Phased array design for optimized directivity behavior in guided wave applications

Abstract

Guided Lamb waves in the ultrasonic range have potential for structural health monitoring of thin structures, e.g. for the detection of impact damages which may cause delamination in carbon fiber reinforced materials. For the emission of guided waves piezoelectric transducers can be used which are applied to the surface of the structure. By using a phased array of transducers a directivity pattern for the inspection of a limited area on the structure can be created with common beam forming algorithms. Line arrays require only a small number of transducers but the main lobe is generated on both sides of the array which means an excitation towards an unwanted direction is produced. In this contribution a 2D array design is introduced which tends to emit only one main lobe towards the direction of interest. The concept basically utilizes two parallel line arrays. Both arrays emit signals with a single burst. The signal emitted by the second line array is meant to suppress the unwanted lobe of the main array by out-of-phase superposition. This requires an appropriate timing of the emission of the signals of the single transducers. The feasibility of the concept has been studied by simulation. Practical experiments on CFRP (carbon fiber reinforced polymer) sheets have been carried out with an array layout with eight single piezoelectric transducers. A PC-controlled electronics system has been used for the actuation of the transducers. Emission and directional behavior of the Lamb waves on the structure has been monitored with a Laser scanning vibrometer.