Using Differential Magnetic Permeability to Estimate Structural and Phase Transformations in Individual Layers of Multilayer Products

In modern mechanical engineering multilayer composite materials are finding increasing application. The use of differently purposed layers considerably expands the performance range of composite materials. The metal layers composing these materials may therewith be essentially different in their structure, strength and physical properties.
Among products made of composite materials there are machine parts with various kinds of surface hardening, welded joints, steel ropes twisted from wires of different types, bimetallic and coaxial products, etc. To ensure proper use and diagnostics of these products, it is very important to know the behaviour of the strength and physical properties of each layer under various operating conditions. A simple application in mechanical engineering is products surface-hardened by high-frequency current quenching or carburising.
The problem of simultaneously determining the properties of both the surface-hardened layer and the bulk of a product can sometimes be solved by using the features of the magnetisation reversal of a two-layer ferromagnet. In the magnetisation reversal of a two-layer ferromagnet, the hysteresis loop has a distorted form, with inflections characterising each layer; however, the dependence of differential permeability on the switching field strength is more informative. The position of the peak of the field of maximum differential permeability characterises the structural state of the layer, the peak height being indicative of its volume content thickness.
The recording of the differential permeability of each layer extends the functional capabilities of magnetic testing techniques and enables one to create devices for the separate determination of the physical-mechanical properties and thicknesses of each layer. Monitoring the changes in the position of the peaks of the differential permeability of each layer during deformation, one can judge about the behaviour of deformation processes in the layers and sometimes estimate phase composition changes, e. g., in austenitic steels with a deformationally unstable structure.

Modelling of wear defects under an external object for the ECT of SG tubes

The vibration of the tubes in steam generators (SG) causes fretting-wear, especially in the tubes bended parts near anti-vibration bars (AVB). Similar degradations, with more unpredictable shapes, can also be caused by foreign objects, like a welding rod located in the secondary area of the SG for example. This joint study between CEA LIST and IRSN aims at efficiently simulating eddy current testing (ECT) signals due to fretting-wears. For this purpose new advanced geometric tools feed the 3D models developed by the CEA LIST allowing to quantify the influence of the various geometric parameters of the configuration. In particular, we want to verify the ability of usual simplified geometric representations to approximate these signatures (typically with a rectangular notch in a straight tube to represent the wear under AVB). This numerical study is accompanied by numerous experimental acquisitions on machined tubes, with and without external object.

ANALYSIS OF RESULTS OF MONITORING THE ST. JAMES’S CATHEDRAL IN ŠIBENIK

The St. James’s Cathedral in Šibenik is one of the most important monuments of Renaissance architecture in Croatia. Monitoring with the 44 sensors lasts for sixteen years. For monitoring, are used: thermometer, hygrometer, strain gauges embedded on iron ties and stone elements and LVDTs embedded over cracks. In progress is analysis of results obtained in the first fifteen years. The goal of the analysis is to determine the trends and irregularities in the behavior of structural elements, and on that basis to conclude if there works carried out near the cathedral, as well as specific interventions on the cathedral, had an impact on the structure. After the preliminary stage of analysis, it was concluded that was necessary to use different kinds of analyses such as structural time series analysis with the temperature as explanatory variable. Analyses of simple and multiple linear regression of interrelations between various sensors and temperature are carried out also. At least neural network analyses were carried out which showed very interesting correlations between temperature changes and results of some strain gauges. Although most of the sensors has a significant correlation with temperature changes, other influences are of the same order of magnitude, and it must not be ignored. There is a statistically significant correlation between most of sensors regardless of the location in the structure. It is interesting to note that some sensors that show relatively low correlation with temperature have a high correlation with each other. This indicates that the structure of the cathedral acts as a single entity regardless of the specific method of construction. In this article will be presented conclusions based on the results, as well as suggestions for further activities on structure testing, to extend and complete the knowledge about the state of the cathedral structure.

THE PRACTICE OF PIPE SCREENING SYSTEM GUIDED WAVE TESTING METHOD

E. O. Paton Electric Welding Institute has developed, manufactured and now tests two diagnostic ultrasonic complexes using low-frequency guided waves for remote examination of corrosion wear of pipelines of up to 330 and 630 mm diameter and resonance frequency 36 kHz and 16.3 kHz.
The system consists of the next blocks:
– acoustic antenna;
– block of excitation of monitoring pulses, reception of reflected signals and software control of modes;
– PC with registration, processing and analysis of received information on pipeline technical state.
Accuracy of measurement of distance from antenna to found defects, welded joints, inserts along the pipe axis is sufficiently high (within the limits of resolving capability of guided wave testing method, not worse than 0.3 m).
Examination of the open sections of gas pipeline of Krasnoperekopsk bromine plant (330 mm pipe, wall thickness 8 mm) showed potential capabilities of this method at up to 100 m distance from set antenna. Examination of the underground section of gas pipeline, coated with reinforced corrosion resistant brizol isolation of 9 mm thickness, was carried out at the same place. In this case acoustic wave attenuates virtually at once.
The elastic waves can sufficiently easy move along the pipes with plastic foam, glass wool thermal insulation with further waterproofing. The areas of corrosion damages and inserts’ holes were determined.
The investigations were carried out on detection of corrosion damages of underground sections of pipes of acting heating main. Hydraulic tests of the heating main detected leakage at one of its sections. After trial boring on open pipeline the acoustic antenna was installed and corrosion damage section was determined at 6 m distance with up to 30 cm accuracy. This experiment showed the potential possibilities of application of guided wave method for pipeline testing with high level of corrosion damage.