in the preferred embodiment of the invention , the object is to detect defects anywhere around the fastener hole wherein the defects are at least 0 . 030 of an inch in length . typical fastener diameters range from 3 / 16 to 5 / 16 of an inch and skin thicknesses from 0 . 117 to 0 . 310 of an inch . fig1 a illustrates fastener 1 holding together layers 2 , 3 and 4 of , typically , aluminum with faying surface crack 5 in first skin layer 2 . fig1 b further illustrates crack 5 in a top view . an angled beam is required in order to direct the ultrasonic beam under the fastener head 6 towards the fastener hole circumference 7 at the faying surface ⅔ interface . phased array ( pa ) ultrasonic beams , as typified by beam 8 , are preferably generated from a number of elements 9 , typically piezoelectric crystals , distributed over a conic surface 10 that surrounds from above ( i . e ., looking down ) a fastener opening placed beneath and substantially in alignment with the axis 11 of such conic surface 10 . in the preferred embodiment , approximately 504 ultrasonic elements are distributed over this conic surface 10 to create conical probe 14 , as illustrated in fig2 . a water path ( not shown in fig2 , but fully detailed below in fig5 ) couples pa ultrasonic beam 8 with aluminum skin layer 1 at point 12 , which is also the point of emergence of the beams at the water / aluminum skin interface on the top surface 2 a of first skin layer 2 ( see fig1 a ). the focal point 13 of beam 8 is close to circumference 7 , hence close to the anticipated location of cracks 1 ( if any ) at the faying surface ⅔ interface . full circumferential scans are performed by programming groups 9 a of elements 9 in sequence so that they fire successively around the conical probe 14 , one such group 9 a being illustrated in fig2 . in this way , they can scan around a fastener opening at the faying surface , using scan patterns that extend over a variety of angles and spatial volumes surrounding the perimeter of the hole . the suppression of grating lobes , which generate misleading false signals , was effected as initially described in the aging aircraft 2000 conference referenced above in the background to the invention section . the design trade - off from an ultrasonic point of view was to select the largest number of elements possible while maintaining sufficient deflection power to generate the required beam angles for all the desired fastener diameters and skin thicknesses , using the smallest number of elements possible while still maintaining acceptably low grating lobes . current state of the art in probe fabrication and cost / size restrictions on the pa electronics imposed a practical limitation of 512 elements 9 on conic surface 10 of conical probe 14 . the classic strategy for reducing grating lobes to an acceptable level was adopted by randomizing the positions of the elements , thereby lowering their peak intensity . this was accomplished by forming pseudo - random groups 15 of 12 elements each , as shown in fig3 a , and then repeating such groups 15 a total of 42 times ( i . e ., 504 elements ) around conical probe 14 to achieve the result illustrated in fig3 b . this randomness configuration of element spacing significantly reduced the peak grating lobe amplitudes by effectively spreading the grating lobe energy over a large spatial area , while maintaining practical manufacturability of the probe . the number of elements that are active for each focal law ( ultrasonic beam ) is a function of the beam diameter at its focal point ( bd ) 8 a , as shown in fig4 , which is 0 . 030 of an inch for the preferred inspection requirement . by working backwards using the angular cone of converging energy ( β ) and the wavelength λ , the intersection of the cone 8 with the conic surface 10 of conical probe 14 gives the approximate number of elements 9 required through the relation bd = λ /( 2 tan β ). these are the elements contained in group 9 a in fig2 and 4 . for the range of fastener hole sizes and skin thickness to be covered , the number of active elements 9 varies from 14 to 38 . as detailed in fig5 , sub - assembly 14 a containing conical probe 14 is first inserted into housing 16 and then hand grip 17 is added to form pa probe assembly 18 . in the preferred embodiment , there are three feet 19 , preferably of teflon ™, on the outer circumference of , and normal to , the forward face of pa probe 18 assembly ( see also fig7 a ). they are required to properly orient the conic surface 10 of conical probe 14 over fastener head 6 . the length of pa probe assembly 18 is approximately 6 . 5 inches and the diameter of housing 16 is approximately 3 . 6 inches . the centering rod 20 may be spring - loaded , and is used to assist in rough centering of the probe over the fastener head 6 . the manner in which the pa probe assembly 18 is precisely centered prior to operation is described in full separately below . flexible boot 21 , preferably made of latex , is filled with a coupling fluid 22 , water in the preferred embodiment , which covers the entire conical surface 10 and the elements 9 located thereon during operation . water 22 is provided to boot 21 by pump 23 through channel 24 in housing 16 and handgrip 17 . further details concerning boot 21 are also given separately below . the 504 elements 9 on conical probe surface 10 of conical probe 14 are all individually connected to a remote system controller and data acquisition system 25 (“ system controller ” 25 ) via a single shielded cable 246 . this is effected by first bundling the 504 wires into four groups of 126 wires in cabling assembly volumes 27 of sub - assembly 14 a prior to being incorporated into a single cable 26 in volume 28 s after passing through the rear of housing 16 . volumes 28 allow cable 26 , which has now become somewhat unwieldy , to exit the pa probe assembly 18 through handgrip 17 . the preferred system controller 25 to drive the phased array probe and process the data is a focus 32 / 256 unit , which is available commercially from rid tech inc . of quebec city , canada . this off - the - shelf system controller is capable , in conjunction with a windows nt operating system , of controlling and monitoring 256 element elements . two such systems may be connected in a master - slave configuration to control up to 512 elements ( 504 are used in the preferred embodiment ). in the configuration of the preferred embodiment , up to 64 element &# 39 ; s may be activated by the focus 32 / 256 unit at any one time to form an ultrasonic beam . more typically , as previously mentioned , 14 to 38 elements may be used to form a beam having a − 6 db focal spot width of approximately 0 . 030 of an inch . fig6 is a rear view of pa probe assembly 18 showing a probe centering light emitting diode ( led ) indicator screen 29 and three actuation buttons . screen 29 has four sectors 29 a , 29 b , 29 c and 29 d , each of which can be independently illuminated . a portion of the electronics 30 for led screen 29 are shown in fig5 , the remainder being located in system controller 25 . returning to fig6 , centering button 31 is pushed to activate the probe - centering algorithm as explained below . once centering is achieved ( normally in fewer than 15 seconds ), inspection button 32 is activated and the inspection routine is completed in a further few seconds . the inspection technique and how it is effected is described separately in a third section below . finally , the next button 33 is pushed to indicate that inspection of the fastener hole is complete so that the location of the hole can be recorded by system controller 25 along with the inspection results before a new file is opened that will allow the same sequence to be repeated for the next fastener hole . fig7 a and 7b are three dimensional views of the pa probe assembly 18 from two different angles , one of which illustrates the back face of the probe . as depicted in fig5 , boot 21 is filled with water , hence it has the appearance of being rigid . in fact , in the preferred embodiment it is made of 0 . 004 to 0 . 008 of an inch thick latex by a standard molding procedure and prior to assembly is a quite flexible membrane 34 , as can be seen in fig8 a ( cross section sketch ) and 8 b ( photograph ). it is assembled to the forward end of conical probe 14 , as illustrated in fig9 with flange 35 firmly anchored and sealed between housing 16 and ring 50 . boot 21 is donut - like with a central sleeve 34 a to receive centering rod 20 . sleeve 34 a terminates in a flange 34 b that is sealing fitted by insert 51 and housing 16 into the central opening of conical probe 14 . prior to operation , membrane 34 is filled with coupling fluid 22 ( usually water ) from pump 23 via channel 24 to form boot 21 . membrane 34 has small perforations 36 in it which allow a small amount of water to pass through it to wet surface 2 a ( see fig1 a ) sufficiently to ensure proper acoustic coupling into the metal being inspected . during operation of pa probe assembly 18 , membrane 34 of boot 21 is in tight contact with the surface 2 a surrounding the fastener hole , thereby deforming membrane 34 . the extent of this deformation is shown in fig1 , which is a photo of membrane 34 under pressure as seen from the top through a plexiglass plate simulating surface 2 a . water is continuously fed into boot 21 at a low flow rate to compensate for the loss of fluid through perforations 36 and to keep boot 21 full . to relax the constraint of having to position the testing device precisely over the center of fastener 1 to be inspected , in the preferred embodiment it is sufficient for the pa probe assembly 18 to be first roughly centered by hand and eye approximation within 0 . 060 of an inch of the true center of the fastener 1 . to assist in the initial manual alignment , pa probe assembly 18 of the invention may optionally be provided with a central , spring - loaded centering rod 20 which may be placed on fasrener head 6 of fastener 1 . depression of this rod into cavity 20 a ( see fig5 ) by steady pressure on pa probe assembly 18 will allow said probe to descend upon fastener 1 , landing preferably with conical probe 14 approximately positioned centrally over fastener 1 . the object of this coarse centering system , with or without centering rod 20 , is to position the phased array probe within 0 . 060 of an inch of the true center of the fastener . this initial coarse manual centering is followed by a more refined procedure wherein subsequent movement of pa probe assembly 18 by the operator to locate its mechanical center within 0 . 030 of an inch of the true center of fastener 1 is guided by an electronic feedback system . to achieve this , the center of the fastener is compared with the geometric center of the conical probe 14 . signals are provided to an electronic feedback display ( e . g ., led screen 29 ) that cause the operator to shift the probe towards having its mechanical center coincide with the center of the fastener opening . this electronic feedback system provides the operator with signals that guide said operator in situating the center of pa probe assembly 18 ( i . e ., the centre of conical probe 14 ) within 0 . 030 of an inch from the true center of the fastener opening . once positioned within this tolerance range , electronic manipulation of the deflection of the ultrasonic beams during inspection is sufficient to accommodate the remaining misalignment . inspection details are provided in the next section below . the intersection of the hole with faying surface 37 forms a corner 38 , as shown in fig1 , which is a good reflector for ultrasonic beams 8 . by measuring the time for the beam 8 to go from the conical probe 14 to the corner 38 and return , the distance between the elements 9 in conical probe 14 that formed the beam 8 and the corner 38 can be calculated . as the angles of the beam 8 inside the water 22 and inside the first skin material 2 are known , it is possible to position the corner 38 with respect to the conical probe 14 . measuring the position of at least three points around the circumference 7 of the hole 1 will provide , after calculation , the diameter and centre position of the hole 1 . to provide signals to this operator - assisting electronic feedback system , at least three beams 8 are used to locate points on the cylindrical surface 7 of the fastener opening 1 at a common depth below the surface of the skin within which such opening 1 is formed . initially , such beams 8 must sweep through the volume of the skin to locate the cylindrical surface 7 . once a scan detects the boundary of the hole 1 by the reflection of ultrasonic beams 8 from the sides of the hole 1 , the position of the source of this reflection is recorded as a point on the side of the cylindrical surface 7 of the hole 1 . once three such points have been located , the location of the true center of the hole is determined by applying standard geometric procedures within a computer - based processing controller 25 . this is illustrated in fig1 a , which depicts a side view of the fastener 1 with conical probe 14 above it ; and in fig1 b and 11c , which both depict the top view of fig1 a looking down through conical probe 14 at the fastener hole 1 below ( fig1 c gives additional detail to fig1 b ). by measuring the position of three points around the circumference 7 of the hole 1 at faying surface 37 , the center of the hole 1 can be determined because only one circle passes through the three points . the estimated skin thickness can be calculated as the average thickness of the three different corner positions 38 , while the diameter and position of the hole 1 are calculated using a specific algorithm called “ best - fit circle ”. this algorithm consists in searching for the circle minimizing the mean - square deviation of the distance between all the three points and the circle . in other words , this algorithm consists in growing a circle centred at different positions until it passes through the three points . using actual computers ( 1 . 8 ghz processors ), this algorithm can be used in real - time despite its high number of iterations with a greater number of points . if it were assumed that the fastener hole 1 is perfectly round , only three measurements would be needed to determine the exact location of the center . in reality , however , fastener holes 1 are not perfect , and the system determines a weighted center of the fastener hole 1 by averaging a series of three - point measurements around the faying surface circumference 7 . it is important to note that , although the rough centering is done using the fastener head 6 as reference , the final centering is done using the fastener hole circumference 7 at the faying surface 37 , which is where the inspection is being performed . for this system to function properly , calibration for two parameters must be conducted prior to use . first it is necessary to know the “ wedge delay ”, which is the time before the beam 8 enters the metal material , because without it measurements of the hole diameter 1 and depth are not accurate when inspecting off - centered holes . that is , knowing the wedge delay ensures that the system will determine whether the hole is perfectly centered or not . this is done using a pre - centered fastener hole and then modifying on - line the wedge delay until both diameter and depth measurements are identical to those of the calibrated hole . this calibration must be done every day before inspection , and is dependent upon the types of holes to be inspected . it takes only a few minutes . second , it is necessary to ensure that the system knows the exact position of the center of the hole when it is off - center . this is done by measuring the position of fastener holes 1 when they are slightly off - center up to 0 . 060 of an inch away from the center of the conical probe 14 . typical results are analyzed in the three b - scans shown in fig1 a , 12 b and 12 c . fig1 a is a b - scan representation of the ultrasonic acquisition with a fastener hole 1 originally positioned in the north - east direction ( distance from probe centre is 0 . 004 of an inch ). even this small deviation can be observed either on the b - scan display ( note the deviation of the time - of - flight curve from a straight line ) or on a reconstructed view on the left hand side of the figure . in fig1 b , the b - scan representation is of an ultrasonic acquisition with a fastener hole 1 off - center in the southwest direction ( distance from probe centre is 0 . 030 of an inch ). here the time - of - flight curve is “ sinus - shaped ”. finally , the b - scan representation in fig1 c is of an ultrasonic acquisition with a fastener hole 1 off - centre in the southwest direction ( distance from probe centre is 0 . 045 of an inch ). these results lead to a comparison of the distance measurement versus the real displacement of the hole , as correlated in fig1 for two hole sizes . by interpolating each series of points by a linear curve , a distance correction factor ( dcf ) can be calculated ensuring a minimal deviation of less than 0 . 1 mm ( dfc = 2 . 778 for a hole diameter of 0 . 1875 of an inch and a depth of 0 . 125 of an inch ; and dcf = 2 . 26 for a hole diameter of 0 . 25 of an inch and a depth of 0 . 25 of an inch ). once the dcf has been calibrated for every type of fastener hole , measurements can be done with accuracy in positioning as tight as 0 . 1 mm (˜ 0 . 004 of an inch ) with fastener holes as far as 1 . 5 mm (˜ 0 . 060 of an inch ) from the center of conical probe 14 . with these two calibrations completed , centering is effected by roughly centering the pa probe assembly 18 over the fastener head 2 a by hand using the centering rod , if present , as described above . the centering button 31 on the rear face of probe 18 ( see fig6 ) is then pushed and fine centering is completed interactively by the operator and the probe - centering algorithm described in the previous section . while a variety of presentation mechanisms may be employed , the preferred embodiment adopts a display in the form of four illuminated sources 29 a , 29 b , 29 c and 29 d arranged in the pattern of a cross appearing on led indicator screen 29 . the illumination of a single source provides a signal that the probe assembly should be displaced in the direction of the illumination ( e . g ., an upper source 29 a on the screen 29 ). when two contiguous sources ( e . g ., 29 b and 29 c ) are illuminated , a signal is provided that the probe assembly should be displaced in an oblique direction , passing between the two illuminated sources . when all four sources are lit , the operator is informed that centering within preset tolerance limit has been achieved . in this manner , an operator is given a ready indication as to positioning the pa probe assembly 18 substantially centrally over the fastener hole 1 to be inspected . this procedure normally takes less than 15 seconds . once centering of the probe assembly 18 has been completed within 0 . 030 of an inch of true center , the inspection button 32 is pushed to activate a scanning scenario that maps the sides of the hole 1 location and determines whether defects are present at the faying surface 37 surrounding the fastener hole 1 . this scenario includes use of a look - up table to download the necessary focal laws , and re - verification of the position of pa probe assembly 18 to ensure that no movement has occurred during the scan . this sequence may be completed in a matter of a few seconds , the data being displayed as a plan view colour map with the fastener hole 1 position indicated and any defects shown along with their relative orientation around the hole 1 . the data may also be captured electronically for a separate analysis and display . the scanning methodology utilizes the fact that small variations in the fastener hole diameter 1 , first layer thickness 2 and crack 5 morphology move the “ best ” interrogation point 39 ( ip 39 ) from the expected intersection of the hole 1 and faying surface 37 . a conglomerate technique has been incorporated that interrogates the suspect region from a variety of incident angles and takes several points in the vicinity of the ip 39 as a localized scan . the circumferential scanning , therefore , cuts a volumetric swath around the base of the hole 1 from many different angles , which improves the reliability of the inspection . by perturbating the actual ip 39 about the calculated “ ideal ” ip , there is a greater chance of getting multiple responses , including the optimal response , from a flaw . fig1 a shows the principle of local scanning . a pattern of pixel points is chosen adjacent to the assumed location of the defect , which for many inspections is at the intersection of the hole and faying surface . this method has the advantage of defocusing to cover a larger area without the resultant loss of defect response amplitude for small defects . fig1 b demonstrates the principle of using several interrogation angles . this approach uses different sections of the conical pa array to strike the ip in such a way as to generate a two or three wall corner trap , and also to generate a direct response from the tip of a crack , if one is present . in practice , the tip diffraction approach would only be used for characterization and sizing of a crack , but is shown here for completeness . the number of points chosen for the local scanning and the number of angles chosen is flexible , but hardware limitations and time constraints put an upper limit on the total number of pulses per ip 39 . one limitation is that a maximum of 1 , 024 focal laws may be downloaded for a single fastener hole inspection . therefore , any combination of points and angles around the circumference cannot exceed this number . for example , a four - point pattern is chosen using four angles for each point . this allows up to 64 ips around the circumference , or one inspection every 5 . 63 degrees . abbreviated inspection sets requiring fewer focal law calculations and downloads are desirable to speed up the inspection process . a preferred scan pattern is to direct the focal points of three probing beams 8 at consecutive locations encircling the fastener opening . thus , each scanned location is preferably sampled by three beams arriving at the sampled location along three distinct paths . to maximize the detection of cracks formed around the fastener opening , such beams are preferentially selected to arrive at the scanned location along paths which are generally tangentially oriented with respect to the side of the fastener opening , as illustrated in fig2 . as a preferred procedure , two sets of more than one probing beams 8 are directed to a scanned location from opposite sides of the fastener opening 1 , arriving along generally nearly tangential paths . as illustrated in fig1 a looking down on the top of the fastener hole 1 , more preferably , two sets of three beams 8 are directed to the scanned location ( or ip ) 39 , each set arriving from an opposite side . thus , a total of six beams 8 are used to sample each ip 39 , preferably in a tangential direction with respect to the hole surface 7 . generally , and preferably , the sampled ips are equally spaced around the cylindrical opening 1 , as described above . this preferred scan pattern may commence initially with a path that follows the circumferential boundary of the cylindrical surface 7 of the hole 1 at the level of the faying surface 37 , as illustrated in fig1 b by ips 39 around fastener 1 . the number of ips 39 shown is illustrative only and does not represent the exact number of ips . second and , optionally , scanning patterns are then preferably directed along encircling paths 40 and 41 located at progressively greater radius from the center of the fastener opening . the number of encircling paths 40 , 41 is discretionary . then optionally , and preferably , these ips 29 may be located radially outwardly from the initial inspected points positioned around the side surface 1 of the hole . the described procedure results in a multi - angle , multi - positional inspection strategy that has been found to efficiently increase the probability of locating a defect . displays of each angle and location would be cumbersome and difficult to interpret , so the peak response data from each pulse is saved and merged into a single value for each ip 39 . this merged data approach greatly simplifies data presentation and interpretation . a colour display similar to a c - scan image is used to represent the ips 39 about the circumference 7 of the hole 1 . a grayscale representation of this simplified display is shown on the left of fig1 , which is an example of the inspection screen . in the colour display , the defect location is clearly indicated as a red sector ( not evident in the grayscale image ). each sector of this display represents the maximum peak value of all of the angles and locations in the interrogation set for that ip 39 . different merging parameters , such as the average value of the sum of all the values , can be shown as well , as in the center and right hand parts of the screen . while the inspection itself , with electronic scanning , takes only a fraction of a second , focal law calculation can take several seconds . once an indication of the possible presence of a defect has been detected , the full power of the system can be utilized to evaluate a smaller region around the possible defect location . during this interrogation , the pa probe assembly 18 can be attached , for example , to an aircraft wing containing the fastener hole 1 under inspection , with mounting fixture 42 , as shown in fig1 . a new group of up to 1 , 024 focal laws can be downloaded to interrogate a much smaller volume , thereby allowing more angles and finer increments to be used . mounting fixture 42 ensures that pa probe assembly 18 remains motionless during the time it takes to characterize a defect , which is considerably longer than the detection procedure ( possibly up to a few minutes ). fixture 42 is positioned over the fastener hole 1 under inspection by manipulating tripod arms 44 before tightening knobs 45 and then rigidly attaching everything to the wing by engaging vacuum cups 47 . next , pa probe assembly 18 is placed into the brake shoe controlled by brake lever 43 , but brake lever 43 remains disengaged so that fine positioning of pa probe assembly 18 can be effected following the centering procedure detailed above . once pa probe assembly 18 is centered , brake lever 43 is tightened and the characterization procedure commences . as illustrated in fig1 , multi - functional test fixture 60 comprises probe support body 61 on tripod 62 with spring - loaded clamp 63 , socket 64 and rotary dial 65 . removable test block 66 , which is shown in cross section in fig1 , is a conically shaped target having a target surface 66 a having the same angle as conical probe 14 of pa probe assembly 18 . test fixture 60 is used for two different tests , the first of which is to verify the functionality of conical array 10 using test block 66 . to do this , pa probe assembly 18 is fitted over test fixture 60 as shown in the photograph of fig2 , being centered by placing feet 19 into sockets 64 and then being firmly held in position by clamps 63 . the test fixture and the forward end of pa probe assembly 18 are then immersed in a water bath to a depth that ensures that conical probe 14 is underwater during the period of testing . to verify the functionality of conical probe 14 , system controller 25 sequentially activates elements 9 of said probe so that precise data may be obtained from the conical target surface 66 a . this data can be used to ensure that the individual elements 9 are functioning properly as well as electronically correcting for slight misalignments or mis - positioning of said elements 9 in the conical probe 14 . the system is also able to compensate for variations in element 9 sensitivity by applying corrections to the received signal for each element 9 . the second test for which test fixture 60 is used is that to verify the uniformity of response of conical probe 10 to cracks at various locations around the fastener hole 1 . other test blocks , as typified by test block 67 shown in cross section in fig2 , are inserted into test fixture 60 in place of conical test block 66 . test block 67 , for example , has a standard edm notch 68 ( 0 . 030 of an inch by 0 . 030 of an inch by 0 . 008 of an inch wide ). such test blocks can then be rotated through 360 degrees using rotary dial 65 so that the same defect is sequentially detected by different parts of conical probe 14 . if the response is not uniform at all test locations , electronic corrective actions may be taken by the system controller 25 to adjust apparent signals obtained in the field in accordance with the calibration data obtained from these tests . advantages of this system of the invention are that it to need not contain any moving parts . it may be constructed in a light compact format . using rapid electronic phased - controlled scanning permits testing to be completed within less than a minute . three - dimensional beams steering allows great flexibility in scanned patterns , contributing to improve reliability . the foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use . these embodiments are only exemplary . the invention in its broadest , and more specific aspects is further described and defined in the claims which now follow . these claims , and the language used therein , are to be understood in terms of the variants of the invention that have been described . they are not to be restricted to such variants , but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein .