Study of phased array techniques for cracks characterization in concrete structures: Fid-Bau Portal

Study of phased array techniques for cracks characterization in concrete structures

Paris, O. / Bredif, P. / Roy, O. / Rambach, J.M. / Nahas, G.

The concrete containment building of nuclear power plant (NPP) structures are susceptible to ageing by various processes depending on the operating environment and service conditions. In order to ensure the structural capacity and leak tight integrity of concrete structures, it is necessary to detect cracking phenomena which is one of the primary sign of ageing. The aim of the present work is to propose a NDT method able to characterize and to represent the crack shape inside the concrete wall. Ultrasonics were identified as having a great potential to detect cracks in such thick concrete structures. Typical frequency range used for ultrasonic control in concrete is low between 50 and 250 kHz. This study drew its inspiration from ultrasonic process applied on steel components. We assume that a crack can be seen as a set of facets with different orientations. Each facet is considered as a reflector. Since the maximum size of the aggregates is 20 mm, we use transducers working at 250 kHz with large bandwith to detect reflectors with a minimum diameter of 20 mm. Such high frequency produces poor signal to noise ratio (SNR), so we use a combination of several transducers to increase the SNR. This paper deals with the results obtained with two different techniques applied to characterize the artificial crack: SAFT processing and Phased array techniques. Multi probes reconstruction using principle of SAFT processing have been tested from results of experiments performed with a set of several transducers. The reconstruction using theoretical parameters of reconstruction is effective to detect with an improved SNR the different facets composing the crack. Tests of focussing of the ultrasonic beam from five transducers have been also performed successfully at different depth for the detection of flat holes and crack. Phase tests show he great potential of phased array techniques applied to concrete structure inspection. Complete control of the array of probes at transmission and at reception will allow to improve the performance at various depths. Next test will concern the detection of tilted reflectors.