US3050991A - Apparatus for determining the drawing quality of sheet steels - Google Patents

Description

Aug. 28, 1962 E. s. MADRZYK ETAL 3,050,991

APPARATUS FOR DETERMINING THE DRAWING QUALITY OF SHEET STEELS Filed March 2, 1959 2 Sheets-Sheet 1 W fierzczlii, 62360 Aug. 28, 1962 E. s. MADRZYK ETAL 3,050,99

APPARATUS FOR DETERMINING THE DRAWING QUALITY OF SHEET STEELS Filed March 2, 1959 2 Sheets-Sheet 2 3,659,991 APPARATUS FOR DETEPMENH IG THE DRAWHNG QUALITY OF MEET STEELS Edmund S. Madrzylr and Francis E. Gibson, Lansing,

Ill., assignors to inland Steel Qornpany, a corporation of Delaware Filed Mar. 2, 195?, fies. No. 7%,407 6 Claims. (Qt. 73-87) This invention relates to apparatus for quantitatively evaluating the forming characteristics of metallic materials of construction. It is more specifically concerned with physically determining the drawing quality of sheet metals.

Although the forming of sheet metal structures by drawing operations is one of the most common sheet metal fabricating operations, no satisfactory method for evaluating the drawability of metal sheet has been developed. The drawability of a metal is rather an obscure property and much effort has been expended to find a method that would adequately measure this property. There have been many tests proposed and devised to obtain an indication of the probable behavior of a sheet under the press. These include simple bend tests, slottedstrip tests, tear-strength tests, mechanical and hydraulic O cupping tests, and tensile tests. None of these tests, however, seem to provide an accurate index of drawability when compared to actual press performance.

Another type of formability test which has been investi gated involves the forming of a bulge on a suitably sized test specimen by means of hydraulic pressure with the sheet metal specimen clamped over a circular or oval opening. The stress-strain relationships involved in a hydraulic bulge test have been mathematically analyzed and expressions that conform very closely to observed data have been derived. The interpretation of bulge data and the application of the results of such hydraulic bulge testing to commercial operations, however, have not been clarified.

According to this invention it has been found that the instability point of a metal under biaxial tension (ap plied by means of a hydraulic bulge tester) can be used as a sensitive criterion of drawability. The instability point is defined as the point at which plastic fiow proceeds without increases in pressure. It has been found that, in bulge testing, the upper portion of the bulge curve, when pressure is plotted against height of bulge, is parabolic in nature which allows the precise location of the instability point. This point so determined has been termed the H value. A drawability index based on this value shows more ability to discriminate between small variations in drawing quality than any of the common tests heretofore employed.

FIGURE 1 is a plan View of one embodiment of the bulge tester of this invention;

FIGURE 2 is a side elevation View of the apparatus shown in FIGURE 1;

'FIGURE 3 is an enlarged fragmentary view illustrating the specimen clamping means employed in the illustrative embodiment; and

FIGURE 4 is an enlarged fragmentary view illustrating the passageway through which the hydraulic operating fluid is admitted to the internal chamber.

In the drawings is illustrated a specific embodiment of an apparatus which can be used for carrying out the bulge testing of a sheet metal specimen to evaluate its drawability qualities. The apparatus in the drawings comprises a heavy duty base ltl having internal chamber 11 with a circular opening. Upstanding wall 12. of chamber 11 is penetrated by a plurality of ports 13 and 14 which connect respectively by means of passageways 13 and 14' (not shown) with hydraulic fluid inlet 15 and outlet 16 means. Suitable flow controllers 17 and 18 are provided to regulate the flow of hydraulic fluid through the system. Base It has Wall thicknesses suificient to withstand safely pressures of 5,000-10 000 p.s.i. depending upon the service in which the bulge tester will be employed.

A portion of the upper surface of sidewall 12 is undercut to provide recessed shoulder 19 which is surrounded by the remaining portion 24 of the upper surface of sidewall 12. A plurality of holes 21 traverse sidewall 12 to receive the holddown clamp fasteners 22 which cooperate with an internally threaded fastener 23 disposed in recess 24 located in the bottom surface of base 16. The sheet metal test specimen S cut in the form of a disc is laid on the surface of recessed shoulder 19 and covers the circular opening in internal chamber Ill. The diameter of the test specimen 8 is sufficiently larger than the diameter of the opening to properly bear on the face of recessed shoulder 19 and provide enough area for clamping ring 25 to engage in a non-slip relationship with the test specimen.

In the illustrated embodiment clamping ring 25 is in the form of an annulus having an inner diameter substantially equal to the diameter of the chamber opening. A portion of the lower surface of clamping ring 25 is undercut. The remaining portion of the lower surface of clamping ring 25" engaging the test specimen is provided with an anti-skid surface consisting of a plurality of lands and groves. The cooperating portion of the face of recessed shoulder 19 is similarly surfaced. The cooperating non-skid surfaces of base member lit and clamping ring 25 hold the test specimen securely in place and prevent slippage and leakage when an adequate clamping means is provided.

A satisfactory clamping arrangement is shown in the drawings and consists of a plurality of segmental clamps 2 6 which contact clamping ring 25 throughout the complete circumference. Ten segments are employed in the illustrated example which permit the use of two fasteners 22 per clamp. It will be noted that the outer marginal edge of clamp 26 rests on the remaining portion 24 of the upper surface of the sidewall face. The inner marginal edge rests on the upper surface of clamping ring 25. When fasteners 22 are drawn up tightly there is a slight bowing of the segmental clamps 26 which is compensated for by the under cut portion of the lower surface of clamping ring 25- and permits the application of a more effective clamping action than is provided by an unbroken surface.

A suitable indicating and measuring device 30 such as an extensometer is suspended over the test specimen. S and the sensing element 3b" brought into contact with the center of the specimen. To insure proper positioning of extensometer 31? at the pole of the test specimen a tripod jig 31 is employed. Jig 31 has three legs 32 having vertically adjustable, pointed feet 33 which register with suitable bench marks located in plate 34 upon which the base member It rests. Legs 32 are connected by spider 35 upon which extensometer 30 is mounted.

Pressure indicating gauges 36 are connected to output means 16 by piping manifold 37. The selection of gauges will depend upon the test program being carried out. Pump means (not shown) is connected to inlet means 15 which forces a suitable hydraulic fluid into chamber 11 to develop the fluid pressure required to bulge the test specimen.

For safety reasons it is preferred that covered shield 4t be used to prevent the spraying of the hydraulic fluid in the event that an unexpected rupture in the test specimen occurs.

Employing the apparatus of this invention a simple testing procedure is employed to determine the drawability index of the sheet metal being tested. After the sheet metal test specimen is securely clamped in place (with the illustrated apparatus a pneumatic impact wrench is used to run in the threaded fasteners and secure the segmental clamps) a hydraulic fiuid is pumped into chamber 11 under sufficient pressure to effect bulging of the test specimen and the bulge height and pressure measurements are correlated and recorded. Pressure is increased until a sufiicient bulge height is produced to permit the determination of the drawability index. This condition is indicated when bulge height increases continue without further increase in pressure.

By employing these simple measurements the biaxial instability point which is the onset of failure is accurately located. This point is located by plotting increments of pressure against increments of bulge height and obtaining a curve, the upper terminal portion of which has a parabolic configuration represented by the mathematical expression Where:

p pressure in pounds per square inch, h=bulge height in inches, and C C C =constants characteristic of metal tested.

In accordance with this invention only this portion of the bulge curve is used for evaluating the drawability of the sheet metal being investigated.

By differentiation, the above formula becomes The maximum was obtained by setting this equal to zero:

The value of the maximum read on the h axis was found by solving for H, where H is the drawability index.

In practice, in order to evaluate the constants, three bulge points are used within the parabolic range; one being near the highest part of the curve, another being at the beginning of the parabolic section of the bulge curve, and still another being intermediate the first and second points.

The correesponding pressure values are read from the actual bulge data and these three sets of points substituted into three parabola equations having three unknown constants, C C and C The H value is then determined by the suitable computation.

It is apparent in determining the H value that the gauge of the metal specimen which is being investigated has an effect. In order to correlate bulge data it is preferred that standard gauge be selected as the norm and compensation be made for any variations from this gauge by the samples which are tested. In establishing an illustrative procedure for employing the instant invention a gauge thickness which approximates the thickness of steel sheet metal employed in a wide variety of drawing operations was selected, namely, 0.050. In setting up the measuring instrument this thickness is established as the standard and any deviation from this gauge, either above or below, is taken into consideration in the calculations for determining the H value by appropriate corrections. In applying this correction it has been found for sheet steel specimens that a correction of 5.636 inches per inch of gauge is to be added to the H value determination if the gauge is less than the standard or subtracted from the H i value if the gauge value is greater than the standard. This correction of 5.636 inches per inch of gauge was determined empirically by correlating the efiect of gauge on H value for a number of samples varying in gauge from 0.010" to 0.060. Accordingly, if the instant invention is to be employed in evaluating the drawability of metals other than steel an appropriate gauge correction will have to be experimentally determined by investigating the effect of gauge of the sample metal on H value and calculating an appropriate correction which can be applied to the specific metal involved.

For example, in determining the drawability index of 2 killed, low carbon, aluminum deoxidized steel sheet 0.046 inch thick, a disc having a diameter of about 14 inches was prepared. The bulge tester used had a base member having an inner chamber 10 inches in diameter and 1% inches deep. The bottom was inch thick and the sidewall was 2% inches high and 6 inches thick. The upper surface of the side wall was undercut inch leaving a inch thick portion of the surface to provide a recessed shoulder enclosed by the outer surface of the sidewall. Clamping ring, /16" thick, having a 10 inch I.D., 14 /8 inch 0.1)., was used to retain the test specimen securely during testing. To prevent slippage a non-skid surface was provided on the cooperating faces of the ring and base. Both the base and clamping ring were fabricated from A.I.S.l. 1065 steel. To hold the clamping ring in place ten segmental clamps were used. Two holes were provided in each segmental clamp to receive 1 inch cap screws which engage with nuts mounted in recesses in the base. After the test specimen was put in place and the apparatus assembled the cap screws were driven to a tightened condition by a pneumatic Wrench. Hydraulic pressure was applied to the test specimen by pumping a hydraulic oil into the inner chamber.

Pressure (p) versus height (11) readings were taken every 10 pounds incremental of pressure change and the following bulge data obtained:

BULGE DATA The data were graphically correlated by plotting p as the ordinate v. h as the abscissa.

Three sets of points were chosen from the parabolic portion of the bulge curve, viz.

=7s3.0 h=3.0 p=707.5 h=2.6 p=625.0 h=2.2

which were substituted in the general parabolic formula. To each of the 11 readings is added the difference in gauge between the standard of 0.050 and the gauge of the sample, i.e., 0.046".

This was solved for H value by a determinant The empirical gauge correction was determined 5.636 inches/inch of gauge .004=0.02

and the corrected H value established STANDARD S Breakage for Min. U H! I percent Part Minimun It H7 1 Plenum Side Panel-Automobile body Instrument Panel-Automobile body Rear Quarter Side Outer-Automobile body Blower Housing Trailer Bathtub Refrigerator Door Liner Oil Pan-Automobile engine Rear Quarter Outer-Automobile body Door Outer-Automobile body It has been found that the best performing steels had the highest bulge value in comparable performance tests. From these tests it was concluded that (1) The drawability index is sensitive enough to discriminate between small difierences in drawing quality.

(2) Drawability index test values are completely reliable.

(3) The Olsen and Rockwell tests are not sensitive enough to discriminate between small or moderate differences in drawing quality.

(4) Drawability index standards for specific parts can be conveniently established for control purposes.

Although the foregoing description of the instant invention is concerned with the testing of steel, other drawable metals such as aluminum, brass, titanium, copper, and others can also be evaluated using the process of this invention. To provide accurate data for steel specimens the diameter of the effective area of the test speciment which is to be bulged should be 6 to 12 inches for example, using gauges of 0.01 to 0.10 inch for example. The size of the specimen is selected to permit the use of convenient pressures and bulge heights. Accordingly, other diameters and gauges can be used for other materials if desired or needed.

In order to facilitate carrying out the determination of the drawability index other apparatus embodying the characteristics of the illustrated device can be used. It is especially helpful if quick acting clamps of suitable elfectiveness be used for clamping the clamping ring which holds the specimen in place. The test can also be made more routine by utilizing conventional analog or digital computers for determining the values of the constants C in the parabolic equation for the selected parabolic portion of a specific bulge curve. For a given set of conditions for the metal specimen, viz, gauge and diameter, it has been found that the parabolic portion of the curve vegin at a substantially constant height. For example, the parabolic section of the curve for 10 inch diameter sheet steel specimens having a gauge in the vicinity of 0.050" begins at a bulge height of about 1.8 inches. Accordingly, by the use of standard test conditions and employing appropriate pressure and height measuring instruments suitable electrical signals can be generated at the three preselected points on the parabolic portion of the curve and the output applied to a suitable automatic computer. This would permit the desired value to be automatically and rapidly obtained and would increase the effectiveness of the invention as a quality control device. In installations employing computers a suitable electronic digital computer which can be employed is the Alwac Model III computer maunfactured and sold by the Alwac Division of El-Tronics, Inc., Redondo Beach, California.

It is apparent from the foregoing discussion that a numer of variations and modifications can be made by those skilled in this art Without departing from the scope of this invention.

What is claimed is:

1. In an apparatus for determining the drawability in dex of a specimen sheet metal disk, a rigid base member being provided With an open-face, internal cavity, an upstanding wall on said base member surrounding said cavity, said Wall being of sufficient thickness to withstand hydraulic pressures encounter in determining said drawability index without distortion, said wall having a recessed portion in the upper face thereof contiguous with the edge of said wall surrounding said cavity, said recessed portion having a non-skid marginal portion adjacent the aforesaid edge, a specimen locking ring comprising a plate adapted to be disposed within said recessed portion and having a curved inner opening with a peripheral configuration similar to the peripheral outline of said cavity, the bottom of said plate having a non-skid marginal portion adjacent the edge of the inner opening cooperating with said non-skid surface on said recessed portion to securely hold a sheet metal specimen clamped therebetween, the thickness of the non-skid marginal portion of said ring being greater than the thickness of the remaining portion of said ring, clamping means resting on said upstanding Wall and said specimen locking ring for securely clamping said ring in said recessed portion, means for applying a clamping force to said clamping means, and means for introducing a hydraulic fluid into said cavity.

2. In an apparatus for determining the drawability index of a specimen sheet metal disk, a rigid base member being provided with an open-face, centrally disposed, circular internal cavity, an upstanding Wall on said base member surrounding said cavity, said wall being of sufficient thickness to Withstand hydraulic pressures encountered in determining said drawability index without distortion, said Wall having a recessed portion in the upper face thereof contiguous with the edge of said wall surrounding said cavity, said recessed portion having a nonskid marginal portion adjacent the aforesaid edge, a specimen locking ring comprising a plate adapted to be disposed within said recessed portion and having a circular opening with a diameter not greater than the diameter of said cavity, the bottom of said plate having a non-skid marginal portion adjacent the edge of the inner opening cooperating with said non-skid surface on said recessed portion to securely hold a sheet metal specimen clamped therebetween, the thickness of the non-skid marginal portion of said ring being greater than the thickness of the remaining portion of said ring, clamping means resting on said upstanding wall and said specimen locking ring for securely clamping said ring in said recessed portion, means for applying a clamping force to said clamping means, and means for introducing a hydraulic fluid into said cavity.

3. In an apparatus for determining the drawability index of a specimen sheet metal disk, a rigid base member being provided with an open-face, centrally disposed, circular internal cavity, an upstanding wall on said base member surrounding said cavity, said wall being of sufficient thickness to Withstand hydraulic pressures encountered in determining said drawability index without distortion, said wall having a recessed circular portion in the upper face thereof contiguous with the edge of said Wall surrounding said cavity, said recessed portion having a non skid marginal portion adjacent the aforesaid edge, an annular, specimen locking ring comprising a plate adapted to be disposed within said recessed portion and having a circular opening with a diameter not greater than the diameter of said cavity, the bottom of said plate having a non-skid marginal portion adjacent the edge of the inner opening cooperating with said non-skid surface on said recessed portion to securely hold a sheet metal specimen clamped therebetween, the thickness of the non-skid marginal portion of said ring being greater than the thickness of the remaining portion of said ring, clamping means resting on said upstanding wall and said specimen locking ring for securely clamping said ring in said recessed portion, means for applying a clamping force to said clamping means, and means for introducing a hy draulic fluid into said cavity.

4. In an apparatus for determining the drawability index of a specimen sheet metal disk, a rigid base member being provided with an open-face, centrally disposed, circular internal cavity, an upstanding wall on said base member surrounding said cavity, said wall being of suflicient thickness to withstand hydraulic pressures encountered in determining said drawability index Without distortion, said wall having a recessed circular portion in the upper face thereof contiguous with the edge of said wall surrounding said cavity, said recessed portion having a non-skid marginal portion adjacent the aforesaid edge, an annular, specimen locking ring comprising a plate adapted to be disposed within said recessed portion and having an outer diameter substantially less than the diameter of said recessed portion, and having a circular opening with a diameter not less than the diameter of said cavity, the bottom of said plate having a non-skid marginal portion adjacent the edge of the inner opening cooperating with said non-skid surface on said recessed portion to securely hold a sheet metal specimen clamped therebetween, the thickness of the non-skid marginal portion of said ring being greater than the thickness of the remaining portion of said ring, clamping means resting on said upstanding wall and said specimen locking ring for securely clamping said ring in said recessed portion, means for applying a clamping force to said clamping means, and means for introducing a hydraulic fluid into said cavity.

5. In an apparatus for determining the drawability index of a specimen sheet metal disk, a rigid base member being provided with an open-face, centrally disposed, circular internal cavity, an upstanding wall on said base member surrounding said cavity, said wall being of sufiicient thickness to withstand hydraulic pressures encountered in determining said drawability index without distortion, said wall having a recessed circular portion in the upper face thereof contiguous with the edge of said wall surrounding said cavity, said recessed portion having a non-skid marginal portion adjacent the aforesaid edge, an annular, specimen locking ring comprising a plate adapted to be disposed within said recessed portion and having an outer diameter substantially less than the diameter of said recessed portion, and having a circular opening with a diameter substantially equal to the diameter of said cavity, the bottom of said plate having a non-skid marginal portion adjacent the edge of the inner opening cooperating with said non-skid surface on said recessed portion to securely hold a sheet metal specimen clamped therebetween, the thickness of the non-skid marginal portion of said ring being greater than the thickness of the remaining portion of said ring, clamping means resting on said upstanding wall and said specimen locking ring for securely clamping said ring in said recessed portion, comprising a plurality of segmental plates forming an annular ring congruent with the face of said upstanding Wall, means for applying a clamping force to said clamping means, comprising threaded fasteners securing said segmental plates to said wall and means for introducing a hydraulic fluid into said cavity.

6. An apparatus for determining the drawability index of a specimen sheet metal disk which comprises a rigid base member being provided with an open-face internal cavity, an upstanding wall on said base member surrounding said cavity, said wall being of sufficient thickness to withstand hydraulic pressures encountered in determining said drawability index without distortion, said wall having a recessed portion in the upper face thereof contiguous with the edge of said wall surrounding said cavity, said recessed portion having a non-skid marginal portion adjacent the aforesaid edge, a specimen locking ring comprising a plate adapted to be disposed within said recessed portion and having a curved inner opening with a peripheral configuration similar to the peripheral outline of said cavity, the bottom of said plate having a nonskid marginal portion adjacent the edge of the inner opening cooperating with said non-skid surface on said recessed portion to securely hold a sheet metal specimen clamped therebetween, the thickness of the non-skid marginal portion of said ring being greater than the thickness of the remaining portion of said ring, clamping means resting on said upstanding Wall and said specimen locking ring for securely clamping said ring in said recessed portion, means for applying a clamping force to said clamping means, means for introducing a hydraulic fluid into said cavity, means for sensing the deformation of said clamped metal specimen, means for indicating said deformation, and means for measuring the pressure of said hydraulic fluid.

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