Ultrasonic Testing. Portable Instruments Industrial inspection systems Air-coupled Testing Transducers

Transcription

1 Ultrasonic Testing Portable Instruments Industrial inspection systems Air-coupled Testing Transducers

2 Ultrasonic testing - overview Ultrasonic non-destructive testing (UT) is commonly used for flaw detection in materials. Ultrasound uses the transmission of high-frequency sound waves in a material to detect a discontinuity or to locate changes in material properties. DIO1000 Industrial system DIO2000 Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include: It is sensitive to both surface and subsurface discontinuities. The depth of penetration for flaw detection or measurement is superior to other NDT methods. Only single-sided access is needed when the pulse-echo technique is used. It is highly accurate in determining reflector position and estimating size and shape. Minimal part preparation is required. Electronic equipment provides instantaneous results. Detailed images can be produced with automated systems. It has other uses, such as thickness measurement, in addition to flaw detection. The Starmans electronic company is mainly focused on development and construction of ultrasonic systems used for material testing. Our constructed systems can be used for material thickness measurement as well as flaw detection. All our systems have implemented the special algorithms which improve the sensitivity and make our systems more accurate.

3 Portable Instruments DEFECTOBOOK DIO1000 DEFECTOBOOK DIO1000 is the newsest instrument fully developed and designed by Starmans electronics Ltd. company. This ultrasonic device is a suitable compromise between high-end ultrasonic testing and dimensions (often limited in industrial applications). The DEFECTOBOOK DIO1000 implies both conventional ultrasonic and EMAT generators for contact and non-contact ultrasonic testing. Main technical specification: Display: Color TFT sunlight, 1024 pixels (W) X 768 pixels (H) Gain control: 110 db Max and reference gain control level sensitivity feature with 6 db, 1 db, 0.5 db and 0.1 db selectable steps Auto Transducer Calibration: Automated calibration of transducer, zero offset and/or velocity Reject: 0% to 80% of full scale in 1% increments Material Velocity: From 100 to m/s in steel Range: Standard 1 mm to 60,000 mm Pulser Type, User Selectable: Tunable square wave, negative spike excitation, burst Rectification: Full Wave, Half Wave Positive or Negative, and rectified RF settings Analog Bandwidth: 0.5 MHz to 30 MHz at 3 db Filters: Broadband, Narrowband, or Custom Selectable Low and High Pass Filters 1 MHz, 2 MHz, 2.25 MHz, 4 MHz, 5 MHz, 10 MHz Operating Temperature: -10 C to 50 C Storage Temperature: -40 C to 70 C Power Requirements: AC Mains: VAC, VAC, Hz Battery: Built-in and external rechargeable LiIon battery pack rated at 3.6 V at 16 Ah USB Communications Port: Hi-speed interfacing with PC Communications ports: RS232 Ethernet Wireles Ethernet Bluetooth Memory: 4 up to 16GB Bscan input: Encoder, A,B pulses, start, TTL 5V, Encoder supply switchable 5V Dimensions: mm Display dimensions: mm DIO 1000 versions (in terms of connectivity): TYPE RS232 USB Ethernet Wifi Encoder input for B?scan DIO 1000 LC X X DIO1000 High X X X X X DIO1000 EMAT X X X

4 Industrial Inspection systems DIO-2000 Hardware System DIO-2000 Complete Solution for Industrial Measuring System DIO 2000 General Features The system s ultrasonic channels are designed as independent electronic plug-in units (modules) with their own microprocessor control and signal processing. The plug-in units (size 100 x 160 mm) are located in frames. Every frame may contain 16 units. Four frames may be assembled in a 19" rack with power supply built-in. In this way the maximal system range consists of 64 channels. The control system and the data evaluating system of DIO 2000 consists of a industrial PC (Pentium IV), 17" SVGA Color monitor, also of industrial type, and the appropriate software under MS Windows This control system is very user friendly, because of applying the well-arranged menu. In order to assure the UT reliability, a synchronization unit may be assembled into the system and a unit for probe localization on the surface of the specimen under test. The output of the device may control a defect marker and, according to the customer s option, an acoustic and luminous signalization. As a matter of course there are printing facilities (ink-jet or laser) for recording the flaw distribution in the material under test (B- and C-scan, thickness or tolerances) or/and an overview during a time interval. According to the customer's requirements it is possible to operate an acoustic alarm. DIO 2000 Channel Unit Features of the basic DIO2000 plug-in-unit Each channel is created by an independent module (card with electronic circuits) containing all components as a complete ultrasonic flaw detector with - adjustable pulse repetition rate; - adjustable amplifier gain and band filters;

5 - digital signal processing DSP (digital filters, averaging and further functions); - three gates with a set-up alarm; - measuring of echo height maximum, minimum, average, echo extent and graphic processing; - 2 analogue outputs (amplitude and time) for C-scan; - automatically renewable freeze mode; - maximum freezing; - external and internal synchronization with adjustable time shift; - possibility of connecting an extern power transmitter to every channel; - possibility of using probes with internal preamplifier. Transmitter: location: in each channel unit transmitted pulse: up to 250 V at 50 Ω loading transmitted pulse width: 60 ns up to1 µs triggering of the transmitted pulse: internal or external synchronization of the transmitted pulse: automatic PC controlled transmitted pulse shift: 0 ns to 10 ms max. transmitter repetition rate: 10 khz operation facilities: normal or TR probes for frequency range from 1 to 20 MHz transmitter output impedance: adjustable - in steps.ωfrom 30 to 1000 Receiver: Location: in every channel unit Max. input echo signal voltage: 1 Vp-p Processable echo signal voltage: < 1 Vp-p to > 0.1 mvp-p at 100% screen height Adjustable dynamic: +20 db to db Gain linearity: 1 % Receiver frequency range: 0.5 MHz to 20 MHz (for -3 db) Input receiver impedance: adjustable from 30 to 1000 Ω Digital Data Processing: Evaluation: by the built-in microprocessor, amplitude and time comparison, leading edge or peak value evaluation in selected gate, statistic elimination of interference, data compression, A-scan Noise suppressor: 0 to 80 % screen height Gates: 3 Threshold levels: 1 in every gate - adjustable Gate triggering: synchronized by the transmitter or selected echo leading edge ( echostart ) Measuring Accuracy: Amplitudes for flaw detection: ±1 % relative to 10 MHz signal Time size measurement: ±1 µm relative to sound propagation velocity (in ferritic steel) for ultrasonic frequency of 15 Hz Entering of measuring parameters: by means of a PC trough RS 232; 422, USB setting of the whole testing system: manual inspection by a PC or by pre-selected parameters stored in the PC (set at some preceding testing). DIO-2000 Unit Versions SF DIO CH SF is the most used type with the frequency band 1-15 MHz containing four frames with 16 channels each, i.e. this type may be broadened till to 64 channels. The automated installations (see thereinafter) supplied for inspection of steel bars, aluminium blocks, axles and wheels for railway vehicles and others is equipped with this DIO 2000 type.

6 HF DIO CH HF is the high frequency type with the frequency band from 15 to 60 MHz, which is being used for inspection of thin components or thickness measurement. LF DIO CH LF+BP is the type provided with a power transmitter foreseen to air coupled inspection of composites, wood, concrete, ceramics and others. Also four frame may be connected with four channels too. In this way for low frequency application with 32 channels in the band from 10 khz to 1 MHz. DIO-2000 Connection and Casing Small Size Version There are cases, where fewer channels are needed. For such purposes, low cost version DIO 2000 with 3 channels only is available. It is a portable industrial which may be used without PC also, e.g. for defect marking and control of other devices. The modifications are the same as said above in connection with the DIO 2000 compact version. Compact Version Table Rack Version, Combined Version Large Rack Version DIO-2000 DS DC servo controller card with PLC

7 DIO-2000 Software SW Possibilities The large screen of a 17 monitor allows simultaneously displaying of several A-scans and of several parameters and channel windows as shown in the pictures below.

8 Example of B-Scan

9 Air coupled Testing Air-Coupled Ultrasonic Testing system Fig. 1: Air-coupled ultrasonic testing principle During the last decade, air coupled ultrasonic testing has advanced from being a laboratory curiosity, of little practical application, to a point where it is a viable technique for many manufacturing inspections. With appropriate equipment, it is possible to perform sensitive inspections for defects such as voids, cracks and dis-bonds in a wide variety of water-incompatible materials. Originally used primarily for aerospace composites, the applications have been extended to cover a variety of materials where conventional NDT methods are not normally considered suitable. Diverse applications of the technique include wood, foams and missile propellants. A variety of frequencies can be used, allowing optimization for resolution or penetration as with "conventional" ultrasonic testing. Most applications of air coupled ultrasound have been for single channel C-scan systems, testing flat panels. In this context it is normally used as a direct replacement for water-coupled "squirter" probes, and installing air-coupled probes and instrumentation on an existing system so as to inspect a wider range of materials is generally a simple procedure. The results are often indistinguishable from those obtained with the water coupling; Figure 1 shows results from inspection of a "knot" in timber. More recently the use has been broadened. In particular multi-channel systems have been developed allowing very rapid inspection of large composite panels at a rate approaching 1 m2 per minute. The current challenge is to overcome the requirements for high transmission power, thus allowing a portable instrument of uncompromised performance to be developed. This technology shows considerable promise as a solution to a number of currently impractical testing problems. Theory of Air-Coupled Ultrasound When sound passes across an interface between two materials only a proportion of the sound is transmitted, the rest of the sound is reflected. The proportion of the sound that is transmitted depends on how close the acoustic impedance of the two materials matches. Water is a fairly good match for most commonly used materials, for example typically around half the sound energy is transmitted at the interface between water and a carbon laminate. After four solid-liquid interfaces (from the probe, to the couplant, to the test piece, and then back again) there is still a few percent of the original energy left so accurate measurement is possible. Conversely if the sound has to move between the test piece and air (which has very low acoustic impedance) only around 1 % of the sound energy is transmitted. Thus after four transitions very little sound energy is left. Typically the overall path loss may be 100 db higher using air as a couplant, than when water is used. The path loss is significantly higher with metals, which have high acoustic impedance compared to plastics which are lower in impedance. It is therefore apparent that we must work hard to minimize losses at every stage if we are to achieve acceptable signal to noise for the inspection.

10 Fig. 2: Transmittion of ultrasonic waves. Fig. 3: Schematic of an ultrasonic transducer showing the critical final acoustic impedance matching layer relative to the piezoelectric element and coupling medium air. Test Configurations Because of the tremendous difference in trans mitted and received signal amplitudes, and the inherent difficulties in achieving adequate transducer/ amplifier isolation and recovery, no current air-coupled NDT systems works in single probe mode. Separate transmit and receive transducers are always used. Fig. 4: Normal through transmission.. Fig. 6: Plate wave. Fig. 5: Shear wave. Where accurate imaging of defects is required the normal through transmission configuration (Figure 1) is most appropriate, as used for conventional water coupled ultrasonic testing. In some materials, particularly laminates, better results are obtained by offsetting the probes and angling them slightly so that shear waves are produced as shown in Figure 2. Because there is no couplant to damp surface vibrations, air coupling lends itself well to production of Lamb or Plate waves as shown in Figure 3. These can travel significant distance in suitable materials. They have two particular advantages: It is possible to have both transmit and receive probes on the same side, or both on the outside of a closed part such as a tube. Because the test checks a line, rather than a single point, it is possible to achieve much greater throughput speeds in applications where a precise image is not required. Effect of Frequency Air coupled ultrasound typically operates at frequencies below 1 MHz, above this the sound transmission in air reduces, and more importantly, scattering losses in many materials become unacceptably high

11 (typically scattering losses increase with the forth power of frequency) STARMANS have worked primarily at 50, 120 and 400 khz. Results at different frequencies are summarized in Table 1. Frequency Typical spot resolution Typical Materials 50 khz 8-10 mm Thick structural foams, Complex multi-layer compo sites, Unprocessed Timber. Comments Will penetrate almost anything, but resolution is inadequate for many purposes. 120 khz 5 mm Foam sandwiches, two or three layer honeycombs, medium thickness timber, and drywall. Good compromise where max resolution not required. Can penetrate most 'possible to test conventionally' materials. 400 khz 1-2 mm Solid laminates, single layer honeycombs. Gives results comparable in resolution to practical production tests. Table 1: Effect of frequency on Air coupled UT tests The majority of the work carried out by STARMANS has been at 400 khz, but increasingly lower frequencies are being employed to test more complex composite materials. Applications Wood products As previously noted the timber industry was among the first that employed air coupled ultrasound commercially. Applications of air-coupled ultrasound fall into four main groups: Assessment of bulk timber for internal decay and voids prior to processing. This requires extremely high penetration. Currently fully air-coupled inspection has not provided adequate signal to noise ratio, although the technology has been applied with soon success in conjunction with dry pressure coupling. Fig.7: Different approaches to scanning of sheet products Detection of delaminations and cracking in composite or processed wood products such as particleboard and pressed wood. This normally involves some form of scanning, although a single line test is sometimes acceptable. By using a lamb wave it may be possible to test the entire width of a board, allowing effectively 100% inspection. Assessment of wood quality by measuring sound transmission velocity. This has been quite successful, although the tone burst limits accuracy of timing measurement. In-service Inspection of wood products for internal decay and cracking, for example structural beams and utility poles. Many different configurations are used, with thin wood panels high frequencies can be used, and extremely good imaging can be obtained, as shown in Figure 8. Fig. 8: Comparison of photograph and C-scan in wood Composites

12 Composite materials, particularly within the aerospace industry, have been a primary area of application for aircoupled (and drycoupled) test methods. The following are among the examples of results from inspection of composite parts. Fig. 9: Scan of part of an aircraft brake disk, dark area appears to be disbonded. Fig. 12: Scan of a complex composite satellite part (carbon/carbon & carbon/honeycomb). Fig. 10: Impact damage in a carbon laminate panel. Fig. 13: As previous figure, tested at a higher gain to optimize results on honeycomb section. Fig. 11: Bonded foam panel, (test at 50 khz). Fig. 14: Light weight armor panel, showing disbonding after impact. These are all through transmission C scans obtaining results which will be reasonably familiar to users of conventional ultrasonic testing, however many production inspections can conveniently use a plate wave configuration, giving simple "go/no go" quality assessments. This has been particularly applied to pultruded composites. Metal Parts The extremely high acoustic impedance of metals, and the corresponding poor transmission coefficients for sound from air, means that metals are generally poor candidates for air-coupled inspection, however, when other circumstances are favourable acceptable results may be obtained. A system is currently being manufactured to inspect bonds in a thin (2 mm) laminated steel tube that is intended for an aerospace application. Accurate Timing Measurement Using Air-Coupled Ultrasound Relevant physical properties of a material can often be correlated to the velocity of sound. This can be calculated by measuring the transit time through a reasonably long section of the material. Water would be incompatible with many of the materials (e.g. wood, where properties are significantly affected by moisture). Air-coupled lamb waves lead themselves well to this, since the results are reasonably predictable, there are no coupling arrangements to interfere with production speed, and the experimental setup can often be arranged so that a fairly long distance in the material can be measured, thus minimizing error. However, as noted earlier, the duration of the tone burst can be a limiting factor in measurement precision. To reduce this it is necessary to use a conventional damped probe driven with a spike or square wave pulse. This greatly reduces the overall efficiency. To counteract this extremely high power can be used (a 1200 V pulser has been tried, and a 4 kv unit is being developed). Since these applications do not typically require a rapid sample rate, signal processing can be used to further improve signal to noise ratio Large Area Scanning As noted above the scanning rate of air-coupled ultrasound is limited by the relatively low PRF which is possible with the long transit times in air. Typically a PRF of around 200 Hz is possible. If we accept a relatively coarse scan pitch of 3 mm as acceptable for a production test this implies a maximum linear

13 speed of 600 mm/s and in two dimensions, an absolute minimum time of around 10 minutes to scan one square meter. The scanning assembly shown in Fig. 11 combines the results from 8 channels, each of which carries out a through trans mission test. Customized soft ware assembles the results from each probe into a single image. The end result is a combined through put of around 1 square meter per minute, able to keep up with a high-volume pro duction line for composite panels. Fig. 15: Example of equipment for inspection of solar panels. Fig. 16: Equipment for non-contact testing of rockets. Equipment for Non-Contact Ultrasonic Testing of Wood 16 channel non/contact system for inspection of boards LSL 13. The system evaluates defects in real time using cluster analysis algorithms. Robotized Air-Coupled Ultrasonic Testing System Portable Equipment for Air-Coupled Ultrasonic Testing

14 Transducers Ultrasonic Probes for Manual and Automated Testing The probes (transducers) are very important part of ultrasonic NDTinstrumentation. The Starmans Company is mostly using its own types of ultrasonic probes being developed in its laboratories and in some but not numerous special applications probes of other manufacturers are connected to the Company s devices as DIO Before the final decision for probe construction the theoretical simulations using mathematical equations are performed. Ultrasonic beam simulations

15 Based on the results from the simulations the real probes are finally developed and constructed. The conventional probes are the single one element and dual (double element) probes for longitudinal and transversal waves. According to the direction of the beam axis, there are straight and angle beam probes. The both kinds of probes are depicted in the picture. For special purposes, as inspection of concrete, wood or for specimens with curved surfaces, special focusing probes have been developed. The angle probes are mostly supplied for transverse (shear) T waves, e.g. for ferrite weld inspection. The most common angles are 45, 60, 70 and 80. Probes for surface waves in different materials are available also. Austenitic welds require use of L waves. Such probes may be ordered as well. In addition, the STARMANS Company may supply some special probe layouts as tandem and LLT probes and special probes for wood and concrete inspecting. Focusing probes are developed according to specific customer s requirements. Air-coupled and EMAT probes (transducers) may be available also. Two piezoelements combined probe with one straight and one oblique beam Focusing probe for small diameter tubing Probes for manual testing are often not suited for automated and mechanized testing carried out with multichannel installations. The laboratory of the Starmans Company is designing and developing probes which are suited for the determined purpose.

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