Abstract:
The disclosure relates to pad printing of video targets on a specimen for tensile or similar testing by a video extensometer. The pad printer may include various improvements such as pneumatic pistons to vary the pad spacing; pad supports which vary their spacing between ink pick-up and printing thereby allowing for a range of printing for a given cliché or printing plate; automatic proportional gauge length calculation; and a combination of a mechanical boss and a magnetic plate to secure the printing pad to the pad carrier.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention pertains to the use of a pad printer to place video targets on a sample for strain measurement by a video extensometer or similar device. 
         [0003]    2. Description of the Prior Art 
         [0004]    Video extensometers are disclosed in patent application Ser. No. 10/683,072 entitled “Testing of Samples”, filed on Oct. 10, 2003 and patent application Ser. No. 11/410,686 entitled “Testing of Samples”, filed on Apr. 24, 2006. These devices generate stress/strain curves for a tested sample. In order to measure the strain, which is calculated through the percentage of linear deformation of the sample under testing, video targets, such as two dots, are placed on the sample. The extensometer uses video methods to determine the change in distance between the targets during testing, thereby calculating the strain. As the cross-sectional area of the target is calculated prior to testing and the force applied to the target is recorded, the resulting stress can be correlated with the strain to generate a traditional stress/strain curve. 
         [0005]    However, in the prior art, the generation of the video targets has been problematic. Manual use of a marking pen to apply the video targets does not have uniform results. Likewise, the use of punched adhesive backed paper to supply the dots is problematic in that the dots may not adhere properly to round or other than flat specimens, may require a considerable application force which may damage certain specimens, may interfere with the test results of thin films in that the rigidity of the dots may artificially increase the rigidity of the sample, and may not adhere evenly to a specimen as it stretches thereby interfering with the strain measurement. Additionally, the dots may fly from the specimen during testing, particularly when the specimen breaks. Dots within the line of sight of the extensometer may cause erroneous test results. At a minimum, such errant dots may clutter the testing area. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0006]    It is therefore an object of the present invention to provide improvements in the marking of video targets on specimens for strain measurements. 
         [0007]    This and other objects are attained by providing a pad printing device for the application of video targets on specimens for strain testing. Additionally, such improvements are attained by providing pneumatic pistons on the pad supports to automatically change the spacing of the pads thereby changing the spacing of the video targets; by providing pad supports which change spacing between the ink pick-up and the ink application thereby providing different video target spacing for a single spacing of ink wells in the printing plate (i.e., cliché) of the pad printer; by providing for automatic gauge length calculation; and by providing a snap-in magnetic configuration for securing the printing pads. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Further objects and advantages of the invention will become apparent from the following description and from the accompanying drawings, wherein: 
           [0009]      FIG. 1  is a perspective view of a video extensometer performing stress/strain testing on a sample which includes video targets. 
           [0010]      FIG. 2  is a plan view of a typical testing specimen with video targets. 
           [0011]      FIG. 3  is a perspective view of a first embodiment of a pad printing device of the present invention. 
           [0012]      FIG. 4  is a perspective view of a second embodiment of a pad printing device of the present invention. 
           [0013]      FIG. 5  is a plan view of a cliché or printing plate for the pad printer of the present invention, showing typical spacing of the ink wells for the printing of video targets. 
           [0014]      FIG. 6  is a schematic of an example of a gauge length pattern of dots on the cliché of  FIG. 5 , including typical dimensions. 
           [0015]      FIG. 7  is a schematic showing the pistons to change the spacing of the printing pads between the ink pick-up and the printing. 
           [0016]      FIG. 8  is a schematic of showing the apparatus for automatic proportional gauge length spacing of the video targets. 
           [0017]      FIG. 9  is an exploded perspective view, partially in phantom, of the magnetic support configuration for the printing pads. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring now to the drawings in detail, wherein like numerals indicate like elements throughout the several views, one sees that  FIG. 1  illustrates a specimen  100  undergoing stress/strain analysis by a video extensometer  200 , such as that disclosed in patent application Ser. No. 10/683,072 entitled “Testing of Samples”, filed on Oct. 10, 2003 and patent application Ser. No. 11/410,686 entitled “Testing of Samples”, filed on Apr. 24, 2006. 
         [0019]    Specimen  100  (shown in detail in  FIG. 2  and discussed below) is held between two sample holders or grips  202 ,  204  which typically exert a pulling force on the specimen to perform tensile testing. By dividing the pulling force by the cross-sectional area of the central part (i.e., the smallest cross-sectional area) of the specimen  100 , the stress is calculated. The measurement of this force and of the cross-sectional area, as well as the calculations, are straightforward. In order to calculate the strain on the portion of interest of the specimen, corresponding to the stress, the relative elongation of the portion of interest of the specimen must be measured. Video targets  102 ,  104  are placed on the portion of interest of the specimen so that the video extensometer  200  can detect the relative deformation of the portion of interest of the specimen  100 . 
         [0020]    A typical shape of the specimen  100  is illustrated in  FIG. 2 . Specimen  100  is typically formed from a planar material of relatively uniform thickness. Specimen  100  includes two ends  106 ,  108  with increased width in order to be engaged firmly by sample holders or grips  202 ,  204 . A central section  110  is formed between two ends  106 ,  108 . Central section  110  typically includes a section with parallel edges and the least width of the specimen  100 , so that relatively uniform stress can be applied to a given section of minimum cross section (and hence maximum stress). Video targets  102 ,  104  are typically placed on central section  110 . 
         [0021]    A first embodiment of a pad printer  10  for the printing of video targets  102 ,  104  is shown in  FIG. 3 . A ground engaging base  12  holds a locating device  14  for positioning specimen  100  for printing and further provides a support for cliché or printing plate  16 . Base  12  further engages support  18  which engages the proximal ends of rails  20 ,  22  and from which rails  20 ,  22  extend. Stop  24  is formed at the distal ends of rails  20 ,  22 . Reciprocating head  30  includes vertical passageways  27 ,  29  through which vertical rails  31 ,  33  pass to provide for vertical movement of carrier plate  32 . This vertical movement can be provided electromechanically or can be provided manually through handle  37  which engages vertical rails  31 ,  33  and pivots at a distal end of pivot extension  39  which extends from reciprocating head  30 . 
         [0022]    Reciprocating head  30  further supports ink cup shaft  35  for inverted ink cup  34 . Inverted ink cup  34  includes magnets to urge the ink cup  34  against the cliché or printing plate  16  and prevent leakage of the ink as the ink cup  34  is moved across the cliché or printing plate  16  by ink cup shaft  35 . These magnets further eliminate the need for ink cup shaft  35  to provide a vertical pressing force to the inverted ink cup  34  as the ink cup  34  is moved across the cliché or printing plate  16 . Pad carriers  36 ,  38  are attached to the underside of carrier plate  32 . Printing pads  40 ,  42  are attached to pad carriers  36 ,  38 . The central portions of printing pads  40 ,  42  are spaced apart by a distance equal to the desired distance between the video targets. Printing pads  40 ,  42  are typically sufficiently soft to conform to any round or non-flat contours of the object receiving the printed pattern. 
         [0023]    As is known in the art of pad printing, cliché or printing plate  16  includes a pattern of indentations  50  (see  FIGS. 5 and 6 ). These indentations  50  collect ink from the inverted ink cup  34  as the inverted ink cup  34  is magnetically urged against the cliché or printing plate  16  during each printing cycle. More particularly, the reciprocating head  30  travels from the distal end of rails  20 ,  22  (the illustrated position in  FIG. 3 ) to the proximal end of rails  20 ,  22  thereby moving the inverted ink cup  34  away from the ink-filled indentations  50  and positioning printing pads  40 ,  42  for vertical movement thereby dipping into indentations  50  and drawing ink therefrom. The reciprocating head  30  then travels back to the distal end of rails  20 ,  22  thereby moving the inverted ink cup  34  over indentations  50  in order to replenish the ink in indentations  50  and positioning printing pads  40 ,  42  for vertical movement thereby printing video targets  102 ,  104  onto specimen  100 . 
         [0024]      FIG. 5  shows a typical pattern of indentations  50  on cliché or printing plate  16  for the printing of video targets  102 ,  104 , while  FIG. 6  gives the dimensions for various typical axial gauge length and transverse gauge length for the printing of video targets  102 ,  104 . Transverse gauge length video targets may be printed by a single printing pad in a central location on the pad printer  10 . 
         [0025]      FIG. 4  shows a second embodiment of the pad printer  10 , with similar elements and further including track  44  allowing for the adjustment of the position and spacing of pad carriers  36 ,  38  and hence printing pads  40 ,  42 . This allows for the automated changing of the spacing or separation of the printing pads  40 ,  42  by the use of pneumatics or motors and is shown in more detail in  FIG. 7 . Solenoid valve  50 , in response to electronic input from line  52 , activates pistons  54 ,  56  to move the positions of printing pads  40 ,  42 , so that various gauge lengths can be achieved, including the illustrated G1 and G2 gauge lengths. Additionally, this configuration can be used to change the spacing of the printing pads  40 ,  42  between the ink pick-up and the printing steps. This is very useful in that different spacing of the printed video targets  102 ,  104  can be achieved without the need to form a new set of indentations  50  in the cliché or printing plate  16  and likewise without the need to partially disassemble pad printer  10  to install a new cliché or printing plate  16  when a different gauge length is needed. More specifically, the indentations  50  can be formed at a first gauge length and the printing pads  40 ,  42  are set to this gauge length during the ink pick-up step, and then the pistons  54 ,  56  can change the spacing of the printing pads  40 ,  42  to a second gauge length prior to the printing of the video targets  102 ,  104 . This is particularly useful when gauge lengths are desired which are wider than the cliché  16  or the ink cup  34 . 
         [0026]    A similar embodiment is disclosed in  FIG. 8  which allows automatic proportional gauge length testing that meets the requirements of metals testing per JIS Z 2201, ISO 6892 Annex D and others that require that the gauge length be proportional to the square root of the cross-sectional area of the specimen  100 . This is normally expressed as GL=k √A, where k is typically 5.65 (no units required for “k” as the square root of an area gives a linear distance). The value of cross-sectional area (A) can be determined using other instruments and methods known in the art and input to controller  70  through a keypad or similar device. Controller  70  generates input to drive and control box  72  which controls motor  74 . Motor  74  drives screw shaft  76  which includes one half left hand threads and one half right hand threads. Pad carriers  36 ,  38  are threadably mounted on screw shaft  76  whereby rotation of screw shaft  76  in a first direction causes pad carriers  36 ,  38 , and hence pads  40 ,  42 , to move toward each other (decreasing the spacing therebetween) and rotation of screw shaft  76  in a second direction causes pad carriers  36 ,  38 , and hence pads  40 ,  42 , to move away from each other (increasing the spacing therebetween). Footswitch  80 , or similar switch, is provided so that the user can initiate the desired operations. Limit switch  82  is provided to prevent movement of pad carriers  36 ,  38  beyond the operating limits of this embodiment and further acts as a reference or home position for the pad mechanism. 
         [0027]      FIG. 9  discloses a configuration for the attachment of a printing pad  40  to a pad carrier  36  in such a way that the connection is sufficiently strong for the operation of pad printing, while still allowing for the simple and fast manual (“snap-in”) installation or removal of the printing pad  40 . Printing pad  40  includes an upper metal plate  82  with a projecting male boss  84 . Pad carrier  36  includes a blind slot  86  of complementary size and shape to the projecting male boss  84 . The position of the projecting male boss  84  and the blind slot  86  can be reversed. Pad carrier  36  further includes magnets  87  to create a strong connection between the upper metal plate  82  and the pad carrier  36 . 
         [0028]    To use pad printer  10 , the user confirms that the appropriate cliché or printing plate  16  is installed and that the printing pads  40 ,  42  are properly spaced. Some embodiments require further programming or adjustment if the spacing of the printing pads  40 ,  42  will be adjusted between ink pick-up and printing. The user places specimen  100  into adjustable locating device  14  and initiates the process so that video targets  102 ,  104  are printed on the specimen  100  in preparation for subsequent tensile or similar testing. 
         [0029]    Thus the several aforementioned objects and advantages are most effectively attained. Although preferred embodiments of the invention have been disclosed and described in detail herein, it should be understood that this invention is in no sense limited thereby and its scope is to be determined by that of the appended claims.