Advances in ultrasonic testing - Research into the application of dry point contact transducers

The advent of dry point transducers has allowed new possibilities for ultrasonic testing of concrete structures and rock specimens. Recent developments in the technology such as single and multi-channel ultrasonic pulse echo technology and the automated determination of the modulus of elasticity have been pushing back the boundaries of the method. The effect of this has been to increase the ease with which traditional applications can be carried out and also to introduce new techniques for structural assessment. This paper sets out to highlight the current application spread of ultrasonic testing methods using dry point contacts. It also describes how the new technology compares with traditional methods and the benefits it brings to the user. Finally it will outline the current limitations of the technology and give some indications about the possible directions for further development. 1. Current Applications Using Dry-point Contacts Dry point contact (DPC) transducers are primarily used in ultrasonic pulse echo equipment. By far the two most important application areas of ultrasonic pulse echo as far as concrete testing is concerned are determination of slab thickness and the detection of voids and delaminations. Slab thicknesses up to typically 1m can be detected. The actual range depends on the quality of the concrete and also on the amount of steel reinforcement present. Other applications include the location of pipes and post-tensioning cables. In order to correctly differentiate between hollow pipes and rebars, phase detection software would be required. 2. Traditional Ultrasonic Measurement of Poisson’s Ratio and E-Modulus 2.1 Theory behind the method This method is based on measurement of the P-wave and S-wave ultrasonic pulse velocities. By measuring a P-wave transmission time and an S-wave transmission time, we are able to determine the P-wave modulus (M) and the Shear modulus (G). P-wave modulus (M): . Where ρ is the density of the material and Vp is the pulse velocity of the P-wave. Shear-modulus (G): . Where ρ is the density of the material and Vs is the pulse velocity of the S-wave. More info about this article: http://ndt.net/?id=19195 2 Knowing any two of the elastic properties of a material allows the others to be calculated. In this case we are interested in calculating Poisson’s Ratio and the Elastic Modulus. Poisson’s Ratio (ν): So Poisson’s ratio can be determined simply by measuring the P-wave velocity and the S-wave velocity and it is not necessary to know the density of the material. Once Poisson’s ratio is known, the elastic modulus can be calculated from the equation: For this it is necessary to know the density of the material. This method determines the dynamic modulus of a material. It is most widely used for rock testing and as such there are well established guidelines including ASTM D2845[1] and in the ISRM guideline for determining sound velocity by ultrasonic pulse transmission technique [2]. 2.2 Dynamic and Static Modulus of Elasticity The method can also be applied to concrete and Proceq have some customers who do this. However structural engineers typically work with the statistical modulus of elasticity. This is typically approximated in practice by the measurement of the secant modulus of elasticity using for example the method described in EN 12390-13 [3]. This is a laboratory test which can only be made on samples or cores. The relationship between the static and dynamic modulus of elasticity can be seen in the following diagram. Fig. 1. Relationship Between Static and Dynamic Modulus of Elasticity of Concrete Empirical relationships have been published between the dynamic and static modulus of elasticity. [3] The advantage of the dynamic modulus of elasticity measurement is that it can be performed in-situ and is totally non-destructive.