DEVICE AND METHOD FOR MATERIAL REMOVAL FROM FIBER-RELATED COMPOSITE MATERIALS, IN PARTICULAR FOR SHEARING

Abstract

The present invention relates to the field of removing material from fibre-composite materials and in particular removal of material to produce a scarf joint, as is provided for example when repairing a fibre composite comprising fibre and resin. A method and a corresponding device are proposed, wherein the method involves providing a pressure gradient in a transport gas, providing a blasting material, accelerating the blasting material through the pressure gradient onto a surface of the fibre-composite workpiece, removing material including fibres and resin by abrasion from the fibre-composite workpiece in a workspace defined by a removal unit and the fibre-composite workpiece, and discharging removed material and blasting material from the work space through the pressure gradient, wherein the blasting material is guided by a blasting tube onto the surface of the fibre-composite workpiece, the blasting material has an average grain size in the range from 200 µm to 500 µm, and a distance of the blasting tube from the fibre-composite workpiece in the workspace lies in a range from 3 to 12 mm.

Description

[0001] Device and method for material removal from fiber-reinforced composites, in particular for scarfing

[0002] The present invention relates to the field of material removal from fiber-reinforced composites, and in particular to material removal for the creation of a scarf joint, as is used, for example, in the repair of a fiber-reinforced composite with fiber and resin. Fiber-reinforced composite structures and laminates are increasingly used in the aerospace, wind energy, and automotive industries. Damage to these structures frequently occurs, for example, due to high-speed impacts. One of the major challenges facing these industries is the traditionally costly and time-consuming repair of the fiber composite. To repair fiber-reinforced composite components, they currently require complex processing, in which the damaged layers are first removed individually. This is achieved by creating scarf joints.Today's predominantly manual material removal processes for repairs and modifications to fiber-reinforced composite components are extremely emission-intensive and require a significant amount of time, from preparation to cleaning the work area. Environmental and health considerations also play a major role. Furthermore, the speed and quality of the material removal process are highly dependent on the operator's experience.

[0003] Currently under development automation solutions have many limitations and disadvantages, particularly regarding the quality and accuracy of the processing. Special emphasis in the industrial use of an automated material removal system is placed on emission-free processing and the most universal applicability possible (location-independent, various fiber-reinforced composite components, different industries) in order to cover a broad spectrum of applications (see, for example, "Composite Repair," publication UTC 102, April 1999, by Hexel Composites, Duxford). From an emissions control perspective, extraction or post-processing must be used when removing individual layers to ensure that all dust and residual contaminants are removed in order to prevent contamination.

[0004] High material removal rates and extensive, deep material removal while maintaining consistent quality are novel challenges in the field of automated machining of repair areas on fiber-reinforced composite components (see, e.g., N. Bhatnagar, N. Ramakrishnan, NK Naik, R. Komanduri, "On the machining of fibre reinforced plastic (FRP) composite laminates." International Journal of Machine Tools & Manufacture 1995; 35(5): 701-716). Standardization and automated, i.e., quality-assured, control are necessary, which offer a significant time advantage. Automated systems are also usually expensive, difficult to transport, and require a large amount of space.

[0005] Currently, challenges exist particularly regarding the large variety of materials, excessively long pre-cleaning, post-cleaning and post-processing times, the lack of standardized procedures for automating the ablation process, the lack of process-accompanying quality control and corresponding sensors, the absence or at least limitation of emissions in automated processes on the surfaces, the weight and space requirements of automated processes, the precision of automated ablation, the still necessary complex manual work and post-cleaning work, the associated safety and health risks, the lack of an automated in-field technology and the lack of an approved automated ablation process for fiber composites.

[0006] The currently approved repair process for fiber-reinforced composites involves removing the damaged fiber composite layers before patch repair layers are bonded to the removed area. Extensive removal of fiber composite material is required around the damaged area. The individual layers of composite structures must be processed and removed until all underlying damaged layers have been eliminated. Layer removal is typically achieved using a stepped or conical scarf joint. As described in the article "Repair of Fiber Composite Components" (F.As outlined in Ellert, Lightweight-Design Journal, 03 / 2015), the process for repairing fiber-reinforced composite components typically includes the following steps: inspection of the damaged area, categorization, pre-cleaning, removal of individual layers, post-cleaning, fabrication of a repair patch, and bonding or laminating the repair patch, including curing. Finally, depending on the application, an NDT test is performed for verification (see, e.g., Wachinger G., Thum C., Scheid P.: “Repairability and Repair Concepts for Structures Made of Fiber-Reinforced Plastics”, in Henning F., “Handbook of Lightweight Construction” Munich: Hanser Verlag, 2011).

[0007] Currently, with few exceptions, the removal of fiber-reinforced composite materials is carried out by manual grinding, which a worker performs using a handheld electric or pneumatic grinder. As a result, the outcome depends significantly on the knowledge and skill of the personnel. Different materials and varying degrees of defects require a treatment intensity adapted to each application. Inspection is performed solely by visual inspection. During manual grinding, the worker must wear protective clothing to shield themselves from fiber dust. This poses a significant health risk. Furthermore, from an ergonomic perspective, manual processing places a considerable physical strain on the worker, as precise material removal with heavy grinding tools and the mechanical forces generated during contact grinding require considerable endurance (see, e.g., F. Cenac, F. Collombet, R.Zitoune and M. Deleris, “Abrasive-water-jet blind-machining of polymer matrix composite materials”, University of Toulouse). Compressed air blasting uses a high-velocity stream of solid abrasive particles to remove material. This is mainly carried out in blasting cabinets to allow for dust extraction. The operator must wear a protective suit.

[0008] Currently, there is no globally approved automated method for removing damage from fiber-reinforced composite components in the aerospace industry, for example, for in-field repairs of CFRP fuselages on the B787 and A350. Automated mechanical material removal is currently being tested using milling machines, almost exclusively at the prototype stage. Disadvantages include complex post-treatment cleaning processes, time, costs, the size and weight of the machines, and tool positioning. To minimize emissions, large milling booths are constructed, and elaborate extraction systems are integrated for cleaning the booths and components (see Dittmar, Hagen; Laser-based repair preparation of composite structures, LZH; 2017). Milling uses carbide or diamond tools to machine and remove the fiber-reinforced composite material (see Fig. 1). The effective speed is tangential to the workpiece surface.The resulting forces cause strong intralaminar and terlaminar shear stresses in the fiber composite, posing a risk of delamination and fiber pull-out, and leading to loose fiber ends emerging from the surface. Furthermore, contact with such tools also generates thermal stresses in the component. Developments in laser ablation, which must be tailored to the specific materials, are time-consuming processes, generate significant heat into the processing area and surrounding environment, and require substantial equipment. The accuracy and precision of the ablation are particularly limited in the edge and transition areas between layers (see also Dittmar, Hagen; Laser-based repair preparation of composite structures, LZH; 2017). Waterjet cutting (AWJ) is a wet ablation process with high equipment costs and a large footprint.The process carries the particular risk of uncontrolled contamination during the repair of the treated structure (see also F. Cenac, F. Collombet, R. Zitoune and M. Deleris, “Abrasive-water-jet blind-machining of polymer matrix composite materials”, Universite de Toulouse). Further information on the repair of fiber-reinforced composites can also be found in “Adaptive Machining for Efficient Manufacture and Repair of CFRP Components” by Claus Bremer (BCT GmbH, 6 / 2012) and “Care and repair of advanced composites” by KB Armstrong, WF Cole, LG Bevan (SAE International, London 2005).

[0009] Approaches for detecting defects are already known and are possible using methods such as radiography, ultrasound, or thermography (see, for example, training material from the SKZ Plastics Center; "Detecting Damage to CFRP Components," www.skz.de). Measuring untreated fiber composite surfaces with 1D / 2D line scanners for distance control and inline path corrections for automated robot movements has been investigated in past and ongoing projects at the Fraunhofer IFAM in Stade. Fully automated monitoring of ablated layers and subsequent path control using sensors during the ablation process is not yet available in industry and presents a particular challenge with the approaches pursued so far. Existing semi-automated approaches rely on camera-based monitoring of the ablated surfaces and are verified by visual inspection by the operator (see, for example, EP 2442 941 B1).

[0010] The previously described technologies for automated or semi-automated scarfing have been known for some time and are also offered on the market. However, due to their inherent technological limitations, disadvantages, and risks, they have not yet been able to prevail over manual scarfing. The vacuum suction blasting process (as an example of a blasting process) has so far only been used for the superficial removal of contaminants or for removing surface layers from fiber-reinforced composites (i.e., without removing the fiber itself), for example, for thin-layer roughening or activation before bonding. A vacuum suction blasting unit comprises three main components: a blasting nozzle unit, an industrial vacuum cleaner, and a material feed system. The blasting medium is conveyed to the workpiece surface to be treated via a pipe and hose system under vacuum.The necessary vacuum is generated via an industrial vacuum cleaner, accelerating the blasting media from a storage container. The blasting media transport system consists of a feed unit and a suction unit, both of which are vacuum-tight. The abrasive action on the surface takes place at the blasting head. After the blasting media impacts the workpiece surface and removes material, the residual particles and removed workpiece particles are extracted via the suction system. This results in a clean workpiece surface after blasting. Advantages of vacuum suction blasting compared to conventional sandblasting methods include lower pressure and therefore reduced stress on the component, as well as emission-free processing.Commercial vacuum suction blasting systems are offered, for example, for deburring metal surfaces (see Ruhland, Sigurd: Vacuum suction blasting in the production line, GP Innovation GmbH, JOT 7, 2014). Manual processing using a hand-held, movable blasting lance is used, for example, for removing paint layers and for roughening surfaces before bonding.

[0011] Previously known approaches in connection with vacuum suction jets are described, for example, in DE 10 2010 020 691 A1 , DE 19 747 838 C2 , DE 10 2010 060 664 A1 , EP 1 136 174 A1 , US 2004 / 053561 A1 or EP 2442 941 B1.

[0012] However, known vacuum suction blasting methods, insofar as they remove fibers in addition to the resin in the composite workpiece, only allow for such a low removal rate that they are unsuitable as alternatives to manual shafting. One objective of the present invention is to provide a method and a corresponding device for material removal from a fiber-reinforced composite workpiece containing fibers and resin, which avoids or at least reduces at least some of the disadvantages of the methods and devices currently used for such material removal, particularly for repair or repair preparation. It is particularly desirable to present a solution thatwhich can serve as the basis for extensive or even complete automation in the repair of a fiber composite workpiece. According to a first aspect of the invention, a method for material removal from a fiber composite workpiece with fibers and resin is proposed, as defined in claim 1, namely comprising the steps of providing a pressure gradient in a transport gas, providing a blasting material, accelerating the blasting material through the pressure gradient onto a surface of the fiber composite workpiece, abrasively removing material including fibers and resin from the fiber composite workpiece in a working space defined by a removal unit and the fiber composite workpiece, and removing the removed material and blasting material from the working space through the pressure gradient.wherein the blasting material is directed through a blasting tube onto the surface of the fiber composite workpiece and the blasting material has a mean grain size in the range of 200 gm to 500 gm and the distance of the blasting tube to the fiber composite workpiece in the working space is in the range of 3 to 12 mm.

[0013] According to a second aspect of the invention, a device for automated material removal from a fiber composite workpiece with fibers and resin is proposed, as defined in claim 16, namely with a pressure unit configured to provide a pressure gradient in a transport gas, a material feed configured to provide a blasting material, and a removal unit with a blasting tube, wherein the device is configured to accelerate and direct the blasting material provided by the material feed onto a surface of the fiber composite workpiece by means of a pressure gradient provided by the pressure unit, so that an abrasive removal of material including fibers and resin from the fiber composite workpiece is effected in a working space determined by a removal unit and the fiber composite workpiece.wherein the device is further designed to remove removed material and blasting material from the working space by means of the pressure gradient, wherein the blasting material has a mean grain size in the range of 200 pm to 500 pm and, in operation of the device, the distance between the blasting tube and the fiber composite workpiece in the working space is in the range of 3 to 12 mm.

[0014] Part of the background to the present invention can be found in the following considerations. It has been found that a method known per se for near-surface material removal (less than 100 pm depth), such as vacuum suction blasting, can be used for material removal relevant for shank cutting, provided the blasting material and the distance of the blasting tube to the surface are appropriately selected. However, vacuum suction blasting can only be used economically for a significant yet sufficiently precise material removal from the fiber composite workpiece by combining a larger grit size and a smaller distance to the surface to be processed compared to the known approach.Using only a blasting material with a larger average grain size while otherwise maintaining the parameters of the known vacuum suction blasting approach (which was only intended for surface treatment) leads to a longer processing time, and the material removal becomes more uneven.

[0015] The mean particle size is determined by sieving (from 10 mm to button size). The particle size corresponding to a 50% sieve pass is considered the "mean particle size" (MP). The percentage by weight of particles between the particle sizes 4 / 3 MP and 2 / 3 MP is referred to as the "degree of uniformity" (DH).

[0016] A smaller distance between the blast tube and the surface has little effect with conventionally used blasting materials, and it has also been observed that the accuracy in the processing sometimes decreases, meaning that the material removal becomes more uneven.

[0017] However, it has now been found that the combination of choosing the grain size of the blasting material together with the appropriate adjustment of the distance of the blasting tube surprisingly leads to a strong and correspondingly rapid material removal, while also achieving the desired accuracy.

[0018] Although a realization of the invention in the form of vacuum suction blasting is preferred, it should be noted that within the scope of the invention a vacuum in the sense of a lower pressure compared to the normal atmosphere is not necessary as such, since basically any sufficiently dimensioned pressure gradient can be used for the transport and removal of blasting material and removed material.

[0019] Since the present invention prevents or at least significantly reduces the release of dust and the like, close control of the material removal results is also possible, which has a beneficial effect on automation possibilities. The inventive approach is robust with respect to hybrid workpieces, i.e., workpieces made of different material types. By design, the forces applied locally to the workpiece surface are very small, especially during vacuum blasting, which is particularly advantageous for the precision machining of thin-walled lightweight structures. In contrast to material removal by the blasting material, the cutting edges of a cutting tool primarily load the workpiece edge zone tangentially in a linear zone. This results in a less concentrated stress.In addition to the desired material removal, inter- and intralaminar microcracks and fiber ends detached from the surface of the composite result, impairing the adhesive properties of the workpiece surface and requiring a subsequent polishing and grinding operation.

[0020] In conventional laser ablation, the extremely diverse thermophysical and optical properties of the components of fiber-reinforced plastics, and potentially the properties of metallic surface layers, are difficult to control. The wavelength must be adjusted to the varying absorption behavior of each material or material combination. The very low decomposition temperature of the resin matrix and the extremely high sublimation temperature of carbon fibers pose a risk of thermal damage to the resin matrix in the vicinity of the processing area, resulting in inter- and intralaminar cracks and delamination. Laser ablation of fiber-reinforced plastics also generates harmful gaseous byproducts, the detection of which is challenging due to the variability of the damage locations being treated and therefore requires considerable effort. These problems do not arise within the scope of the invention.

[0021] In contrast to methods with a tangential movement to the component surface (e.g., milling, grinding), the approach according to the invention is characterized by an exclusively or predominantly normal movement to the component surface. This enables the fibers to be separated without or largely without shear or tensile stresses on the fiber-matrix interface. This prevents a gradual detachment of fiber ends from the matrix, which impairs the quality / strength of subsequent adhesive repairs and / or requires a lengthy intermediate fine grinding / polishing process, so that this process can generally be omitted when using the invention.

[0022] The detrimental impairment of the component surface in processes with a tangential action to the component surface is also highly dependent on the wear condition of the tool. In contrast, this detrimental influence is eliminated in the present invention due to the blasting material (abrasive medium) supplied (see Freese, J. de et al.: “End milling of Carbon Fiber Reinforced Plastics as surface pretreatment for adhesive bonding - effect of interlaminar damages and particle residues” TFIE JOURNAL OF ADHESION, https: / / doi.org / 10.1080 / 00218464.2018.1557054; or Hintze, W.; Hartmann, D.; Schubert, U. “End milling of CFRP for surface preparation and repair preparation”. Z. wirtschaftlichen Fabrikbetrieb., Jun, 2012, 107, 462-466. DOI: 10.3139 / 104.110775).

[0023] Unlike abrasive water jet removal, the present invention avoids unwanted moisture ingress into the damaged component zone. Furthermore, in contrast to abrasive water jet removal, the invention allows for a largely emission-free removal process, meaning the surrounding area is not contaminated by abrasive particles and removal products.

[0024] Until now, conventional methods such as the well-known use of vacuum blasting could only remove thin layers of the top layer of fiber composites with removal rates of 0.2 mm. 3 / s are removed. In contrast, it was found that the present invention enables an automatic and controlled process for the removal of uniform and large-area fiber composite layers at removal depths in both the thin-film and cm ranges with removal rates of 5 mm. 3 / s and more can be removed. Larger areas can thus be removed faster, more evenly, and homogeneously with an accuracy down to the pm range, and in particular, resin and fibers can be removed over large areas. A decisive advantage over previous ablation methods and thin-layer grinding, besides the lateral adjustment of the blast area, lies in the faster and greater ablation rate and a defined scarification profile with a greater and more defined depth of removal. Different composite materials and geometries can be processed with this method. The possible and preferred automation improves ergonomics, safety, and process time compared to manual grinding.Another important advantage of the invention is that the abrasive blasting media, along with the resulting FRP grinding dust, is immediately extracted, thus preventing the release of dust as with conventional methods. The suction blasting unit therefore operates with low emissions, eliminating the need for coolants or post-cleaning steps. A further significant advantage of the invention when removing fiber composite layers is the avoidance of delamination, since the material separation mechanism is perpendicular to the surface and not tangential. Only negligible mechanical stresses are applied to the component, and no thermal stress is generated (as, for example, with laser processes). Depth and width control, supported by a 2D sensor, and the regulation of the removed fiber layers allow for precise monitoring and assurance of quality during the process, thereby saving rework time.Vacuum blasting also applies very low forces to the material being treated when processing lightweight surfaces, thus eliminating the force-deformation problems associated with repair work. The process is also suitable for various fibers and resin systems and, unlike laser ablation, can be used on conductive materials.

[0025] The invention offers, in particular, a system and process solution for the repair sector, and thus for an in-field solution. The application area includes, but is not limited to, the assembly of fiber composite components in the aerospace industry (primarily manufacturers and suppliers) in the repair sector, especially maintenance facilities such as the Maintenance and Repair Operations of Lufthansa Technik. The present invention is suitable for in-field machining and repair of fiber composite components, including the machining of large fiber composite structures, as are frequently found in aircraft construction. Furthermore, the experience gained in the aerospace sector, for example, can be easily applied to machining processes on rotor blades in wind energy or automotive manufacturing. The process can be used directly on-site, thereby saving considerable costs and time.Another area of ​​application is the reworking of manufacturing defects on components during production, where inline (in rep shops and FAL) rework during assembly enables fast and clean processing.

[0026] In an advantageous embodiment of one aspect of the invention, the pressure gradient lies in the range of 200 to 500 hPa, preferably in the range of 275 to 350 hPa. It has been found that good results can be achieved with a pressure gradient in this range. In another advantageous embodiment of one aspect of the invention, the pressure gradient is provided such that a negative pressure exists in the working chamber relative to the surrounding atmosphere. In this embodiment, the invention essentially corresponds to the approach of vacuum suction blasting, whereby the pressure gradient can be easily achieved by a suitable suction unit relative to the ambient air.In another advantageous embodiment of an aspect of the invention, the ablation unit is configured such that the working space has a supply of transport gas and / or air between the ablation unit and the fiber composite workpiece, wherein the supply preferably comprises a gap between the ablation unit and the fiber composite workpiece with a width in the range of 1 / 4 to 1 / 20 of the distance between the jet tube and the fiber composite workpiece, particularly preferably with a width of less than 0.5 mm. It has been found that such an additional supply of air or transport gas does indeed influence the pressure gradient itself, but that the additional flow results in better material removal.

[0027] In another advantageous embodiment of an aspect of the invention, the blasting material has a angular grain shape. The more or less pronounced irregularity, together with the sharp edges of the angular material, has a positive effect on the abrasive capacity of the blasting material. In another advantageous embodiment of an aspect of the invention, the blasting material comprises a ceramic material with a Mohs hardness in the range of 6 to 7.5, silica, and / or crushed glass. It has been found that good properties can be achieved with such blasting material.

[0028] In another advantageous embodiment of an aspect of the invention, the blasting material has a mean grain size of 200 to 325 pm. The range for the mean grain size specified here is particularly preferred.

[0029] In another advantageous embodiment of an aspect of the invention, the blasting material is accelerated at a mass throughput in the range of 120 to 200 g / min, preferably in the range of 140 to 160 g / min. With such a mass throughput, good material removal rates can be achieved along with economical use of the blasting material.

[0030] In another advantageous embodiment of an aspect of the invention, the blasting material strikes the fiber composite workpiece at an angle of 40° or less to a respective surface normal of the fiber composite, preferably at an angle of 25° or less, and particularly preferably at an angle of 5° or less, wherein the respective surface normal is an averaged surface normal of a region around an impact point with a diameter at least 10 times larger than the mean grain size of the blasting material. Although an approximately perpendicular impact of the blasting material is preferred, it has been found that good results can still be achieved even with deviations from the normal.Due to the surface roughness caused by material removal and possibly even by the fiber composite workpiece itself, the surface normal is not to be considered in the microscopic range or even finer.

[0031] In another advantageous embodiment of an aspect of the invention, the jet tube has a nozzle with a round or rectangular nozzle outlet, preferably with an inner diameter of less than 50 mm, preferably in the range of 10 to 15 mm.

[0032] In another advantageous embodiment of an aspect of the invention, the nozzle of the jet tube is a Venturi nozzle with a constriction area whose inner diameter is in the range of 50 to 75% of the nozzle outlet. In another advantageous embodiment of an aspect of the invention, the ablation unit is moved laterally relative to the fiber composite workpiece at a speed in the range of 1 to 10 mm / s, preferably in the range of 3 to 5 mm / s. It has been found that good results can be achieved with such a movement speed of the ablation unit and thus of the area in which material is ablated.

[0033] In another advantageous embodiment of an aspect of the invention, the ablation unit is moved laterally relative to the fiber composite workpiece such that an overlap of adjacent ablation tracks with a width in the range of 5 to 10% of a track width is achieved. The ablation in the ablation tracks is generally somewhat less in the edge region, so that with a corresponding overlap, a good overall uniformity can be achieved.

[0034] In another advantageous embodiment of an aspect of the invention, the distance of the blast tube relative to the fiber composite workpiece is adjusted according to a local material removal rate. Given the importance of the distance of the blast tube from the surface, it can be advantageous—depending on the locally existing depth—to adjust the position of the blast tube relative to the surface (or relative to the "bottom" of the removal area). This relates, among other things, to generating uniform material removal over the entire path length and preventing excessive material removal at the path end during movement stops / dead times. Upon reaching the endpoint of the removal path, after the abrasive feed is switched off, the blast nozzle's upward movement prevents further material removal at the stopping point of the blast effector due to the delayed extraction of residual abrasive. This preferably occurs abruptly to a specific height approximately 0.5 s after reaching the endpoint, e.g., from approximately...60 mm, to remove material evenly but not excessively. The nozzle is preferably raised automatically by means of a stroke and spring stop.

[0035] In another advantageous embodiment of an aspect of the invention, the material removal according to the invention is described as a material removal process in a method for repairing a fiber composite workpiece with fibers and resin, which also includes subsequent lamination or bonding of a repair piece to the fiber composite workpiece in the area of ​​material removal.

[0036] In another advantageous embodiment of an aspect of the invention, the device according to the invention comprises at least one sensor unit for detecting a removal result and a control unit which is designed to control the device based on a detection result of the sensor unit.

[0037] Features of advantageous embodiments of the invention are defined in particular in the dependent claims, with further advantageous features, embodiments and configurations also being apparent to the person skilled in the art from the above explanation and the following discussion.

[0038] The present invention will now be further illustrated and explained with reference to exemplary embodiments shown in the figures.

[0039] Fig. 1 schematic representations of the effects of a blasting process and a machining process on a composite material,

[0040] Fig. 2 shows a schematic representation of a mobile, manually operated beam unit,

[0041] Fig. 3 shows a schematic representation of a vacuum suction jet unit with a connection to a robot,

[0042] Fig. 4 is a schematic representation to illustrate an embodiment of the invention.

[0043] Fig. 5 is a schematic representation to further illustrate the embodiment example.

[0044] Fig. 6 Examples of typical shank variations,

[0045] Fig. 7 schematic representations to illustrate a path path in a step-shaped joint,

[0046] Fig. 8 schematic representations of target shaft geometries and respective damage, Fig. 9 a schematic top view of a material removal system according to the invention with continuous measurement,

[0047] Fig. 10 shows a flowchart of an embodiment of the method according to the invention.

[0048] In the accompanying drawings and the explanations relating to these drawings, corresponding or related elements are marked with corresponding or similar reference symbols, where appropriate, even if they are found in different embodiments.

[0049] Fig. 1 shows schematic representations of the effects of a blasting process and a machining process on a composite material 1. In a blasting process, such as the process according to the invention, the primarily normal working motion of the blasting medium 2 (represented by arrow 4) results in a corresponding normal stress and a material removal directed perpendicular to the workpiece 1. In contrast, machining with a cutting edge 3 results in a primarily tangential working motion (represented by arrow 5) and a shear stress, leading to damage 6. Fig. 2 shows a schematic representation of a mobile, manually guided blasting unit, which is essentially already known from the prior art. The blasting unit comprises a blasting nozzle unit 11 or 12, which is coupled on one side to a material feed 13 and on the other side to an industrial vacuum cleaner 14.The negative pressure created by the industrial vacuum cleaner 14 draws blasting material from the material feed 13 and conveys it to the blast nozzle unit 12, where the blasting material hits the surface to be processed and is then carried away by the negative pressure of the industrial vacuum cleaner 14 together with the removed material.

[0050] Fig. 3 shows a schematic representation of a vacuum suction blasting unit connected to a robot. Similar to the blasting unit from Fig. 2, blasting material is fed from a blasting material supply 15 to a blasting head 17 held by a robot 16, in order to be directed onto the workpiece 16.

[0051] While sandblasting accelerates material onto the surface at a pressure of typically 6 bar, vacuum suction blasting conventionally uses approximately 0.2 bar. After impacting the workpiece 18, the blasting material, along with material removed from or detached from the workpiece 18, is conveyed away by the negative pressure of the vacuum suction device 19.

[0052] Fig. 4 shows a schematic representation to illustrate an embodiment of the invention, wherein a vacuum suction jet unit is shown.

[0053] The vacuum suction blasting unit comprises a blasting media container 21, which serves as a material feed for supplying blasting media, a feed hose 22, a blasting tube 23, a suction tube 24, a suction hose 25, and a suction device 26, which serves as a pressure unit for providing a pressure differential. The blasting tube 23 and the suction tube 24 together form a removal unit for removing material from a surface 27 to be processed or from a workpiece. The respective distances of the blasting tube 23 and the suction tube 24 to the surface 27 are not to scale with each other or with the dimensions of the tubes 23 and 24.

[0054] Fig. 5 shows a schematic representation to further illustrate the embodiment. Only the blast tube 23, the extraction tube 24, and the workpiece surface 27 are illustrated here. Abrasive material 28 is directed through the blast tube 23 onto the surface 27, as indicated by the downward-pointing arrow in the drawing. The abrasive material 28 impacts the surface 27 and removes material. Due to the prevailing pressure gradient, the abrasive material 28 and the removed material from the workpiece are carried away through the area between the extraction tube 24 and the blast tube 23.

[0055] If, as schematically indicated here, a small gap exists between the end of the suction tube 24 and the surface, air can enter, and the resulting airflow assists in the removal of the blasting material and the abraded material. However, it is important to note that this gap should be kept small (e.g., less than 0.5 mm, preferably significantly smaller), as excessive air ingress would otherwise negatively affect the pressure gradient to the suction device 26 and hinder or even prevent the removal of the blasting material.

[0056] Fig. 6 shows examples of typical scarf joint variants. The left part of Fig. 7 shows a step machining operation where steps are present in the scarf joint area of ​​the workpiece 31, whereas in a machining operation with a transition, illustrated in the right part, such steps are not present. An adhesive 32 is typically provided between a patch 33 and the workpiece 31.

[0057] Fig. 7 shows schematic diagrams illustrating the path of a step-scarring process. In the automated removal process of step-scarring, as illustrated in Fig. 7, the first layer of the entire scarification surface is removed first (see left part of Fig. 7), followed by the area one step width smaller (see middle and right parts of Fig. 7). The geometry of the treatment area is determined by the geometry of the damage. Round or oval scarification paths can be produced more easily and precisely at the edges of the steps than rectangular paths. The upper part of Fig. 7 indicates the direction of movement and the shape of the scarification path in a top view, while the lower part shows a schematic cross-sectional view.

[0058] Within the scope of the invention, an abrasive current of preferably silicon oxide blasting media can be used to blast the fiber composite surfaces and achieve layer-by-layer removal of fiber composite structures and laminates.

[0059] Fig. 8 shows schematic representations of the intended joint geometries and the respective damage. For a more point-like damage 41, the corresponding joint geometry has a substantially round shape, as shown in the left part of Fig. 8. For an elongated damage 42, as shown in the right part of Fig. 8, a more oval joint geometry is preferably provided.

[0060] Fig. 9 shows a schematic top view of a material removal system according to the invention with continuous measurement.

[0061] Within the scope of the present invention, a process control system for controlled material removal is provided. The global location and type of damage geometry are assumed to be known before the start of the process, and the methods of damage detection already correspond to the prior art.

[0062] The blast head (with blast tube 23 and outer tube 24) is positioned using a robot end effector. The local starting point of the burr is determined using upstream sensors (e.g., a 2D line scanner) in order to then follow the programmed path. The direction of movement of the blast head across the surface (initially the surface 53 to be processed) is indicated by arrow 55.

[0063] For continuous monitoring of the blasting process, an optical 2D line sensor 51 with a resolution of 2 pm is used, positioned downstream of the blasting head. This sensor detects layer depths of at least 0.05 mm within a measuring width of 25 mm. The achieved blasting depth and width are checked and corrected if necessary to react to changes in blasting performance within a 2 ms timeframe during the process. A real-time program in the controller adjusts the robot movements and blasting parameters. An algorithm corrects the process if a layer still needs to be removed. The normal distance to the surface is measured directly behind the outer blasting nozzle. Similarly, if the blasting process is interrupted, the blasting head can be moved and the removed surface area 54 subsequently measured.Continuous tracking measurement is the most time-saving option, as measurements can be taken directly during operation, eliminating the need for an additional check run. Since the sensor's measuring point is located outside the blast head, the distance 56 between the measuring point and the working area is minimized by a thin-walled outer tube 24 (approx. 1 mm) to allow for rapid correction of the material removal (e.g., if the target depth is not reached, the program automatically adjusts the blast head's movement and speed). A leading sensor 52 serves as a reference.

[0064] Unlike the known vacuum suction blasting method, which is only intended for the surface pretreatment of fiber composite workpieces, where only the matrix of the top layer is to be removed, but not the fibers, the inventive method aims to remove the fibers in particular, and to remove them over a greater depth of the component.

[0065] An exemplary embodiment of the invention is explained below.

[0066] A mixture of air and abrasive particles is drawn in through a feed channel via a suction channel that is sealed or largely sealed to the component surface. Unlike the suction channel, the feed channel is located at a distance from the component surface (see Fig. 5). The abrasive particles cause material removal through individual impacts on the surface, due to a portion of their kinetic energy. The particles are reflected from the component surface by the remaining energy and, assisted by the vacuum, enter the suction channel. The corresponding device comprises (see also Figs. 2 and 3) a blasting media container, a feed hose, a blasting nozzle, a suction pipe, a suction hose, and a vacuum suction device.

[0067] In the present embodiment, particles preferably of a scrabbled grain shape, i.e. with sharp-edged partial surfaces, preferably of a ceramic of high clarity, further preferably of SiO2, preferably as glass granules, are used, wherein their density is less than 3.2 g / cm³. 3 , preferably less than 2.7 g / cm³ 3 , specifically about 2.5 g / cm² 3 the mean grain diameter is at least 200 gm, preferably at least 250 gm, wherein the generated negative pressure in the blast hood at the point of action is between 200 hPa and 500 hPa, preferably between 280 hPa and 330 hPa, wherein the distance of the supply channel (blast tube) to the point of action on the component surface is 4 to 11 mm, preferably 6 to 9 mm, specifically 7 to 8 mm.

[0068] The operating principle according to the invention is based on energy-dependent material removal by a gas-guided particle jet with a primarily normal direction of action, which has an angle of at most 40° to the surface normal, preferably at most 25° to the surface normal, specifically with an approximately normal direction of action to the workpiece surface (see Fig. 1). The material removal mechanism is based on the fact that the individual jet particles, through their kinetic energy, exert a primary point-like compressive load on the material to be removed superficially in a very limited local area. In the case of brittle materials such as carbon and glass fibers, the stress concentration leads to brittle fracture. In the case of ductile materials, such as resin matrix and copper mesh for lightning protection, repeated impacts of the abrasive particles lead to local fatigue of the treated material area and ultimately to the surface removal of individual particles. Deeper fiber or...Laminate layers are therefore not damaged (see Fig. 1).

[0069] The invention implements the vacuum suction blasting process for targeted, large-area material removal in width and depth from fiber-reinforced composite components. The material removal includes both fibers and matrix. The vacuum suction blasting method is a novel approach for the precise removal of defects in fiber-reinforced composite components by means of scarfing. The automation of this process is therefore suitable as a technological solution for the repair of fiber-reinforced composite components in industry. The process covers various repair area sizes and depths. A material removal geometry adapted to the damaged area can be generated, whereby basic scarfing patterns are produced on the scale of a typical scarfing area of ​​approximately 300 mm x 300 mm. This size covers approximately 90% of damage in industry.

[0070] For this purpose, target joint geometries are created and executed using CAM routines for pocket milling (see Fig. 6). In aerospace repairs, material-dependent specifications for layer thickness are typically used to adapt the process and process parameters to the respective material. Typical material thicknesses of fiber composite components (e.g., fuselage shells) in aerospace range from approximately 1 mm to 25 mm with individual layers of approximately 0.15 mm. A stepwise material removal process is preferred, the step widths of which depend on the damage depth.An example of a material removal procedure includes selecting a program for the respective removal process, positioning the robot's blast head on the surface, switching on the industrial vacuum cleaner via a program command in the robot program, starting the abrasive supply via a signal in the program, moving the blast head on the workpiece surface, and controlling the removal depth and width using sensors and a corresponding control system for the program parameters, with the program running until the end of the blasting process.

[0071] One embodiment of the inventive method is dry processing using an abrasive blasting medium conveyed in the blast tube by means of a vacuum within the transport gas. The blasting medium is sealed off from the surface being processed by an outer tube, which includes a controlled, small gap for an intake air that accelerates the flow of the blasting medium. The method is distinguished by the fact that, for volumetric material removal from fiber-reinforced resin laminates, the arrangement of the blast tube, the pressure range, the mass flow rate, and the particle size can be appropriately adjusted as the main parameters. The vacuum to be set is between 200 hPa and 500 hPa, preferably at 300 hPa. The mass flow rate required for material removal can be set, in particular, between 120 g / min and 200 g / min, preferably at 150 g / min. Particle sizes between 200 g / m² and 500 g / m² are provided; a glass-crushed abrasive with a particle size of 315 µm is preferred.

[0072] It was found that the corresponding device offers adjustment options, particularly regarding the nozzle geometries. The nozzle geometries used for material removal are especially suitable, consisting of round nozzles, preferably Venturi nozzles (Venturi diameter min. 8 mm, outlet 13.5 mm to max. 20 mm). The inner tube diameter can be up to 50 mm, depending on the hose diameter, but preferably 10 mm to 15 mm, to enable, for example, an optimal and efficient processing width of 20 mm. Nozzles with round or rectangular outlets can be used. A nozzle distance of 3 mm to 10 mm from the surface is possible, preferably 7-8 mm. The traverse speed at the aforementioned preferred values ​​should be between 1 mm / s and 9 mm / s, preferably 4 mm / s. To ensure a seamless transition between two removal tracks, a track overlap width of 1 mm to 2 mm must be set.A flexible sealing material, adaptable to various component geometries, is used to seal the blast head against the component, thereby creating the necessary negative pressure to separate the repair area from its surroundings. In a more specific embodiment, the system is designed for a CFRP plate measuring 300 mm x 300 mm, with a layer thickness of 0.125 mm and a top layer of 0.06 mm. A processing width with 20 mm steps and the removal of three layers are specified.

[0073] In the automated material removal process of step-wise scarfing, the first layer of the entire scarfing surface is removed first, followed by the area one step width smaller. For CFRP components with a cover layer, this layer is removed first; a separate speed is selected for this layer because it is thinner.

[0074] An initial state includes, for example, an industrial robot with a suction blast deflector and blasting media container on a tender trolley, a hose guide connected to an industrial vacuum cleaner, 45 mm hoses, a 30 m extraction hose, a 10 m 13 mm blast hose, with blast pipe geometries consisting of round Venturi nozzles (Venturi diameter min. 8 mm, outlet 13.5 mm to max. 20 mm).

[0075] The process involves clamping the plate securely onto a flat table. Two levers on the industrial vacuum cleaner are set to maximum vacuum (approx. 300 hPa). The abrasive container is filled with GB 315 blasting media, and the inner tube of the blasting nozzle is adjusted to a distance of 7 mm from the surface. A program is selected for the respective removal process, and the robot positions the blasting head on the surface. This involves moving to the initial position with the outer nozzle 1 mm from the surface, with a flexible seal in contact with the surface.

[0076] The robot program activates the industrial vacuum cleaner via a program command, thus initiating the extraction process. The conveying rate is approximately 60% (150 g / min). Subsequently, the abrasive feed is started via a signal in the program, thus initiating the conveying process. A robot movement of 8 mm / s is used to remove the top layer in the desired area, while a robot movement of 4 mm / s is set for the remaining layers. During the movement, the distance between the path centers is 13 mm (path overlap width of 1 mm to 2 mm). The blasting head is moved along the workpiece surface according to pre-programmed paths. The sensors monitor the removal depth and width and adjust the program parameters until the blasting process is complete.To prevent excessive material removal at the end of the abrasive path during movement, stop, or dead times, the blast nozzle is preferably raised abruptly to a specific height approximately 0.5 seconds after the abrasive feed is switched off at the endpoint of the abrasive path. This ensures even, but not excessive, material removal. The blast tube is preferably raised automatically by means of a stroke and spring stop. Other options include raising the entire end effector to interrupt the blasting process, switching a valve at the blast outlet to close the passage to the surface, or prematurely stopping the abrasive feed during movement.

[0077] Fig. 10 shows a flowchart of an embodiment of the method according to the invention.

[0078] This process is used for material removal from a fiber-reinforced composite workpiece containing fibers and resin. In step 61, a pressure gradient is established in a transport gas. In step 62, a blasting material is provided. Steps 61 and 62 are shown here in parallel, but can also be performed sequentially or only partially overlapping.

[0079] As a result of the pressure gradient, in step 63, blasting material is accelerated towards a surface of the fiber composite workpiece, so that in step 64, material, including fibers and resin, is abrasively removed from the fiber composite workpiece within a working space defined by a removal unit and the fiber composite workpiece. In step 65, the removed material and blasting material are carried away from the working space by the pressure gradient.

[0080] According to the invention, the blasting material is directed onto the surface of the fiber composite workpiece through a blasting tube, wherein the blasting material has a mean grain size in the range of 200 g / m² to 500 g / m² and the distance between the blasting tube and the fiber composite workpiece in the working area is in the range of 3 to 12 mm. Even if the figures show various aspects or features of the invention in combination, it is obvious to those skilled in the art – unless otherwise indicated – that the combinations shown and discussed are not the only possible ones. In particular, corresponding units or sets of features from different embodiments can be interchanged.

[0081] List of references

[0082] 1 Composite material

[0083] 2 Abrasive media

[0084] 3 cutting edges

[0085] 4. Action motion of the blasting medium

[0086] 5. Cutting motion

[0087] 6 Damage 11, 12 Jet nozzle unit

[0088] 13 Material feed

[0089] 14 industrial vacuum cleaners

[0090] 15 Provision of blasting material

[0091] 16 robots

[0092] 17 Beam head

[0093] 18 workpieces

[0094] 19 vacuum suction cups

[0095] 21 blasting media containers

[0096] 22 Supply hose

[0097] 23 nozzle

[0098] 24 Suction pipe

[0099] 25 Suction hose

[0100] 26 suction cups

[0101] 27 surface

[0102] 28 blasting material

[0103] 31 workpiece

[0104] 32 Adhesive

[0105] Patch 33

[0106] 41 point-like damage

[0107] 42 elongated damage

[0108] 51 trailing sensor

[0109] 52 Leading sensor 53 Surface to be removed

[0110] 54 worn surface area

[0111] 55 Direction of movement

[0112] 56 Distance 61 Providing a pressure gradient

[0113] 62 Provision of blasting material

[0114] 63 Accelerating beam material

[0115] 64 Abrasive Removal

[0116] 65 Removal of material

Claims

Claims 1. Method for material removal from a fiber composite workpiece (1 , 31) with fibers and flark, comprising the steps: Providing (61) a pressure gradient in a transport gas, providing (62) a blasting material (28), Acceleration (63) of the blasting material (28) by the pressure gradient onto a surface (27, 53) of the fiber composite workpiece (1, 31), abrasive removal (64) of material including fibers and flaking from the fiber composite workpiece (1, 31) in a working space defined by a removal unit (24) and the fiber composite workpiece (1, 31) and 2. Removal (65) of removed material and blasting material (28) from the working space by means of the pressure gradient, wherein the blasting material (28) is directed through a blasting tube (23) onto the surface (27, 53) of the fiber composite workpiece (1, 31), characterized in that the blasting material (28) has a mean particle size in the range of 200 g / m² to 500 g / m² and the distance of the blasting tube (23) to the fiber composite workpiece (1, 31) in the working space is in the range of 3 to 12 mm.

2. Method according to claim 1, wherein the pressure gradient is in the range of 200 to 500 hPa, preferably in the range of 275 to 350 hPa.

3. Method according to one of the preceding claims, wherein the pressure gradient is provided such that there is a negative pressure in the working space relative to a surrounding atmosphere.

4. Method according to one of the preceding claims, wherein the removal unit (24) is configured such that the working space has a supply of transport gas and / or air between the removal unit (24) and the fiber composite workpiece (1, 31), wherein the supply preferably comprises a gap between the removal unit (24) and the fiber composite workpiece (1, 31) with a width in a range of 1 / 4 to 1 / 20 of the distance of the jet tube (23) to the fiber composite workpiece (1, 31), particularly preferably with a width of less than 0.5 mm.

5. Method according to one of the preceding claims, wherein the blasting material (28) has a ragged grain shape.

6. Method according to one of the preceding claims, wherein the blasting material (28) comprises a ceramic material having a Mohs hardness in the range of 6 to 7.5, silicon dioxide and / or crushed glass.

7. Method according to one of the preceding claims, wherein the blasting material (28) has a mean grain size of 200 to 325 pm.

8. Method according to one of the preceding claims, wherein the acceleration (63) of the blasting material (28) is carried out with a mass throughput in the range of 120 to 200 g / min, preferably in the range of 140 to 160 g / min.

9. Method according to one of the preceding claims, wherein the blasting material (28) strikes the fiber composite workpiece (1, 31) at an angle of 40° or less to a respective surface normal of the fiber composite workpiece (1, 31) during the ablation (64), preferably at an angle of 25° or less, particularly preferably at an angle of 5° or less, wherein the respective surface normal is an averaged surface normal of a region around an impact point with a diameter that is at least 10 times larger than the mean grain size of the blasting material (28).

10. Method according to one of the preceding claims, wherein the jet tube (23) has a nozzle with a round or rectangular nozzle outlet, preferably with an inner diameter of less than 50 mm, more preferably in the range of 10 to 15 mm.

11. Method according to one of the preceding claims, wherein the nozzle of the jet tube (23) is a Venturi nozzle with a constriction area, the inner diameter of which is in the range of 50 to 75% of the nozzle outlet.

12. Method according to one of the preceding claims, wherein the removal unit (24) is moved laterally relative to the fiber composite workpiece (1 , 31) at a speed in the range of 1 to 10 mm / s, preferably in the range of 3 to 5 mm / s.

13. A method according to any one of the preceding claims, wherein the removal unit (24) is moved laterally relative to the fiber composite workpiece (1, 31) such that an overlap of adjacent removal tracks with a width in the range of 5 to 10% of a track width is achieved.

14. A method according to any one of the preceding claims, wherein the distance of the jet tube (23) relative to the fiber composite workpiece (1, 31) is adjusted according to a local removal rate.

15. Method for repairing a fiber composite workpiece (1, 31) with fibers and flark, comprising the steps of: material removal (64) according to the steps of the method according to one of the preceding claims, Laminating or gluing a repair piece (33) to the fiber composite piece (1 , 31) in the area of ​​material removal.

16. Device for automated material removal from a fiber composite workpiece (1, 31) with fibers and resin, comprising: a pressure unit (26) configured to provide a pressure gradient in a transport gas, a material feed (21) configured to provide a blasting material (28), and a removal unit (24) with a jet tube (23), wherein the device is configured to accelerate (63) and direct the blasting material (28) provided by the material feed (21) onto a surface (27, 53) of the fiber composite workpiece (1, 31) by means of a pressure gradient provided by the pressure unit (26), so that an abrasive removal (64) of material including fibers and resin from the fiber composite workpiece (1, 31) is carried out by a removal unit (24) and the fiber composite workpiece (1 , 31) is caused by a specific workspace,wherein the device is further configured for the removal (65) of removed material and blasting material (28) from the working space by means of the pressure gradient, characterized in that the blasting material (28) has a mean grain size in the range of 200 gm to 500 gm and, in operation of the device, the distance of the blasting tube (23) to the fiber composite piece (1, 31) in the working space is in the range of 3 to 12 mm.

17. Device according to claim 16, further comprising at least one sensor unit (51, 52) for detecting a removal result and a control unit designed to control the device on the basis of a detection result of the sensor unit (51, 52).