Introduction to Phased Array Testing of Composites
Phased array ultrasonic testing (PAUT) of aerospace composites during the fabrication stage is an important quality control step to assure that composite structures do not contain delaminations, fiber defects, interlayer bonding issues, voids and ply cracking. Phased array testing of composites is comparably more challenging that testing metals due to the material anisotropy, high attenuation, and thin material thickness. However, conventional ultrasonic and phased array testing are deeply embedded in the aerospace composite inspection industry. The non-destructive testing is mainly applied using highly sophisticated fully automated multi-axis immersion, waterjet, bubbler systems and semi-automated inspection using roller probes. However, some non-destructive testing applications still require manual inspection to confirm inspection findings.
Phased Array Linear Transducers
Aerospace composite PAUT is largely performed with linear versus sectoral scans due to the difficulty of steering beams through anisotropic materials. Additionally, composite structures generally lend themselves to straight beam inspection due to material geometry and thickness. A phased array linear scan inspects the component by sweeping a fixed angle aperture across the width of the transducer. The RollerFORM wheel probe offered by Olympus is excellent inspection platform that allows for semi-automated inspections. The PAUT probe comes with 64 or 128 elements, 0.8 and 1 mm pitches, respectively. Experience has shown that a minimum aperture of 8 elements is optimal for composite material backwall and defect signal-to-noise ratio (SNR) and resolution. Smaller phased array apertures will decrease SNR, increase beam spread, but improve index axis resolution. Larger PAUT apertures will increase SNR, decrease beam spread, but decrease index axis resolution. PAUT linear probe footprint is another important consideration. By increasing the PAUT footprint, inspection may be completed more efficiently. A 64 element 1 mm pitch PAUT probe will cover half are compared to a 128 element 1 mm pitch PAUT probe. However, larger probes are more expensive.
Figure 1: Phased array roller probe used for composite inspection.
Phased Array Inspection of Aerospace Composite
Automated inspection of composites, conventional or phased array ultrasonic testing, can be delivered using traditional full immersion, water jet, bubbler, or roller probe inspection platforms. The data acquisition and analysis fundamentals are the same, however. The cited solutions use automation to generate high precision – high accuracy scans, water coupling to assure transfer of ultrasoound to and from the tested composite, and A-scan, S-scan, and C-scan data analyses.
The inspection data shown in the above video displays the PAUT A-scan, S-Scan, and C-scans. The A-scan in the top left displays the current beam or focal law selected in the S-scan. During this automated phased array inspection, three different reflections are observed. The first reflection is the interface echo received from the top surface of the composite. Due to the large difference in acoustic impedance between the water path and composite this reflection will always be high amplitude.
The second PAUT reflection, periodically observed at lower amplitude, are from the various delaminations located between the front and back surfaces of the composite. The ultrasonic reflections range from approximately 30 to 80 %FSH depending on size. These reflection may be sized in 2-D using the phased array index and scan axes.
Last, but not least, is the phased array back wall reflection (BWR). In the absence of a delamination, the BWR is very consistent at approximately 25 %FSH due to water coupling. In the presence of a delamination, the backwall becomes undetectable as shown in Figure 2. Sometime near surface delaminations are only detectable via loss of BWR since the actual reflection is too difficult to discriminate from the interface echo.
The phased array red data and yellow interface echo gates are shown in the A-scan. Notice in the video, that the interface echo is constantly shifting due to small changes in composite surface position relative to the transducer. The same shifts are observed in the for the delamination and backwall reflections. In order to maintain accurate defect depth and wall thickness measurements, these reflectors must be measured relative to the moving interface echo. This is accomplished by synchronizing the red data gate to the edge of the interface echo crossing the yellow interface echo gate.
Figure 2: Phased array back wall reflection loss.
The phased array S-scan, L-scan or B-scan is shown in the top right. The L-scan is constructed from many amplitude color coded A-scans and is dependent on the number of PAUT probe elements and aperture selected. This phased array immersion test used a 5 MHz, 64 element, 0.80 mm pitch probe. The total active probe width is 51 mm. Using 8 elements, the active aperture is 6.4 mm or approximately 0.25”. Therefore, the L-scan shown consists of 57 different A-scans (No. of element – active aperture + 1). The interface echo is observed at the top of the L-scan and is present across all apertures. The PAUT BWR is observed at approximately 6-7 mm at lower amplitude. In Figure 2, the BWR reflection is lost and a 6 mm, or 0.25”, wide delamination is observed at the lower PAUT probe apertures.
The PAUT C-scan on the bottom was generated at 1 mm scan axis resolution and was 650 mm in length. Therefore, a total of 650 L-scans and 37,050 A-scans were used to generate the C-scan. The intermittent delaminations are observed at the lower PAUT probe apertures of the C-scan. The length and width of the delaminations may be measured using the scan and index axes measurement and reference cursors. In the Figure 2 example, the delamination located at approximately 360 mm is 8 mm long.
Summary
Phased array ultrasonic testing (PAUT) using linear array probes in immersion, waterjet, bubbler, and roller probe automated and semi-automated configurations is a precise and accurate technique to test newly fabricated composites for a variety of defects. These advanced NDT methods provide high quality ultrasonic A-scan, S-scan, and C-scans that allow for accurate 2-D sizing. For automated scans, it is important to synchorinze the data gate to the edge of the interface echo crossing the interface echo gate. Consult our expert team to learn more about PAUT applications, Level 2 phased array 80-hours training course, and phased array inspection services.
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