Abstract:
A method of testing metal samples is provided to simulate stretch flanging of trimmed surfaces. A sample having planar sides and a sidewall extending between the planar sides is placed in a fixture so that the planar sides are confined. The sidewall of the sample is bent about a curved surface while restricting buckling out of a planar area. One testing machine is disclosed in which a test specimen is clamped in a die with a nesting slot while a punch bends a sample against a forming surface that has a radius. In another testing machine, a mandrel and clamping element engage the planar sides of the sample while first and second bending members engage first and second end portions to bend one of the sidewalls of the sample about the mandrel.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a testing method for determining the performance of metal sample in a stretch flanging after trimming operation. 
       BACKGROUND 
       [0002]    In sheet metal stamping operations on materials having reduced ductility, such as advance high strength steels, dual phase steel and aluminum alloys, one issue is that trimmed surfaces tend to split when they are formed into a flange. When a flange is formed on a contoured trimmed surface, the flange is stretched. Current testing methods have proven unreliable in predicting the formability of materials in stretch flanging operations. 
         [0003]    In the prior art, a hole expansion test has been used to predict formability. In the hole expansion test, a small diameter hole is formed in a sheet metal blank and then a larger diameter punch is driven through the smaller diameter hole to form flanges. The larger diameter punch is driven through the smaller diameter hole until a through thickness crack appears on the edge of the hole. A limiting hole expansion ratio is calculated based upon the original hole diameter and the expanded hole diameter. Alternatively, a limiting forming ratio may be determined as the ratio of the expanded hole diameter to the original hole diameter. 
         [0004]    Tensile testing of a sheared edge may be conducted on sample strips. Samples are trimmed to have a dog bone shape, half dog bone shape or straight sides. The samples are stretched in a direction parallel to the trimming line until failure. 
         [0005]    A high degree of correlation is not generally exhibited between the tensile test and the hole expansion test. The expansion test and tensile test methodologies do not provide reliable data on the ability of a particular material specimen to stretch in a specific direction across the width of the flange in a flanging operation. Stretching the flange in the flanging operation may cause significant strain localization in a fracture zone. 
         [0006]    Applicant&#39;s proposed test methodology has been developed to provide more reliable predictions of metal sample performance in a stretch forming operation that is performed after drawing and trimming operations. 
       SUMMARY 
       [0007]    In the proposed testing method, a sample strip of sheet material is bent in the plane of the strip about a radius that is a function of the stretch flanging radius and height of the flange to be formed in the flanging operation to be simulated. Out of plane bending of the strip is prevented by supporting both planar sides of the strip during the bending operation. Applicant&#39;s methodology can be applied to stretch flanging or stretch hemming operations and provide reliable data for predicting the performance of a sample in a stretch flanging operation. 
         [0008]    The test sequence may include the following steps:
       pre-stretching the sheet metal to simulate the strain applied to the metal in a preceding draw operation;   trimming strip samples to simulate conditions in a production tool;   marking the sample with a grid or series of marks at a known regular small interval;   bending the strip in plane with the trimmed edge corresponding to the edge undergoing maximum stretching;   measuring the distance between the interval markings near the fracture area to compare the measurements with analytical calculations and finite element measurement results;   repeating the test on samples cut in the longitudinal, transverse and 45° to determine the effect of sample orientation.       
 
         [0015]    The proposed test method may be used to provide a quick screening test in production stamping plants, especially where advanced high strength steels, dual phase steels and aluminum alloys are formed. The screening test can detect material property variation quickly before placing a coil or stack of material into production. By being better able to simulate the stretch flanging and stretch hemming stresses in the material, materials that lack desired properties or suffer from variation can be avoided. It may also be possible to provide a material specification that more closely predicts production performance. In addition, blank orientation may be optimized to prevent splits in stretch flanging and stretch hemming operations by aligning the stretch flanging direction with the direction of maximum formability for the trimmed surface. 
         [0016]    According to one aspect of the disclosure, a method of testing metal for a flanging operation includes providing a sample of metal having planar sides and a sidewall that extends between the planar sides. The planar sides of the sample are confined in a fixture to a planar area. The sidewalls of the sample are then bent about a curved surface while restricting buckling out of the planar area. 
         [0017]    According to other aspects of the method, the sample may be positioned in the confining step with the length of the sample being centered relative to the curved surface. A first lengthwise end and a second lengthwise end of the sample are engaged in the bending step to move the first and second ends to bend the sample about the curved surface. The curved surface preferably has a test radius that is a function of the stretch flanging radius and height of the flange to be formed in the flanging operation. 
         [0018]    According to other aspects of the disclosure, the step of confining the planar side of the sample may include clamping a first lengthwise end of the sample with two opposed side edges of the sidewall being clamped between a clamping member and a die that define a slot. The slot is slightly larger than the width of the sample so that the planar sides of the sample are restrained, but not clamped within the slot. The step of bending the sidewall of the sample may include engaging a second lengthwise end of the sample on one of the opposed side edges of the sidewall with a punch that bends the sample about a curved surface. 
         [0019]    Alternatively, the method disclosed may include the step of confining the planar sides of the sample by clamping a central portion of the planar sides of the sample with a portion of the sidewall adjacent to a mandrel. The central portion of the sample may be disposed between two spacing elements with first and second lengthwise ends of the sample extending from the spacing elements. The step of bending the sidewall of the sample may then include engaging the first and second lengthwise ends of the sample and moving both of the ends relative to the central portion. 
         [0020]    According to other aspects of the disclosure, a test machine is disclosed for testing the elongated planar metal sample for suitability for use in a flanging operation in which a flange is stretched while the flange is formed. The test machine may include a die that has a forming surface and a clamping member that cooperates with the die to clamp a first longitudinal end of the sample. The die and clamping member engage two opposed portions of a sidewall of the sample and may also define a nesting slot that confines the planar sides of the sample. A punch engages a third portion of the sidewall on the opposite end of the forming surface from the two opposed portions to bend the sample against the forming surface. The sidewall engaged by the clamping member is stretched while the punch bends the sidewall of the sample against the forming surface. 
         [0021]    The forming surface may have a radius that is a function of the stretch flanging radius and height of the flange to be formed in the flanging operation. The nesting slot may be defined by the die, the clamping member or by the combination of die and the clamping member. A clearance space may be defined between each of the planar sides of the sample and the nesting slot. 
         [0022]    An alternative embodiment of the test machine is also disclosed for testing an elongated planar metal sample for suitability for use in a flanging operation in which a flange is stretched while the flange is formed. The alternative embodiment of the test machine includes a mandrel and first and second clamping elements that engage opposite planar sides of the sample. The clamping elements engage an intermediate portion of the sample that is secured with the sidewall against the mandrel. First and second bending members engage end portions of the sidewall of the sample on opposite ends of the intermediate portion of the sample to bend the sample about the mandrel. The sidewall between the first end portion and the second end portion is stretched while the first and second bending members bend the sample. 
         [0023]    Additional aspects relating to the alternative embodiment of the test machine may include providing a mandrel with the first and second clamping elements comprising a plurality of washers assembled to the mandrel. The mandrel may then be secured to a fixture that retains the mandrel in a fixed location while the bending member bends the sample about the mandrel. The bending member may be clamps, a press apparatus, or a manual force that bends the sample around the mandrel. 
         [0024]    According to another aspect of the disclosure, the sample may be cut from a quantity of material that is to be used in the flanging operation. The above aspects and features of the disclosure will be better understood in view of the attached drawings and the following detailed description of the illustrated embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a diagrammatic cross-sectional view of a test machine with a sample retained in the machine prior to testing according to one embodiment of the present invention; 
           [0026]      FIG. 2  is a diagrammatic cross-sectional view taken along the line  2 - 2  in  FIG. 1 ; 
           [0027]      FIG. 3  is a diagrammatic elevation cross-sectional view of the test machine shown in  FIG. 1  after the test sample is bent over a forming surface that has a radius; 
           [0028]      FIG. 4  is a partially exploded diagrammatic perspective view of an alternative embodiment of a test machine with a test sample loaded prior to testing; 
           [0029]      FIG. 5  is a fragmentary diagrammatic perspective view of the embodiment of the test machine shown in  FIG. 4  after completion of the bending operation; and 
           [0030]      FIG. 6  is a perspective view of a test specimen after bending to simulate a stretch forming operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0032]    Referring to  FIG. 1 , a test machine  8  is shown with a test strip sample  10  loaded into the test machine  8  for testing. A first planar side  12  of the test strip sample  10  is shown to have a width b. The test strip sample  10  has a sidewall  14  that extends about the sample  10  between the first planar side  12  and a second planar side (not shown in  FIG. 1 ). A first portion  16  of the sidewall  14  is supported on a die  18 . A second portion  20  of the sidewall  14  is engaged by a clamp  22  that holds the test strip sample  10  in place during the test procedure. A punch  26  is driven into contact with the test strip sample  10  to form the test strip sample  10  about a forming surface  28  that has a radius. The forming surface  28  has a radius R that may either be a standard radius or a radius selected to correspond to a function of the stretch flanging radius and the height of the flange to be formed in the stretch flanging operation. 
         [0033]    A relief radius  30  may be formed on the punch  26 . Another relief radius  32  may be formed on the clamp  22 . The relief radii  30 ,  32  are intended to reduce any tendency of the punch  26  or clamp  22  from tearing the test strip sample  10  during the testing procedure. 
         [0034]    Referring to  FIGS. 1 and 2 , a punch clearance slot  36  is provided to receive the punch  26 . The test strip sample  10  is received in a nesting slot  38  that is provided to confine the first planar side  12  and a second planar sidewall  40  during the test procedure. In the test procedure, the punch  26  is driven into engagement with the test strip sample  10  and moved through the punch clearance slot  36  while the punch  26  bends the test strip sample  10  over the forming surface  28 . 
         [0035]    The test machine  8  is shown with the test strip sample  10  prior to bending. The first planar side  12  and second planar side  40  of the test strip sample  10  are shown with an exaggerated clearance between the test strip sample and the nesting slot  38 . A second portion  20  of the sidewall  14  is visible in  FIG. 2  that is engaged by the clamp  22  shown in  FIG. 1 . The die  18  defines the nesting slot  38 , but it should be understood that the nesting slot could also be provided in the clamp  22  or partially in the clamp  22  and the die  18 . The punch  26  is shown disposed above the punch clearance slot  36 . 
         [0036]    Referring to  FIG. 3 , the test machine  8  is shown to include the die  18  and the clamp  22 . The test strip  10  is partially retained on the first portion of the sidewall  16 , while a second portion of the sidewall  20  is engaged by the clamp  22 . In  FIG. 3 , the punch  26  is shown after engaging the test strip sample  10  (shown in  FIGS. 1 and 2 ). A bent sample  42  is formed after the test strip sample  10  is bent about the forming surface  28 . Fractures  44  may or may not be formed in the bent sample  42  depending upon the formability of the bent sample  42  after the simulated stretch flanging operation test. Fractures  44  tend to form in the part of the sidewall  14  that is opposite the portion of the sidewall  14  that engages the forming surface  28 . 
         [0037]    Referring to  FIG. 4 , an alternative embodiment of a test machine  48  is shown to include a mandrel  50 . The mandrel  50  may be a bolt, as illustrated, or may take another form. A first clamping element  52  is shown as a plurality of washers, but could take an alternative form, such as a cylindrical spacer. A second clamping element  54  is also shown as a plurality of washers. An intermediate portion  56  of a test strip sample  10  is clamped between the first and second clamping elements  52  and  54 . The mandrel  50  is supported in a fixture  58  that may be a vice or other type of fixture capable of holding the mandrel  50  in place while the test strip sample  10  is bent about the mandrel  50 . A first bending member  60  and a second bending member  62  are shown diagrammatically by arrows in  FIG. 4 . The bending members  60 ,  62  could be a pair of pliers or a fixture ram that exerts equal pressure on a first end portion  64  and a second end portion  68  of test strip sample  10 . 
         [0038]    Referring to  FIG. 5 , the test machine  48  is shown with a bent sample  42  after the bending test has been performed. The mandrel  50  is retained by the fixture  58  and the bent sample  42  is retained between the first clamping element  52  and the second clamping element  54 . The intermediate portion  56  of the bent sample  42  is bent by the force applied by the first bending member  60  and second bending member  62 , as shown in  FIG. 4 . The first end portion  64  and second end portion  66  of the bent sample  42  are shown while they are bent during the test procedure. 
         [0039]    Referring to  FIG. 6 , a bent test sample  44  is shown with the first end portion  64  and second end portion  66  being bent toward the same direction. The sidewall  14  is shown to include a stretched sidewall  68  with a plurality of small fractures  44 . 
         [0040]    According to the method of testing metal for a stretch flanging operation, the sample  10  is shown to have two planar sides  12  and  40  that have a sidewall  14  that extends between the planar sides. The sides  12 ,  40  of the sample  10  are retained in a fixture  8 ,  48  with the side being confined either in the nesting slot  38  or by the first and second clamping elements  52  and  54 . The sidewall  14  of the sample  10  is bent about a curved surface, such as the radius forming surface  28  or mandrel  50 . In-plane bending of the strip  10  is performed with the strip either being retained in a nesting slot  38  or between the first and second clamping elements  52  and  54 . Buckling of the sample  10  is restricted outside of the planar area defined by the nesting slot  38  or the first and second clamping elements  52 ,  54 . 
         [0041]    The samples  10  may be trimmed transversely across a coil of metal or longitudinally relative to the direction that the coil is rolled. The test specimens may be trimmed at a 45° angle or at another angle that corresponds to the orientation of the stretch formed flange in a production part. The test may be repeated for rolling, transverse and 45° test sample orientation to determine whether the sample orientation has any effect on the strip flanging test. 
         [0042]    The stretched portion of the sidewall  40  may be marked with a grid or other evenly spaced marking to permit measurement of the deformation of the bent sample  42 . After the bending operation, the distance between the elements of the grid or other markings may be measured for subsequent analytical calculations. 
         [0043]    The test can be used as a production test in stamping plants where advanced high strength steels, dual phase steels or aluminum alloys may be used to perform production parts. The test can determine the properties of material samples to provide evidence of property deviations from material specifications or to test for material quality variations. By testing the material prior to running a production unsuitable material may be returned to a material supplier and more detailed specification may be developed to assure the performance of difficult to form material in stretch flanging operations. In product design and development, samples may be tested that are taken with different elongation orientations, such as rolling, transverse, 45° orientations, or the like. Panels may be oriented for optimal performance during die design so that splits in stretch flanging operations may be avoided even when forming difficult to flange trimmed surfaces. 
         [0044]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.