Akira Sasamoto

On impedance measurement of reinforced concrete on the surface for estimate of corroded rebar

In an estimate of health monitoring for reinforced concrete, corrosion degree of rebar is important parameter but is not easy to be estimated by non destructive testing. A few test method such as half cell method or polarization resistance method could be a ’perfect’ nondestructive method if luckily having had wired connection to rebar without destructing target concrete. In this presentation it is reported the experimental result that an impedance measurement on surface of reinforced concretes is able to distinguish corroded rebar from healthy rebar. The contact electrode on concrete surface are simple structure made of urethane sponge and needle. Impedance measurement are carried out with frequency response analyzer with frequency range from 0.01Hz to 1MHz, typical amplitude of imposed voltage are 10 volt. We made concrete specimens under two different corrosion process. One process(pre corrosion) has rebars corroded by electrolysis in salty water before concrete casting and another process (post corrosion) has concrete specimens being corroded during the curing. The results of application of developed method to these corroded specimens show the method is useful to estimate corrosion level of rebars.

Feasibility study on 3 axis magnetic sensor for flux leakage method

Most of NDT system by magnetic field sensing has employed coil or semiconductor as sensor which has one axis sensitivity. Recent development of semiconductor technology can makes a chip that enable us to measure 3 axis magnetic field in a 1mm square. This vector information is expected to show a new insight in NDT testing. This presentation will show a basic experimental feasibility study for application of magnetic sensor to flux leakage and eddy current testing method by using a sensing system with the chip.

A novel methodology to determine needle position for DC potential difference method to evaluate hardening depth from surface

DC potential difference method are simple and fast non destructive testing method. It measures voltage between two points on surface touched by each needle. In some kind of engineering structural member, neighborhood of iron surface is required to have higher hardness than normal iron ’s one. To meet this demand, quench hardening process is a popular process to make iron hard from surface and hardening depth is important parameter to be ensured in target member after process. Because quench iron increases its resistance by 20 %, D.C.potential method is able to be used for evaluating hardening depth. However output voltage was insensitive for change of deep hardening depth over 5.0mm, measurement would fail over 2.0 mm with improper needle array probe in reality. We developed a novel methodology to determine needle position to improve its applicability over 5.0 mm depth which is demanded in some member. In this methodology, needle position are optimized for cost function that is voltage gradient to hardening depth. Throughout this optimization, (1) an analytical expression of surface voltage which are obtained solve Laplace equation in three dimensional space. (2) CAS system (Maple / MAXIMA) . are invoked. Experimental data shows good coincidence to those numerical computation, and monotone increasing property of voltage to depth are kept up to 10 mm hardening depth. Proposed needle positions have made DC potential difference method possible to evaluate hardening depth over 5.0 mm. The algorithm is most likely to work well with only one response function for this type probe.

Combination of a GMR sensor and reconstruction algorithm: a novel magnetic sensing system

Over the past few years, the authors have developed a reconstruction algorithm that can accurately reconstruct images of flaws from data obtained using conventional ECT sensors. The algorithm is simple and fast and involves few steps, thus making it suitable for implementation on a PC. The algorithm can be applied to study eddy current systems; it can also be used in conjunction with non-destructive testing methods involving a magnetic field. However, there is one inherent limitation related to sensor design. In eddy current or magnetic flux leakage, a conventional sensor is used to detect flaws in damaged areas. This sensor is designed in such a manner that when the magnetic field is imposed on the target surface, the strength of the magnetic field is maximized. This measurement method has remained unchanged since the introduction of the technique. The developed reconstruction algorithm is designed for data obtained by imposing a uniform magnetic field on the target surface. Recent developments in computer technology have enabled the integration of computing and testing equipment; in this context, the authors believe that a new sensor for use with reconstruction algorithm will be required. Therefore, the authors have developed a prototype sensor for applications to magnetic flux leakage. The developed sensor comprises a GMR magnetic field sensor to detect a static magnetic field and two magnets adjacent to the GMR sensor to magnetize the target specimen. The results of the combined use of the sensor and the reconstruction algorithm are presented in this paper.

An image reconstruction from ECT data of complex flaws

A lack of safety and the reliability of the existing metallic structure can threaten peoples lives today. It is getting stronger to demand to ensure reliable safety in society. Non Destructive Testing(NDT) can support to public safety with finding damaged structure. Eddy Current Testing (ECT) is a one of NDT for metallic or conductive materials. It already plays an important role in very wide field such as airline and power plants for maintenance, ironworks for production. Though ECT is considered as a finished testing method,it has the unwanted property that flaw blur in ECT signal.This defect partly comes from the essential principle of ECT. In order to obtain fine image of flaw, the authors proposed a method with signal processing to reconstruct more finer image of flaw from ECT signal. The method is based on simple relationship that signal are expressed as a convolution of response function and flaw shape. Many obtained results, more fine images of points flaw and both short and long line flaw than images of those ECT signal were reconstructed, show validity of the method for those flaws. Nevertheless its aim was fundamental survey on validation of the method so that tested flaws were limited in shape.In this paper,beyond that limitation, the authors wish to report the results of applications to complex shape flaws that are likely to be found in actual inspection site. The obtained reconstructed images show notable results indicate that the validity is kept even for complex flaw.

An image reconstruction method by deconvolution for ECT

Eddy Current Testing(ECT) has been used in wide field such as airline and power plants for maintenance, ironworks for production. However original flaw shape blur in image by signal of ECT. In our previous work an image reconstruction method from signal had been proposed. The method is based on that simple relationship between signal and source are described by a convolution of response function and flaw shape. The method was able to show more fine image of points flaw, short line flaw, long line flaw than images of those original signal. One difficulty in the method was to determine empirical parameter by trial and error. In this paper, we propose a concept of modified response function and signal that enable to make empirical parameter unnecessary. Those modification process is fully programmable and is carried out automatically. Validity of introducing those modification are considered from mathematical view point. Numerical results shows the method with this concept reconstructed image as same as empirical parameter method.

On an image reconstruction method for ECT

An image by Eddy Current Testing(ECT) is a blurred image to original flaw shape. In order to reconstruct fine flaw image, a new image reconstruction method has been proposed. This method is based on an assumption that a very simple relationship between measured data and source were described by a convolution of response function and flaw shape. This assumption leads to a simple inverse analysis method with deconvolution.In this method, Point Spread Function (PSF) and Line Spread Function(LSF) play a key role in deconvolution processing. This study proposes a simple data processing to determine PSF and LSF from ECT data of machined hole and line flaw. In order to verify its validity, ECT data for SUS316 plate(200x200x10mm) with artificial machined hole and notch flaw had been acquired by differential coil type sensors(produced by ZETEC Inc). Those data were analyzed by the proposed method. The proposed method restored sharp discrete multiple hole image from interfered data by multiple holes. Also the estimated width of line flaw has been much improved compared with original experimental data. Although proposed inverse analysis strategy is simple and easy to implement, its validity to holes and line flaw have been shown by many results that much finer image than original image have been reconstructed.