New Highly Productive Phased Array Ultrasonic Testing Machine for Aluminium Plates for Aircraft Applications

Transcription

1 19 th World Conference on Non-Destructive Testing 2016 New Highly Productive Phased Array Ultrasonic Testing Machine for Aluminium Plates for Aircraft Applications Christoph HENKEL 1, Markus SPERL 1, Walter DE ODORICO 2 1 AMAG rolling GmbH, Ranshofen, Austria 2 GE Sensing & Inspection Technologies GmbH, Alzenau, Germany Contact - Christoph.henkel@amag.at markus.sperl@amag.at walter.deodorico@ge.com Abstract. AMAG rolling is an approved supplier of aluminium plates for all major aircraft manufacturers. Because of the very good experience with the first use of phased array ultrasound in the inspection process a second new and more productive system was installed as part of the AMAG investment project for a new hot rolling mill and plate production line. With the new configuration a throughput of 1 to 2 m²/min can be reached, depending on the thickness of the plates. To reach this level of performance, the GE equipment needed to ensure that all virtual probes in the ultrasonic test have the same characteristics and sensitivity. Several adjustments were performed and validated in advance. The new phased array system operates in pulse/echo mode in immersion. The inspection of the plate is done by a linear scan. Interface and back wall echoes are automatically monitored and tracked. With the preconfigured Distance Amplitude Correction (DAC)-curve, all indications are displayed immediately with the correct size and depth. A search scan, as recommended in few current specifications such as provided by means of a paint-brush sensor, is no longer necessary. Reference blocks according to ASTM E127 are used for sensitivity adjustment and periodic validation. In this application the back wall echo has its own amplification, providing gain adjustment for the back wall separately from the defect gate. The probe follows the contour deviations of the plate caused by the production process ensuring perpendicular sound intromission on the centre axis of the ultrasonic beam. Thus, a stable back wall echo can be achieved. Maximizing the indication as required by all well-established specifications is carried out in form of a fully automated rescan with the same phased array transducer as used for the main scan. The beam is steered in an angle from -5 to 5 in both direction of the plane. The system runs in an automatic mode. All indications are automatically evaluated and recorded in a defect-list and shown graphically in a C-image. Borderline indications are automatically re-tested and maximized by angulation. This new system provides automated defect recognition based on AMAG s failure criteria. The equipment is designed to meet all requirements to provide appropriate testing and performance required for obtaining necessary aircraft approvals. License: 1 More info about this article:

2 1. Introduction AMAG rolling is an approved supplier of aircraft plates of the alloys 2024, 2014, 61, 75 and. In 2005 one of the first phased array systems in the world was installed for the US-testing of these plates. Because of the extension of the plate production in the course of the investment in a new hot rolling mill it was necessary to extend the capacity for ultrasonic testing, too. Based on the good experiences with the first equipment a second phased array system was installed in It utilizes a large number of features offered by the phased array technique in order to provide a highly productive system for ultrasonic testing of aircraft plates. The new system can inspect plates with the following dimensions: thickness: 12 to 1 mm width : to 2.4 mm length: to mm An overview of the system is shown in Figure 1. All necessary aircraft approvals have been achieved until now. Fig. 1. General layout of the system 2. Description of the System 2.1. Layout The system consists of three areas. At the entrance side the plates are provided in stacks. Each single plate is separated by crane and positioned on a roller table, where the plates are moved to the ultrasonic part and immersed in the tank. After testing, the plate is lifted again, relevant indications are marked and the plate is dried. At the outlet side the plates can be stored in separate stacks for acceptable and non-acceptable plates. Before stacking, a semiautomatic measuring system for electrical conductivity is integrated additionally. 2

3 2.2. Configuration of the US-System The ultrasonic testing is carried out according to AMS 2154 [ 1 ], ASTM B 594 [ 2 ] and other customer specifications. The system operates in pulse/echo mode in immersion. The probe consists of 128 elements. For class A inspection 39 virtual probes with a nearly square cross section are used in order to guarantee a round unfocussed ultrasonic beam, which is required in some specifications. The index is 2.1 mm in the active direction. The speed in the passive direction is depending on material thickness adjusted also to 2.1 mm. This pixel-size is only achievable with a flat rounded sound field, especially in the near field. Figure 2 shows the schematic impact of a narrow and a wide sound filed on the index and so subsequently on the throughput. A real example of the sound filed of the used ultrasonic beam is shown in Figure 3. vp1 vp2 vp3 vp4 vp5 vp1 vp2 vp3 vp4 vp5 vp1 vp2 vp3 vp4 vp5 Narrow Sound Field - Index 2 Sensitivity Variation acceptable Narrow Sound Field - Index 3 Sensitivity Variation not acceptable Flat round Sound Field - Index 3 Sensitivity Variation acceptable Echo Height [ % FSH ] or Sound Pressure [ db ] 40 1 db Echo Height [ % FSH ] or Sound Pressure [ db ] 40 1 db Echo Height [ % FSH ] or Sound Pressure [ db ] 40 1 db ,8-4,7-3,5-2,3-1,2 0,0 1,2 2,3 3,5 4,7 5,8 7,0 8,2 9,3 10,511,712,8-5,8-4,7-3,5-2,3-1,2 0,0 1,2 2,3 3,5 4,7 5,8 7,0 8,2 9,3 10,511,712,8-5,8-4,7-3,5-2,3-1,2 0,0 1,2 2,3 3,5 4,7 5,8 7,0 8,2 9,3 10,511,712,8 Distance from vp to vp (Index) [ mm ] Distance from vp to vp (Index) [ mm ] Distance from vp to vp (Index) [ mm ] Fig. 2. Impact of a narrow and wide sound field to the index (schematic) Fig. 3. Sound field, measured at 1.2 mm FBH at 3 mm depth 3

4 2.3. Sensitivity Adjustment Because of the use of 39 virtual probes, it has to be guaranteed, that all vp s have the same sensitivity level within specific limits. To cover the full thickness range, this check is carried out at notches in ¼, ½, and ¾ of the maximum thickness. For this purpose, the probe is moved in the active direction across each single notch. As a result, all virtual probes are checked on the same point of the notch. Therefore, tolerance-related variations in the notches have no impact on this check. Figure 4 shows the typical results for this configuration with 39 vp s. At all three notches a deviation in sensitivity of less than 5 % of the average was determined. These checks have to be repeated on a regular basis to guarantee that no loss of sensitivity has occurred since the last successful test. FSH [ % ] notch 120 mm depth upper limit for adjustment upperlimit for retesting average lower limit for retesting lower limit for adjustment Tolerance for adjustment: Tolerance for retesting: +/- 8% of the average +/- 10% of the average Chanal [ vp ] FSH [ % ] notch mm depth upper limit for adjustment upperlimit for retesting average lower limit for retesting lower limit for adjustment Tolerance for adjustment: Tolerance for retesting: +/- 8% of the average +/- 10% of the average Chanal [ vp ] FSH [ % ] notch 45 mm depth upper limit for adjustment upperlimit for retesting average lower limit for retesting lower limit for adjustment Tolerance for adjustment: Tolerance for retesting: +/- 8% of the average +/- 10% of the average Chanal [ vp ] Fig. 4. Results of the alignment of the used virtual probes at the notches in different depths 4

5 The uniformity of the ultrasonic beam was measured at 5 vp's distributed uniformly over the width in active and passive direction at all metal travel distances available in the reference blocks. The deviations in both directions are clearly less than 10% of the average of the measurements in one direction and at the same depth. Horizontal and vertical linearity, near field and far field resolution as well as the signal to noise ratio are determined in accordance with ASTM E 317 [ 3 ] and ASTM B 594 [ 2 ]. All requirements are met with significantly better values compared to the limits given in the specifications. After the adjustment of a Distance Amplitude Correction (DAC)-curve, all indications are displayed immediately with the correct size and depth. A search scan, as recommended in few current specifications by means of a paint-brush sensor, is no longer necessary. Because of the dimensions of the plate in length and width flatness-deviations within the tolerances of the material specifications cannot be prevented completely. A surface tracking system makes sure that the probe follows the surface contour in the active as well as in the passive direction. This system also works in the areas of the plate-edges, where the probe is not longer completely above the plate. This feature guarantees a very stable interface and back wall echo Reference Blocks and Reference-Scan Standard reference blocks according to ASTM E 127 [ 4 ] are used in the system. Each 13 blocks with 1.2 and 2.0 mm FBH cover the entire discontinuity range from 3 to mm for class A according to ASTM B 594 [ 2 ]. Each block stands on 2 ceramic cylinders arranged perpendicularly to each other and thus galvanically isolated in a stable and stiff stainless-steel fixture. The adjustment can be done easily by three-point support. The scan of all these reference blocks is carried out with full line speed and takes less than one minute. This very short time for the reference scan could be realized by using the ALOK-procedure (Amplituden-Laufzeit-Orts-Kurven) for data acquisition. The gates for the single FBH s with different metal travel distances are set during data analysis. Figure 5 shows the results of few reference- scans from the commissioning phase and some actual values. The deviation for all metal travel distances is less than +/- 1 db Echo Height [ %FSH ],0 95,0,0 85,0,0 75,0,0 65,0,0 55,0,0 45,0 40,0 35,0 30,0 25,0 20,0 15,0 10,0 5,0 0,0 FBH :40:51 FBH :39:43 FBH :53:42 FBH :44:56 FBH :22:52 FBH :23:37 FBH :11:00 FBH :11: Metal Travel Dictance [ mm ] Fig. 5. Results of reference scans for class A in week 32 and 44 of FBH 2.0 mm Upper Acceptance Limit ±1 db FBH 2.0 mm Lower Acceptance Limit FBH 1.2 mm Upper Acceptance Limit ±1 db FBH 1.2 mm Lower Acceptance Limit

6 2.5. Scanning and Evaluation The arrangement of scanning and evaluation area related to the plate is shown in Figure 6. The actual position of the plate on the roller table in the tank is measured by a laser system directly before the plate is immersed in the tank. The scanning area is defined in a way that the plate is completely over-scanned in the length and width. The evaluation area is automatically arranged inside the measured plate dimensions, so that influences of the edges can be excluded completely. For the case that the plate is skewed a little bit during the movement on the roller tables the evaluation area is skewed too and adjusted parallel to the edges of the parent plate. After inspection the corners of the evaluation area are marked on the surface. After the final plates have been swan, the marked corners of evaluation area must have been removed. In this case, one can be sure that the final plates do not contain any untested or unevaluated area. The system runs in an automatic mode. All indications are automatically evaluated and recorded in a defect-list and shown graphically in a C-image. Borderline indications are automatically re-tested and maximized by angulation. A re-scan-manager supports the operator by the choice and decision, which indications have to be maximized by angulation. Finally a decision proposal is made automatically by the system, based on size, extension, distance to each other and number of indications. bridge with US-probe 2.6. Angulation Fig. 6. Definition of scanning and evaluation areas Most specifications require a maximization of indications above a certain level by angulation. For this purpose, the probe shall be mechanically tilted in two vertical planes that are perpendicular to each other in 1 -steps in the material. This angulation is normally done with a conventional probe, tilted in both directions by means of a joystick by an operator. In the present new system, the angulation is carried out with the same phased array transducer as used for the main scan but using a higher resolution. At first the probe is centred above the indication that has to be checked. By means of an automatic mechanical procedure the ultrasonic beam is adjusted perpendicularly to the surface. In both rolling direction and perpendicular to this the beam angle is changed from in 1 -steps, 6

7 covering the variation of natural defect orientation. It is necessary to cover the area of interest in the material in all depth (Figure 7). This procedure results in 121 C-images (Figure 8). In this summarized C-image, the maximized response and the angular deviation in both directions can be read out and overtaken in the re-scan report of this indication. The time for the whole automated procedure of angulation of one indication is much less than the time a manual angulation takes. sound path (water + aluminum) [ mm ] Area of Interest extension longitudinal and transverse to rolling direction [ mm ] 1/+5 2/+5 3/+4 3/+5 4/+4 4/+5 5/+3 5/+4 5/+5 6/+3 6/+4 6/+5 7/+2 7/+3 7/+4 7/+5 8/+2 8/+5 9/+1 9/+5 10/+1 10/+5 11/0 11/+5 12/-4 12/+5 13/-5 13/+5 14/-5 14/-4 14/-3 14/-2 14/-1 14/0 14/+4 14/+5 15/-5 15/-4 15/-3 15/-2 15/-1 15/0 15/+1 15/+2 15/+4 15/+5 16/-5 16/+4 25/-4 25/-5 26/-5 27/-5 Fig. 7. Theoretical calculation for the recan of indications in various depths using angulation with different skew angles Fig. 8. Summarized C-Image of 121 different variations of angles from +5 to -5 in 1 -steps in the material 2.7. Throughput As mentioned in point 2.2 one of the main parameters, which impact the throughput, is the shape of the ultrasonic beam. Only with a flat round sound field (see Figure 2) an index of 2.1 mm is achievable. This index of 2.1 mm is the basis of the high throughput. But with an optimized shot frequency the performance can be increased even further, so that line speed of 0 mm/s and a throughput of up to 2 m²/min can be achieved. This enables to inspect a parent plate of x 8,200 x 1,6 mm in about 8 to 10 minutes. The complete inspection including re-scans and marking takes between 15 and 20 minutes. 7

8 3. Summary The phased array technique is based on the same physical principals as a conventional system. Because of the higher number of virtual probes, more effort, surveys and verification checks are necessary to implement the system and to guarantee correct testing in normal production. Although the phased array technique is not covered by all specifications so far, it meets all requirements of these standards. Furthermore, the phased array technique offers a huge benefit concerning test speed, angulation, throughput and degree of automation. The new highly productive phased array system for aluminum plates for aircraft applications at AMAG rolling meets all requirements of the specification and is approved by the main aircraft customers. 4. References [ 1 ] AMS-STD Process for Inspection, Ultrasonic, Wrought Metals [ 2 ] ASTM B 594 Standard Practice for Ultrasonic Inspection of Aluminum-Alloy Wrought Products [ 3 ] ASTM E 317 Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse- Echo Testing Instruments and Systems without the Use of Electronic Measurement Instruments [ 4 ] ASTM E 127 Standard Practice for Fabrication and Control of Aluminum Alloy Ultrasonic Standard Reference Blocks 8