Phased array ultrasonic testing is a widely used method of sub-surface inspection in NDE, and is able to locate and size defects to a greater degree of confidence and precision than standard ultrasonic testing due to its ability to perform multiple inspections from one location . Data is acquired using Full Matrix Capture (FMC), and an image of the ultrasonic response can be produced using the Total Focussing Method (TFM) , providing an indication of flaws in the inspection region. In addition to the direct ultrasonic wave from array to defect, there will be a wave which reflects from a back wall in the inspection material, arriving at the defect later in time. This appears distinct from the direct signal in the FMC data. These reflected waves can be imaged using TFM , enabling inspection of the component from multiple angles to increase the rate at which defects are detected, and improve characterisation. Typically, applications of the TFM inspect regions of the component which are either directly below or adjacent to the array [4-7] – in other words, within the array’s line-of-sight (LoS).
It is common practice in NDE to prioritise inspection of areas most likely to develop defects. In a complex part such as an aircraft engine, it is likely that these areas will not be LoS. This leads to the requirement for disassembly prior to inspection, increasing cost of the inspection process. This research project aims to develop and validate a method which is able to inspect non-line-of-sight (NLoS) areas by reflecting an ultrasonic wave from the geometry of a component. The initial goal is to evaluate how well a phased array can assess these areas using the simple test case shown in figure 1. Subsequent goals are to target the inspection of increasingly complex cases, including abstract part geometry, anisotropic materials or samples whose geometric or material properties are not precisely known. Ultimately, a method capable of inspecting real, industrially relevant samples such that its deployment in an industrial setting will be justified.
Figure 1: Simple NLoS geometry demonstrating accessibility of a phased array in contact with a component. Shaded region indicates the part which cannot be accessed by direct rays.
Work so far has focussed on developing a ray-based model to produce FMC data including reflections of the ultrasonic wave from any individual wall in the component geometry rather than just the back wall, as well as the expected sensitivity of the probe to side-drilled holes (SDH) in these NLoS TFM images. The sensitivity of a 32-element 5MHz probe to a SDH with diameter 0.4mm is shown in figure 2, where multi-mode TFMs are focussed from side wall reflections. These results indicate that there are views which have non-negligible sensitivity to an ideal defect in NLoS areas. While this is a useful tool for determining the expected signal from a defect in ideal conditions, it does not account for random and coherent noise expected from a real system. These results were therefore validated against finite-element simulations by comparing the expected signal response from defects in a range of positions within the geometry.
Figure 2: Sensitivity maps of unique views of a SDH when rays are reflected from a side wall located at x = 40 mm, using a 32-element 0:5mm pitch, 5MHz centre frequency array in contact with an aluminium block. Decibel scale has been normalised by the maximum intensity across all views.
Next steps in the project are investigate how dependent this sensitivity is to the probe location with respect to the geometry, to ensure that defects can be reliably detected, as well as performing validation against experimental results. Following this, the complexity of the models will be increased, as the assumptions of a polygonal isotropic solid are relaxed to include an arbitrary, anisotropic geometry whose dimensions may not be exactly known. Ultimately, it will be a requirement to validate the tools developed against real, industrially relevant samples.
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