Patent Publication Number: US-2021190729-A1

Title: Non-destructive testing using phased arrays

Description:
FIELD 
     The present invention relates to non-destructive testing, particularly phased array ultrasonic scanning. 
     BACKGROUND TO THE INVENTION 
     Non-destructive testing (NDT), also known as non-destructive examination (NDE), non-destructive inspection (NDI) and non-destructive evaluation (NDE), is the development and application of technical methods (also known as techniques) to examine materials and/or articles, for example components, in ways that do not impair future usefulness and serviceability in order to detect, locate, measure, and evaluate flaws; to assess integrity, properties, and composition; and to measure geometrical characters. Typical NDT techniques include eddy-current, magnetic-particle, liquid penetrant, radiographic, ultrasonic, and visual testing. NDT is typically used to determine indications, discontinuities, flaws and/or defects in the materials and/or the articles. 
     NDT is typically applied in industries where failure of the materials and/or the articles would cause significant hazard or economic loss, such as in transportation including aerospace, pressure vessels, building structures, piping, and hoisting equipment. In use, the materials and/or the articles are typically subject to loads that may result in at least initiation of failure, for example crack formation and/or growth thereof. Fatigue may occur due to repeated (typically cyclic) loading and unloading. If the loading is above a threshold, microscopic cracks begin to form at stress concentrators such as the surface, persistent slip bands (PSBs), interfaces of constituents in the case of composites, and grain interfaces in the case of metals. If a crack reaches a critical size, the crack will propagate suddenly, resulting in failure. A fatigue life, N f , of a specimen (for example, of a material) may be defined as a number of stress cycles of a specified character that the specimen sustains before failure of a specified nature occurs. 
     A shape of an article, for example a component, may adversely affect a fatigue life thereof, by locally increasing stresses (i.e. the loading), for example above the threshold. Additionally and/or alternatively, use of a mechanical fastener in the article (i.e. a fastened article) may locally increase stresses in the article, similarly adversely affecting a fatigue life thereof. If the article is a component in an assembly, for example an aerospace component mechanically fastened in an aircraft, NDT of the component in situ (i.e. in the original place, as assembled) is difficult. For example, physical access to the component may be restricted or the component may be inaccessible, thereby limiting NDT thereof. Typically, disassembly of the assembly may be required in order to perform NDT of the component, thereby increasing complexity, duration and/or cost of the NDT, with subsequent reassembly and optionally, requalification of the assembly also required. However, disassembly may not always be possible in practice, due to operational constraints, for example. Furthermore, mechanical fasteners used in some assemblies are not reusable (i.e. single use), thus increasing cost associated with new mechanical fasteners. 
     Hence, there is a need to improve NDT of articles, for example components included in assemblies. 
     SUMMARY OF THE INVENTION 
     It is one aim of the present invention, amongst others, to provide a method of non-destructive testing which at least partially obviates or mitigates at least some of the disadvantages of the prior art, whether identified herein or elsewhere. For instance, it is an aim of embodiments of the invention to provide a method of non-destructive testing that provides detection of flaws associated with passageways in articles, for example for fasteners. For instance, it is an aim of embodiments of the invention to provide a method of non-destructive testing that provides detection of flaws associated with passageways in articles in situ. 
     A first aspect provides a method of non-destructive testing of an article having a first surface, wherein the article comprises a set of passageways, including a first passageway, and wherein respective sets of flaws are associated with respective passageways of the set of passageways, including a first set of flaws, optionally including a first flaw, associated with the first passageway, wherein the method comprises: 
     phased array ultrasonic scanning of the article using a phased array probe communicatively coupled thereto through the first surface; and 
     detecting the first flaw, if included in the first set of flaws associated with the first passageway. 
     A second aspect provides use of phased array ultrasonic scanning, preferably in situ phased array ultrasonic scanning, for non-destructive testing of an aerospace component. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention there is provided a method of non-destructive testing, as set forth in the appended claims. Also provided is use of phased array ultrasonic scanning for non-destructive testing of an aerospace component. Other features of the invention will be apparent from the dependent claims, and the description that follows. 
     Method 
     A first aspect provides a method of non-destructive testing of an article having a first surface, wherein the article comprises a set of passageways, including a first passageway, and wherein respective sets of flaws are associated with respective passageways of the set of passageways, including a first set of flaws, optionally including a first flaw, associated with the first passageway, wherein the method comprises: 
     phased array ultrasonic scanning of the article using a phased array probe communicatively coupled thereto through the first surface; and 
     detecting the first flaw, if included in the first set of flaws associated with the first passageway. 
     In this way, NDT of the article is facilitated, enabling NDT thereof even if physical access to the article, particularly the first passageway, is restricted. In this way, if the article is a component in an assembly, for example an aerospace component mechanically fastened in an aircraft, NDT of the article in situ (i.e. in the original place, as assembled) is facilitated such that disassembly of the assembly may not be required in order to perform NDT of the article, thereby decreasing complexity, duration and/or cost of the NDT. Furthermore, if disassembly is not required, new mechanical fasteners (i.e. to replace removed mechanical fasteners) are also not required. 
     Terminology 
     It should be understood that terms used herein are generally according to ASTM E-1316-18a: 
     defect: one or more flaws whose aggregate size, shape, orientation, location, or properties do not meet specified acceptance criteria and are rejectable; 
     discontinuity: a lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component; 
     evaluation: a review, following interpretation of the indications noted, to determine whether the indications meet specified acceptance criteria; 
     false indication: an indication that is interpreted to be caused by a discontinuity at a location where no discontinuity exists; 
     flaw: an imperfection or discontinuity that may be detectable by NDT and is not necessarily rejectable; 
     flaw characterization: a process of quantifying a size, a shape, an orientation, a location, a growth, and/or other properties, of a flaw based on an NDT response; 
     imperfection: a departure of a quality characteristic from an intended condition; 
     indication: evidence of a discontinuity that requires interpretation to determine a significance thereof; 
     instrument calibration: comparison of an instrument with, or the adjustment of an instrument to, a known reference(s), which may be traceable to the National Institute of Standards and Technology (NIST), for example; 
     instrument standardization: adjustment of an instrument, prior to use, to an arbitrary reference value; 
     interpretation: determination of whether an indication is relevant or non-relevant; 
     non-relevant indication: an indication that is caused by a condition or type of discontinuity that is not rejectable; false indications are non-relevant; 
     relevant indication: an indication that is caused by a condition or type of discontinuity that requires evaluation; 
     Fatigue, particularly in aerospace components 
     As outlined above, fatigue of components included in assemblies, particularly aerospace components included in aircraft, is particularly problematic. Fatigue is typically characterised by four phases: 
     Phase 1: Early fatigue damage (no visible cracks); 
     Phase 2: Crack initiation; 
     Phase 3: Crack growth; 
     Phase 4: Fracture. 
     It is desirable to identify onset of fatigue at the earliest possible phase, preferably at Phase 2 or Phase 1. Conventional NDT techniques, however, are typically only able to identify fatigue at Phase 3. It should be understood that cracks are flaws and some of the cracks may be defects, according to their aggregate size, shape, orientation, location, and/or properties, for examples. 
     Fasteners, Particularly for Aerospace Components 
     When a conventional mechanical fastener, for example a threaded fastener such as a bolt, is installed in an article through a passageway (i.e. a fastener hole) therein, tightening thereof results in an axial tensile strain in a shank of the mechanical fastener and a corresponding reduction in cross-sectional area of the shank. This reduction in cross-sectional area means that the shank, in use, is not radially in contact with a wall of the passageway, resulting in a loose fit. This loose fit is undesirable. 
     Special fasteners have been developed to eliminate this loose fit, by modifying stress and/or strain distributions in the fasteners and/or fastened article. Typically, such a special fastener includes a collar that is compressed into position. Such special fasteners include pin rivets, Taperlock fasteners, Tigue fasteners and turnlock fasteners. Taperlock fasteners are preferred. 
     Pin (also known as hi-shear) rivets are non-blind rivets with access to both sides required for installation. Pin rivets have the same shear strength as bolts of equal diameters, though at about only 40% weight, are faster to install. Pin rivets are may be considered as threadless bolts: the pin is headed at one end and is grooved about a circumference at the other end, around which a metal collar is swaged, effecting a firm, tight fit. 
     Taperlock fasteners are the strongest special fasteners used in aircraft construction. A taperlock fastener exerts a force on the tapered walls of the fastener hole (i.e. passageway) because of its tapered shape. The taperlock fastener is designed to completely fill the hole, but unlike a rivet, the tapered shank fills the tapered hole without deforming the shank. Instead, the washer head nut compresses the shank against the tapered walls of the hole. This creates radial compression in the article around the shank and axial compression in the article as the fastener is fastened. The combination of these stresses provides superior strength compared with other mechanical fasteners and may improve fatigue life. Hence, taperlock fasteners are suitable for high interference fit applications in high loaded structure junctions, particularly for restricted locations, where conventional fasteners do not fit in the space available. 
     A Hi-Tigue fastener has a bead that encircles the base of its shank. The bead preloads the fastener hole in which this fastener is received, resulting in increased joint strength. At installation, the bead presses against the sidewall of the hole, exerting radial force that strengthens the surrounding area. Because it is preloaded, the joint is not subjected to the constant cyclic action that normally causes a joint to become cold worked and eventually fail. 
     Turnlock fasteners (also known as quick opening, quick action, and stressed panel fasteners) are used to secure inspection plates, doors, and other removable panels on aircraft. 
     A Dzus turnlock fastener comprises a stud, grommet, and receptacle. A quarter of a turn of the stud (clockwise) locks the fastener. The fastener may be unlocked only by turning the stud counterclockwise. A Dzus key or a specially ground screwdriver locks or unlocks the fastener. 
     Passageways, Particularly Fastener Holes for Fasteners 
     The ability to detect flaws associated with passageways, for example fastener holes, without disassembly, for example without removal of fasteners from the fastener holes, presents significant challenges to conventional NDT techniques. Aircraft wing structures, for example wing diffusion joints, are complex, multi-layered assemblies of components, coupled using mechanical fasteners. For example, fastener hole inspections, by eddy current inspection, of wing diffusion joints in Tornado aircraft requires disassembly thereof, including removal and replacement of 81 Taperlock fasteners, as described below, per wing diffusion joint, at an estimated cost of £500,000 per aircraft. Increasing the longevity of these aircraft and/or allowing operational use thereof beyond original design limits necessitates regular and/or frequent inspection, to reduce risk. Flaws associated with passageways, for example fastener holes, arising from fatigue, are typically cracks that initiate at and/or proximal a wall of the passageway and grow away therefrom, into the article. NDT of such cracks, particularly Phase 2 cracks, may be further complicated by the discontinuities presented by the wall of the passageway, for example of the fastener hole, and/or the fastener received therein. 
     In one example, a longitudinal axis of the first passageway is transverse, preferably orthogonal, to the first surface and optionally, the first passageway intersects with the first surface. In one example, the first passageway is a blind hole (i.e. extending from one surface, for example the first surface, partly through the article). In one example, the first passageway is a through hole (i.e. extending completely through the article, from the first surface to an opposed, second surface, for example). 
     In one example, the set of passageways is arranged as an array. In one example, the array is a regular array. In one example, the array is an irregular array. 
     In one example, a cross-sectional dimension D, preferably a diameter, of the first passageway is in a range from 1 mm to 100 mm, preferably in a range from 5 mm to 50 mm, more preferably in a range from 8 mm to 20 mm. In one example, a cross-sectional shape of the first passageway is substantially circular or circular. 
     In one example, a spacing between the first passageway and an adjacent passageway of the set of passageways is in a range from 0.5D to 10D, preferably 1D to 7D, more preferably 2D to 5D. In one example, a centre spacing between a centre of the first passageway and a centre of an adjacent passageway of the set of passageways is in a range from 1.5D to 11D, preferably 2D to 8D, more preferably 3D to 6D. That is, the passageways are relatively closely spaced, such that physical access to first passageway, is restricted. For example, an area of the first surface between the passageways may be relatively limited, such that scanning using a conventional single element probe, for example and as described below, would not sweep a complete volume of the article. 
     In one example, the first passageway comprises a cylindrical passageway and/or a frustoconical passageway (i.e. a tapered passageway, for example arranged to receive a Taperlock fastener). In one example, the first passageway comprises a threaded first passageway (i.e. having a thread formed, for example machined or cut, in a wall thereof). In one example, the first passageway comprises a spline and/or a keyway, formed, for example machined or cut, in a wall thereof. In one example, the first passageway comprises a smooth wall (i.e. surface or inner surface), having no convexities and/or concavities provided therein. It should be understood that flaws and/or defects, for example cracks, are not provided in the wall of the first passageway, for example during manufacture, but may be initiated at and/or proximal the wall of the first passageway in use, for example due to fatigue. 
     In one example, the first passageway is arranged to receive a mechanical fastener and optionally, the article comprises the mechanical fastener received therein. In one example, the first passageway comprises a frustoconical passageway (i.e. a tapered passageway) arranged to receive a Taperlock fastener therein and optionally, the component comprises the Taperlock fastener received therein. 
     In one example, the set of passageways includes N passageways, wherein N is a natural number in a range from 2 to 2,000, preferably in a range from 10 to 1,000, more preferably in a range from 20 to 200, for example 50, 100, 150. In one example, each passageway of the set of passageways is as described with respect to the first passageway. 
     Phased Array Ultrasonic Scanning 
     Conventional single-element probes, also known as monolithic probes, for ultrasonic scanning, emit beams in fixed directions. It should be understood that an element is typically an ultrasonic transducer. The element may comprise a single active element that both generates and receives high frequency sound waves, or two paired elements: one for transmitting and one for receiving. To test a volume of material, a conventional single-element probe must be physically moved, for example by translating or turning, to sweep the fixed beam through the volume. 
     In contrast, a phased array probe for phased array ultrasonic scanning comprises a plurality of elements, which are individually pulsed in a coordinated manner. In this way, a beam emitted by the phase array probe may be focused and swept electronically without moving the phase array probe. By varying a timing of pulsing of each element, for instance by making a pulse from each successive element progressively delayed by a progressive time delay, a pattern of constructive interference is set up that results in radiating a quasi-plane ultrasonic beam at a set angle depending on the progressive time delay. In contrast to conventional single-element probes, phased array probes may be used to hence electronically sweep a sound beam through ranges of refracted angles, along linear paths, and/or dynamically focus at different depths, thus increasing flexibility and capability of testing. In this way, by changing the progressive time delay, the beam may be steered and hence swept through the volume of the material, without moving the phase array probe. Response data from multiple beams may be combined to provide an image of a slice through the material. 
     Phased array probes may include typically from 16 to 256 elements, arranged in a strip (linear array), a ring (annular array), a circular matrix (circular array), or a more complex shape. As with conventional single-element probes, phased array probes may be designed for direct contact use, as part of an angle beam assembly with a wedge, or for immersion use with sound coupling through a water path. Transducer frequencies are typically in a range from 2 MHz to 10 MHz (i.e. radio frequency, RF). 
     Typically, response data are based on time and amplitude information derived from processed RF waveforms. These waveforms and the information extracted therefrom may be typically presented in one or more of four formats: A-scans, B-scans, C-scans, or S-scans. 
     An A-scan is a simple RF waveform presentation showing time and amplitude of an ultrasonic signal, as commonly used with conventional single-element probes. A B-scan is an image showing a cross-sectional profile through one vertical slice of the test piece, showing the depth of reflectors with respect to their linear position. B-scan imaging requires sweeping of the beam, either by moving a conventional single-element probe in a direction or using a phase array probe, as described above, without movement thereof. A C-scan is a two dimensional presentation of response data displayed as a plan view, in which colour may represent a gated signal amplitude (i.e. response) at each (x, y) coordinate. C-scan imaging requires sweeping of the beam, either by moving a conventional single-element probe in two rastering directions or using a phase array probe, as described above, with movement in only one direction. An S-scan or sectorial scan image represents a two-dimensional cross-sectional view derived from a series of A-scans that have been plotted with respect to time delay and refracted angle. S-scan imaging requires sweeping of the beam through a series of angles to generate an approximately cone-shaped cross-sectional image. 
     The inventors have determined that the benefits of phased array ultrasonic scanning, over conventional single-element probe ultrasonic scanning, are particularly advantageous for NDT of flaws associated with passageways in articles, especially for in situ NDT. The benefits of phased array ultrasonic scanning result, at least in part, from use of multiple elements to steer, focus and scan beams. Beam steering (also known as sectorial scanning) may be used for mapping components at appropriate angles, thereby simplifying NDT of components having complex geometries. A small footprint (i.e. physical size) of the phased array ultrasonic probe and the ability to sweep the beam without moving the phased array ultrasonic probe also aids inspection of such components in situations where there is limited access for mechanical scanning. The ability to perform ultrasonic scanning at multiple angles significantly increases the probability of detection of anomalies (i.e. flaws). Particularly, a larger volume may be swept and/or a volume may be swept more completely compared with conventional scanning at a single angle. Electronic focusing permits optimizing the beam shape and size at the flaw location, thus further optimizing probability of detection. The ability to focus at multiple depths also improves the ability for sizing critical flaws and/or defects for volumetric inspections. Focusing can significantly improve signal-to-noise ratio in challenging applications, and electronic sweeping allows for C-Scan images to be produced very rapidly. 
     Translating Phased Array Probe in a Set of Directions 
     In one example, the phased array ultrasonic scanning of the first surface using the phased array probe comprises translating the phased array probe in a set of directions, including a first direction, across the first surface and optionally, wherein the set of directions includes a second direction orthogonal to the first direction. In this way, a volume of the article swept by the scanning is increased, thereby increasing probability of detecting flaws. By translating in the second direction, flaws may be scanned from orthogonal directions, thereby increasing probability of their detection, since a maximum response due to a flaw may be due, at least in part, to an orientation of the flaw. 
     In one example, the set of directions includes a third direction at angularly displaced from the first direction, for example by π/4 radians, and optionally, a fourth direction orthogonal to the third direction. In this way, a probability of detecting the flaws is increased, since a maximum response due to a flaw may be due, at least in part, to an orientation of the flaw. Hence, by increasing the number of directions of scanning, a probability of detecting the flaws is increased. 
     In one example, the phased array ultrasonic scanning of the first surface using the phased array probe comprises scanning around a periphery, for example a circumference i.e. through 2π radians. In one example, the phased array ultrasonic scanning of the first surface using the phased array probe comprises sweeping a volume of the article surrounding the first passageway. 
     In one example wherein the passageway defines a circular rim at the surface and the first direction in which the phased array probe is translated is parallel with a tangent to the rim. Particularly, the probe may moves from a first position illuminating one side of the passageway, to a second position illuminating the other side. 
     Standardization and Calibration 
     In one example, the method comprises standardizing and/or calibrating an instrument for the phased array scanning using the phased array probe. Suitable instruments include the Olympus Ominscan SX. 
     In one example, the method comprises standardizing and/or calibrating an instrument for the phased array scanning using the phased array probe using a reference article. In one example, the reference article comprises and/or is a replica (i.e. a copy) of at least a part of the article, having a first reference surface, wherein the reference article comprises a set of reference passageways, including a first reference passageway, and wherein respective sets of reference flaws are associated with respective reference passageways of the set of reference passageways, including a first set of reference flaws, including a first reference flaw, associated with the first reference passageway. That is, the reference article includes deliberately introduced flaws, for example by machining such as electrical discharge machining (EDM). In one example, the first reference flaw has a predetermined size, shape, orientation, location and/or property. In this way, standardizing and/or calibrating the instrument, based on the first set of reference flaws, may be improved, thereby increasing probability of detection of the first flaw and/or enhancing characterisation thereof. In one example, each reference passageway of the set of reference passageways is as described with respect to the first reference passageway. In one example, the reference article is otherwise as described with respect to the article. 
     Phased Array Probe 
     In one example, the phased array probe has a nominal refracted beam angle (in steel) in a range from 0° LW to 60° LW, for example 45° LW or 60° LW. 
     In one example, the phased array probe has a nominal refracted beam angle (in steel) in a range from 0° SW to 60° LW, for example 45° LW or 55° LW. 
     In one example, the phased array probe has a sweep in a range from −30° to 70°, preferably in a range from 0° to 70°, more preferably in a range from 30° to 70°. 
     In one example, the phased array probe operates at a frequency in a range from 2.5 Mhz to 25 MHz, preferably in a range from 5 MHz to 12.5 MHz. 
     In one example, the phased array probe has a footprint in a range from 2 mm×2 mm to 24 mm×24 mm, preferably in a range from 8 mm×8 mm to 16 mm×16 mm. That is, the phased array probe may have a relatively small footprint. In this, way, the phased array probe may be moved between closely-spaced passageways, for example. 
     In one example, the phased array probe has a pitch and/or an elevation in a range from 2 mm×2 mm to 24 mm×24 mm respectively, preferably in a range from 8 mm×8 mm to 16 mm×16 mm respectively. That is, the phased array probe may have a relatively small pitch and/or an elevation. In this, way, the phased array probe may be moved between closely-spaced passageways, for example. 
     In one example, the phased array probe comprises a wedge. In one example, the wedge has an angle in a range from 15° to 60°, preferably in a range from 30° to 45°. 
     In one example, the method comprises operating the phased array probe according to one, preferably more than one, more preferably all operating parameters and corresponding values tabulated in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Phased array probe operating parameters and values. 
               
            
           
           
               
               
               
            
               
                 Operating 
                   
                   
               
               
                 parameter 
                 Values 
                 Preferred values 
               
               
                   
               
               
                 Beam delay 
                 0 μs to 20 μs 
                 2.5 μs to 10 μs 
               
               
                 Start (half path) 
                 −1.00 mm to +1.00 mm 
                 −0.50 mm to +0.50 mm 
               
               
                 Range (half path) 
                 5 mm to 100 mm 
                 25 mm to 75 mm 
               
               
                 Acquisition rate 
                 10 Hz to 1 kHz 
                 25 Hz to 100 Hz 
               
               
                 Type 
                 PA 
                 PA 
               
               
                 Averaging factor 
                 0.1 to 10 
                 0.5 to 2 
               
               
                 Scale type 
                 Compression 
                 Compression 
               
               
                 Scale factor 
                 0.1 to 100 
                 1 to 20 
               
               
                 Video filter 
                 On 
                 On 
               
               
                 Pretrigger 
                 0 μs to 20 μs 
                 0 μs to 10 μs 
               
               
                 Rectification 
                 Full wave (FW), 
                 FW 
               
               
                   
                 Half wave (HW) 
               
               
                 Filter 
                 Band pass 2.0 MHz to 
                 Band pass 4.0 MHz to 
               
               
                   
                 32 Mhz 
                 16 Mhz 
               
               
                 Voltage 
                 10 V to 200 V 
                 20 V to 100 V 
               
               
                 Gain 
                 10 dB to 100 dB 
                 20 dB to 50 dB 
               
               
                 Mode 
                 Pulse echo (PE) 
                 PE 
               
               
                 Wave type 
                 Shear 
                 Shear 
               
               
                 Sound velocity 
                 100 ms −1  to 
                 1000 ms −1  to 
               
               
                   
                 10,000 ms −1   
                 5,000 ms −1   
               
               
                 Pulse width 
                 10 ns to 200 ns 
                 50 ns to 100 ns 
               
               
                 Scan offset 
                 −1.00 mm to 1.00 mm 
                 −0.50 mm to 0.50 mm 
               
               
                 Index offset 
                 −50 mm to 50 mm 
                 −25 mm to +25 mm 
               
               
                 Probe skew 
                 45° to 135° 
                 60° to 120° 
               
               
                 C-Scan time 
                 0.1 ns to 25 ns 
                 1 ns to 10 ns 
               
               
                 resolution 
               
               
                 Digitizing 
                 10 MHz to 1 GHz 
                 50 MHz to 250 MHz 
               
               
                 frequency 
               
               
                 A-scan time 
                 10 ns to 250 ns 
                 25 ns to 100 ns 
               
               
                 resolution 
               
               
                 Gate start 
                 I: Off; A: 1 mm to 
                 I: Off; A: 5 mm to 
               
               
                   
                 25 mm; B: Off 
                 10 mm; B: Off 
               
               
                 Gate width 
                 I: Off; A: 10 mm to 
                 I: Off; A: 20 mm to 
               
               
                   
                 100 mm; B: Off 
                 50 mm; B: Off 
               
               
                 Gate threshold 
                 I: Off; A: 5% to 
                 I: Off; A: 10% to 
               
               
                   
                 75%; B: Off 
                 50%; B: Off 
               
               
                 Gate 
                 I: Off; A: Pulse; 
                 I: Off; A: Pulse; 
               
               
                 synchronization 
                 B: Off 
                 B: Off 
               
               
                 Gate peak 
                 I: Off; A: Max Peak; 
                 I: Off; A: Max Peak; 
               
               
                 selection 
                 B: Off 
                 B: Off 
               
               
                   
               
            
           
         
       
     
     Flaw 
     In one example, the first flaw has a maximum dimension d in a range from 0.5 mm to 7 mm, preferably in a range from 1 mm to 5 mm, more preferably in a range from 1.5 mm to 3 mm. In this way, cracks may be detected at the earliest possible phase. 
     In one example, the method comprises characterizing the first flaw. In one example, the characterizing the first flaw comprises quantifying a size, a shape, an orientation, a location, a growth, and/or a property, of the first flaw based, at least in part, on the phased array ultrasonic scanning response. 
     In one example, the method comprises determining that the first flaw is and/or comprises a first defect. In one example, the determining comprises comparing a size, a shape, an orientation, a location, a growth and/or a property of the first flaw with respective predetermined acceptance criteria. 
     Article 
     In one example, the article has a second surface, opposed to the first surface, and wherein the phased array ultrasonic scanning of the article using the phased array probe communicatively coupled thereto is only through the first surface. That is, physical access to the article, particularly the first passageway, may be restricted. 
     In one example, the article has a thickness, measured normal to the first surface, in a range from 1 mm to 200 mm, preferably in a range from 5 mm to 100 mm, more preferably in a range from 10 mm to 50 mm. In one example, the article comprises a multi-layered article, for example comprising a plurality of layers. Generally, interfaces between the layers are presented as discontinuities during NDT, thereby complicating the NDT of the first passageway, for example. 
     In one example, the method comprises stressing the article during the phased array ultrasonic scanning thereof. In one example, the stressing of the article is due, at least in part, to a mechanical fastener, for example a Taperlock fastener, received and fastened in the first passageway. In one example the stressing of the article comprises compressive stressing, for example proximal the first passageway and/or the first surface. 
     In one example, the article is a component, preferably an aerospace component, included in an assembly, preferably an aircraft, and wherein the phased array ultrasonic scanning of the article comprises in situ phased array ultrasonic scanning of the article included in the assembly. In this way, NDT of the component in situ (i.e. in the original place, as assembled) is facilitated such that disassembly of the assembly may not be required in order to perform NDT of the article, thereby decreasing complexity, duration and/or cost of the NDT. Furthermore, if disassembly is not required, new mechanical fasteners to replace removed mechanical fasteners are also not required. In one example, the aircraft is a fixed wing aircraft, a swept wing aircraft or a rotary wing aircraft. In one example, the article is a wing component, for example a wing diffusion joint or a lower wing diffusion joint. 
     Aerospace Materials 
     Aerospace materials are materials, typically metal alloys, developed for, or have come to prominence through, aerospace applications. 
     Aerospace Alloys 
     Typically, aerospace alloys include Al-, Mg-, Ni-, Co- and Ti-based alloys. 
     In Al-based alloys, Al is the predominate metal in the alloy along with alloying elements such as Cu, Zn, Mn, Si and Mg. Al-based alloys may be classified as cast and wrought alloys, both of which are subdivided into heat-treatable and non-heat-treatable categories. More than 80% of Al alloys are wrought alloys, provided in the form of rolled sheets and foils. Al-based alloys typically used in aerospace applications include 7075, 6061, 6063, 2024 and 5052 and/or L93 2014, L95 7075, L97 2024 or L115. In one example, the article comprises and/or is formed from L97 2024, for example in a T351 condition (equivalent to T4). In one example, the article and the reference article are formed of the same material, for example L97 2024 for example in a T351 condition, 
     Use 
     A second aspect provides use of phased array ultrasonic scanning, preferably in situ phased array ultrasonic scanning, for non-destructive testing of an aerospace component. 
     The phased array ultrasonic scanning, the non-destructive testing and/or the aerospace component may be as described with respect to the first aspect. 
     Definitions 
     Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of” or “consists essentially of” means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention, such as colourants, and the like. 
     The term “consisting of” or “consists of” means including the components specified but excluding other components. 
     Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to include the meaning “consists essentially of” or “consisting essentially of”, and also may also be taken to include the meaning “consists of” or “consisting of”. 
     The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein are also applicable to all other aspects or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or exemplary embodiment of the invention as interchangeable and combinable between different aspects and exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which: 
         FIG. 1  schematically depicts a method of non-destructive testing of an article according to an exemplary embodiment; 
         FIG. 2  shows a photograph of a method of non-destructive testing of an article according to an exemplary embodiment; 
         FIG. 3  shows a photograph of the method, in more detail; 
         FIG. 4A  schematically depicts a cross-sectional view of a conventional fastener in an article, in use;  FIG. 4B  schematically depicts a cross-sectional view of a taperlock fastener, in use; 
       and  FIG. 4C  schematically depicts a cross-sectional view of the first passageway in the article of  FIG. 2  for a countersunk taperlock fastener; 
         FIG. 5  schematically depicts a phased array ultrasonic scanning signal response from a base of the first passageway in the article of  FIG. 2 , tested according to an exemplary embodiment; 
         FIG. 6  schematically depicts a phased array ultrasonic scanning signal response from a first flaw associated with the first passageway of  FIG. 5 ; 
         FIG. 7  schematically depicts a plan view of the article of  FIG. 2 , comprising a first passageway, tested according to an exemplary embodiment, in more detail; 
         FIG. 8  schematically depicts a phased array ultrasonic scanning signal response from a second flaw associated with a second passageway in the article of  FIG. 2 , tested according to an exemplary embodiment; 
         FIG. 9  schematically depicts a phased array ultrasonic scanning signal response from the second flaw of  FIG. 8 , in more detail; 
         FIG. 10  schematically depicts an aircraft; 
         FIG. 11  schematically depicts an aerospace component of the aircraft of  FIG. 10 , in situ; 
         FIG. 12A  schematically depicts a plan view of an upper surface of the aerospace component of  FIG. 11 ; and  FIG. 12B  schematically depicts a plan view of a lower surface of the aerospace component of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically depicts a method of non-destructive testing of an article according to an exemplary embodiment. 
     The article, having a first surface, comprises a set of passageways, including a first passageway. Respective sets of flaws are associated with respective passageways of the set of passageways, including a first set of flaws. The first set of flaws optionally includes a first flaw, associated with the first passageway. 
     At S 101 , the article is phased array ultrasonic scanned using a phased array probe communicatively coupled thereto through the first surface. 
     At S 102 , the first flaw, if included in the first set of flaws associated with the first passageway, is detected. 
       FIG. 2  shows a photograph of a method of non-destructive testing of an article  10  according to an exemplary embodiment and  FIG. 3  shows a photograph of the method, in more detail. 
     The article  10 , having a first surface  11 , comprises a set of passageways  100 , including a first passageway  100 A. Respective sets of flaws  1000  (not shown) are associated with respective passageways of the set of passageways  100 , including a first set of flaws  1000 A (not shown). The first set of flaws  1000 A optionally includes a first flaw  1000 AA, associated with the first passageway  100 A. 
     In this example, the article  10  is a reference article, comprising a replica (i.e. a copy) of at least a part of an article, particularly a wing diffusion joint  20 , as described below in more detail. The reference article is formed from L97 2024 in a T351 condition, particularly plate thereof having a thickness of 20 mm. The reference article  10  has the first reference surface  11 , wherein the reference article  10  comprises the set of reference passageways  100 , including the first reference passageway  100 A, and wherein respective sets of reference flaws  1000  are associated with respective reference passageways of the set of reference passageways  1000 , including a first set of reference flaws  1000 A, including a first reference flaw  1000 AA, associated with the first reference passageway  100 A. That is, the reference article  10  includes deliberately introduced flaws, for example by machining such as electrical discharge machining (EDM). In this example, the first reference flaw  1000 AA has a predetermined size, shape, orientation, location and/or property. In this way, calibrating the instrument, based on the first set of reference flaws  1000 A, may be improved, thereby increasing probability of detection of the first flaw  1000 AA and/or enhancing characterisation thereof. In this example, the article is otherwise as described with respect to the reference article  10 . 
     In this example, a longitudinal axis of the first passageway  100 A is orthogonal to the first surface  11  and the first passageway  100 A intersects with the first surface  11 . In this example, the first passageway  100 A is a through hole (i.e. extending completely through the article  10 , from the first surface  11  to an opposed, second surface, for example). 
     In this example, the set of passageways  100  is arranged as an irregular array. That is, the set of passageways  100  are not regularly arranged such that a spacing between adjacent passageways of the set of passageways  100  varies. 
     In this example, a diameter D of the first passageway  100 A is in a range from 11 mm to 14 mm. In this example, a cross-sectional shape of the first passageway  100 A is circular. 
     In this example, a spacing between the first passageway  100 A and an adjacent passageway of the set of passageways  100  is in a range from 2D to 5D. In this example, a centre spacing between a centre of the first passageway  100 A and a centre of an adjacent passageway of the set of passageways  100  is in a range from 3D to 6D. That is, the passageways  100  are relatively closely spaced, such that physical access to first passageway  100 A, is restricted. 
     In this example, the first passageway  100 A comprises a frustoconical passageway (i.e. a tapered passageway) arranged to receive a countersunk taperlock fastener  50 A therein and optionally, the article  10  comprises the taperlock fastener  50 A received therein. 
     In this example, the set of passageways  100  includes N passageways  100 A to  100 T, where N is 20. The passageways  100 A to  100 P are shown and the passageways  100 Q to  100 T are not shown (obscured). In this example, the passageways  100 A to  100 D comprise countersunk taperlock fasteners  50 A to  50 D, respectively, received therein. In this example, each passageway of the set of passageways  100  is generally otherwise as described with respect to the first passageway  100 A. 
     Particularly, the method is performed using an Olympus Ominscan SX (reference sign I), a compatible phased array probe (reference sign P) and a corresponding wedge (reference sign W) for the phased array probe. 
     Prior to carrying out NDT of an article, as described below, the reference article  10  including simulated cracks (EDM notches) from bores is used to check the set-up sensitivity of the instrument I and act as a good to go check. 
     Generally, the first surface  11  is cleaned, degreased and ensured free from corrosion. Preferably, the first surface  11  is as smooth as possible to prevent acoustic signal deformation and reduce resultant measurement error. The geometry of the scan surface  11  should be as parallel as possible to the surface of the probe shoe, to enable an optimal acoustic reflection. This will prevent difficulties in obtaining a return signal from the reflector surface. Any sharp dents, gouges, nicks, scratches or machine marking must be dressed as far as practicable to allow the ultrasonic inspection to be conducted on an acceptable surface. 
     A coupling medium, for example Sound Clear Soundclear® 60, available from Magnaflux, or any other couplant approved by the UT Level  3 , is applied to the first surface  11 . Response is optimised for the first flaw  1000 AA. The coupling medium is removed from the first surface following NDT. 
       FIG. 4A  schematically depicts a cross-sectional view of a conventional fastener B in an article  10 , in use;  FIG. 4B  schematically depicts a cross-sectional view of a taperlock fastener  50  in an article  10 , in use; and  FIG. 4C  schematically depicts a cross-sectional view of a first passageway  100 A in an article  10  for a countersunk taperlock fastener  50 A. 
     The conventional fastener B, for example a bolt, exerts forces on the opposed first surface  11  and second surface  12  of the article  10 , with stresses raised therein proximal the opposed surfaces  11 ,  12  around the cylindrical walls of the fastener hole H. 
     In contrast, as described above, the taperlock fastener  50  exerts a force on the tapered walls of the fastener hole (i.e. the first passageway  100 A) because of its tapered shape. The taperlock fastener  50  is designed to completely fill the first passageway  100 A, but unlike a rivet or the bolt B for example, the tapered shank fills the tapered first passageway  100 A without deforming the shank. Instead, the washer head nut compresses the shank against the tapered walls of the first passageway  100 A. This creates radial compression in the article  10  around the shank and axial compression in the article  10  as the taperlock fastener  50  is fastened. However, while stresses are not raised as with conventional mechanical fasteners, fatigue, corrosion and/or stress-corrosion cracking may be problematic following extended operation and/or adverse operating conditions. Hence, NDT of such articles is required. 
       FIG. 5  schematically depicts a phased array ultrasonic scanning signal response from a base of the first passageway  100 A in the article  10 , tested according to an exemplary embodiment. 
     Particularly,  FIG. 5  shows: (A) an A-scan; (B) an S-scan of the response due to the base of the first passageway  100 A in the article  10 . 
       FIG. 6  schematically depicts a phased array ultrasonic scanning signal response from a first flaw  1000 AA associated with the first passageway  100 A of  FIG. 5 . 
     Particularly,  FIG. 6  shows: (A) an A-scan; (B) an S-scan of the response due to the first flaw  1000 AA associated with the first passageway  100 A. The first flaw  1000 AA is a notch, having dimensions 3 mm×3 mm, deliberately introduced by EDM. 
       FIG. 7  schematically depicts a plan view of the article  10 , comprising the first passageway  100 A and its associated circular rim, tested according to an exemplary embodiment, in more detail. The phased array probe P is shown in 4 positions, 1 to 4, wherein the 4 positions are mutually angularly spaced apart and equispaced from the passageway  100 A. 
     The phased array probe P is translated in respective directions parallel to respective tangents to the passageway  100 A (i.e. tangents to the circular rim). Over the course of a translation the probe moves from a first position illuminating one side of the passageway  100 A to a second position illuminating the other side, thereby sequentially scanning the entire passageway  100 A. 
     In this example, the phased array ultrasonic scanning of the first surface  11  using the phased array probe P comprises translating the phased array probe P in a set of directions, including a first direction Y, as shown at position 1, across the first surface  11 . The set of directions includes a second direction X, as shown at position 2, orthogonal to the first direction Y. 
     In this example, the set of directions includes a third direction Y′ angularly displaced from the first direction Y, as shown at position 3, by Tr/4 radians, and, a fourth direction X′, as shown at position 4, orthogonal to the third direction Y′. 
     Each of directions X, Y, X′ and Y′ is equispaced from the passageway  100 A (i.e. the line extending orthogonally from the direction to the centre of the passageway  100 A is the same length). 
     The spacing of the directions from the passageway is selected with regard to the radiation pattern of the probe. Particularly, the spacing is selected such that the beam from the phased array ultrasonic scanner tends to spread onto a sufficient range of depths of the passageway. 
       FIG. 8  schematically depicts a phased array ultrasonic scanning signal response from a first flaw  1000 BA associated with a second passageway  100 B in the article  10 , tested according to an exemplary embodiment. 
     Particularly,  FIG. 8  shows an S-scan of the response due to the first flaw  1000 BA associated with the second passageway  100 B. 
       FIG. 9  schematically depicts a phased array ultrasonic scanning signal response from the first flaw  1000 BA associated with the second passageway  100 B in the article  10 , tested according to an exemplary embodiment. 
     Particularly,  FIG. 9  shows an S-scan of the response due to the first flaw  1000 BA associated with the second passageway  100 B, in more detail. 
       FIG. 10  schematically depicts an aircraft A. In this example, the aircraft is a military aircraft, particularly a swept wing aircraft, specifically a Tornado. Regions of interest for NDT are indicated by dashed circles. 
       FIG. 11  schematically depicts an aerospace component  20  of the aircraft A of  FIG. 10 , in situ. Particularly, the aerospace component  20  is a wing diffusion joint, formed from L97 2024 in a T351 condition, particularly plate thereof. In this example, the article  20  is generally as described with respect to the reference article  10 , description of which is not repeated, for brevity. 
     The article  20 , having a first surface  21 , comprises a set of passageways  200 , including a first passageway  200 A. Respective sets of flaws  2000  (not shown) are associated with respective passageways of the set of passageways  200 , including a first set of flaws  2000 A (not shown). The first set of flaws  2000 A optionally includes a first flaw  2000 AA, associated with the first passageway  200 A. 
     In this example, the set of passageways  200  includes N passageways, where N is 81, comprising countersunk taperlock fasteners  50 , respectively received therein. In this example, each passageway of the set of passageways  200  is generally otherwise as described with respect to the first passageway  200 A. NDT is performed for the 81 passageways, as described above. 
       FIG. 12A  schematically depicts a plan view of an upper surface (i.e. the first surface  21 ) of the aerospace component  20  of  FIG. 11 ; and  FIG. 12B  schematically depicts a plan view of a lower surface (i.e. a second surface  22 ) of the aerospace component  20  of  FIG. 11 . The aerospace component  20  is disassembled from the aircraft A, for visual inspection of the lower surface  22 . 
     Particularly, corrosion at or proximal the lower surface  22  may increase occurrence of flaws associated with the first set of passageways  200 , for example, due to stress corrosion cracking. Hence, in situ NDT, as described herein, increases longevity of these aircraft and/or allows operational use thereof beyond original design limits without disassembly. 
     Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above. 
     In summary, the invention provides a method of non-destructive testing and use of phased array ultrasonic scanning. In this way, NDT of the article is facilitated, enabling NDT thereof even if physical access to the article, particularly the first passageway, is restricted. In this way, if the article is a component in an assembly, for example an aerospace component mechanically fastened in an aircraft, NDT of the article in situ (i.e. in the original place, as assembled) is facilitated such that disassembly of the assembly may not be required in order to perform NDT of the article, thereby decreasing complexity, duration and/or cost of the NDT. Furthermore, if disassembly is not required, new mechanical fasteners to replace removed mechanical fasteners are also not required. 
     Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.