Abstract:
A sheet metal assembly includes first and second metal sheets having flanges extending from the sheets. The flanges overlap and are in engaged parallel facing relation for a width between outer edges and flanges. At least one pair of self-piercing rivets joins the flanges at spaced locations along the width. The rivets each have a head and a shank extending into the flanges from opposite sides. The rivet heads of each pair are located in opposite flanges and are spaced further from the outer edges of their respective flanges (nearer the load-application ends) than are the shank portions of other rivets of the pair.

Description:
TECHNICAL FIELD 
     This invention relates to joints produced by the self-piercing riveting (SPR) process. More particularly, it relates to maximizing the strength of an SPR lap joint that consists of two rows of self-piercing rivets without increasing the width of the overlap region (flange width) or the quantity of material used in any manner. 
     BACKGROUND OF THE INVENTION 
     In self-piercing riveting (SPR), a tubular rivet made from a high-strength steel alloy is forced through a pair of partially overlapping sheets that are supported by a rigid circular die with an axisymmetric cavity. The diameter of the die and rivet are similar. The sheet material is typically an automotive aluminum alloy such as AA6111-T4 or AA5754-O. The joint is cold-formed with the rivet walls experiencing large amounts of compressive plastic deformation. The upper sheet is pierced through its entire thickness by the rivet, predominantly in shear, and the lower sheet is pierced only partially. Piercing forces cause the lower sheet to flow into the die cavity locally and conform to the cavity shape. The entire process is completed in about one second. 
     Any means of increasing the mechanical (static and fatigue) strengths of SPR joints through the modification of only particular values of process parameters within the existing set is highly desirable. Current methods for producing SPR joints generally involve riveting from one direction only. Even when practical constraints force riveting from opposite directions, the influence of different rivet orientations on joint mechanical (static and fatigue) strength is unknown. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the maximization of joint strength (static and fatigue) by selecting a particular combination of riveting directions, i.e., rivet orientations. It also provides design-guidelines relating to the variation in joint mechanical strength for different combinations of riveting orientation. 
     The present invention provides an assembly and method, which increases the mechanical (static and fatigue) strength of a self-piercing riveted lap joint without increasing flange width, that is, material used in the overlap region. Strength is increased by inserting pairs of adjacent rivets on opposite sides of the flange in a particular configuration. The rivets are driven into the opposite sides of the flange by using multiple rivet driving apparatuses, which are capable of driving rivets in opposite directions. 
     When a tension load is put upon a riveted flanged assembly, the area surrounding a rivet becomes a high stress area. Testing shows the area around the head of the rivet tends to be the area of highest stress. As a result, the high stress area around the head of the rivet tends to break before any other part of the assembly. It has been determined that the mechanical strength of a joint can be maximized by placing the heads of rivets on opposing sides of a flanged assembly near the interior portions of the flanges, closest to the loaded ends of the sheets. 
     By dividing the number of high stress areas between both flanges of the assembly, the stress is shared between the metal sheets. As a result, the strength of the joint holding the assembly together is increased without increasing the width of the flange. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a double riveted lapshear joint assembly illustrating the placement of rivets according to the present invention; 
     FIG. 2 is a pictorial view showing the riveted assembly of FIG. 1; 
     FIG. 3 is a diagrammatic view of a first test embodiment of double riveted lapshear joint; 
     FIG. 4 is a diagrammatic view of a second test embodiment of double riveted lapshear joint; 
     FIG. 5 is a diagrammatic view of a third test embodiment of double riveted lapshear joint; 
     FIG. 6 is a graph comparing fatigue performance of the double riveted lapshear joints of FIGS. 3-5; and 
     FIG. 7 is a graph comparing static performance of double riveted lapshear joints of FIGS.  3 - 5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first FIGS. 1 and 2 of the drawings, numeral  10  generally indicates a structural assembly such as a panel or portion of a frame for automotive use. Assembly  10  includes first and second sheets  12 ,  14  made of a light metal such as aluminum or aluminum alloy. The sheets have overlapping portions defining linearly extending flanges  16 ,  18  overlapped by a width dimension  19 . The flanges of assembly  10  are joined by self-piercing rivets  20  spaced longitudinally along the flange to form a riveted joint  22 . 
     One end of a rivet forms a head  24  and the opposite end forms a shank  26  extending from the head of the rivet. The rivet shank  26  is typically hollow or partially hollow and cylindrical in shape. Shank  26  is deformed by the die when the rivet is pressed into the flanges  16 ,  18 . 
     The process of self-pierce riveting a lapshear joint riveting involves overlapping the flanges  16 ,  18  and inserting the overlapped flanges into a rivet driving apparatus, not shown. The rivet driving apparatus clamps the flanges between a die and a rivet driver. The driver presses a rivet into the metal flanges  16 ,  18  causing a localized portion of the flanges to deform into the die. Pressure from the rivet  20  compresses deformed metal into the die, causing the metal to take the form of the die. The deformed metal and rivet form a mechanical joint  22 , which holds the assembly together. 
     In accordance with the present invention, multiple rivets  20  are used to form each joint  22 . In a preferred embodiment, at least two rivets  20  are used at each riveting location along the length of the joined flanges. The two rivets at each location are spaced laterally along the width dimension  19  of the flanges and are positioned inward from opposite edges  28 ,  30  of the flanges  16 ,  18 . 
     As shown in FIGS. 1 and 2, the two rivets  20  are located symmetrically, but facing in opposite directions with the heads  24  on opposite sides of the flanges. The heads  24  of the two rivets are located at interior portions  31 ,  32  of the flanges  16 ,  18  as far as possible from the adjacent edges  28 ,  30  of their respective flanges  16 ,  18 . 
     For example, one row  33  of rivets  20  is driven through the upper flange  16  into the lower flange  18 . The rivets  20  of row  33  have their heads  24  in the upper flange  16  and at the interior portion  31  of the flange spaced farthest from its edge  28 . The shanks  26  of these rivets  20  extend into the lower flange  18  and are spaced closest to the edge  30  of the lower flange. A second row  34  of rivets  20  are driven through the lower flange  18  into the upper flange  16 . The rivets  20  of this row  34  have their heads  24  in the lower flange  18  at the interior portion  32  and spaced farthest from its edge  30 . The shanks of these rivets  20  extend into the upper flange  16  and are spaced closest to the edge  28  of the upper flange. 
     In use of the riveted assembly  10  of FIGS. 1 and 2, when the metal sheets  12 ,  14  are loaded in tension linearly in the direction of arrows  36 , the offset metal sheets begin to rotate about the joint  22 . They rotate because they are overlapping and do not lie in the same plane. If the bending forces pulling the assembly  10  apart are excessive, the joint  22  fails. The amount of force needed to break the joint depends upon the placement of the rivets  20 . The relationships between rivet placement and joint strength are shown in the following two tests. 
     In the case of a 2 rivet-row joint with identical sheets (material and thickness) there are 3 possible combinations of rivet orientations, as shown in FIGS. 3-5, owing to the asymmetry of the rivet geometry. Tensile tests were conducted on these three embodiments. 
     In FIG. 3, numeral  40  indicates a first test embodiment of double rivet lapshear joint having first and second sheets  41 ,  42  having overlapping upper and lower flanges  43 ,  44 . The flanges are joined by two rivets with heads  45 ,  46  countersunk in the upper flange  43  while the shanks  47 ,  48  extend into the lower flange  44 . 
     A second test embodiment  50  is shown in FIG. 4 having first and second sheets  51 ,  52  having overlapping upper and lower flanges  53 ,  54 . The flanges are joined by a first rivet with head  55  countersunk in the upper flange  53  while the shank  57  extends into the lower flange  54 , and a second rivet with head  56  countersunk in the lower flange  54  and its shank  58  extends into the upper flange  53 . The countersunk heads  55 ,  56  of the two rivets are spaced farthest from the edges of the flanges  53 ,  54  where they are under direct loading when the sheets are pulled in the direction of arrows  59 , similar to assembly  10 . 
     A third test embodiment  60  is shown in FIG. 5 having first and second sheets  61 ,  62  having overlapping upper and lower flanges  63 ,  64 . The flanges are joined by a first rivet with head  65  countersunk in the lower flange  64  while the shank  67  extends in the upper flange  63 , and a second rivet with head  66  countersunk in the upper flange  63  and its shank  68  extends into the lower flange  64 . The countersunk heads  65 ,  66  are nearest the edges of the flanges  63 ,  64  where they are shielded from direct loading when the sheets are pulled in the direction of arrows  69 . 
     FIG. 6 is a graph illustrating the effects of rivet head placement relating to fatigue performance of double rivet lapshear joints. The graph illustrates fatigue performance by showing the number of cycles an assembly can endure for a given load before failure. The test results graphically illustrate that test embodiment  60  is the weakest because it fails at less than 40,000 cycles. Test embodiment  40  is stronger than embodiment  60  and is able to withstand up to 60,000 cycles. Embodiment  50  is the strongest because it is able to withstand between 70,000 and 90,000 cycles before failing. 
     FIG. 7 of the drawings is a graph presenting the results of static performance tests on embodiments  40 ,  50  and  60 . In these tests, embodiments  40 ,  60  were able to withstand approximately 1800 lbs. before failing. Embodiment  50  is again the strongest, which was able to withstand approximately 2000 lbs. before failing. 
     Embodiment  50  has the most static and cyclic (fatigue) mechanical strength because the heads  55 ,  56  of the rivets are located at the inward portions of the flanges  53 ,  54  to provide the greatest strength and support to the highest stressed areas of the joint. The bending stresses are thus equally divided between the sheets  51 ,  52  at their highest stresses points, resulting in the maximum static and fatigue performance. Embodiment  40  is weaker than embodiment  50  apparently because the location of the rivet heads  45 ,  46  gives inadequate support to the lower sheet  42 , causing it to fail. Similarly, embodiment  60  is weaker than embodiment  50  because the rivet heads  65 ,  66  are placed at the ends of the flanges, away from the zone of highest bending stress where the rivet shanks  66 ,  67  are located. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.