Masahiro Kusaka

Automobiles are highly convenient and indispensable as a means of transportation in our lives, however traffic accidents are an important issue and casualties must be significantly reduced. Technology for automobile safety is required to reduce casualties caused by traffic accidents, and impact energy absorbing members, such as a crash box and a side member, are used to protect occupants in the event of an accident. These members are required to be lightweight in order to improve running distanse and be required to absorb more impact energy. The crash box requires the ability to suppress load fluctuations, and has high strength and large deformation to improve the impact energy absorption performance. In this study, the shapes of lattice structure imitating Miura-ori, which is a kind of origami, were examined with the aim of increasing the amount of impact energy absorbed by increasing the amount of deformation. From results obtained by the impact crushing analysis, it was found that lattice structures imitating Miura-ori with appropriate strut angles and diameters have small load fluctuations and excellent impact energy absorption performance.

Researches on crash boxes for automobiles are being carried out to reduce occupant damage caused by traffic accidents. We have been studying a crash box consisting of many cells with an elliptical hole for the purpose of improving energy absorption by increasing the amount of displacement during crushing. Our previous studies have showed that the tapers on the side plane of the crash box reduces load fluctuations, however the densification comes from middle part of the crash box. In this study, models that crushed from the impact face of the crash box were examined to reduce damage to the side members connected to the crash box. In addition, by comparing the performance of a crash box model consisting of cells with a circular hole and a crash box model with an elliptical hole, it was clarified that the crash box consisting of cells with elliptical holes had a large crushing displacement and was excellent in impact energy absorption.

A stent is a tubular network that is placed inside a blood vessel to support the narrowing of the blood vessel and keeps the blood vessel in an expanded state. The stent needs to have high extension rate since the stent is attached to the tip of the catheter and is carried through the blood vessel to the affected part. Our propose of this study is to develop a stent that can be expanded in the radial and longitudinal directions. We created a stent model imitating the Namako-ori, which was a type of folding origami, and analyzed the expansion behavior of the stent using FEM analysis. From the analysis results, it was confirmed that the proposed stent model could be extended in the radial and longitudinal directions.

In recent years, space debris has been on the increase. Space debris orbits around the earth at a maximum speed of 15 km/s(1). If space debris collides with a space station, it will cause enormous damage. Therefore, countermeasures against space debris collisions are indispensable for advancing space development. The purpose of this study is to clarify how the defending performance of the bumper shield can be improved to reduce damage to the space structure. Reimerdes et al.(4) showed that it was advantages to halve the thickness and install two bumpers than to install one bumper. In this study, the defending performance of the bumper shield in which one bumper was divided into plural bumpers in the thickness direction and those bumpers were arranged was examined by using SPH analysis. In addition, the bumpers with protrusions were examined to scatter fragments of debris and bumpers. From analysis results, it was found that the defending performance of the bumper shield was improved as the number of bumpers increased. The method of scattering fragments of debris and bumpers was effective for improving the defending performance. It was also found that the defending performance of the bumper with protrusions depended on the place where debris collided with the bumper. Therefore, it is considered that bumpers with protrusions are less reliable than flat bumpers. It is necessary to consider an effective method for scattering debris.

An energy absorbing member called a crush box is mounted on automobiles to absorb shocks in the event of an accident. These members are required to suppress an extraordinarily high load at an initial stage of collision, to have a high energy absorption capacity, and to be lightweight. In this study, we focused on the crush box with the lattice structure as the energy absorbing member. Then, the impact crushing analysis was performed on four types of structures based on the crystal structure and polyhedral shape. As a result, it was found that the lattice structure was suitable as an energy absorbing member, becuase the extraordinarily high load at an initial stage of collision was not generated and load fluctuation during crushing was small. Furthermore, it was found that the FCC structure had the best impact absorption characteristics, and the amount of energy absorption could be increased by increasing the aspect ratio of the unit cell. In addition, from comparison with the conventional tubular models, the FCC lattice structure is excellent in reducing damage of the occupants. Additionally, we conducted impact compression tests on AlSi12 lattice structure and heat-treated AlSi12 lattice structure and examined their crushing charactaristics and the validity of the analysis method. As a result, it was found that the lattice structure could be used as a energy absorbing member by performing heat treatment.

In recent years, the demand for weight reduction in the field of transportation equipments has been increasing along with the rise of global environmental issues. Although, due to issues such as productivity, CFRP is rarely used as a stand-alone material and is generally used in the right places for the right materials. Therefore, the joining of dissimilar materials with metallic materials is essential. In this study, dissimilar material joints between CFRP and A6061 were fabricated using the punching rivet method with the impulsive load. Compared to the conventional riveting method, the punching rivet method does not require drilling of the material to be joined, and the punching and fastening can be performed almost simultaneously. To increase the seating pressure and improve joint strength, the tip of the rivet shaft was inclined to promote the outward deformation of the tip of rivet shaft. The part of rivet holder hole was chamfered. From the experimental result, joint strength was improved.

Shock absorbing members such as crash boxes are mounted on the front and rear of an automobile to absorb the shock in the traffic accident. This shock absorbing member is required to have a large amount of energy absorption while suppressing the peak load at the initial stage of collision, and to suppress the load fluctuation during crushing. In this study, impact crushing analysis is performed using the finite element analysis software LS-DYNA® for a lattice structure having cubic unit cells positioned at different angles, and investigated the arrangement angle of unit cells that can have a large amount of energy absorption in both frontal collision and oblique collision. As a result, it was found that the lattice structure is suitable for the shock absorbing member of an automobile because the initial peak load is suppressed, and the load fluctuation is suppressed at any collision angle by changing the arrangement angle of unit cells.

Joint strength of dissimilar thin pipe friction welded joints between 5052 Al alloy (A5052) and 304 stainless steel (SUS304) was investigated. Pipes had the outer diameter of 16 mm and the inner diameter of 12 mm, and those were welded with a friction speed of 27.5 s⁻¹, a friction pressure of 30 MPa, and a friction time of 1.2 s. When joints were made with as-received pipes, the joint strength at a forge pressure of 60 MPa had approximately 64% in the tensile strength of the A5052 base metal although that had scattering. Almost all joints fractured between the weld interface and the A5052 side though some joints fractured in the A5052 side. Thus, the fractured portion of joints had scattering at the same friction welding condition. On the other hand, the joint strength at a forge pressure of 50 MPa had approximately 77% in the tensile strength of the A5052 base metal when joints were made by pipes with the machined of the inner and outer diameter parts from solid bars. In addition, all joints fractured from the A5052 side. That is, to obtain good joint such as the fracture in the A5052 side without scattering of the fractured portion of joints, the joint should be made without the affected layer on the pipe surface at the manufacturing of itself.

In automobiles, the crash box is provided to absorb impact energy in the traffic accident. The impact energy is absorbed by deforming the crash box when a collision accident occurs, and the safety of the occupant is protected. Crash boxes are required to reduce sudden changes in acceleration transmitted to passengers by the low initial impact load, and to absorb a great deal of impact energy. Currently, a simple square tube structure is used for the crash box. However, the simple tube structure has some problems such as insufficient strength and buckling. To solve this problem, this study proposed a model in which the inside space of the thin polygonal tube is reinforced by a honeycomb structure. By analyzing the model using finite element analysis software LS-DYNA , we examined^® changes in load, displacement and deformation which are caused by reinforcement with a honeycomb structure. From these analysis results, effects of the honeycomb structures inserted to the inside space of the thin polygonal tube on the energy absorption characteristics were demonstrated.

This study presents investigation of analytical technique using the smoothed particle hydrodynamics (SPH) method and the finite element method (FEM) for micro drilling by water jet technology. Water jet is used for processing various materials, but simulation method of water jet has not been established. In this study, motion of water was analyzed by SPH method and deformation of solid body was analyzed by FEM. In order to increase the quantity of water that can be handled, the analytical technique was also examined for the case where both water and the work material were expressed as the SPH model. Validity of analysis methods was clarified.

In this study, the punching rivet method was applied to joining of a CFRP sheet and an aluminium alloy sheet. The conventional riveting method requires drilling process of sheets. In the punching rivet method, punching and joining of two sheets can be carried out at the same time, so it is not necessary to drill in advance. Therefore, production efficiency is excellent. A tensile test was conducted to examine the strength. All joints fractured from the CFRP sheet side in tensile testing. From the experimental results, ( )1 The strength of the rivet with a hole up to the head was lower than that of the hollow rivet, ( )2 The strength was increased by eliminating the chamfered part of the rivet holder. A cross-section of the joints was observed to examine the fastening condition. In both cases, delamination was observed in the CFRP sheet around the rivet shaft. This damage may cause reduction in the joint strength. Therefore, it is necessary to examine the countermeasures for suppressing the delamination.

The interest for a wider range of usable materials for the technology of Selective Laser Melting (SLM) is growing. In this study, the manufacturing of AC8A parts using SLM technology was systematically investigated. The effect of processing parameters on the density of the fabricated AC8A specimens was studied. It shows that the laser energy density plays a significant role in the densification behavior of the AC8A powder during the SLM process. The laser energy density value of 100 J/mm3 is found to be the threshold, above which high density specimens without imperfections and cracks can be obtained.

Joining of plastics and light metals contributes to the reduction of a product weight. In this study, the punching rivet method was applied to joining of an acrylic resin sheet and an aluminum alloy sheet. The punching rivet method can join the sheets without drilling. The riveting process of this method is constituted of the punching process of the sheets using the rivet shank and the fastening process of the sheets using the rivet and the rivet holder. The sheets are fastened by using the plastic deformation of the rivet shank. From the observation of the joints made by the punching rivet method, it was found that the acrylic resin sheet of the joint had no crack and out-of-plane deformation of the joint was small. From the results of the joint strength tests, it was considered that the joint made by the punching rivet method had high strength due to the effect of the pressures on seating faces of the rivet and the rivet holder. As a result, the punching rivet method was effective to join the acrylic resin sheet and the aluminum alloy sheet.

The conventional riveting method requires drilling process of sheets. In order to solve this problem, the punching rivet method has been proposed. In the punching rivet method, drilling is unnecessary in advance, and since the punching and joining of sheets are carried out at the same time, the production efficiency is good. In this study, an Aluminum alloy sheet and a GFRP sheet were joined using the punching rivet method. After joining, the cross sections of the joints were observed, and the deformed conditions of the rivets were examined. For the riveted and bolted joints, tensile tests were carried out to compare their strength. From experimental results, the strength of riveted joint was almost same as strength of bolted joint because of the seating pressure working on the sheet surface of the riveted joint. Though the shape of rivet and maximum load in joining were changed to examine the effect of them, strength of riveted joints were almost same. The cause due to the fact that the deformed shapes of the rivet shafts was almost same, when the cross sections of joints were observed.

To improve the performance of magnetic drive torque actuator that consists of a superelastic material and a ferromagnetic material, it is necessary to increase the operation torque, and to reduce the gradient of torque in a superelastic region. The structure of the composite material which consisted of soft iron and superelastic wires were examined by the FEM analysis. To increase the operation torque, the superelastic wires were arranged in two rows. As a result of the FEM analysis, the operation torque when the superelastic wires are arranged in two rows compared with the case arranged in a row has increased greatly. Furthermore, it became clear to be able to decrease a gradient of torque by increasing the number of superelastic wires of the inside row than an outside row. In addition, we carried out the analysis that the number of superelastic wires changed in the longer direction of the composite. In conclusion, the operation torque decreased by gradually increasing the number of superelastic wires from the revolving shaft side towards a peripheral side, but it became clear to be able to decrease a gradient of torque.

Friction welding method was applied to improve the productivity in manufacturing for a dissimilar pipe joint. In this research, a joining experiment for using a circular pipe of A5052 Al alloy and SUS304 stainless steel was conducted and the effects of the welding condition on the joining phenomena and the joint strength were investigated. Even if the thickness changes, good joints with fracture on the A5052 side are obtained by setting suitable friction welding conditions.

Linear friction welding machine with simple components system was developed to join the square members. Reciprocating motion of linear friction welding was produced by two cams which were placed on outsides of specimen chucking vice. It was found that this machine was able to perform linear friction welding when the experiments of joining pure aluminum were carried out by changing the friction pressures variously.

There is a possibility that the roundness of the big end hole changes when the fracture splitting method is applied to the aluminum alloy die-cast connecting rod. In this study, it is examined by FEM analysis whether the roundness of the big end hole is kept by tightening the bolt after the process of the fracture splitting. As a result of the analysis, the big end hole was axially extended with the plastic deformation by the fracture splitting process. In addition, it became clear that the connecting rod was deformed by the bolting in elasticity and the big end hole shortened axially. As a result, the possibility that the fracture splitting method was able to be applied to the aluminum alloy die-cast connecting rod was shown.

This study presents investigation of simulation method using SPH method and FEM for waterjet penetration .Waterjet is used for processing various materials, but simulation method of water jet has not been established. In this study, motion of water was analyzed by the smoothed particle hydrodynamics (SPH) method and deformation of solid body was analyzed by the finite element method (FEM). Validity of analysis results was examined.

When the fracture splitting method is applied to aluminum alloy die-cast connecting rod, the ruggedness on the splitted surface may crash while driving the engine. A fatigue endurance test was conducted by repeatedly applying a load to a connecting rod fabricated by the fractured splitting method. That was investigated whether it could be safely used during driving the engine. As a result, when the load amplitude was severer than the actual working load, the connecting rod ruptured from the small end of the connecting rod before crushing occurred on the splitted surface. There was also a trace like rubbing on the splitted surface. As a result of FEM analysis, it was found that if the load does not exceed the driving load, there is no gap between the splitted surfaces and aluminum alloy die-cast connecting rod can be safely used.

In order to ensure the safety of passengers in the event of an accident, side member and crash box are mounted on automobiles. Cylindrical tubes, rectangular pipes and hat-shaped members have been examined as structural members that subjected to an axial compressive load. However, these structures have problems that the initial peak load is very high and the load rapidly decreases due to buckling during crushing. To solve the problems, we proposed a cellular solid with mimetic woody structure as a new structural member. Some woods have no initial sharp peak load and have a plateau region which the load is constant in the relationship between the load and the displacement, when the impulsive load are applied to them. We considered that those features were suitable for structural members like a side member or a crash box. The basic cell was a square block with a side length of 10 millimeters and it had a hole in the center. The cellular solid was constituted by combining some basic cells. Therefore, a homogeneous cellular solid was fabricated by making small holes in the aluminum cube. From results obtained from the impact crushing test and simulation by the FEM software LS-DYNA^®, it was demonstrated that the proposed cellular solid had crushing characteristics similar to the wood, and the energy absorption characteristics were influenced by the shape and arrangement of the cells. As a result, it was shown that the results of experiment and analysis substantially corresponded. Since the load during crushing depended on the shape and arrangement of the cells, the possibility of controlling the energy absorption characteristics was shown.