1. Introduction Laser speckle strain imaging has long been used in NDT studies of materials, components and structures. This technique was developed in the 1970s by Loughborough University by replacing holographic plates with television cameras and subsequently became known as Electronic Speckle Pattern Interferometry (ESPI). This technique exploits the phenomenon of laser ‘speckle patterns’. ESPI involves frst recording the objects speckle pattern with a television camera. This reference speckle pattern is then electronically combined with a 2 nd speckle pattern from the deformed object to produce computer generated fringes. Each subsequent television picture is compared digitally in real time with the initial stored reference image; changes in displacement are seen as movements in these fringes across the object. A previous paper (1) considering the applicability for non-destructive testing of composites demonstrated is very capable of accurate sub-surface defect location, but of limited industrial value The high sensitivity of ESPI results in fringe numbers which are far too great for any meaningful engineering structure undergoing normal loading. Furthermore, displacement maps are not that useful since components generally respond to variation in areas of higher rates of change in displacement ie in strain fields. Numerical differentiation of interferometric displacement data is inherently noise prone and of very limited dynamic range. To overcome this, the data is optically differentiated prior to it being imaged by the digital camera, giving rise to Electronic Speckle Pattern Shearing Interferometry (ESPSI) as shown in Figures 1 and 2. This in-plane technique, used exclusively by LOE highlights 100% of all faults due to its in and out-of-plane sensitivity and its unique instrument configuration. This approach has a component of in-plane and out-of-plane strain. To explicitly resolve both components it is essential that two sets of data are simultaneously acquired, this is achieved using the dual-head system shown in Figure 3. LASER TECHNIQUES Residual lifetime prediction in aerospace structures using wholefeld laser strain techniques J Tyrer, J Petzing, J Ibrahim, J Jones and L Lobo Paper presented at NDT 2004, the 43 rd Annual British Conference on NDT, September 2004, Torquay, UK. The manufacture of aircraft components requires quality control procedures and mechanisms to identify potentially defective units during manufacture, testing stages as well as during service life. Defects can be formed due to random or systematic errors within the manufacturing process and comprise of tiny fatigue fractures within the wheel structure. Use of such defective wheel and tyre units by an aircraft operator could for example lead to growth of these fractures, which in turn can generate sudden unexpected catastrophic failure on landing. Qualifcation and approval of all wheel units is mandatory under UK Civil Aviation Authority (CAA) and US Federal Aviation Authority America (FAA) regulations as well in the aircraft manufacturer’s standards. The regulatory authorities and manufacturers were requesting improvement in testing and traceability of, analysis methods during manufacture and during routine in service maintenance checks. They were thus considering alternative technologies. Speckle shearing interferometry is a non-contact wholefeld sensor system used primarily within the aerospace industry, for analysis of defects in composite materials, including the analysis of manufacturing defects in aircraft tyres. This methodology has traditionally been used qualitatively to identify the location of faults such as disbonds. Through laser technology and image processing technology advances, it has become more common for this type of instrumentation to deliver quantitative data. However there has been concern about data integrity, data quality and issues of defects being missed due to instrument sensitivity. John Tyrer, Jon Petzing and Jamal Ibrahim are at Loughborough University. John Jones and Leon Lobo are at Laser Optical Engineering. Corresponding author contact: j.r.tyrer@lboro.ac.uk; Tel: +44 (0)1509 227531. Figure 1. The LOE SM-10 strain mapper Figure 2. Layout of the LOE SM-10 strain mapper 74 Insight Vol 47 No 2 February 2005