New challenges for NDT

Dr Christophe Mattei, senior scientist based at Exova, Linköping, Sweden looks at how integrated composite structures are challenging the traditional ways of performing NDT. The strength analysis is approved, the process is developed, but what if this elegant and innovative composite design will never fly because a technique for checking the component for porosity is unavailable? Will NDT really be the bottleneck of the manufacturing process?

The design and manufacturing process of composite components for aerostructures is in constant evolution. Driven by cost optimisation and weight reduction objectives, manufacturers and material suppliers develop innovative solutions to integrate structures into complex single components that are manufactured in ‘one hit'. 3D lay-ups, infusion processes, co-curing of laminate and adhesive bond lines are all examples of these new design and manufacturing process that show great cost potential and weight reduction by eliminating a number of costly steps in the production process.

The specifics of integrated composite structures centre on components and materials that are manufactured simultaneously. This calls for quality testing and if needed, repair procedures that are performed right after the curing process. Early generations of aircraft involved mainly composite laminates and stringers and once developed, qualified NDT and repair procedures could often be reused for other components.

The application of complex 3D structures has greatly changed this situation and new procedures, often based on the development of dedicated equipment and operator skills, need to be redeveloped to a certain degree each time. An example is the case of ultrasonic inspection of composite structures.

Inspection of composite material in production is mainly performed using ultrasonic inspection due to the fact that the technique is sensitive to all traditional defect types that can potentially occur in a composite structure. In the presence of inconsistencies such as delamination, foreign objects, voids or porosity, reflection or scattering of part of the incident waves are used to detect the size and quantify the extent of the defective zone. One important requirement for ultrasonic-based inspection of composite structures is that the ultrasonic beam must be carefully aligned to the surface of the component in order to ensure that a constant amount of energy is radiated into the material. Robust and precise mechanical systems are usually needed to hold and maintain the ultrasonic probe with controlled positioning and orientation while scanning composite components.
 
Clearly, the development of fully integrated composite structures where the use of sharp radii, corners and stepping predominate, push the requirements of inspection procedures and instrumentation one step further. Complex gantry- or robot-based systems designed for scanning doubled curved skin panels have shown limitations for highly integrated structures. Complex contour following, access constraints inside integrated structures and the need for representation and interpretation of 3D ultrasonic data are some of the technical challenges requiring an evolutionary shift in technology.

Within the aerospace industry, the NDT community is answering these new challenges by implementing production technologies recently developed in research laboratories. The production applications of phased array systems - similar in principle to medical ultrasonic imaging systems - illustrate that an innovative approach was needed.

Phased array systems enable the automated control, steering and shape of the ultrasonic beam, giving much needed flexibility for adapting the inspection parameters to the shape of the component.

It's clear that the complex geometries and material structures of 3D integrated composite components can be a challenge for quality assurance. Therefore, the ‘inspectability' of a new design becomes an important parameter when evaluating its feasibility. Will the NDT technology available today be able to detect and size the relevant defects types anywhere in the component? An early and quantitative answer to this question is that for most of the time, there is a requirement to avoid bottlenecks further down in the development process.

Evaluating the ‘inspectability' of a complex composite structure is not always a trivial task. Traditionally, optimisation and demonstration of the performance of the NDT process is done by manufacturing components with imbedded reference defects simulating ‘real-life' process variations. If these reference mock-ups were relatively easy to manufacture for aircraft laminates, it would be a real challenge to manufacture components with controlled levels of porosity in a 3D structure. A good understanding of both manufacturing processes and NDT techniques is required to develop the steps needed to define, optimise and validate an inspection procedure.

A way forward is by using simulation tools. Simulation of the processes involved in composite manufacturing for optimisation and evaluation is an approach that has seen increasing development in recent years. In the nuclear energy industry, simulation of NDT inspection based on physical models is now well accepted for the development and technical support during qualification of procedures. Relevant material models for composite structures were recently introduced in some of these tools and interfaces with CAD tools are now being implemented. This opens the door for the complete virtual inspection of new designs that could be instrumental in evaluating the ‘inspectability' of a design.

Another issue associated with integrated structures is the growing interest in ‘new defect types'. An example is the potential need for characterisation of fibre angle deviation, waviness, gaps and cuts that can occur when draping 3D complex structures. In most cases these process variations were not seen in aircraft composite laminates. Detecting and characterising these types of defects implies techniques that can image the inner structure of the material. Ultrasonics, pulsed thermography and eddy currents are techniques that have shown great potential for this purpose.

Within the NDT community, development teams are focused on finding tools and strategies to help NDT operators quickly detect and quantify these potential structural variations hidden in the material.

If it was expected that the drive for cost and weight optimisation of aerostructures would lead to the development of new processes in the manufacturing chain, an unexpected effect for the NDT function may well be its new position as the potential ‘weak link'. Elegant and cost-effective design solutions may just end up in the bin if they cannot be inspected according to the requirements in place.

The realisation of these challenges has pushed the technology forward in recent years, but thoroughly assessing the possibilities and limitations of NDT at the early design stage in the development of new components is clearly the key to successful designs.

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