SE1551214A1 - Method and system for inspecting plate-like structures usingultrasound - Google Patents

Abstract

fi) 2G 23 Abstract A method of inspecting a piate-iike structure (2) by means ofuitrasound for detecting anernaiies (3) in the structure, cornprising: positioningat least three uitrasonic transmitting/receiving probes (4) in Herizian contactwith a surface of the structure (2) and in a poiygonai configuration such thatan inspection area (6) of the structure is defined by the configuration;successiveiy drive one probe (4) at the tirne as an uitrasound transmittihgprobe generating an uitrasound wave propagating into a piane of the structurein the inspection area (6), whiie at ieast one of the other probes (4) isarranged to be used as an uitrasound receiver detectirrg a resuiting waveprovided by the uitrasound wave propagating through the inspection area (6);acquiring signals corresponding to waves detected by the second probes (4)and comparing the acquired signais with reference signais for acorresponding inspection area of a reference structure without anornaiies. Eiected for pubiication: Fig. t

Description

1G 2G 3G iviETi-ECD AND SYSTEM FÛR iNSÉECTiNG PLATE-LiKE STRUCTURESUSiNG ULTRÅSGUND, Technicai Fieid The present disciosure reiates to a method of inspecting piateiikeStructures by means of uitrasound for detecting ancmaiies in the structureand to a system for inspecting piate-iike structures using uitrasound.

Background of the inventšon inspection of objects, such as structures of metais, aiioys, compcsitesetc, efter manufacturing or for maintenance purposes, is commoniy performedusing non-destructive uitrasonic testing. Such methods reiy on scanning ofthe structure or part of the structure with a probe transmitting an uitrascnicsignai into the materia! and measuring a resuiting uitrasonic signa! that hastraveiied through the thickness of the materia! or has been refiected at aninterface or at an imperfection in the materiai before arriving at the measuringpoint. Anaiysis of the resuiting uitrasonic signa! provides information about theinterior of the materiai.

The inspection time is reiated to the size of the part to be scanned butaiso to the geometricai compiexity of the object as the technique requires agood contra! of the probe orientation over the object. This requirernent ieadsto compiex, expensive and reiativeiy siow inspection systems.

Some of these dravvicacks can be minimized by using phased-arrayuitrasonic probes aiicwing inspection of the area iocated under the probewithout physicaiiy moving the prcbe. in US ?,654,'i42 the measuring probecomprises a piuraiity of aiigned uitrasonic transducers. The transducerstransmit an uitrasonic puised wave, one transducer at a time, whiie the othertransducers are used to receive the refiected signais. A first uitrasonicmeasurement is performed on a fiawiess reference part and a seconduitrasonic measurement is performed on the part to be inspected. Asubtraction is performed between the measurements of the inspected partand the reference part, and the topcicgica! energy at each position in the part is deterniined and an indication of defects or modification in the inspectedpart is obtained.

A phased-array technique is thus potentiaiiy faster than mechanicaiscanning. Stiii, however, it shows iimitations for handiing cornpiex geometricaivariations (such as radii Variations, certain ioggie configuraticns, etc). Thetechnique also requires that the probs is positioned close to the inspectedpart. This particuiar requirement often resuits in access iimitations especiaiiyin the case of integrated Structures.

Summary of the invention it is an object of the present disciosure to provide a method ofinspecting piate-iike structures by means of uitrasound for detectinganornaiies in the structure, Which method aiiows for a fast inspection of astructure and aiiow for inspection of structures comprising compiexgeornetries. it is aiso an object to provide a system for inspecting piate-iikestructures using uitrasound.

The invention is defined by the appended independent ciairns.Embodiments are set fcrth in the dependent ciaims, in the attached drawingsand in the foiiowing description.

According to a first aspect there is provided a method of inspecting apiate-iike structure by means of uitrasound for deteoting anomaiies in thestructure, the method comprising the steps of: a) positioning at east three uitrasonic transmittingireceiving probes ini-iertzian contact with a surface of the structure and in a poiygonaiconfiguration such that en inspection area of the structure is defined by theconfiguration; b) using a first one of the probes in a transmitting mode to generate anuitrasound wave propagating into a piane of the structure in the inspectionarea; c) using at ieast a second one of the probes in a receiving mode to detecta resuiting wave provided by the uitrasound wave propagating through theinspection area; d) repeating steps b) and c) using another one of the probes as the firstprobe; e) acquiring signals corresponding to waves detected by the secondprobes; f) comparing the acquired signals with reference signals for acorresponding inspection area of a reference structure without anomalies.

A plate-like structure is here a structure in which the area of a surfaceof the structure is at least 10-25 times, at least 25-50 times, at least 50«1 OOtimes or at least 100-1000 times larger than a maximum cross-sectional areaof the structure. Plate-like structures include plates which are confined by twoprinciple surfaces which may be planar or curved (single curved or doublecurved). The plate-like structure could form part of a complex object such asl-beams, U-beams etc., and could also be non-homogenous and for examplecomprise integrated stiffeners. The material of the plate-like structure couldbe metals, alloys, composites etc. The plate-like structure could for examplebe a part in aircraft, such as airplanes, or a part in a car or a boat.

The inspection area of the structure defined by the configuration couldfor example include plane structures, structures with a radius (single- and/ordouble curved surfaces), structures with one or more holes, structures withrivets, nails etc, or structures with an attached patch.

Anomalies may here include any deviation from a thought/intendedstate which may occur in the plate-like structure during use or which is built-inin the structure. Non-limiting examples of such anomalies are delaminations,cracking, fiber fracture, fiber pullout, matrix cracking, inclusions, voids,impact-damages, hole-defects, corrosion, pits, thickness Variations, fatiguecracks, stiffness changes, local stiffness Variations, patches coming loose,etc. That the probes are positioned in Hertzian contact with the surface of thestructure is here meant that they are positioned in reversible contact with thesurface, i.e. the probes are not glued or bonded to the surface of the structureor embedded in the structure.

Due to the positioning of the ultrasonic transmitting/receiving probes ina polygonal configuration such that an inspection area of the structure isdefined by the configuration, mechanical movement of the probes needed to 3G scan a given area is minimized, thereby reducing the inspection time. in orderto cover a iarger inspection area of the structure it may be needed to movethe probe configuration on the structure surface repeating the method at morethan one position on the structure surface. The probs configuration couid bemoved manuaiiy, by means of a robot, or the probe configuration couid beintegrated in a robot.

With pciygonai configuration is here meant that the probes are piacedin the nodes of a poiygonai configuration and imaginary iines betweenneighboring probes define the poiygonai shape and the inspection area.

Cornpared to traditionai uitrasonic testing, which makes use of voiumewave propagation through the thickness of the structure, the present method,acousto-uitrasonic testing, utiiizes piate-iike ornriidirectionai waves (aisoknown as Lamb waves) that propagates in the piane of the structure andinteract with the structures geometricai features and potentiai anomaiiespresent in the structure in the inspection area. The frequency of the piatedikewave shcuid be adapted to the thickness of the piate-iike structure in order tocontroi the waveiength of the plate-fika wave. For exarripie for an aiurninumpiate-iike structure of thickness 2rnm a frequency of 250 kHz can be used forthe propagation of the first fiexurai mode in the piane of the structure with aveiocity of ZÛOG rn/s ieading to a 8 mm waveiength .

Piate-iike waves have the abiiity to propagate in a piate-iike structureeven when the structure is not piariar, why inspection of a structure having aradius is possibie. The wave generated by a transmitting probe on one side ofthe radius propagates through the radius and is detected by receiving probeson other sides of the radios. This configuration eiirninates the need for ascanning device in the radius as the wave itseif probes the materiai in theradius and the method, hence makes it possibie to inspect aii areas of acomponent even if direct access to the area is iirriited and enabies inspectionof compiex geornetries without having to adept and change the physicaiparameters of the equipment (changing probs, re-orienting the probe. re-positioning the structure etc).

By comparing the acquired signais with signais for a correspondinginspection area of a reference structure it is possibie to handie compiexstructuresigeornetries where the wave path and veiocity is not a priori known.

The step of positioning the probes in contact with the surface of thestructure may comprise providing a iiciuid between the structure surface andthe probes.

Thereby, the probes and the structure are in a wet contact with eachother, The step of positioning the probes in contact with the structure mayaiternativeiy cornprise positioning the probes in direct dry contact with thestructure.

Positioning the probes in a poiygonai configuration may comprisearranging the probes in a probe fixture having a predetermined configurationprior to positioning the probes in contact with the structure.

The fixture may be fiexibie enough to adapt to structure geometriesinciuding fiat structures, curved structures or branched structures, such thatthe probes may be positioned in the same ptane or aiong a non-pianarsurface. in the case of repetitive inspection of a sirniiar component in aproduction context, the fixture rnay be designed to be iockabie in certainconfigurations and included in a dedicated tooi. The distance betweenneighboring probes in a fixture may vary between 10 rnm up to 100 rnm, andis typicaiiy 30 to 80 min.

The probes may be positioned in a poiygonai or substantiaiiy poiygonaiconfiguration seiected from a group comprising a trianguiar configuration, arectanguiar configuration, a square configuration, a diamonddikeconfiguration, a pentagonai configuration, a hexagonai configuration, aheptagonat configuration, en octagonai configuration, a nonagonaiconfiguration, a decagonai configuration, a poiygonai configuration with 1145nodes, a poiygonai configuration with 15-25 nodes, a poiygonai configurationwith 25-35 nodes, or a poiygonai configuration with 35-50 nodes.

The method may comprise causing the probes to generate, when usedin a transmitting mode, uitrasound waves at a frequency of iess than 1 ivii-iz,preferabiy 50 ki-iz-500 kHZ or 100 ki-iz-250 id-iz. 6 The system is. hence, a so oaiied low-frequency uitrasonic system.The frequency range used depends on the thickness of the structure underinspection.

The probes may be caused to generate and receive a toneburst of 1to 1G cycies.

The probes may be caused to generate, when used in transrnittingmode, uitrasound waves comprising a fiexurai mode.

The plate-iike wave, lamb way, generated should preferabiy be the firstantisymmetric mode (or flexurat mode) often referred to as A0 Lamb mode.However, the presence of other modes mixed in the signai does not affect theperformance of the method.

The reference signais may be obtained by using the method steps a)-e) of the method described above for a separate reference structure or areference zone of the structure under inspection.

The reference structure or reference zone shouid then have similargeometry and be made of similar material as the inspection area of thestructure under inspection and be without anornalies. For obtaining thereference signais a similar probs configuration and similar area of inspectionshould be used as for the structure under inspection.

The area of inspection shouid as ciosely as possible be the same forthe structure under inspection and the reference structure or zone. Preferabiy,the location of a feature of interest in the area under inspection should notdiffer with more than 1-3 mm, 1-2 mm or 9.54 mm between the structureunder inspection and the reference structure or reference zone.

The sensitivity of the anomaly detection is depehdent on therepeatability of the positioning of the probes on the structure under inspectionand the reference structure.

The reference signais may aiternatively be obtained from theoreticaianalysis of the structure without any anomaiies in the inspection area.

Such a theoretical analysis may be a Finite Element Method analysisof the structure.

The step of cornparihg the acquired signais with the reference signaismay cornprise quantifying a difference between the acquired signals and the 2G 39 reference signals by caiculating a time shift between the acquired signals andreference signals, oalculating a difference in amplitude between the acquiredsignals and reference signals, andlor caicuiatlng a correlation ef the signals.

The method may further cemprise a step of identifying an anornaly inthe inspection area based en the comparison in step f) of the methoddescribed above.

With based on is here meant, as a nen~limited example, thatidentification of an anomaly is being made using the comparison betvveenacquired signals and reference signals, for example a comparison of height ofamplitude, or a function of such a comparison and comparing this with atabulated or preset value.

The method may further comprise a step of visuaiizing en identifiedanemaiy.

The visualization may comprise visuaiization on a screen indicating theanomaly on the screen as a color difference, or the anornaly being marked onthe screen with a box, ring or arrow. Tomographic imaging is one example ofsuch visualization. Alternatively, en anomaly may be marked directly in theinspection area of the structure under inspection by means of for example abeam of light.

According to a second aspect there is provided a systern for inspectingpiate~like Structures using uitrasound, the system comprising: - an ultrasonic generator; - at least three uitrasonic transmittingireceiving probes connected toand driven by the ultrasonic system, the probes being arranged in a polygonaiconfiguration in a fixture in such a way that when the probes are positioned incontact with a surface of the structure an inspection area of the structure isdefined by the configuration; -the ultrasonic generator being arranged to successiveiy drive oneprobe at the time as an ultrasound transmitting prebe generating anultrasound wave propagating into a piane of the structure in the inspectionarea, while at least one of the other probes is arrangecl to be used as anuitrasound receiver detecting a resulting wave provided by the ultrasounclwave propagatlng through the inspection area; 3G -the system further comprising a signai acquisition unit arranged toacquire detected waves from the probes and to transrnit signaiscorresponding to detected waves to a processing device of the system; -the processing device being configured to compare signais from theacquisition unit for a structure under inspection with signais for acorresponding inspection area of a reference structure without anomaiies,providing a comparison index.

The system may further cornprise an anomaiy identifying unit arrangedto identify any anomaiy in the structure under inspection based on thecomparison index.

The fixture may be arranged to hoid the proioes in a poiygonaiconfiguration seiecteci from a group comprising a trianguiar configuration, arectanguiar configuration, a square configuration, a diamond-iikeconfiguration, a pentagonai configuration, a hexagonai configuration, aheptagonai configuration, an octagonai configuration, a nonagonaiconfiguration, a decagonai configuration, a poiygonai configuration with 'ii-ionodes, a poiygonai configuration with 15-25 nodes, a poiygonai configurationwith 25-35 nodes, or a poiygonai configuration with 35-59 nodes.

The fixture may comprise probe hoiding eiements hoiding the probesin the fixture.

Probe hoiding eiements may hoici the probes in such a way that an endsurface of the probs ensures fuii contact with the object to be inspected.

The fixture may comprise distance eiernents hoiding the probe hoidingeiements together in the fixture.

The distance eierrients may be used for adapting and changing thegeometry of the fixture.

The probs hoiding eiement may comprise a biasing mechanismaiiowing for perpendicuiar movement of the probes reiative the structuresurface.

Thereby, aiiowing for appiication of a controiied force on the probe as itis in contact with the structure. The spring mechanism may aiiow a movementof the probs between Git mm and 5 mm.

The fixture may have a iockabie geometry.

The connection between probs hoiding eiements or probe hoidingeiernents and distance elements may be iockabie such that a rigid probsconfiguration is obtained, which is a prerequisite for repeatabiiity when usingthe system. The iocking between the probe hoiding elements or the probehoiding eiements and the distance eiements may be obtained by means of forexampie dovetaii ioints.

The distance eiements couid aiternativeiy provide for at ieast somemobiiity around an axis paraiiei with the surface piane andlor perpendicuiar tothe surface piano. Aiso for a fixture without distance eiements there probehoiding eiements are directiy connected to each other mobiiity around an axisparaiiei and/or perpendicuiar to the surface piane couid be provided.

The probs may comprise a piezoeiectric active eiemerit excited in itsthickness resonance mode.

The uitrasonic generator may be arranged to provide a drive signai tothe probes such that the probes generate, when used in a transmitting mode,uitrasound waves at a frequency of iess than 1 ivii-iz, preferabiy 50 ki~iz-500ki-iZ, or 100 ktiz-250 id-tz.

The at ieast three uitrasonic transmittinglreceiving probes maycomprise 3-10 probes, 10-15 probes, 15-20 probes, 20-25 probes, 25-30probes, 30-35 probes, 35-40 probes, 40-45 probes or 45-50 probes.

A tip area of the probe for contacting with a surface of the structuremay have a conicai shape, which conicai shape may have contact surfacearea with a diameter of at most 1/3 of a waveiength of the generateduitrasound wave.

This requirement is iinked to excitation/detection efficiencyconsiderations. The diameter of the contact surface may vary between 1/5 to113 of a waveiength of the generated wave.

The piezoeiectric eiement may be arranged in physicai contact with thetip area. The uitrasonic generator may be arranged to provide a drive signa!to the probes such that the probes generate and receive a tone-burst of 1 to10 cycies. 2G 'EÛ The uitrasonic generator may be arranged to provide a drive signai tothe prohes such that the probes, when used in transmiiting mode, generatesound waves comprising a fiexurai mode.

The system may further cornprise a visuaiization unit visuaiizing anidentified anomaiy of the structure under inspection. ârief Ûescršotion of the tšrawings Fig. 'i shows a systern for inspecting piate~iii Figs Sa and 3b shows tvvo different probs fixtures from shove.

Fig. da shows an acoustc-uitrasonic signai for a reference piate-iike structurewithout any anomaiy, Fig. Lib an acousto~uitrasonic signai for a piate~iikestructure with an anomaiy in the inspection area, and in Fig. 4c detectedsignais from the structures in Figs da and 4b are shown in the same timedomain graph.

Fig. 5 iiiustrates damage index computation and dispiay after tomograhicreconstruction.

Fig. d shows appiication of the system shown in Fig. 'i on a structurecomprising a radius.

Fig. 7 shows different probe configurations.

Fig. 8 shows an uitrasonic transmittinglreceiving probs from different views.

Detaiied Descrigtion of the Brawinds A system 1 for inspecting piateiike structures 2 using uitrasound fordetecting anomaiies in the structure is shown in Fig 1. The structure 2 may bepianar, Fig. 'i and Fig. 4, or non-oianan i.e. a structure having a radius, asshown in Fig. 6. The structure may comprise one or more hoies, rivets, naiis,attached patches etc. (not shown). The piate-iike structure couid eg. be ofmetai, aiioy or composite. Anornaiies 3 may be deiaminations, cracking, fiberfracture, fiber puiiout, matrix cracking, inciusions, voids, impact-damages,hoie-defects, corrosion, pits, thickness Variations, fatigue cracks, stifinesschanges, iocai stiffness Variations, patches coming ioose etc. e 1G 11 The system 1 ccmprises at ieast three uitrasonic transmittinglreceivingprobes 4 connected to and driven by an uitrasonic generator 5. in Fig. 1 thenumber of probes 4 in the system 1 is four but in other ernbodirnents of thesystem the number of probes may very between three up to fifty probes. Theprobes 4 are in Fig. 1 arranged in a sduared configuration. The probes may,however, be arranged in a range of different poiygonai configurations. in Fig.7 different possibie poiygonai configurations with eight probes are shown.

When piaced in contact with a surface of a piate~iike structure 2 aninspection area 3 of the structure 2 is defined by the configuration, indicatedby the dotted iine in Fig. 1 and Fig. 2. A typicai inspection area 6 defined by asquared configuration with eight probes is 256 mm x 256 mm.

As a generai ruie, the more nodes, the more preciseiy it wiii bepossibie to identify the position and extent of the anomaiy.

The probes 4 are positioned in reversibie contact with the surface ofthe structure 2, and the contact may be a t-iertzian contact (non-adhesivecontact), either through direct dry contact or wet contact with a fiirn of iiguidbetvveen the surface of the structure 2 and the probe 4.

The probes 4 may be arranged in a probe fixture 39. in Figs 3a and 3bfixtures 30 with sguared configuration are shown, which fixtures hoid fourteenand ten probes 4, respectiveiy, giving the probes a predetermined squaredconfiguration. A range of different fixture configurations are possibte givingthe probes 4 heid by the fixture 3G the different configurations discussedabove. The fixture in Fig. Sa comprises probe hoiding eiernents 31 directiyconnected to neighboring probs hoiding eiernents 31. The probs hoidihgeiements 31 couid be iockabiy connected to each other for exampie bymeans of dovetaii joints (as shown in Fig. Se) such that a rigid, iockabie,fixture and probe configuration is obtained. Qther types of iockabie joints areaiso possibie. The fixture shown in Fig. Bb comprises probs hoiding eiernents31 and distance eiements 32 iinking the probe hoiding eiements 31 togetherin the fixture 3G. The connection between probs hoiding eiernents 31 anddistance eiements 32 may aiso be iockabie such that a rigid fixture and probeconfiguration is obtained. The iocking between the probe holding eiements 31and the distance eiements 32 may aiso be obtained by means of dovetaii 1G 12 joints but other joints are equaiiy possibie. The distance elements 32 mayaiso be used for adapting and changing the geomatry of the fixture Bb.

The fixture 39 may aiternativeiy be fiaxibie enough to adept to differentgeometries of the structures 2 to be inspected, from fiat structures, curvedstructures or branched Structures, such that the probes may be positioned inthe same piane or aiong a non-pianar surface.

The fixture 30 with probes 4 may be portabie, such that it can bemoved on the structure surface covering different inspection areas d of thestructure 2 under inspection, or be moved from one structure to another byhand or by means of a robot.

The distance between neighboring probes 4 in a fixture 3G may varybetween 1G mm up to 199 mm, and is typicaiiy 30 to 8G mm.

The probe hoiding eiaments 31 hoid the probes 4 in such a way that anand surface of the probe 4 ensures fuii contact with the surface of thestructure 2 to be inspacted. A good repeatabiiity of the contact duaiity is aprerequisite for a good sensitivity of the method.

The probe hoiding eiernent 31 may comprise a biasing mechanism,such as a spring-ioaded mechanism (not shown), which aiiows the probe 4 tomove perpendicuiariy to the surface of the structure. in particuiar, it may bedesirabie to bias the probe towards the surface to be testad and preferabiywith a predetermined force. Thereby, aiiowing for appiication of a controiiedforce on the probe 4 as it is in contact with the structure 2. The springmechanism may aiiow a movement of the probs between 0.1 mm and 5 mm.

The uitrasonic generator 5 successiveiy drivas one probe 4 at the timeof the transmittinglreceiving probes 4 positioned in contact with the surface ofthe structure 2 as en uitrasound trahsrnitting probs generating an uitrasouhdsound wave propagating into a piane of the structure 2 in the inspection area6, as shown in Figs 4a and 4b and in Fig. 6. To create the uitrasound waves,the Lamb Waves or piate-iike waves, the frequency and wave iength areseiected based ort the eiastic properties of the materia! of the inspectedstructure 2 and on the thickness of the structure. The piate-iike structure 2 wiiithen act as a guide for the propagating Lamb waves. 2G 3G 13 The other probes 4, not used as the transmitting probs, are used in areceiving mode, detecting a resuiting wave provided by the uitrasound wavepropagating through the inspection area 6. The uitrasound wave propagatesin the structure and interact with the structures geometrica! features andpotentia! anornaiies 3 present in the structure. Anomaiies 3 in the structureaffect the sound wave propagating there through. in Fig. 7 it is exempiifiedhow signais are generated and detected by the probes 4 in the configuration.

The materia! of the structure under inspection may be composite.

For example couid the composite structure be a fiber reinforced poiymer.Deiaminations in composites do not refiect an incoming voiurne sound waveused in traditionai uitrasonic testing but affect the signa! propagating therethrough, i.e. with the system 'i shown in Fig. 1 such deiaminations may bedetected.

The system i comprises a signa! acquisition unit 7 which is arranged toacquire detected waves from the probes 4 used in receiving mode, and totransmit signais corresponding to the detected waves to a processing device8 of the system 1. The processing device 8 is arranged to compare signaisobtained from the acquisition unit 7 for a structure under inspection withsignais for a corresponding inspection area of a reference structure withoutanomaiies, providing a comparison index.

The reference signais may be obtained by using the system idescribed above on a separate reference structure or in a reference zone ofthe structure 2 under inspection. The reference structure or reference zoneshouid then have sirniiar geometry and be made of sirniiar materia! as theinspection area 6 of the structure 2 under inspection and be withoutanornaiies 3. For obtaining the reference signais a sirniiar probs configurationand simiiar inspection area shouid be used for the reference structure orreference zone as for the structure under inspection. The reference signa! canbe registered at the time of inspection or be pre-saved from an eariierinspection. Aiternativeiy, the reference signais may he obtained fromtheoretica! anaiysis of the structure 2 without any anornaiies 3 in theinspection area d. Such a theoreticai analysis may be a Finite EiementMethod anaiysis of the structure. 1G 29 3G 14 The processing device 8 guantifies a difference between the acquiredsignals and the reference signals by caicuiating a time shift between theacquired signals and reference signals, by caicuiating a difference inamplitude between the acquired signals and reference signals, aridior bycaicuiating a correiation of the signals. ln Fig. 4c the time domain reference signa! (solid line) for the referencestructure shown in Fig. 4a and an acquired signal (dotted line) for a teststructure with an anomaly 3 shown in Flg. db are piotted in the same graph.As can be seen in this graph, where the anomaiy is exempiified by adelamination, the signal shows a time shift AT and an amplitude variation AAdue to the fact that that wave Velocity and attenuation are localiy differentover the anomaly in the structure.

The difference between the reference and the test signal may bequantifled by calculating a so called damage index. The damage index (Di)can be calculated from time shift AT, amplitude variation AA, difference of thetime domain signal andlor through a correlation of the two signals.

One way of calculating a damage index (St) is using a correlationbetween the reference signals and acquired signals. Ali signals detected bythe probes 4 for the structure 2 to be inspected are saved in a test signalmatrix St,- and reference signais are saved in a reference signal matrix Ru. D!may then be calculated with the foliowing formula in;or e :gi ~ cs~i~i:i=fg¿.-,.r¿u,r} i . .

Other damage index expressions have been reported in the iiteraturefor Structure! Health Monitoring and can potentiaily be applied also here.

The system i comprises an anomaly identifying unit id, which uses thecalculated damage index to identify if there is an anomaiy 3 in the structure 2under inspection. For example if DE=AT or DE=AA is larger than zero this isindicative of an anomaly 3 in the inspection area 6 of structure 2. The system”i may further cornprise a visuaiization unit 'lt visuaiizing en identifiedanomaly 3 in the structure. 3G The visuaiization may comprise visuaiization on a screen 11 indicatingthe anornaiy 3 on the screen for exampie as a coior difference, or theanornaiy 3 being marked on the screen with a box, ring or arrow.Tomographic imaging is one exarnpie of such visuaiization. Aiternativeiy, ananomaiy 3 may be marked directiy in the inspection area 6 of the structureunder inspection by means of for exampie a beam of iight. in Fig. 5 is shown the principie of damage index computation betweena reference structure without anomaiies a) and b) test structure with ananomaiy 3. The differences in acquired signais from the test structure and thereference structure are correiated and a Di caicuiated. i-tere Di is caicuiatedc) using the formuia Bi=t-(CQV(X1, X2). X1 is an acquired signai matrix for thereference structure and X2 an acquired signa! matrix for the test structure. Anirnaging aigorithm is then based on a tomographic reconstruction of thevariation of the damage index in the inspection area defined by the probsconfiguration and iiiustrated in d), e). in Fig. 5 a non~pianar piate-iike structure 2 having a radios is shown.Anomaiy inspection of such structures is possibie as the transmitting probe 4is driven by the uitrasonic generator 5 to generate uitrasound waves whichhave the abiiity to propagate in a piate-iike structure 2 even when thestructure is not pianar, why inspection of a structure having a radius ispossibie. in Fig. 6 the uitrasound wave is generated by a probs 4 used intransmitting mode on one side of the radius, propagates through the radiusand is detected on the other side of the radius by other probes 4 used inreceiving mode. This configuration eiirninates the need for a scanning devicein the radius as the wave itseif probes the materiai in the radius.

The system t may be suitabie for detection of anornaiies in differentkind of features of the piate-iike structure under inspection as iong as thefeature is iocated in the area of inspection defined by the probe configuration.Such features inciude, but are not iimited to, areas with rivets, naiis, hoies,attached patches etc (not shown).

The performance of the anornaiy detection depends on the number ofprobes 4 and their position in the configuration, which consequentiy 2G 16 determines the number of paths used for reconstruction of the inspection area5 defined by the probes 4.

The fact that the system and method reiies on a comparison with areference structure impiies that differences such as thickness, stiffness orattenuation between the reference structure and the structure underinspection wiii be detected. The effect of these structurai differences has beenfound negiigibie for the toierances used for typicai aerospace compositeparts. in Fig. 5 a transrnitting/receiving probe 4 is shown from various angies.A tip area 12 of the probe 4 has a conicai shape, which conicai shape has acontact surface area 13 with a diameter d of at most 1/3 and between ti5 and“H3 of the waveiength of the generated wave. This requirement is iinked toexcitationidetection efficiency considerations. Due to the srnaii diameter of thecontact surface area 13 of the probe 4, the probe acts as a point source andgenerates an omnidirectionai uitrasound wave in the piataiike structure 2.

The probe 4 comprises a piezoeiectric active eiement 14 excited in itsthickness resonance mode. The piezoeiectric eiement 'i4 may be in physics!contact with the tip area 12.

When used in a transmitting mode, the probes may be driven a by theuitrasonic generator 5 to transmit uitrasound waves at a frequency of iessthan t itdi-iz, preferabiy 50 ki-iz-50Û ki-iZ, or 100 kHz-25G ki-tz. The frequencyrange used depends on the thickness of the structure under inspection. Theuitrasonic generator 5 may drive the probes 4 to generate and receive a tone-burst of t to 10 cycies.

The uitrasonic generator 5 may be a muiti-channei uitrasonic systemwith as many channeis as probes 4 used in the configuration, Fig. t.

The signa! acquisition unit 7 may transmit acquired signais in reai timeto the processing device 8. it is to be understood that the different parts of the system 1 describedabove and iiiustrated in Fig. t, i.e. the probes 4, the uitrasonic generator 5,the signai acquisition unit 7, the processing device 8, the anornaiy identifyingunit 10 and the visuaiizing unit tt couid be separate parts communicatingwith each other. Aiternativeiy, some or more of the parts may be integrated as 1? funetšene in ene n? the parts. Far exarnnåe annie the signa! aequšaätäen unit 'Fand the ušâraeenše generater 5 aa funetšena än the preeeeešng aevšee 8 er bešntegratea in eaen arena 4. The nreeeeeing device 3 eauåa eernpnae theanamaiy identšfyšng unit 1G and všauaiizäng unit 11 ae integrated funetšana.

Claims (20)

1. 3G 18 CLAliViS t. A rnethod of inspecting a plate-like structure (2) by means ofultrasound for detecting anomalies (3) in the structure, the method cornprisingthe steps of: a) positioning at least three ultrasonic transmitting/receiving probes (4) ini-iertzian contact with e surface of the structure (2) and in a pclygonaiconfiguration such that an inspection area (6) of the structure (2) is defined bythe configuration; b) using a first one of the prcbes (4) in a transrnitting mode to generate anultrasound wave propagating into a piane of the structure in the inspectionarea (6); c) using at least a second one of the probes (4) in a receiving mode todetect a resuiting wave provided by the uitrasound wave prcpagating throughthe inspection area (6); d) repeating steps b) and c) using another one of the probes (4) as thefirst probs; e) acquiring signals corresponding to waves detected by the secondprobes (4); f) contparing the acquired signals with reference signals for acorresponding inspection area of a reference structure without anontalies.

2. The method of oiairn t, vvherein the step of positioning theprobes (4) in contact with the structure (2) comprises providing a liquidbetween the structure surface and the probes.

3. The method of claini t, wherein the step of pcsitioning theprcbes (4) in contact with the structure (2) cornprises positioning the probes indirect dry contact with the structure.

4. The method according to any of the preceding ciainis, whereinthe step of positioning the probes (4) in a polygonal configuration comprisesarranging the probes in a probe fixture (30) having a predeterrnined 3G 19 configuration prior to positioning the probes in contact with the surface of thestructure (2).

5. The method according to any of the preceding ciairns, whereinthe step of positioning the probes (4) in a poiygonai configuration comprisespositioning the prohes in a suhstantiaiiy poiygonai or semi-poiygonaiconfiguration seiected from a group comprising a trianguiar configuration, arectanguiar configuration, a square configuration, a diamond-fikaconfiguration, a pentagonai configuration, a hexagonai configuration, aheptagonai configuration, an octagoriai configuration, a nonagonaiconfiguration, a decagonai configuration, a poiygonai configuration with 11-15nodes, a poiygonai configuration with 15-25 nodes, a poiygonai configurationwith 25-35 nodes, or a poiygonai configuration with 35-56 nodes.

6. The method according to any of the preceding ciaims,comprising causing the prohes (4) to generate, when used in a transmittingmode, uitrasound waves at a frequency of iess than 1 iVii-iz, preferabiy 50id-tz-šüü ktiZ, or 100 kHz-Eou kHz.

7. The method according to any of the preceding ciaims,comprising causing the prohes (4) to generate and receive a tone-hurst of 'ito 1G cycies.

8. The method according to any of the preceding ciaims,comprising causing the probes (4) to generate, when used in transmittingmode, uitrasound waves comprising a fiexurai mode.

9. The method according to any of the preceding ciaims, whereinthe reference signais for a reference structure are obtained hy using themethod steps a)-e) of the method of ciaim 1 for a separate reference structureor a reference zone of the structure under inspection.

10. The method according to any of the ciaims 1-8, wherein thereference signals are obtained from theoretical analysis of the structurewithout any anornalies in the inspection area. tt.the step of comparing the acquired signals with the reference signals The method according to any of the preceding clairns, wherein comprises quantifying a difference between the acquired signals and thereference signals by oalculating a time shift between the acquired signals andreference signals, calculating a difference in amplitude between the acquiredsignals and reference signals, and/or calculating a correlation of the signals. 12. The method according to any of the preoeding claims furthercornprising a step of identifying an anornaiy in the inspection area based onthe comparison in step f) of the method. 13.comprises a step of visualizing an identified anomaly. The method according to ciairn 12, wherein the method further ”i4. A system (t) for inspecting plate-like structures (2) usingultrasound, the system comprising: - an ultrasonic generator (5); - at least three ultrasonic transmitting/receiving probes (4) connected toand driven by the uitrasonic generator (5), the probes (4) being arranged in apolygonai configuration in a fixture (30) in such a way that when the probesare positioned in contact with a surface of the structure (2) an inspection area(d) of the structure is defined by the configuration; -the ultrasonic generator (5) being arranged to successively drive oneprobe (4) at the time as an uitrasound transrnitting probs generating anultrasound wave propagating into a plana of the structure in the inspectionarea (6), while at least one of the other probes (4) is arranged to be used asan ultrasound receiver detecting a resulting wave provided by the ultrasoundwave propagating through the inspection area (6); 2G 3G 21 ~ the system (t) further comprising a signai acquisition unit (T)arranged to acquire detected waves from the probes (4) and to transmitsignais corresponding to the detected waves to a processing device (8) of thesystem (t), -the processing device (8) being configured to compare signais fromthe acquisition unit (7) for a structure under inspection with signais for acorresponding inspection area of a reference structure without anomaiies,providing a comparison index. 15. The system (1) according to ciaim 14, further comprising ananomaiy identifying unit (1 G) arranged to identify any anomaiy in the structure(2) under inspection based on the comparison index. 16. The system (1) according to any of the preceding ciaims 14 or15, wherein the fixture (36) comprises prooe hoiding eiements (31) hoidingthe probes in the fixture (âü). 17. The system (1) according to ciairn 16, wherein the prohe hoidingeiement (31) oomprises a hiasing mechanism aiiowing for perpendiouiarmovement of the probes heid by the fixture reiative to the structure surface. 18. The system (1) according to any of the preoeding ciaims 14-172wherein the probs (4) comprises a piezoeiectric active eiement (14) excited inits thickness resonance mode. 12. The system (1) according to any of the preceding ciaims 14-18,wherein a tip area (12) of the prohe (4) for oontacting with a surface of thestructure (2) has a conicai shape, which conicai shape has a contact surfacearea (13) with a diameter of at most 1/3 of a waveiength of the generateduitrasound wave. 22 2G. The system (1) according to any of the preceding ciaims 14-19,whereån the system (t) further cernpršses a våsuatizetion unit (11) visuaiizšngan identified anernaiy of the structure (2) under inspection.