Abstract:
A method and apparatus for detecting and inspecting composite structures to discover inconsistencies by the force and displacement caused by extension of a stylus driven against the composite structure a fixed distance. The measurement of the force on the stylus and the deflection is made from the exterior surface of the composite structure, which eliminates the need to remove interior panels for inspection.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This disclosure relates to systems and methods for inspecting composite structures, and in particular to a system and method for inspecting composite structures for inconsistencies. 
         [0003]    2. Description of the Related Art 
         [0004]    Composite material has a life cycle much like other materials. Inspection is part of the process used to track the condition of composite materials during its life cycle. 
         [0005]    Inconsistencies in the resin of a composite structure may be difficult to detect using nondestructive testing without accompanying inconsistencies in the fibers of the composite. Inconsistencies in the resin of a composite structure may be caused by many sources including, but not limited to, exposure to high temperature for short time periods or moderate temperatures for long periods, lightning strikes and electrical arcing. 
         [0006]    At present, there are no approved nondestructive test methods to assess inconsistencies in the resin of composites, particularly thermally induced resin inconsistencies. 
         [0007]    Another problem may include the detection of inconsistencies in composite stringers, longerons, frames, spars, caps, and other support structures, for example hat section stringers. In such situations, non-destructive inspection may typically require the time-consuming removal of interior structure, such as panels and/or insulation blankets 
         [0008]    Accordingly, there is a need for a fieldable in-service method that enables aircraft maintenance personnel to assess and determine the disposition of composite structures that have been thermally exposed or otherwise suspected to be have inconsistencies that might affect performance. 
       SUMMARY OF THE INVENTION 
       [0009]    To address the requirements described above, embodiments of the disclosure illustrate a method and apparatus for analyzing a composite structure for hidden structural inconsistencies. The method comprises the steps of deflecting an interior portion of a test area of the composite structure a fixed distance relative to an exterior portion of the test area by securing a surface of the exterior portion from motion in a first direction and driving a rigid member against a surface of the interior portion at a first location in the first direction, then measuring the force applied to deflect the interior portion of the test area the fixed distance relative to the exterior portion of the test area, and/or measuring the deflection at a distance d from the driven stylus. Next, determining if the composite structure has inconsistencies by comparing the measured force and/or deflection to an expected force and or expected deflection. 
         [0010]    In a representative embodiment, the apparatus is a portable, single-sided nondestructive inspection apparatus for analyzing a composite structure, comprising a frame having a horizontal member, a plurality of suction cups attached to the frame and configured to releasably attach the frame to a surface of the composite structure, a drive mechanism, coupled to the horizontal member of the frame, for urging a rigid stylus against the surface of the composite structure at a first location to flex the composite structure, and a force sensor for measuring a force applied to the surface of the composite structure at the first location by the driven stylus and a distance gage for measuring the deflection of the surface at a distance d from the driven stylus. 
         [0011]    This foregoing provides a single sided method of detecting inconsistencies caused by thermal exposure or structural effects by monitoring the force and displacement caused by extension of a stylus driven against the composite structure a fixed distance. A measurement of the force on the stylus (and hence the composite structure) and the deflection is made from the exterior surface which eliminates the need to remove interior panels for inspection. 
         [0012]    If the structure is without inconsistencies, the structure will be relatively stiff, and the measured force will be large and the displacement, small. If the structure has inconsistencies caused thermal exposure or other factors, the structure below the stylus will flex, and the measured force decreases while the displacement increases. The information generated by these two measurements can be compared to the values from a similar structure in adjacent areas to enable a relative comparison of the severity of the inconsistencies. 
         [0013]    This provides a method to quantify thermal inconsistencies to composite material, a method to detect “hidden” inconsistencies in the supporting structure that is not in intimate contact with the exterior skin, and may be hidden by insulation and interior panels. It also can help determine the location of subsurface structures by monitoring the force/extension values taken as the device is moved across a surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
           [0015]      FIG. 1  is an illustration of a perspective view of an exemplary embodiment of an inspection apparatus; 
           [0016]      FIGS. 2A-2C  are illustrations of side, bottom, and front views of the inspection apparatus shown in  FIG. 1 ; 
           [0017]      FIG. 3  is a flow chart illustrating exemplary method steps that can be used to practice one embodiment of the present invention; and 
           [0018]      FIG. 4  is an illustration of the application of the inspection apparatus to an external surface of a composite structure; 
           [0019]      FIGS. 5A-5E  are illustrations of the operation of the inspection apparatus; and 
           [0020]      FIG. 6  is an illustration of a further application of the inspection apparatus. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
         [0022]      FIG. 1  is an illustration showing a perspective view of an exemplary embodiment of an inspection apparatus  100 . In the illustrated embodiment, the inspection apparatus  100  comprises a frame  102  having a horizontal member  104 , and a plurality of suction cups  108 A- 108 D hereinafter alternatively referred to as suction cup(s)  108 ) attached to the frame by members  106 , and configured to releasably attach the frame  102  to the surface of a composite structure  150 . 
         [0023]    The inspection apparatus  100  also comprises a drive mechanism  140  coupled to the horizontal member  104 , for urging a rigid stylus  120  against the surface of the composite structure  150  at a first location  122  to flex the composite structure  150  and a force sensor  128  for measuring a force applied to the surface of the composite structure at the first location  122  by the stylus  120 . In one embodiment, the drive mechanism  140  employs a structure that causes the rigid stylus  120  to be gently driven or urged against the surface of the composite structure  150  a fixed distance. The drive mechanism  140  can comprise a handle  114  and a cam  202  (illustrated in  FIG. 2A ), or an equivalent structure such as an elbow joint coupled to a plurality of arms (not shown). 
         [0024]    In the illustrated embodiment, the inspection apparatus  100  also comprises a second stylus  118  that is offset from the first stylus  120  by a distance d. The second stylus is in communication with a measurement device  116 . The second stylus  118  may be spring loaded so that when the inspection apparatus  100  is attached to the composite structure  150 , the second stylus  118  may be in light contact with the composite surface  150 . When the first stylus is driven against the composite structure  150 , the second stylus  118  remains in contact with the structure  150  so that any displacement of the structure  150  may result in a longitudinal displacement of the stylus  118 . The longitudinal displacement of the stylus  118  is measured by the displacement measuring device  116 , and used to determine the displacement of the composite surface at a distance d from where the first stylus  120  is driven against the composite structure. 
         [0025]    This displacement measurement may also be taken at a location coincident with the first location  112 . This can be accomplished by including a displacement measuring device or sensor in the drive mechanism  140 . 
         [0026]    The inspection apparatus  100  may be secured to the external surface of the composite structure  150  by use of the plurality of suction cups  108 , which are attached to a distal end of cross members  106 A and  106 B. Each suction cup is secured to the frame by bosses  110 A- 110 D. Each of the bosses  110 A- 110 D includes a hollow section therethrough, from which air within the cavity  210 A- 210 D formed by each of the cups  108  can be evacuated. Pneumatic connectors  112 A- 112 D are attached to each of the bosses  110 A- 110 D and the hollow portion of each pneumatic connector  112 A- 112 D is in sealed pneumatic communication with the hollow section of the bosses  112 A- 112 D. Each pneumatic connector  112 A- 112 D also includes nipples that permit the connection of pneumatic tubing so that vacuum can be applied, ultimately evacuating air from the inside cavity of the suction cups  108 , affixing them (and thereby, the inspection apparatus  110 ) to the surface of the composite structure  150 . 
         [0027]    The illustrated embodiment of the inspection apparatus  100  also includes a plurality of rigid stops  126 A- 126 B coupled to the horizontal member  104 . As the air in the inside cavities  210 A- 210 D of the suction cups  108  is evacuated, the apparatus  100  is drawn towards the surface of the composite structure  150 . The rigid stops  126  prevent the inspection apparatus  100  from being drawn any closer to the composite structure  150 . 
         [0028]      FIGS. 2A-2C  are further illustrations of the embodiment shown in  FIG. 1 .  FIG. 2  shows additional details regarding one embodiment of the drive mechanism  140 . In this embodiment, the drive mechanism  140  comprises a handle  114  having an integral cam  202 . The cam  202  has an eccentric surface and is placed in contact with a force-measuring device  128  such as a load cell. The cam  202  may be secured from all but rotational movement in the plane of the paper by pins  123  that extend through apertures  124 . When the handle  114  is lowered, the handle  114  is secured to the location of the aperture  124  by the pin, and thus, rotates around the aperture  124 . The eccentric surface of the cam  202  applies a force to the force measuring device  128 , which applies the force to the stylus  120 , directing the stylus in the downward direction and against the surface of the composite structure  150 . 
         [0029]    In one embodiment, the surface of the cam  202  is also shaped so that it has a constant radius after being rotated a given amount (instead of an increasing radius before that time), so that the stylus is driven against the surface of the composite structure by a fixed amount (a fixed deflection). This may also be implemented by using a cam  202  having an increasing radius, and adding a stop to the handle  114  or to the drive mechanism supporting structure  117  so that the handle  114  can only be rotated a maximum amount. 
         [0030]    There are several apertures in the supporting structure  117  and the handle  114 , and by moving the pin  123  to a specific set of apertures  124 , the amount of fixed deflection can be selected. The current embodiment has a choice of 0.050″, 0.100″, 0.150″ and 0.200″, but other deflections are within the scope of this disclosure. 
         [0031]      FIG. 2A  also illustrates that the stylus  120  may also comprise a thumbwheel  204  that may be used to extend the length of the stylus as it is mounted in the inspection apparatus  100 . This allows the user to attach the inspection apparatus to the surface of the composite structure  105 , then, adjust the nominal depth of the stylus (e.g. by rotating thumbwheel  204 ) so that the stylus  120  contacts the composite structure  150  before the handle  114  is rotated and the load is applied. 
         [0032]      FIG. 2B  is an illustration of a bottom view of one embodiment of the inspection apparatus  100 .  FIG. 2B  shows the rigid stops  126 , and the apertures  212 A- 212 D that permit pneumatic communication between the cavities  210 A- 210 D and the hollow portion of the bosses  110 A- 110 D and hence, the nipples  112 A- 112 D. 
         [0033]      FIG. 2C  illustrates a side view of the inspection apparatus  100 . This view illustrates the spatial relationship between the rigid stops  126  and the suction cups  108 , the design of the nipples  112 A- 112 D. 
         [0034]      FIG. 3  is a flow chart illustrating exemplary method steps that can be used to practice an embodiment of the invention.  FIG. 3  will be discussed with reference to  FIGS. 1 ,  2 A- 2 C,  FIG. 4 , which illustrate the application of the inspection apparatus  100  to the composite structure  150 , and  FIGS. 5A-5E , which illustrate the process. 
         [0035]    As shown in step  302  of  FIG. 3  and in  FIGS. 5A and 5B , an inspection apparatus having the suction cups  108  peripherally disposed thereto are attached to the exterior surface of the composite structure  150 . This can be accomplished by placing the suction cups  108  against the surface of the composite structure  150  and applying a vacuum to the suction cups  108  to evacuate the suction cup cavities  210 A- 210 D and urge the inspection apparatus towards the surface of the composite structure  150 . In one embodiment, the suction cup cavities  210 A- 210 D are evacuated a sufficient amount so that the rigid stops  126  are in contact with the surface of the composite structure  150 . As shown in  FIG. 5C , the thumbwheel  204  can be used to bring the stylus  120  to contact with the surface of the composite structure  150 . The second stylus  118  may also be brought into contact with the composite structure  150 . 
         [0036]    Next, as shown in step  304  of  FIG. 3 , an interior portion  402  (shown in  FIG. 4 ) of a test area  404  of the composite structure  150  is deflected a fixed distance relative to a portion exterior to the test area  404 . This can be accomplished by securing a surface of the exterior portion  406  from motion in a first direction (e.g. in a direction perpendicular to the composite structure  150 ), and driving a rigid member  120  against a surface of the interior portion  402  of the test area  404  in the same direction, as illustrated in  FIG. 5D . 
         [0037]    Next, as shown in step  306  of  FIG. 3 , the force applied to deflect the interior portion  402  (shown in  FIG. 4 ) of the test area  404  the fixed distance relative to the portion  406  external to the test area  404  is measured. This is also illustrated in  FIG. 5D . In this embodiment, the force is measured by the sensor  128  disposed between the cam  202  and the stylus  120 . 
         [0038]    Next, as shown in step  307  of  FIG. 3 , the deflection of the of the interior portion  402  of the test area  404  is measured at a distance d from the stylus  120  relative to the portion  406  external to the test area  404  is measured. This is also illustrated in  FIG. 5D . In this embodiment, the distance is measured by the sensor  116 . 
         [0039]    Finally, as shown in step  308  of  FIG. 3 , a determination is made as to whether the composite structure  150  has inconsistencies by comparing the measured force to the expected force and/or the measured deflection to the expected deflection. “Inconsistencies,” as the term is used in the appropriate context throughout this disclosure, refers to the difference between one or more measured characteristics of a composite structure under test (and potentially effected by exposure to factor(s) including thermal load(s), structural load(s), lightning, or electrical arcing) with expected values for the same characteristics of an analogous composite structure unaffected by exposure to those factors. 
         [0040]      FIG. 5D  is a diagram illustrating the typical response of a test area  404  without inconsistencies in the composite structure to the driven stylus, and  FIG. 5E  is a diagram illustrating the typical response of test area with inconsistencies in the composite structure to a driven stylus. Typically, a composite structure  150  that is without inconsistencies will be stiffer than one that has inconsistencies. Hence, when the stylus  120  is driven the fixed distance against the composite structure to deflect it, the force sensor  112  will register a higher reading for a composite structure  150  ( FIG. 5D ) without inconsistencies than for a structure with inconsistencies ( FIG. 5E ). This higher reading is an indication that the underlying structure has inconsistencies. 
         [0041]    In the foregoing embodiment, stylus  120  is moved a fixed distance and the force applied to the stylus  120  may be monitored to determine whether the composite structure  150  has inconsistencies. Other embodiments can use a stylus  120  that is driven a variable distance against the composite structure and a deflection measurement device to measure the deflection of the composite structure  150  in response to the driven stylus  120 . 
         [0042]    For example the drive mechanism  140  may include a vernier scale or other device permitting measurement of the displacement of the stylus  120  driven against the composite structure  150  as well as the force sensor  128  to measure the force applied to the composite structure  150 . The relationship between displacement and applied force can be stored and compared to measurements of nearby test areas of the composite structure that are known to be without inconsistencies to assess whether the test area includes inconsistencies. 
         [0043]    In addition to or as an alternative to measuring the deflection of the driven stylus  120 , a separate deflection measurement device  116  can be used to measure the deflection at a second location laterally offset from the location where the driven stylus  120  contacts the composite structure  150 . Since the displacement of the driven stylus  120  is known (it is either fixed or can be measured), the deflection of the composite structure at the second location can be used to provide a measure of the shape of the composite structure  150  when the driven stylus  120  is forced against it&#39;s surface, as shown in  FIG. 5E . This data can also be compared with expected results to determine whether the composite structure is sufficiently uniform in its pertinent characteristics, or if the inconsistencies in those measure characteristics that require further investigation. 
         [0044]    The deflection measurement device  116  can also be used to assure that the rigid stops  126  remain in contact with the surface of the composite structure  150  as the stylus  120  is driven against the composite structure  150 , or to provide additional measurement information that can be used to assess possible inconsistencies in the composite structure even if the stops separate from the surface. Typically, if the suction cups  108  become extended and the rigid stops  126  are drawn away from the surface of the composite structure  150 , the measured force on the stylus is less than it might have been if suction cup  108  extension had not occurred, but typically, the force is still greater than that which might occur if the composite structure  150  was without inconsistencies. 
         [0045]      FIG. 6  illustrates the use of the inspection apparatus  100  to externally determine the location of stringers and other structures in multi-layer composite structures. The illustrated composite structure includes one stringer  602  bonded to a sheet of composite material  604 , to strengthen the composite material. The location of the stringer is invisible from the external side  606  of the structure  150 , however, if the inspection apparatus  100  were passed over different sections of the structure  150  from left to right, the location of the stringer  602  can be determined. For example, if the inspection apparatus  100  were passed in the direction of the arrow labeled “A” while taking measurements as described above, the measured force applied to the stylus might follow a pattern much like that of plot “A”, showing increased resistance to deflection in the location where the stringer  602  pattern and the sheet of composite material  604  combine, and less resistance to deflection in those areas where the stringer  602  is not in contact with the sheet of composite material  604 . If the inspection apparatus were passed along path “B” over an area of the stringer  603  having inconsistencies, the composite material  604  and stringer  602  combine so that there is less variation in the measured deflection from the driven stylus  120 . Using such measurements, the location and configuration of stringers along internal surfaces can be determined. This information can also be used to compare expected forces and deflections with measured values, to determine whether further investigation or replacement is warranted. 
       CONCLUSION 
       [0046]    This concludes the description of the preferred embodiments. The foregoing description of the preferred embodiment has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the rights granted under this disclosure be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the embodiments. Since many embodiments can be made without departing from the spirit and scope, the claims hereinafter appended are submitted for consideration.