32 Transparent tape measurement of deformed circles. Source: Ref 78.
Grid circle analyzers use a solid-state digital array camera with a built-in light source, a minicomputer, keyboard, CRT
display, and printer. An image of a given deformed circle is displayed on the CRT, and a least squares curve fitting
program selects the most suitable ellipse, which is displayed simultaneously. The major and minor strains, computed from
the equation for the ellipse and the diameter of the original circle, are displayed on the screen and printed. A typical
layout for the equipment is shown in Fig. 33.
Fig. 33 Layout of a grid circle analyzer. Source: Ref 79.
References cited in this section
78.
S. Dinda, K.F. James, S.P. Keeler, and P.A. Stine, How to Use Circle Grid Analysis for Die Tryout,
American Society for Metals, 1981
79.
R.A. Ayres, E.G. Brewer, and S.W. Holland, Grid Circle Analyzer: Computer Aided Measurement of
Deformation, Trans. SAE, Vol 88 (No. 3), 1979, p 2630-2634 (SAE Paper 790741)
80.
D.N. Harvey, Optimising Patterns and Computational Algorithms for Automated, Optical Strain
Measurement in Sheet Metal, in Efficiency in Sheet Metal Forming,
Proceedings of the 13th Biennial
Congress, Melbourne, Australia, International Deep Drawing Research Group, Feb 1984, p 403-414
Formability Testing of Sheet Metals
Brian Taylor, General Motors Corporation
Drawbead Forces
It is common practice in production stamping operations to control the movement of the edges of the blank into the die
cavity by means of drawbeads placed in the blankholder. These consist of a semicylindrical ridge in the upper part of the
blankholder and a corresponding groove with rounded shoulders in the lower part, or a similar but opposite configuration.
The drawbeads cause the periphery of the blank to bend and straighten three times as it passes through each bead, as
shown in Fig. 34.
Fig. 34 Repeated bending and straightening of a blank edge in a drawbead.
The repeated bending and straightening produces a restraining force in addition to that caused by surface friction. A
method has been devised for measuring the restraining force due to deformation independently of the effects of friction,
using a drawbead simulator with low-friction rollers instead of a fixed bead and groove (Ref 81, 82). A second drawbead
simulator with nonrotating parts can be used to measure the combined effects of friction and deformation. Figure 35
shows both types of simulators.
Fig. 35 Drawbead simulators. (a) Frictionless simulator. (b) Standard simulator. Source: Ref 82.
Strips of 0.75 to 1.00 mm (0.03 to 0.04 in.) thick and 50 mm (1.97 in.) wide rimmed and aluminum-killed steels and two
aluminum alloys were tested using simulators and a universal testing machine. The contribution from deformation to the
total restraining force depended on the lubricant used and ranged from an average of 60% with poor lubrication to 85%
with very good lubrication. The required clamping forces, surface strains in the workpiece at various locations in the
drawbead simulators, effect of drawbead radius, and effect of rate of testing were also investigated.
References cited in this section
81.
H.D. Nine, Drawbead Forces in Sheet Metal Forming, in Mechanics of Sheet Metal Forming,
Plenum Press,
1978, p 179-211
82.
H.D. Nine, The Applicability of Coulomb's Friction Law to Drawbeads in Sheet Metal Forming,
J. Appl.
Metalwork., Vol 2 (No. 3), July 1982, p 200-210
Formability Testing of Sheet Metals
Brian Taylor, General Motors Corporation
Lubricants
The use of lubricants is essential in most forming operations (see the article "Selection and Use of Lubricants in Forming
of Sheet Metal" in this Volume). An effective lubricant provides the following advantages:
• Reduction or elimination of direct sheet metal to die contact and the associated wear and galling
• Control of friction
• More uniform distribution of strain and therefore an increase in the overall level of deformation
• Reduction of heating
Lubricants must meet many requirements to be used in a production operation, such as:
• Suitable viscosity over the ranges of temperatures and pressures encountered
• Chemical and physical compatibility with work and die materials
• Ease of application, removal, and disposal
• Compatibility with welding operations, sealants, and paint systems
No single lubricant is optimal for all types and rates of forming and all combinations of work and die materials. Rankings
of lubricants change considerably for different types of operations and material combinations; this necessitates evaluation
on an individual basis. Differences in the performance of lubricants are to be anticipated in view of the differences in
surface composition, roughness, and texture of work and die materials and the different strain paths and rates of different
forming operations. For example, some stretching operations involve local increases in area in excess of 100%, but in
many drawing operations, a negligible or negative change in area occurs. In addition, the rate at which the work material
slides over the dies varies widely.
A simple test for evaluating lubricants measures the frictional force exerted on a lubricated strip of sheet metal when it is
pulled between two rectangular blocks of die material. The force used between the blocks and the test rate can be varied.
The number of strips that can be tested before the onset of galling provides an additional measure of the effectiveness of
the lubricant. However, this test does not include some of the important aspects of actual forming operations, such as
plastic deformation of the work material, the ranges of sliding speeds and rates of straining involved, die geometry (which
influences the amount of residual lubricant in various locations as the operation progresses), and the heating that occurs in
high-volume production operations.
The drawbead simulator described earlier is more realistic and has been used to measure friction forces with various
lubricants (Ref 81, 82). Under most of the conditions tested, friction was described by Coulomb's law, which states that
the friction force, F, is directly proportional to the normal force, N, between the contacting surfaces:
F = μN
(Eq 32)
where μ is the coefficient of friction. Coulomb's law did not apply at the highest contact pressure tested. The value of the
coefficient of friction for mill oil, a poor lubricant, was found to be 0.17; for the best lubricant tested--a soap-based
lubricant--it was 0.06.
In addition, simulative tests can be used as lubricant evaluation tests. The production operation should be characterized in
terms of the principal types of forming operations involved and their relative severities, and the appropriate simulative
test selected. The Swift flat-bottomed cup test has been extensively used for the evaluation of lubricants for deep drawing,
and the Swift round-bottomed and Fukui conical cup tests have been used for stretch-drawing operations. The 100 mm
(3.94 in.) hemispherical dome test can be used for stretching operations.
The various regimes of lubrication--that is, thick film, thin film, mixed, and boundary lubrication--and their
characteristics are reviewed in Ref 83, which also discusses the limited validity of Coulomb's law, methods for lubricant
evaluation, and some of the current limitations in this area.
References cited in this section
81.
H.D. Nine, Drawbead Forces in Sheet Metal Forming, in Mechanics of Sheet Metal Forming,
Plenum Press,
1978, p 179-211
82.
H.D. Nine, The Applicability of Coulomb's Friction Law to Drawbeads in Sheet Metal Forming,
J. Appl.
Metalwork., Vol 2 (No. 3), July 1982, p 200-210
83.
W.R.D. Wilson, Friction and Lubrication in Sheet Metal Forming, in Mechanics of Sheet Metal Forming,
Plenum Press, 1978, p 157-177
Formability Testing of Sheet Metals
Brian Taylor, General Motors Corporation
References
1.
S.P. Keeler, "Understanding Sheet Metal Formability, Vol 2," Paper 350A, Metal Fabricating Institute,
1970
2. J.H. Holloman, Tensile Deformation, Trans. AIME, Vol 162, 1945, p 268-290
3. W.A. Backofen, Massachusetts Institute of Technology I
ndustrial Liaison Symposium, Chicago, March
1974
4. A.K. Ghosh, The Influence of Strain Hardening and Strain-Rate Sensitivity on Sheet Metal Forming,
Trans.
ASME, Vol 99, July 1977, p 264-274
5. I.S. Brammar and D.A. Harris, Production and Properties of
Sheet Steel and Aluminum Alloys for Forming
Applications, J. Austral. Inst. Met., Vol 20 (No. 2), 1975, p 85-100
6.
S.P. Keeler and W.A. Backofen, Plastic Instability and Fracture in Sheets Stretched Over Rigid Punches,
Trans. ASM, Vol 56 (No. 1), 1963, p 25-48
7.
G.M. Goodwin, "Application of Strain Analysis to Sheet Metal Forming Problems in the Press Shop,"
Paper 680093, Society of Automotive Engineers, 1968
8. S.P. Keeler, Determination of Forming Limits in Automotive Stampings, Sheet Met. Ind., Vol
42, Sept
1965, p 683-691
9. S.P. Keeler and W.G. Brazier, Relationship Between Laboratory Material Characterization and Press-
Shop
Formability, in Microalloying 75 Proceedings, Union Carbide Corporation, 1977, p 517-530
10.
H. Naziri and R. Pearce, The Effect of Plastic Anisotropy on Flange-
Wrinkling Behavior During Sheet
Metal Forming, Int. J. Mech. Sci., Vol 10, 1968, p 681-694
11.
K. Yoshida and K. Miyauchi, Experimental Studies of Material Behavior as Related to Sheet Metal
Forming, in Mechanics of Sheet Metal Forming, Plenum Press, 1978, p 19-49
12.
W.F. Hosford and R.M. Caddell, in Metal Forming, Mechanics and Metallurgy, Prentice-
Hall, 1983, p 273,
309
13.
J. Havranek, The Effect of Mechanical Properties of Sheet Steels on the Wrinkling Behavior
During Deep
Drawing of Conical Shells, in Sheet Metal Forming and Energy Conservation,
Proceedings of the 9th
Biennial Congress of the International Deep Drawing Research Group, Ann Arbor, MI, American Society
for Metals, 1976, p 245-263
14.
J.S.H. Lake, The Yoshida Test--A Critical Evaluation and Correlation with Low-
Strain Tensile Parameters,
in Efficiency in Sheet Metal Forming,
Proceedings of the 13th Biennial Congress, Melbourne, Australia,
International Deep Drawing Research Group, Feb 1984, p 555-564
15.
J.L. Duncan, R. Sowerby, and M.P. Sklad, "Failure Modes in Aluminum Sheet in Deep Drawing Square
Cups," Paper presented at the Conference on Sheet Forming, University of Aston, Birmingham, England,
Sept 1981
16.
G. Glover, J.L. Duncan, and J.D. Embury, Failure Maps for Sheet Metal, Met. Technol.,
March 1977, p
153-159
17.
J.D. Embury and J.L. Duncan, Formability Maps, Ann. Rev. Mat. Sci., Vol 11, 1981, p 505-521
18.
J. Datsko, Materials in Design and Manufacturing, J. Datsko Consultants, 1977, p 7-16
19.
N. Kuhn, On the Springback Behavior of Low-Carbon Steel Sheet After Stretch Bending, J. Austral.
Inst.