Use of Flexible Ultrasonic Arrays in InspectionJocelyn Langlois
Sigma Transducers, Inc., Kennewick, WA. 99336
Exponent, 149 Commonwealth Drive, Menlo Park, CA 94025
Corresponding Author Contact:
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Ultrasonic inspection is frequently used to detect defects and degradation in structures during manufacturing, maintenance, and in-service operation. Traditionally, the inspection is performed using a single transducer either manually or with a robotic scanning unit. As regulations become increasingly demanding regarding scope and frequency of testing critical components, NDT professionals are challenged by problems including limited access, complex geometries, and harsh environments - making manual or robotic scanning difficult if not impossible. A remotely operated flexible ultrasonic transducer array represents an innovative and cost-effective solution to many of the problems associated with ultrasonic inspection of complex or hard to access components. The ultrasonic array is applicable to a wide variety of materials and structures for which ultrasound is the appropriate non-destructitest (NDT) method.
Employing a vacuum coupling technique, the flexible array provides the unique ability to conform to complex curved surfaces. Because flexible ultrasonic arrays are lightweight, portable and easy to use, they are particularly well suited to field inspection applications.
Promising applications of the ultrasonic array include the folowing; composites, which are formed into complex shapes to satisfy difficult design criteria, complex piping systems (including T's and elbows), and pipelines. For example, a flexible ultrasonic array can be permanently attached (under insulation) to high velocity piping systems to continuously monitor wall thinning resulting from flow assisted corrosion. The device can be used to periodically or continuously monitor the condition of a pipe in harsh environments such as radiation areas without manual application. Because the array elements are at fixed locations the inspection results can always be displayed as easy to interpret C-scan images. Significant benefits of the flexible array include ease of use, rapid and automated application and continuous and remote operation.
The flexible ultrasonic array consists of a two dimensional array of ultrasonic transducer elements. Figure 1 shows an ultrasonic array with 1024 .25 x .25 inch transducer elements arranged in an 32 x 32 element matrix, vacuum coupled to a 12 inch diameter pipe.
The dimensions of the active area of this array are 8 by 8 inches providing a total inspection area of 64 square inches. The geometry and ultrasonic characteristics of the flexible array can be designed and fabricated to meet specific inspection requirements. For example, the array used to inspect metal and thin composite structures operates at 2.25MHz-5MHz while the array used to inspect thick, highly attenuative composites operates at less than 1MHz. A flexible array can be built with a shape that conforms to the complex geometry of a part, such as a pipe elbow or leading edge of an aircraft wing.
Use of the ultrasonic array requires very little skill or training. The array is placed over the suspect area and the inspection performed by automatically pulsing each of the transducers in the array under computer control. Because the flexible array is vacuum coupled to the surface, couplant may not be necessary for relatively smooth surfaces. In some cases a light water spray is all that is needed. It was found in testing at Sigma transducers, Inc. that in some cases, particularly those using higher frequency arrays, that additional couplant may cause spurious signals which interfere with testing. The display and control unit, which is typically built into a portable computer, has a menu-driven operating system. The resulting digital ultrasonic data can be viewed as a C-scan, compared with a database of allowable defects on site, sent via telemetry for subsequent evaluation, and/or archived, whichever is appropriate to the specific situation.
An ultrasonic array inspection system represents an innovative solution to many of the problems associated with manual ultrasonic inspections using a single transducer . Full area coverage is guaranteed because the test area is inspected electronically by sequencing through array elements. Flexibility insures that individual transducer elements self-align to the component surface, thereby maximizing the amount of sound transmitted into the component. Uniform acoustic coupling across all the transducer elements is insured by use of a vacuum system or by external force to affix the array to the component. Since the array elements are at fixed locations, the inspection results can easily be displayed as time-of-flight and amplitude C-scan images. The ultrasonic array is quick and easy to use. Inspection time for an 8x8 inch array of 1024 transducers is under one minute. The ultrasonic array inspection system can be effectively employed in the factory, at maintenance facilities, and in the field.
Applications of the ultrasonic array inspection system include a wide variety of materials and structures for which convenient and rapid inspection of an area are required. Therefore, ultrasonic array inspection is particularly attractive for field applications. The ultrasonic array can also be permanently attached to a structure and used for long term structural health monitoring. These features of the ultrasonic array make it particularly attractive for piping inspection.
One traditional application of ultrasonic inspection is in thickness measurement, which is important in detecting wall thinning due to erosion and corrosion in metallic structures, such as petrochemical piping and storage tanks. The flexible ultrasonic array inspection system was used to measure wall thinning in carbon steel piping . The test specimen was a 14-inch diameter pipe elbow with a 0.375-inch diameter wall. A region of the specimen with visually observable wall thinning was inspected with an 8x8 inch ultrasonic array consisting of 1024 0.25 inch square elements. The results of the inspection are displayed as time-of-flight C-scan and B-scan images in figure 2. Dark pixels in the C-scan correspond to areas of wall thinning. The B-scan shows the through thickness variation in the pipe at a location indicated by the cursor in the C-scan. Notice how the wall is thinner at the locations corresponding to the dark pixels in the C-scan.
Ultrasonic inspection of petrochemical piping structures is complicated because of their large size, complex geometry, limited access, and location in harsh environments. For such structures, the ultrasonic array provides the capability to rapidly inspect a relatively large area on a curved surface. Arrays can be designed and built for inspecting specific geometries such as elbows and tees. Figure 3 illustrates how ultrasonic arrays can be designed for complex geometries and used for limited access inspection.
An ultrasonic array can be permanently attached to a structure and used as a structural health monitoring device. The array can detect the presence of an event, such as an impact, or monitor changes in a structure, such as wall thinning over time. Telemetry can be used to send the data from a remote site to a central data collection and evaluation station. Figure 4 represents a concept for attaching arrays to pipelines, so that wall thinning data over an area can be continuously obtained automatically, rather than by manually scanning the pipeline in a harsh environment, such as the North Slope or the Arabian desert.
For years, advanced composite materials have been used in weight critical structures such as aircraft and spacecraft. Composites are widely used in the latest commercial aircraft, such as the A340 and Boeing 777. Figure 5 illustrates the composite components of the A340 . The 777 incorporates 9 percent composites or about 10 times the amount used on the 757 or 767 aircraft. The weight saved in the carbon/toughened epoxy horizontal stabilizers alone adds 60 additional miles to the flying range of the 777 . The increased use of composites in aircraft is explained, in part, by the decreasing material and manufacturing costs .
The structural characteristics and economic viability of composites have led to greater usage in a wide range of non-aerospace applications including automobiles, marine structures, and sporting goods. In addition to reduced vehicle weight, which improves fuel economy, composites provide other benefits in automotive applications, including design flexibility, durability, tailorable appearance, and consistent fabricability . Composites are also a key element to producing practical alternative fuel vehicles, such as electric cars, where low structural weight is a primary consideration. The US Army is evaluating composite materials for use in armored vehicles. Such materials exhibit structural and ballistic characteristics that meet or exceed those of conventional armor at a significant savings in weight.
Fiberglass reinforced composite materials have been used in marine applications for over 50 years, due to their high specific strength and stiffness, durability, and resistance to the marine environment. More recently, the Navy is evaluating a variety of applications of composites for surface ships and submarines, including hulls, topside structures, and shipboard machinery [6/7]. Composites are finding increased usage in the infrastructure, as replacements for wooden piers and as reinforcement for concrete highway supports. In the future concrete structures including roadways and bridge decks, that have been traditionally reinforced with steel, may be reinforced with composite materials producing lighter, stiffer, stronger, more durable and earthquake resistant structures.
As the use of composites in aerospace and other industry increases, the need for technology to support these structures during manufacturing and service also increases. Charles E. Harris, head of the Mechanics of Materials Branch of the NASA Langley Research Center, reported on an assessment of the current practices in supporting composite structures in the commercial air transport fleet . Among the current and future technology needs identified by the study was non-destructive testing (NDT) to assess damage and to qualify repairs. Harris states that "the technology necessary to identify, characterize, and assess damage and the technology to repair a damaged structure are essential partners in aircraft supportability and cost effective fleet management". As composites find greater use in a broader range of applications, it is likely that similar technology will be required to support these structures.
There are a number of factors that make composite structures difficult to inspect. Damage in composite structures is much more difficult to detect and assess as compared to metallic structures. For composites, low velocity impact such as foreign object damage may leave no visible indication of damage but extensive internal damage. Ballistic impact, which leaves a hole in the structure, will often produce damage extending well beyond the vicinity of the hole.
There are material and design characteristics that complicate inspection of composite structures. For example, the reinforcing fibers produce scattering of ultrasonic signals, causing signal amplitude to fall off rapidly with increasing frequency.
Many aircraft composite structures are quite large, which makes 100% inspection of the structure a time-consuming, expensive operation. Frequently only one surface of a composite structure, such as an aircraft wing skin is accessible for inspection. Complex shapes, such as the leading edge of a wing, further complicate the inspection problem. Composite structures do not always have surfaces that are flat and smooth which makes surface inspection techniques more difficult to apply to composites.
As with metallic structures, there is a need to inspect composite structures throughout their lifetime, during manufacturing maintenance, and service. Inspection techniques that are appropriate for the factory inspection environment, such as robotic scanners may not be applicable to field inspection. Regardless of the inspection environment, digital inspection techniques support convenient acquisition, display, evaluation, and storage of NDT data. Figure 6 shows the C-scan and A-scan data from the debonds in a composite plate.The various inspection environments share a common need for a practical inspection system that reliably provides accurate inspection data for a wide variety of aircraft composite structures.
Ultrasonic inspection is frequently used to detect defects such as delaminations and porosity in composite structures. Traditionally the inspection is performed by manually scanning the surface with a single ultrasonic transducer. Once again a flexible ultrasonic transducer array represents an innovative solution to many of the problems associated with manual scanning. Full area coverage is guaranteed, the flexibility insures that individual transducers self align to the component surface, and uniform accoustic coupling can be assured by use of the vacuum system or by external pressure used to affix the array to the component. The flexible array ultrasonic inspection system is a digital system, which facilitates post processing, storage, and transmittal of inspection data. When indications are found, the images can be transmitted anywhere in the world by modem for evaluation by experts. If a component is scanned with the array immediately after manufacture, the data can be archived and used as a baseline for subsequent field inspections. The system can be used at maintenance facilities and deployed in the field. Arrays can be permanently attached to structures to monitor integrity, to detect damage, and to inspect difficult to access surfaces. Finally, an array based system can be developed that incorporates the advantages of a large fixed scanner into a portable instrument. The instrument can be used at maintenance facilities and deployed in the field.
The ability of the ultrasonic flexible array inspection technique to measure wall thinning in piping, and to detect flaws and delaminations in various thicknesses of composite materials has been successfully demonstrated. For inspection, the array offers several practical advantages over traditional ultrasonic inspection techniques, including ease of use, rapid inspection, use in limited access areas, and on complex geometries. Postulated applications include field and factory inspection of pipelines and piping structures, the transportation industries, (civilian and military), and marine applications. Remote structural health monitoring by telemetry can be used for any application in harsh environments such as the North Slope or the Arabian Desert.