Development of an Ultrasonic System for C-Scan of Aircraft Components

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1 Development of an Ultrasonic System for C-Scan of Aircraft Components More info about this article: Abstract K. Chandran, Tejas Tadpatrikar, G. Prashanth, Pradeep Sangvikar Tata Advanced Materials, Bangalore Aircraft manufacturers all over the world are increasingly making use of Carbon Fibre Composites for primary structural parts. Tata Advanced Materials Limited Bangalore is manufacturing composite parts for the world s leading commercial aircraft manufacturers Boeing and Airbus. One of the primary structural components is a set of honeycomb sandwich panels called wing lower panels. The Lower panels are subjected to 100% ultrasonic inspection to ensure defect free components. The C- Scan inspection is done twice, first with 1 MHz frequency for evaluation of Delamination, Debond, Voids and Inclusion and scanned again with 5 MHz frequency for Porosity evaluation in the laminate area. The requirement of panels is likely to be doubled in the future. This will put a lot of capacity constraint on the C Scan equipment schedule and it is decided to develop a C- Scan equipment that can be used for scanning 5 MHz frequency for the monolithic area which will save the C Scan machine time. This paper highlights the stages of developing an NDT Ultrasonic Inspection System as alternate primary inspection for these wing panels. These panels are required to qualify strict acceptance criteria during manufacturing, ensuring critical airworthiness requirement. Keywords: Prepreg, C-Scan, Attenuation, Acoustic impedance, Squirter. 1. Introduction The aircraft industry continues to increase its use of composite materials for principle structural elements. The extreme damage tolerance and high strength-to-weight ratio of composites have motivated designers to expand the role of fiberglass and carbon graphite in aircraft structures. The expanded use of composite materials has placed greater emphasis on the optimization of current inspection practices and the development of improved NDI techniques that are more reliable and sensitive than conventional NDI. TATA Advanced Materials Limited (TAML), a subsidiary of TATA Industries Limited, is a pioneer and leader in Composite Manufacturing and Solutions, catering to the demands of the Aerospace, Space, Defence and Industrial Composites realms across the globe, with a mission of providing value to customers and stakeholders by offering innovative production & engineering solutions using advanced composite materials. Composite Parts for Space Applications, Structural Components, Engine Components, Personal and Vehicle Armour and Composite parts for various Industrial Applications are some of the

2 products that TAML manufactures for various OEMs. One of the important composite structural component manufactured for Airbus A 350 aircraft is a set of Honeycomb Sandwich panel called Wing Lower Panel (see Fig 1 for a typical Lower Panel part). These panels are made of carbon fabric skins bonded to a non- metallic Honeycomb with adhesive layers. Ultrasonic testing remains the most preferred NDT technique to detect almost all the defects in composites. The Lower panels are subjected to 100% ultrasonic inspection to ensure discontinuity free components. Special purpose C- Scan equipment has been used to inspect these parts. This paper brings out the details of a new Ultrasonic C-Scan system developed to replace the costly conventional C-Scan equipment. Sandwich area Fig. 2 : Sandwich part constructional details State-of the-art facility and machines like Contamination Controlled Area (Ref Fig.3), Prepreg cutting Machine (Ref Fig.4), Computer Controlled Autoclave (Ref Fig.5), 5axis CNC machine for trimming (Ref Fig.6), Coordinate measuring machine for dimensional inspection (Ref Fig.7) and Through-transmission automatic C-Scan Equipment for NDI Inspection (Ref Fig. 8) are employed for production and stringent quality checks of these panels. Monolithic area Fig 3: Contamination Controlled Layup area Fig. 1 : A typical Lower Panel Sandwich part 2. Manufacturing details The Lower Panels are fabricated with carbon fabric prepreg skins bonded to a non-metallic Honeycomb (Nomex) with adhesive layers (Ref Fig.2). The monolithic laminate area thickness varies from 4mm to 6mm (approximately). Fig 4: Prepreg cutting machine

3 3. Defects in composites Fig 5: Computer controlled autoclave for curing The probable defects encountered normally during manufacturing of these panels are: Delamination in laminate skins, Debond between core and adhesive, Inclusion Void Porosity 4. Ultrasonic Testing Fig 6: CNC 5 axis machine for trimming & drilling The most common method of NDE for composite materials is ultrasonic inspection. The measurement of ultrasonic parameters can provide a wealth of information on the quality of composites. Ultrasonics can generally detect delaminations, inclusions, debond, porosity, and voids in composites structures (See Fig 9 for typical inspection of composite structure using ultrasonics) Fig 7: Coordinate measuring machine (CMM) for dimensional check Fig 8: TTU C-Scan Equipment Fig 9: Squirter ultrasonic C-scan machine for NDT of composites Ultrasonic testing is a Non Destructive Testing technique in which sound beams of frequency ranging from 20 KHz to 20 MHz are introduced into materials for the detection of defects, which lie in surface or in the subsurface. The basic principle behind this technique is the impedance mismatch between materials. Ultrasonic beam gets

4 reflected or diffracted once it reaches a point where impedance mismatch due to material change arises. The reflected/diffracted ultrasonic wave from a discontinuity (delamination, porosity, or back-wall of the medium) in the material is often used to characterize a flaw or the internal structural features in a material, a component, or a structure. Conventional ultrasonic methods refer to three types of the transducer configurations i.e. (a) pulseecho(fig 10), (b) through transmission(fig. 11) and (c) pitch-catch.(fig. 12) Fig 12 Pitch Catch method 5. Requirement of new C scan equipment Conventional through transmission C-Scan machine was used to do C-Scan inspection of Lower panels two times (1 MHz for Sandwich area and 5 MHz for monolithic area) Refer Fig 1 for typical configuration of lower panel part. The conventional inspection consumes more time and even cost of operation is more. This will put a lot of capacity constraint on the conventional C Scan equipment schedule in the near future. So it is decided to procure a cost effective C Scan system which can be used for scanning 5 MHz frequency for the monolithic area. Fig 10 Pulse Echo method Fig 11 Through Transmission method 6. Criteria for selection of new equipment It is decided that the new C-Scan equipment for scanning the lower panels will be Phased Array equipment in place of a conventional gantry based C-Scan equipment. The following factors are considered for selecting the Phased Array equipment. The total cost of Phased array equipment is only 1/10th of conventional C-Scan equipment. Operating cost is negligible. Phased array equipment is battery operated while conventional C Scan equipment consumes minimum 10 KW power. Availability of A-scan, B-scan, C-scan images instead of only A-scan and C-scan of conventional system (Ref Fig 13). This allows better interpretation of signals inside the composite structures resulting in better defect detection and defect sizing

5 housing edge). They are well suited for C-scan inspections of composites. Fig.13: A-scan, B-scan and C-scan presentation 7. Omniscan MX2 and accessories The OmniScan device, shown in Figure 14, is manufactured by Olympus. The one-line scan capability of the OmniScan allows inspectors to collect data in one axis and visualize it using the top view. This feature is easy to set up and allows the data to be played back after the acquisition for offline analysis and reporting. Data can be encoder or time based, and phased array images can be displayed in real time. Transducers are available with up to 128 elements. Fig. 15 : Phased Array Probe 5L64 NW1 SNW1-0L-IHC-C Rexolite wedge is procured. The above probe is designed to work with this wedge (Ref Fig 16). Fig. 16: SNW1-0L-IHC-C Rexolite wedge Mini-wheel encoder ENC1-2.5-LM is procured for positioning and dimensioning of defects in the scan axis, and can synchronize data acquisition with probe movement (Ref Fig 17). Fig. 17: Mini wheel encoder. Fig. 14 : Omniscan MX2 5L64 NW1 linear phased array near wall probe (Ref. Fig. 15) is procured. Near-wall probes offer a shortened dead zone at both ends of the probe (1.5 mm between center of first or last element and RollerFORM was procured for scanning composite parts more precisely and in a faster way than mini wheel encoder connected with conventional phased array probe. Olympus new phased array wheel probe designed to address the inspection of composites and other smooth-surfaced materials, such as those

6 commonly used by the aerospace industry. An affordable and easy-to-implement replacement for full 2-D encoding systems, the RollerFORM also offers a viable alternative to immersion techniques. The RollerFORM, combined with a phased array instrument such as an OmniScan uses zero-degree ultrasonic beams for composite inspections (Ref. Fig. 18). 6 db starting at least at 40% FSH. After normalization: All values in a range of 2 db starting at least at 70% FSH. The water coupling was a big problem with the Rexolite wedge. It was very difficult to get db variations within 6dB envelope mentioned as scanning uniformity in Table 2 (Refer Fig 19). As well as porosity evaluation was not possible due to db variations within the C-Scan plot. Fig. 18: RollerFORM 8. Problems Encountered Qualification of the equipment and its accessories for inspection of Airbus A-350 structural components was very challenging task. It has to pass the stringent acceptance criteria and requirements mentioned in AITM_6-0013, some of them are mentioned in below Table 2 Table 2: Acceptance criteria Particulars Acceptance value Backlash < 3mm Scanning uniformity Values within 6 db Normalization Before normalization: All values in a range of Fig.19: Amplitude plot showing db variations in the part using Rexolite wedge The mini wheel encoder as well as RollerForm was having problem of backlash which was more than 3 mm (Refer Fig 20) and time consumed was more for scanning than existing conventional C- Scan machine (Details given in Table 1). Using mini wheel encoder the scan plan was too complex; it was divided in many patches and later stitched using software (Ref. Fig. 21). Table 1: C-Scan Inspection time for 5Mhz Time on Conventional equipment Time using mini wheel encoder Time using RollerForm

7 30minutes 60 minutes 50 Minutes The Glider Scanner is a 2 axis X-Y scanner with encoders for semi-automatic inspection of slightly curved or flat composite parts. To measure the probe position there are two waterproof encoders, one on scan axis and the other on index axis. For completing the inspection in 2 patches we procured a special Scan axis track of 1.50M which is not a standard item (Ref Fig. 22). Index Axis Track Scan Axis Track 3.5 mm Fig.22: Glider Scanner Fig. 20: Backlash using mini wheel encoder as well as Roller form Fig. 21: Scan Plan using mini wheel encoder as well as Roller Form We also decided to replace Rexolite wedge with Aqualene wedge. This is the latest wedge from Olympus for the composite inspection. Aqualene wedge uses soft rubbery material designed to have the same sound speed as water. Aqualene insert is recessed into the face of the wedge hence it does not contact the scan surface. This design effectively removes the echo usually associated with the end of the wedge, allowing for better near surface resolution. Care should be taken to tight the screw heads evenly, otherwise the Aqualene may deform and the image may not be level, giving bad results (Ref. Fig. 23). 9. Final solution to the problems encountered The breakthrough has come when we integrated a scanner (glider scanner) to make OmniScan a semi-automatic equipment.

8 Fig.23: Aqualene Wedge Fig.25: Depth(TOF) plot for a Lower Panel Fig 24 shows the Omniscan integrated with glider scanner and aqualene wedge The Fig. 26 shows plot Amplitude plot for a lower panel produced by Omni-Scan integrated with glider scanner. The amplitude plot shows the db variations within 6 db envelope which is acceptable as per Airbus specifications. Fig.24: Ultrasonic Scanning System Fig.26: Amplitude plot for a Lower Panel 10. Results The Fig. 27 shows Depth (TOF) plot for a lower panel part with a detected inclusion. Inclusion The following are the results produced by Omni- Scan integrated with glider scanner, aqualene wedge. The Fig. 25 shows depth (TOF) plot for a lower panel produced by new integrated setup Fig.27: Depth (TOF) plot for a Lower Panel with inclusion detected

9 The Fig. 28 shows Amplitude plot for a lower panel part with a detected Volume Porosity. 4. TAML/NDT-UT/3/Tech-07 Ultrasonic inspection Technique sheet A350 XWB panels Volume Porosity Fig.28: Amplitude plot for a Lower Panel with Volume Porosity detected 11. Conclusions Easier detection of defects Better reliability and reproducibility Substantially reduce inspection times as well as intuitive displays and data storage for later re-evaluation. Cost Savings than conventional C-Scan equipment. Integrated system has approved by AIRBUS References Flight Standards Information for Airworthiness (FSAW 03-10B), Fuselage Skin Scribe Mark Damage on Boeing 737 Aircraft, November AITM_ AITM Airbus Test Method For Inspection Processes Evaluation of conventional ultrasonic inspection facilities, equipment and probes