Since magnesium alloy is the lightest metal material, use of magnesium alloy is extended from the point of cost and fuel economy, of automobiles, in recent years. Therefore, the fatigue characteristics of magnesium alloy are indispensable in order to create the design which ensures the reliability and the safety of vehicles especially in long-term use. However, most of the fatigue data for magnesium alloys are S-N curves dating from the 1930s to 1960s and there are few researches on the local strain in the fatigue behavior of magnesium alloy. In this research, some fatigue tests of magnesium alloy ware performed. Local strain in the vicinity of the notch root and crack-tip was measured in real time by means of the fine-grid method. The relationships between local strain, crack initiation from the notch and crack propagation have been studied. As a result, also in the magnesium alloy, it was made sure that it was possible to estimate crack initiation and progress by local crack-tip strain.

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Stable and unstable fracture behavior of PLA was studied using a high-speed extensometer. The single-edge-notched tensile specimens were fractured with a special loading jig so that it could split and fly away after the fracture. The load and displacement diagram, i.e. the external work applied to the specimen was partitioned into four parts: the stable and unstable fracture energy for creating new surfaces, the elastic energy left in the fractured specimen, and the nonelastic energy due to viscoplastic deformation. These energies were then determined and correlated with the fracture load and external work. Energy release rates were also evaluated, and the results were discussed.

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Displacement measurement was conducted on compact normal and shear specimens made of acrylic homogeneous material subjected to mixed-mode loading by digital image correlation. The intelligent hybrid method proposed by Nishioka et al. was applied to the stress-strain analysis near the crack tip. The accuracy of stress-intensity factor at the free surface was discussed from both viewpoint of experiment and 3-D finite element analysis. The surface images before and after deformation were taken by a CMOS camera, and we developed the system which enabled the real time stress analysis based on digital image correlation and inverse problem analysis. The great portion of processing time of this system was spent on displacement analysis. Then, we tried improvement in speed of this portion. In the case of cracked body, it is also possible to evaluate fracture mechanics parameters such as the J integral, the strain energy release rate, and the stress-intensity factor of mixed-mode. The method of 9 points elliptic paraboloid approximation could not analyze the displacement of submicron order with high accuracy. The analysis accuracy of displacement was improved considerably by introducing the Newton-Raphson method in consideration of deformation of a subset. The stress-intensity factor was evaluated with high accuracy of less than 1% of the error.

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It is very important to clarify fracture behavior of the materials under low temperature atmosphere. In this study, we first constructed the experimental fracture apparatus by combining cooling furnace with the method of caustics for investigating fracture behavior of various materials in the temperature range from room temperature to about 93K. Then, fundamental points of applicability of the caustics technique in this experimental system are studied. It is shown that a clear caustic pattern can be observed at very low temperature by using this experimental apparatus. Influences of low temperature on fracture toughness K_IC and caustic constant c₀ are analyzed for some polymers, glass and ceramic plates by using the experimental apparatus constructed here. Moreover, we tried to measure the value of c₀ of ceramics by strain gage technique. There is not so large variation in the value of c₀ of ceramics and glass plates within the temperature tested, but the values c₀ of polymers are decreased at low temperature. Fracture toughness values K_IC of glass, Pyrex glass and PMMA plates become large at low temperature.

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To evaluate the 3-D stress field inside the specimen from displacement data on the free surface obtained from the 2-D intelligent hybrid method, we developed the 3-D local hybrid method based on the inverse problem analysis. The accuracy of the 3-D local hybrid method varies depending on the depth of the plane of error assessment, hybrid domain size and specimen thickness. Hence the optimal analysis condition was discussed for the acrylic homogeneous material and the acrylic-aluminum dissimilar material. The 3-D local hybrid method can estimate the stress intensity factor with high accuracy, if error assessment is carried out between the displacement matrix at 1.5 mm depth of the 3-D local hybrid model and the modified displacement data obtained by the 2-D intelligent hybrid method from the displacement data measured in the experiment. Accurate analyses can be conducted if hybrid domain size is taken more than two times of the thickness of the specimen with a straight crack of homogeneous and dissimilar materials. The stress intensity factor at the central part of the specimen can be estimated with less than 1.0 % error. The 3-D local hybrid method can evaluate the stress field inside the specimen of homogeneous material with high accuracy. On the other hand, in dissimilar material, accuracy goes down slightly. In an actual structure, a surface crack occurs in many cases. Then, the applicability to the surface crack problem of the 3-D local hybrid method was examined.

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High speed holographic microscopy is applied to take successive photographs of fast propagating cracks in uniform tensile loading specimens and pin-loading specimens. From the photographs, crack opening displacements (CODs) are measured along the cracks as a function of r from the crack tips. The measurement results show that the CODs of branch cracks in uniform tensile loading specimens are not proportional to √r. However, the CODs of branch cracks in pin-loading specimens are proportional to √r and energy release rate can be measured from the CODs. The energy release rates at the bifurcation are the same in both specimens and are continuous across the bifurcation point. As branch cracks propagate, the energy release rates increase in uniform tensile loading specimens but decrease in pin-loading specimen.

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The authors developed a new hybrid method combining digital image correlation (DIC) with finite element method (FEM) in case of elastic deformation fields. DIC and FEM can calculate the displacement, strain and stress distribution in the full-fields. But DIC includes inevitably experimental error, for example, correlation error and limit of resolution. On the other hand FEM involves serious problem whether selected boundary condition is correct or not. In the present paper, first, the proposed hybrid technique is demonstrated based on the intelligent hybrid technique developed by Nishioka for reducing experimental error. This proposed technique can reconstruct the boundary condition from results of full field displacement distribution obtained by DIC, then calculate the full field displacement using determined boundary condition. This result can assure the satisfaction of the equilibrium equation. In addition, in order to verify the effectiveness of proposed hybrid method, the authors perform experiment of tensile test plate with a hole using DIC method, and comparison of the results obtained by theory, experiment, current hybrid and proposed hybrid. As a result, proposed method is indicated a more effective than experiment and current hybrid technique.

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Recently, there is a tendency that number of the lumbago patient is increased. In order to cure the patients who suffer from the lumbar disease, external load is applied on the intervertebral substitute in the medical treatment. It is important to clarify the stress distribution in the intervertebral substitute based on the research of biomechanics. In this study, we analyzed the stress distribution in the intervertebral substitute by using photoelastic stress-freezing technique. Three-dimensional stress distribution is obtained for intervertebral substitute under vertical compressive load and the effect of loading angle on the stress distribution is clarified. Moreover, stress distribution for intervertebral substitute inserted in carbon cage is analyzed and load transmission ratio in intervertebral substitute is discussed.

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