Surface-Adaptive Ultrasound (SAUL)

Designed for complex composite structure inspection, the Surface-Adaptive Ultrasound (SAUL) technique enables the inspection of components with varying geometries (flat, concave, convex surfaces), using a unique configuration. SAUL is a development tool for experienced users aiming to design inspection procedures.

A lot of the composite structures encountered in the aerospace industry have complex and variable geometries. Traditional ultrasound NDT requires either specific probes, specific coupling tools for each reference and/or high-resolution surface profilometer tools, making the whole process difficult to automate. SAUL is a new technique implemented for industry by M2M, that adds adaptability to any given phased-array probe.



SAUL is an iterative process that first learns the surface profile and then sends normal-incident waves into the component.

The advantage of this technique, is that it can inspect with one probe, different geometries (stringer, radius,…) in a full automated immersion technique. Combined with the parallel architecture of the MultiX++ SAUL enables fast inspection of complex composite components.SAUL_detouréSAUL legende


Surface-Adaptive ULtrasound (SAUL) Nondestructive testing for Airbus A350 composite parts ©Stelia Aerospace Composites

First Shot: all elements are fired without delay law.



Wave propagation




The signals reemitted by the specimen are recorded by all the elements. A new delay law is computed in real time taking into account of the retrodiffused signals and applied.


Wave propagation after SAUL convergence

Radius inspection (1st shot)

Radius inspection (1st shot)

Radius inspection after SAUL convergence

Radius inspection after SAUL convergence








SAUL_composite inspection_logo stelia ©M2M_BD

saul composite inspection_imaging



The SAUL technique enables the transmission of an incident wave-front parallel to any complex surface. One specimen presenting variable geometries (flat, concave or convex surfaces) can be entirely controlled using a unique probe (such as a matrix array with a flat active aperture). This is achieved by means of an iterative algorithm that does not require the knowledge of the geometrical and acoustical properties of the component undergoing inspection. The real-time adaptive processing is illustrated through measurements obtained with typical aircraft composite structures (e.g., CFRP stringers, stiffeners).

The principle of the SAUL algorithm may be decomposed as follows. The first step consists in transmitting a plane wave with the full array and recording elementary signals in parallel for all channels. The B-scan obtained is displayed in the figure below.


As the exited wave is not normally incident on the surface, the consecutive plies inside the material (each ply being approximately parallel to the surface) deviate the transmitted field. This results in a poor image where the back-wall interface of the specimen cannot be identified.

The second step consists in adapting the incident wave to the surface geometry. This process relies on measuring the times of flight between all elements of the array and the surface (e.g., by detecting the maxima of the surface echo envelope). These times of flight are used to extract a delay law that will be applied to a second transmission. A reception delay law can also be applied in order to synchronize received elementary signals and to create several coherent summations of signals via electronic scanning of a sub-aperture. This algorithm is iterative and converges after a couple shots (e.g., 3 shots per position).


Figure: example of surface adapting using the SAUL algorithm

The SAUL algorithm has been implemented on M2M MultiX
systems. An industrial application has been achieved with CONTOUR DYNAMICS (formerly MECNOV) and EADS (AIRBUS parts). This method may be extended to many other industrial projects, such as the inspection of turbine blades, wavy plates, and all types of metallic/composite structures presenting irregular surfaces.


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.

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

S. Robert, O. Casula, M. Njiki, O. Roy, ‘Assessment of Real-Time Techniques for Ultrasonic Nondestructive Testing’, Review of Progress in QNDE, in press, 2012.

S. Robert, O. Casula, A. Nadim, ‘Procédé de Commande de Transducteurs d’une Sonde à Ultrasons, Programme d’Ordinateur et Dispositif de Sondage par Ultrasons’, France Patent No FR 10/56217, July 2010.

S. Robert, O. Casula, E. Iakovleva, ‘Procédé de Détermination d’une Surface d’un Objet par Sondage Echographique, Programme d’Ordinateur Correspondant et Dispositif de Sondage à Ultrasons’, France Patent No FR 11/60399, November 2011.