Abstract:
A rivet configured to provide a strengthened rivet joint capable of acting as a load-bearing joint, such as in a vehicle, is provided. A method of joining a multiple member work-piece utilizing the rivet is also provided. The rivet includes a head portion and an annular body portion extending from the head portion. The body portion has an open end opposite the head portion. The head portion and the annular body portion define an interior cavity that opens at the open end. The annular body portion has an inner surface at the interior cavity and an outer surface opposite the interior cavity. The body portion has passages extending through the body portion from the inner surface to the outer surface, and grooves running along the outer surface intersecting with the passages.

Description:
TECHNICAL FIELD 
     The invention relates to a self-piercing rivet and a method of joining members using the rivet. 
     BACKGROUND OF THE INVENTION 
     Self-piercing rivets are fastening mechanisms that have been widely used for the joining of non-ferrous materials and have demonstrated capability in joining ferrous materials as well. Self-piercing rivets join overlapping members by driving the rivet under pressure into the members, and creating a mechanical interference between the members and the rivet by using a die to modify the rivet geometry. 
     SUMMARY OF THE INVENTION 
     A riveted joint may be subjected to periodic loading which may cause long term loosening and incremental local deformations that reduce the direct contact between the rivet and the joined members which is integral to maintaining the joint strength. Additionally, if the materials of the joined members are dissimilar, such as with a ferrous and a non-ferrous metal, corrosion may occur if a gap is formed due to loosening of the joint. Thus it would be desirable to further strengthen and seal a riveted joint by combining the mechanical strength of the riveted structure with bonding between the rivet and the joined members. 
     A rivet configured to provide a strengthened rivet joint capable of acting as a load-bearing joint, such as in a vehicle, is provided. A method of joining a multiple member work-piece utilizing the rivet is also provided. The rivet includes a head portion and an annular body portion extending from the head portion. The body portion has an open end opposite the head portion. The head portion and the annular body portion define an interior cavity that opens at the open end. The annular body portion has an inner surface at the interior cavity and an outer surface opposite the interior cavity. The body portion has passages extending through the body portion from the inner surface to the outer surface, and grooves running along the outer surface intersecting with the passages. Thus, when adhesive is stored within the interior cavity, at an appropriate viscosity, the adhesive will flow through the passages and along the grooves when pressure is applied to the adhesive through the open end. When the rivet interlocks the work-piece, the adhesive will create a bond between the rivet and the members of the work-piece, strengthening and sealing the joint. 
     A method of joining first and second members, which is especially useful if the members are dissimilar materials, includes providing the rivet described above, driving the rivet into the first and second members to establish a riveted joint, thereby forcing the adhesive through the passages to the grooves to bond the riveted joint. The method may include heating the adhesive prior to forcing the adhesive through the passages, in order to establish an appropriate viscosity to allow such flow. For example, the heating may be resistance heating via an electrical current running through the die used to drive the rivet. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective illustration of a rivet; 
         FIG. 2  is a schematic cross-sectional illustration of the rivet of  FIG. 1  taken at line  2 - 2  shown in  FIG. 1  showing adhesive stored in the rivet and passages and grooves in the rivet; 
         FIG. 3  is a schematic plan view of the rivet of  FIGS. 1 and 2 ; 
         FIG. 4  is a schematic perspective view of a work-piece of stacked metallic sheets; 
         FIG. 5  is a schematic cross-sectional view of the rivet of  FIGS. 1-3  interlocking and bonded to the sheets of  FIG. 4  with the adhesive bonding the outer surface of the rivet to the sheets; 
         FIG. 6  is a schematic cross-sectional view of a die set and rivet assembly clamping the sheets, which are shown in fragmentary cross-sectional view; 
         FIG. 7  is a schematic cross-sectional view of the die set of  FIG. 6 , with an upper punch causing the rivet to pierce the sheets; 
         FIG. 8  is a schematic cross-sectional view of the die set and rivet assembly of  FIGS. 6-7 , with an electric current applied to the upper punch and lower die to heat the adhesive; 
         FIG. 9  is a schematic cross-sectional view of the die set and rivet assembly of  FIGS. 6-8 , with a movable die portion of the lower die pressing upward to force or expel some of the adhesive from the interior cavity through the passages and grooves of the rivet; 
         FIG. 10  is a schematic cross-sectional view of the die set and rivet assembly of  FIGS. 6-9  with the movable die portion, the punch and the clamp withdrawn to allow release of the riveted sheets; 
         FIG. 11  is a flow diagram illustrating a method of joining the rivet of  FIG. 1  to the sheets of  FIG. 4  using the die set of  FIGS. 6-10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,  FIG. 1  shows a rivet  10  with a head portion  12  and a generally cylindrical hollow body portion  14 , also referred to as a shank portion. The body portion  14  is tapered to a piercing edge  16 , shown in  FIG. 2 , that is sufficiently sharp to pierce a work-piece  18  of  FIG. 4  when subjected to a driving force, such as may be achieved by a punch acting on the head  12 , in order to mechanically interlock a top sheet  20 , also referred to as a second member, and a bottom sheet  24 , also referred to as a first member, of the work-piece  18 . The shank  14  pierces and enters all of the sheets, passing through the top sheet  20 , but does not completely pass through bottom sheet  24 . The shank  14  and the material of bottom sheet  24  immediately adjacent to shank  14  are deformed through the action of a lower die  32  in order to mechanically interlock the two sheets  20 ,  24 . The rivet  10  is referred to as a “self-piercing” rivet, as the piercing edge  16  is sufficiently sharp to penetrate the sheets  20 ,  24 , under the force of a die set, and the body portion  14  is configured to deform to create a mechanical joint or interface, without requiring a lead hole for the rivet  10  in the sheets  20 ,  24 . As shown in  FIG. 5 , the rivet  10  is forced through the top sheet  20  and only penetrates but does not pass through the bottom sheet  24 . It should be appreciated that more than two sheets may be stacked together; in any such embodiments, the rivet pierces and passes through all sheets except for the bottom sheet, which it only penetrates, but does not pass completely through. 
     The rivet  10  is configured to enable the rivet  10  and sheets  20 ,  24  to be bonded to one another at the outer surface  25  of the rivet  10 . Specifically, the annular body portion  14  and head portion  10  form an interior cavity  28  surrounded by an interior surface  30  of the body portion  14 . The body portion  14  has an open end  32  at the piercing edge  16 . Substantially equally spaced passages  34  extend through the body portion  14  from the interior surface  30  to the outer surface  25 . As best seen in  FIG. 3 , the passages  34  are spaced about the circumference of the cylindrical body portion  14 . At the outer surface  25 , similarly spaced grooves  36  extend substantially from the head portion  12  to the piercing edge  16 . The grooves  36  may be U-shaped or squared channels that appear as recesses in the surface  25 , or may have any other suitable cross-sectional shape. The grooves  36  intersect with the passages  34 ; that is, each passage  34  empties into a different one of the grooves  36  at the surface  25 . 
     An adhesive  40  is stored within the cavity  28  and is of a viscosity that will cause it to remain within the cavity  28  when the surroundings are within a predetermined temperature range typical of pre-riveting conditions, i.e., during shipping, and storage of the rivet  10 . However, when subjected to the joining methods illustrated in  FIGS. 6-10  and in the flow diagram of  FIG. 11 , the adhesive  40  is forced through the passages  34  and along the grooves  36  to bond the outer surface  25  of the rivet  10  to the members  20 ,  24 , thus bonding the mechanical joint. Some of the adhesive remains in the interior cavity  28  and bonds the inner surface  30  to the members  20 ,  24 , as well. 
     Referring to  FIG. 6 , the stacked sheets  20 ,  24  are rested on a lower composite die  42  of a die set  44 . The lower composite die  42  has a stationary annular outer portion  46  and a movable die portion  48  within the annular portion  46 . The die set  44  also includes an upper die  50  that has a clamp  52  and a punch  54  movable within the clamp  52 . A downward clamping force F 1  clamps the sheets  20 ,  24  between the dies  42 ,  50 . The rivet  10  of  FIGS. 1-3  is positioned within a die cavity  56  and the punch  54  is driven downward by a force F 2  to pierce the stacked sheets  20 ,  24 , as illustrated in  FIG. 7 , until the upper surface of the top sheet  20  contacts the adhesive  40 , causing a rapid increase in force F 1 . The rapid increase in force F 1  is monitored, and acts as a signal to cease driving. The increase in force F 1  is independent of the length of the rivet  10 , the volume of adhesive  40 , the nature of the adhesive  40 , etc., and so serves as a reliable indicator of the appropriate time to cease driving the rivet  10 , so that the adhesive  40  may be heated prior to being forced from the interior cavity  28 . Deformation of the top sheet  20  creates a slight dome on the upper surface thereof within the cavity  28 , permitting access for the adhesive  40  to later flow outward through channels  34  (numbered in  FIG. 2 ). 
     Referring to  FIG. 8 , an electric circuit  60  is created when a normally open switch  62  is closed (closed position shown in phantom) to allow current from a power supply  64  to flow through the punch  54  and the movable die portion  48  that is in contact with the adhesive  40 , as well as through the rivet  10  and sheets  20 ,  24 . (If a non-conductive rivet is used, the circuit may be modified to allow current flow that bypasses the rivet  10 , but that generates heat sufficient to heat the adhesive  40 .) The punch  54  is electrically insulated from the clamp  52  by isolator  65 . The movable die portion  48  is electrically insulated from the annular portion  46  by isolator  67 . The current causes resistive heating of the rivet  10 , leading to conductive heating of the adhesive  40 . At an elevated temperature, the adhesive  40  has a lower viscosity. Thus, after a predetermined amount of time or when the temperature of the die portion  48  reaches a predetermined temperature, the current is ceased by opening the switch  62 . The downward force of the punch  54  is then resumed. Simultaneously, the movable die portion  48  is moved upward in the cavity  28  of the rivet  10  with a force F 3 . The opposing forces on the rivet  10  causes the sheets  20 ,  24  to be drawn about the shape of the die portion  48  and the body portion  14  of the rivet  10  to deform outward and interlock the sheets  20 ,  24 , as illustrated in  FIG. 8 , while adhesive  40  is forced out of the cavity, through the passages  34  and along the grooves  36 , as illustrated in  FIG. 9  and best shown in  FIG. 5 , to bond the sheets  20 ,  24  at the rivet  10 , including at the outer surface  25 . The forces are controlled so that the rivet  10  passes into but not through the bottom sheet  24 . 
     The clamp  52  and punch  54  are then withdrawn with forces F 4 , F 5 , respectively, and the movable die portion  48  is withdrawn with a force F 6 , as illustrated in  FIG. 10 . When the adhesive  40  is sufficiently cured at the bonded outer surface  25  of the rivet  10 , the assembled sheets  20 ,  24  with bonded rivet  10  are then withdrawn from the die set  44 . 
     Referring to the flow diagram of  FIG. 11 , a method  100  of joining a multiple member work-piece is illustrated and is discussed herein with respect to the embodiments of  FIGS. 1-10 , although other embodiments may be used within the scope of the claimed invention. The method  100  includes step  102 , stacking a first member  24  and a second member  20  of a work-piece  18  on a lower composite die  42 . Additionally, under step  104 , a rivet  10  is provided with adhesive  40  stored in an interior cavity  28 . The rivet  10  has an outer surface  25  with grooves  36  and has passages  34  leading from the interior cavity  28  to the grooves  36 . Under step  106 , the stacked members  20 ,  24  are clamped between an upper die  50 , which includes a clamp  52 , and the annular outer portion  46  of the lower die  42 . 
     With the stacked members clamped, the method progresses with step  108 , in which the rivet  10  is driven into the sheet  20  until the adhesive  40  contacts the upper surface of sheet  20 , causing the rapid increase in force F 1  discussed above. At this point, the rivet  10  extends through the top sheet  20  and may or may not penetrate the bottom sheet  24 , depending on the length of the body portion  14 , thickness of the sheet  24 , etc. but does not go through the bottom sheet  24 . Next, under step  110 , the adhesive  40  is heated using electric current running through the punch  54  and through the movable die portion  48  in contact with the adhesive  40 . Once the adhesive  40  is heated to establish a sufficiently low viscosity, then, in step  112 , punch  54  and movable die portion  48  are simultaneously forced toward the rivet  10 , thereby deforming the rivet  10  to interlock the sheets  20 ,  24  and forcing the adhesive  40  through the passages  34  and along the grooves  36  of the outer surface  25  to bond the sheets  20 ,  24  to the outer surface  25  of the rivet  10  as well. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.