Joseph Moysan

Applying nonlinear resonant ultrasound spectroscopy to improving thermal damage assessment in concrete

Nonlinear resonant ultrasound spectroscopy (NRUS) consists of evaluating one or more resonant frequency peak shifts while increasing excitation amplitude. NRUS exhibits high sensitivity to global damage in a large group of materials. Most studies conducted to date are aimed at interrogating the mechanical damage influence on the nonlinear response, applying bending, or longitudinal modes. The sensitivity of NRUS using longitudinal modes and the comparison of the results with a classical linear method to monitor progressive thermal damage (isotropic) of concrete are studied in this paper. In addition, feasibility and sensitivity of applying shear modes for the NRUS method are explored.

Modelling the grain orientation of austenitic stainless steel multipass welds to improve ultrasonic assessment of structural integrity

Knowledge of the grain orientation quantifies the material anisotropy which helps to ensure the good ultrasonic testing of welded assemblies and the assessment of their mechanical integrity. The model described here concerns the weld solidification of 316L stainless steel. The solidification of multipass welds made with a shielded electrode raises many unsolved modelling questions as it involves heat and fluid flow modelling in addition to solute redistribution models. To overcome these difficulties we have developed the MINA model to predict the resulting grain orientations without using a complete solidification model. This model relies upon a phenomenological description of grain orientations from macrograph analysis. One important advance of this model is to include data reporting in the welding notebook that ensures the generality of the model. This model allows us to accurately simulate the ultrasonic testing of welded components and to propose a new tool to associate welding design with the ultrasonic assessment of structural integrity.

Bscan image segmentation by thresholding using cooccurrence matrix analysis

Abstract In this paper we present innovative research for the processing of an uktrasonic image obtained in non-destructive testing (NDT). We also present an algorithm for automatic thresholding segmentation. The image analysis is based on the calculation of cooccurrence matrices. The method we used is the Spatial Grey-Level Dependence Method (SGLDM); it shows the distribution of the intensities of pairs of pixels separated by a vector d . We show that it is necessary to build optimized matrices in order to improve the existing techniques while adapting to the specifics of the ultrasonic image obtained in NDT. We solve for the vector d through an automatic analysis of the matrix behaviour for several vectors based on the characterization curves. The developed method allows us to present an efficient thresholding algorithm that is tested on real images.

Measurement of ultrasonic scattering attenuation in austenitic stainless steel welds: realistic input data for NDT numerical modeling.

Multipass welds made of 316L stainless steel are specific welds of the primary circuit of pressurized water reactors in nuclear power plants. Because of their strong heterogeneous and anisotropic nature due to grain growth during solidification, ultrasonic waves may be greatly deviated, split and attenuated. Thus, ultrasonic assessment of the structural integrity of such welds is quite complicated. Numerical codes exist that simulate ultrasonic propagation through such structures, but they require precise and realistic input data, as attenuation coefficients. This paper presents rigorous measurements of attenuation in austenitic weld as a function of grain orientation. In fact attenuation is here mainly caused by grain scattering. Measurements are based on the decomposition of experimental beams into plane-wave angular spectra and on the modeling of the ultrasonic propagation through the material. For this, the transmission coefficients are calculated for any incident plane wave on an anisotropic plate. Two different hypotheses on the welded material are tested: first it is considered as monoclinic, and then as triclinic. Results are analyzed, and validated through comparison to theoretical predictions of related literature. They underline the great importance of well-describing the anisotropic structure of austenitic welds for UT modeling issues.

Microbubble cloud characterization by nonlinear frequency mixing.

In the frame of the fourth generation forum, France decided to develop sodium fast nuclear reactors. French Safety Authority requests the associated monitoring of argon gas into sodium. This implies to estimate the void fraction, and a histogram indicating the bubble population. In this context, the present letter studies the possibility of achieving an accurate determination of the histogram with acoustic methods. A nonlinear, two-frequency mixing technique has been implemented, and a specific optical device has been developed in order to validate the experimental results. The acoustically reconstructed histograms are in excellent agreement with those obtained using optical methods.

Potential of Nonlinear Ultrasonic Indicators for Nondestructive Testing of Concrete

In the context of a growing need for safety and reliability in Civil Engineering, acoustic methods of nondestructive testing provide answers to a real industrial need. Linear indicators (wave speed and attenuation) exhibit a limited sensitivity, unlike nonlinear ones which usually have a far greater dynamic range. This paper illustrates the potential of these indicators, and evaluates its potential for in situ applications. Concrete, a structurally heterogeneous and volumetrically, mechanically damaged material, is an example of a class of materials that exhibit strong multiple scattering as well as significant elastic nonlinear response. In the context of stress monitoring in pre-stressed structures, we show that intense scattering can be applied to robustly determine velocity changes at progressively increasing applied stress using coda wave interferometry and thereby extract nonlinear coefficients. In a second part, we demonstrate the high sensitivity of nonlinear parameters to thermal damage as regard with linear ones. Then, the influence of water content and porosity on these indicators is quantified allowing to uncouple the effect of damage from environmental or structural parameters.

Adapting an ultrasonic image threshold method to eddy current images and defining a validation domain of the thresholding method

This paper presents a generalization to eddy current images of a thresholding method developed for ultrasonic imaging. It is also shown how to improve the thresholding strength through the definition of a cooperation rule between two threshold assessment measures. This rule is also used to reduce time of calculus. These measures are calculated from the image cooccurrence matrix. The aim is to build a thresholding method that gives a threshold value but also an indicator which allows the operator to know if the methodology applies to the processed image. Examples of results are presented.

Crack-like defect detection and sizing from image segmentation through co-occurrence matrix analysis

Abstract The inspection of austenitic welds used in the nuclear industry using ultrasound poses problems in interpretation: strong grain noise makes the detection of the crack top and the crack bottom difficult. Since corresponding echoes enable defect sizing, defect sizing also becomes difficult. The formation of two-dimensional images (BSCAN) and their processing enable an increase in the effectiveness of testing. This paper presents a new segmentation method, based on the analysis of the co-occurrence matrix. A threshold is automatically calculated by the extremum of a curve, called the Average Grey-level Variance Measure. This curve is an analysis of the distribution of the matrix coefficients. Examples of segmentation improvement applied to BSCAN images of artificial defects are presented.

Ultrasonic image data processing for the detection of defects

Abstract Ultrasonic testing is widely used in material inspection. Forming an ultrasonic image helps the operator in decision making. This image, however, can result in common interpretation difficulties during the testing of heterogeneous or noisy zones. A new method of detection of defects located in a noisy medium is presented: it is based on considering the ultrasonic grey-level image from the beginning of the analysis. The data processing used here is searching for a certain determinism in the spatial and temporal evolution of the image in the presence of a defect: a first criterion studies the horizontal stability of the gradients in the image and a second considers the transient temporal nature of the defect echo. Results obtained on artificial defects located in real welds are shown. Conclusions with respect to the quality of this defect detection method are drawn.

Advances in ultrasonic testing of austenitic stainless steel welds. Towards a 3D description of the material including attenuation and optimisation by inversion

A precise description of the material is a key point to obtain reliable results when using wave propagation codes. In the case of multipass welds, the material is very difficult to describe due to its anisotropic and heterogeneous properties. Two main advances are presented in the following. The first advance is a model which describes the anisotropy resulting from the metal solidification and thus the model reproduces an anisotropy that is correlated with the grain orientation. The model is called MINA for modelling anisotropy from Notebook of Arc welding. With this kind of material model1ing a good description of the behaviour of the wave propagation is obtained, such as beam deviation or even beam division. But another advance is also necessary to have a good amplitude prediction: a good quantification of the attenuation, particularly due to grain scattering, is also required as far as attenuation exhibits a strong anisotropic behaviour too. Measurement of attenuation is difficult to achieve in anisotropic materials. An experimental approach has been based both on the decomposition of experimental beams into plane waves angular spectra and on the propagation modelling through the anisotropic material via transmission coefficients computed in generally triclinic case. Various examples of results are showed and also some prospects to continue refining numerical simulation of wave propagation.