# Dr. Nakahata

**Handouts of collegiums are available(sorry Japanese only).**

2nd symposium in 2009 of Research Committee of Acoustic Imaging

Inverse scattering imaging method for ultrasonic testing

Department meeting of JSNDI 2010 in Matsuyama, Ehime

Image-based FIT and its application to ultrasonic testing

## Recent researches(ultrasonic imaging, acoustic imaging)

Development of diagnosis technique of structures by nondestructive testing

We are developing ultrasonic reconstruction methods of flaws such as crack due to stress concentrations and inclusions in the manufacture of steel. Recently we are focusing on using array transducers which can transmit ultrasonic wave to arbitrary directions by electronic scanning.

### (1) Flaw reconstruction using time domain method

There is a phased array technique which makes use of scattered longitudinal waves, measured by every two-element combination as a pulser and a receiver, to synthesize high amplitude beams for any arbitrary angle and/or focal depth. Using this full waveform sampling technique, we show a new flaw reconstruction method with strong focused beams at all pixels of the imaging area. This technique is called “Full-waveform sampling and processing (FSAP)”. An apodization technique can also be incorporated in our imaging method.

The FSAP has potential to reconstruct crack-like flaws. Here we utilize processed wave data, called “scattering amplitude”, which is extracted from each flaw echo. In the following figure, a flaw reconstruction is demonstrated with an array transducer of center frequency 2MHz. An apodization technique can also be incorporated in our imaging method.

### (2) Flaw reconstruction using frequency domain method

We develop here a new imaging technique to reconstruct flaw shapes from waveforms measured by an array transducer. The imaging technique is based on the linearized inverse scattering method and here an algorithm of the synthetic aperture focusing technique in the frequency domain is incorporated into the inverse method. Since we adopt FFT to the shape reconstruction process, the high-speed imaging of flaw shapes is possible at small computational cost. An example of the inverse scattering imaging method (ISIM) is shown in the following figure. Here 3-D artificial flaws in aluminum are reconstructed well.

## Recent researches(Image-based modeling of wave propagation)

Large-scale simulation of elastic wave propagation

Time domain simulation tools for ultrasonic wave (elastic wave) propagation in materials with a complex outer surface or various inclusions are developed by combining the elastodynamic finite integration technique (EFIT) and the finite element method (FEM) with an image-based modeling approach. The EFIT is a grid-based spatial discretization method that works in conjunction with a leap-frog time marching scheme. On the other hand, the FEM adopts a voxel-shaped spatial element and the calculation is updated by an explicit scheme without solving system equations. In our simulation, geometries of targets are determined by digital images such as CT and CAD data and the processed images are directly fed into the wave simulation by the image-based EFIT and FEM.

### (1) Elastic wave in concrete structure

Concrete material has randomly distributed aggregates and entrained air in cement. We can make elements (meshes) of the EFIT and FEM from a digital picture of a cross section of concrete. With this image-based technique, we can easily model shapes and distributions of aggregates and air more precisely in concrete. The following figure shows a modeling of contact ultrasonic testing of concrete. We utilize a parallel calculation technique of the EFIT with a cluster PC of share and distributed memory system (Open MP and MPI).

The following figure shows 3D ultrasonic wave propagations. The model is made from X-ray CT pictures. The CT pictures are built up and then the 3D image of the concrete is discretized into a series of voxels.

### (2) Ultrasonic propagation in anisotropic and heterogeneous material

The nondestructive testing (NDT) of an austenitic steel with welds is difficult due to the acoustical anisotropy and local inhomogeneity. The ultrasonic wave in the austenitic steel is skewed along crystallographic directions and scattered by boundaries of the weld. For reliable NDT, a straightforward simulation tool to predict the wave propagation has been desired. Here a combined method of the elastodynamic finite integration technique (EFIT) and the digital image processing is developed as a wave simulation tool for NDT. An example of a two dimensional simulation of a phased array ultrasonic testing is demonstrated. In our simulation, a cross-sectional picture of steel with a V-groove weld including a stress corrosion crack (SCC) is scanned and fed into the image based EFIT modeling.

### (3) Simulation of ultrasonic wave in air, fluid and solid (Air-coupled UT, Non contact UT）

Ultrasonic wave propagates as both pressure and shear waves in solid, however as only pressure wave in air and fluid. Our study presents a numerical time domain modeling of acoustic, elastodynamic, and coupled waves. Our simulation tool is based on the finite integration technique (FIT). The FIT for acoustic field (air and fluid) is called the AFIT and for elastic field (solid) is called the EFIT. Here the modeling and simulation of an air-coupled UT including fluid-solid interactions is demonstrated. Acoustic impedance is totally different between air and solid, therefore very weak ultrasonic transmits into solid. It is shown that leaky waves from solid to air are also very weak.

### (4) 3-D visualization of ultrasonic wave in metal or human with complicated outer surface

A time domain simulation tool for ultrasonic wave propagation in a homogeneous material is developed using the 3-D elastodynamic finite integration technique (EFIT) and an image based modeling. Here, the geometry of a model is determined by a CAD data and the processed voxel model is fed into the 3-D EFIT. Although the ultrasonic wave simulation in such a complex material requires much time to calculate, we here execute the 3-D EFIT by a parallel computing technique using the hybrid OpenMP and MPI of a cluster computer system. The following example is a modeling of a turbine blade in electric generating plant. In this analysis, we need about 3 hundred million voxel elements and it takes 3 hours for the simulation using 32 CPUs. You can see fast components of pressure waves and subsequent shear waves in the turbine blade.

The following figure is a simulation of ultrasonic propagation from human nose. This surface shape was made from a projected pattern. Using a shape data from projected pattern or laser interferometer, etc, we can model wave propagation in solid with a complicated surface. This technique can be applied to not only nondestructive testing but also a medical ultrasonic testing.

## Recent researches (Advanced health monitoring, MEMS sensor)

Development of structural health monitoring using advanced micro sensors

According to development of micro and nano technologies, small, robust and cheap sensors go on sale. We are focusing on developments of multi-point sensing by means of these sensors. The point of our technique is that we can measure multiple information using different type sensor such as acceleration, sound level, gyre, temperature, etc. At the moment, we are manufacturing by ourselves a sensor module equipped with acceleration and sound level sensors for health monitoring of bridges. In this system, waveforms of acceleration and sound level are acquired, processed and output in real time. Now we are applying our system to a wireless format.

The following figure shows waveforms of accelerations and displacements of a gallery at Ehime University’s building when a person jumps down from a stepladder. Here the displacement is calculated from the acceleration with a digital filter processing.