Shuichi Wakayama

Comparison of 2D and 3D carbon/carbon composites with respect to damage and fracture resistance

Abstract The static mechanical responses of two- and three-dimensionally reinforced carbon/carbon composites (2D- and 3D-C/Cs) were compared. The mechanical properties examined included tensile and shear stress–strain (S–S) relations, and fracture behavior using compact tension and double edge notch configurations. Compared with 2D-C/Cs, 3D-C/Cs were shown to possess a similar tensile S–S relation, lower shear strength, higher ultimate deformation in shear, and much higher fracture resistance. The differences in shear and fracture resistance were shown to be derived from a weaker fiber/matrix interface and weaker bonding between fiber bundles in the 3D-C/Cs. These weak interface characteristics of 3D-C/Cs are due to the high value of residual stresses caused by the three-dimensional fiber constraint of 3D-C/Cs.

Characterization of 2-dimensional crack propagation behavior by simulation and analysis

Crack propagation behavior in 2-dimensional polycrystals is simulated and analyzed as a function of the fracture toughness of the grain boundary. The path of a crack impinging on a grain boundary is determined by the competition theory between intergranular and transgranular propagation. With decreasing boundary toughness, the tendency of intergranular propagation increases and the apparent fracture toughness of the polycrystal decreases. The results of the 2-dimensional analysis are compared with the simulation, and the advantages and limitations are discussed. The grain boundary toughness is evaluated by comparing the simulated crack paths with direct observations, resulting in a reasonable value for alumina ceramics. The fracture behavior is characterized in a macro-scale by the percentage of transgranular fracture and also in a micro-scale by the distribution of crack deflection angles.

Characterization of mechanical properties and microstructure of high-energy dual ion implanted metals

Abstract The dual implantation of silicon and carbon ions into copper and iron was carried out with an MeV ion accelerator. Analysis by transmission electron microscopy (TEM) revealed that the ion implanted layer of a Cu substrate is crystalline, while that of an Fe substrate is amorphous. The hardness was measured as a function of the depth by a continuous stiffness measurement method with a nano indenter. Dual ion implantation was found to enhance the hardness of a substrate, and the peak hardness occurred at a smaller depth than the peak concentration of the implanted layer. Cross-sectional TEM images of ion implanted layers taken under the indentations with various depth showed that the indenter did not fracture the implanted layer, but rather deformed it plastically. These data provide us with a qualitative understanding of the hardening mechanism.

Critical Stress of Microcracking in Alumina Evaluated by Acoustic Emission

Microfracture process during the bending tests of alumina ceramics were evaluated by acoustic emission technique. Different size specimens were used for the bending tests in order to investigate the dependence of microfracture process on the specimen size. A remarkable point in the AE generation pattern of each specimen, at which both AE events and energy increased rapidly, was observed before the final unstable fracture. It is important that the apparent stress at those points were independent of the AE threshold level and specimen size. Using the Fluorescent dye penetrant method, the fracture process on the surface was observed, then it has been understood that the stress corresponds to the critical stress for the maincrack formation due to the coalescence of microcracks and/or pores. Considering the microfracture process, statistical treatment for the strength of brittle materials has been discussed. Consequently, it was concluded that the critical stress can be the advanced evaluation parameter, which is equivalent to yield strength in metals, for ceramic materials.

Simulation of microfracture process of brittle polycrystals: Microcracking and crack propagation

Abstract Microcracking and crack propagation behavior are simulated for 2-dimensional alumina polycrystals which have thermal anisotropy within a grain. Microcracks are generated by thermally induced residual stresses at the grain boundary. Stress redistribution due to microcracking and stress intensity factors at the microcrack tip are obtained numerically by the body force method. The location at which microfracture occurs is determined by a competition between microcracking and crack propagation under external stresses. The microfracture stress increases with the progress of fracture and decreases after the maximum indicating a fracture strength. In many cases, the extension of microcracks induces an unstable fracture. With both increasing grain size and decreasing grain boundary toughness, the number of microcracks prior to the unstable state increases and the stress concentration due to the microcracks plays a significant role in the stable crack extension, resulting in lower strengths than the fracture-mechanical predictions.

Characterization of fracture process during ring burst test of FW-FRP composites with damage

The strength of two- and three-ply FRP composite with artificial damage under internal pressure was evaluated using the ring burst test developed by the authors [1, 2]. Ring specimens were fabricated by a filament winding (FW) machine. In this study, the simulated damage of the scratch and the combination of scratch and pre-existing interlaminar delamination, i.e. teflon sheet inserted between the outermost and the inner FRP layers, were introduced artificially into the FW-FRP specimen. The fracture behavior during the tests of damaged FRP was characterized by observation using an 8-mm video camera and a high magnification video scope, strain measurements and AE analysis. It was confirmed that AE analysis is exellent for monitoring the propagation of delamination in an FRP pressure vessel with damage. The pressure of a notched specimen showed the maximum when the delamination propagated to the half region of the specimen. The maximum pressure was larger than the predicted pressure based on the premise tha...

Evaluation of damage progression and mechanical behavior under compression of bone cements containing core-shell nanoparticles by using acoustic emission technique.

In this work, the effect of the incorporation of core-shell particles on the fracture mechanisms of the acrylic bone cements by using acoustic emission (AE) technique during the quasi-static compression mechanical test was investigated. Core-shell particles were composed of a poly(butyl acrylate) (PBA) rubbery core and a methyl methacrylate/styrene copolymer (P(MMA-co-St)) outer glassy shell. Nanoparticles were prepared with different core-shell ratio (20/80, 30/70, 40/60 and 50/50) and were incorporated into the solid phase of bone cement at several percentages (5, 10 and 15 wt%). It was observed that the particles exhibited a spherical morphology averaging ca. 125 nm in diameter, and the dynamic mechanical analysis (DMA) thermograms revealed the desired structuring pattern of phases associated with core-shell structures. A fracture mechanism was proposed taking into account the detected AE signals and the scanning electron microscopy (SEM) micrographs. In this regard, core-shell nanoparticles can act as both additional nucleation sites for microcracks (and crazes) and to hinder the microcrack propagation acting as a barrier to its growth; this behavior was presented by all formulations. Cement samples containing 15 wt% of core-shell nanoparticles, either 40/60 or 50/50, were fractured at 40% deformation. This fact seems related to the coalescence of microcracks after they surround the agglomerates of core-shell nanoparticles to continue growing up. This work also demonstrated the potential of the AE technique to be used as an accurate and reliable detection tool for quasi-static compression test in acrylic bone cements.

Thermal Fatigue Behavior of 3D-Woven SiC/SiC Composite with Porous Matrix for Transpiration Cooling Passages

The effect of porous matrix on thermal fatigue behavior of 3D-orthogonally woven SiC/SiC composite was evaluated in comparison with that having relatively dense matrix. The porous matrix yields open air passages through its thickness which can be utilized for transpiration cooling. On the other hand, the latter matrix is so dense that the air passages are sealed. A quantity of the matrix was varied by changing the number of repetition cycles of the polymer impregnation pyrolysis (PIP). Strength degradation of composites under thermal cycling conditions was evaluated by the 1200°C/RT thermal cycles with a combination of burner heating and air cooling for 200 cycles. It was found that the SiC/SiC composite with the porous matrix revealed little degradation in strength during the thermal cycles, while the other sample showed a 25% decrease in strength. Finally it was demonstrated that the porous structure in 3D-SiC/SiC composite improved the thermal fatigue durability.

Improvement of the burst strength of FW-FRP composite pipes after impact using low-modulus amorphous carbon fiber

Impact tests of filament-wound carbon fiber reinforced plastic (FW-CFRP) composite pipes were carried out using a drop-weight impact test device developed by the authors. Burst pressure of composite pipes after impact tests was also measured in order to investigate the burst strength degradation due to impact damage. Depth of fiber breakage region caused by impact loading was evaluated after burst strength test. The burst strength of FW-CFRP composite pipes after impact was demonstrated to decrease with the increase in cross-sectional area of damage. Furthermore, lowmodulus carbon fibers were wound on the surface of the pipes in order to suppress the impact damage. Consequently, it was understood that the residual burst strength was enhanced due to the reduction of impact damage such as fiber breakages.

Damage accumulation behavior of non-crimp fabric-reinforced epoxy composite under static and cyclic tensile loading

A non-crimp fabric with a stacking sequence of [0°,+45°,90°,−45°] embedded in epoxy resin matrix was analyzed. Samples for mechanical test were obtained from laminates at different orientations depending on the textile architecture direction at 0°, 45°, and 90° in order to study the relationship between damage initiation and propagation with fabric geometry. Tension mode tests (static and cyclic) were carried out to evaluate the evolution of damage using as a main tool, the acoustic emission technique that allows monitoring the mechanical behavior of the materials during the test in ‘real time’. Results show that there is a remarkable mechanical influence of the reinforcement textile in the composite and that damage generation and progression (mechanisms of fracture) is highly dependent of the direction in which the stress is applied in relation with the architecture of the fabric. In static tensile mode, samples at 0° exhibited better mechanical parameters than samples at 90°, while at 45° orientation was lesser. The effect of textile geometry was demonstrated to have an influence in such mechanisms. In cyclic tensile mode, Kaiser effect is observed at low stresses, while the experimental results showed that the Felicity effect became clearer along with the increasing of stress level.