Evolution of The Process
Traditional evaluations of formability are based on fundamental and simulative tests. Within the first category are direct measurements are mechanical properties derived from a standard tensile test, such as yield stress, tensile strength, yield point, elongation, and total elongation, and measurements of hardness. (3-8) Property levels required for successful stampings are determined either from an accumulation of many past trial and error attempts on similar stampings, or from long statistical correlations with press performance data.
Unfortunately, the relationship between test results and press performance data is often unclear; specifications so established are only partially valuable for selected stampings. Recent work by members of the International Deep Drawing Research Group (4,5, 9-15) has contributed to a better understanding of this problem. Three new property measurements have evolved as being directly related to press performance: the coefficient of work hardening n (if hardening is parabolic), the coefficient of anisotropy r, and the circle are elongation e ca. These three measurements are discussed at junction points making measurements more difficult: open space also occurs in this pattern. The overlapping circles of pattern C are popular because all of a given area is included in at least one circle; however, some areas are duplicated in measurements which could strain distributions additionally, visualization of individual circles is difficult. The double overlapping in pattern D provides cross marks which act as locators for the centers of the circles and avoids wide lines at junction points.
In the past scribing methods have restricted grids to squares which can easily be ruled from parallel lines. Circle patterns generally cannot be scribed. Can you imagine giving a shop man the following assignments: scribe a pattern of 5000 0.10 in. diameter circles, accurate to 1%, on each of six blanks and have them ready for the press line in 15 minutes? Such a requirement demands some type of imprinting system. First attempts consisted of using a rubber stamp and marking ink. (20) Resolution and accuracy of grids prepared in this manner are limited and the ink markings are easily erased.
A photographic process has also been used. (20,21) A photosensitive emulsion is placed on a metal sheet, exposed to an illuminated negative, and developed. Very fine and accurate grid systems, such as 100 lines to the inch, have been created in this manner. However, the grid is easily removed when rubbing over a die radius, and the time to produce one grid can be greater that 30 minutes. In addition, the technique is generally applied only to small parts evaluated in the laboratory. Similar comments are applicable to silk screen processes for applying grids.
The system currently being used for producing small diameter circular grids, or, any other pattern desired, is electrochemical marking. (1,20,22) In this process, first used to produce grids by R. H. Heyer, a nonconducting sheet is treated so that the lines forming the desired pattern will conduct electricity. This "electrical stencil" is placed on the blank to be gridded (Fig 3). Then a felt pad soaked in electrolyte, an electrode, and a weight are then placed on in that sequence. For large 9 by 9 inch grid areas a person standing on a platform placed on the electrode provides the weight. A 14 V a-c current is passed through the system for approximately 5 seconds. The amperage required is a function of total conducting area and may reach 200 amp or more. An alternative technique requiring less amperage but more time utilizes a felt pad soaked in electrolyte (placed on the stencil) over which is presses on a rocker type electrode (fig. 3).
With either electrode, the grid pattern is etched into the blank and a black deposit replated into the grid lines. The depth of etching is proportional to the time of application. The gird remains visible after abrasion, yet does not introduce stress concentrations (preferential failure sites) created by scribed lines. Since the equipment is portable, a grid may be applied rapidly at the press line on any production size blanks. The electrical stencil, power source, and chemicals are commercially available from several companies in the marking business.
An illustration of the grid system on an automotive part is shown in Fig. 4. The grid was applied twice to the flat blank to cover an area larger than 9 by 9 inches. Subsequently the blank was lubricated and formed into the final stamping. The grid remains visible even after severe forming operations.
PRESENT STATE OF THE ART - The circular grid system is being used today by an increasing number of companies in the automotive and appliance industry to analyze deformation patterns. There are presently two major areas of utilization: visual display, and estimates of failure proximity.
VISUAL DISPLAY - A grid of small diameter circles visually displays the strain state of the formed stamping. Areas of severe deformation are revealed with the direction and magnitude of the principal (maximum) strains graphically displayed from point to point. The distribution of strain also indicates how localized the high strain area may be. With this system, strains at different stages in the forming process can be measured by deforming blanks to various degrees of completion. These are then plotted as a function of stamping depth to develop a graphic picture of the strain history at each location. In the example shown in Fig. 5, the high strain is concentrated in area A. Grids also help when two sides of a supposedly symmetrical stamping do not strain identically and one side fails. Observed variations in strain distribution can often be traced to unequal die radii, misgauging of the blank, variation of draw beads, and the like. Correction of these variations will be reflected by changes in the strain distribution patterns. Similar pattern changes can also be observed when changes in material and lubrication occur.
ESTIMATES OF FAILURE PROXIMITY - When a stamping tears in the press, it is obvious that some change in material, lubrication, or tooling is required to produce a successful part. Frequently, however, a stamping will not fail during die tryout because the die maker often uses slow presses, excellent materials, hand lubrication, and properly set dies. If a stamping has not yet failed but is extremely close to failure, the stamping is said to be critical. In production, however, conditions other than optimum may exist and breakage of critical stampings may occur. Reworking the dies during this period would cause costly downtime. It would therefore be extremely valuable to identify, during die tryout, those stampings which are critical and make necessary modifications.
Such identification is possible by combining the information obtained from the small diameter circle grids with a failure curve which has been empirically developed for the more common ductile metals used in the automotive industry.
When a sheet of metal is stretched over a punch, the stain increases. If the stretching limit of the material is reached, the stamping tears. First however, a trough of very localized deformation or local measurements are being recorded, an arrest in the load record are being detected. Fracture is more realistically these events than by the usual concept of physical separation or tearing of the material. Strains have been measured at the onset of this fracture both in laboratory specimens of annealed tough pitch copper, 1100 aluminum, 70/30 brass and aluminum killed steel (11) and on production automotive steel stampings. (1, 19) These results are plotted in Fig. 6.
The y-axis of the graph is the largest percentage strain found on the surface of the stamping. With a circular grid system, this would be the major axis of the resulting ellipse. The x-axis is the surface strain perpendicular to the largest strain, or the minor axis of the same ellipse. The band drawn through these points separates failure and non-failure conditions and is labeled the critical strain level. By measuring the strains on any given stamping and relating them to Fig. 6, the proximity to failure may be determined for each region of the stamping.
A new concept has therefore been established. One can no longer argue that steel A will stretch more than steel B, or that brass will stretch more than steel. The maximum strains, measured in a small length, are identical for equal perpendicular strains. Instead, one must evaluate how well each material distributes the strain in the presence of a stress gradient. Note that the emphasis has changed from which material will stretch more to which material will better distribute the strain. Modification of material properties is now just one of many methods which are available to redistribute the strain more uniformly and therefore permit a deeper stamping before failure or prevent failure at a given depth of formation. Changing material properties, however, may not be the best solution. Slight modifications of lubrication or tool and die geometry are often more effective in distributing the strain more uniformly than large changes in material properties. Goodwin (2) and Heyer (16) also discuss this point.
An interesting feature of the curve is that the critical strain level slopes upward. From this is would seem that one could restrict metal flow* perpendicular to the maximum strain before fracture. It also means that the location of maximum strain may not be the fracture site. As an illustration, the central portion of an automotive bumper had a rounded dome like nose which had biaxial tensile strains of 54% by 30%. This strain level (shown as A in Fig. 6) was near critical but had not caused failure because of the high perpendicular strain. At a location somewhat removed from the central portion was a sharp ridge and no strain along the ridge (point B in Fig. 6). Critical conditions were satisfied here for a lower peak strain and the bumper failed at that location.
LIMITATIONS - The circular grid system will not presently solve all forming problems, nor is it probable it will do so in the future. The technique, even after two years, is just in its infancy and is in use by only a limited number of companies. More extensive trials are required. Some of the successes and failures achieved by various companies using this technique have not been reported.
The critical strain level has been investigated for only a few common ductile metals. One wonders how broad is the base of application, and are there similar curves at Even though wide ranges of normal cleanliness do not affect the curve, very large inclusions or other imperfections can lower the failure strain. The critical strain level was obtained for annealed and lightly skin passed materials; cold work of metal is known to lower the curve. Presently the critical strain level is well defined for only strains which are tension tension, although Goodwin (2) describes preliminary work for the tension compression combination of strains. The strain must also not be reversed, such as a tension strain imposed on material previously compressed.
Even with these limitations, the grid analysis system is a valuable aid for evaluating sheet metal formability.