Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.engstruct.2022.115324

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

The mechanical performance of bolt joints in wooden structures is known to vary significantly with the initial tightening force (axial force) of the bolts during the construction work. When a joining method that considers axial force is applied to a structure, the control of the axial force is important. This study attempts to evaluate the elastic stiffness of bolted timber joints during tightening for the purpose of applying the turn-of-nut tightening method, one of the methods for controlling axial force. Considering the elastic stiffness of joints during tightening as a series spring model using the spring constant of the bolt and the embedment spring constant of the washer, elastic stiffness is calculated using the washer embedment spring constant obtained through a washer embedment test. This result is compared with the elastic stiffness obtained through the tightening test using four species of timber. It was found that while the washer embedment spring constant varies greatly with timber species, the calculated values agreed well with the test results.

DOI： 10.1186/s10086-022-02038-1

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Other Link： https://link.springer.com/article/10.1186/s10086-022-02038-1/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

The management of axial forces is an important issue when a joining method that takes into account the axial forces generated by tightening bolts is applied to the bolted joints of a wooden structure during construction. This study focuses on elastic interactions in which the axial force of adjacent bolts changes as a result of sequential tightening of each bolt in multiple bolted joints affecting the deformation around each of the other bolts. To this end, tightening experiments are conducted within the elastic range, with the tightening sequence, bolt spacing, and wood thickness set as parameters. From the results, it was found that variations in axial force tended to decrease as bolt spacing increases. In addition, an evaluation formula for calculating fluctuations in axial force due to elastic interactions was derived. By comparing the calculated value to the experimental value, it was found that as bolt spacing was increased, the calculated value tended to capture the experimental values well.

DOI： 10.1186/s10086-022-02060-3

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Other Link： https://link.springer.com/article/10.1186/s10086-022-02060-3/fulltext.html

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：SPIE-Intl Soc Optical Eng  

DOI： 10.1117/1.oe.61.10.105107

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s00107-022-01827-1

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Other Link： https://link.springer.com/article/10.1007/s00107-022-01827-1/fulltext.html

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.mechrescom.2021.103815

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>The pretensioning force in bolted joints enhances the lateral strength of the connections, and causes the embedment of metal washers into wood. Despite the significance of embedment behavior in the design of bolted joints, its mechanism has yet to be fully understood. In this study, the mechanism of the embedment of a metal washer into wood along the radial direction was examined through three-dimensional nonlinear finite-element analysis (FEA). The FEA results were validated by comparing them with experimental results for nine metal washers with different geometries. Moreover, the sensitivity of embedment stiffness and yield load to wooden material constants was also investigated. The numerical results showed good qualitative and quantitative agreement with the experimental results. In addition, the embedment stiffness and yield load were sensitive to the yield stress and Young’s modulus of wood in the radial and tangential directions. The determination of these mechanical properties of wood through material testing is important for reproducing the behavior of the embedment of a metal washer into wood and accurately estimating the yield load and initial stiffness using FEA. This will play a significant role in designing bolted joints.

DOI： 10.1186/s10086-021-01973-9

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Other Link： https://link.springer.com/article/10.1186/s10086-021-01973-9/fulltext.html

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jcsr.2021.106923

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：SAGE Publications  

In previous studies on optimized single-layer latticed domes, the inner space and external shape of the optimized dome is different from those of the initial dome. This difference may result in loss of structural functionality and aesthetics intended by the designers, making it difficult to separately evaluate the mechanical properties of the grid patterns and shape of the surface. In this study, 64 types of single-layer latticed domes having different geometric properties are optimized to obtain mechanically effective grid patterns. Six types of objective functions are employed. The nodal coordinates of the domes serve as the design variables under geometrical constraints, where the nodes of the domes can be shifted on the surface area. The geometric and mechanical properties of the optimized grid patterns are evaluated quantitatively against the objective functions. Moreover, interactions between the geometric and mechanical properties are investigated. The results show that the optimized grid pattern has superior mechanical properties and geometric imperfection sensitivity. This optimization scheme can be applied for designing mechanically effective grid patterns for single-layer latticed domes.

DOI： 10.1177/1369433220956813

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Other Link： http://journals.sagepub.com/doi/full-xml/10.1177/1369433220956813

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Authorship：Lead author   Language：English  

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Authorship：Lead author   Language：English  

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Language：English  

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Authorship：Lead author   Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Fatigue crack in cast aluminum alloys subject to low-cycle fatigue is initiated mainly by the fracture of Si particles. To investigate the mechanism of Si particle fracture, we carried out the geometrical measurements and finite element analysis for a lot of Si particles in their actual geometrical configuration. The synchrotron X-ray CT was used for the geometrical measurement and the image-based finite element modeling. The fracture of Si particles were detected as a source of crack initiation from the CT images captured during the fatigue test. The large-scale voxel finite element model contains the outer surface, pores, Si and intermetallics. The number of elements was about 150 million. Two cycles of loading were calculated. The finite element analysis and its post-processing were performed on the supercomputers by the massively-parallel computing. The result of finite element analysis showed that the first principal stress was concentrated around the aligned or gathered Si particles, and the gradual increase by cyclic loading in the stress of the Si particles appeared near the outer surface and pores. Moreover, the result of geometrical measurements showed that the volume, shape index and adjacency index of each Si particle could be used to evaluate the likelihood of particle fracture. The probability of particle fracture was statistically evaluated with respect to those geometrical and mechanical parameters. The relationship between mechanical and geometrical parameters were also examined. As the result, the criteria of fractured Si particle was proposed by using the combination of geometrical and mechanical parameters.

DOI： 10.1299/transjsme.18-00028

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Authorship：Lead author   Language：Japanese  

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.msea.2016.10.004

Web of Science

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Authorship：Lead author   Language：Japanese  

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Authorship：Lead author   Language：Japanese  

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Authorship：Lead author   Language：English  

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Language：Japanese  

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J-GLOBAL

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Crack initiation in cast aluminum alloy under low-cycle fatigue loading is mainly caused by the fracture of Si particles. The fracture of Si particles is influenced by pores, aluminum particle, specimen surface and their interactions. Thus, the stress-strain field on the large part of specimens should be investigated to elucidate the fracture mechanics of Si particle. In our previous study, the large-scale image-based finite element analysis was conducted by using CT images for the large part of a specimen, where the voxel element was employed to generate the finite element mesh definitely and automatically. However, the voxel element caused the lack of accuracy of the stress along the material interface. In this study, we addressed the zooming analysis by using the result of large-scale finite element analysis to achieve the high precision of stress field. The small volume image for zooming analysis was cropped from the entire CT images, where the Si particle for the crack initiation was contained. By using the extracted CT images, the finite element model with ten-node tetrahedral element was generated. The nodal displacement on the surface of the zooming small model was imposed from the result of the finite element analysis. The comparison between the large-scale, zooming small and local small model was performed to investigate the validation of the zooming analysis. As the result, the zooming analysis could reproduce the mechanical effect on the fracture Si particle observed in the large-scale analysis and mitigate the lack of accuracy of the stress due to the voxel element.

DOI： 10.1299/jsmemm.2019.os1602

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Crack initiation in cast aluminum alloy under low-cycle fatigue loading is mainly caused by the fracture of Si particles. In this study, the large-scale image-based finite element analysis was conducted to reveal the mechanism of Si particle fracture. The fatigue test and in situ CT scanning were performed at SPring-8. The fracture of Si particle was identified by chronological CT observation. The CT images used for the model was extracted into quarter of horizontal cross section where the fractured Si particle for crack initiation was included. The image-based finite element model with voxel element was generated semi-automatically using the extracted CT images. The number of elements for the finite element model was about 130 million. To evaluate the incremental rate of stress for Si particles under cyclic loadings, 10 cycles of loading were applied in the analysis. The large-scale finite element analysis and its post-processing were performed on the supercomputers by the massively-parallel computing. The result of the finite element analysis showed that the value of the first principal stress for Si particles and its incremental rate under cyclic loadings did not only depend on their shapes but also their position in the specimen. Moreover, it was observed that the increment of the first principal stress for the fractured Si particle near a pore under cyclic loadings was drastically changed.

DOI： 10.1299/jsmemm.2019.os1601

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Language：Japanese   Publisher：日本建築学会  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

We have addressed the mechanism of fatigue crack initiation in cast aluminum alloys. To numerically simulate the actual mechanical condition around the crack initiation site, the finite element model has to include a lot of inclusions along with micro and macro pores and material surface. We are trying to realize this kind of large-scale image-based finite element analysis by using the synchrotron CT and voxel finite element techniques. The finite element model of AC4CH-T6 cast aluminum alloy was constructed and its fatigue simulation of ten cycles loading was carried out. From the comparison of the experiment and the simulation, we found that the silicon particle in the crack initiation site underwent a high stress concentration and a low geometrical constraint leading to a gradual increase in the stress.

DOI： 10.1299/jsmemm.2018.ps19

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Fatigue crack in cast aluminum alloys subjected to low-cycle fatigue is initiated mainly by the fracture of Si particles. We addressed the mechanism of Si particle fracture through fatigue testing and <i>in situ</i> CT scanning, and the large scale image-based finite element analysis. The fracture of Si particle in the crack initiation site was detected from chronological CT observation. The large scale finite element analysis, where the number of element was about 150 million, was conducted by using CT images which was extracted to contain crack initiation site. To generate the finite element mesh automatically, the heterogeneous voxel element was employed. Moreover, the small scale analysis with 10-node tetrahedral element was performed to compare the results of large scale analysis. The large scale finite element analysis and its post-processing were performed on the supercomputers by the massively-parallel computing. The result of finite element analysis showed that the first principal stress of the interested Si particle was higher inside the specimen than in the surface of the specimen since the geometrical constraint was generated from the normal strain to the loading axis. Moreover, the first principal stress of the interested Si particle increased in neighbor of the surface of specimen under cyclic loading while the stress of Si particles inside the specimen and at the corner of specimen decreased.

DOI： 10.1299/jsmemm.2018.os1102

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Image-based modeling technique enables us to create the finite element model with real shape by using CT images. Moreover, the large-scale finite element model with billions of voxel elements can be meshed semi-automatically while the meshing of tetrahedral element in large-scale FEA is considerably difficult. However, interface between dissimilar materials of voxel element has the stepwise shape, where the incorrect stress concentration is generated. To overcome the shortcoming of the voxel element, we proposed the voxel element the material properties of which are defined in each integration point of the voxel element. Material properties of each integration point are determined based on the bi-linear interpolation of the intensity of CT images. Image-based finite element analysis was performed by using CT images of the cast aluminum alloy containing Si particle, intermetallic compounds, aluminum matrix and pore, where the interface of them is complex curved surface. Results among the 10-node tetrahedral element, the homogeneous voxel element and the heterogeneous voxel element were compared to verify the effectiveness of the proposed method. As the result, it was found that the integration point stress of the proposed model vibrated more violently than that of homogeneous voxel model in the stress concentration region of dissimilar interface, however the average element stress of both model was almost same there. Moreover, the average stress of the proposed model is similar distribution of that of 10-node tetrahedral model in the low stress region of particle/matrix interface.

DOI： 10.1299/jsmemm.2018.os1101

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Fatigue crack is initiated by the fracture of Si particles or debonding of Al-Si interface in low-cycle of cast aluminum alloys. To fully utilize the synchrotron X-ray CT image and evaluate the precise stress and strain in the microstructure under a cyclic loading, a large-scale finite element model was constructed based on the CT image. The outer surface, pores, Si particles and intermetallics were fully considered in this model. The voxel finite element was employed for simulation with stabilization. The number of finite elements was about 150 million. The analysis and post-processing were performed on the supercomputer by the massively-parallel computing with the domain decomposition techniques. The material and geometrical nonlinearities were considered, and the two cycle of loading was calculated. The result showed that the effect of free surface was predominant on the accumulation of plastic strain in matrix phase and the gradual increase of stress in silicon phase. The availability of massively-parallel computing to the large-scale micromechanical analysis was demonstrated. Moreover, the geometrical analyses of silicon particles were performed, then the correlation between mechanical and geometrical parameters were investigated statistically, where we found that the high stress concentration was occurred on silicon particles with complex shape and ones at the near of which other particle was present.

DOI： 10.1299/jsmemm.2017.os0908

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

In cast aluminum alloys, it is found from an experiment that a fatigue crack is initiated from the fracture of Si particles. We have been investigating a methodology to evaluate the particle fracture life from the geometrical parameters of Si particles. By using the synchrotron radiation CT, the geometrical parameters of Si particles were measured, and the image-based finite element analyses were also performed to evaluate the mechanical parameters of Si particles. A few effective geometrical and mechanical parameters were selected to determine the fracture of Si particles. The particle fracture life of Si particle was determined from the chronological observation of CT images. Then, the relationship between the particle fracture life and the geometrical and mechanical parameters were learned by the artificial neural network to predict the particle fracture life of unknown Si particles. In practice, forty three Si particles including the fractured and nonfractured particles were selected from five fatigue specimens, and used for the finite element analysis to evaluate the mechanical parameters. Their geometrical and mechanical parameters were used as the training data for the neural network. The validity of the predicted particle fracture life was examined through the comparison with the actual parameters of fractured particles.

DOI： 10.1299/jsmemm.2017.os0905

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

The fatigue crack initiation mechanism of cast aluminum alloy has been addressed by using the synchrotron radiation which gives a high resolution CT images to visualize the microstructure. By the 4D imaging, we have observed the crack initiation and propagation by low-cycle fatigue where the source of crack was the fracture of Si particle or debonding of Al-Si interface. The micromechanical finite element analysis can be performed with the precise microstructure based on the CT images. To investigate the effect of a small uncertainty in the applied cyclic loading on the stress and strain around the silicon particles which were fractured and initiated a crack in the fatigue test, the elastic-plastic finite element analyses was carried out with ten cycles of loading. The fractured silicon particles were identified from chronological CT observation. A method to make a reasonable waveform with precise mean and standard deviation was proposed. The small uncertainty coming from the experimental error was considered, and two types of cyclic loading waveform was made and used in the simulation. From those results, we found that the small uncertainty of applied stress had an significant effect on the maximum stress in silicon particles and increased it gradually by the cyclic loading.

DOI： 10.1299/jsmemm.2017.os0907

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Cast aluminum alloys are promising from the perspective of its high specific strength and high cost effectiveness. However, cast aluminum alloys inevitably contain casting defects which degrade its fatigue strength, and it leads to the low reliability. It was observed in the experiments that the cracking was initiated by the fracture of Si particles or debonding of Al-Si interface. In this study, the image-based finite element analyses were carried out by using synchrotron X-ray CT images to elucidate the crack initiation mechanism of cast aluminum alloy under low-cycle fatigue. To examine the difference of mechanisms between fracture of Si particle and debonding of Al-Si interface, the finite element elastic-plastic analyses of five models for the fractured or debonded particles were performed under ten cycles of fatigue loading. From the numerical results, the Si particle with large volume was fractured because of high stress concentration, while the Si particle neighboring closely to intermitallics was debonded since the large plastic deformation of the aluminum matrix between Si and IMC particles.

DOI： 10.1299/jsmemecj.2017.g0300404

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Cast aluminum alloys have been widely used in transportation equipment because of its high productivity and high specific strength. However, the cast aluminum alloys have low reliability of fatigue strength caused from inevitable casting defect. In our previous work, from the low cycle fatigue test and in situ synchrotron X-ray CT observation of the cast aluminum alloy, it was found that the crack initiation of the cast aluminum alloy was occurred by the fracture of Si particles. In this study, the fracture of Si particle was regarded as the crack initiation, and the geometrical characteristic of fractured Si particle was investigated. The fractured and intact Si particles were identified by chronological CT observation. Geometrical analyses of Si particles were conducted by using high resolution CT images, while Si particles were extracted by an appropriate threshold value of brightness. Geometrical parameters of a few hundreds of thousand Si particles in a specimen were analyzed, then they were evaluated statistically. The effective geometrical parameters were suggested to determine the condition of particle fracture.

DOI： 10.1299/jsmemecj.2017.g0300405

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To understand the crack nucleation mechanism, the image-based finite element analysis was employed. The crack propagation mechanism have been studied, while the crack initiation mechanism is not revealed. Through the fatigue test and <i>in situ</i> CT observation, we have observed that the fatigue crack was nucleated from the breakage of Si particles and, the broken and intact Si particles were identified. By using the image-based modeling technique, the finite element models were generated so as to include the interested Si particle in the analysis domain. A few hundreds of Si particles and intermetallic compounds were contained in the finite element models. We examined the cause of crack initiation with the breakage of Si particles and the effect of intermetallic compounds against Si particles through numerical results.

DOI： 10.1299/jsmemm.2016.os15-11

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Fatigue test and <i>in situ</i> observation were performed using a cast aluminum alloy AC4CH-T6, the fatigue cracking behavior was observed. Then the geometry and distribution of inclusions in the cast aluminum alloy is measured by using the CT images. The relationship between the geometrical parameters of the particles and the particle fracture life is evaluated and learned by the artificial neural network. Using the trained artificial neural network, the particle fracture life of particles in other CT image was predicted.

DOI： 10.1299/jsmemm.2016.os15-10

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Language：Japanese   Publisher：Architectural Institute of Japan  

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Language：Japanese   Publisher：Architectural Institute of Japan  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

The crack initiation mechanism of cast aluminum alloys is addressed through the image-based finite element analysis. By using the synchrotron radiation CT, the initiation and extension of fatigue crack has been observed. The fatigue crack is initiated by the break of silicon particles near the pores, while its mechanism is not clear. To understand the stress and strain fields around silicon particles under cyclic loading, the finite element elasto-plastic finite-strain analyses were carried out. From the fatigue test and its in situ CT observation, the broken and unbroken silicon particles were identified. Their finite element models were semi-automatically generated from the initial CT image, where a few tens of silicon particles were included. By comparing the history of stress and strain between broken and unbroken silicon particles, we examined the trigger of breakage of silicon particle, that is, the cause of crack initiation.

DOI： 10.1299/jsmemm.2015._os0808-36

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Language：Japanese   Publisher：Architectural Institute of Japan  

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