1、 SURFACE VEHICLE RECOMMENDED PRACTICE J863 APR2015 Issued 1963-06 Reaffirmed 2015-04 Superseding J863 JUN1963 Methods for Determining Plastic Deformation in Sheet Metal Stampings RATIONALE J863 has been reaffirmed to comply with the SAE 5-year review policy. _ SAE Technical Standards Board Rules pro
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5、CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/J863_201504 ForewordThis Document has not changed other than to put it into the new SAE Technical Standards BoardFormat. T
6、he preferred method for determining plastic strain is the circle grid and the severity curve. The scribed square andchange in thickness methods may also be used to evaluate deformation during the forming of a flat sheet into thedesired shape.1. ScopeThis SAE Recommended Practice describes methods fo
7、r determining plastic deformationencountered in the forming or drawing of sheet steel.2. ReferencesThere are no referenced publications specified herein.3. Methods3.1 Circle Grid MethodThe test system employs electrochemically etched circle patterns on the surface of asheet metal blank and a severit
8、y curve for the evaluation of strains developed by forming in press operations.It is useful in the laboratory and in the press room. Selection from the various steels which are commerciallyavailable can be done effectively by employing this technique. In addition, corrective action in die or partdes
9、ign to improve performance is often indicated.The severity curve in Figure 1 has been developed from actual measurements of the major (e1) andassociated minor (e2) strains found in critical areas of production type stampings. Strain combinations whichlocate below this curve are safe, while those whi
10、ch fall above the curve are critical. The left of zero portion ofthe curve (tension-compression) represents 25% change in unit area. The right side (tension-tension) definesa severity limit since no constant percent change in area will be found to be critical.FIGURE 1SEVERITY CURVE4. Procedure1. Obt
11、ain or prepare a stencil with selected circles in a uniform pattern. The circles may be 0.100.25 in(2.56.4 mm) in diameter; the most convenient diameter is 0.20 in (5.1 mm) because it is easy to readand the gage spacing is short enough to show the maximum strain in a specific location on the part.2.
12、 The sheet metal blanks should be cleaned to remove excess oil and dirt; however, some precoatedsheets can be etched without removing the coating. The area(s) to be etched should be determinedfrom observation of panels previously formed; generally, the area which has a split problem is selectedfor e
13、tching. Normally, the convex side of the radius is gridded. If sufficient time is available, the entireblank may be etched, since valuable information can be obtained about the movement of metal instamping a part when strains can be evaluated in what may appear to be noncritical areas.Additionally,
14、for complex shapes it may be desirable to etch both surfaces of blanks so that the strainswhich occur in reverse draws can be determined.3. The etch pad is saturated with an appropriate electrolyte. Various electrolytes are available fromsuppliers of the etching equipment. Some electrolytes are more
15、 effective than others for etchingcertain surfaces, such as terne plate and other metallic coated steels. A rust inhibiting solution ispreferred for steel sheets._ SAE INTERNATIONAL J863 Reaffirmed APR2015 2 of 74. A ground clamp from the transformer of suitable amperage (1050 A is usually used) is
16、fastened to theblank and the second lead is attached to the etch pad. Although the current may be turned on at thistime, caution should be taken not to lay the pad on the sheet blank as it will arc. It is advisable torefrain from touching the metal of the etch pad and the grounded sheet blank.5. The
17、 stencil is placed with the plastic coating against the sheet surface in the area to be etched.Wetting the stencil with a minimum amount of electrolyte will assist in smoothing out the wrinkles andgives a more uniform etch. The etch pad is now positioned on the stencil and the current turned on, ifi
18、t is not already on. Apply suitable pressure to the pad. Only the minimum time necessary to producea clear etched pattern should be used. The etching time will vary with the amperage available from thepower source and the stencil area, as well as the pad area in contact with the stencil. Rocker type
19、 etchpads give good prints and require less amperage than flat surfaced pads. Excessive current causesstencil damage.6. The etching solution activates the surface of the metal and may cause rusting unless it is inhibited.After the desired area has been etched, the blank should be wiped or rinsed, dr
20、ied, and neutralized.7. The etched blank is now ready for forming. The lubricants and press conditions should simulateproduction situations.8. If a sequence of operations is used in forming a part, it is desirable to etch sufficient blanks so thateach operation can be studied.4.1 Measurement Of Stra
21、in After FormingAfter forming, the circles are generally distorted into ellipticalshapes (Figure 2). These ellipses have major and minor strain axes. The major strain (e1) is always defined tobe the direction in which the greatest positive strain has occurred without regard to original blank edges o
22、r thesheet rolling direction. The minor strain (e2) is defined to be 90 deg to the major strain direction.There are several methods for determining the major and minor strains of the formed panel. Typical tools are apair of dividers and a scale ruled in 50ths of an in (0.5 mm). For sharp radii, a th
23、in plastic scale, which canfollow the contour of the stamping, can be used to determine the dimensions of the ellipses. (Scales areavailable to read the percent strain directly.)_ SAE INTERNATIONAL J863 Reaffirmed APR2015 3 of 7FIGURE 24.2 Evaluation Of Strain MeasurementThe e1 strain is always posi
24、tive while the e2 strain may be zero,positive, or negative, as indicated on the severity curve chart (Figure 1). The maximum e1 and associated e2values measured in critical areas on the formed part are plotted on the graph paper containing the severitycurve by locating the point of intersection of t
25、he e1, e2 strainsIf this point is on or below the severity curve, the strain should not cause breakage. Points further below thecurve indicate that a less ductile material of a lower grade may be applied. Points above the severity curveshow the fabrication has induced strains which could result in b
26、reakage. Therefore, in evaluations onstampings exhibiting high strains, efforts should be made to provide an e1, e2 strain combination which wouldlie on or below the severity curve. A different e1, e2 strain combination can be obtained through changes ofone or more of the forming variables such as d
27、ie conditions, lubricants, blank size, thickness, or material grade.When attempting to change the relationship of e1 and e2 strains, it should be noted that on the severity curvethe most severe condition for a given e1 strain is at 0% e2 strain. This means the metal works best when it isallowed to d
28、eform in two dimensions, e1 and e2, rather than being restricted in one dimension. A change in e2to decrease the severity can be made by changing one of the previously mentioned forming variables or the diedesign, for example, improving lubrication on the tension-tension side will increase e2 and de
29、crease theseverity._ SAE INTERNATIONAL J863 Reaffirmed APR2015 4 of 7In addition to the severity curve, the e1, e2 strain measurements may be used to evaluate the materialrequirements on the basis of strain gradients, as illustrated in Figure 3, or by plotting contours of equivalentstrain levels on
30、the surface of the formed part. Even when the level of strain is relatively low, parts in which thee1 strain is changing rapidly either in magnitude or direction over a short span on the surface may require moreductile grades of sheet metal, change in lubrication, or change in part design.FIGURE 34.
31、3 Example Of Major And Minor Strain DistributionA formed panel (Figures 4 and 5), with a cross sectionas shown in Figure 3, is used to illustrate major and minor strain combinations. A plot of the major straindistribution is made by finding the ellipse with the largest major strain (circle 7 in Figu
32、re 3), corresponding tothe fracture area shown in Figure 5, original contour, and measuring the major and minor strains in the row ofellipses running in the direction of the major strain. The solid dots (Figure 3), are the measured major strainsfor each ellipse. The xs are the critical major strains
33、 as determined from the severity curve at thecorresponding minor strain (intersection of the measured minor strain and the severity curve, the numbers onFigure 1 are the major and minor (e1, e2) strains for the measured circles of 3).Usually, a row of ellipses will suffice to determine the most seve
34、re strain distribution. The resulting straindistribution plot (Figure 3), illustrates both severity of strain compared to the critical strain limits and theconcentration of the strain in the stamping. Steep strain gradients should be avoided because they areinherent fracture sites._ SAE INTERNATIONA
35、L J863 Reaffirmed APR2015 5 of 7FIGURE 4FIGURE 5The lower picture in Figure 4 is a top view of the original contour shown in Figure 5. The final contour (upperpicture) shows the line of dots indicating the row of ellipses measured for the strain evaluations. Acorresponding row was measured on the or
36、iginal contour with the results as shown in Figure 3.4.4 Example For Reducing Splitting TendencyThe splitting tendency in such an area as represented inFigures 3 and 5 (original contour) can be reduced as follows: If the radius of the part in the region of circle 1 isincreased, some strain can be in
37、duced to take place in this area which will allow the strain combination in circle7 to be modified to bring it within a “safe“ range. By changing the radius as shown in Figures 4 and 5, the straincombination in the critical area was changed from 40 x 5% to 35 x 17.5% (dot in circle of Figure 1) whic
38、h is nowjust in the “safe“ area.This course of action requires no building nor re-shaping of the punch, only grinding the radius of the die._ SAE INTERNATIONAL J863 Reaffirmed APR2015 6 of 7The average major strain required to make this formation is only 17.5% (the average major strain is calculated
39、by adding each of the major strains for the individual circles and dividing the total by the number of circlesinvolved). Yet, in a 0.2 in (5.1 mm) circle, the localized strain is as high as 40%. The strain distribution curveputs forth graphically the steep strain gradient and the need to distribute
40、the strain by some means asdescribed in this section.A change in lubrication can also improve the strain distribution of a stamping. If the strain over the punch iscritical, the amount of stretch (strain) required to make the shape can be reduced by allowing metal to flow inover the punch. This is a
41、ccomplished by decreasing the friction in the hold down area through the use of amore effective lubricant in this area.If the part shape is critical, a change in material may help, that is, a material having a better uniform elongationwill distribute the strain more uniformly, or a material having a
42、 higher “r“ value will make it possible to “draw“ inmore metal from the hold down area so that less stretch is necessary to form the part.5. Scribed Square MethodThe basic technique is to draw a panel from a blank which has been scribed bothlongitudinally and transversely with a series of parallel l
43、ines spaced at 1 in (25.4 mm) intervals. The lines onthe panel are measured after drawing and the stretch or draw calculated as the percent increase in area of a1in (25.4 mm) square. This is a fairly simple procedure for panels having generous, radii and fairly evenstretch or draw. A great many majo
44、r panels fall in this category and in these instances it is quite easy to pickout the square area exhibiting the greatest increase.If the square or line to be measured is no longer a flat surface, place a narrow strip of masking (or othersuitable tape) on the formed surface and mark the points which
45、 are to be measured. Remove the tape, placeon a plane surface, and determine the distance between the points with a steel scale.There will be cases of minor increase in area with major elongation in the one direction. In these instances, thepercent elongation should be recorded.6. Thickness MethodTh
46、ere are instances when the maximum stretch is confined to an area smaller than 1 in2(25.4 mm2) or the shape of the square has been distorted irregularly, making measurement difficult andcalculation inaccurate. When either of these conditions exists, an electronic thickness gage may be used atthe are
47、a in question or this area may be sectioned and the decrease in metal thickness measured with a ballpoint micrometer. The increase in unit area can be calculated by dividing the original thickness by the finalthickness.EXAMPLEAssuming the blank thickness to be 0.035 in (0.889 mm) and the final thickness to be 0.028 in(0.7112 mm), the increase in unit area would be 0.035/0.028 = 1.25 (0.7112/0.889 = 1.25) or 25%increase in unit area.PREPARED BY THE SAE IRON AND STEEL TECHNICAL COMMITTEEDIVISION 32_
