This paper deals with applicability of non-linear fracture mechanics to crack growth by stress corrosion cracking (SCC) under large-scale yielding and in a plastically deformed area. Crack growth test by compact tension specimen is carried out to evaluate crack growth rate under small-scale and large-scale yielding conditions. To evaluate the crack growth behavior from a crack initiated in a plastically deformed area, crack growth test is also carried out for a very short pre-crack in a plastically deformed four-point bending specimen. Conventional stress intensity factor (K) and equivalent stress intensity factor (K_J) defined by J integral are used as fracture mechanics parameters which characterize the crack growth rate. On da/dt-K diagram, a data band shows wide scatter, especially the crack growth rate in a plastically deformed area is higher than that under small-scale yielding condition. On the other hand, da/dt-K_J diagram exhibits narrower scatter on a data band than da/dt-K diagram. The equivalent stress intensity factor is appropriate for characterization of crack growth rate by SCC under small-scale yielding through large scale yielding conditions and in a plastically deformed area.

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Peening techniques are used to introduce compressive residual stress into the surface of a metal in order to improve the fatigue properties and resistance to stress corrosion cracking of the metal. The present paper focuses on the effect of peening on the fatigue crack growth behavior in austenitic stainless steel. An indicator which can be used to determine by how much the fatigue crack growth can be suppressed was found. Regardless of the type of peening technique used, a much closer correlation to the number of cycles to failure was found with the area of integration under the compressive residual stress curve with respect to depth than with the compressive residual stress at the surface. Moreover hybrid peening, which is a combination of different peening techniques, was developed in this study in order to enhance the favorable effects of peening. Hybrid peening increased the area of integration under the compressive residual stress curve and, as a result, a further increase in the number of cycles to failure compared to single peening was obtained.

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Changes in surface roughness during low cycle fatigue loading were investigated on austenitic stainless steel, SUS316NG, commonly used in the piping systems of nuclear power plants. The fatigue damage process, including crack initiation and propagation, was observed using cellulose acetate replicas, and the relationship between the process and changes in surface roughness was discussed. Strain-controlled fatigue tests were conducted on mirror polished specimens at constant strain range conditions Δε = 8, 4, and 1%. During the cyclic loadings, the surface roughness was measured at cycles determined with respect to the usage factor UF_pre. As a result, the surface roughness was found to increase roughly linearly until approximately UF_pre = 0.4 regardless of the strain range conditions. The rate at which surface roughness increased with UF_pre became smaller with decreasing applied strain range. In the damage process, small cracks were observed to initiate very early in fatigue life. The cracks propagated slowly until about the middle of fatigue life, however it grew rapidly after around UF_pre = 0.6. These results showed that the change in surface roughness is sensitive to fatigue loading even when cracks are very small and crack detection is difficult. The obtained results suggest that surface roughness can probably be used to assess fatigue damage until the middle of fatigue life because of its linear increase with respect to the number of cycles.

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In the present study, the impact damage of CNF/CFRP hybrid laminates and influence of carbon nanofiber (CNF) interlayer on impact damages were investigated by the drop weight impact tests. Vapor grown carbon fiber (VGCF) has been employed for the toughener of the interlayer on the CFRP laminate. Drop weight impact tests were carried out using “Dynatup” impact test equipment. Damaged area occurring in the interlayer of the CFRP laminate was observed by ultrasonic flaw detection system. The damage properties of the CNF/CFRP hybrid laminates were considered from the viewpoint of the relation between damaged area and impact energy. Moreover, by compression after impact (CAI) tests, the relation between residual compression strength, impact energy and damaged area was investigated. It was confirmed that the damaged area could be reduced, and CAI strength became higher by inserting CNF interlayer. Moreover, the optimal additive amount of VGCF for the interlayer was about 20 g/m².

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Multi-walled carbon nanotube (MWCNT)/alumina composites with different amounts of MWCNTs were prepared by a simple mechanical mixing method followed by a pressureless sintering. The effects of microstructures including grain size, MWCNT location, fracture patterns of the matrix and dispersibility of MWCNTs on the mechanical properties of the resultant composites were investigated. It was demonstrated from microstructural observations and fracture tests of the composites that grain size, MWCNT location and fracture patterns had no significant impact on the mechanical properties of the composites prepared under the processing conditions used in this research. In contrast, the state of MWCNT dispersion within the composites was shown to be one of the important factors affecting the strength and toughness enhancement in our composites. The dispersibility of MWCNTs decreased monotonically as a function of MWCNT volume fraction. The degradation in the mechanical properties observed for the composites with a higher amount of MWCNTs was shown to be associated with the dispersibility of MWCNTs, and a MWCNT agglomerate was anticipated to act as imperfections and led to a reduction of mechanical properties.

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In order to discuss the origin of the buckling of carbon nanotubes from the atomic level, we have performed the compressive simulation of non-defective and defective triple-walled carbon nanotubes (TWCNT) by the molecular dynamics method using the adaptive intermolecular reactive empirical bond order potential, and observed changes in atomic stresses until the buckling. In the non-defective TWCNT, standard deviations of atomic axial stresses rise drastically before the buckling. The transition from homogeneous stress distributions to inhomogeneous ones play an important role in the occurrence of the buckling. In TWCNTs with a vacancy-type defect or a Stone-Wales defect, buckling stresses differ according to location of the defect. Repulsive interlayer interactions caused by the constriction of the outer layer including a defect reduce significantly the buckling strength. On the other hand, constrictions of the middle or inner layer including a defect produce slightly attractive interaction with the outer layer. Therefore, whole layers is buckled at the same time as the buckling of the outer non-defective layers. TWCNTs including many defects that are generated by the heat treatment simulation show smaller buckling stresses than that of TWCNTs including a defect. Defect configurations have a significant influence on distribution of atomic stresses until the buckling. The buckling occurs from constriction parts located close to defects.

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We propose a pattern position correction method for obtaining a fringe pattern in speckle interferometry to measure deformations of both small enough displacements and large displacements which exceed the limit in the same observation region. Intensity values at each point of the deformed state speckle pattern illuminated by dual beam are shifted to initial state location. In this method, not only speckle images illuminated by dual beam, but also speckle images illuminated by single beam are captured at the initial state and the deformed state. Shift amounts for each point are obtained by local least squares for data obtained by digital image correlation using the initial and the deformed state speckle images illuminated by single beam. Shift amounts vary continuously on each point. Intensity values at each point of the corrected speckle image are obtained by bilinear interpolation. Image subtraction is performed to the dual illuminated initial state spackle image from the corrected deformed state image. An interference fringe can be observed on not only the region that the displacement is small enough, but also the region that the displacement exceeds the range of conventional spackle interferometry, signifying the successful application of our proposed technique.

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This paper reports state of progress for development of high-performance CFRP flywheel. For the development, high specific energy density and durability are required. A new fabricating method suggested in our previous work (Mechanics of Time-Dependent Materials 2012) is employed for the CFRP flywheel. According to the method the fabricated flywheel does not have radial stress and the ultimate failure is dominated by hoop stress. A spin test is conducted for the fabricated CFRP flywheel. Strain gauge and digital image correlation methods are employed for the rotational strain measurement. The both methods verify very little rotational radial strain, which means the radial stress is also little. As a first step of the development of high-performance CFRP flywheel, the feasibility of no-radial stress flywheel fabrication is indicated by this study.

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In this study, bast fibers of kenaf (Hibiscus cannabinus L.), in the form of randomly dispersed short-fiber reinforcement, were loaded into poly(lactic acid) (PLA) or simply polylactide. Twin-bladed melt mixing followed by hot-pressing was chosen as the present composite molding with fiber weight fractions varied from 0wt% (i.e. neat PLA) to 20wt% by 5wt% increment. From the optical microscopic surface images, kenaf fibers were found to evenly be dispersed in the present composites with random fiber orientations in the two-dimensional flat-wise directions. Tensile strength and strain at break of the composites were found to decrease as compared to those of neat PLA. On the other hand, the tensile moduli both at initial and at several stress levels up to the final breakage were found to be improved via kenaf fiber loading. Two-parameter Weibull plots in terms of tensile strength and SEM fractography of the fractured surfaces were also examined and complicated and multiple sources for the fracture initiation and propagation in the present composites were implied. Finally, flexural properties, Charpy impact toughness, plain strain fracture toughness and S-N curves for the present composites were also shown and then discussed to sustain some benefits of the kenaf fiber loading.

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Gels are soft and wet materials, which have unique properties such as low surface friction, material permeability, and biocompatibility. These superior properties of gels can possibly be applied to develop novel artificial blood vessels and can also be used at knee joints. However, the mechanical properties of the gels have not been clarified and controlled so far due to the lack of standard type of gels for mechanical engineering research. Here we try to develop a novel type of gel named the standard gel engineering materials (standard GEM) via simple bulk polymerization, i.e. dry synthesis without any solvent. In the numerous previous studies of gels, almost all gels have been prepared by solution polymerization, i.e. wet synthesis with various solvents. The mechanical properties of the wet-synthesized gels, however, strongly depend on the concentrations of the monomer, crosslinker, initiator, and accelerator in pre-gel solutions. Therefore, it is difficult to control the mechanical properties of gels causing rough reproducibility of mechanical experiments. In the present paper, the simple bulk polymerization is proposed as another candidate to create a standard GEM. The key point of the present work is to use the liquid type of highly reactive monomer, which plays the role of solvent in preparation, and thus we do not have to consider both the ratio concentrations of the monomer and the accelerator to the solvent in preparation. In this case we can focus only on the ratio concentration of the crosslinker to the monomer. This ratio concentration is directly related to the contour length of the polymer chain between the crosslinking points. We have found that the mechanical properties of the dry-synthesized GEM depend hardly on the ratio concentration of the initiator to the monomer, but mainly on that of the crosslinker. The Young's modulus is simply, well described as a power function of the crosslinker concentration. We hope this dry-synthesized GEM will be useful as another novel standard sample for mechanical engineering research.

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Porous scaffolds play important roles in tissue engineering. Biodegradable synthetic polymers such as poly(ε-caprolactone) (PCL) are frequently used in the preparation of porous scaffolds. In this study, a reinforcing composite scaffold was fabricated using the freeze-drying technique by embedding porous PCL core within collagen shell. Controls of pure collagen and PCL scaffolds were also investigated for comparison in this study. Microstructure and mechanical properties of the scaffolds were investigated and related to their morphologies examined by SEM. Fracture micromechanism under tension was also characterized on the basis of microstructural deformation behavior. It was found that the composite scaffold possessed an appropriate mechanical property, which was greater than that of pure collagen scaffold. This study demonstrated that the reinforcing technique could modulate the mechanical properties and porous structure of the composite scaffold, which could be potential factors in ligament tissue engineering.

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