Abstract:
A method an apparatus for testing shear strain response of a test specimen having a thick adhesive bond line is disclosed. Method and apparatus permit the use of readily available clip-on extensometers. The apparatus and method may include the use of knife edges secured to each of two adherend components on a standard ASTM D 5656 thick adherent test specimen.

Description:
[0001]     This invention was made with Government support under contract number SCRA 2001-508 awarded by the United States Navy. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention is related to systems and methods for measuring shear stress-strain response, and more specifically, to systems and methods for measuring shear stress-strain response for structures that include thick adhesive bondlines.  
         [0004]     2. Description of the Related Art  
         [0005]     The standard method (ASTM D5656) that is typically used to measure the shear stress-strain response for thin adhesive bondlines (e.g., having bondline thicknesses of 0.005 to 0.020 in or 0.127 to 0.508 mm) employs the American Cyanamid KGR-1 gage. One improvement to ASTM D5656 has been proposed in Yang et al., “Evaluation and Adjustments for ASTM D5656 Standard Test Method for Thick-Adherend Metal Lap-Shear Joints for Determination of the Stress-Strain Behavior of Adhesives in Shear by Tension Loading,” Journal of Testing and Evaluation, JTEVA, volume 29, no. 1, January 2001, pp. 36-43. In this article, the authors set forth an arrangement that uses four pins and mounting holes to mount a KGR-1 device to a test specimen, to reduce errors due to slippage of the mounting pins. However, the KGR-1 gage has limited deflection travel (d=0.020 in., see  FIG. 1   a ) and was designed for use in testing of bondline thicknesses (ta) less than 0.040 in. and therefore cannot be used for testing of thick bondlines.  
         [0006]     This disclosure is directed toward overcoming one or more problems or disadvantages associated with the prior art.  
       SUMMARY OF THE INVENTION  
       [0007]     According to one aspect of the invention, a method of testing shear strain response of a test specimen having a thick adhesive bond line disposed between two adherend components is provided. The method includes providing a first pair of knife edges secured to each of two sides of a first adherend component, providing a second pair of knife edges secured to each of two sides of a second adherend component, applying a tensile load to the first and second adherend components, and measuring a displacement of the first and second pairs of knife edges with respect to one another.  
         [0008]     In accordance with another aspect of the invention, shear strain response of thick adhesive bondlines may be measured using standard clip-on extensometers.  
         [0009]     The features, functions, and advantages can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a diagrammatic prospective view of a test apparatus according to one exemplary embodiment of the disclosure;  
         [0011]      FIG. 2 a  diagrammatic side view of a test specimen that may be used in the test apparatus of  FIG. 1 ;  
         [0012]      FIG. 3  is an enlarged diagrammetric opposing side view of a portion of the test specimen of  FIG. 2 ;  
         [0013]      FIG. 4  is a graphical representation of a typical stress-strain curve;  
         [0014]      FIG. 5  is a diagrammatic side view of a knife edge component that may be used in the test apparatus of  FIG. 1 ;  
         [0015]      FIG. 6  is a bottom view, taken along lines  6 - 6  of  FIG. 5  of the knife edge component of  FIG. 5 ;  
         [0016]      FIG. 7  is an end view, taken along lines  7 - 7  of  FIG. 5 , of the knife edge component of  FIG. 5 ;  
         [0017]      FIG. 8  is a diagrammatic side view of another knife edge component that may be used in the apparatus of  FIG. 1 ;  
         [0018]      FIG. 9  is a bottom view taken along lines  9 - 9  of  FIG. 8 , of the knife edge component of  FIG. 8 ;  
         [0019]      FIG. 10  is a diagrammatic end view, taken along lines  10 - 10  of  FIG. 8 , of the knife edge component of  FIG. 8 ;  
         [0020]      FIG. 11  is a diagrammatic front view of the knife edge components of  FIGS. 5-10 , attached to a channel member; and  
         [0021]      FIG. 12  is a diagrammatic cross-sectional view of the channel member, taken along lines  12 - 12  of  FIG. 11 . 
     
    
     DETAILED DESCRIPTION  
       [0022]     With reference to initially to  FIG. 1 , a test apparatus  10  for measuring shear strain response of thick adhesive bondlines may include a first extensometer  12 , a second extensometer  14 , and a test specimen  16 . The test specimen  16  may be constructed in accordance with ASTM D5656 and may have a width, W, of 1.0 in (2.54 cm). The first and second extensometers  12  and  14  may be clip-on type extensometers, such as, for example, MTS model 632.02 clip-on gages, available from MTS Systems Corporation.  
         [0023]     The first and second extensometers  12  and  14  may each include a pair of cantilevers  18  that engage a starboard upper knife edge component  20  on the starboard side of the test specimen  16 , a lower starboard knife edge component  22  on the lower starboard side of the test specimen  16 , an upper port knife edge component  24 , on the port side of the test specimen  16 , and a lower port knife edge component  26  on the lower port side of the test specimen  16 .  
         [0024]     The test specimen  16  may include a lower front adherend component  28 , an upper front adherend component  30 , a lower rear adherend component  32 , and an upper rear adherend component  34  (shown in isolation in  FIG. 2 ), that may each be made from a metallic material (e.g., 6A1-4V Titanium). The test specimen  16  may be, for example, attached to a tension testing machine by upper and lower pinned joints  36  and  38 .  
         [0025]     As shown in  FIG. 2 , the test specimen  16  may be constructed in order to obtain a measurement of shear strain response of a thick adhesive bond line  50  when the test specimen  16  is subjected to a tensile load, as indicated by the arrows  54  in  FIG. 2 .  
         [0026]     In operation, the test apparatus  10  permits the use of simple clip-on extensometers to measure shear strain response of the thick adhesive bond line  50 , by providing knife edge surfaces from which shear displacement may be measured. With reference to  FIG. 3 , this testing methodology has a deflection capability of d=0.15 in. or (8 times greater than KGR-1) and can accept a bondline thickness of t a =0.30 in. or (˜8 times greater than KGR-1). The important adhesive design data obtained from this test is the shearing strain (gamma,  FIG. 4 ) obtained at failure which is a measure of the adhesive ductility and is obtained by dividing the maximum deflection travel (d) by the adhesive thickness (ta), see  FIG. 1   a . Also, unlike with the KGR-1 gage, the new method does not require the use of a metal correction factor due to the location of the set screws  48  ( FIG. 1 ), therefore eliminating the need for a metal correction test specimen as specified in ASTM D5656. The test equipment should have capability to measure a ultimate shearing strain of 1.0 in./in.  
         [0027]     Each of the four knife edge components  20 ,  22 ,  24 , and  26  may be attached to the test specimen  16  in any suitable fashion.  
         [0028]     For example, each of the lower knife edge components  26  and  22  may be secured to a lower channel member  40  by threaded fasteners  42  that pass through end bores  44  and side bores  46  formed in the lower knife edge components  22  and  26  (the bores  44  and  46  are best seen in  FIGS. 5-10 ). The lower channel member  40  may be attached to the lower front adherend component  28 , for example, by set screws  48  ( FIG. 1 ), that pass through set screw bores  52  (shown in  FIG. 1 ).  
         [0029]      FIGS. 5-7  show the lower port knife edge component  26  in isolation. As best seen in  FIG. 6 , the lower port knife edge component  26  may have a substantially L-shaped geometry when viewed from below. The upper port knife edge component  24  may have identical geometry to that of the lower port knife edge component  26 . Accordingly, it is not necessary to show the upper port knife edge component  24  in isolation.  
         [0030]     Similarly,  FIGS. 8-10  show the lower starboard knife edge component  22  in isolation, and the upper starboard knife edge component  20  may have identical geometry to that of the lower starboard knife edge component  22 .  
         [0031]      FIG. 11  shows the placement of the lower knife component  26  and  22  on the lower channel member  40 , and  FIG. 12  is a cross-sectional view of the channel member  40 .  
         [0032]     Other aspects and features of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.