Ensuring integrity

ensuring-integrity
ensuring-integrity

Increasing demand for aerospace composite structures has resulted in a dramatic boost in orders for Aldershot-based Ultrasonic Sciences Ltd (USL), which develops automated ultrasonic testing systems entirely in-house.


Composite companies throughout the world have placed orders with USL for large testing machines to support the new commercial and military programs. For more than 20 years it has been designing and manufacturing these systems, which are required by a wide range of industries, including aerospace, metal manufacturing, road and rail transport and integrated circuit manufacture. Companies such as Rolls-Royce and BAE Systems are major users of this equipment for inspection of advanced fabricated products in both metals and composites, to ensure that they are free from manufacturing defects. From the initial starting point of supplying small desk top sized units, the machines have gradually increased in size and now include large multi-axis systems up to 30m long. The USL machines for ultrasonic testing of composites are based on a number of standard designs which are built according to the size range and complexity of the parts to be tested. These designs are modified and added in line with special customer requirements and can include additional manipulator axes, special adaptors to access difficult areas and custom software processing for automated analysis. Machines are designed and manufactured at USL’s factories where there are facilities for welding of large structures, CNC sheet metal fabrication and assembly as well as mechanical design. The motion drive electronics, ultrasonic and data acquisition systems are entirely designed and produced by USL using specialised UK sources for PCB manufacture and assembly. Software is written in-house to control the complex motion profiles as well as the data acquisition and imaging routines. In addition to manufacture of complete testing systems, USL also undertakes upgrades of existing machines which require new ultrasonic, motion control and imaging functions to bring them up-to-date. Sound methodology In ultrasonic testing, pulses of high frequency sound energy are transmitted into the part under test by scanning a transducer over the surface of the component. The sound energy reflected from internal features or transmitted through the part is analysed and used to generate an ultrasound plan view image, known as a C Scan, which reveals internal features such as material structure and defects. Any defects can be measured and compared with acceptance standards, which are specified according to the service stresses, fatigue life and similar criteria. The result will determine whether a basic material or manufactured part is fit for purpose. High frequency ultrasound is not transmitted to a significant degree through air, so the testing has to be done by immersing the part in a liquid or by using water jets to transmit and receive the sound. Metals are usually inspected by full immersion. Composites can be tested both by immersion or water jet methods, but in the case of large panels the latter method, also known as squirter inspection, is usually used. Parts with honeycomb or foam cores are invariably tested using this method. The transducer, or transducers in the case of through transmission testing, must be scanned over the test part at a pre-determined angle for a valid ultrasound image to be produced. For through transmission testing, the transducers on each side must also be perfectly aligned. Scanning speeds of up to 1m/second are used to minimise the total inspection time. Sometimes multiple individual transducers or arrays are used to reduce the time further. Composite parts in particular are seldom flat and in many cases have very complex shapes. The transducers must follow the surface profile with high accuracy if the inspection is to be valid and reliable. The USL systems incorporate sophisticated motion control hardware and software to ensure that this can be achieved at high scanning speeds. The trajectory of the transducers is programmed using teach and learn procedures, or preferably by CAD data input, which in the case of aerospace composite parts is usually generated from CATIA. From frames to fans One application typical of the methodology is in the inspection of parts for the F35 Lightning II Joint Strike Fighter – a collaboration between Lockheed Martin and BAE Systems. USL first supplied a multi-axis squirter inspection system in 2004 and additional scanners for the project have been supplied to companies in UK, The Netherlands and Denmark. In the civil aircraft arena USL systems are in use by manufacturers of composite structures for Airbus, Boeing and others in Europe, Malaysia, USA and China. The latest 11-axis machine was supplied to Hindustan Aeronautics in India and currently six large scanners are in production for Chinese and European customers. These USL units up to now have been used primarily for inspection of composite parts for airframe applications such as fuselage panels, leading edge/trailing edge and other wing or tailplane structures. A USL immersion testing system is now being used at the CTAL joint venture between GKN Aerospace and Rolls-Royce, which has been set-up to develop pioneering manufacturing processes for aero-engine fan blades and composite fan cases. In addition to C Scan machines at Rolls-Royce factories throughout the UK, eight systems have recently been installed at a new manufacturing facility in Singapore. Solving blade inspection Inspection of composite fan blades requires different ultrasonic techniques, especially in the development phase. For example the USL system uses three methods simultaneously during a single scan: through transmission, pulse echo and full waveform capture. The through transmission method generates a ‘C scan’ image which reveals structural defects such as de-laminations, disbonds and porosity but it gives no information about the depth of defects and features. Pulse echo also produces images showing defects and structural features but adds depth information. A combination of the two methods can help to give an insight into the cause of defects and the possibility of repair. Then, with full waveform acquisition, the inspection can be replayed at a later date using different software processing methods. For example, cross-sectional images can be generated, similar to those used for unborn babies and known in industry as B scans. These images can be ‘scrolled’ across the component to reveal variations in the structural features and ply lay-up. This technique is not normally used for production applications, where the acceptance criteria are well established, but is invaluable in development environments. For now, and particularly with the increasing use of composites in aerospace construction, USL anticipates that the demand for ultrasonic testing systems like this will increase further. At the same time, the complexity of the structures is getting greater, especially as processes such as resin transfer moulding become more widely applied. This places great demands on manufacturers like USL to develop testing systems which can keep pace with the production rates and maintain close to 100% coverage. www.ultrasonic-sciences.co.uk

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