In this study, FDEM (ECZM), which is the combined finite-discrete element method (FDEM) based on an extrinsic cohesive zone model (ECZM), was improved by incorporating a novel algorithm that makes it easier to insert the cohesive elements than the conventional adaptive remeshing and a scheme to avoid the generation of dormant cohesive element, which is generated as a result of inserting a single cohesive element within the FDEM mesh. The improved FDEM (ECZM) was implemented in a self-developed 2-dimentional code, and it was first validated against a numerical experiment assuming a virtual rock. Subsequently, the proposed FDEM was applied to model the uniaxial compression and the Brazilian tests of siliceous mudstones under quasi-static loading. The results of the proposed FDEM simulations reasonably captured the trends of fracturing and stress-strain curves observed in the experiments well. The proposed scheme can also be extended to parallel computation based on general-purpose-graphic-unit (GPGPU) and 3-dimentional FDEM (ECZM) with minimal efforts.

Currently, the effect of pore water on the thermal diffusivity of rocks at low temperatures is unclear. In this study, the thermal diffusivities of Berea sandstone (12.8%-porosity) and Himekami granite (1.09%) were measured using an optional heating method. Consequently, the change in the thermal diffusivity of saturated Himekami granite (approximately 100%-water saturation) increased continuously and gradually, as observed in dry granite (approximately 0%). However, when cooling from room temperature to −10°C, the thermal diffusivity of saturated Berea sandstone increased discontinuously and rapidly, in contrast to the dry sandstone in which it increased continuously and gradually. Subsequently, the effect of porosity and pore structure on the thermal diffusivity of saturated rocks was examined during the freezing process by comparing the results of Kimachi sandstone (20.2%-porosity) and Ogino tuff (28.7%) measured using the same method by the Kamoshida et al. (2013). Results show that the change in the thermal diffusivity during the cooling process of saturated rocks is depended by the pore structure (pore size distribution) instead of the rock’s water content (porosity). Therefore, rocks containing macropores, such as Berea and Kimachi sandstones, can be assumed particle-dispersed complexes (the rock-forming minerals are the matrix, and the pores are the particles). Thus, the changed behavior of thermal diffusivity during the cooling process can be estimated by measuring the pore size distribution and thermophysical properties of rock-forming minerals.

This study developed an original constant-head permeability apparatus for rocks. This apparatus can freely adjust the confining pressure and hydraulic pressure (hydraulic gradient), and can directly measure the runoff volume using an electronic balance. This apparatus can measure the hydraulic conductivity more accurately than the conventional constant-head method that measures the runoff volume using a measuring cylinder. Second, this study measured the hydraulic conductivity of Inada granite specimens (50 mm in diameter and 40 mm in length) under confining pressure conditions using the constant-head permeability apparatus. The test was performed at 22±1°C in a temperature-controlled room. As result, we found that the hydraulic conductivity of Inada granite, which was measured using this apparatus, is quite similar to that reported by a previous study using transient pulse method. Furthermore, the observed decline in hydraulic conductivity due to the confining pressure is consistent with the observations of the previous study. Therefore, the permeability measurement system used in this study is established and reliable. In addition, this study measured the hydraulic conductivity of various seven intact rocks (granite, basalt, dacite, sandstone, and tuff) under confining pressure conditions using a constant-head permeability apparatus, and we presented how the hydraulic conductivity changes as the confining pressure increases.

Information on the permeability of materials for radioactive waste disposal in geological repositories is essential. High strength and ultra low permeability concrete (HSULPC) is being considered as a material used to package transuranic (TRU) waste. HSULPC would be reinforced by steel fiber to increase its tensile strength and toughness, but the effect of reinforcing steel fiber on permeability is unclear. Permeability tests need to be highly accurate to determine the hydraulic properties of low permeability materials. In this study, the hydraulic conductivity of HSULPC with steel fiber using the transient pulse method. The hydraulic conductivities of HSULPC with/without steel fiber were determined to be around 10⁻¹³ to 10⁻¹¹ m/s under the confining pressures between 2 and 10 MPa and pore pressure of 1 MPa constant. The results further showed that the permeability of these materials had a hysteretic dependence on the effective confining pressure. There was no remarkable difference of hydraulic conductivity between HSULPC with and without steel fiber. SEM observation revealed that HSULPC with steel fiber has isolated voids to some extent included during mixing of concrete but there are few voids on the interface of steel fiber for water channel. However, due to the higher porosity, the hydraulic conductivity of HSULPC with steel fiber is relatively higher than that without fiber. Still, the permeability of fiber reinforced HSULPC is low enough compared with Horonobe mudstone and Toki granite and would enable it to effectively confine ¹⁴C radionuclides in TRU waste.

Calculation of elastic wave velocity using digital rock models is a powerful method to understand physical properties of rocks. However, one difficulty of the approach is to construct a proper digital model due to the uncertainty by the segmentation process of X-ray CT image. Recently, the segmentation-less method has been proposed to avoid the uncertainty, and has shown to be useful in application to Berea sandstone. In this study, we assessed the method through the application to Shirahama sandstone in addition to Berea sandstone, and confirmed whether the method can be applied to other types of sandstones. We also applied and compared two different rock-physics models to obtain elastic properties of a digital rock model. Our results showed that the segmentation-less method can be successfully applied to Shirahama sandstone as well as Berea sandstone. However, when the pore diameter of rock sample is smaller than the spatial resolution of CT image, we found that a correction is required when mapping the CT number to a density. Our results would be useful when applying the segmentation-less method to other types of sandstones.

Porosity of rocks is one of the most fundamental physical properties and is required to quantitatively evaluate the characteristics of rocks in fault zone drilling projects. In the drilling project of Futagawa fault, which ruptured during the 2016 Kumamoto earthquake mainshock, although porosities of intact rock core samples were measured, there was not a continuous porosity profile because core samples could not be obtained in fractured zones. Therefore, we estimated a vertical, continuous porosity profile for a depth interval of about 300–660 m except 383–399 m in the borehole FDB-1 of the project using sonic log data. First, we tested several different empirical equations proposed in previous studies for both sedimentary and volcanic rocks and proposed a new equation considering the effects of compaction and lithology for sedimentary rocks. Second, we compared the estimated porosities with the core porosities at the depths of the measured core samples. As a result, our new equation showed better estimations for sedimentary rocks, but a previous one called Li et al.’s equation gave closer estimations for volcanic rocks. The porosities estimated by our new equation for sedimentary rocks were approximately 50% at the depths of about 300–330 m and approximately 20–40% at about 330–350 m and 510–660 m. The porosities by Li et al.’s equation were approximately 15% for volcanic rocks (massive lava) at the depths of about 380–460 m, and approximately 30–40% for volcanic rocks (autobrecciated lava) of about 350–380 m and 460–510 m. Obviously, the porosity derived from a sonic log of volcanic rocks was higher than those measured using intact core samples due to fracture porosity existing and alteration. Therefore, the derived porosity profile might reflect a reasonable in-situ state in the borehole of the Futagawa fault drilling project.

For comprehensive understanding of Nankai Trough seismogenic zone, IODP (Integrated Ocean Drilling Program / International Ocean Discovery Program) has conducted a deep ocean drilling project referred to Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) from 2007 to 2019 in off the Kii peninsula. As an investigation of physical properties of the subduction zone, to reveal compressive strength of the sediments we carried out consolidated-undrained triaxial compression tests with pore water pressure measurements on NanTroSEIZE core samples. The core samples used were retrieved from depths around 400 meters below seafloor at site C0006 and C0007 with ~4000 m water depths. The rocks are the Pliocene siltstones with a porosity of ~50%, classified as sedimentary soft rocks. As results of the triaxial compression tests, the cohesion ccu and internal frictional angle ϕ_cu by the total stress analyses ranged in 1.8–1.9 MPa and 15–17°, respectively. In addition, the cohesion c’ and internal frictional angle ϕ’ by the effective stress analyses were within 1.6–2.2 MPa and 18–28°, respectively. All the specimens tested caused brittle failure and formed clear shear fractures. In case of such brittle failures, we found that specimen area correction in post failure for calculating differential stress might cause a non-negligible underestimation to the differential stresses. Therefore, we suggest the area correction for undrained triaxial compressive tests objected to soft rocks should be conducted until their peak strength if brittle failure occurs.

It is difficult to mine-widely, routinely measure absolute stress in the highly stressed rock mass in deep mines in the world because the drilled holes or cores are damaged during and immediately following the drilling. We evaluate the use of the Diametrical Core Deformation Analysis (DCDA) method, developed by Funato and Ito. The method can evaluate non-destructively the absolute differential stresses and measurement errors in the planes orthogonal to the core axes by precisely measuring the ellipticity of the core section orthogonal to the borehole axis. The five readings required to evaluate a single core take only about ten minutes to make. The measurement system is compact enough for a single regular courier parcel or flight check-in luggage. Absolute differential stresses were determined for thirty-five core samples from fourteen holes in different directions in the highly stressed rock mass in a South African gold mine. We were able to constrain an average 3D differential stress field, consistent with the 3D stress field measured in-situ with an overcoring method, with the maximum principal stress larger than 100 MPa, with a root mean square of residuals of several MPa. Interestingly, the results represent relaxation in shear stress near the fault intersected by the boreholes. The measurements require a core diameter larger than ca. 40 mm, and a core is longer than ca. 10 cm. The method assumes that there is no significant inelastic deformation and that the rock is isotropic.

The small punch (SP) test for brittle materials is superior method compared to the three and the four point bending tests because the strength properties can be investigated from thin and small specimens. Note that the SP test can be measured the equibiaxial flexural strength. However, effects of the test conditions such as shape and thickness of specimen, and the SP diameter on the equibiaxial flexural strength have not been studied experimentally and theoretically. In this study, biaxial flexural strength characteristics of the small and thin glass specimen were theoretically and experimentally investigated via SP and modified SP (MSP) tests. First, the relationship between 1- and 2-axis strengths and effective volumes was derived on the basis of competitive risk model. Second, SP and MSP tests were performed for square and circular glass specimens. Thereafter, the F (fracture probability) – S (flexural strength) – V_eff (effective volume) diagrams predicted using 1-axis bending strength data was compared with the experimental results. As the result, the predicted diagram value reasonably agreed with the experimental data of the circular specimen. In contrast, the predicted diagrams was not able to well estimated with the experimental data of the square specimen because Hertzian fracture due to the punch contact occurred during SP and MSP tests.

Probabilistic property associated with debonding of CFRP-bonded steel plate under bending load is theoretically discussed by the use of a probabilistic model based upon a spatially random differential equation. Further, a statistical experiment for debonding of CFRP-bonded steel plate under bending load is performed to verify the theoretically estimated probabilistic property of debonding. First, a system of statically equilibrium equations is formulated to describe axial and shear forces of a steel plate bonded by a CFRP plate. Next, it is extended to a probabilistic model consisting of a system of spatially random differential equations where spatially random variation of the adhesive thickness is introduced as a random field, whose solution leads to a spatially random variation of a principal stress of the adhesive. A Monte Carlo simulation technique is then applied to estimate the probability of debonding under bending load based upon the proposed probabilistic model, where the debonding of adhesive is supposed to occur when the maximum principal stress exceeds a critical value. Further, statistical experiment is also performed to clarify the probability that the debonding of CFRP-bonded steel plates occurs under bending load. Finally, the experimental result is compared with the probability of debonding estimated through the Monte Carlo simulation. The result shows that the probability of debonding obtained through the statistical experiment can be quite well reproduced by the Monte Carlo result based upon the proposed probabilistic model.

Fatigue cracks in steel structures often occur near welds where stress concentration is high. Fatigue cracks are inspected by visual inspection, and if necessary, non-destructive testing such as magnetic particle testing, and penetrant testing are used to confirm the presence of cracks. However, visual inspection has the disadvantage that it depends on inspectors’ experience and skills, and non-destructive testing may not be able to deal with fatigue cracks inside steel plates, so cracks may be overlooked. We are trying to develop auxiliary means to solve these problems. In this study, we attached strain gauges near the suspected fatigue damage position, and we confirmed stress amplitude ratio in two directions around the fatigue cracks, examined fluctuations around fatigue cracks occurrence and progress. We investigated whether cracks can be detected quantitatively by comparing and examining the results of vibration fatigue test and FEM analysis for out-of-plane gusset weld joint and the welding lines of U-rib in orthotropic steel decks. As a result, fluctuations in the stress amplitude ratio are confirmed due to the occurrence and progress of cracks. This method isn’t dependent on inspectors’ experience and skills, may be able to deal with damage inside the structure, so it can be an auxiliary mean of the existing inspection method.

Expectations for solar power generation, one of the renewable energy sources, are increasing in order to reduce greenhouse gas emissions. However, solar panels are industrial products, and there is always a risk of local failure of the products. Several automatic anomaly detection systems have been developed for solar panels and their roofs. However, there is no precedent for automatic anomaly detection of joint clamps supporting roofs and panels, and visual inspection by technicians is still performed as a regular inspection. Thus, accurate and quick evaluation of the joint clamps of solar panels will contribute to reducing the labor and time required for inspections. In this study, we attempted to develop an automatic anomaly detection system for joint clamps, using object detection and recognition technologies based on deep learning. In the first stage, the joint clamp is detected by object detection technology. In the second stage, the system judges the abnormality from the detected the joint clamp by recognition technology. The effective of the proposed system was evaluated through the field experiments.

In industrial, agricultural, and chemical plants, rotating machinery is the most regularly used and important equipment, and its troubles and failures have a significant impact on production and quality. Therefore, the development of technology for detecting abnormalities and diagnosing the soundness of rotating machinery has been an important topic of study for many years. Recently, anomaly detection using unsupervised learning methods of machine learning has been studied in various fields. In this study, we attempted to develop an unsupervised anomaly detection method that does not require damage data in advance. Using a Variational Autoencoder (VAE), which is one of the machine learning techniques, we conducted an anomaly detection experiment through vibration experiments simulating some typical damages of rotating machines and investigated whether the system can recognize situations different from normal appropriately.

It is said that about 50% of all bridges in service will be in service for more than 50 years by 2030. Therefore, measures against aging bridges are an issue. Bearing which is one of the bridge members has a role to support the dead load and live load of the superstructure. Furthermore, it has a function to cope with expansion and contraction of the girder due to temperature changes. However, the quantitative evaluation of the deterioration of the bearing and the effect on other members are unknown. In this study, we targeted bridges erected in Yamagata Prefecture. We measured the behavior of the target bridge due to temperature changes and conducted a static loading test using a 14tf truck. The deck of target bridge was replaced in 2019, and experiments were conducted before and after that. When the behavior with temperature change was compared before and after the replacement, it was confirmed that the deck replacement had a certain effect. The authors conducted a reproduction analysis of behavior due to temperature changes. As a result, it was confirmed that the measured value was between the analysis value considering deterioration and the analysis value not considering deterioration. The authors introduced a spring element during the analysis to quantify the degree of deterioration of the bearings. The authors performed an analysis on a model with a spring element and compared it with the experimental results of a static loading test. As a result, it was confirmed that the analysis value and the measured value of the natural frequency and the vertical displacement are approximated by changing the spring constant. From this, it is considered that this method is effective for quantifying bearing deterioration.

In this study, as a fundamental study of alkali activated material mortar using blast furnace slag fine aggregate, evaluation of fresh properties were carried out. The compressive strength and drying shrinkage after hardening were measured. As a result, in the case of using blast furnace slag fine aggregate, when the mixing volume ratio of fly ash and ground granulated blast furnace slag was 60: 40, the setting time was ensured, and sufficient workability was obtained. And, by using both ground granulated blast furnace slag and blast furnace slag fine aggregate, the alkali activated material can be increased in strength over 90 MPa, and the drying shrinkage can be reduced by 25 % compared with the case where blast furnace slag fine aggregate is not used. In addition, the drying shrinkage of alkali activated material can be reduced by using expansive additive and shrinkage reducing additive, and the expansion effect is equivalent to the general mixing quantity to concrete using Portland cement as a binder.

A complex structure composed of piping is modeled as a simple frame system with an arbitrary curved beam with branches. The transfer matrix method (TMM) is one of the methods used to analyze the displacement and stress fields in such modeled structure. The transfer matrix method (TMM) is highly popular because it reduces the stiffness matrix and facilitates high-speed calculation. However, the application of the TMM has been limited to structural vibration and static problems for a simple straight beam without branches. In this study, we generalized the theory of the TMM for application to binary tree structure, which is a branching structure without a closed paths and with at most three elements connected to branch points. Additionally, a problem based on a statically indeterminate beam with branch was analyzed to verify the generalized theory in this study.