# Real-Time Nondestructive Testing of Composite Aerospace Structures with a Self-Adaptive Technique

Sébastien Robert, Olivier Casula, Olivier Roy, Guillaume Neau

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Sébastien Robert, Olivier Casula, Olivier Roy, Guillaume Neau. Real-Time Nondestructive Testing of Composite Aerospace Structures with a Self-Adaptive Technique. 4th International Symposium on NDT in Aerospace 2012, Nov 2012, Augsburg, Germany. pp.1-8. hal-05049565

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# Real-Time Nondestructive Testing of Composite Aerospace Structures with a Self-Adaptive Technique

Sébastien ROBERT*, Olivier CASULA*, Olivier ROY**, Guillaume NEAU**

*CEA-LIST, F-91191, Gif-sur-Yvette cedex, France

**M2M, 1 rue de Terre-Neuve, 91940, Les Ulis, France

**Abstract.** In aerospace industry, composite structures under testing have complex and variable geometries. In this case, an optimal use of ultrasonic phased arrays requires specific algorithms in electronic systems in order to achieve rapid and reliable inspections. To fulfil such requirements, a new real-time adaptive inspection method is presented. This technique enables the transmission of an incident wave-front parallel to any complex surface. It allows to inspect a given component presenting variable geometries in its totality using a same transducer array. This is achieved by means of an iterative algorithm that does not require the knowledge of the geometrical properties of the component undergoing inspection. In this paper, the real-time adaptive processing is illustrated through acquisitions performed with different typical aerospace composite structures.

## Introduction

New commercial aircrafts incorporate more and more composite materials in their structures to ensure both lightness and robustness. This increase in composite surfaces motivates the development of new techniques in ultrasonic nondestructive testing (NDT) to achieve fast and reliable inspections.

Ultrasonic phased arrays are the most promising technological solution to reduce scanning times and to also improve the characterization of defects. In general, these sensors are used to control manufacturing defects (porosities, foreign bodies, delaminations, cracks...) introduced in composite materials during manufacturing processes [1]. Inspection is perfor-med in a tank filled with water, and a maximum scanning speed is desired to reduce NDT costs, especially for large composite structures.

One of the most famous acquisition modes is the so-called 'paintbrush' acquisition, that consists in transmitting once with all elements of a phased array, without delay law, and receiving elementary signals in parallel on all elements [2, 3]. This mode leads to high scanning speeds that do not depend on the number of used elements. In order to improve the signal to noise ratio, a sub-aperture of few consecutive elements can be used in reception. This sub-aperture is electronically moved along the whole aperture to obtain a moving average of the received elementary signals.

For complex composite parts, such as the corner of a stiffened part (a corner radius), a solution consists in using shaped probes, such as cylindrical phased arrays. The probe is positioned over the convex surface and its curvature radius ensures a normal incidence transmission of the ultrasonic field at any point of the corner surface [4, 5]. Inspection is

<figure><img src="image_3.png" /></figure>
performed by mechanically adjusting the lateral position during scanning to master the normal incidence transmission. However, the main drawback of shaped probes is that they are valid for one geometry type. Other parts of a given component (with flat, concave, shoulder geometries) may require a large inventory of probes with different shapes, which leads to expensive NDT costs and reduces inspection speeds when a complex component must be inspected in its entire totality.

In this paper, a new real-time adaptive inspection method is presented. The technique allows to master a normal incidence transmission through various complex surfaces using a conventional no-shaped phased array (typically a linear or matrix array with a flat active aperture). A given component presenting variable geometries can be inspected in its totality using a same probe. The self-adaptive SAUL technique ('Surface Adaptive Ultrasounds') has been implemented in the MultiX systems, designed by the M2M Company, and the real-time processing is illustrated in the present paper through acquisitions performed with different composite structures.

# 1. Principle of the SAUL method

## 1.1 Paintbrush acquisition mode

A typical corner radius of Carbon Fiber Reinforced Plastic (CFRP) is schematized in figure 1. Its thickness is about 5 mm. A linear phased array with a central frequency of 5 MHz is positoned over the convex surface, with a water path of 16 mm. The probe is composed of 42 elements of 0.5x8 mm², with 0.6 mm pitch. The curvature radius of the front interface is 14 mm.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** A linear transducer array placed in front of the convex surface of a corner radius</figcaption></figure>

The paintbrush acquisition mode consists in transmitting a plane wave with the whole array and in recording the elementary signals in parallel with all elements. The experimental Bscan (channels/time) acquired with a MultiX system is presented in figure 2 (no electronic scanning is applied in reception).

As the incident wave is not normally incident on the surface, the successive plies inside the material (each ply being approximately parallel to the surface) refract the transmitted field during its propagation and practically no energy reaches the back surface. This results in a poor image in figure 2 where both the plies and the back interface of the corner radius cannot be identified.
<figure><img src="image_5.png" /><figcaption>Figure 2. Bscan image obtained in paintbrush acquisition mode</figcaption></figure>

## 1.2 Principle for adapting a wave to a complex surface

The incident field can be partially adapted to the surface according to the processing described in [2] and [6, 7]. It consists in measuring the times of flight between all elements of the array and the surface (for instance, by detecting the maximum of the surface echo envelope). Next, these times of flight are used to extract a delay law that will be applied to a second shot. Noting $t_i$ the time of flight between element i and the surface ($1 \le i \le 42$), the transmission delay applied to this element is defined as:

$$E_i = \frac{1}{2} [\max(t_i) - t_i]. \qquad (1)$$

A reception delay law can also be applied in order to synchronize received elementary signals and, for instance, to create several coherent summations of signals using an electronic scanning of a sub-aperture. The reception delay applied to element i is deduced from the emission delays as follows:

$$R_i = \max(E_i) - E_i. \qquad (2)$$

The times of flight measured from the surface echo and the corresponding delay laws are respectively presented in figures 3(a) and 3(b). The new Bscan (channels/time) when both the transmission and reception delay laws are applied is displayed in figure 4.

<figure><img src="image_6.png" /><figcaption>Figure 3. Times of flight measured from the surface echo (a) and transmission and reception delay laws deduced from these data (b)</figcaption></figure>

Compared to the previous result, the bottom echo now can be identified in figure 4, which confirms that the delay law calculated with (1) transmits an incident wave front
approximately parallel to the surface; the anisotropy effects due to the fibrous nature of the material are minimized by forcing a normal incidence transmission. However, as the incident wave is not completely adapted to the surface, the bottom echo has a low amplitude level and the internal composite structure still can not be identified. The plies are not clearly visible. In addition, the 42 signals are not exactly synchronized, which could result in a poor image if an electronic scanning with a sub-aperture of several elements was used in reception.

<figure><img src="image_7.png" /><figcaption>Figure 4. Bscan image when applying adaptive delay laws</figcaption></figure>

For a geometry as complex as a corner radius, the incident wave is not perfectly adapted to the surface since the times of flight used in the delay law computation do not correspond to specular reflections. A technique for measuring these specular times of flight consists in using an electronic scanning in emission with a sub-aperture and a scanning step of 1 element (all elements are exited one by one). Nevertheless, in practice, this technique is not applicable to industrial controls for two reasons. First, as a single element is used at each transmission, the amplitude level of the surface echo is much lower than the one obtained by transmitting with the full array and times of flight may be difficult to be measured. Second, the large number of shots (43 in the present case) may dramatically decreases inspection rates compare to a conventional paintbrush acquisition mode.

### 1.3 The SAUL algorithm

The SAUL method (acronym for ‘Surface Adaptive Ultrasounds’) is an iterative processing of a conventional paintbrush acquisition [8]. The processing thus begins with the transmission of a plane wave by simultaneously exiting all elements and recording elementary signals in parallel with the whole array. As previously described, the adaptive delay laws are then calculated and applied to a second ultrasonic shot.

In the SAUL processing, this processing is iterated as many times as necessary, until the incident wave ideally fits the front surface, i.e.: new delays laws are calculated after the second shot and are added to the previous ones; then, the resulting delay laws are applied to a third shot; and so on, until the iterative processing converges. More precisely, the transmission delay applied to element i (with $1 \le i \le 42$) is calculated as follows:

$$E_{i}^{(j+1)} = \frac{1}{2} \left[ \max(t_{i}^{(j)}) - t_{i}^{(j)} \right] + E_{i}^{(j)}, \quad (3)$$

$$E_{i}^{(j+1)} = E_{i}^{(j+1)} - \min[E_{i}^{(j)}]. \quad (4)$$
where j (j = 1, 2,...) denotes the shot number in the iterative processing (j = 1 corresponds to the first transmission without delay law, i.e. $E_i^{(j)} = 0$ for j = 0 $\forall i$). (3) expresses an accumulation of delay laws iteration after iteration, while (4) is an offset correction ensuring that the minimum delay in the accumulated delay law is still zero. In reception, the delays are simply defined as:

$$R_i^{(j+1)} = \max[E_i^{(j+1)}] - E_i^{(j+1)}. \qquad (5)$$

For an optimal implementation in electronic systems, it is more convenient to express $E_i^{(j+1)}$ and $R_i^{(j+1)}$ only with delays related to previous shot j. Substitution of (4) into (3) yields to the following expression for the emission delays:

$$E_i^{(j+1)} = E_i^{(j)} - \frac{t_i^{(j)}}{2} - \min\left(E_i^{(j)} - \frac{t_i^{(j)}}{2}\right). \qquad (6)$$

Next, substituting (6) into (4), the reception delays may be expressed as:

$$R_i^{(j+1)} = \max\left(E_i^{(j)} - \frac{t_i^{(j)}}{2}\right) - \left(E_i^{(j)} - \frac{t_i^{(j)}}{2}\right). \qquad (7)$$

Expressions (6) and (7) are the ones implemented in the MultiX system hardware in order to accelerate inspection rates. In below figure 5, the successive Bscans (channels/times) obtained for the first 4 shots are presented. The experimental set-up is unchanged compared to the previous paragraph.

<figure><img src="image_8.png" /><figcaption>Figure 5. Successive Bscan images in the iterative SAUL processing</figcaption></figure>

The set of Bscans illustrates the gradual adaptation of the incident wave to the surface of the corner radius. Iteration after iteration, the amplitude levels of the front and back surface echoes gradually increase in amplitude, and the received elementary signals progressively become time coherent. At convergence, the Bscan is identical to the one provided by an electronic scanning in transmission.
## 2. Real time SAUL processing

The experimental set-up is presented in figure 6. It consists in a parallel acquisition system with 128 channels, and a PC computer that drives both the system and a linear motorized positioning system to set the probe position. A CFRP corner radius, with the same geometrical properties than the one presented in figure 1, is immersed in a tank. A linear phased array, with 5 MHz central frequency, is positioned in front of the convex surface of the component that presents an artificial defect (a flat bottom hole) at mid-length. This probe is composed of 128 elements of 0.5x8 mm², with 0.6 mm pitch. In our experiment, only 48 active elements are necessary to inspect the curve part of the component.

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Experimental set-up including a parallel acquisition system, a PC computer driving the system, and a composite structure immersed in a tank</figcaption></figure>

Both acquisitions in figure 7 have been performed by translating the transducer to a length of 280 mm, and data have been recorded in 2 mm steps at a speed of 100 mm/s. In figure 7(b), the SAUL processing has been applied by using a cycle of 4 shots for each position, while no processing has been applied in figure 7(a). For each acquisition, an electric scanning with a sub-aperture of 4 adjacent elements and a scanning step of 1 element has been used in reception (which corresponds to 45 sequences in reception). For each scanning step, the mean of 4 elementary signals is calculated and represented in the form of an electronic Bscan (sequence/time).

Each electronic Cscan (position/sequence) in figure 7 results from the concatenation of the electronic Bscans recorded at each acquisition step. The comparison of both images reveals a larger surface echo when the SAUL processing is enabled. This is mainly due to the time coherence of the elementary signals rather than an increase in their amplitude. This surface echo is large enough to enable triggering during scanning and eliminate water path variations. A triggering is essential in order to set an acquisition time window at a given inspection depth below the surface, and to eliminate the surface echo that could hide an echo of interest, such as in figure 7(a). For instance, the mechanical Bscan (position/time) in figure 8(a) represents the evolution of a single sequence (i.e. the average of 4 received elementary signals at the 19th sequence) as function of the probe position. This image shows that the arrival time of the surface echo varies due to a slight misalignment between the scanning axis and the corner radius one. After triggering, in figure 8(b), the water path variations are corrected, so that a 2 µs time window at 3 µs after the surface echo (which
corresponds to an inspection depth of around 4 mm) can be set to detect defects near the back surface.

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Electronic Cscan without processing (a) and by applying the SAUL technique with 4 shots in each cycle of iterations (b)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Mechanical Bscans displayed for the 19th sequence: without triggering (a) and after triggering (b). A 2 µs acquisition time window is fixed at 3 µs after the surface echo</figcaption></figure>

The real time SAUL processing, associated with a correction of water path variations, provides the new electronic Cscan displayed in figure 9. The artificial defect is detected with a suitable signal-to-noise ratio (around 12 dB) at 180 mm from the initial scanning position. Thus, this experimental result clearly demonstrates that it is possible to detect a
defect without any prior accurate knowledge of the geometry and/or the position of a
composite component. Nevertheless, it should be noted that the method does not allow to
characterize a defect (especially, its size) because the SAUL imaging is only a represent-
tation sequence/scanning of received amplitudes. A substantial improvement would be to
reconstruct ‘true’ images inside the component, which requires an implementation of fast
surface reconstruction algorithms in the imaging MultiX systems. Developments are in
progress and a better characterization is expected soon.

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Electronic Cscan provided by a SAUL MultiX system: a defect is detected at 180 mm from the initial scanning position</figcaption></figure>

Conclusion

In this paper, a new adaptive inspection technique implemented in MultiX systems has been presented. This technique is based on an iterative processing of a paintbrush acquisition and requires only a small number of ultrasonic shots to master a normal incidence transmission through a complex surface of a composite structure. All different parts of a given component can be inspected using a same transducer array, such as a conventional probe with a flat active aperture. An acquisition result performed with a corner radius has been presented and shows that the SAUL method is promising in terms of scanning rate and detection ability. Future works will focus on the geometry reconstruction from the SAUL algorithm in order to improve the defect characterization.

Acknowledgement

This work was supported by grants from DGCIS/CG92/CG91/IDF in the framework of the
competitiveness cluster System@tic Paris-Region (CORTEX 3D project).

References

[1] R. D. Adams, P. Cawley, ‘A review of Defect Types and Nondestructive Testing Techniques for Composites and Bonded Joints’, NDT International, Vol 21, No 4, pp. 208-222, 1988.

[2] G. Ithurralde, 'Advanced Functions of PAUT in Aeronautics', ECNDT Proceeding, Berlin, 2006.

[3] N. Dominguez, G. Ithurralde, ‘Ultra-fast Ultrasonic Inspection for Aeronautical Composites using Paintbrush Acquisitions and Data Processing on GPU’, European Conference on NDT, Moscow, 2010.

[4] R. Meier, J. Becker, T. Rehfeld, Method for the Ultrasonic Testing of a Workpiece within a Curved Region of its Surface and Device Suitable for the Execution of the Process, Patent No US 7516664 B2, 2009.

[5] F. D. Young, C. T. Uyehara, Hien T. Bui, ‘Ultrasonic Testing of Corner Radii Having Different Angles and Sizes, Patent No US 0211361 A1, 2009.

[6] A. Maurer, W. Haase, W. De Odorico, ‘Phased Array Application in Industrial Scanning Systems’, ECNDT Proceeding, 2006.

[7] A. Maurer, M. Strauss, W. De Odorico, 'Roman Koch, Method and Circuit Arrangement for Disturbance-Free Examination of Objects by Means of Ultrasonic Waves', Patent No US 0195273 A1, 2006.

[8] S. Robert, O. Casula, M. Njiki, O. Roy, ‘Assessment of Real-Time Techniques for Ultrasonic Nondes-
tructive Testing’, Review of Progress in QNDE, pp. 1960-67, 2012.