Patent Publication Number: US-2022212417-A1

Title: Fatigue life improvement of adhesively bonded joints

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates to adhesively bonded structures and to methods of manufacturing adhesively bonded structures. 
     BACKGROUND 
     Adhesive bonding is a low cost, simple manufacturing method that is typically applied to thin metal assemblies. Advantages include bonding and sealing in one step, reduction in noise transmission, low cost to join mixed materials, long fatigue life, and high impact resistance. 
     There are limitations, though, with adhesive bonding. Like many other joining methods, adhesively bonded joints have a finite life when experiencing cyclic loads. This is especially true when cycles to failure are low (i.e. loads are very high). High loads result in crack initiation and propagation, whereas low loads result in creep of the adhesive layer. Fatigue life and ultimate joint strength are directly related to the steel thickness and begin to plateau at a given thickness. Methods to improve strength and/or fatigue life include adding material (by extending the overlap) or fixing the edge of the overlap (by tack welding). These methods increase cost and weight of the assembly. 
     In case of adhesive joints, the stress distribution is relatively more uniform than with other conventional methods of joining which enables a reduction in weight. But even in adhesive joints the stress distribution is not perfectly uniform (due to localized stresses). From the joint mechanics point of view, the major limiting factor for adhesive joints is peel or cleavage stress. These should be reduced if strong joints are to be designed. This situation of high peel stress concentrations at the edge of the overlap is exacerbated in assemblies with unbalanced adherend stiffnesses. Fatigue cracks almost always initiate at the highly stressed edges of the adhesive joints. 
       FIG. 1  schematically illustrates a typical prior art adhesively bonded metal structure  100  including first and second metal sheets  102  and  104  bonded together by an adhesive layer  106 . The application of a tensile force across the joint is schematically indicated by the arrows F and the load is applied generally in the direction of the dashed line  108 . 
       FIG. 2  schematically illustrates the same structure of  FIG. 1  after the load F is sufficient to deform the joint.  FIG. 2  includes a superimposed graphic representation of the stress distribution within the adhesive layer  106 . High magnitude tensile stresses peak at the edges  110  and  112  of the overlap, and small compressive stresses are located in the area  114  towards the center of the overlap. It is in these areas  110  and  112  where fatigue cracks typically initiate in prior art structures. 
     Accordingly, there is a continuing need for improved methods of manufacturing adhesively bonded metal structures having improved strength and fatigue life without increasing weight of the structures and at low cost. 
     SUMMARY OF THE DISCLOSURE 
     In one embodiment a method is provided of manufacturing an adhesively bonded structure including first and second components including first and second outer surfaces, respectively, at least the first component being a first metal component, the first and second outer surfaces facing one another and partially overlapping, and the adhesively bonded metal structure including an adhesive layer received between overlapping portions of the first and second outer surfaces. The method may include the steps of:
         (a) deforming the first outer surface of the first metal component along a first isolated path;   (b) adhesively bonding the overlapping portions of the first and second outer surfaces to form the adhesive layer such that the adhesive layer includes a first edge laterally facing a non-overlapping portion of the first outer surface and the adhesive layer includes a second edge laterally facing a non-overlapping portion of the second outer surface; and   (c) wherein the first isolated path extends beside the first edge of the adhesive layer along at least a majority of a length of the first edge of the adhesive layer.       

     In another embodiment an adhesively bonded structure may include first and second metal components including first and second outer surfaces, respectively, the first and second outer surfaces facing one another and partially overlapping. An adhesive layer may be received between overlapping portions of the first and second outer surfaces, the adhesive layer including a first edge laterally facing a non-overlapping portion of the first outer surface and a second edge laterally facing a non-overlapping portion of the second outer surface. The first outer surface of the first metal component may be deformed along a first isolated path extending beside the first edge of the adhesive layer along at least a majority of a length of the first edge of the adhesive layer. The second outer surface of the second metal component may be deformed along a second isolated path extending beside the second edge of the adhesive layer along at least a majority of a length of the second edge of the adhesive layer. 
     Numerous objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a review of following description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic representation of a prior art adhesively bonded joint. 
         FIG. 2  is a schematic representation of the prior art adhesively bonded joint of  FIG. 1  under load, with a superimposed diagram representative of the stress distribution within the adhesive layer 
         FIG. 3  is a schematic perspective view of an adhesively bonded metal structure formed by the methods of the present invention. 
         FIG. 4  is a schematic representation of an adhesively bonded metal structure wherein the deformation of the metal structure adjacent the adhesive layer is performed after the construction of the adhesively bonded metal structure. 
         FIG. 5  is a schematic representation of an adhesively bonded metal structure wherein the deformation of the metal structure adjacent the adhesive layer is performed before the construction of the adhesively bonded metal structure. 
         FIG. 6  is a schematic representation of a high frequency impact tool performing the deformation of the metal sheet like that of  FIG. 4 . 
         FIG. 7  is a schematic representation of a high frequency impact tool performing the deformation of the metal sheet like that of  FIG. 5 . 
         FIG. 8  is a graphical representation of fatigue testing data using a first example adhesive with a 4.5 mm thick steel sample. 
         FIG. 9  is a graphical representation of fatigue testing data showing adverse results for an adhesive joint using a relatively thin 1.0 mm thick metal sheet. 
         FIG. 10  is a graphical representation of fatigue testing data using a second example adhesive with a 3.0 mm thick steel sample. 
         FIG. 11  is a graphical representation of fatigue testing data using the first example adhesive, this time with a 3.0 mm thick steel sample. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings and particularly to  FIG. 3  an adhesively bonded structure  200  includes first and second components  202  and  204  including first and second outer surfaces  206  and  208 , respectively. The first and second outer surfaces  206  and  208  face one another and at least partially overlap. The first outer surface may include an overlapping portion  212  and a non-overlapping portion  213 . The second outer surface  208  may include an overlapping portion  214  and a non-overlapping portion  215 . The adhesively bonded structure  200  includes an adhesive layer  210  received between the overlapping portions  212  and  214  of the first and second outer surfaces  206  and  208 . 
     At least the first component  202  may be a first metal component  202 . In an embodiment the first component  202  may be a metal component and the second component  204  may be a non-metal component. In another embodiment both the first and second components may be metal components. In an embodiment one or both of the metal components may be steel sheet material having thicknesses of at least about 2.0 mm. 
     The adhesive layer  210  includes a first edge  230  laterally facing the non-overlapping portion  213  of the first outer surface  206  and a second edge  232  laterally facing the non-overlapping portion  215  of the second outer surface  208 . 
     In  FIG. 3  the geometry and dimensions of the components  202  and  204  may be described within an x, y, z co-ordinate system that is represented in the drawing. The components may be described as having a length along the x axis, a width along the y axis, and a thickness along the z axis. 
     Thus the first component  202  has a length  216 , a width  218  and a thickness  220 . The second component  204  has a length  222 , a width  224  and a thickness  226 . The overlapping portions  212  and  214  have an overlap length  228 . 
     We have discovered the surprising result that certain surface treatments on the metal components  202  and  204  can improve the fatigue life of the adhesive joint  210  between those metal components. 
     In one embodiment the first outer surface  206  of the first metal component  202  may be deformed along a first isolated path  234  extending beside the first edge  230  of the adhesive layer  210  along at least a majority of a length of the first edge of the adhesive layer. The second outer surface  208  of the second metal component  204  may be deformed along a second isolated path  236  extending beside the second edge  232  of the adhesive layer  210  along at least a majority of a length of the second edge  232  of the adhesive layer  210 . In the example shown the lengths of the first and second edges  230  and  232  are equal to the widths  218 ,  224  of the adjoined components  202  and  204 . 
     As used herein the term “isolated path” refers to a path of deformed metal which lies between regions of non-deformed metal on either side of the path. Thus a metal component  202  having its entire outer surface  206  deformed, for example by shot peening, would not have an isolated path of deformed metal. 
     The deformation of the metal components to form the isolated path  234  may for example be achieved by high frequency mechanical impact using an impact tool such as the HiFit pneumatic tool available from HiFIT Vertriebs GmbH, Adam-Opel-Straße 4 D-38112 Braunschweig, Germany. Such a pneumatic tool is schematically illustrated in  FIGS. 6 and 7  and is identified by the number  242 . The pneumatic tool  242  includes a pneumatic driver  244  which reciprocates an impact tool  246  having a tip  248  which may be generally semi-spherical in shape. 
     The isolated paths  234  and  236  may be in the shape of a rounded groove as is formed by the semi-spherical tip  248  of an impact tool like the HiFit tool referenced above.  FIGS. 4 and 5  schematically illustrate two embodiments of the isolated path  234 . The groove may have a groove depth  238  of at least about 0.1 mm and a groove width  240  in a range of from about 1.0 to about 3.0 mm. 
     In  FIG. 4  the isolated path  234  has been formed after the adhesive joint  200  is created by bonding the two components  202  and  204  together with the adhesive layer  210 . Thus the isolated path  234  has been formed as closely adjacent the first edge  230  as is practical. The isolated path of  FIG. 4  may be described lying laterally outside the adhesive layer. Preferably the isolated path  204  either touches the edge  230  or is spaced from the edge  230  by no more than the thickness  220  of the first metal component  202  perpendicular to the first outer surface  206 . 
     In  FIG. 5  the isolated path has been formed before the adhesive joint  200  is created, thus the adhesive layer  210  may partially or completely overlap with the isolated path  234 . In  FIG. 5  the adhesive layer  210  is shown partially overlapping the isolated path  234 . 
     Collectively the isolated path  234  as shown in  FIGS. 4 and 5  can be described as either at least in part underlying the adhesive layer  210  as seen in  FIG. 5  or lying laterally outside the adhesive layer  210  as seen in  FIG. 4  by no more than the thickness  220  of the first metal component  202  perpendicular to the first outer surface  206 . 
       FIGS. 6 and 7  schematically illustrate how the pneumatic tool  242  may be used to form the isolated path  234  prior to or after formation of the adhesive joint  200 , thus corresponding to the resulting structures seen in  FIGS. 4 and 5 , respectively. 
     If the isolated path  234  is formed in the metal component  202  after formation of the adhesive joint  200 , the impact driver is preferably at an angle such as  250  with the tip  248  as close as practical to the edge  230  of adhesive layer  210 . The angle  250  may be in a range of 30 degrees to 80 degrees. In an embodiment the angle  250  may be in a range of 60 degrees to 75 degrees. In a further embodiment the angle may be about 67 degrees. The isolated path  234  extends beside the edge  230  along a majority of the length of the edge  230 , and preferably along the entire length of the edge  230 . In an embodiment the isolated path  234  may touch the edge  230  or is separated from the edge  230  by a distance no greater than the thickness  220  of component  202  along a majority of the length of the edge  230 , and preferably along the entire length of the edge  230 . 
     If the isolated path  234  is formed in the metal component  202  before formation of the adhesive joint  200 , the impact driver is preferably at an angle such as  252  with the tip  248  essentially perpendicular to the surface  206 . The angle  252  may be in a range of 80 degrees to 100 degrees. When the isolated path is formed prior to the construction of the adhesive joint  200  the adhesive layer  210  may extend into the isolated path as seen in  FIG. 5 . The edge  230  may lie within the adhesive path  234  as seen in  FIG. 5 , or the edge  230  may even lie on the far side of isolated path  234 . If the edge  230  does lie on the far side of isolated path  234  so that path  234  is completely covered, preferably the edge  230  extends past the isolated path  234  by no more than the width  220  of the component  202  along a majority of the length of the edge  230 , and preferably along the entire length of the edge  230 . 
     Using either of the methods illustrated in  FIG. 6 or 7 , there may be preferred parameters for operation of the pneumatic tool  242 . 
     The tip  248  may have a tip diameter in a range of from about 0.5 mm to about 3.0 mm. 
     The pneumatic drive  244  may be operated to apply a force to the impact tool  246  in a range of from about 0.5 pound to about 15.0 pounds. 
     The pneumatic tool  242  may be moved along the isolated path at a travel speed in a range of from about 0.1 to about 20.0 inches per second. In a further embodiment the pneumatic tool  242  may be moved along the isolated path at a travel speed in a range of from about 0.1 to about 10.0 inches per second. 
     The isolated path  234  may be formed in a single pass or in multiple passes of the pneumatic tool  242 . 
     In one example the HiFit tool was oriented perpendicular to the steel sheet as shown in  FIG. 7  and applied on a part before applying adhesive. The HiFit tool had a tip diameter of 1.5 mm. The steel sheet was 3 mm thick 350 MPa yield strength steel. Three passes were used. The first pass created a rough groove with a depth of 322 microns and peak-to-peak width of about 1 mm. A second pass increased the depth to 338 microns and width of about 2 mm. A third pass resulted in a depth of 514 microns and width of about 2.4 mm. 
     In another example the Hifit tool was applied after formation of the adhesively bonded joint. The HiFit tool was oriented as shown in  FIG. 6  at an angle  250  of about 67 degrees. The HiFit tool had a tip diameter of 1.5 mm. The steel sheet was 3 mm thick 350 MPa yield strength steel. A single pass was used resulting in an asymmetric trough or groove having a peak of 349 microns high on one side closest to the adhesive layer  210 , and a peak of 186 microns on the other side. The trough had a width  240  of about 2.6 mm. 
     The pneumatic tool  242  may be operated at a tip impact frequency in a range of from about 100 impacts per second to about 400 impacts per second. In an embodiment the tip impact frequency in a range of from about 150 impacts per second to about 300 impacts per second. In a further embodiment the tip impact frequency in a range of from about 200 impacts per second to about 250 impacts per second. 
     The action of the pneumatic tool  242  impacting the surface  206  of metal component  202  with tip  248  creates a compressive residual stress in the metal below and adjacent the isolated path  234 . There is a local work hardening and/or strain hardening of the metal. It is not fully understood how this deformed metal area of the isolated path  234  interacts with the adhesive layer  210  to increase the fatigue life of the adhesive layer  210 , but as the test data discussed below shows there is such an improved fatigue life in some instances. 
     It further appears that the improved fatigue life is more consistently achieved when the isolated path  234  is combined with an adhesive layer  210  formed from a preferred adhesive material. In an embodiment the adhesive material may be selected from the group consisting of an epoxy adhesive, a polyurethane adhesive and an acrylic adhesive. In another embodiment the adhesive may be a high crash resistant epoxy adhesive. 
     Methods of Manufacture: 
     In an embodiment a method is provided of manufacturing an adhesively bonded structure  200  including first and second components  202  and  204  including first and second outer surfaces  206  and  208 , respectively, at least the first component  202  being a first metal component, the first and second outer surfaces  206  and  208  facing one another and partially overlapping, and the adhesively bonded structure including an adhesive layer  210  received between overlapping portions  212  and  214  of the first and second outer surfaces, the method comprising:
         (a) deforming the first outer surface  206  of the first metal component  202  along a first isolated path  234 ;   (b) adhesively bonding the overlapping portions  212  and  214  of the first and second outer surfaces  206  and  208  to form the adhesive layer  210  such that the adhesive layer includes a first edge  230  laterally facing a non-overlapping portion  213  of the first outer surface  206  and the adhesive layer includes a second edge  232  laterally facing a non-overlapping portion  215  of the second outer surface  208 ; and   (c) wherein the first isolated path extends  234  beside the first edge  230  of the adhesive layer  210  along at least the majority of a length of the first edge of the adhesive layer.       

     In one embodiment step (a) may be performed before step (b) as schematically illustrated in  FIG. 7 . In a further embodiment step (b) may be performed before step (a) as schematically illustrated in  FIG. 6 . 
     When the second component  204  is also a metal component the method may further include deforming the second outer surface  208  of the second metal component  204  along a second isolated path  236 , wherein the second isolated path  236  extends beside the second edge  232  of the adhesive layer  210  along at least a majority of a length of the second edge  232  of the adhesive layer  210 . 
     In an embodiment in step (a) the deforming may be performed by high frequency mechanical impact with a tool such as the pneumatic impact tool  242 . 
     In a further embodiment in step (a) the high frequency mechanical impact may be performed using an impact tool  246  having a tip diameter in a range of from about 0.5 mm to about 3.0 mm. 
     In a further embodiment in step (a) the high frequency mechanical impact may be performed using an impact tool having an impact force in a range of from about 0.5 lbs to about 15 lbs. 
     In a further embodiment in step (a) the high frequency mechanical impact may be performed using an impact tool having a travel speed in a range of from about 0.1 inch/second to about 20 inch/second. 
     In a further embodiment in step (a) the high frequency mechanical impact may be performed by multiple passes of the impact tool. 
     In a further embodiment in step (a) the deforming may create a groove  230  in the first outer surface. The groove  230  may have a depth of at least 0.1 mm and a width in a range of from about 1.0 mm to about 3.0 mm. 
     In an embodiment in step (a) the deforming of the first outer surface  206  of the first metal component  202  may increase a fatigue life of the adhesive layer  210 . 
     In an embodiment in step (b) the adhesive may be selected from the group consisting of an epoxy adhesive, a polyurethane adhesive and an acrylic adhesive. 
     Examples 
     A number of fatigue tests were performed to evaluate the disclosed methods of manufacturing adhesive joints. In the fatigue tests, the samples were always in tension (at both the high and low stress levels during the cycle). Two different load levels (Minimum 5%/Maximum 50% or Minimum 4%/Maximum 40%) of static joint strength were investigated. This means an R-Ratio of 0.1 was used. Test frequency was 5 Hz. The tests were run using a closed loop servo-hydraulic 100 kN two post frame with a Model  8801  fatigue test machine manufactured by Instron. The adhesively bonded joints were shaped substantially as shown in  FIG. 3  having widths  218 ,  200  of about 38 mm and having an overlap length  228  of about 25 mm. The adhesive layer  210  had a thickness of about 0.25 mm. 
       FIG. 8  graphically shows fatigue life of one type of adhesive (named Epoxy 1) with and without deformation of the metal component using the HiFit impact tool at two different load levels. HiFit was applied after bonding of the 4.5 mm thick steel coupons. The fatigue life of the HiFit treated samples increased by an average of 3.5× at the 40% load level and 2× at the 50% load level. 
       FIG. 11  graphically shows fatigue life of the Epoxy 1 adhesive, this time using 3.0 mm thick steel coupons, with and without deformation of the metal component using the HiFit impact tool at two different load levels. The fatigue life of the HiFit treated samples increased by an average of 3.3× at the lower load level and 1.5× at the higher load level. 
     Epoxy 1 was a 1-part epoxy adhesive available from Henkel under the tradename Teroson EP 5089. The Teroson EP 5089 adhesive is described by Henkel as having a high crash resistance of greater than 20 N/mm up to −40 degrees C. It is described as having a nano-dispersion embedded into an epoxy matrix. It is described as having a very high static strength, an E-modulus greater than 1600 MPa, and a low temperature curing capability. 
       FIG. 9  represents a test run to determine the applicability of the process to very thin steel sheets. In the test of  FIG. 9  the HiFit process was applied to an adhesively bonded structure made from 1 mm thick steel sheets. It is seen that there was a significant reduction in fatigue life after the HiFit treatment. For this reason, we have concluded that the process should only be applied to steel sheets having a thickness of at least about 2.0 mm. 
       FIG. 10  shows fatigue life of one type of adhesive (named Epoxy 2) with and without deformation of the metal component using the HiFit impact tool at one load level. HiFit was applied before bonding of the 3.0 mm thick steel coupons. The fatigue life of the HiFit treated samples increased by an average of 7.5×. Epoxy 2 was a 2-part epoxy adhesive available from Henkel under the tradename Teroson 5065. 
     On the other hand, it is noted that tests similar to that of  FIG. 10  were run using several other adhesives which did not result in an increase in fatigue life of the HiFit treated samples. Those other adhesives included: (1) a 2-part epoxy adhesive available from Sika under the tradename Sikapower 1277; (2) a 2-part methyl methacrylate adhesive available from Lord under the tradename 850/25 GB; and (3) a 2-part methyl methacrylate adhesive available from ITW under the tradename Plexus MA422. 
     It is not yet understood why the disclosed process of deforming the metal component leads to an increase in fatigue life of the adhesive layer. And it is not understood why the improvement in fatigue life occurs with some adhesives and not with others. But any proposed combination of metal components and adhesive materials for an adhesively bonded joint can be readily tested by the techniques disclosed herein to determine whether the methods disclosed herein are applicable. 
     Thus, it is seen that the apparatus and methods of the present disclosure readily achieve the ends and advantages mentioned as well as those inherent therein. While certain preferred embodiments of the disclosure have been illustrated and described for present purposes, numerous changes in the arrangement and construction of parts and steps may be made by those skilled in the art, which changes are encompassed within the scope and spirit of the present disclosure as defined by the appended claims Each disclosed feature or embodiment may be combined with any of the other disclosed features or embodiments.