Method and device for inspecting a joining surface

Abstract

A method for inspecting a joining surface (14) of a substrate, wherein a component is to be adhered to the joining surface of the substrate by means of an adhesive material (27), wherein the method comprises the following steps: •—providing at least one planar test textile (20), which has a fiber material (21) and an adhesive primer (22), •—applying the planar test textile to at least one part of the joining surface of the substrate to which the component is to be adhered so that the adhesive primer of the planar test textile contacts the joining surface of the substrate, •—at least partially curing the adhesive primer of the planar test textile in order to integrally bond the planar test textile to the substrate by means of the adhesive primer, •—pulling off the planar test textile after at least partially curing the adhesive primer and inspecting the joining surface by means of a qualitative evaluation of the fracture pattern between the cured adhesive primer and the planar test textile and/or by means of a quantitative evaluation of the pull-off force determined when pulling off the planar test textile.

Claims

1. A method for inspecting a joining surface of a substrate, comprising: providing at least one sheetlike test textile which comprises a fiber material and an adhesive primer, applying the at least one sheetlike test textile to at least part of the joining surface of the substrate to which a structural element is to be adhered so that the adhesive primer of the at least one sheet test textile makes contact with the joining surface of the substrate, at least partly curing the adhesive primer of the at least one sheetlike test textile to connect the at least one sheetlike test textile to the substrate using the adhesive primer, wherein the at least partly curing step produces a cured adhesive primer, removing the at least one sheetlike test textile after the at least partial curing of the adhesive primer, and inspecting the joining surface to ascertain whether there is a defect-free attachment of the adhesive primer over a full area of the joining surface by at least one of either: a) performing a qualitative evaluation of a fracture pattern formed between the cured adhesive primer and the at least one sheetlike test textile during the removing step of the at least one sheetlike test textile, and determining the joining surface as having defect-free attachment when the fracture pattern is uniform, and as having defective attachment when the fracture pattern is nonuniform; or b) performing a quantitative evaluation of ascertaining whether the removal force determined during the removing step of the at least one sheetlike test textile has a uniform force profile, and determining the joining surface as having defect-free attachment in the case of ascertaining the uniform force profile, and defective attachment in the case of ascertaining a nonuniform force profile.

2. The method as claimed in claim 1, wherein the substrate is a fiber composite component produced from a fiber composite material comprising a fiber material and a matrix material.

3. The method as claimed in claim 1, wherein the substrate is formed of one or more of a polymer material, metal material, a metal alloy, an organic material, and an inorganic material.

4. The method as claimed in claim 1 further comprising recording the fracture pattern between the cured adhesive primer and the at least one sheetlike test textile using a camera, a uniform or nonuniform fracture pattern being ascertained from recorded image data using an image analysis unit, and/or in that the removal force during the removal of the sheetlike test textile is ascertained using a force sensor wherein a uniform or nonuniform force profile is ascertained using a force analysis unit.

5. The method as claimed in claim 1 further comprising conditioning the at least one sheetlike test textile during and/or after the curing of the adhesive primer by exposing the at least one sheetlike test textile to a predetermined test temperature over a predetermined timespan and/or by associating the at least one sheetlike test textile with a medium over a predetermined timespan, wherein the step of removing the at least one sheetlike test textile is performed after the step of conditioning.

6. The method as claimed in claim 1 wherein the adhesive primer is completely cured before the at least one sheetlike test textile is removed.

7. The method as claimed in claim 1 wherein the providing step is performed such that the at least on test textile comprises one or more spacers disposed on the fiber material of the at least one test textile and/or one or more spacers contained in the adhesive primer.

8. The method as claimed in claim 1 wherein only a) is performed.

9. The method as claimed in claim 1 wherein only b) is performed.

10. The method as claimed in claim 1 wherein both a) and b) are performed.

Description

(1) Further explanation of the invention is given, by way of example, using the appended figures, in which

(2) FIGS. 1a-1c—show schematic representation of the key method steps for the inspection;

(3) FIG. 2—shows schematic representation of a repair based on the inspection;

(4) FIG. 3—shows an exemplary embodiment of an inspection; and

(5) FIG. 4—shows schematic-macroscopic representation of a fracture pattern.

(6) FIGS. 1a-1c show a possible course of the inspection method in a schematic representation. Provided first of all in the first step (a) is a substrate in the form of a fiber composite component 10, which is composed of plural plies of fiber material 11. The fiber composite component 10, represented here in a cross section, exhibits damage 12 which is to be repaired. This damage 12 extends here over plural fiber material plies 11.

(7) In the next step (b), the damaged region 12 of the fiber composite component 10 is removed, by the introduction of a taper 13 into the fiber composite component 10 over a large area.

(8) This taper 13 for removing the damaged region 12 may be made manually or mechanically, and is commonly also referred to as surface preparation. It is followed by the surface pretreatment, which may be accomplished by abrading, blasting or milling, for example. This operation may also include cleaning or activating operations (by plasma, for example). The result of the second step (b) is then a joining surface 14 which is to be introduced later into a repair patch. This joining surface 14, following step (b), has been made ready to an extent such that the step of adhesive bonding can commence. Now, in accordance with the invention, the joining surface 14 is first inspected, in order to ensure the effectiveness of the bonding.

(9) For this purpose, in step (c), a sheetlike test textile 20 is inserted onto the joining surface 14 of the taper 13, with the sheetlike test textile 20 having a fiber material 21 which is impregnated by an adhesive primer 22.

(10) In edge regions, the sheetlike test textile can be wrapped around, so as to generate a tab 23 for the subsequent removal. In order to prevent the tab 23 sticking to the test textile 20, a release film 24 may be provided between tab 23 and test textile. Also conceivable is for a tab preventing attachment to be generated by means of a release film like that placed onto the fiber composite component 10 in edge regions.

(11) When the test textile 20 has been applied to the joining surface 14, the next, subsequent step (d) sees the test textile being consolidated, i.e., cured. For this purpose the test textile 20 is exposed to heat and optionally pressure. The treatment of the applied sheetlike test textile 20 ultimately depends on the manner in which the adhesive primer of the test textile 20 cures. If the adhesive primer used was a matrix material corresponding to the matrix material of the fiber composite component, or corresponding to the adhesive of the later bond, then in general the test textile is exposed to a temperature and a pressure corresponding, for example, to the elevated temperature and pressure involved in a vacuum system customary for fiber composite technology.

(12) In a subsequent, optional step, the test textile may be conditioned for more in-depth testing, by exposing it to a desired test temperature and optionally to a medium. The medium may for example be water or other substances with which the fiber composite component regularly comes into contact. The medium is able to diffuse, for example, through a thin film into the boundary layer within a short timespan. The subsequent testing takes place under the influence of media and temperature, therefore, and so critical scenarios for the bond can also be generated.

(13) Then, in the subsequent step (e), the test textile 20 is removed by means of the tab 23 from the joining surface 14, either manually or mechanically. The entire test area, i.e., the joining surface 14, may also be covered with a plurality of planar test textiles in a defined width, which are removed individually. As a result, a comparable removal force based on the textile width is measured. The removal angle, the removal rate, and the removal direction in relation to the fiber orientation may be defined here according to requirements.

(14) In one advantageous configuration, the edges of these strips of test textile are configured in such a way as to prevent tears extending into the strips. For this purpose it is conceivable in particular for the strips to be thermally sealed at the edge or to have, for example, a woven margin.

(15) In the normal scenario, the removal of the test textile is accompanied by cohesive fracture in the adhesive primer, with the consequence that parts of the adhesive primer remain on the joining surface 14 and a surface is formed that is defined, clean, and ready for the subsequent bonding. If it has been ascertained, after the removal of the test textile 20, that the entire primer attaches uniformly, completely, and without defect on the joining surface 14, then the adhesive primer has been fractured cohesively over the entire area of the test textile, suggesting that the boundary face to the fiber composite component 10 has withstood the applied loading. In this case, the adhesive and the repair material can be applied directly to the primed joining surface 14.

(16) If, however, the fracture pattern exhibits irregularities, damage or defects, then the prepared/pretreated joining surface 14 present was inadequate, and reliable bonding cannot be carried out on such a surface. Resultant defects of this kind which can be seen from the fracture pattern are recognizable in step (f). Shown here, schematically, is the removed test textile, which, in a first region 25, contains not only parts of the adhesive primer 22, which ought actually to attach on the joining surface 14, but also parts of the fiber composite component 10, thus producing an irregularity or defect site in the fracture pattern in the region 25. In this region, the base of the joining surface 14 lacked sufficient stability, and so the attachment of the adhesive primer 22 on the joining surface 14 resulted in a greater attachment than commonly in the region 25 in the fiber composite component 10. The surface pretreatment in the region 25 was therefore not sufficient.

(17) In a further region 26, on the other hand, the adhesive primer 22 did not attach sufficiently on the joining surface 14, again suggesting an insufficient surface pretreatment of the joining surface 14. In the region 26 as well, the fracture pattern, in the case both of the fiber composite component and of the test textile, shows an irregularity or defect site which is immediately apparent to the naked eye.

(18) Accordingly, on the basis of the fracture pattern, it is possible to carry out a qualitative evaluation for the purpose of inspecting the joining surface, by ascertaining whether the fracture pattern, on the joining surface 14, for example, exhibits defect sites or irregularities which manifest themselves in a lack of consistent attachment of the attaching adhesive primer on the joining surface 14.

(19) Alternatively or additionally, it is also possible for the removal force (F) on removal of the test textile 20 to be found and then dissipated over the test path, and this is shown in step (g). The regions 25 and 26 here correspond to the regions 25 and 26 in step (f), where the fracture pattern exhibits corresponding defect sites. In these regions, the removal force subsides suddenly, resulting from deficient attachment of the adhesive primer or of the fiber composite material of the fiber composite component. Here again, deviations from a uniform removal force may be used to conclude the presence of corresponding defect sites. It becomes possible, accordingly, to inspect the joining surface not only directly in relation to its nature, for the purpose of adhesive bonding, but instead also at the same time, in the case of a successful inspection, to ready the joining surface for the subsequent bonding operation to an extent such that a defined adhesive area is formed, on which a corresponding bond can be guaranteed in a defined way.

(20) The repair of the fiber composite component 10 shown in FIGS. 1a-1c can be seen in FIG. 2. When the inspection steps of FIGS. 1a-1c have been carried out, a part of the adhesive primer 22 remains on the joining surface 14 of the fiber composite component 10. The corresponding adhesive 27 for the repair is then applied to this adhesive primer, and subsequently the repair patch 28 is introduced into the taper. The repair patch 28 may likewise, for example, be a fiber composite material. The layer 27 of adhesive in this case may be introduced simultaneously with the repair patch 28, in the form of a prepreg.

(21) Optionally, in a secondary test region 29, an end of the test textile 20 and/or an end of the adhesive 27, with a test fabric, may be disposed. By way of these it is possible to inspect the attachment of the adhesive to the primer. Moreover, this region may be utilized for a long-term test of the attachment of the primer to the fiber composite component, and also of the adhesive to the primer. For this purpose, the test fabric remains in the corresponding material (primer or adhesive) and is not removed until a later point in time. In the interim, the secondary test region can be painted over, so that it has no influence visually and aerodynamically.

(22) It is also conceivable, however, for the attachment of the adhesive 27 to the repair patch 28 to be inspected using the test textile. For this purpose, a release film is placed under the desired region, the adhesive is provided with the test textile, and the repair patch is applied over it. After the curing of the repair patch, the region is separated from the component by the release film. The test textile is subsequently removed from the separated region, and the interface between adhesive and the repair plies lying above it is loaded and therefore tested. It is possible accordingly to test all of the interfaces of the connection system: component/primer, primer/adhesive, and adhesive/repair patch. Also conceivable, however, is the inspection of the attachment of the adhesive 27 to the material of the repair patch 28 by means of the test textile. For that purpose, alongside the patch, a region is covered with release film, the adhesive is provided with the test textile, and the material of the repair patch is applied over it. After the curing of the repair patch, the additional region is separated from the component by the release film.

(23) All of the test elements in the secondary test region 29 may be tested either directly after the repair (or after optional conditioning), or else only at a later point in time, for the implementation of a long-term test. Before the long-term test is carried out, the test sections may likewise be conditioned.

(24) The test elements in the secondary region 29 may be connected directly to the actual bond or may be separated from it by an interruption/gap.

(25) For process control in component fabrication and/or bonding, tag-along specimens with a control attachment may be fabricated, serving to validate the process and the correct attachment of adhesive to the components. For this purpose, in addition to the regular joining partners, a test connection is constructed as is shown in FIG. 3. In the case of this test connection, a first test element 31 and a second test element 32 are used, a first test textile 33 being disposed on the first test element 31, and a second test textile 34 on the second test element 32. The first and second test elements are then joined to one another by means of their two test textiles, with a release film 35 being disposed between the two test textiles. Then, in the next step, a regular bond is performed, both of the component actually under production and of the test connection 30, so that the control bond is carried out in one operating step together with the actual component bond.

(26) Subsequently, by means of the release film 35, the two joining partners 31 and 32 of the control connection 30 are separated, and subsequently the test textile 33, 34 can be removed from each of the two test elements 31 and 32, as described above, by the application of a removal force. Subsequently, both test elements can be inspected by qualitative and/or quantitative evaluation of their joining surface, with the result of this testing being representative of the joining process of the fabricated component join.

(27) In this context it would be conceivable for the test connection 30 not to be a control fabrication, but rather the component connection to be produced; in that case, in a first step, the two joining partners are joined to one another as in the test connection, and then, subsequently, the test textile is removed from both joining partners 31 and 32. If the two fracture patterns of the removed joining surface and of the test textiles, respectively, are without findings, then in a next step the two joining partners can be bonded to one another using a layer of adhesive. The test textile is required, during removal, to transmit a force of defined height to the adhesive primer or to the joining surface, respectively, and so to leave behind a thin primer layer whose surface is clean and is amenable to attachment for a subsequent bonding operation. In order for the test textile to be nevertheless removed conveniently from the surface, with little application of force, the removal may take place, as described above, in narrow strips of defined width. The requirements imposed on the test textile are achieved by employing a woven fabric having an open structure.

(28) Via the architecture of the test textile (type of weave, cross-sectional construction, thread configuration, single/multiple filaments, linear thread density, free cross-sectional area, etc.) it is possible, among other things, to adjust parameters such as the extent of the test force or test tension (the tension with which the interface in question is loaded), the extent of the removal force (the force required to remove the textile), resolutions for any defects, areal proportion of the cohesive fracture, film thickness of the primer left behind, and surface topology of the priming coat generated.

(29) The test textile may in principle consist of any materials. Examples are plastics, natural materials, or metals. The particular material is processed to a sheetlike structure, which may be a textile in the conventional sense—that is, for example, a woven or knitted fabric. However, sheetlike structures such as, for example, expanded metal or lattices or meshes are also conceivable.

(30) The test textile may in principle have been impregnated with primers beforehand or else may be in dry form and may not be impregnated until during application, in situ, by the application of adhesive or primer to the test textile.

(31) With regard to FIG. 3, the following scenarios are conceivable: firstly, both test elements may be present in uncured form prior to control bonding, and may be cured in this operation with the test textile. Prior to the control bonding, furthermore, one of the test elements may be uncured and the other in consolidated form. Lastly, both test elements may be cured.

(32) FIG. 4 shows, schematically, the process of removal for an open fabric made from single-filament threads with a circular cross section.

(33) In the removal operation, the fiber material 21 detaches from the adhesive primer 22, and the free surfaces between the individual fibers of the fiber material 21 undergo cohesive fracture.

(34) The precise topography of the surface generated is adjustable through the properties of the test textile and/or through the form of the fiber material. For example, an angular cross section of the fiber material, with undercuts, can also be utilized in order to generate a relatively uniform, thin priming coat.

(35) As a result of the removal of the test textile and the formation of a thin priming coat on the joining surface, a particularly adhesive surface is generated. The surface is therefore very well prepared for the subsequent adhesive joining operation.

(36) For automated operation, it is possible here to provide a computer unit 40, which is connected to a camera 41 and/or to a force sensor 42, as represented schematically in FIG. 4 in a greatly simplified representation. The camera 41 in this case records the fracture surface of the cohesively fractured primer, and corresponding irregularities and also attachment of the primer 22 not over the full area on the component can be recognized with the aid of an image analysis on the computer unit 40. If the test textile is removed in an automated way using a device, then the force F can be detected by means of the sensor 42. Through analysis of the force profile it is then possible to inspect the surface as described above.

(37) With the aid of the method of the invention, it also becomes possible here to test highly curved surfaces. This is not a possibility with the great majority of conventional approaches. Moreover, the method of the invention allows the joining surface to be inspected independently of ply and/or independently of position. After the testing for defects and the ascertainment of a defect-free surface, the method of the invention produces a clean, defined surface to which, subsequently, bonding can be carried out directly. In contrast to conventional repair procedures, however, the surface quality is not dependent on the human factor. The method of the invention makes it possible, furthermore, to fix the condition of the pretreated joining surface.

(38) Application of adhesive need not therefore take place immediately after the end of the surface pretreatment. Accordingly, a repair process can be made more flexible in terms of time. Lastly, the invention also allows the detection of contaminations which take effect only during the bonding operation at elevated temperature, by diffusion from the substrate material interior to the surface, for example. This risk cannot be recognized by customary surface analysis methods, prior to bonding.

(39) The adhesive primer may constitute a reactive substance, based on epoxy, for example, which has a similar profile of properties to the adhesive used for the repair, but whose precise properties can be tailored to the specific task (in the simplest case: adhesive primer=adhesive). This includes, among other things, the implementation of a very strong attachment to the substrate, good tolerance of contamination, and the capacity to enable particularly effective attachment to the adhesive in a downstream step. The color of the adhesive primer is made such that it is readily distinguishable from the substrate with the test textile so that defects can be easily recognized visually. The formulation of the primer may be such that its property changes during the later curing of the adhesive film, and takes on the desired final property. This corresponds to a heating step during the curing of the adhesive film at elevated temperature. Thus, for example, it is possible for the primer still to possess a residual reactivity after the curing of the test textile, in other words not to be completely reacted, and as a result to crosslink with the adhesive in the subsequent bonding operation and therefore to enter into particularly strong chemical bonds.

(40) The textile in this case is in particular not provided with additional coatings such as release agents, which after testing could remain as an adhesion-inhibiting residue on the surface of the primer. In terms of the filament density or linear thread density and the weave and/or areal distribution, the textile is configured so that it leaves behind a regular surface and a defined, thick layer of the primer after removal.

(41) The removal textile may have designated fracture sites at regular width intervals, with these sites segmenting the textile into strips. This allows application to take place on one piece, and at the same time the individual segments can be removed as strips of defined width, with the force being recorded in relation to a defined width.

(42) The test textile with the fiber material and the adhesive primer is adapted such that, on correct surface preparation and surface pretreatment, the attachment between substrate and primer, and also the strength of the substrate, are higher than the force required to bring about the cohesive fracture in the adhesive primer. This means that, in the case of correct implementation, the interface between removal textile and primer always represents the weak point. In the case of relevant defects/contaminations in the substrate surface, these regions represent the weak point—in other words, the strength of these regions is below that between removal textile and primer, this being manifested in the change in the fracture pattern, since corresponding defects are then perceptible in said fracture pattern.

LIST OF REFERENCE NUMERALS

(43) 10—fiber composite component 11—fiber material plies 12—damage 13—taper 14—joining surface 20—test textile 21—fiber material of the test textile 22—adhesive primer 23—tab 24—release film 25—first irregularity in the fracture pattern 26—second irregularity in the fracture pattern 27—adhesive 28—repair patch 29—secondary test region 30—test connection 31—first test element 32—second test element 33—first test textile 34—second test textile 35—release film