TITANIUM IS A relatively new engineering material: it was discovered in the late nineteenth century, much later than other commonly used materials, and was not used in commercial applications until the late 1940s. Since the introduction of titanium and titanium alloys, these materials have, in a relatively short time, become the backbone materials for a wide range of industries. Because of titanium's excellent corrosion resistance, it is used for chemical processing, desalination, power generation equipment and prosthetic devices. Its high strength-to-weight ratio and ability to withstand extreme temperatures makes it especially well suited to Structurally Significant Item (SSI) applications in aerospace engineering.
Such SSI components originate from the initial manufacture of a titanium billet, a circular solid bar of up to 400mm in diameter and several metres in length. The manufacturing process may produce sub-surface defects that are particularly difficult to detect at the stage of production. Failure to detect these incipient defects can lead to subsequent in-service failure, the consequences of which can be disastrous. One such catastrophe was the crash of flight UA232 in 1989, which resulted in 111 fatalities and was due to a titanium engine disc failure. The cause of this disaster was directly attributable to metallurgical defects not detected during the manufacturing process, and whilst it is the most well-known of such incidents it is by no means the only one. Indeed, the current level of service failures indicates that detection methods are still inadequate, with low sensitivity, low repeatability, high cost and high intrusion on the production process. It is to this situation that Dr Patel's project - 'Development of New and Novel Quality Control System for the Inspection of Titanium Components in Safety Critical Applications in the Aerospace Industry' - responds. Appropriately abbreviated to 'QualiTi', the project addresses the obvious and urgent need to develop improved and advanced non-destructive testing(NDT) technologies for application during the manufacture of titanium destined for aircraft components. QualiTi brings together partners from across Europe to develop a new and novel NDT system to fully inspect titanium billets using combined phased array ultrasonic and eddy current technology. The QualiTi system employs sensors applied by an automated system, minimising the use of manual inspections. The system employs phased array (PA) technology as an alternative to the conventional, multizone inspection ultrasonic which is currently used. The PA ultrasound inspection system is being developed at the TWIN DT Validation Centre (Wales), UK. The PA probe was designed with the aid of Acoustic Ideas Inc. using Probe Designer software, and is currently being manufactured by Vermon SA, France. The probe uses a total of 255 elements and has an ultrasonic centre frequency of 5MHz. The design uses a customised contour represented by a fifth order set of cosine basis functions. The probe is an elliptic shape with a long axis of 98mm and a short axis of 78mm.The probe was designed to deliver a 2.5mm diameter ultrasound beam spot at all inspection depths from just beyond the blind zone (5mm) to half an inch past the centre of the 10" billet (139mmfrom the surface).
With ultrasonic testing (UT) inspection, the strong interface echo from the front face of the billet makes it impossible to reliably detect defects near the surface; the interface echo creating a blind zone of approximately 5mm at the edge of the billet. QualiTi has developed an innovative solution to this problem, as Dr Patel explains: "We employ a complementary eddy current inspection system using a hybrid probe of 5 coils, which has been developed at West Pomeranian University of Technology, Poland". This complementary inspection system is able to inspect the titanium within 5mm of the surface, which when combined with PA ensures complete inspection coverage. The benefits of using state-of-the-art phased array technology over conventional UT stems from its ability to use multiple elements to steer, focus and scan beams with a single transducer assembly. Beam steering, commonly referred to as sectorial scanning, can be used for mapping components at appropriate angles, which can greatly simplify the inspection of components with complex geometries. The small footprint of the transducer, and the ability to sweep the beam without moving the probe, also aids inspection of components in situations where there is limited access for mechanical scanning. The ability to test materials with multiple angles from a single probe greatly increases the probability of detection of anomalies. Electronic focusing permits optimising the beam shape and size at the expected defect location, thus further optimising probability of detection. The ability to focus at multiple depths also improves the ability for sizing critical defects in volumetric inspections.
A COLLABORATIVE AND OUTWARD-LOOKING APPROACH
QualiTi is funded in part by the European Commission (EC) under the Research for the Benefit of Small to Medium Enterprises (SME) programme. SMEs represent 99 per cent of all enterprises in Europe, contributing more than two thirds of European GDP and providing 75 million jobs in the private sector. They are therefore a key to the implementation of the renewed Lisbon strategy for economic growth and employment. The EC programme aims to help SMEs outsource research, increase their research efforts, extend their networks, better exploit research results and acquire technological know-how, narrowing the gap between research and innovation. An important aspect of all EC funded projects is the dissemination and exploitation of the results and this is certainly true for QualiTi. For SMEs to benefit from the research and development undertaken within the project, all participants are required to commit to dissemination activities. These activities included the creation of this website in 2008, the publication of peer-reviewed scientific papers, presentations of the project to industry conferences and an interactive video for further internet dissemination.
SECURING A FUTURE FOR EUROPEAN SMES
QualiTi has undertaken important research and development work with its partners, and made very valuable advancements in inspection technologies, in addition to the development of a more integrated and accurate system of inspection for titanium billets. By the end of the project it is expected to have a number of products commercially available through all SME partners; these products include: 3D beam steerable ultrasonic PA sensor and calibration surface, an EC inspection sensor and system and the QualiTi integrated titanium inspection system. These are considerable accomplishments and a remarkable model of what collaborative research can achieve. QualiTi is a good example of how effectively small groups of innovative SMEs and their RTD partners can solve technological problems and produce outcomes that will make them more competitive in the global marketplace.