detachable active array hehi

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285 Detachable Active Array Head (DAAH): A Proposed Solution to Help the Proliferation of Phased Array Manual UT François Mainguy 4975 rue Rideau, Suite 140 Québec City, Québec, G2E 5H5 – Canada +1 (418) 683-6222 x105; fax +1 (418) 683-7032; e-mail [email protected] ABSTRACT As the price of portable phased array instrumentation drops, the current phased array probe technology doesn’t allow for a major cost reduction, to a point where the proliferation of phased array for manual UT may be limited or impossible. The current manufacturing process is presented to better understand where the high costs come from. A Detachable Active Array Head (DAAH) technology is proposed to solve many problems related to manual phased array UT, but also to semi-automated and automated UT. Results show that the technology has a great potential to become the long-awaited solution for phased array massive adoption. INTRODUCTION Since the early 1990s, the interest in phased array technology and equipment increased in the NDT community. Though the price of the instruments has constantly decreased in the past years, the price of phased array probes hasn’t followed the same price trend. The cost of good transducers today is approximately 20% of the instrument price (Figure 1). Figure 1: Evolution of the probe-to-instrument price ratio (rough estimates). Both the price and delivery time have been causing problems to the industry, which has limited the proliferation of phased array technology. The major issue is that most potential users will invest in phased array once they have had the proof their application can be solved. Of course, one or several probes are needed for the proof. On a second order, the actual owners of phased array instrumentation are slowed down in the process of resolving new applications because it takes a long time for them to acquire new probes. Amongst the current limitations to the proliferation of manual phased array inspection, probe supply is now the most critical one. With the cost of portable instruments nally decreasing, it’s now becoming a serious subject the industry should resolve. At the root of the problem is the array technology itself which is much more complicated and expensive than the mono-transducer technology. Another problem is the context in which phased array has been evolving in for the last 15 years. For the entire 1990s it remained a very expensive technology that most NDT departments couldn’t afford. In most cases, the “lucky ones” had enough money for phased array equipment within a strategic technology surveillance program or research and development funding. Quite naturally, those programs are driven by researchers and top-managers, who are often removed from daily pragmatic considerations. Still, the latter have enjoyed the benets of working with phased array technology because of its power to resolve applications. Due to the fact they had plenty of budget, only few compromises were made in the probe design. In the past 15 years, we have seen array probe designs that are no less than exotic: rho-theta probes, segmented annular arrays, exible probes, Fresnel arrays, and much more. They have been reported in numerous papers and conferences. ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. © Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

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  • 285

    Detachable Active Array Head (DAAH):A Proposed Solution to Help the Proliferation of Phased Array Manual UT

    Franois Mainguy4975 rue Rideau, Suite 140

    Qubec City, Qubec, G2E 5H5 Canada+1 (418) 683-6222 x105; fax +1 (418) 683-7032; e-mail [email protected]

    ABSTRACTAs the price of portable phased array instrumentation drops, the current phased array probe technology doesnt allow for a major cost reduction, to a point where the proliferation of phased array for manual UT may be limited or impossible. The current manufacturing process is presented to better understand where the high costs come from. A Detachable Active Array Head (DAAH) technology is proposed to solve many problems related to manual phased array UT, but also to semi-automated and automated UT. Results show that the technology has a great potential to become the long-awaited solution for phased array massive adoption.

    INTRODUCTIONSince the early 1990s, the interest in phased array technology and equipment increased in the NDT community. Though the price of the instruments has constantly decreased in the past years, the price of phased array probes hasnt followed the same price trend. The cost of good transducers today is approximately 20% of the instrument price (Figure 1).

    Figure 1: Evolution of the probe-to-instrument price ratio (rough estimates).

    Both the price and delivery time have been causing problems to the industry, which has limited the proliferation of phasedarray technology. The major issue is that most potential users will invest in phased array once they have had the proof their application can be solved. Of course, one or several probes are needed for the proof. On a second order, the actual owners of phased array instrumentation are slowed down in the process of resolving new applications because it takes a long time for them to acquire new probes.

    Amongst the current limitations to the proliferation of manual phased array inspection, probe supply is now the most critical one. With the cost of portable instruments fi nally decreasing, its now becoming a serious subject the industry should resolve. At the root of the problem is the array technology itself which is much more complicated and expensive than the mono-transducer technology.

    Another problem is the context in which phased array has been evolving in for the last 15 years. For the entire 1990s it remained a very expensive technology that most NDT departments couldnt afford. In most cases, the lucky ones had enough money for phased array equipment within a strategic technology surveillance program or research and development funding. Quite naturally, those programs are driven by researchers and top-managers, who are often removed from daily pragmatic considerations. Still, the latter have enjoyed the benefi ts of working with phased array technology because of its power to resolve applications. Due to the fact they had plenty of budget, only few compromises were made in the probe design. In the past 15 years, we have seen array probe designs that are no less than exotic: rho-theta probes, segmented annular arrays, fl exible probes, Fresnel arrays, and much more. They have been reported in numerous papers and conferences.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 286

    Despite a valid theoretical justifi cation and very exciting conference presentations, their geometry and beamforming capabilities dont necessarily appear simple. Additionally when you start to include the odd example of phased array terminology like focal law and beamforming, the recipe is perfect to make one think nice, but not for me.

    Theres been of course a great deal of applications solved with simple linear 1D arrays, most of them providing a great benefi t when compared to other UT or NDT techniques. But they also suffered from the same science fi ction aura as perceived by most NDT practitioners worldwide. NDT fi eld practitioners are, to a large extent, down-to-earth, time-pressurized individuals who dont necessarily value new technology when results are not readily available. They have limited spare time for designing and qualifying array search units and they often lack experience to write procedures based on phased array apparatus. It is also a sad truth that NDT, at least in North America, is struggling with lack of qualifi ed personnel and very tight budgets, therefore the natural reaction towards phased array is Well, looks pretty sleek, but I dont have much time or money to explore this further. Plus Im not even sure I want to pay so much money for a single probe!

    Without a doubt, it is essential to convince manual UT operators that phased array is an excellent technology, but manufacturers need to speak their language and eliminate as many barriers as possible.

    In average, the mono-channel fl aw detectors sell from 7,000 to 14,000 USD, while mono-transducers sell from 200 to 500 USD. The transducer-to-instrument ratio is roughly 3%. The current ratio for manual phased array is about 15 to 25%. An array transducer is currently about 10 times more expensive than a mono-transducer and this is not acceptable to most users. It is true that a single phased array probe can replace a few mono-transducers, but most customers have a psychological barrier with such expensive probes.

    Furthermore, there are 5000 to 7000 manual fl aw detectors sold each year, worldwide (excluding China). Phased array sales are grabbing only a minor fraction of this market. Immense marketing efforts were made around the systems themselves, promoting hardware specifi cations and software features. There are now about 4 proactive competitors in portable phased array and about 10 in semi-automatic or automated phased array. As the market evolves, more and more competitors compete, and the marketing will increasingly become more focussed on specifi cations and price. This is the natural evolution of a market when in transition into a stage of maturity.

    But is the market ready for maturity? It is strongly believed that the market cant wait any longer to have affordable and standard probes [1].

    This paper will fi rst summarize what phased array is all about. Then it will discuss the technical diffi culties of array manufacturing and the different cost issues. It will also present the current typical workfl ow for solving applications and the impracticality of it. Lastly, it will propose a new workfl ow based on a new probe technology, for which results are provided.

    PHASED ARRAY UT

    Phased Array BeamformingOver the past years, many papers have presented the underlying principles of phased array technology. Following is a quick overview.

    A piezoelectric crystal is split into many small elements, each individually driven by a pulser-receiver circuitry. On transmission, elements are excited with a high-voltage pulse, but at different moments (different phases). The phase pattern adjusts the relative propagation of wavefronts. It is creating a transmit beam that is just like any mono-transducer beam from a catalogue probe. The beam has a focal spot, a beam width, a depth-of-fi eld, a divergence, and of course an angle (Figure 2). The electronic activation and phasing of elements can quickly change the position, the angle and the focus of the beam, which are the most attractive advantages of phased array technology.

    On reception, a synthetic beam is created by the use of programmable delay lines on each channel (

    N in Figure 2) and a summation. Sometimes, channel deactivation and

    Figure 2: A phased array beam is just like any other beam, with same defi nition of beam width

    and depth-of-fi eld.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 287

    apodization (N in Figure 3) are used to weigh the contribution of elements from extremities. The desired effect is to reduce

    side-lobes and extend the depth-of-fi eld, while enlarging the beam width.

    When refl ecting on material discontinuities, the backpropagating echoes are collected by all elements and the parallel delay lines act like a spatial fi lter. It means that echoes coming from a specifi c location - the focus point - will emerge at the same time at the end of the delay lines thus maximizing the sum amplitude. Off-focus echoes will be out of phase and will produce a weak summation, barely distinguishable from the noise. This process is known as delay-and-sum beamforming to which a spatial fi lter rejection ratio can be associated. As an example, a linear array probe can lead to about 60 dB rejection ratio in the lateral vicinity of the focal spot, which can provide very sharp focusing. It is good to recall that phased array beamforming can only carve the focusing within the near fi eld of the overall array aperture. In the farther range, only defl ection remains controllable. At the time of writing this paper, the available portable phased array instruments on the market feature a delay-and-sum beamformer ranging from 16 to 32 active channels, with a multiplexing stage to address 64 or 128 elements.

    Imaging CapabilitiesPhased array can provide two real-time images, both based on several A-scans that are generated sequentially by the beamformer. If the beamformer is programmed to sweep the beam angles (i.e. 35 to 70 degrees in shear waves), a sectorial scan image will be displayed on screen (Figure 4).

    This image is also known as S-scan and its well known from the medical industry. However, if the beamformer is programmed to move the beam linearly at a constant angle (i.e. 30 degrees in longitudinal wave), a parallelogram image will be displayed on screen. This image is known as L-scan for Linear scanning in opposition to sectorial scanning, and its also very much used in medical and veterinary applications. Some NDT manufacturers refer to the latter as E-scan for electronic scan, but it leaves open a debate as to what is performed electronically: phasing or activation?

    Most portable phased array instruments are able to produce more than one real-time image, a feature known as multiscan. In this case, the beamformer is able to manage multiple hundreds if not thousands of beam parameters, also known as focal laws.

    Both S- and L-scans present color-encoded A-scan amplitudes that are juxtaposed (Figure 4). It must always be noted that each displayed A-scan line is in fact the integration of the energy collected from the acoustic sensitivity fi eld. Therefore is most likely to include echoes coming from refl ectors not on the nominal beam orientation, mainly because of beam width and potential side effects (i.e. side lobes, mode conversions, etc.) [2].

    Figure 4: Sequential beams are generated and produce A-scans. A-scans are collected, color-coded, and juxtaposed to generate a

    sectorial scan image.

    It should also be pointed that both on S-scan and L-scan images, the phased array system can extract and display a particular A-scan corresponding to a specifi c beam. From the latter conventional UT views can be generated like B-scan, D-scan, C-scan, strip charts, plus projected views such as top, side, and end [3]. In fact, most commercial systems will allow many A-scans to be extracted at once.

    Figure 3: Delay-and-sum beamforming.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 288

    Imaging ApproachesPhased array leads to three different approaches for scanning a weldment volume. The fi rst method is to use a shear wave S-scan imaging. The second is shear and/or longitudinal wave L-scan imaging. The third is zone-discrimination using D-scan imaging and/or strip charts, as proposed by ASTM E-1961 for automated ultrasonic testing of girth welds [4]. The latter uses phased array to generate multiple discrete beams that will impact the weld bevel areas using a pulse-echo or a pitch-and-catch method. This is using phased array without its full imaging capabilities of continuous sweeping, it also requires a constant and precise positioning of the probe away from the weld. For those reasons, this method is not considered manual per se and it falls outside the scope of this paper.

    Most, if not all, phased array techniques refer to weld length axis as the scan or D-scan axis. The distance from the weld is referred to the offset, or surface distance, or the B-scan axis (Figure 5).

    Figure 5: Defi nition of D-scan weld length axis (dotted arrow) and L-scan/S-scan surface distance axis (plain arrow).

    The search unit is moved manually in different patterns, depending on the sound coverage of the imaging, as well as the related probability of detection (PoD) [5]. Of course, both sides of a weldment should be scanned. For thin components using S-scan, a single line pass could be enough, like the dotted line on Figure 5. For thicker components, most will use multiple line passes, away from the weld [6], or use a raster scan technique, as with a mono-transducer. Of course, the greatest advantage of manual imaging is the fact the operators dexterity is not as crucial to see the fl aws, and therefore the scanning speed is higher.

    Common ErrorsIt is very common to see arrays that are oversized for the need of the application. The fi rst consequence is an unstable couplant layer. This can sometimes be solved using forced couplant irrigation. The other consequence is the use of too many elements to perform sector scanning. The number of active elements defi ne the active aperture and therefore the beam width and depth-of-fi eld. It is very common to see arrays with 2 or 3 times the active surface than what would have been used as a mono-transducer to solve the application. The result is over-focusing with a very short depth-of-fi eld, turning into blindness in most of the depth of interest. This extremely sharp focusing is sometimes nice, and it also shows clearly an advantage of phased array. But rarely it is desirable in the context of manual UT like weld inspection. Keeping the number of elements to the minimum will keep the cost reasonable. The last consequence is that a bigger probe will inevitably position the index points away from the weld, therefore the region of interest couldnt be inspected with the direct incidence of steep angles. Second leg will have to be used.

    Another common mistake is to count on circular geometries to solve applications, like rho-theta and segmented annular arrays. It is perhaps the best geometries to do beamforming with because the symmetry of revolution will provide nice acoustic beams in 3D, without edge effects. It is the very reason why mono-transducers are often circular. But to manufacture separate disk elements or a circular array geometry is a lot more diffi cult and expensive than fabricating rectangular ones. Since cost is always an issue for manual UT, those geometries shall be considered for special, well funded projects only, at least for now due to the state-of-the-art. The next section explains the array manufacturing process.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 289

    ARRAY MANUFACTURING

    ReviewThe vast majority of the market is currently for 1D linear arrays between 1 and 10 MHz. The pitches are generally higher than 300 m and the kerf (inter-element spacing) is normally in the range of 50 m. Most probes manufactured so far feature less than 128 active elements. EPRI and a few other players have been successfully using 1.5D (i.e. 8 x 4) or 2D arrays, showing the clear advantage of exploiting the second dimension for skewing the beams [7]. Despite the superiority of results and the reusability of matrix probes for other applications, their cost and high-channel count have made most people reluctant to follow this option.

    CostProbes are still expensive and take a long time to manufacture. As an example, a 32-element probe at 5 MHz with 65% relative bandwidth is approximately 4000 to 5000 USD and 5 weeks delivery under normal times, in North America. The retail price is determined by the following cost implications:

    1. Connector,2. Multi-coaxial cable length,3. Man-hours for assembly of cable and connector,4. CAD time and machining of the probe frame,5. Material processing,6. Quality-assurance measurements and report,7. Distribution fees,8. Profi t.

    The connector (1), the cable (2), the machining (4) and the material processing (5) have a cost that is diffi cult to decrease other than by high quantity production. The quality-assurance process varies a lot from one manufacturer to another. When performed manually, it can be a lengthy process and therefore it will impact the pricing negatively. The following lines describe the major steps of array manufacturing, without going into detail.

    Material Processing1-3 piezocomposite is the most popular material used to design high-performance ultrasound arrays with. Its been used for almost 25 years in medical arrays and for about 15 years in NDT.

    A cylindrical bar is made of piezo-ceramic powder like PZT5H (lead zirconate titanate). A diamond saw is used to cut very thin slices of the bar. The thickness of the slice is usually half the wavelength of the desired central frequency, including a correction factor.

    In most cases, the slice is diced into miniature rectangular pillars using a diamond saw with orthogonal passes (Figure 6). The size and spacing of the pillars are such that an integer number of pillars, used in cluster, will become an element of the array. It should be said that this technique called subdicing is used to maximize compression wave generation while reducing other wave modes that are incombant to the array performance. A polymer will be poured in to fl ood the inter-pillar spacing. Various polymers can be used like urethane. The acoustic properties of the piezocomposite can be adjusted by setting the volume fraction of the polymer (Figure 7). Parametric fi nite element simulations with tools like PZFlex can prove to be very practical to avoid trial and error [8]. This whole process is known as the dam-and-fi ll technique [9].

    The processed slice is then polished on a lapping or grinding machine until the exceeding polymer is removed, until a very precise thickness and surface fi nish are obtained. The process requires a very high parallelism in order to ensure the same thickness and therefore frequency of resonance.

    The next step is to perform metallization of the slice surfaces. The evaporation or sputtering process will deposit a thin layer of gold that will act as an electrode. Gold will be deposited in the sub-diced channels, therefore causing a short-circuit of all elements. A very precise

    Figure 6: The sub-dicing process improves the

    electro-acoustic performance.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 290

    laser-vaporizing process is normally used to isolate the individual elements of the array. The slice is then machined to create one or many probe apertures. The actual machined area will be larger to include dead elements on the perimeter to avoid aperture side effects.

    Impedance MatchingThe next step is to add acoustic matching. The inner impedance of the piezocomposite is in the range of 15-20 MRayls while the impedance of typical Rexolite wedges is in the 2.5 MRayls range. One or two quarter-wavelength (/4) layers should be added to help the acoustic energy transfer into the wedge material, to create a smooth transition to the ten-fold impedance mismatch. The matching layer(s) play a crucial role in determining the resonance of the elements (frequency and damping) [10].

    The aperture is then cleaned and put into a production gig to avoid damaging the thin piezocomposite substrate in the numerous subsequent steps. The last step of material processing is to pole the substrate. A high DC voltage is used to create a high-intensity electric fi eld. The piezoelectric dipoles will be aligned permanently in the axis of electric fi eld, perpendicular to the electrodes.

    InterconnectThe next step is to electrically connect the electrodes. For high-end processes, a fl ex circuit will be used to link the electrodes to one or many interconnect printed circuit boards (PCB), on which individual micro-coaxial wires will be soldered. The micro-coaxial wires will have a gauge typically between 36 to 42 AWG, which is extremely small and requiring a protective epoxy poting.

    At the other end, the micro-coaxial wires are soldered to another set of interconnect PCBs that will link to the multi-pin connector which will mate to the ultrasound instrument. As soon as some connectivity is available, a capacitance check is performed on the entire array of elements in order to track for bugs such as short-circuits or high cross-talk. The consistency of results along the array shall be observed at this point. The critical measurements are element sensitivity, central frequency and bandwidth. Defective elements may imply to abandon further array processing.

    At this point, we call the assembly an acoustic stack. Again, consistency along the array must be monitored. Figure 8 shows a piezocomposite assembly with 7 pillars per element electrode (lateral dimension), featuring a single matching layer.

    Figure 8: A typical 1-3 piezocomposite with a single quarter-wavelength matching layer.

    Figure 7: A polymer is fi lling the inter-pillar spacing. Thats where the

    composite word comes from.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 291

    PackagingThe last mechanical step is to insert the acoustic stack and its interconnect into the probe frame. The volume between the back of the acoustic stack and the frame top is fi lled with damping material which is normally an epoxy mixture that is optimized for maximum attenuation of back-propagating sound waves and acoustic matching with the stack. This backing material will fi ne-tune the electro-acoustic response of the array elements. The backing plays an important role in the pulse compression (or bandwidth) of a transducer. The epoxy potting will also protect all the micro-coaxial wires and the fragile soldering work, while providing a stiff support for the acoustic stack while in operation.

    Quality-AssuranceThe last step is to validate the overall performance of the array transducer. Typically, all elements are monitored for their frequency, bandwidth and sensitivity. A report is created and a print version is provided with each delivered unit.

    OBSERVATIONSThe described process may differ from one manufacturer to the other, but in essence, the previous steps are used in the fabrication of phased arrays for NDT. One can predict that such a process infers a lot of manual operations requiring dexterity and time. It also often leads to defective arrays, which will give rise to a higher retail price for the functional units. The yield is particularly bad when a new combination of pitch, frequency, bandwidth and impedance matching is designed. Therefore, the industry would benefi t in standardizing the pitches and frequencies, being able to reuse them in many array designs.

    Another dead cost in each probe is its interconnection. Its role is simply to link the instrument to the piezo-elements. Clearly, it has no value-added in an inspection. Interestingly, it represents a substantial portion of the probe manufacturing costs, essentially because it takes time to solder micro-coaxial cables one by one. Unfortunately, low-cost countries are not interested in the quantities involved in NDT. They can barely cope with the demand and quantities in medical.

    Medical array probes are produced well above 100,000 units per year and have a cost of approximately 500 to 1000 USD. It is unlikely that the NDT array probe production will some day reach this level. At the same time, NDT array manufacturers have always said the cost of probe would drop only when the quantities would be signifi cant. Standardization is obviously required in order to yield increased quantities in manufacturing.

    Due to the state-of-the-art and to fi nancial reasons, the only way NDT could benefi t of much cheaper probes is to restrict the number of acoustic stack designs and by keeping the interconnect separate from the active piezo-elements. Another possibility is to simply reduce the profi t. The problem is that the manufacturers gross margins will become too tight and there will be an impact in the long term such as poor quality-assurance or poor service.

    CURRENT WORKFLOWThe most common workfl ow to solve an application with phased array is the following (or something very similar).

    The user:1. Describes the geometry of the component to inspect.2. Reports any previous experience with monotransducer ultrasound on the component, either as a trial or real inspection.

    Then the application-solver:3. Depicts the insonifi cation requirement.4. Depicts the array design.5. Depicts the wedge design.6. Verifi es the practicality of the array and wedge design, and a redesign may be compulsory.7. Tries to get access to a probe and a wedge of comparable specifi cation in order to validate the insonifi cation and

    procedure. If not available, jump to step 9.8. Optimizes the probe and wedge design.9. Orders the probe and wedge from manufacturer.10. Uses the new probe and wedge to validate the insonifi cation and fi nalizes the procedure.11. Re-designs the probe and wedge to better fi t needs, if ever required. (Repeat steps 9 and 10.)12. Finalizes and publishes procedure.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 292

    And lastly, the user:13. Proceeds with on-site inspection with procedure.14. Provides feedback to application-solver (is there a better term for this?) on the quality of results and eventual

    improvements to probe and wedge design.

    Although steps 1 to 3 are relatively quick and inexpensive for most cases, steps 4 to 12 represent a long, tedious, and expensive adventure, especially for the non-initiated. It should be noticed that many times, multiple probes are required to solve an application, perhaps discouraging the user to pursue the work adequately. A new probe technology is proposed in order to simplify this workfl ow.

    NEW PROBE TECHNOLOGYThe new probe concept being proposed consists of standardizing as much things as possible in a probe design. Looking at previous section III.b), the connector (1), the cable length (2), the man-hours of assembly (3), and the mechanical components (4) can all be standardized very easily. A study of all popular arrays and applications can also lead to some standardization of the acoustic stacks (5). Automation can also decrease the cost of quality-assurance report generation (6).

    The new concept is a search unit that has low-cost, Detachable Active Array Heads (DAAH) of standard connectivity and footprint. The heads are available in two versions. The fi rst version features the fl at array itself with holes for external wedge mounting. The second version features an integral wedge. The integral wedge can be made with the proper cut angle for longitudinal or shear wave generation, with or without contouring.

    The heads are designed to be rugged and as compliant as possible with popular manual UT codes. The goal is to offer a low-cost, industrial, hassle-free phased array solution to very common UT inspections, without an absolute need for customization. The new technology has several advantages over the current monolithic approach (all-in-one cable and array).

    The fi rst advantage is to lower production costs and therefore it allows suppliers to build up inventory in many geographical locations, closer to the end-user. It allows the retail price to drop from 20% down to 5 to 8% of the portable instrument value.

    The second advantage is to reduce the cost of the inspection solution. It saves both the cost of the search unit, but also avoids spending precious time in the design and qualifi cation process of an array.

    The new concept leads to a third advantage. The user could try, at low cost, different probe heads to solve an application. Once the user fi nds the probe head that works best, an optional optimization phase can be started in order to get a fi nal search unit design.

    Figure 8: The different parts of a DAAH search unit. Figure 9: The DAAH technology allows for integrated interconnect solutions such as Y-splitting, keeping the

    heads available for other interconnect contexts.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 293

    A fourth advantage is that the adaptor can be declined with different instrument connectors like Hypertronics, ITTCannon, TCZIF, and I-PEX (formerly known as MajorLeague). Therefore, the users can invest their money in the acoustics without being tied to a certain instrument manufacturer.

    The fi fth advantage is about the interconnect. The concept reduces the number of insertion cycles in the instrument, which may reduce the risk of damaging its pins. Also, adaptors can be produced for multi-head setups, such as Ysplitters, therefore avoiding multi-level connectivity that is bulky for on-site inspections.

    The new concept leads to a sixth advantage. The standard footprint leads to the standardization of external wedge footprint and scanner attachment methods, which is currently a bit hectic and vendor-specifi c.

    The concept by itself is rather simple. For decades, monotransducers have been detachable from the cable. The proposed design is just a phased array equivalent, as shown in Table 1.

    The only perceivable disadvantage is the limitation imposed by the footprint size. For the sake of ruggedness, the footprint is limited to a discreet size in which elements should be contained. This can be circumvented by the creation of several footprint families.

    The new concept aims at the 80/20 rule, where models and families should be created for 80% of the market needs. There will always be a need for custom array designs with special pitches, frequencies and mechanical dimensions.

    The fi rst footprint family (Type 1) allows for a certain degree of code compliance (Table 2).

    Table 1: DAAH concept compliance with mono-transducer.

    Table 2: Code compliance of Table 1 DAAH as per popular manual UT codes.*

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

  • 294

    NEW PROPOSED WORKFLOWThe new probe technology allows for a new proposed workfl ow described hereunder.

    The user:1. Finds any previous experience with monotransducer ultrasound on the component, either as a trial or real inspection.2. Orders a detachable probe head with specifi cations and/or performance that are close to the monotransducer(s).3. Uses the probe head to validate the insonifi cation and fi nalizes the procedure, if results are satisfactory. If not, workfl ow

    continues with step 4. Then the phased array expert:4. Re-designs the search unit to better suit end user needs, if required. The new search unit may be designed as a custom

    probe head or a monolithic probe.5. Finalizes and publishes procedure.

    And lastly the user:6. Proceeds with on-site inspection using procedure.7. Provides feedback to the phased array expert on the quality of results and eventual improvements to the search unit

    design.

    The new technology should allow for steps 1 to 3 to be accomplished within 2 to 3 weeks realistically. In comparison, steps 1 to 9 of the normal workfl ow (IV) takes generally 7 to 10 weeks in practice. Major sites and serious customers may also want to build their own DAAH inventory, as they already do with mono-transducers. This would allow urgent inspection needs to be met with phased array. It is believed the new workfl ow will enable higher levels of reactivity for the customers projects, providing a new reputation for quick turn-arounds, simplicity and low-cost to phased array manual UT.

    RESULTSMany prototypes were fabricated and at the time of publishing this paper, the production units were available in different frequencies, pitches and wedge integrations. In this section, well talk about the results obtained for the 5 MHz array model T1-PE-5.0M32E0.8P. The array has 32 elements and a pitch of 0.8 mm, with an elevation of 12 mm.

    The array had the following targeted performances:1. Central frequency of 5.0 MHz +/- 0. 5.2. Bandwidth of no less than 80% for all elements.3. Maximum sensitivity deviation of 1.5 dB.4. Electro-acoustic performance.

    The prototypes were tested using an automated test bench that records all impulse responses and measures all the important characteristics of each element. The probe was installed on a Rexolite block and echoes from the 25 mm backwall were analyzed. Figure 10 shows the consistency among elements for frequency, bandwidth and sensitivity.

    Figure 10: Uniformity of the elements.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

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    The results obtained surpassed the design goals. Four probe units were tested and the following results account for all of them. The average frequency was 5.00 MHz with maximum 0.1 MHz deviation for the worst element. The impulse response was virtually identical for all elements. The average bandwidth was 100% with no less than 98% for the worst element. The sensitivity of each element was within 0.42 dB of the average for all four probes, which is near the best the state-of-the-art can offer.

    Figure 11: A typical impulse response from an element array.

    ImagingThe prototypes were tested with a shear wave wedge for imaging on carbon steel blocks. Unlike the medical industry, there is no reference imaging block for NDT. An AWS resolution block was used as it minimizes the imaging artifacts due to bouncing waves on close refl ectors.

    The 60-degree cluster of three side-drilled holes was imaged with an optimized focal point located on the center hole. The sector scan was set to 0.25 degree of resolution and it was zoomed in. Table 3 compares the DAAH results obtained with the results obtained with a well-known probe manufacturer. The DAAH prototype needed 14 dB less gain to provide the same A-scan amplitude. The latter probe didnt have the exact same aperture size, so the relative sensitivity should be computed.

    In this case, the DAAH prototype showed a better sensitivity of 10.4 dB. For applications with high-attenuation materials or when tip diffraction is essential, such a boost in sensitivity is very helpful.

    We also compared the sweep range on an IIW block. The DAAH prototype had a -6 dB sweep range from 34 to 78 degrees, in shear waves. The peak amplitude occurs at 45 degrees. It showed excellent sensitivity at 45, 60 and 70 degrees, which are the most common angles used in manual UT.

    Table 3: Imaging compared with a well-known manufacturer.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.

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    Figure 12: The -6 dB sweep range goes from 34 to 78 degrees (SW).

    RuggednessThe design was targeting an industrial protection ranking of IP-66. The mechanical design of the mating socket and head had to be resistant to fi ne dust and high-pressure water jets.

    There was no apparent variation of the above results after exposing the units to this environmental stress. We also tried a full immersion at 15 cm with success. The unit was also drop-tested on a concrete fl oor from 3 meters of altitude. The mating of the socket and the detachable head remained perfect and the only perceivable effect was an aesthetical degradation of the casing.

    Figure 13: The DAAH sealing was tested in immersion at 15 cm.

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    CONCLUSIONThe new Detachable Active Array Head (DAAH) technology is very promising. The detachable acoustic head becomes the through value-added of the search unit and provides freedom in the selection of the instrument model and manufacturer.

    Among the numerous advantages of the new concept, the low price and quick availability should induce a new reputation of affordability and simplicity to phased array manual UT. The technology performs as well as the state-of-the-art can offer, showing electro-acoustic and imaging results that are among the best available currently.

    REFERENCES1. A Practical Proposal for Designing, Testing and Certifi cation Phased Array Probes Used in Nuclear Applications,

    J. Poguet et al., 4th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, London, December 2004.

    2. Multi-Zone Imaging, F. Mainguy, HARFANG Microtechniques inc., 9th European Conference on NDT (ECNDT), September 2006.

    3. Phased Array is not the solution to all problems, T. Armott, Lavender International, American Society of Nondestructive Testing (ASNT) - Fall Conference, October 2005.

    4. E1961 -1998, Standard Practice for Mechanized Ultrasonic Examination of Girth Welds Using Zonal Discrimination with Focused Search Units, American Society of Testing Materials.

    5. Phased Array and TOFD: When they Score, Where they Dont, M. Moles, R/D Tech inc., American Society of Nondestructive Testing (ASNT) - Fall Conference, October 2005.

    6. Use of Ultrasonic Examination in Lieu of Radiography, Section I and VIII, Division 1 and 2, ASME Code Case 2235-8, Cases of ASME Boiler and Pressure Code, October 2005.

    7. Procedure for Manual Examination of Pressure Vessel Welds from the Outside Surface Using Phased Array Ultrasonic Technology, G. Selby et al., Electric Power Research Institute (EPRI), Charlotte, May 2005.

    8. PZ Flex software, Weidlinger Associates Inc., contact Paul Reynolds and Robert Banks, Los Altos (CA).9. The Role of Piezocomposites in Ultrasonic Transducers, W.A.Smith, IEEE Proceedings Ultrasonics Symposium, 1989.10. Computer Modeling of Diced Matching Layers, G. Wojcik et al., IEEE International Ultrasonics Symposium

    Proceedings, 1996.11. AWS D1.1:2006, Structural Welding Code Steel, American National Standards Institute (ANSI).12. AWS D1.5:2002, Structural Welding Code Steel, American National Standards Institute (ANSI).13. BS EN1712:1997 Non-Destructive Testing of Welds Ultrasonic Testing of Welded Joints, European Standards.14. BS EN1714:1998 Non-Destructive Testing of Welds Ultrasonic Testing of Welded Joints Acceptance Levels,

    European Standards.15. API Recommended Practice 5UE, American Petroleum Institute, June 2005.

    ASNT Fall Conference and Quality Testing Show 2007 [Las Vegas, NV, November 2007]: pp 285-297. Copyright 2007, 2011, American Society for Nondestructive Testing, Columbus, OH.