METHOD AND APPARATUS FOR PREPARING A WORKPIECE FOR TESTING

Technical Field The present invention relates to cutting a workpiece and more particularly to a method for testing the properties of the workpiece and an apparatus for preparing the workpiece for testing.

Background Art

In both the aircraft and aerospace industries, coatings are essential protection for structures, such as the fuselage, from destructive environmental conditions. Typically, the structure has a coating bonded to a substrate. If adhesion is poor between the coating and the substrate, then the protective properties of the coating may not work as well or at all. In certain cases, a second coating of a different material is bonded on top of an initial coating. In the case of the Space Shuttle solid rocket boosters, the aluminum booster's initial coating is a conventional paint coating and the second coating is a special Thermal Protective System (TPS), which helps protect the substrate from overheating. If the initial coating debonds from the substrate, and takes the special TPS with it, then catastrophic failure of the substrate can occur and an entire mission can be lost. in order to assess and maintain an acceptable level of coating adhesion, the wet/dry tape test is performed on workpieces of all types having a coating bonded to a substrate. The wet tape test is more severe than the dry tape test and is used when moisture intrusion into the coating is possible during use. The dry tape test is used when moisture intrusion is not a factor or the quality level of the process has been verified to allow the use of the dry tape test. The American Society for Testing Materials (ASTM) has developed the industry standard procedure for performing the tape test. Specification ASTM D 3359 outlines the ASTM procedure for measuring the coating adhesion by tape testing. The specification requires the following steps for dry tape testing: (1) cutting the coating down to the substrate with an "X" or cross-hatch pattern using a knife blade, (2) applying a piece of pressure-sensitive tape over the pattern, (3) removing the tape from the pattern, and (4) measuring the results. The test results are measured by rating on a scale from five to zero the relative amount of coating

remaining bonded to the substrate, after removing the tape. For example, a five rating indicates no coating was removed from the substrate or transferred to the tape. A four rating indicates that a small amount of the coating was removed from the substrate or transferred to the tape adjacent to the intersection of the cut lines. A zero rating indicates excessive coating bond failure. A five or four rating is usually considered acceptable.

In the case of wet tape testing, the moisture-related steps of emerging the substrate in water or exposing the substrate to 100% humidity for twenty-four hours and wiping dry the coating within two hours of applying moisture are performed before steps (1) to (4). The ASTM specification demands that the operator make only one attempt at each cut.

The current method for cutting the pattern in the coating in preparation for the tape test involves an operator using a template and manually cutting the coating around the perimeter of the template using a knife blade. This method results in several problems ranging from inconsistency in results and damaging the substrate or coating to safety hazards and poor visibility. The first problem is the inconsistency of results on the same sample. The ASTM specification acknowledges that the most accurate results are achieved when the cut line goes through the entire coating at a uniform depth. The cut depth varies because of the operator's technique and the nature of the blade.

Operator technique in applying pressure to the knife blade is critical to the depth of the cut. The pressure necessary to cut the coating can vary throughout the cut as well as from blade to blade. The variation from blade to blade is related to the blade angle, which is the grind angle of the blade itself. For instance, a small blade angle of approximately 15-22° would be possessed by a razor blade.

As the blade angle decreases, the mechanical advantage possessed by the knife decreases; therefore, the force necessary to cut the coating increases. Because the operator may not know how much pressure to use at first with a particular knife blade, the pressure applied and the depth of the cut may vary. T e nature of the blade itself can cause the depth of the cut to vary on a microscopic level. Referring to Fig. 1 , a prior art cross-sectional view of the microscopic effect of using a knife blade 10 to cut a workpiece 11 having a coating 12 bonded to a substrate 14. The arrow D indicates the motion of the knife blade 10 during cutting. On a microscopic level, the surface of the substrate 14 has peaks 14a and valleys 14b. If the knife blade 10 were to cut the coating 12 along a line A, a clean cut avoiding the substrate peaks 14a would be made. The cut along line A would leave a great deal of coating 12 undistributed in the substrate

valleys 14b. Testing such a shallow cut may measure a false four or five. Thus, test results may report that the adhesion is acceptable even though the coating/substrate bond has not been weakened by the cut as the test requires. In an attempt to cut away additional coating 12, the knife blade 10 can cut along a line B. However, this leads to a damage zone between lines A and B, where the substrate peaks 14a' have been removed while still leaving coating 12' in the substrate valleys 14b. Testing such a deep cut may report that the adhesion is unacceptable even though the coating substrate bond may be failing prematurely due to the additional stress introduced into either the coating or substrate by the knife blade 10. Variability of results has been a major concern of the tape test inspectors because they may be unsure which results are an accurate representation of the coating's bond strength.

The second problem of possibly damaging the substrate or the coating has resulted in the tape test being performed on a test part in lieu of the actual hardware. Refeπing to Fig. 1, damage can be done to the substrate peaks 14a', if the operator applies so much pressure that he cuts along the line B or below, and this is routinely done in order to remove a sufficient amount of coating 12.

The damage done to the coating is in the form of coating fracture and can occur in two situations. The first situation occurs because initiating a cut with the knife blade begins as a notch. Since a notch is a stress concentration in the coating, it can lead to coating fracture. The second situation occurs if the knife edge dulls enough to cause coating fracture rather than a clean cut. Although the ASTM specification points out that when a knife blade dulls it should be replaced, it is still possible that the precautions taken against dulling may fail and fracture occurs anyway. Certain parts once coated are very expensive and difficult to repair, or if significantly damaged, must be replaced. Because of the possibility of damage to the substrate or coating, a test part may be used for testing. The testing done on the test part may not be representative of the quality level of the adhesion on the actual hardware, thus, leaving the inspectors to extrapolate the results on the actual hardware from the results on the test part.

The third problem with using a knife blade involves a potential safety hazard. When cutting the pattern, the operator uses a template or straight edge. The operator can slip using the knife blade during cutting and injure himself. Also, the sharp edge of the blade, if not covered properly during transportation, can injure the operator. Although safety precautions may be in place to minimize injuries, a potential hazard is always present when a sharp instrument is involved.

The last problem is visibility into the bottom of the cut. Visibility is important so that the operator can see to the substrate in order to verify that the entire coating has been cut. Using the knife blade to cut the coating makes visibility difficult because the cut lines are so thin the operator may not be able to see into them.

What is needed in the art is an improved method for testing the adhesion strength between the coating and the substrate on which it is applied, and an apparatus for preparing a workpiece for adhesion testing. The method and the apparatus should allow for more consistent results; decrease the possibility of damage to the substrate and coating so that the actual hardware can be tested; decrease the potential safety hazards associated with a sharp instrument; and increase the operator's visibility into the depth of the cut line.

Disclosure of Invention

According to the present invention, a method for testing the properties of a workpiece having a coating bonded to a substrate is disclosed. The method comprising the steps of directing an abrasive media stream against the coating thereby removing that portion of the coating from the substrate, and performing a test on the properties of the workpiece. The properties that can be tested are the adhesion strength of the coating to the substrate and corrosion. The present invention also includes an apparatus for preparing the workpiece for testing. The apparatus comprises a guide for directing the nozzle of a grit blast gun in a pattern. The guide forms an opening in the shape of the desired pattern. The guide is disposed such that an abrasive media stream exiting the nozzle follows the perimeter of the opening and causes the abrasive media stream to contact the workpiece such that a portion of the coating is removed in the shape of the perimeter of the opening.

Brief Description of the Drawing

Fig. 1., a prior art cross-sectional view of the microscopic effect of using a knife blade to cut a workpiece having a coating bonded to a substrate. Fig. 2., a cross-sectional view of the microscopic effect of utilizing the present invention to prepare a workpiece having a coating bonded to a substrate.

Fig. 3., an exploded view of one embodiment of the present invention having an apparatus for preparing a workpiece for adhesion testing.

Fig. 4., a partially broken away front perspective view of an apparatus for preparing a workpiece for adhesion testing of the present invention.

These figures are meant to be exemplary and not to limit the generally broad scope of the present invention.

Best Mode of the Invention

Referring to Fig. 2, a cross-sectional view of the microscopic effect of utilizing the present invention to prepare a workpiece 11 having a coating 12 bonded to a substrate 14. The abrasive media stream 16, which can be an abrasive media, such as crushed glass, entrained in pressurized air, wears away the coating 12 in a progressive manner to an approximately uniform depth illustrated by the line C. The substrate valleys 14b have little coating 12' remaining in them after the abrasive media stream 16 has contacted the coating 12. The abrasive media stream leaves rounded substrate peaks 14a". These rounded peaks 14a" roughen the substrate in preparation for re-coating, rather than damage the substrate. The damage that may be done by a knife blade (not shown) is illustrated with the phantom lines 14a'.

Refeπing to Fig. 3, an exploded view of one embodiment of the present invention having an apparatus 20 for preparing a workpiece 11 for adhesion testing. The workpiece 11 has a coating 12 bonded to a substrate 14. Fig. 3, depicts a single coating 12 bonded to the substrate 14. However, the apparatus 20 can be used with a substrate having multiple coating layers bonded to it, or a substrate with a primer applied prior to applying the coating. The apparatus 20 is for use with a means for directing the flow of an abrasive media stream onto the coating. Ideally, this is performed using a grit blast gun 22. However, other mechanisms may be used as well, so long as, they can direct an abrasive media through a nozzle with enough force to remove the coating. In this embodiment the grit blast gun 22 has a nozzle 24 and a means for delivering 26 an abrasive media stream entrained in an air stream (not shown) to the nozzle 24.

The apparatus 20 is a guide which in this embodiment comprises a housing 28, a gun retaining assembly 30, a mounting assembly 32, and a debris outlet assembly 34. The housing 28 for enclosing a portion of the workpiece 11 has front, rear, left and right vertical side walls 36, 38, 40, 42, respectively, a top wall 44 and an open base 45. The front and rear vertical side walls 36, 38 are spaced apart from and parallel to one another. The left and right vertical side walls 40, 42 are spaced apart from and parallel to one another and perpendicular to the front and rear vertical side walls 36, 38. In this embodiment, the left and right vertical

side walls 40, 42 each may have a notch 46 for retaining the top wall 44. In another embodiment, at least two of the vertical side walls may have grooves instead of notches for receiving the top wall. A hole 47 is disposed through the left vertical side wall 40. In addition, gasketing 48 for sealing the housing 28 is secured to the bottom of all of the vertical side walls 36, 38, 40, 42. The gasketing 48 should be any chemically resistant rubber, like neoprene or its equivalent, that has a durometer that allows gasketing to contour to a curved workpiece.

The housing must have a means for guiding the nozzle that forms a pattern. Preferably, this means is the top wall 44 forming an opening 50 in the shape of a pattern. However, any structure can be used that can form the necessary pattern such as a template. In this embodiment, the top wall 44 is removable. Since the top wall 44 is removable, the same housing 28 can be utilize with any combination of top walls 44 with various patterns. However, the top wall 44 can be permanently attached to the vertical side walls 36, 38, 40, 42 by any means such as fasteners or adhesive or injection molded into the housing 28. In another embodiment, using the template, the template can be disposed in the open base 45 of the housing. The template can be independent of the housing and held to the workpiece in some way, such as taped, or the template can be attached to the housing with tape or incorporated into the housing with fasteners, adhesive, or injection molding.

The top wall requires a means for preventing the abrasive media stream (not shown) from exiting the housing. Preferably, this means is a bearing seal 52, bonded to the perimeter of the opening. The bearing seal 52 in this embodiment is Plexiglas; however, any material, such as polycarbonate or acrylic, can be used as long as the material has plenty of rigidity for stability and wear. However, a flexible dust boot can also be used with the top wall or a template.

In this embodiment, the vertical side walls 36, 38, 40, 42 form a rectangular housing 28 for ease of manufacture, but the housing 28 can be any shape, such as cylindrical. The size of the vertical side walls 36, 38, 40, 42, top wall 44, opening 50 and the bearing seal 52 depend on the size of the workpiece 11 and the grit blast gun 22. Additionally, in the embodiment shown, the housing is built from separate front, rear, left and right vertical side walls 36, 38, 40, 42 that are coupled with adhesive. However, where the vertical side walls could be manufactured by a process, such as injection molding, the housing may be a molded unit with the top wall 44 insertable. As an alternative, the housing, including the top wall 44, can be a molded unit. The housing should be made of any transparent material to increase the operator's visibility into the housing 28;

however, an opaque material can be used, knowing that this will decrease the operator's visibility. Plexiglas is preferred; however, any plastic, such as a polycarbonate or acrylic, can be used because they are transparent. Metal can also be used so long as the structural requirements are met. The gun retaining assembly 30 for holding the grit blast gun 22 has an upper plate 54 and a retainer 56. The upper plate 54, which is the support member of the gun retaining assembly 30, forms a hole 58. The upper plate 54 should be transparent for visibility and made of the same material as the housing 28. The dimensions of the upper plate 54 depend on the dimensions of the housing 28 and require that the upper plate be able to support the elements of the gun retaining assembly 30.

The retainer 56 for holding the means for delivering 26 is coupled to the upper plate 54 and disposed around the hole 58 in the upper plate 54. The retainer 56 must fixedly hold the means for delivering 26. The retainer in this embodiment is an adjustable split bushing.

The mounting assembly 32 for holding the housing 28 against the workpiece 11, in this embodiment, has at least two brackets represented by the bracket 60 and at least two suction cup assemblies, represented by the suction cup assembly 62. The bracket 60 for coupling the housing 28 to the suction cup assembly 62 is attached to the front vertical side wall 36 by adhesive; however, any fastener such as a nut and bolt can be used. Each bracket 60 has a first portion 60a substantially parallel and connected to the front side wall 36, and a second portion 60b substantially perpendicular and integral with the first portion 60a. There is a hole 64 disposed through the second portion 60b. The suction cup assembly 62 for holding the housing 28 against the workpiece 11 with positive pressure has a suction cup 66, an integral bolt 68 extending from the suction cup 66, and a pump 70 connected to the suction cup 66. The suction cup assembly 62 is commercially available and manufactured by Woods Power-Grip Co. Inc. under the name "gripper". The integral bolt 68 extends through the hole 64, and a nut 72 is screwed to the integral bolt 68. In other embodiments, the mounting assembly 32 can be a magnetic device, adhesive or even tape, so long as the housing 28 is connected to the workpiece 11 in a manner which allows the operator to keep both hands free during the removal of the coating. The debris outlet assembly 34 removes a debris, which is the spent abrasive media stream and removed coating (not shown), from the housing 28 by creating a lower pressure outside the housing 28 than within. The debris outlet

assembly in this embodiment has a removal tube 74, a debris bag (not shown), a vacuum (not shown), and a sealing means (not shown). The debris outlet assembly 34 used is commercially available and manufactured by Mc Master-Carr under the name Mini-Vac. In this embodiment, the removal tube 74 for transporting the debris from the housing 28 to the debris bag is disposed through the hole 47, so that the removal tube 74 is close to the debris (not shown) on the workpiece 11. In another embodiment, the removal tube 47 can be disposed through the top wall 44; however, this is not the best configuration because the operator's visibility would be decreased by the upward moving debris within the housing 28, and additional suction might be necessary in order to lift the debris from the workpiece 11 to the removal tube 74.

The removal tube 74 should be disposed through a sealing means (not shown). The sealing means can be any device which allows an airtight seal around the removal tube 74, such as rubber o-rings on both the interior and exterior sides of the left vertical side wall 40 or a grommet pressure fit around the removal tube 74 within the hole 47 in the left vertical side wall 40. The grommet is preferred over the o-rings because the grommet is less likely to fall out of the hole 47 or lose seal.

Referring to Fig. 4, a partially broken away front perspective view of an apparatus 20 for preparing a workpiece 11 for adhesion testing of the present invention. In Fig. 4, the apparatus 20 is fully assembled and functioning. The upper plate 54 is disposed on the bearing seal 52. The means for delivering 26 is disposed through the retainer 56 and the opening 50. A abrasive media stream 16 is exiting the nozzle 24 and contacting the coating 12. in operation, the operator (not shown) should conduct an apparatus 20 set-up to determine all the variables necessary to use the apparatus. The set-up should be done on a test workpiece 11. Refeπing to Fig. 3, first, the operator sets the housing 28 on the workpiece 11. If a wet tape test is to be performed, the operator must apply moisture in the appropriate fashion and dry the workpiece before setting the housing on the workpiece. Next, the operator must engage the mounting assembly 32 by pumping up each suction cup 66 using the pump 70 until the housing 28 is firmly attached to the workpiece 11. Then, the top wall 44 with the opening 50 is inserted into the notches 46 in the vertical side walls 36, 38, 40, 42. Referring to Fig. 4, next, the operator sets the upper plate 54 on top of the top wall 44 such that the upper plate 54 is in contact with the bearing seal 52. The

operator must select the grit blast gun 22, the nozzle 24, and the abrasive media for the abrasive media stream as known by those of ordinary skill in the art.

The grit blast gun 22 is commercially available and well known in the art. The grit blast gun used was the Micro-Jet 200 manufactured by Hunter Products, Inc. The grit blast gun 22 is placed such that the means for delivering 26 is disposed through the retainer 56 on the upper plate 54 and through the opening 50 in the top wall 44. The nozzle chosen will depend on the desired width of the cut.

The abrasive media utilized will be a function of the coating and the substrate. Selection of the appropriate abrasive media is well known in the art. For example, a crushed glass abrasive media is preferred with an urethane or epoxy coating on metal or composite substrates, and an aluminum-oxide abrasive media is preferred on harder coatings, such as ceramic bonded lubricant on substrates of metal, composite or ceramic. The particle size of the abrasive media selected should be a function of the particle density and the nozzle inside diameter as known by those of ordinary skill in the art.

The standoff distance or the distance from the nozzle 24 to the coating 12, should be adjusted by and raising or lowering the nozzle 24. The standoff distance should be adjusted so that the distance allows the abrasive media stream 16 to reach a certain speed and shaping at impact with the workpiece 11 and so that the nozzle 24 does not hit the workpiece 11 at any given position on the workpiece 11. This standoff distance is a function of the nozzle chosen, and calculation of the standoff distance is well known in the art. During flow of the abrasive media stream, the standoff distance can be adjusted as needed. Now, the debris outlet assembly 34 is started so that positive flow of the vacuum (not shown) pulls the upper plate 54 into closer contact with the bearing seal 52 in the top wall 44. Now, the operator should begin the abrasive media stream flow. The abrasive media and air will enter the housing 28 through the means for delivering 26 from the abrasive media and air sources (not shown) and exit the nozzle as the abrasive media stream 16. The air pressure should be selected to allow progressive removal of the coating, minimal damage to the substrate upon contact with the abrasive media stream, and compatibility with the nozzle selected. Generally, the air pressure ranges from approximately 38 to 100 psi.

Once the abrasive media stream 16 has been directed against the coating such that a portion of the coating 12 is removed, the operator begins traversing the workpiece 11 along the perimeter of the opening 50. The travel speed or the speed at which the operator traverses the workpiece is deveiopmentally

determined by sight. Since the coating is removed progressively, the operator should allow the abrasive media stream 16 to contact the same area of coating until the operator sees the substrate 14; then, the operator should move the nozzle 24 along the perimeter of the opening 50. If the operator wants to go slowly, the operator can remove a small portion of the coating in one area, move on to another area and come back to finish the previous area. If the operator accidentally allows the abrasive media stream 16 to contact the substrate 14 for a small time, this is not detrimental to the part. This results from choosing the abrasive media appropriate for the substrate 14; thus, with a little contact, the abrasive media stream 16 will only roughen the substrate 14. The roughening of the substrate 14 will not affect the tape test results or damage the workpiece 11. Roughening will only prepare the workpiece 11 to be re-coated. The nozzle 24 movement continues until the coating has been removed in the form of the pattern, then the media flow is stopped. Next, the vacuum is turned off and the mounting assembly 32 disengaged from the workpiece by pinching each suction cup 66 at the predetermined pinching area (not shown).

Now, the operator can perform the coating removal on the actual hardware with the parameters determined by the set-up. Once this removal is done, the operator is free to perform a the test on the properities of the workpiece. One such property that can be tested is the adhesion strength of the coating. This can be done by applying a tensile force to the portion of the coating remaining bonded to the substrate around the removed portion. There are several conventional ways to apply the tensile force. The preferred method is tape testing as defined in the ASTM Specification. However, any method in which a tensile force is applied to the coating such that the adhesive strength of the coating can be measured can be used.

Corrosion testing may also be performed after using the apparatus by exposing the workpiece to a corrosive environment, like salt spray, after coating removal. After exposing the workpiece, the effects of the corrosive environment on the workpiece can be measured. The use of abrasive media stream to remove the coating instead of a knife blade in corrosion testing may have the advantage of increased consistency of the testing. Unlike the knife blade the abrasive media stream is non-metallic and neutral, therefore, would not accelerate corrosion where the knife blade can leave metallic residues on the surface of the substrate that could enhance corrosion.

While a particular invention has been described with reference to illustrated embodiments, this description is not meant to be construed in a limiting sense. It

is understood that although the present invention has been described in a preferred embodiment, various modifications of the illustrative embodiments, as well as additional embodiments of the invention, will be apparent to persons skilled in the art upon reference of this description without departing from the spirit of the invention, as recited in the claims appended hereto. It is therefore contemplated that the appended claims will cover any such modification or embodiments that fall within the true scope of the invention. We claim: