Patent Publication Number: US-2018031624-A1

Title: Fibre composite material inspection method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from British Patent Application Number 1612981.9 filed 27 Jul. 2016, the entire contents of which are incorporated by reference. 
     Field of Disclosure 
     The present disclosure concerns a fibre composite material inspection method. 
     Background 
     Fibre composite materials such as carbon fibre reinforced plastic (CFRP) comprise a plurality of fibres embedded within a matrix. In the case of CFRP, the fibres comprise carbon fibre, while the matrix comprises a curable resin. The composite has bulk material properties such as strength which exceeds that of its components. 
     Such materials have greater strength to weight ratios than many traditional aerospace materials such as aluminium. Furthermore, composite materials are less susceptible to corrosion. However, damage to fibre composite components may still occur. 
     Several methods have been proposed for detecting defects in fibre composite materials. These include X-ray and ultrasound scanning and Magnetic Resonance Imaging (MRI). However, these techniques frequently require direct line of sight of the component, which makes inspection difficult without removing the component from larger subassemblies. Furthermore, X-ray, ultrasound and MRI equipment tend to be relatively bulky, which makes onsite inspection impractical. It is also known to provide on-board sensors such as laser diffraction sensors, which detect strain in use. However, such methods are heavy and expensive, and only detect strains rather than actual damage. Similarly, where the composite part comprises a plurality of layers of material, defects may be hidden beneath the surface, making visual inspection inadequate. 
     SUMMARY 
     Consequently, the present disclosure seeks to provide a method of detecting a defect in a fibre composite material that seeks to overcome the above problems. 
     According to a first aspect of the disclosure there is provided a method of determining the position of a defect within a fibre composite article comprising a plurality of electrically conductive fibres embedded within a non-conductive matrix, the method comprising: 
     providing an array of spaced electrical contacts in electrical contact with fibres of the article; 
     conducting an electrical current between two or more spaced electrical contacts of the array to determine a measured electrical profile of the article comprising measurements of at least one of electrical resistance, inductance, capacitance and self-resonant frequency of the article; and 
     comparing the electrical profile with a nominal electrical profile representative of an undamaged article to determine the presence of a defect within the article. 
     Advantageously, from a comparison of a measured electrical profile to a nominal profile of an undamaged part, damage can be readily detected. The method is highly repeatable, and so do not require direct line of access to the part, while being relatively inexpensive. 
     The conductive fibres may comprise non-metallic fibres, such as carbon fibre, and the non-conductive matrix may comprise a polymer such as a thermoset resin such as any of epoxy, phenolic, polyester resin, vinyl ester resin and urethane. 
     The fibre composite article may comprise a plurality of layers of fibres. Each layer of fibres may comprise a plurality of woven fibres. 
     The array of electrical contacts may comprise a solder contact which may comprise a low temperature solder such as one or more of C-Solder™ a tin, lead, bismuth, antimony, silver, indium, zinc or copper based solder . The solder contact may comprise solder bonded to the electrically conductive fibres. 
     The method may comprise drilling a hole through a plurality of layers of fibres and applying a molten solder into the hole. Advantageously, each contact is electrically connected to multiple plies (layers) of carbon fibre, which may have different orientations. 
     The array of electrical contacts may comprise at least first and second contacts electrically connected to separate fibre layers, which may be spaced from one another within the plane of each fibre layer. 
     The step of determining an electrical profile may comprise providing a further electrical connection between the first and second contacts, and determining one of a resistance, a capacitance and an inductance between the first and second contacts. 
     The method may comprise determining a first electrical path within the composite material between the first and second electrical contacts, and determining the presence of a defect along the electrical path where the determined electrical profile of the first and second contacts differs from the nominal electrical profile. 
     The method may comprise providing at least third and fourth electrical contacts spaced from the first and second contacts which define a second electrical path through the composite component that intersects the first electrical path. 
     The method may comprise determining the presence of a defect at the point of intersection of the first and second paths where the determined electrical profile of the first and second contacts and the third and fourth contacts both differ from the nominal electrical profile. Step change differences in point-to-point electrical properties over time indicate the presence of damage or defects. 
     The electrical current may comprise one of a DC electrical current and an AC electrical current. Where a DC current is used, electrical resistance can be reliably measured to determine bulk material properties. On the other hand, where an AC current is used, one or both of inductance and capacitance can be determined, to determine one or more of the parallel resonance (of inductance and capacitance), the impedance, the resonance and the admittance. 
     The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       An embodiment will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a plan view of a composite component comprising a defect detection apparatus; 
         FIG. 2  is a cross sectional view of the component of  FIG. 1  through the through the line A-A of  FIG. 1 ; 
         FIG. 3  is a plan view of the composite component of  FIG. 1  illustrating a plurality of electrical paths between an array of contact points; 
         FIGS. 4  is a plan view of the composite component of  FIG. 1  having a defect; 
         FIG. 5  is a cross-sectional view of the composite component of  FIG. 1  along the line A-A in  FIG. 1 ; and 
         FIGS. 6 a - b    show cross-sectional of the composite component of  FIG. 1  at first, second and third steps of an electrical contact forming process; and 
         FIG. 7  is a process flow diagram illustrating a method of detecting a defect in a composite component. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the figures, there is provided a method of detecting a defect in a composite article  10  shown in  FIGS. 1 to 5 , the method being outlined in  FIG. 6 . 
     Referring to  FIGS. 1 and 2 , the article  10  comprises a composite material comprising a plurality of electrically conductive, non-metallic fibres such as carbon fibres  14 , embedded within a matrix  15  comprising a polymer thermoplastic such as epoxy. The carbon fibres  14  are arranged in a plurality of generally planar layers  16   a ,  16   b . The fibres in each layer  16   a ,  16   a  are arranged such that the fibres  14  have a long axis aligned with the plane of the respective layer  16   a - d . Fibres within adjacent layers  16   a ,  16   b  are generally oriented at an angle to one another, such as 90 and 45 degrees, though other relative orientations can be provided for. In some cases, fibres within a layer may be woven, with approximately half of the fibres being oriented in a first direction parallel to the plane of the layer  16   a ,  16   b , with the remaining fibres being aligned to the plane of the layer  16   a ,  16   b , and normally to the other half of the fibres  14 . In such a case, the fibres in adjacent layers are arranged approximately 45 degrees relative to each other. Consequently, the strength of the bulk material in directions parallel to the planes of the layers  16  is improved. The carbon fibres may be in the form of carbon nanotubes such as multi-walled carbon nanotubes (MWNT). 
     The article  10  comprises an array of spaced electrical contacts  12   a - k . The electrical contacts  12  are provided in electrical contact with the electrically conductive fibres  14  of the composite article  10  within the plane of layers  16 , and are generally provided in electrical contact with a plurality of layers, as described below. 
       FIGS. 6 a  and 6 b    show the contacts  12  in more detail, and illustrate a method of bonding the contacts  12  to the fibres  14  to ensure a consistent electrical contact. In a first step, shown in  FIG. 5 a   , a hole  18  is drilled into the article  10 , in a direction generally normal to the plane of the layers  16 , and to a depth extending through several layers  16   a - d.    
     In a second step, a quantity of solder  20  is introduced into the hole  18  in a molten phase. The solder  20  is of a type that provides a consistent electrical contact with the fibres  14  of the article  10 . The calculated surface free energy of multi walled carbon nanotubes is around 20-50 mJ m −2 , meaning that only liquid metals with a low surface tension of around 100-200 mN m −1  will wet the surface of carbon nanotubes and carbon fibres. The inventors have found that a tin-based solder compound sold commercially as C-Solder™ is particularly suitable, in view of the strong mechanical bond provided between the solder  20  and the fibres  14 , and the consistent electrical contact that is provide by such a bond. This process is repeated for each contact  12 . However, it will be understood that other metallic, low temperature solders may be suitable, such as any of tin, lead, bismuth, antimony, silver, indium, zinc or copper based solder. In multi-layer carbon fibre composite materials, electrical conductivity is highest in the fibre direction (X), somewhat lower in the transverse direction (Y), and is very low in the stacking direction (Z), i.e. there is little conduction between plies. Thus to detect a fault or defect it is preferable to penetrate at least two and ideally four plies to get a good averaged conductivity over a range of plies and orientations without having to rely on poor and random connections in the Z direction. 
     Referring now to  FIG. 3 , a plurality of intra-plane electrically conductive paths  22  are defined between each spaced pair of electrical contacts  12   a - j . The electrically conductive paths  22  extend along the plane of the first layer  16   a . In view of the separation of the contacts  12   a - j  within the plane of the first layer  16   a , a plurality of the paths  22  intersect with one another at a plurality of intersection points  24 . Consequently, an array of intersection points  24  is defined. 
     Referring to  FIG. 5 , one or more further electrical contacts  12   k  may be provided, which are in electrical contact with one or more layers  16   e  spaced from the first layer  16   a . Consequently, a further inter-plane electrical path  26  is defined extending between a first contact  12   a  provided in electrical contact with electrically conductive fibres  14  of the first layer  16   a , and a further contact provided in electrical contact with electrically conductive fibres  14  of the further layer  16   e.    
     As shown in  FIG. 5 , a further electrical connection  28 is provided between the contacts  12   a ,  12   k . Consequently, an electrical circuit is defined by the contacts  12   a ,  12   k , path  26  and connection  28 .An LCR meter  30 ,(a type of electronic test equipment used to measure the inductance (L), capacitance (C), and resistance (R) of an electronic component) is provided, to determine one or more of an electrical inductance, capacitance and resistance of the electrical circuit. The LCR meter  30  is provided within the electrical circuit, in electrical contact with the connection  28 .The LCR meter  30  may comprise a power source such as a battery, transformer or variable frequency supply configured to provide both AC and DC electrical current through the electrical circuit, and sensors configured to measure each of inductance, capacitance, resistance and voltage and current at the LCR meter  30 . The LCR meter  30  can be used to provide a range of AC frequencies to determine the mutual coupling and harmonic behaviour of the inductance and capacitance of the electrical paths  34 ,  36 . Harmonics will provide a unique electrical identification for the condition of the article. 
     At some frequency, the circuit achieves the “self-resonant frequency” or SRF, where the capacitance (C) forms a parallel resonance with the inductance (L) and the inductor becomes a tuned circuit. At frequencies below the Self-Resonate Frequency, the model appears to be inductive; at frequencies above the SRF it appears to be capacitive and at the SRF it is resistive, as the inductive and capacitive reactance are equal in magnitude but opposite in sign and thus cancel each other out. 
     At the SRF of an inductor, all of the following conditions are met: 
     the input impedance is at its peak; 
     the phase angle of the input impedance is zero, crossing from positive (inductive) to negative (capacitive); 
     the effective inductance is zero, since the negative capacitive reactance (X C =1/2πfC) cancels the positive inductive reactance (X L =1/2πfL), where X L  is positive inductive reactance (ohms), f is frequency (Hertz), L is inductance (Henrys), X C  is capacitive reactance (ohms), and C is capacitance (Farads). 
     The following equation shows how the SRF is related to inductance and capacitance in the inductor model circuit. 
     
       
         
           
             
               F 
               SR 
             
             = 
             
               1 
               
                 ( 
                 
                   2 
                    
                   
                       
                   
                    
                   π 
                    
                   
                     
                       ( 
                       
                         LC 
                         p 
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     where: 
     F SR  is the Self-Resonant Frequency (in Hz); 
     L is the inductance in Henrys; and 
     C p  is the parasitic capacitance in Farads. 
     From this equation, it is clear that increasing inductance or capacitance lowers the measured SRF. Reducing inductance or capacitance raises the SRF. Since the capacitance depends on the spatial separation between conductors, if the structure is damaged, the capacitance and hence the SRF will also change. If fibres are damaged or broken, then the inductance changes and hence the SRF will also change. Both of these alter the electrical profile of the material indicating damage. 
     Further electrical connections (not shown) are provided between each pair of electrical contacts  12   a - j . It will be understood that the electrical contacts do not have to be provided simultaneously, and can be made for the purpose of determining the electrical profile as described below, then broken for determining further electrical profiles. Again, in each case, the LCR meter  30  is provided in electrical series with the circuit defined by the electrical contact pairs, electrical paths  22 , and electrical connection, and can be used to determine one or more of a resistance, inductance and capacitance in the respective electrical circuit. 
     Referring now to  FIGS. 4, 5 and 7 , a method of determining a defect in the article  10  is as follows. 
     In a first step, a nominal electrical profile of the undamaged article  10  is made. This includes electrical resistance (DC) and inductance and capacitance (AC) and resonances in the inductance and capacitance (Variable frequency AC). The nominal electrical profile comprises an array of measurements of the electrical properties of each electrical path  22 ,  26  of the undamaged article. The LCR meter  30  is electrically coupled to each circuit via the respective connection  28 , to thereby determine the electrical properties of each path  22 ,  26  between each contact pair  12 . 
     In a second step, after a period of time in service, a subsequent electrical profile is determined by repeating the above first step, and recording the measured values. 
     In a third step, the electrical profiles of the nominal (start of life) and service-run components are compared. If an element in the array differs, i.e. if the electrical properties along a path  22 ,  26  of the subsequent profile is different to the electrical properties along a path  22 ,  26  of the nominal profile, then a defect is determined to be present. 
     For example, a change of resistance from the nominal to the measured profile indicates a change in the bulk material properties such as one or more broken fibres  14 , and can be measured with the application of a DC voltage. On the other hand, applying an AC voltage along the paths  22 ,  26  allows measurements of inductance and capacitance to be made. Applying a variable frequency of AC voltage to the article allows resonances (points in the AC frequency spectrum where the inductance and capacitance cancel each other out) to be identified. These resonances are fundamental properties of the article and will alter if the article is damaged or degraded. Since the capacitance depends on the spatial separation between conductors if the structure is damaged, the capacitance and hence the SRF will also change. If fibres are damaged or broken, then the inductance changes hence the SRF will also change. Both of these alter the electrical profile of the material indicating damage. 
       FIG. 4  illustrates how the position of a defect can be determined using the method of the present disclosure. In  FIG. 4 a   , a defect  32  is located at the position shown. By performing the above first through third steps, it is determined that the subsequent electrical profiles of the paths  22  shown as dotted lines differ to the nominal profile, while the subsequent electrical profiles of the paths  22  shown as solid lines have the same electrical property values (i.e. each of resistance, inductance and capacitance) as the nominal resistance profile. Consequently, it will be understood that the defect  32  can be determined to lie within the shaded region by knowledge of the positions of the contacts  12 , and the subsequent resistance profile. 
     Similarly,  FIG. 5  illustrates how a defect may be determined to lie underneath the surface layer  16   a . Since the defect lies within the path  26 , the electrical properties of a subsequent resistance profile in this case will differ from that of the nominal profile, and so a defect can be deduced to be present along this line. The spatial resolution of the method can be increased by introducing further contacts  12 . 
     It will be understood that the disclosure is not limited to the embodiment above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.