Abstract:
A thermal and vacuum assessment method includes providing a composite part having a repair area, installing a vacuum bag over the repair area, drawing a vacuum on the vacuum bag, testing for air leaks in the vacuum bag, heating the vacuum bag and obtaining a thermal image of the repair area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and incorporates by reference in its entirety U.S. provisional application No. 61/205,799, filed Jan. 23, 2009 and entitled “THERMAL AND VACUUM ASSESSMENT METHOD”. 
    
    
     TECHNICAL FIELD 
     The disclosure generally relates to methods for determining thermal variations and vacuum leak paths in a damaged structure. More particularly, the disclosure is generally directed to a time-efficient thermal and vacuum assessment method for determining the thermal variations and vacuum leak paths in a damaged structure prior to installing a bonded repair to ensure uniform heating and minimal porosity of the repair. 
     BACKGROUND 
     Currently, getting a damaged metallic or composite component ready for a bonded repair requires the technician to gain knowledge of the thermal characteristics of the repair area. Structural stiffeners, frames, thick doublers, and thin skins all accentuate the thermal variations during the cure process. Adhesive materials and composite material resin systems require heat to be cured. The resin chemistry requires the heated area to be within a very narrow temperature range for the resins to cure properly. The presence of heat sinks can cause the actual temperature variation to fall outside of the specified range. Any un-cured, undercured, or overcured resins will cause a failure in the repair. By understanding the thermal anomalies of the structure a cure plan can be developed that includes selection of the heating source and insulation methods needed to equalize the cure temperatures within the specified range prior to curing the part thus ensuring a well fully cured component. 
     As part of the thermal survey a vacuum leak check is often performed. Vacuum leaks around fasteners, between attached structures, and through holes in the structure can filter air and other gases through the adhesive/resin materials being cured thus creating porous bondlines and resins. By understanding and sealing the leak paths in the repair area, porosity will be minimized and the repair will pass any subsequent inspections. 
     Existing solutions for bonding a repair onto a structure includes a thermal survey and a vacuum leak test prior to the actual repair. To perform a survey the repair in reality needs to be performed two times. The first “repair” is the survey where a full-size doubler is constructed out of actual repair material using one of the templates created that identifies each ply boundary. A 60-ply repair can take hours cutting and orienting repair plies into a suitable doubler. This new doubler is then placed onto the prepared repair area using a separator ply (so the survey doubler does not bond to the structure). Numerous thermocouples are placed in the curable and adjacent areas to monitor temperatures during the cure. A vacuum bag and heat blanket stack-up is then prepared and placed over the doubler. The survey doubler is then cured using the actual equipment and process spelled out in the repair procedure. The thermocouples are monitored and insulation is added to low temperature regions of the repair and insulation is taken away from hotter sections of the repair. In some cases supplemental heat is applied to the backside of the repair or to the perimeter to act as a thermal dam thus reducing the cure temperature variation. In addition the thermal monitoring, vacuum gauges are monitored to ensure adequate vacuum is available to apply the required cure pressure to the repair. Low vacuum would indicate a vacuum leak and an opportunity for air or other gases to permeate the resin or bondline thus reducing the repair performance to the point of a failed repair. After the survey is complete and a cure plan is ready for the actual repair, the second set of repair materials can now be used to complete the actual repair. 
     Existing solutions double the time necessary to perform a bonded repair, require two complete sets of repair consumables including very expensive repair materials, and add another heat cycle to the parent structure. Parent composite structures are often limited to five heat cycles during the life of the structure before replacement is necessary due to heat degradation of the polymers in the composite material. 
     A time-efficient thermal and vacuum assessment method for determining the thermal variations and vacuum leak paths in a damaged structure prior to installing a bonded repair to ensure uniform heating and minimal porosity of the repair is needed. 
     SUMMARY 
     The disclosure is generally directed to a thermal and vacuum assessment method. An illustrative embodiment of the method includes providing a composite part having a repair area, installing a vacuum bag over the repair area, drawing a vacuum on the vacuum bag, testing for air leaks in the vacuum bag, heating the vacuum bag and obtaining a thermal image of the repair area. 
     In some embodiments, the thermal and vacuum assessment method may include providing a composite part having a repair area; installing a vacuum bag over the repair area; drawing a vacuum on the vacuum bag; testing for air leaks in the vacuum bag; heating the vacuum bag a first time; obtaining a first thermal image of the repair area; assessing a difference in temperature across the repair area based on the first thermal image; heating the vacuum bag at least a second time; obtaining at least a second thermal image of the repair area; and repairing the repair area. 
     In some embodiments, the thermal and vacuum assessment method may include providing a composite part having a repair area; installing a vacuum bag over the repair area; drawing a vacuum on the vacuum bag; testing for air leaks in the vacuum bag; detecting at least one leak in the vacuum bag; heating the vacuum bag a first time; obtaining a first thermal image of the repair area; assessing a difference in temperature across the repair area based on the first thermal image; sealing the at least one leak in the vacuum bag; heating the vacuum bag a least a second time; obtaining at least a second thermal image of the repair area; and repairing the repair area. 
     In some embodiments, the thermal and vacuum assessment method may include providing a composite part having a repair area; placing a separator film over the repair area; placing a repair doubler on the separator film; installing a vacuum bag over the repair area; drawing a vacuum on the vacuum bag; testing for air leaks in the vacuum bag; detecting at least one leak in the vacuum bag; heating the vacuum bag a first time; obtaining a first thermal image of the repair area; assessing a difference in temperature across the repair area based on the first thermal image; correlating cold indications on the first thermal image with locations on the repair area; sealing the at least one leak in the vacuum bag; heating the vacuum bag a second time; obtaining a second thermal image of the repair area; assessing a difference in temperature across the repair area based on the second thermal image; heating the vacuum bag a third time; obtaining a third thermal image of the repair area; assessing a difference in temperature across the repair area based on the third thermal image; determining that the repair area is outside an acceptable temperature range; preparing insulation; heating the vacuum bag a fourth time; installing the insulation on a backside of the vacuum bag; heating the vacuum bag at least a fifth time; obtaining at least a fourth thermal image of the repair area; assessing a difference in temperature across the repair area based on the fourth thermal image; performing a validation scan; preparing a thermal heating plan for final repair on the repair area; removing the vacuum bag from the repair area and cleaning the repair area; and repairing the repair area. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a block diagram of a system which is suitable for implementation of an illustrative embodiment of the thermal and vacuum assessment method, with a composite part to be repaired shown in cross-section. 
         FIG. 2  is a thermal image of a repair area in a part and which indicates a need for supplemental backside heating to achieve uniform heating of the composite part prior to installing a bonded repair on the part. 
         FIG. 3  is a cross-section of the part with the bonded repair in place. 
         FIG. 4  is a block diagram of an illustrative embodiment of the thermal and vacuum assessment method. 
         FIG. 5  is a flow diagram of an aircraft production and service methodology. 
         FIG. 6  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to practice the disclosure and are not intended to limit the scope of the appended claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented 
     The disclosure establishes a new methodology for performing a pre-repair vacuum and thermal survey using rapid heating and infrared cameras. The process may be performed using an actual repair doubler placed in a prepared repair area of a composite part, which may be vacuum-bagged without any insulation, heat blankets, and non-flyaway ply layers (peel plies, separator films, caul sheets, etc). 
     Referring initially to  FIG. 1 , an exemplary system which is suitable for implementation of an illustrative embodiment of the thermal and vacuum assessment method, hereinafter method, is generally indicated by reference numeral  100 . A composite part  101  on which the method may be performed includes a repair area  102 . Prior to forming a bonded repair  103  ( FIG. 3 ) in the repair area  102  of the composite part  101 , the composite part  101  may be subjected to the method using the system  100 . The system  100  may include a heat source  110  which emits heat  111  that heats the composite part  101 . The heat source  110  may utilize any of numerous technologies including but not limited to induction, infrared, high-intensity flashed light or heat gun. 
     The system  100  may include an IR camera  114  which emits IR radiation  115  against the composite part  101  and takes thermal images of the composite part  101 . A database  120  may be adapted to receive thermal images  118  from the IR camera  114 . In some embodiments, the IR camera  114  may be a video camera. In other embodiments, the IR camera  114  may be a still-shot digital camera. The IR camera  114  may be any type of IR camera which has enough video or rapid single shot frames to record/display/sense thermal decay of the heated repair area  102  of the composite part  101 . In some embodiments, the system  100  may include a backside heat/insulation source  122 . 
     Referring next to  FIG. 4  in conjunction with  FIGS. 1-3 , a method  400  of an illustrative embodiment of the thermal and vacuum assessment method is shown. Fabrication of the repair doubler for the bonded repair  103  ( FIG. 3 ) may start with preparation of the repair area  102  for repair. In block  402 , the repair area  102  of the composite part  101  may be prepared for the bonded repair  103 . Preparation of the repair area  102  may include taper sanding of the repair area  102  in preparation for a scarf repair or removing damage and sanding the repair area  102  to create a bondable surface. A template (not shown) may then be prepared to identify each ply of the bonded repair  103  as well as the size, geometry and ply orientation of the bonded repair  103 . Using the template, each ply may be cut from a wet layup or prepreg material (not shown). The plies may be stacked and oriented onto each other to create the repair doubler for the bonded repair  103 . 
     In block  404 , a temporary separator film (not shown) may be prepared and laid onto the prepared repair area  102 , after which the repair doubler may be installed onto the separator film. This may ensure that the ply paths of the repair doubler are aligned. 
     In block  406 , a vacuum bag  106  may be installed over the entire repair area  102  without any extra materials between the repair doubler and the vacuum bag  106 . In block  408 , a vacuum may be drawn on the vacuum bag  106  and all obvious vacuum leaks may be sealed to ensure a vacuum pressure of at least 18-20 in. of mercury. In block  408   a , a determination may be made as to whether there exist any undetectable leaks in the vacuum bag  106 . If there are no undetectable leaks in the vacuum bag  106   a  in block  408   b , the method may proceed to block  410 . If there are undetectable vacuum leaks in the vacuum bag  106 , the method may proceed through blocks  408   c - 408   k , which will be hereinafter described. 
     If there are no undetectable vacuum leaks in the vacuum bag  106  in block  408   b , then in block  410 , the vacuum bag  106  may be rapidly heated to at least 20 degrees above ambient using the heat source  110 . A sufficient quantity of heat  111  which is necessary to heat the repair doubler and composite part  101  may be used. In some applications, delta temperatures of two or more degrees may be sufficient since most IR cameras can detect delta temperatures of two or more degrees within the composite part  101  in a subsequent step. A 20-degree delta may provide enough dwell time for multiple scans and still not permit a chemical reaction needed to cure the repair doubler. 
     In block  412 , an IR camera snapshot or video of the heated repair area  102  may be taken to obtain a thermal image  118  of the repair area  102 . One to two seconds may be required to obtain a good thermal image  118  without being influenced by the thermal decay. Longer video times may increase the error factor and may lead to thermal anomaly decisions that don&#39;t support a good thermal survey. The thermal image  118  taken by the IR camera  114  may be stored in the database  120 . 
     In block  414 , the thermal image  118  which was taken from the IR snapshots or the video in block  412  may be used to assess the delta temperatures across the heated repair area  102 . Any visual features on the composite part  101  may be used to help isolate or understand the thermal image  118 . The high and low temperature zones in the repair area  102  may be documented. Block  414  may be performed by comparing the decay signatures with signatures from a database of similar signatures/structures or by reviewing and comparing the thermal deltas with the actual structural geometry. Anomalies ranging from vacuum leaks to heat sinks can be minimized by repairing the vacuum leaks and knowing where to place supplemental heat or insulation. 
     In block  416 , using the documented thermal image anomalies from block  414 , some supplemental heating or insulation may be applied to the backside of the repair area  102  using a backside heat/insulation source  122  to reheat the composite part  101 . The composite part  101  may be tested again by repeating blocks  410 ,  412  and  414 . In block  416   a , a determination may be made as to whether the composite part  101  is outside the acceptable temperature range. If the composite part  101  is still outside the acceptable temperature range in block  416   b , the process may proceed through blocks  416   c - 416   i , as will be hereinafter described. This procedure may be performed after making the appropriate physical adjustment and retesting, thus achieving the expected result. 
     If the determination in block  416   a  reveals that the composite part  101  is not outside the acceptable temperature range in block  416   j , a validation scan may be performed in block  418 . Based on the final thermal image that shows thermal uniformity, a thermal heating plan for the final repair may be prepared in block  420 . The thermal survey findings may be documented. 
     In block  422 , the vacuum bag  106  may be removed from the composite part  101  and repair doubler may be removed from the repair area  102 . The repair area  102  may be cleaned for the actual bonded repair. In block  424 , the repair area  102  may be prepared for actual repair and the repair may be carried out following the approved repair process to form the bonded repair  103  in the repair area  102 , as shown in  FIG. 3 . 
     As was described herein above, after a vacuum is drawn on the vacuum bag  106  and obvious leaks in the vacuum bag  106  are sealed in block  408   a , a determination may be made as to whether there are any undetectable leaks in the vacuum bag  106  in block  408   a . In block  408   c , if undetectable leaks are found in the vacuum bag  106  then the vacuum bag  106  may be heated rapidly in block  408   d . In block  408   e , an IR camera snapshot or video of the heated repair area  102  may be taken. In block  408   f , the delta temperature across the heated repair area  102  may be assessed and the resulting thermal image documented. 
     A vacuum leak may be present in the vacuum bag  106  if there are any cold spots on the thermal image that cannot be related to a heat sink. Therefore, in block  408   g , the cold indications on the thermal image may be correlated to the actual locations on the composite part  101 . An operator (not shown) may then listen (with or without supplemental listening devices) for air leaks in the vacuum bag  106 . In block  408   h , a sealant or putty may be used to seal any probable leak locations. In the event that additional leaks are detected in blocks  408   i  and  408   j , blocks  408   e - 408   h  may be repeated as many times as are necessary to seal all leaks in the vacuum bag  106 . In the event that no additional leaks are detected in block  408   k , the process may continue to block  410 . Block  408   g  may be performed by comparing the decay signatures with signatures from a database of similar signatures/structures or by reviewing and comparing the thermal deltas with the actual structural geometry of the composite part  101 . Anomalies ranging from vacuum leaks to heat sinks can be minimized by repairing the vacuum leaks and knowing where to place supplemental heat or insulation. 
     As was described herein above, after the vacuum bag  106  is heated and the composite part  101  retested in block  416 , a determination may be made as to whether the composite part  101  is outside the acceptable temperature range in block  416   a . If the composite part  101  is found to be outside the acceptable temperature range in block  416   b , as indicated in the thermal image shown in  FIG. 2 , insulation  122  in the size and shape which is needed to apply to the backside of the repair area  102  may be prepared in block  416   c . With the insulation  122  ready to install, the vacuum bag  106  may be rapidly heated in block  416   d . In block  416   e , the newly cut insulation may be rapidly installed to the outside of the vacuum bag  106  using tape. If no vacuum leaks remain in the vacuum bag  106  in blocks  416   f  and  416   g , the process may proceed to block  410 . If there are still indications that vacuum leaks remain in the vacuum bag  106  (such as areas that are still cold and not adjacent to heat sinks) in blocks  416   f  and  416   h , the process may proceed through blocks  408   g - 408   k  until all leaks are sealed, after which the process may proceed to block  410 . 
     The existing thermal and vacuum assessment method solutions may require that two repair doublers be fabricated and cured. The thermal survey may be a dry run of the repair that includes an exact duplicate of every repair feature thus mimicking all cure details. The detection of vacuum leaks may be a hit or miss proposition in that there may be no direct correlation between low vacuum pressures and where the vacuum is actually leaking. TC spacing and TC placement may be very subjective; thus, the actual temperature distribution may be distorted. The second repair doubler may then be cured as the actual repair. 
     The method of the disclosure may require that only the actual repair doubler be fabricated. The actual thermal test may use very low heat and since the entire repair area may be exposed to the IR camera, thermal variations and vacuum leaks may show up as image anomalies. In addition, the IR cameras may use a very small pixel size and thus, minute temperature variations may be easily detected. Since the test may require less than a minute, changes in the supplemental heating, vacuum leak sealing, placement of insulation may be made rapidly. 
     Referring next to  FIGS. 5 and 6 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  78  as shown in  FIG. 5  and an aircraft  94  as shown in  FIG. 6 . During pre-production, exemplary method  78  may include specification and design  80  of the aircraft  94  and material procurement  82 . During production, component and subassembly manufacturing  84  and system integration  86  of the aircraft  94  takes place. Thereafter, the aircraft  94  may go through certification and delivery  88  in order to be placed in service  90 . While in service by a customer, the aircraft  94  may be scheduled for routine maintenance and service  92  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  78  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 6 , the aircraft  94  produced by exemplary method  78  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  78 . For example, components or subassemblies corresponding to production process  84  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  94  is in service. Also one or more apparatus embodiments may be utilized during the production stages  84  and  86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  94  is in service, for example and without limitation, to maintenance and service  92 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.