NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

Ultrasonic waves offer the possibility to non-destructively test civil engineering structures. But due to the inhomogeneityof the material non-destructive testing of concrete is difficult for all methods based on wave propagation. Ultrasonic measurements are disturbed by scattering at the random arrangements of aggregates which are a typical content of the material. Scattering at aggregates leads to grain noise and to a speckle structure in SAFT-images. Detection of objects embedded in concrete becomes difficult and depends more or less on subjective criteria of the operator.

The aim of the investigations presented here is to establish a more objective way for image interpretation utilising statistical image processing. The task of distinguishing obstacle indications from artifacts is formulated as a detection problem similar to methods used in radar target detection. The investigations yield to a technique which does not use any morphologic information of the signals. An algorithm is developed which adjusts a detection threshold to maintain a constant false alarm probability over the depth of the material. The value of the threshold depends on the length of the sound path and on the aggregate size distribution of the structure under test. Weibull and lognormal distributions are used to model the amplitude statistics of the received signals. In a number of controlled experiments the dependency of the parameters of the distributions from the properties of concrete materials is examined.

The proposed technique can be used to reduce the distortion of SAFT-images by speckle and to help detecting embedded objects. It is a step towards introducing objective measures into ultrasonic testing of a material which up to date poses many difficulties on standard evaluation methods. Further enhancements are planned by including morphological information.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

Publication Contact:
7th ECNDT, Park Alle 345, DK-2605 Broendby, Denmark, Fax: +45 46 26 70 11, Email: 101373.3414@compuserve.com