Deep draw test


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Deep draw test

  1. 1. Deep drawing Test 10.1 Objective To determine the ductility of sheet using Erickson Cupping test 10.2 Apparatus  Sheets of different materials (Aluminum and steel)  Deep drawing test machine  Venire caliper 10.3 Theory A mechanical test used to determine the ductility and stretching properties of sheet metal. It consists of measuring the maximum part depth that can be formed before fracture. The test is typically carried out by stretching the test piece clamped at its edges into a circular die using a punch with a hemispherical end A cupping test used to assess the ductility of sheet metal. The method consists of forcing a conical or hemispherical-ended plunger into the specimen and measuring the depth of the impression at fracture. 10.3.1 Sheet Metal Forming Sheet Metal Forming is the process of converting a flat sheet of metal into a part of desired shape without fracture or excessive localized thinning. The process may be simple, such as a bending operation, or a sequence of very complex operations such as those performed in high-volume stamping plants. In the manufacture of most large stampings, a sheet metal blank is held on its edges by a blankholder ring and is deformed by means of a punch and die. The movement of the blank into the die cavity is controlled by pressure between the upper and lower parts of the blankholder ring. This control is usually increased by means of one or more sets of draw beads. These consist of an almost semi cylindrical ridge on the upper part of the blankholder and a corresponding groove in the lower part (the positions are sometimes reversed). The drawbeads force the periphery of the blank to bend and unbend as it is pulled into the die; this increases the restraining force considerably. Presses with capacities to 17.8 MN (2000 tonf) are commonly used for the manufacture of large stampings, and presses to 26.7 MN (3000 tonf) are used for heavy-gage parts. Sheet metal forming operations are so diverse in type, extent, and rate that no single test provides an accurate indication of the formability of a material in all situations. However, knowledge of materials properties and careful analysis of the various types of forming involved in making a particular part are indispensable in determining the probability of successful part production and in developing the most efficient process 10.3.2Types of Forming Many forming operations are complex, but all consist of combinations or sequences of the basic forming operations—bending, stretching, drawing, and coining. Inspection and Testing Lab report Page 1 of 7
  2. 2. Deep drawing Test Bending is the most common type of deformation, and it occurs in almost all forming operations. Bending around small radii can lead to splitting in the early stages of a forming process because it localizes strain and prevents its distribution throughout the part. Ideally, strain should be distributed as uniformly as possible to maximize the amount of deformation that can be obtained. Even a slight increase in the radius in a given location can sometimes significantly improve strain distribution. Frequently, designs specify smaller radii than necessary, which results in manufacturing problems and increased costs. Stretching Stretching is caused by tensile stresses in excess of the yield stress. These forces produce biaxial stretching when they are applied in perpendicular directions in the plane of the sheet. Balanced biaxial stretching occurs when the perpendicular forces are equal. Much higher levels of deformation, as measured by an increase in area, can be reached in balanced biaxial stretching than in any other forming mode. Many forming operations involve stretching of some areas within the stamping. Automotive outer body panels are typical examples of parts formed primarily by stretching. Parts with regions containing domes, ribs, and embossments also involve stretching. Plane-strain stretching Plane-strain stretching results in elongation in one direction and no dimensional change in the perpendicular direction. It frequently occurs when a wide, flat area of sheet metal is stretched longitudinally—for example, in the sidewall of a stamping. In this case, strain in the transverse direction is prevented by the adjacent metal. Plane-strain stretching is an important type of deformation because most materials fracture at a lower level of strain in plane strain than in any other condition. Many of the fractures that occur in stamping operations are in the plane-strain region. Drawing Drawing of sheet metal causes elongation in one direction and compression in the perpendicular direction. The simplest example is the drawing of a flat-bottomed cylindrical cup. In this process, a circular disk is held between two flat annular dies and impacted in the center by a flat-bottomed punch. This draws (pulls) the edges of the disk inward to form the wall of the cup. The metal is stretched radially by the tensile forces produced by the punch, but it is compressed circumferentially as its diameter decreases. Many other forming operations involve substantial drawing. Coining Coining occurs when metal is compressed between two die surfaces. It is used extensively for making coins and parts with similar surface features, for flattening, and for reducing springback upon removal of parts from a die. In many stretching and drawing operations, coining is undesirable because it restricts metal movement, localizes strain, and produces surface damage. Much of the die preparation for these operations concentrates on locating and eliminating coining. 10.3.4 Formability Problems Inspection and Testing Lab report Page 2 of 7
  3. 3. Deep drawing Test The major problems encountered in sheet metal forming are fracturing, buckling and wrinkling, shape distortion, loose metal, and undesirable surface textures. The occurrence of any one or a combination of these conditions can render the sheet metal part unusable. The effects of these problems are discussed next. Fracturing Fracturing occurs when a sheet metal blank is subjected to stretching or shearing (drawing) forces that exceed the failure limits of the material for a given strain history, strain state, strain rate, and temperature. In stretching, the sheet initially thins uniformly, at least in a local area. Eventually, a point is reached at which deformation concentrates and causes a band of localized thinning known as a neck, which ultimately fractures. The formation of a neck is generally regarded as failure because it produces a visible defect and a structural weakness. Most current formability tests are concerned with fracture occurring in stretching operations. In shearing, fracture can take place without prior thinning. The most common examples of this type of fracture occur in slitting, blanking, and trimming. In these operations, sheets are sheared by knife edges that apply forces normal to the plane of the sheet. Shearing failures are sometimes produced in stamping operations by shearing forces in the plane of the sheet, but they are much less common than stretching failures. Buckling and Wrinkling. In a typical stamping operation, the punch contacts the blank, stretches it, and starts to pull it through the blankholder ring. The edges of the blank are pulled into regions with progressively smaller perimeters. This produces compressive stresses in the circumferential direction. If these stresses reach a critical level characteristic of the material and its thickness, they cause slight undulations known as buckles. Buckles may develop into more pronounced undulations or waves known as wrinkles if the blankholder pressure is not sufficiently high. This effect can also cause wrinkles in other locations, particularly in regions with abrupt changes in section and in regions where the metal is unsupported or contacted on one side only. In extreme cases, folds and double or triple metal may develop. These may in turn lead to splitting in another location by preventing metal flow or by locking the metal out. Therefore, increasing the blankholder pressure often corrects a splitting problem. Shape Distortion In forming operations, metal is deformed elastically and plastically by applied forces. Upon removal of the external forces, the internal elastic stresses relax. In some locations, they can relax completely, with only a very slight change in the dimensions of the part. However, in areas subjected to bending, through-thickness gradients in the elastic stresses will occur; that is, the stresses on the outer surfaces will be different from those on the inner surfaces. If these stresses are not constrained or “locked in” by the geometry of the part, relaxation will cause a change in the part shape known as shape distortion or springback. Springback can be compensated for in die design Inspection and Testing Lab report Page 3 of 7
  4. 4. Deep drawing Test for a specific set of materials properties but may still be a problem if there are large material property or process variations from blank to blank. Loose metal Loose metal occurs in undeformed regions and is undesirable because it can be easily deflected. A phenomenon usually referred to as oil canning, in which a local area can be either concave or convex, can also be encountered. In stampings with two or more sharp bends of the same sign in roughly the same direction, such as a pair of feature lines, a tendency exists for the metal between them to be loose because of the difficulty involved in pulling metal across a sharp radius. It is sometimes possible to avoid the problem by ensuring that the metal is not contacted by both lines at the same time; thus, some stretching can occur before the second line is contacted .There is a tendency for loose metal to occur toward the center of large, flat, or slightly curved parts. Increasing the restraining forces on the blank edges usually improves this condition. Undesirable Surface Textures. Heavily deformed sheet metal, particularly if it is coarse grained, often develops a rough surface texture commonly known as orange peel (see the article “Press Forming of Coated Steel” in this Volume). This is usually unacceptable in parts that are visible in service. Another source of surface problems occurs in metals that have a pronounced yield point elongation, that is, materials that stretch several percent without an increase in load after yielding. In these metals, deformation at low strain levels is concentrated in irregular bands known as Luders lines (or bands), or stretcher strains. 10.3.5 Effect of Temperature on Formability A change in the overall temperature alters the properties of the material, which thus affects formability. In addition, local temperature differences within a deforming blank lead to local differences in properties that affect formability. At high temperatures, above one-half of the melting point on the absolute temperature scale, extremely fine-grain aluminium, copper, magnesium, nickel, stainless steel, steel, titanium, zinc, and other alloys become superplastic. Superplasticity is characterized by extremely high elongation, ranging from several hundred to more than 1000%, but only at low strain rates (usually below approximately 10-2/s-1 at high temperatures. The requirements of high temperatures and low forming rates have limited superplastic to low-volume production. In the aerospace industry, titanium is formed in this manner. The process is particularly attractive for zinc alloys because they require comparatively low temperatures (~270C, or ~520 F). At intermediate elevated temperatures, steels and many other alloys have less ductility than at room temperature. Aluminium and magnesium alloys are exceptions and have minimum ductility near room temperature. Alloys of these metals have been formed commercially at slightly elevated temperatures (~250 C, or ~480 F). The strain rate Inspection and Testing Lab report Page 4 of 7
  5. 5. Deep drawing Test sensitivity and post-uniform elongation for aluminium magnesium alloys have been found to increase significantly in this temperature range. A method of improving drawability by creating local temperature differences has been developed and is being used commercially. It involves water cooling the punch in a deep-drawing operation. This lowers the temperature of the blank where it contacts the punch, which is the principal failure zone, and increases the local flow stress. Heating the die in order to lower the flow stress in the deformation zone at the top of the draw wall has also been found to be beneficial. The combination of these procedures has produced an increase of over 20% in the drawability of an austenitic stainless steel 10.4 Factors affecting the formability of materials 1) Grain size 2) Grain size relation to sheet thickness. 3) Preferred orientation 4) Structure 5) Prior deformation 6) Work hardening rate 7) Tensile strength 8) Ductility 10.5 Forming Limit Diagrams A traditional forming limit diagram (FLD) indicates 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 than the second and provides slightly different results. Good correlation has been obtained between FLDs determined in the laboratory and production experience. 10.5.1 Circle Grid Analysis and Use of FLDs 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 81, 82). The forming limit diagram Inspection and Testing Lab report Page 5 of 7
  6. 6. Deep drawing Test 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 curve (FLC), problems with the tooling, lubrication, blank size or positioning, or press variables are indicated, whether 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. If the greatest levels of strain in the stamping are more than 10% below the FLC, the dies are considered “green,” or safe. Many companies require this type of analysis, proving that the new stamping dies are capable of producing a part with strain levels in the “green” zone of the FLD, before buying off on the dies). When new dies cause levels of strain between 0 and 10% below the FLC, this is considered a “yellow” zone, where caution requires further development of either the dies themselves, the drawbeads, or other process conditions including lubrication. Any strain readings that exceed the FLC indicate a “red” condition, where the likelihood of failure is high if the dies are not modified prior to being used for production. The material used in die tryout should 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. 10.6 Erichsen Cupping test In tests, the cup height at fracture is used as the measure of stretchability. The preferred criterion for determining this point is the maximum load. When this cannot be determined, the onset of a visible neck or fracture can be used, but this yields a slightly different value. The cup height measured by means of a visible fracture is 0.3 to 0.5 mm (0.012 to 0.020 in.) greater than the height measured at the maximum load. These tests, as indicators of stretchability, should correlate with the n value, but the correlation is not satisfactory. Improved correlations with the total elongation and reduction in area have been reported. Some investigators have reported poor reproducibility of results in the Olsen and Erichsen tests and poor correlation with production experience. Satisfactory reproducibility and correlation in specific cases have been reported when experimental conditions were carefully controlled. The variability in tests has been attributed to the small size of the penetrator, uncontrolled drawing-in of the flange, and inconsistent lubrication. The small size of the penetrator leads to excessive bending, particularly Inspection and Testing Lab report Page 6 of 7
  7. 7. Deep drawing Test in thicker sheet, and is generally unrepresentative of production conditions. Drawing-in can be controlled somewhat by standardizing the specimen size and by using a high (~71 kN, or ~8 tonf) clamping force. Even greater control can be achieved by using lock beads or serrated dies (dies with concentric circular ridges of triangular cross section that dig into the specimen and prevent slippage). Consistent lubrication can be achieved by using oiled polyethylene between the specimen and penetrator. The problems with the Olsen and Erichsen tests have led to the development of stretching tests that use a much larger diameter punch and a lock bead to prevent drawing-in. 10.7 Data 10.7.1 Sample no 01 Sheet material = Aluminium Thickness of sheet = 0.5mm Erichsen number = 9.5 10.7.2 Sample no 02 Sheet material = Steel Thickness = 0.5mm Erichsen number = 4.5 10.8 Result As more distance is covered by ball its means aluminium sheet is more ductile as compare to steel which covered less distance. As indicated by Erichsen number. 10.9 Reference    ASM Handbook, Volume 14B: Metalworking: Sheet Forming S.L. Semiatin, editor, p673-696 Copyright © 2006 ASM International® Handout lecture Inspection and Testing Lab report Page 7 of 7