Tomonori Yamada

Experimental characterization of concrete removal by high-power quasicontinuous wave fiber laser irradiation

The performance of high-power quasicontinuous wave fiber laser irradiation on concrete has been investigated in the downward (along the gravitational direction) and the upward laser directions (against it). Mechanism of material removal without an assist gas is dominated by vaporization and melts expulsion induced by recoil pressure evaporation. Also, the melt concrete ejection is greatly enhanced by expansion via gaseous bubbles and breakup. The performance of laser irradiation on concrete is significantly enhanced in the upward direction by the assistance of gravity. In addition, the laser parameters can control the quality and performance of laser drilling and cutting of concrete. Furthermore, the thermal effect analysis determines the impact of laser power on the concrete surface, which may help to consider the heat affected zone coupled with the analyses of material property around the laser treated region.

Construction of Vibration Table in an Extended World for Safety Assessment of Nuclear Power Plants

Computational issues of the seismic simulation of nuclear power plants are discussed in this chapter. The safety requirement of nuclear power plant is high enough to conduct real experiments to evaluate the structural integrity of mechanical components. However, such experiments are performed on independent sets of components because the ability of experimental facilities is limited. Hence, we are constructing a vibration table in an extended world for simulation. In our vibration table, the balancing domain decomposition method is adopted to reduce the computation cost of large mechanical components. Because the computation cost of balancing domain decomposition method depends much on the number of subdomains, a prediction methodology of optimal number of subdomains is introduced. Numerical validation is performed with a component of an actual nuclear power plant.

Component-Wise Meshing Approach and Evaluation of Bonding Strategy on the Interface of Components for Assembled Finite Element Analysis of Structures

The finite elements are extensively utilized to solve various problems in engineering fields with the growth of computing technologies. However, there is a lack of methodology for analyses of huge assembled structures. The mechanics on the interface of each components, for instance, contact, bolt joint and welding in assembly is a key issue for important huge structure such as nuclear power plants. On the other hand, it is well known that as finite element models become large and complex, construction of detailed mesh becomes a bottleneck in the CAE procedures. To solve these problems, the authors would like to introduce component-wise meshing approach and bonding strategy on the interface of components. In order to assemble component-wise meshes, the penalty method is introduced not only to constrain the displacements, but also to introduce classical spring connection on the joint interface, although penalty method is claimed that it is not suitable for iterative solver. In this paper, the convergence performance of an iterative solver with penalty method is investigated and the detailed component-wise distributed computation scheme is described with numerical examples.

Development of laser cladding system with process monitoring by x-ray imaging

We have been developing a new laser cladding system to repair the damages of parts in aging plants. It consists of some devices which are a laser torch, composite-type optical fiber, QCW fiber laser and etc. All devices are installed in a mobile rack, so we can carry it to plants, laboratories or anywhere we want to use. We should irradiate the work with the best accuracy of laser beam and filler wire in laser cladding. A composite-type optical fiberscope is useful. This fiberscope was composed of a center fiber for beam delivery surrounded by 20000 fibers for visible image delivery. Thus it always keeps target on center of gun-sight. We succeeded to make a line laser cladding on an inside wall of 1-inch tube by our system. Before this success, we solved two serious problems which are the contamination of optics and the deformation of droplet. Observing laser cladding process by X-ray imaging with Spring-8 synchrotron radiation, we found that the molten pool depth was formed to be under a hundred micrometers for 10 milliseconds. A Quasi-CW fiber laser with 1 kW was employed for a heat source to generate the shallow molten pool. The X-ray shadowgraph clarified that a molten droplet was formed at the edge of a wire up to a millimeter size. It grew up if the wire didn’t contact with the tube wall in initial state. Here we succeeded to measure the thermo-electromotive force voltage between a wire and a tube metal to confirm whether both came in contact. We propose to apply the laser cladding technology to the maintenance of aging industrial plants and nuclear facilities.

Real-Time Observation of Laser Heated Metals with High Brightness Monochromatic X-Ray Techniques at Present and Their Future Prospects

We present the x-ray techniques for characterizing laser heated metals for welding and cutting techniques. At present, with an undulator (70 keV) as well as bending magnet (30 keV) sources at SPring-8 as a probe source, CW 300 W Ytterbium fiber laser irradiates an Aluminum slab as a sample. Simultaneously the x-ray beam probes the sample for real time observation of a molten pool. We observe the convection indicated by the motion of tungsten based particles as a tracer in the molten pool. During the cooling phase, the molten metal is solidified with residual stresses which are affected by the heating and convection processes. In this experiment the time and space resolution are ∼milli-second and several tens of μm, respectively. On the other hand, microscopic short transient phenomena also play a significant role for the quality of a solidified material. For this purpose, we need high energy short pulse x-ray sources. We try to discuss on the capability and limitation of present x-ray sources and the prospect of an ultra high brightness x-ray source as a complementary source for full characterization of the laser heated and cooling processes of metals.

Visualization Technique for Quantitative Evaluation in Laser Welding Processes

To improve phenomenological understanding of laser welding processes and to control residual stress, we have to characterize the molten pool properties. We succeeded to observe a convection, molten pool shape, and bubbles in situ using an intense X-ray beam and tracer particles during laser spot welding. During the cooling phase, the molten metal was solidified and bubbles were confined in the weld metal. The numerical simulation code has been newly developed to evaluate the effect of molten pool convection to the temperature distribution including phase change, melting and solidification based on the in situ observation results. The numerical code can simulate the laser welding phenomena. We have found that the Marangoni effect on the molten pool surface gives considerable influence to temperature distribution not only on the surface but also in the molten pool. Both the experimental and numerical results provide us useful knowledge about laser welding phenomena for quantitative evaluation.

Intense X-Ray Measurement of Transient Hydraulic Phenomena in Molten Pool During Laser Welding Process

Spatial temperature distribution during the laser welding process has a huge effect on any residual stress distribution. Therefore, understanding of the transient hydraulic phenomena which affect the temperature distribution in the molten pool is very important.In this work, intense X-ray measurement at the Super Photon ring-8 GeV (SPring-8) facility well carried out to document the transient hydraulic phenomena in the molten pool during the laser welding process. Based on in-situ observation of inside material, the experimental results confirmed that the molten pool shapes, hydraulic condition such as flow velocity, etc.. In the case of laser power is 330W and spot diameter is 1mm, we observed the steady flow which consisted of downward flow and upward flow. The flow velocities were about 19.5 mm/s and 9.0 mm/s, respectively. Moreover, the rate of phase change was obtained from molten pool shape during laser welding. The rate of phase change was not constant during laser welding. Thus the interface shape might change at all time. Therefore, to evaluate the temperature distribution, it is necessary to consider not only convection but also the interface shape.These results indicate that the intense X-ray measurement during laser welding is very effective for the understanding the molten pool phenomena.Copyright

Instrumentation device and surface control technology for coolant piping system of nuclear power plants

Instrumentation devices using laser technologies were developed for maintenance of coolant piping system. For instrumentation devices, seismic monitoring by optical fiber sensors was able to detect vibration of piping system. X-ray absorption contrast by synchrotron radiation clarified convection motion inside a molten pool produced by laser welding. For surface control technologies, a new probing system was under development for heat exchanger tubes by laser cladding. And laser ablation could be used for surface cleaning. These technologies are expected for maintenance of the piping system of nuclear power plants.Copyright