Abstract:
The present invention provides an apparatus and a method to apply a load to an adherend, which is bonded to a substrate, to allow an accurate measurement of the bond between an adherend and substrate. The strength of the bond is tested by shearing the adherend from the substrate and measuring the force per unit area required to fail the bond. This is accomplished by positioning a crosshead at the base of the adherend and applying a load, which is parallel to the surface of the substrate against the base of the adherend. The strength of the bond is the force per unit area required to shear the adherend from the substrate. The accuracy of the measurement is enhanced by minimizing the surface area of the crosshead that is in contact with the substrate and by ensuring that the adherend is not fractured during testing. Fracturing and deformation of the adherend is limited by using a notch to test with rather than a straight chisel. The present invention is configured to load the adherend such that the strength of the bond may be measured accurately.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to a device that is used to test the bond strength of an adherend, which is formed from a hardenable material, and is bonded to a test piece. More particularly, the present invention is related to testing bond strength between an adherend and a substrate by shearing the adherend from the substrate. 
     2. The Prior State of the Art 
     The goal of forming a bond between restorative dental materials and dentin, enamel, or other dental substrates is to withstand the significant shear forces created in the oral environment and to reinforce the remaining tooth structure. For this reason, high bond strengths are desirable. Measuring the strength of a bond between restorative dental materials and dental substrates requires two steps. First, the bond between the dental materials must be formed and second, the restorative dental material must be sheared or pulled until failure occurs and then the peak force per unit area is determined. 
     The formation of a bond has several steps. First, the type of dental substrate, adhesive and adherend to be tested are selected. If the dental substrate is irregularly shaped, such as a tooth, then the tooth is mounted in resin to form a bonding substrate test piece. The test piece is cut or polished to create a smooth, flat top surface with a portion of the tooth exposed, preferably at the same level as the resin. The exposed portion of the tooth or other dental substrate is referred to herein as a test sample. Next, the top surface of the test sample is etched and typically rinsed to remove any contaminates. The test sample, or at least a portion thereof which will be the bond site, is then coated with a primer and/or adhesive, which will either be light cured or be allowed to chemical cure. A secondary restorative dental material (adherend) is then placed on the bond test site and is also light cured or chemical cured. The curing process hardens both the adhesives and the adherend. Once this process is completed, a bond has been formed between the restorative dental materials and the test sample or dental substrate such that a bond assembly now exists. The process of creating the bond assembly in the prior art presents several problems, which prevent the strength of the bond from being accurately measured. 
     The first problem associated with the bond assembly is related to the shape of the adherend. A cylindrical shaped adherend is the most conducive for obtaining an accurate measurement of the bond strength. Any other geometric shape, as well as deviations in the cylindrical shape lead to less accurate measurements of bond strengths. A perfect cylindrical shape however, has proven difficult to form as illustrated by the prior art. If the adherend is not formed to have a uniform cross-sectional shape as taken along its length, then a shear device used to shear the adherend for bond strength testing may not be able to properly interface with the adherend. 
     One prior art method involves bonding a composite filled gelatin capsule onto a testing substrate. In this method, a slightly overfilled gelatin capsule is overturned and manually or mechanically held in place on the substrate. The resulting adherend has a number of problems. First, a manually or mechanically held gelatin capsule is to some extent compressed. This compression deforms the cylindrical shape of the adherend. The second problem is that the gelatin capsule must be held immobile during the hardening process, which is difficult to do manually. This factor further deforms the shape of the composite material. The third problem is that the gelatin capsule must be slightly overfilled to ensure the proper adaptation of the composite to the substrate. When the composite filled capsule is placed onto the substrate the excess composite must be removed before curing takes place. This process creates difficulty in keeping the capsule stationary before curing which leads to further deformation. The combination of these deforming factors produces a composite material that is not perfectly cylindrically shaped and will not fit a shear device perfectly, which results in inaccurate bond strength measurements. A further difficulty is ensuring that the gelatin capsules are held perpendicularly to the dentin. If the adherend is not perpendicular to the substrate then the test loads will not be able to be applied properly. 
     Another attempt to eliminate these problems is the use of a small stainless steel nozzle, which is attachable to a guide fixture. The nozzle has small windows through which composite material can be added and through which the composite material is light cured. This method eliminates deformities in the shape of the composite material due to compression. However, it still has problems because the small windows cause difficulty in composite placement and curing. The windows limit the amount of light that can enter the nozzle to cure the composite material, which leads to inaccurate measurements of the bond strength because the composite material may not be completely cured. Removing the nozzle, after the curing process is difficult because the cured composite often extends into the windows, thus binding the nozzle in place. The difficulty in removing the nozzle creates stress on the newly formed bond and can weaken or fracture the bond. Note, the nozzle is held perpendicular to the substrate by means of a guide fixture, which also limits the user in choosing a suitable test site. Additionally, the stainless steel mold will not allow use of some restorative materials such as glass ionomers, copolymers, luting cements, and amalgam, thus limiting the ability to gather information related to such dental restorative materials. An example of such stainless steel instruments is the system sold under the name Bencor Multi-T. Information regarding this system is provided by a distributor, Danville Engineering at “www.danvilleengineering.com” which is linked to “www.edoc.co.za/dentalnet/research/microgrip/bencor.shtml” to provide more detailed information. 
     The next problem in the prior art is that the bond is not limited to the area between the test site and the adherend. When adhesives are applied to the substrate, it is typically applied to the entire surface of the substrate. This excess adhesive is not removed before the curing process occurs and results in a resin snowshoe or shelf, which encompasses more surface area than the test site. This resin snowshoe can distribute test loads out over the surface of the substrate; similar to the way a snowshoe spreads out the load of a human over a broader area. This resin snowshoe prevents the true strength of the bond from being measured. In essence the resin snowshoe bonds the adherend to more than just the test site. For this reason, the measurement of the bond strength is not accurate. The use of split molds, straws, or tubing such as TYGON™ tubing to create an adherend without creating a “resin snowshoe” requires the application of the primer and adhesive through the openings of these devices. When adhesives are applied in such a manner, capillary action occurs and some of the adhesive is drawn up the interior walls. Since this negatively influences the accuracy of test data, the result is inaccurate measurement of material properties. 
     Once the bond is formed between the substrate and the adherend, the strength of the bond can be tested. Testing the bond strength means measuring the force per unit area required to shear the adherend from the substrate. In addition to the factors related to the formation of the bond that effect test results, the actual testing of the bond could introduce inaccuracies. The prior art demonstrates additional problems that can influence the measurement of the bond strength. 
     The frictional force between the shear device and the test piece must be taken into account in order to obtain accurate bond strength measurement. The frictional force is typically greater when the shear device has a large amount of surface area in contact with the surface of the test piece or is held in place with guide fixtures. The shear device must load the bonded specimen until the adhesive fails without fracturing the adherend. If the adherend fractures first, then the adhesive is not the reason for failure and an accurate bond strength measurement cannot be obtained. The problem with fracturing the adherend rather than shearing the adhesive is more prominent when the adherend is not perpendicular to the substrate. If the shear device is too thin, then the adherend is, once again, more likely to be fractured and the resulting bond strength measurement is resultantly inaccurate. The shear force must be applied as close to the bond interface as possible or at the base of the adherend. If the shear force is not applied at the base of the adherend, a lever arm will be created and the force required to shear the bond will be measured inaccurately. 
     There are other methods of creating bond assemblies which maintain the controls necessary to obtain fairly accurate measurements but they are very cumbersome to use which limits productivity. These prior art systems do not offer the user ease of use, freedom of material choice, and choice of bond location while still maintaining the accuracy needed. 
     Researchers are employing many different methods of testing shear bond strength. Many of the methods involve complex fixturing which usually introduce more errors than benefits. An example of a method that involves complex fixturing is the method developed by Larry Watanabe, which is identified as ISO TR 11405. 
     There is a need in the industry for a method for both creating a bond assembly and testing the bond between adherend and the substrate such that the measurements of the bond strength actually represent the bond strengths. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems in the prior art by creating a bond assembly and testing the bond between a substrate and an adherend. The bond assembly is shaped and formed such that it will yield accurate bond strength measurements upon being tested. Typically, the bonding substrate is dentin, enamel, metal, porcelain, or composite and the adherend is a composite, copolymer, glass ionomer, unfilled resin, or amalgam material. 
     The present invention shapes the adherend into a cylindrical shape without the deformities caused from being hand held. The cylindrical shape is important, as this shape results in the most accurate measurements of shear bond strength. The invention also permits the light cure restorative materials to be completely cured by providing easy access to the uncured restorative composite material. Additionally, in light cure and chemical cure situations, excess primer or adhesive is prevented from being exposed to the curing light, thereby eliminating the formation of a “resin snowshoe.” As a result, the bond to be tested only encompasses the surface area under the adherend, which is the bond site. The present invention forms an adherend that can be completely hardened from exposure to high intensity light, is perpendicular to the substrate, and is uniformly cylindrical in shape. 
     The present invention allows for accurate measurements of the strength of a bond between an adherend and a substrate to be obtained by shearing the adherend from the substrate with a unique shear device or a crosshead, as disclosed herein below. An accurate measurement of the bond strength is obtained, in part, as a result of the reduced frictional force between the crosshead and a bonding substrate. The frictional force is reduced when compared with other testing systems as the surface area of the crosshead that comes in contact with the substrate is minimal. 
     The crosshead shears the adherend from the bonding substrate by applying a force that is parallel to the substrate and is directed at the base of the adherend, which is at the site of the bond. By pushing at the base of the adherend with the proper crosshead, the force measured is the shear force required to cause the bond to fail unless fractures occur in the adherend first. Applying a force at a point higher on the adherend is more likely to break the adherend due to leverage, rather than cause the adhesive between the substrate and the adherend to fail. The present invention further enhances the accuracy of the measurement of the bond strength by having a minimal amount of contact between the substrate and the crosshead and by ensuring that the adherend is not fractured off of the substrate or deformed. 
     Additional objects and advantages of the invention will be set forth in the description, which follows and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings as listed herein below. 
     FIG. 1 is a perspective view of a test piece with an adherend bonded to a test sample embedded in the test piece. 
     FIG. 2 is a bottom view of a bonding and molding platform. 
     FIG. 3 is a cross-sectional view of the bonding and molding platform in FIG. 2 taken along section line  3 — 3  of FIG.  2 . 
     FIG. 4 is an expanded perspective view of the clamping assembly, the bonding and molding platform and the test piece. 
     FIG. 5 is a perspective view of the clamping assembly, the test piece, and the bonding and molding platform configured to receive a hardenable material to form an adherend. 
     FIG. 6 is a perspective view of FIG. 5 taken along the section line  6 — 6  of FIG.  5 . 
     FIG. 7 is a perspective view of a crosshead. 
     FIG. 8 is a top view of the crosshead. 
     FIG. 9 is a cross-sectional view of the crosshead in FIG. 8, taken along the section line of  9 — 9  of FIG.  8 . 
     FIG. 10 is a perspective view a crosshead positioned to shear an adherend from the top surface of a test sample embedded in a test piece. 
     FIG. 11 is a cross-sectional view of FIG. 10 taken along the section line of  11 — 11  of FIG.  10 . 
     FIG. 12 is a perspective view of another embodiment of a crosshead. 
     FIG. 13 is a top view of the crosshead shown in FIG.  12 . 
     FIG. 14 is a cross-sectional view of the crosshead in FIG. 13, taken along the section line of  14 — 14  of FIG.  13 . 
     FIG. 15 is a perspective view the crosshead shown in FIG. 12 positioned to shear an adherend from the top surface of a test sample embedded in a test piece. 
     FIG. 16 is a cross-sectional view of FIG. 15 taken along the section line of  16 — 16  of FIG.  15 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to methods and apparatus for forming a bonding assembly as well as methods and apparatus for testing the strength of a bond between an adherend and a substrate. FIGS. 1 through 6 illustrate an exemplary embodiment of the components needed to form an adherend and bond the adherend to a bond site of a test piece. FIGS. 1 through 6 are also employed to describe an exemplary method of forming an adherend and bonding the adherend to a bond site of a test piece. FIGS. 7 through 11 illustrate an exemplary embodiment of an apparatus and a method used to test the strength of a bond between an adherend and a bond site of a test piece. FIGS. 12 through 16 depict another embodiment of an apparatus and a method used to test the strength of a bond between an adherend and a bond site of a test piece. Please note that while the methods and apparatus are particularly useful with dental substrates and dental restorative materials, the methods and apparatus may also be utilized with other substrates and adherends. 
     FIG. 1 is a perspective view of test piece  20  after adherend  10  and a test bond have been formed. Adherend  10 , which is discussed in greater detail herein below, is typically formed from resins that have cured or hardened such as composite resins, glass ionomer, or combinations thereof known as compomers and also from amalgam material. Test piece  20  and its components are discussed first in relation to FIG. 1 to better understand the methods and components illustrated in later figures. Adherend  10  is bonded on a bond site  22  which is a specific portion of test piece  20  identified in conjunction with a molding platform  30  discussed in detail herein below with reference to FIGS. 2-6. 
     As depicted in FIG. 1, test piece  20  has a top surface  23  opposite a bottom surface  24 . Top surface  23  is preferably flat and smooth, as shown, to provide for consistent testing. While bottom surface  24  is preferably flat and parallel, as shown, it can have any shape which enables the top surface to have a desired orientation. 
     The test piece is shown at  20  as being formed from two components, a tooth shown at  25  and a holding material  28 . When it is desired to test the bond strengths for adherends to substrates which are naturally irregularly shaped, such as teeth, it is preferable to form a test piece by at least partially encasing the substrate in a holding material. The holding material may be any suitable resinous material such as various methacrylates. After the irregular shaped substrate is encased or embedded in the holding material and the holding material has been allowed to harden, then both the irregularly shaped substrate and the holding material are shaped until both are substantially smooth and flat. The smooth and flat configurations may be achieved by any suitable method such as grinding. Together the smoothed and flattened substrate and holding material form the top surface of the test piece. Note that, as indicated in the Background section herein above, substrates having irregular shapes, such as teeth, which have been flattened after being embedded in a holding material to produce a two-component test piece are referred to herein as test samples. 
     While test piece  20  is shown in FIG. 1 as having two separate components, tooth  25  and holding material  28 , the present invention can also be utilized with test pieces which are formed from only one material such as porcelain, a particular metal, metal alloys and amalgams, or a composite material. Since materials such as porcelain or metals can be easily formed to have opposing flat surfaces and be sized for easy handling, it may be useful to form such materials into a solid, single component test piece. 
     Whether the test piece is formed from two components or a single component, the portion of the test piece under the adherend is the bond site. If the test piece includes a test material such as a tooth, as in FIG. 1, then the bond site is the portion of the test material under the adherend as shown at  22 . More particularly, the potential bond site may be any part of the test material while the actual bond site is the portion of the test material under the adherend. The dimensions of the bond site are determined, as described in greater detail herein below, by the structure which forms the adherend. 
     Tooth  25  is positioned on its side such that a small amount of enamel  26  is exposed while exposing as much dentin  27  as possible. The large surface area of dentin combined with the relatively small surface area of enamel enables the adherend to be bonded to the dentin without contacting the enamel. This is particularly desirable since many restorative dental materials are bonded only to dentin. Since it is easier to bond to enamel than to dentin, the bond site preferably includes only dentin when testing the bond strength of restorative materials intended for use with dentin as contact with enamel may provide an inaccurate indicator of bond strength. Accordingly, any portion of dentin  27  which enables adherend  10  to be on dentin without being over enamel is a potential bond site; however, the actual bond site is essentially only the portion of the dentin under the adherend. As described below, the apparatus and systems disclosed herein can be configured to be particularly useful for testing bond strengths of adherends to most dental materials. 
     FIG. 2 is a bottom view of a bonding and molding platform  30  and FIG. 3 is a cross sectional view of platform  30  taken along the section line  3 — 3  of FIG.  2 . Platform  30  is used to form adherend  10 , as shown in FIG. 1, such that adherend  10  is bonded perpendicularly to bond site  22  of the test piece substrate  20 . The method of forming adherend  10  and the method of bonding adherend  10  to the substrate  26  or  27  are mentioned in reference to FIGS. 2 and 3, but are more fully discussed in reference to FIGS. 5 and 6. 
     Platform  30 , as shown in FIGS. 2-6, comprises body  31  having top surface  35  opposite a bottom surface  37 . Platform  30  has a mold  34  extending downward from bottom surface  37 . Platform  30  also has a perimeter support member  38  extending down from bottom surface  37 . Preferably, perimeter support member  38  is inset from perimeter  40 . Platform  30 , as described in relation to FIGS. 2-6, is an example of bonding means for receiving a hardenable material and for enabling light energy to be directed to the hardenable material to yield an adherend on a test piece. 
     As shown best in FIG. 3, mold  34  is an integral portion of platform  30 . Mold  34  has a top end  41  opposite an outlet end  42 . A conduit  39  extends through from top end  41  to outlet end  42 . Conduit  39  is accessed via portal  33  which is centrally located in countersunk portion  32  of top surface  35 . While portal  33  is the inlet opening into conduit  39  the outlet opening is defined by outlet rim  36 . 
     The primary function of mold  34 , and particularly conduit  39 , is to form and shape an adherend as shown in FIG. 1 at  10 . Mold  34  is an example of molding means for forming an adherend on a bond site of a test piece from a hardenable material delivered into the molding means. 
     Adherend  10  is formed by filling conduit  39  through portal  33  with a hardenable material, such as composite material. The configuration of countersunk portion  32 , located in top surface  35  of platform  30 , enables conduit  39  to be easily filled with a hardenable material. After conduit  39  is filled with a hardenable material, any hardenable material remaining in countersunk portion  32  is removed. 
     When the hardenable material in mold  34  is hardened with light irradiation or allowed to chemical cure, adherend  10  is formed. In other words, adherend  10  is a hardenable material that has been cured while inside of conduit  39 . Mold  34 , more specifically, conduit  39 , is designed to properly shape the hardenable material. Accordingly, conduit  39  is preferably cylindrical in shape but may also embody other shapes and conduit  39  preferably has a uniform diameter along its length. Since the hardenable material takes the shape of conduit  39 , the cylindrical shape of the conduit yields a cylindrically shaped adherend  10 . Not only does conduit  39  ensure that adherend  10  is substantially cylindrical, it also ensures that adherend  10  is substantially perpendicular to top surface  23  of test piece  20 . 
     The depth of mold  34 , more particularly, the depth of conduit  39 , is sufficient to permit complete cure by light irradiation. The preferred length is in a range from about 1 mm to about 3 mm and the range is more preferably from about 1 mm to about 2 mm. 
     The width of the conduit and the resulting adherend varies depending on the particular materials to be tested. While any width may be used, the width may, for example range from about 1 mm to about 5 mm. However, less force is required in testing when a smaller diameter is used. Additionally, when testing the shear bond strength on dentin use of a smaller diameter reduces the likelihood of bonding onto enamel. A diameter of about 2.5 mm has been found useful in bonding on dentin without bonding to enamel. Further, for ease in calculating the force required to shear the adherend in Pascals, the diameter is preferably 2.3798 mm. 
     Conduit  39 , which is essentially the inner surface of mold  34  has a diameter which is the same throughout its length. While the exterior surface of mold wall  43  may have a portion which is parallel to conduit  39 , as shown at top end  41 , the thickness of mold wall  43  of mold  34 , particularly in the region of outlet end  42 , decreases. The thickness of mold wall  43  decreases until conduit  39  and the exterior surface of mold wall  43  meet to form a point at outlet end  42 , which is outlet rim  36 . Stated otherwise, outlet end  42  is cone shaped and tapers downward and inward toward conduit  39  such that conduit  39  has a thin circular outlet rim. Since outlet rim  36  is where conduit  39  and the exterior wall of the mold come together or coterminate, it should be understood that outlet rim  36  is integral with and defined by both conduit  39  and the exterior surface of mold wall  43 . The advantages of the thin configuration of outlet rim  36  are discussed below in reference to FIG.  5  and FIG.  6 . 
     The function of perimeter  40  and perimeter support member  38  are also best understood, as described herein below, in reference to FIGS. 5 and 6. Perimeter  40  is designed to removably mate with a device that will hold platform  30  immobile while adherend  10  is formed and bonded to bond site  22  of test piece  20 . Perimeter support member  38  is designed to withstand the pressure of the device which holds platform  30  such that the shape of mold  34  is not changed, but remains cylindrical while adherend  10  is formed and bonded to bond site  22 . 
     In FIGS. 2-6, platform  30  is shown having a portion cut off to form front  44 . Front  44  is designed to facilitate visual access to mold  34  and bond site  22  as adherend  10  is being formed. Front  44  facilitates the alignment of mold  34  over the test piece  20  so that the mold is over the desired bond site. More particularly, by minimizing the distance between the perimeter of the platform and the mold it is easier to see the position of the mold over the test piece to select the location of the bond site. This is particularly important when testing the strength of an adherend on dentin of a tooth embedded in a holding material without contacting enamel. 
     While the platform is shown in FIGS. 2-6 as having an essentially circular perimeter with a portion cut away to yield a front for visual access, the platform may of course have any shape such as a circle or a square. The preferred shape and embodiment of platform  30  is as described in reference to FIGS. 2-6. Additionally, when the platform has a circular perimeter, the mold can be concentrically or eccentrically located. Similarly, if the platform is noncircular, the mold can be centered or offset. 
     FIGS. 4-6 depict a clamping assembly  60  which is a device used to hold platform  30  relatively immobile during formation of adherend  10 . The clamping assembly is first described in reference to FIG. 4 which provides an expanded perspective view of test piece  20 , platform  30  and clamping assembly  60 . The procedures for forming an adherend with platform  30  and clamping assembly  60  are described primarily in reference to FIGS. 5-6, as are the functions and advantages of some of the features of platform  30 . 
     Clamping assembly  60  comprises base  62 , posts  64 , plate  68  and retention nuts  66 . Clamping assembly  60  also preferably includes disc springs  61  and guide bushings  69 . Base  62 , in the depicted embodiment, is rectangular and has two permanently attached, upward extending posts  64 . Plate  68  has two similar apertures  67  on each side of plate  68 . Each aperture  67  has a guide bushing  69  press fit into it which is configured to removably mate with post  64  as plate  68  is mounted. Guide bushings may also alternatively be integrally connected to plate  68  and extend downward around each aperture such each post  64  extends through both a guide bushing and an aperture when the plate is mounted. Additionally, in a less preferred embodiment, a plate may be utilized without guide bushings. Each retention nut  66  is configured to removably mate with each respective post  64  to secure plate  68 . Posts  64  and retention nuts  66  are preferably threaded, as shown, such that each retention nut  66  can securely and removably mate with each post  64 . Disc springs  61  are designed to fit freely on posts  64  such that when retention nuts  66  are tightened against springs  61  a consistent load is applied to plate  68  and in turn to bonding platform  30 . Retention nuts  66  may be tightened and loosed such that plate  68  is securely and removably connected to base  62 . Clamping assembly  60  is an example of means for holding a platform and a test piece in a fixed position with respect to each other while adherend is formed and bonded to the substrate, such as tooth  25 . 
     Plate  68  also includes plate slot  63  and plate groove  65 . The perimeter of plate slot  63  comprises plate groove  65 . Plate groove  65  is defined by opposing track members  65   a  and  65   b  and by a plate groove face  65   c.  The configuration of plate groove  65  enables it to receive a portion of perimeter  40  of platform  30 . As shown in FIG. 5, platform  30  is slidably and removably inserted in plate groove  65 . Note that the width of platform  30  is substantially the same as the width of the space between the opposing sides of plate groove face  65   c.  While platform  30  and plate groove  65  can be designed to have a relatively close tolerance to ensure that platform  30  is securely held by the tight fit with plate groove  65  other mechanisms can also be utilized. For example, plate groove may have crimps, as shown at  65   d  in FIG. 4, which are very small portions of plate groove  65  wherein the opposing track members  65   a  and  65   b  are closer together than along the remainder of plate groove  65 . Crimps  65   d  are intended to bind perimeter  40  of platform  30  into plate groove  65  so that platform  30  maintains its position in plate slot  63 . 
     The assembly of the pieces shown in FIG. 4 is demonstrated in FIG. 5 while FIG. 6 is a cross sectional view of FIG. 5 taken along section line  6 — 6 . FIGS. 5 and 6 are discussed together. In FIGS. 5 and 6, test piece  20  is placed on base  62 . Platform  30  is inserted in plate slot  63  with perimeter  40  slidably and removably positioned in plate groove  65 . Plate  68  is then connected to base  62  by inserting each post  64  through each corresponding plate aperture  67 . Each retention nut  66  is then screwed onto each post  64  against spring discs  61 . Test piece  20  is oriented such that outlet rim  36  of mold  34 , as seen in FIG.  1  and FIG. 2, is directly above test piece  20 . When clamping assembly  60  is tightened, outlet rim  36  will be firmly in contact with test piece  20 , more particularly with dentin  27  of tooth  25 . Note in FIG. 5 that test piece  20  is not securely held until clamping assembly  60  is tightened. This permits test piece  20  to be oriented so that outlet rim  36  is above the desired bonding site, which in this instance is a portion of dentin  27 . For this purpose, platform  30  has front  44 , as described above, which provides visual access to outlet rim  36  and test piece  20  as clamping assembly  60  is being tightened. 
     Once test piece  20  is properly oriented on base  62  and platform  30  is positioned in plate slot  63  and held in plate groove  65 , retention nut  66  is tightened against disc springs  61  such that test piece  20  is securely and removably held in clamping assembly  60 . Due to the light pressure applied to the top of plate  68  by disc springs  61 , platform  30  does not flex or warp. In this position, mold  34  will not move relative to test piece  20  and the cylindrical shape of conduit  34  will not be deformed. 
     The purpose of perimeter support member  38  is evident at this point in FIGS. 5 and 6. Preferably, perimeter support member  38  and mold  34  extend downward from platform  30  an equal distance. Thus, as each retention nut  66  is being tightened, against disc springs  61 , outlet rim  36  and perimeter support member  38  will touch top surface  23  of test piece  20  at the same time. As clamping assembly  60  is tightened, perimeter support member  38  prevents the interface between outlet rim  36  and test piece  20  from being changed due to the bowing of body  31 . Accordingly, perimeter support member  38  prevents mold  34  from distorting or deforming and maintains the circular shape of outlet rim  36 . While the platform can be used without a perimeter support member, the platform preferably has a perimeter support member to ensure that the shape of the mold is not distorted and remains cylindrical such that the resulting bond strength measurements are accurate. It is also possible to use the clamping assembly without disc springs  61 . It is preferable, however, to use disc springs  61  as disc springs  61  ensure that uniform pressure is applied in each test. More particularly, disc springs  61  prevent over tightening of the assembly from occurring as over tightening can cause the bonding platform to warp. Perimeter support member  38  is an example of supporting means for bracing a platform on the top surface of a test piece. 
     Next, the method of forming adherend  10  and the bond between substrate  26  and restorative material  90  is described. After obtaining a test piece such as test piece  20  which already has a relatively flat and smooth top surface, then the top surface  23  of test piece  20  can be prepared as necessary. More specifically, the test sample or tooth  25  is prepared by first coating tooth  25  with a layer of etch and then rinsing the etchant off after the prescribed time. Second, a thin layer of primer and or adhesive is placed on the etched surface of tooth  25 . Preparing test piece  20  includes any step necessary to replicate conditions where the bond is relied upon, such as a restorative material in a tooth. The steps discussed above such as priming and chemically etching at least a portion of a test piece are examples of different steps for preparing a test piece. The primers or adhesives are cured as discussed below, which is another step of preparing the test piece. Of course, flattening or grinding a substrate embedded in a holding material to yield a two component test piece, such as is shown at  20 , is another example of a step necessary for preparing a test piece for use in a shear bond strength test. 
     After obtaining a clamping assembly and a test piece, then the clamp assembly is assembled as shown in FIGS. 4-6 with test piece  20  in position under platform  30 . Before clamping assembly  60  is tightened, test piece  20  is moved around until conduit  39  is directly over the desired bonding site. The ability to select the location of the bonding site by moving test piece  20  relative to platform  30  held by clamping assembly  60  is very advantageous, particularly when the potential bond site is the dentin of a tooth. Many prior art systems do not permit such movement. 
     Once clamping assembly  60  is tightened with platform  30  and test piece  20  oriented such that outlet rim  36  is in contact with test piece  20 , primers or adhesives on test piece  20  are light or chemical cured. More particularly, primers or adhesives visible through conduit  39  of mold  34  are cured. Platform  30  only permits the primers or adhesives visible through conduit  39  of mold  34  to be cured. Platform  30  prevents any excess primer from curing and prevents the formation of a “resin snowshoe” as the path of the curing light is blocked from the excess primer/adhesives by top surface  35  of platform  30 . The presence of a “resin snowshoe” on top surface  23  would interfere with the measurement of the bond strength for reasons explained in reference to FIG.  11 . 
     As indicated herein above, outlet end  42  is conical shaped to taper downward to form outlet rim  36 . One of the functions of outlet end  42  is to prevent excess primer or adhesives, which are applied to top surface  23  of test piece  20 , or more particularly the test sample, before composite material is bonded to bond site  22 , from being pushed inside conduit  39 . Note that if mold  34  did not have outlet rim  36 , and conduit  39  terminated instead with a flat surface, then pressing mold  34  tightly against top surface  23  of test piece  20  might cause excess primer on top surface  23  to be pushed inside mold  34 . Excessive primer or adhesive can result in capillary action causing the primer or adhesive to pool around the perimeter against conduit  39 , thereby resulting in a defective test method. The shape of outlet end  42 , particularly outlet rim  36 , prevents this from occurring. Despite the foregoing, primer or adhesive can in some instances be delivered through mold  34 , particularly when the primer or adhesive has a low viscosity. 
     Adherend  10  can be formed in mold  34  after preparing test piece  20  as needed, positioning test piece  20  and then securing test piece  20  in the desired position. To this end, FIG. 5 shows restorative material  90  being placed in conduit  39  of mold  34  via countersunk portion  32 . Applicator  92  is representative of any means for delivering restorative material to a conduit of a mold. Conduit  39  is filled with restorative material  90  up to the desired level. Excess restorative material  90  is removed from countersunk portion  32  such that restorative material  90  is essentially in only conduit  39 . As discussed above, mold  34  has a depth sufficient to permit or that does not prevent the curing light from curing all restorative material  90  inside of conduit  39 . At this point, restorative material  90  inside conduit  39  is light or chemical cured and cylindrically shaped adherend  10  is formed. The curing of restorative tooth material  90  finalizes the formation of the bond between restorative material  90  and the substrate, tooth  25 . 
     In the process of forming adherend  10 , outlet rim  36  of mold  34  rests on the bonding substrate. More particularly when a two component test piece is used which includes a test sample such as a flattened tooth, then outlet rim  36  rests on the test sample. When forming adherend  10 , outlet rim  36  is held securely against test piece  20  such that no hardenable material escapes from conduit  39  when conduit  39  is filled with hardenable material. The pointed shape of outlet rim  36  enhances its ability to prevent hardenable material from flowing out of conduit  39 . The tight contact between test piece  20  and outlet rim  36  also ensures that the bottom end of adherend  10  is cylindrical in shape. Because the strength of the bond between test piece  20 , particularly bond site  22 , and adherend  10  is tested at the bottom end of adherend  10 , the top surface of adherend  10  need not be necessarily flat. For this reason, excess composite material can be removed from countersunk portion  32  by any suitable device. 
     As indicated above, the pointed shape of outlet rim  36  combined with the ability of plate  68  to press platform  30  against test piece  20  as nuts  66  are tightened, provides a secure barrier against passage of the hardenable material out of conduit  39 . So after selecting the location of the bonding site, platform  30 , particularly outlet rim  36 , also ensures that the hardenable material does not flow beyond the bond site. This ability of outlet rim  36  to tightly interface with test piece  20  and prevent restorative material from spilling out of conduit  39  ensures that flashing does not form around adherend  10 . Overflow can skew test data in general. 
     After forming adherend  10 , retention nuts  66  are loosened and plate  68  is lifted. The bond assembly, which is adherend  10  as bonded on test piece  20 , is then disconnected from platform  30 . This can be achieved by pushing on the top of adherend  10  with a suitable instrument while holding platform  30  or plate  68  or by pulling the bond assembly away from platform  30 . The configuration of platform  30  ensures that the structure and position of the adherend is not altered when platform  30  is removed. Excess primer and adhesive on top surface  28  of test piece  20  may be removed as necessary. 
     FIGS. 7 through 11 illustrate methods and apparatus for shearing and testing the strength of a bond formed between adherend  10  and test piece  20 . One embodiment of the present invention is a crosshead  50 , illustrated in FIG.  7 . FIG. 8 is a top view of crosshead  50  and FIG. 9 is a cross-sectional view of FIG. 8 along section line  9 — 9 . FIGS. 7,  8  and  9  are discussed simultaneously. 
     The force required to shear adherend  10  from bond site  22  on test piece  20  with crosshead  50  is measured and recorded. Crosshead  50  is designed to push against the base of adherend  10  such that the measured force is the force required to shear adherend  10  from bond site  22  on test piece  20  rather than the force required to fracture or deform adherend  10 . 
     The primary parts or surfaces of crosshead  50  include a top surface  57  opposite a bottom surface  54 . At one end of bottom surface  54  is a contact surface  55  which slants up toward top surface  57  at a shallow angle. Contact surface  55  can angle up from bottom surface  54  at any degree which enables only contact surface  55  to contact a test piece during a bond strength test. For example, the angle may be about 5°. As described in greater detail herein below, this configuration as well as the design of the embodiment shown in FIGS. 12-16, enable only contact surface  55  to contact and rest flush on the test piece. 
     Crosshead  50  has a face  5  that extends between contact surface  55  and top surface  57 . Crosshead  50  also has an aperture  59  used to attach crosshead  50  to a particular device or arm  80  which provides the shearing force. Crosshead  50  must be mounted to arm  80  at an angle where contact surface  55  would be able to be aligned parallel to top surface  23  of test piece  20 . 
     Contact surface  55  has a groove  52  cut into it which extends perpendicularly from contact surface  55  through to top surface  57  such that groove  52  is visible in its entirety in face  51  which extends between contact surface  55  and a top surface  57 . Although, groove  52  preferably has a semicircular shape as shown, it may also have other configurations. Groove  52  has a top end at top surface  57  and a bottom end at contact surface  55 . Groove  52  is an example of groove means for receiving an adherend. 
     At the bottom end of groove  52 , where it meets contact surface  55 , is a section which has a slightly smaller radius than the majority of groove  52 . This smaller diameter section creates a lip  53  which extends inward from groove  52 . The front or inside radius of lip  53  is also perpendicular to contact surface  55 . The configuration and size of lip  53  relative to groove  52  enables an adherend to be sheared from a bond site due to contact from lip  53  and not due to other structures such as groove  52 . While the lip is shown having a semicircular configuration, it may have any configuration corresponding with that of the groove. More particularly, the lip is sized and shaped to correspond with the size and shape of the adherend while the groove is complimentary to this shape and slightly larger in dimensions. Lip  53  is an example of means for contacting an adherend to shear the adherend from the bond site on the test piece when the crosshead is pushed against an adherend. 
     The functions of the parts of crosshead  50  are described in relation to FIGS. 10 and 11. FIG. 10 is perspective view of crosshead  50  positioned to test the strength of a bond between adherend  10  and bond site  22  on a test piece  20 . FIG. 11 is a cross-sectional view of FIG. 10 taken along section line  11 — 11  and illustrates lip  53  pushing against the base of adherend  10 . 
     Arm  80  is representative of any device capable of connecting to crosshead  50  such that crosshead  50  can be moved to shear adherend  10  from test piece  20  or more particularly from dentin  27  of tooth  25 , which is embedded in holding material  28 . To test the strength of a bond, crosshead  50  is oriented such that contact surface portion  55  is flush with top surface  23  of test piece  20 . Contact surface portion  55  is aligned such that when contact surface portion  55  is flush with top surface  23 , groove  52  and lip  53  are essentially perpendicular to top surface  23 . This permits lip  53  to push directly against the base of adherend  10  and prevents groove  52  from pushing against the top of adherend  10 . If groove  52  were to push against adherend  10 , there would be an increased likelihood that adherend  10  would fracture rather than test the adhesive. More particularly, if adherend  10  were pushed at the top, adherend  10  is likely to break rather than the adhesive. Because pushing at the base of adherend  10  with lip  53  prevents adherend  10  from being used as a lever, a more accurate measurement of the bond strength can be taken. Lip  53  also has a thickness suitable to prevent lip  53  from fracturing adherend  10 . 
     Only lip  53  contacts adherend  10  and pushes against the base of adherend  10 . If groove  52  did not have a larger diameter than lip  53  then groove  52  would contact adherend  10 . As indicated herein above, excessive contact between groove  52  and adherend  10  would enable groove  52  to lever or push or adherend  10  and any measurement of the bond strength would be skewed accordingly. 
     Contact surface portion  55  also helps to minimize the amount of surface area of crosshead  50  in contact with test piece  20  such that the force of friction is minimized as the strength of the bond between adherend  10  and test piece  20  is tested. Also when contact between crosshead  50  and top surface  23  is minimized there is less opportunity for obstructions to hinder placing contact surface  55  flush with top surface  23 . Once crosshead  50  is oriented correctly, a vertical force is applied until adherend  10  is sheared from bond site  22  of test piece  20 . The strength of this force is measured and recorded. 
     Note that arm  80  is coupled to crosshead  50  by screw  82  in aperture  59 . Aperture  59  is an example of attachment means for attaching a crosshead to an arm. Arm  80  is an example of pushing means for pushing a crosshead vertically against an adherend to shear the adherend from a test piece. 
     FIGS. 12-16 depict another embodiment of a crosshead identified at  70 . FIGS. 12-16 respectively depict crosshead  70  in the same views as crosshead  50  is depicted in FIGS. 7-11. 
     Crosshead  70  has essentially the same parts as crosshead  50  however the configurations differ. Crosshead  70  has a face  71 , which extends between a contact surface  75  and a top surface  77 . Crosshead  70  also has an aperture  79  used to attach crosshead  70  to a particular device which provides the shearing force, such as arm  80 . 
     Contact surface  75  is offset but parallel to bottom surface  76  and is connected to bottom surface  76  by connecting surface  74 . Having contact surface  75  offset from bottom surface  76  accomplishes the same thing for crosshead  70  that contact surface  55  being at an offset angle from bottom surface  54  does for crosshead  50 ; namely this allows for only the contact surfaces of these crossheads to be in contact with top surface  23  of test piece  20  during testing. 
     Contact surface  75  also had a groove  72  cut into it which extends perpendicularly from contact surface  75  through to top surface  77  such that groove  72  is visible in its entirety in face  71 . Groove  72  preferably has a semicircular shape like groove  52  and is another example of a groove means. A lip  73  is located at the bottom end of groove  72  which extends out from beyond the bottom end of groove  72  as groove  72  has a slightly larger radius than lip  73 . Lip  73  is another example of a means for contacting an adherend to shear the adherend from the bond site on a test piece. 
     Face  71 , groove  72 , and lip  73  are all substantially perpendicular with top surface  77  and bottom surface  76 . In contrast to the configuration of contact surface  55 , groove  52  and lip  53  of crosshead  50 , it is unnecessary for contact surface  75  and groove  72  to be angled due to contact surface  75  being offset from bottom surface  76 , as described below. 
     Bottom surface  76  is recessed but perpendicular to contact surface portion  75 , which is configured to rest flush on the top surface of a test piece and for movement or sliding on the test piece as lip  73  is urged against an adherend. Bottom surface  76  is recessed from contact surface  75  to minimize contact of crosshead  70  to top surface  23  of a test piece  20 . This minimizes friction and allows for flush placement of contact surface  75  so that shear forces are applied to adherend  10  rather than leverage forces as would be the case if crosshead  70  were obstructed from being placed directly against top surface  23 . The offset from flat section  76  and contact surface portion  75  is typically about 0.025 inches. 
     As shown in FIG. 16, the configuration of contact surface  75  enables crosshead  70  to approach and contact an adherend with bottom surface  76  essentially parallel to the top surface  23  of test piece  20  as well as top surface  77 . Additionally, groove  72  is essentially parallel to adherend  10 . By resting contact surface portion  75  on top surface  23 , it is virtually impossible for groove  72  to contact adherend  10  which is similar to the function of crosshead  50 . More particularly, the configuration of crosshead  50  requires that crosshead  50  be attached to arm  80  at an angle to allow contact surface  55  to be placed parallel to top surface  23  and to allow groove  52  and lip  53  to be parallel to adherend  10 . With crosshead  70 , the same alignment to adherend  10  and top surface  23  are accomplished but without the angles involved with crosshead  50 . This makes crosshead  70  much easier for machinists to fabricate because basically all of its surfaces are parallel or at right angles to each other. These crossheads are preferably made of hardened steel to reduce deformation and wear. 
     Note that in another embodiment, the crosshead is similar to that shown at  70  except the bottom surface and the contact surface are not offset with respect to each other such that there is a single flat surface. In such an embodiment, it may be necessary to identify the effect of friction and misalignment or leverage forces to correctly determine the bond strength. 
     In review, some advantages of the features of the platform and the crosshead are described herein below. Platform  30  illustrated in FIGS. 2 and 3 increases the likelihood of obtaining an accurate bond strength measurement for numerous reasons. First, platform  30  prevents the formation of a resin snowshoe. If a resin snowshoe is formed, then the bond tested is not between adherend  10  and bond site  22 , but between adherend  10  and more surface area of test piece  20 . Platform  30  ensures that the bond does not extend beyond the area within mold  34  of platform  30 . The configuration of conduit  39  yields a cylindrically shaped adherend  10 . Perimeter support member  38  prevents body  31  of platform  30  from bowing and thereby distorting the shape of adherend  10 . Front  44  of platform  30  provides visibility to the interface between outlet rim  36  and test piece  20  to ensure proper placement of the bond. Body  31  prevents the curing light from curing the excess primer/adhesives thereby eliminating the formation of a resin snowshoe. Outlet rim  36  allows for better isolation and less displacement of adhesives upon placement of platform  30 . 
     Crosshead  50 , illustrated in FIGS. 7 through 11, and crosshead  70  in FIGS. 12-16 also increases the likelihood of obtaining an accurate bond strength measurement. First, contact surface portion  55  or  75  reduces the force of friction by limiting the amount of surface area of the bottom surface of the crosshead that is in contact with top surface  23  of test piece  20 . Second, lip  53  or  73  pushes against adherend  10  at a point near the bond between adherend  10  and test piece  20 . Third, the lip has a thickness sufficient to load adherend  10  without fracturing or levering it from test piece  20 . Fourth, the groove  52  or  72  is larger than the lip such that contact is prevented between adherend  10  and the groove when the crosshead is positioned or oriented to shear adherend  10  from test piece  20 . Fifth, the lip has the same essentially the same diameter as adherend  10  and is a means of distributing the applied test load over the base of adherend  10  such that adherend  10  is not fractured. If the lip was not present, then a very small point of contact would be made as a straight line contacted adherend  10  thus increasing the likelihood of fracturing adherend  10 . Sixth, the contact surface portion facilitates easy alignment of crosshead  50  to be parallel and flush with surface  23  of test piece  20 , particularly contact surface portion  75 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.