Abstract:
Inspection methods and systems for detecting leaks in a vacuum bag assembly used in the manufacture of a composite part based on spraying helium over the vacuum bag and detecting the helium drawn by the vacuum inside the vacuum bag through a leak. The inspection methods comprise creating a local helium atmosphere in a chamber over the vacuum bag assembly by zones, following a programmed zonal sequence, for a first leak detection test of each zone and a subsequent second leak detection test if the first test gives a positive result. The inspection systems comprise a helium spraying system and helium detecting equipment arranged to implement the inspection methods.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the European patent application No. 15382281.2 filed on May 28, 2015, the entire disclosures of which are incorporated herein by way of reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the detection of leaks in vacuum bag assemblies used in the manufacture of composite parts in, particularly, the aeronautic industry. 
       BACKGROUND OF THE INVENTION 
       [0003]    The intensive introduction of advanced composites or composite materials in primary aircraft structures has become one of the priority objectives in the design and manufacture of a new generation of aircraft because of the possibilities they provide for structural optimization. 
         [0004]    As is well known, the manufacture of composite parts made from polymeric fibrous reinforcements and resins for aircraft structures involves its curing within a vacuum bag under certain conditions of temperature and pressure. It is therefore essential that no pressure loss occur in the vacuum bag during curing processes. 
         [0005]    Vacuum bags used for manufacturing composite parts consist of different types of plastic materials depending on the maximum temperature of use and their elongation capacity until their rupture. The temperature range can be between 420° and 120° and the elongation capacity can reach 500%. Different materials such as nylon, polyolefin, polyamide or thermoplastic elastomers in one or more layers are used. 
         [0006]    As these materials are susceptible to breakage, the manufacturing processes of composite materials must necessarily consider the use of leak detection methods in vacuum bags. 
         [0007]    The existence of such leaks can be easily detected through measurements of pressure within the vacuum bag but their location can be very difficult particularly in the case of large bags using known methods and in particular ultrasonic detectors. The method probably most used in industry for locating leaks in vacuum bags is based on the use of ultrasound, and in fact, manufacturers of vacuum bags often include in their catalogs ultrasonic detectors that convert the ultrasound produced by pressure losses in audible sound that is amplified as closer is to the detector. 
         [0008]    There are other proposals, such as that described in WO 2008/088435 A1, which is based on the incorporation into the vacuum bag of a film sensitive to oxygen that changes appearance when a leak occurs. 
         [0009]    The use of thermograph equipment (particularly IR cameras) for detecting leaks is also known. 
         [0010]    Another method of detecting leaks in tools includes the use of a helium detector wherein a helium emitter is moved along the tool when placed under vacuum. When helium is drawn into a leak, the detector is activated in order to indicate the location of the leak. 
         [0011]    These detection techniques have significant costs and do not adapt equally to various sizes of vacuum bags that are used particularly in the aircraft industry. Ultrasound detectors require a complete inspection of the part approaching the ultrasonic detector to the areas of the part susceptible to have leaks, which entails significant costs and limits its use in large pieces. The incorporation into the vacuum bag of a film sensitive to oxygen significantly increases its cost. 
         [0012]    In any case, there is a need of inspection methods and systems for detecting leaks that produce pressure loss of small entity in vacuum bag assemblies for manufacturing large composite parts, particularly in the case of manufacturing methods where the vacuum bag covers directly all the components of the composite part (called “direct bag” methods). 
       SUMMARY OF THE INVENTION 
       [0013]    In one aspect, the invention provides an inspection method for detecting leaks in a vacuum bag assembly used in the manufacture of a composite part based on spraying helium over the vacuum bag and detecting the helium drawn by the vacuum inside the vacuum bag through a leak. 
         [0014]    The inspection method comprises creating a local helium atmosphere over the vacuum bag assembly by zones, following a programmed zonal sequence for a first leak detection test of each zone. The local helium atmosphere is created inside a chamber placed over each zone. The first leak detection test gives a positive result when the helium detected inside the vacuum bag by a helium detecting equipment exceeds a predetermined first threshold, and in that case, the zone is submitted again to a new local helium atmosphere for a second leak detection test. The second leak detection test gives a positive result when the helium detected by the helium detecting equipment inside the vacuum bag exceeds a predetermined second threshold. 
         [0015]    The inspection method allows, therefore, a quick scanning of the whole surface of the vacuum bag assembly, discarding those zones where the first leak detection test gives a negative result, and identifying the zones where the second leak detection test gives a positive result as those zones where there is a high likelihood of finding a leakage. 
         [0016]    Advantageously, the creation of a local helium atmosphere over a zone comprises a first step of helium injection and a second step of helium exposure, their duration times being control variables of the inspection method, with different values for the first and second leak detection tests (longer for the second test). 
         [0017]    Advantageously, the inspection of a zone further comprises a third step of helium removal, its duration time being a control variable of the inspection method with different values for the first and second leak detection tests. 
         [0018]    Advantageously, the beginning of the inspection of a new zone requires a waiting time after finishing the inspection of the previous zone, the waiting time being a control variable of the inspection method with different values for the first and second leak detection tests. 
         [0019]    In an embodiment, the vacuum bag assembly comprises a cured panel, a plurality of uncured stiffening stringers, curing tools for the uncured stiffening stringers with fastening clamps and a vacuum bag arranged directly over the composite part. The vacuum bag is formed joining individual bags for the uncured stiffening stringers and other bag pieces to cover the whole panel. The zones of the vacuum bag assembly to be covered by the chambers are set so that, in the direction of the stiffening stringers, have a predetermined length and in the direction transverse to the stringers comprise, for each stringer, the space occupied by the stinger&#39;s individual bag, the sealing area of two contiguous individual bags being assigned to the zone of one of the individual bags, so that the sealing area can be covered by one chamber. 
         [0020]    In another aspect, the invention provides a system for implementing the above mentioned method. Two basic components of the system are a helium spraying system for spraying helium over the vacuum bag assembly and helium detecting equipment to detect helium drawn by the vacuum inside the vacuum bag through a leak. The helium spraying system comprises apparatus configured to create a local helium atmosphere over individual zones of the vacuum bag assembly, so that the helium detecting equipment can detect the helium drawn by the vacuum in the individual zones through a leak, particularly a chamber to be positioned over the individual zones for housing the local helium atmosphere and helium injection apparatus in the chamber. The helium spraying system is configured for carrying out the inspection of the vacuum bag assembly by zones following a programmed zonal sequence. 
         [0021]    In an embodiment for the above mentioned vacuum bag assembly, the helium spraying system comprises a portal configured and arranged to be displaced over the vacuum bag assembly in the direction of the stiffening stringers and a plurality of arms aligned with the stiffening stringers so that each arm can position a chamber over a zone of the vacuum bag assembly, the arms been connected to a helium bottle through pipes. 
         [0022]    Other characteristics and advantages of the present invention will be clear from the following detailed description of embodiments illustrative of its object in relation to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIGS. 1 a  and 1 b    are, respectively a schematic plan view and a partial schematic elevation view of a vacuum bag assembly of a composite part comprising a cured panel and uncured stiffening stringers to be submitted to an autoclave cycle for co-bonding the stiffening stringers to the panel, the vacuum bag being applied directly over the stiffening stringers and the panel. 
           [0024]      FIG. 2  adds to  FIG. 1 a    the zoning to be followed by the inspection method of the invention. 
           [0025]      FIG. 3  is an elevation view of a zone of the vacuum bag assembly covered by a chamber where a local atmosphere of helium is created. 
           [0026]      FIG. 4  is a schematic view of the helium detecting equipment used in an embodiment of the method of the invention. 
           [0027]      FIG. 5  is a schematic view of the helium spraying system used in an embodiment of the method of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    The invention is described in connection with an assembly of the components of a composite stiffened panel in a vacuum bag before subjecting the components to a curing process in an autoclave. 
         [0029]    The vacuum bag assembly  11  (see  FIGS. 1 a  and 1 b   ), arranged on a mold  14 , comprises a cured panel  13  and several ensembles  19  formed by uncured stringers  15  disposed inside a vacuum bag  31  and between curing tools  21 ,  21 ′ provided with fastening clamps  33  for controlling the distance between them. The vacuum bag  31  for the whole assembly  11  is formed joining individual bags  23  for each ensemble  19  and other parts  25 ,  26  to cover the rest of the panel  13 . The joining of the various parts of the vacuum bag  31  is made using sealing tapes  32 . 
         [0030]    Vacuum will be pulled on the vacuum bag assembly  11  and a standard vacuum leakage detection test shall be performed. If the vacuum bag assembly  11  passes the test it will go to the autoclave for co-bonding the stringers  15  to the panel  13 . If the test is failed, the inspection method according to the invention, which is based on inspecting the vacuum bag assembly  11  by zones, shall start. Typically, it will begin inspecting the first row starting in zone Z 1 , 1  and finish inspecting the last zone Zm, 2  in the last row (see  FIG. 2 ). 
         [0031]    In each zone a first leak detection test using helium to rule out any leakage will be performed in the first place. If its result is positive, a second leak detection test using helium will be performed to confirm whether or not helium is detected inside the vacuum bag at a certain level. 
         [0032]    Carrying out the inspection of the vacuum bag assembly  11  in the above-mentioned two steps allows a very quick inspection procedure. 
         [0033]    The inspection is based on spraying helium at each of the zones in a chamber located over the zones so that a local atmosphere is created with the helium injected into the chamber, and to establish a correspondence between the detection of helium within the vacuum bag  31  and the particular zone where the helium local atmosphere is created, indicating a probable presence of a leak in the zone (or close to the zone). The use of this chamber is especially necessary where, as in the vacuum bag assembly  11 , there are areas of the vacuum bag  31  not visible, as those that are located between the curing tools  21 ,  21 ′ and the uncured stringers  15 . 
         [0034]    In an embodiment, the equipment for the detection of helium inside the vacuum bag comprises (see  FIG. 4 ), a leak detector  41 , a gas dosing valve  43 , and two auxiliary pumps  45  connected to hoses  53 ,  53 ′ and two hubs  47 ,  49  connecting a plurality of vacuum mold probes  51  with the hoses  53 ,  53 ′. 
         [0035]    The auxiliary pumps  45  are needed for two mean reasons:
       Get the highest possible vacuum level inside the vacuum bag  31 . High vacuum level on the bag is required to increase the sensitivity for leakage detection.   Recover, in a short period of time, the vacuum level inside the vacuum bag  31 , when it is required to open it (as when it is necessary to reduce the amount of helium accumulated inside).       
 
         [0038]    The main pump will be on during the most time of the leak test, whereas the second pump will be used mainly for quick vacuum recovery and as a backup pump in case of maintenance of the main pump. 
         [0039]    The gas dosing valve  43  is necessary to maintain the minimum required vacuum level at the input of the leak detector  41 , independently of the health of the vacuum bag  31 . 
         [0040]    Especially at first stages of helium leakage detection, the vacuum bag  31  may have too many leakages, and the pump is not able to create a vacuum level so low for the spectrometer to measure. 
         [0041]    During the detection process and in order to increase helium detection sensitivity, this valve should be kept as open as possible. 
         [0042]    The leak detector  41  is based upon mass spectrometers and quartz window sensors. Mass spectrometers ionize a gas mixture and isolate the desired tracer gas (helium in this case) on the basis of their mass-to-charge ratio. Quartz window sensors are based on the selective permeation of light gases through a quartz membrane. 
         [0043]    Distance from the leak detector  41  to the vacuum bag assembly  11  shall be minimized. Therefore, length and volume of hoses  53 ,  53 ′ will be minimized to avoid water vapor condensation. 
         [0044]    A computer system (not shown in the Figures), connected to the leak detector  41  and to the helium spraying management system  61  would provide the results obtained in the above-mentioned detection tests. 
         [0045]    The computer system will have the following information:
       Positions of chambers  55 .   Criteria used to determine leakages in each zone of the vacuum bag assembly  11 .   Parameterization of leakages and thresholds for each criterion.       
 
         [0049]    During the inspection, a leakage map will be shown in screens and printed on paper. In the images, a color code will be used to indicate the chambers  55  where helium has been sprayed with different colors for different leakage levels and the chamber  55  where helium is being injected at present. 
         [0050]    The chambers  55  may have a length (in the stringer direction) of, for instance lm and the width needed to cover one ensemble  19  and one of the adjacent sealing tapes  32  (see  FIG. 3 ). The internal volume of the chambers will be minimized as much as possible to reduce helium consumption and improve chamber performance. For a rigid chamber, the volume can be minimized with interior curable foam. 
         [0051]    The chambers  55  are made of a transparent material to allow a view of possible interferences with the curing tools  21 ,  21 ′ or the fastening clamps  33 . 
         [0052]    The chambers  55  should be as closed as possible to improve helium concentration and facilitate helium recovery. 
         [0053]    The material used in contact with the surface will be flexible to accommodate different geometries of panel  13  and soft to avoid damage to the vacuum foil, but shall not seal if there is a pore in the vacuum foil. 
         [0054]    In an embodiment, the helium spraying system  61  (see  FIG. 5 ) comprises a portal  63  configured and arranged to be displaced longitudinally over the vacuum assembly  11  and a plurality of arms  65  aligned with the stringers (seven in this case) so that each arm  65  can position a chamber  55  over a zone of the vacuum bag assembly  11  to spray helium and to retrieve it at the end of the inspection of the zone. The arms  65  are connected to a helium bottle  67  and have pipes  69  to carry the helium to the chambers  55 . 
         [0055]    Helium should be sprayed along a chamber  55  at the same time, especially if the chamber is long, to get a uniform helium concentration. As helium is lighter than air, it shall be sprayed from the lowest rigid part, so it will naturally go to the upper part. It is essential to spray helium from both sides of the chamber  55 . 
         [0056]    The quantity of helium released in each chamber  55  shall be controlled. One way to do it is controlling the helium pressure on the distribution pipes  69  and releasing the gas for a period of time with an electric valve. Time shall be controlled at least in 1 second intervals. Typical application time used in tests is 2 seconds. 
         [0057]    After applying the helium it is necessary to wait a few seconds for the gas to diffuse over the entire surface of the vacuum bag  31  and to penetrate inside if there is an aperture in the vacuum foil. After this time, the helium chamber should be evacuated from inside. For helium removal at least one wide port will be situated on the top part of the chamber. After evacuation the chamber will be lifted from the surface. 
         [0058]    As said before, the inspection method of the invention includes a first leak detection test to be performed on all the zones subject to inspection and a second leak detection test to be performed on those zones where the first leak detection test gives a positive result, immediately after the first leak detection test. 
         [0059]    The control variables for both tests are the same and include:
       Time of helium injection in each chamber.   Time for helium exposure in each chamber.   Time for helium removal in each chamber.   Waiting time for helium injection in next chamber.       
 
         [0064]    Logically the values of the variables for both tests are adapted to their objectives and are in a range of a few seconds. 
         [0065]    The inspection method of the invention can be automated to a high degree so that a first test of an assembly of a large composite part, such as a cover of an aircraft wing, can be performed in less than one hour. 
         [0066]    It is important to note that the objective of the inspection method of the invention is not identifying exactly the position of a leak in a vacuum bag assembly but to identify the zones in which the application of helium through the chambers is creating the highest helium detection in the spectrometer. It can happen that a zone registers a leakage signal, but the leakage does not have to be in that zone. Once the potential leakage areas are identified they are recorded for later manual inspection and rework. 
         [0067]    Finally, it should be noted that the present invention can identify leaks in non-visible areas of the vacuum bag, which exist when “direct bag” manufacturing methods are used as they are covered by curing tools. This type of leak cannot be detected with known methods. 
         [0068]    Although the present invention has been described in connection with various embodiments, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention as defined by the appended claims. 
         [0069]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.