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The effect of current peening techniques that modify the surface layer of metallic materials on the fatigue strength of those materials has been enhanced by employing a micro-size, high-rate shot impact. The present paper evaluates the effect of impact velocity and impact size on work hardening on the surface of polycrystalline metals subjected to peening. A single-shot impact was modeled based on a polycrystal plasticity finite-element analysis in order to address the effect of grain-order work hardening. Using the finite-element analysis, the effect of the relative size of the shot and the individual grains on the surface work hardening of polycrystalline metal was investigated. Simulated results reveal that the deformation progresses preferentially along the grain boundary rather than inside the grain after a large shot impact, while peening with a small shot can introduce intense work hardening inside the grain just beneath the surface. Moreover, we compared a shot impact and a static indentation whose dimples were almost the same size and confirmed that a high-rate shot impact can generate a significantly work-hardened layer beneath the impacted surface.

The usual way for reduce outgassing of a large structure in vacuum is to bake the whole vacuum chamber containing the structure. However, this method needs a huge heater capacity and there are limits caused by the heat expansion of the chamber. The solution is to raise the temperature of the structure inside without heating the vacuum chamber. This is achieved by installing a heat source inside the chamber and by inserting the heat shield between the structure and the chamber walls to direct the heat to the structure. In the particle accelerator field, it is often required to reduce outgassing of structures inside vacuum chambers. One example is a kicker magnet, which is installed in a vacuum chamber and consists mainly of ferrite and aluminum alloy. As known from former experience the main outgassing component from ferrite is water. We applied the above mentioned method to the outgassing reduction of such a kicker. We are able to direct most of the heat flow toward the kicker magnet by inserting the heat shielding plates and thus outgassing was successfully reduced.

The effect of fiber arrangement on transverse tensile failure in unidirectional carbon fiber reinforced composites with a strong fiber-matrix interface was studied using a unit-cell model that includes a continuum damage mechanics model. The simulated results indicated that tensile strength is lower when neighboring fibers are arrayed parallel to the loading direction than with other fiber arrangements. A shear band occurs between neighboring fibers, and the damage in the matrix propagates around the shear band when the interfacial normal stress (INS) is sufficiently high. Moreover, based on the observation of Hobbiebrunken et al., we reproduced the damage process in actual composites with a nonuniform fiber arrangement. The simulated results clarified that the region where neighboring fibers are arrayed parallel to the loading direction becomes the origin of the transverse failure in the composites. The cracking sites observed in the simulation are consistent with experimental results. Therefore, the matrix damage in the region where the fiber is arrayed parallel to the loading direction is a key factor in understanding transverse failure in unidirectional carbon fiber reinforced composites with a strong fiber/matrix interface.

Cavitation normally causes severe damage in hydraulic machinery such as pumps and turbines by the impact produced by cavitation bubbles collapsing. Although cavitation is known as a factor of erosion, Soyama et al. succeeded in utilizing impacts of cavitation bubble collapsing for surface modification by controlling cavitating jet in the same way as shot peening. The local plastic deformation caused by cavitation impact enhances the fatigue strength of metallic materials, and the surface modification technique utilizing cavitation impact is called “cavitation peening (CP)”. It is well known that the peening improves fatigue strength by introducing compressive residual stress on the surface, but little attention has been paid to the behavior of fatigue crack growth of the material which was modified by CP. In the present study, the fatigue behavior of austenite stainless steel with and without CP was evaluated by a plate bending fatigue test, and the results revealed that the compressive residual stress introduced by CP suppresses fatigue crack growth rate by 70 % compared to that without CP.

Shape memory polymers (SMPs) are currently investigated as potential materials for large deployable space structures [1–3]. The thermomechanical properties of these polymers significantly change on reaching their glass transition temperature, which yields the excellent feature of shape fixity and shape recovery [4]. As another aspect, the modulus of these materials is not sufficient since they are polymeric materials. In actual applications, the fiber reinforcement is effective for ensuring the sustainability of the deployed structures. However, while the fiber reinforcement has advantages for increasing the stiffness, it has a negative influence on the shape recovery behavior of SMPs. Experimental studies for shape memory polymer composite were conducted by Gall et al. [5, 6] for SiC powder-reinforced nanocomposite, Ohki et al. [7] for short-glass-fiber reinforcement, and Lan et al. [8] for SMP reinforced with plain-weave fabrics, and the increase of residual strain after shape recovery process was confirmed. In order to maximize both the stiffness and shape-recovery behavior of fiber-reinforced SMPs, it is essential to know how the fibers block the shape recovery behavior of polymers. However, the mechanism was not modeled. Therefore, we focus on the mechanism underlying the degradation of shape recovery behavior due to fiber reinforcement. To investigate this mechanism, we first conducted thermomechanical cycle tests for pure SMP and SMP reinforced with short-carbon fibers. We used polyurethane series of thermoset SMP, Diary MP-5510 (curing temperature 100 C), provided by SMP Technologies Inc. The cured polymer has a glass transition temperature of 55 C, and the temperature range of glass transition between glassy state and rubbery state is 30 C. Short carbon fibers T700S (Toray Industries Inc.) were embedded in the polymer. The fibers with approximately 5 mm length were randomly embedded in the prepolymer before curing, and the weight fraction of fibers was set to 0, 2, and 4 wt%. It should be noted that the bundle of carbon fibers (12,000 fibers) was embedded and not dispersed in these model experiments, as also modeled later. After curing, strip specimens were cut out (approximately 40 mm gauge length, 20 mm width, and 2 mm thickness in average). The following thermomechanical cycle was applied to these specimens using a tensile test machine (INSTRON 5566), as shown in Fig. 1

This paper examines the stress distribution around a fiber break in alumina-fiber reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts the tensile strength using tensile failure simulations. In particular, we discuss the effect of the matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic-perfect plastic matrix and an elastic-plastic hardening matrix using finite element analysis. Second, the procedure for simulating fiber damage evolution in the Al2O3/Al composites is presented. The simulation incorporates the analytical solution for the axial fiber stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites, and discuss the effect of matrix hardening on the tensile strength of the Al2O3/Al composites. Coupled with size-scaling analysis, the simulated results express the size effect on the strength of the composites, which is seen in experimental results.

This paper proposes a numerical model of the fatigue damage process in short fiber-reinforced plastics. In the fatigue fracture of these composites, the microcracks in the polymer matrix increase with fatigue cycles and dominate the fatigue damage process. Therefore the matrix crack was modeled by the continuum damage mechanics approach while considering the microscopic fatigue damage process in the polymer matrix based on a Kachanov-type damage-evolution law. We applied the model to addressing the fatigue-cycle experiments of short glass-fiber reinforced polycarbonate conducted by Ha et al. The simulated results agreed well with the experimental results. Moreover, the simulation revealed that the dependence of the damage accumulation on the fiber orientation remarkably changes the fatigue life of the short glass-fiber reinforced plastics.

We investigate the effects of the vacancy defects (i.e., missing atoms) in carbon nanotubes (CNTs) on the interfacial shear strength (ISS) of the CNT-polyethylene composite with the molecular dynamics simulation. In the simulation, the crystalline polyethylene matrix is set up in a hexagonal array with the polymer chains parallel to the CNT axis. Vacancy defects in the CNT are introduced by removing the corresponding atoms from the pristine CNT (i.e., CNT without any defect). Three patterns of vacancy defects with three different sizes are considered. Two types of interfaces, with and without cross-links between the CNT and the matrix are also considered here. Polyethylene chains are used as cross-links between the CNT and the matrix. The Brenner potential is used for the carbon-carbon interaction in the CNT, while the polymer is modeled by a united-atom potential. The nonbonded van der Waals interaction between the CNT and the polymer matrix and within the polymer matrix itself is modeled with the Lennard-Jones potential. To determine the ISS, we conduct the CNT pull-out from the polymer matrix and the ISS has been estimated with the change of total potential energy of the CNT-polymer system. The simulation results reveal that the vacancy defects significantly influence the ISS. Moreover, the simulation clarifies that CNT breakage occurs during the pull-out process for large size vacancy defect which ultimately reduces the reinforcement.

Numerical simulation by finite element analysis was used to investigate the relationship between the strength of glass fiber reinforced plastic (GFRP) and fiber length. Load speed dependability was also investigated, since thermoplastic resin used for GFRP exhibits much nonlinear stress—strain behavior and strong dependency on load speed. For this purpose, we conducted a periodic-cell simulation to address the effect of composite microstructure, matrix viscoplasticity, and microscopic damage (fiber break and matrix crack). When the fiber length was varied, the damage pattern was divided into two patterns: fiber-avoiding propagation and fiber-breaking modes of the matrix crack from fiber ends. When the matrix crack easily propagated in a fiber-avoiding way for shorter fiber lengths, the rate-dependent effect of the matrix was significant. Moreover, we considered the length at which the fracture mode changed based on this analysis, and compared it with the conventional critical length given by Kelly. Since the conventional critical length does not ensure improved composite strength, the consideration of the damage mode transition is essential for selecting the appropriate fiber length for strength improvement.

Discontinuous-glass-fiber-reinforced plastic (GFRP) composite material is widely used in industrial fields, mainly because glass fiber improves the strength and stiffness of polymer and because it is much less expensive than carbon fiber. It is thought that the use of long fiber is important in more efficiently improving the strength and stiffness of composites. In our previous study [1], we reported that the fracture mode of the discontinuous-fiber-reinforced composite changes from the matrix-cracking mode to the fiberbreaking mode (Fig. 1), when the aspect ratio for the fiber length to the fiber radius exceeds about 150. We also demonstrated that the strength is dramatically improved compared to the strength of the short fiber-reinforced composites and that the global load-sharing (GLS) model can roughly predict the strength (Fig. 2). Recently, Thomason [2, 3] produced the long discontinuous-fiberreinforced composites where the aspect ratio was about 250 and reported the stiffness and strength of the composites. In this article, we applied the GLS model to his experiments and discuss the validity of our models. First, we discuss the strength of unidirectional (UD) discontinuous-fiber-reinforced composites, rUD, based on the GLS assumption [4, 5]. We applied two types of GLS approaches to predict the composite strength. The GLS model focuses on one fragmented fiber (i.e., discontinuous fibers), aligned in the fiber axial direction, and neglects the interaction among fibers in the fiber cross-sectional direction. It predicts the composite’s strength by simulating the fiber damage evolution in such a fiber. One approach is based on Monte Carlo simulation [6] for fragmentation in a fiber in the composites. The other is based on the analytical model by Duva et al. [5]. (Hereafter, we refer to this as the DCW model.) Monte Carlo simulation deals with a detailed fiber stress distribution and fragment distribution, though multiple calculations are required for the prediction because it is a probabilistic approach. In contrast, the DCW model assumes an approximate stress distribution and fragment distribution, but it predicts the composite strength analytically. In simulating the fiber-damage evolution, the first approach utilized Monte Carlo simulation with the elastic– plastic hardening shear-lag model given by Okabe and Takeda [7]. The schematic of the elastic–plastic shear-lag model is illustrated in Fig. 3. The axial length of the model was set to 25 9 lf (lf is the length of discontinuous fiber), and the axial length was divided into 10,000 segments. The fiber ends in discontinuous-fiber-reinforced composites were represented by setting some random segments to the initially broken segments. Thus, the averaged length lf of the discontinuous fibers was related to the density of the initially broken segments introduced in the model. The transverse length of the matrix shear region in the model

This paper attempts to quantify the fracture properties (strength and toughness) of the fiber–matrix interface in composites, using the fragmentation process and debonding growth for HI-Nicalon™ SiC single-fiber and T300 carbon single-fiber epoxy (Bisphenol-A type epoxy resin with triethylenetetramine (TETA) as curing agent) composite systems. This method is based on the numerical modeling for the microscopic damage and fragmentation process in single-fiber composite (SFC) tests, with a cohesive zone model (CZM). For the HI-Nicalon™ SiC single-fiber epoxy composite in which the major damage near a fiber break is interfacial debonding, interface properties were reasonably determined as ( T II,max , G IIc ) = (75 MPa, 200 J/m 2 ). In contrast, for T300 carbon single-fiber epoxy composite, we could not determine unique interfacial properties, since the variation of the cohesive parameters hardly affects the microscopic damage process due to the transition to the damage pattern dominated by matrix cracking.

This paper proposes a numerical simulation of interlaminar damage propagation in FRP laminates under transverse loading, using the finite element method. First, we conducted drop-weight impact tests on CFRP cross-ply laminates. A ply crack was generated at the center of the lowermost ply, and then a butterfly-shaped interlaminar delamination was propagated at the 90/0 ply interface. Based on these experimental observations, we present a numerical simulation of interlaminar damage propagation, using a cohesive zone model to address the energy-based criterion for damage propagation. This simulation can address the interlaminar delamination with high accuracy by locating a fine mesh near the damage process zone, while maintaining computational efficiency with the use of automatic mesh generation. The simulated results of interlaminar delamination agreed well with the experiment results. Moreover, we demonstrated that the proposed method reduces the computational cost of the simulation.

This paper proposes a numerical simulation of interlaminar damage propagation in FRP laminates under transverse loading, using the finite element method. First, we conducted drop-weight impact tests on CFRP cross-ply laminates. A ply crack was generated at the center of the lowermost ply, and then a butterfly-shaped interlaminar delamination was propagated at the 90/0 ply interface. Based on these experimental observations, we present a numerical simulation of interlaminar damage propagation, using a cohesive zone model to address the energy-based criterion for damage propagation. This simulation can address the interlaminar delamination with high accuracy by locating a fine mesh near the damage process zone, while maintaining computational efficiency with the use of automatic mesh generation. The simulated results of interlaminar delamination agreed well with the experiment results. Moreover, we demonstrated that the proposed method reduces the computational cost of the simulation. � 2006 Elsevier Ltd. All rights reserved.

Superfluid transition and solidification of 4 He in aerogel has been studied by longitudinal ultrasound. The superfluid transition temperature was determined up to the solidification pressure. The superfluid transition temperature in aerogel which was identified as a sharp absorption peak was suppressed from that in bulk liquid. The magnitude of suppression in aerogel was a few milli-Kelvin from the saturated vapor pressure to the solidification pressure and was much smaller than that in other porous media. The onset of solidification of 4 He in aerogel was identified as an increase in sound absorption. It was shown that the overpressure required to initiate solidification was 0.2–0.3 MPa between 1.1 and 1.8 K. Small hysteresis was observed at the solidification and melting.

Acoustic properties have been studied in 4 He–aerogel system. The longitudinal ultrasound velocity and attenuation have been measured in 92.6 and 94.0% porous aerogels at elevating pressures from the saturated vapor pressure to the melting point for both normal and superfluid phase. The temperature dependence of the sound velocity was similar to that of bulk liquid. The superfluid transition temperature was determined by a dip in velocity and a sharp peak in attenuation. The sound velocity was analyzed using a hydrodynamic theory for both phases. The pressure and temperature dependencies of the sound velocity in the normal phase can be rather well explained by a simple hydodynamic theory. However, at the lowest temperature where no normal component is expected, the sound velocity behavior differs significantly from the case of bulk case. It is shown that coupling mechanism between acoustic phonons in the liquid and aerogel matrix other than viscosity is necessar to elucidate the sound velocity below 1 K.

The single-fiber composite (SFC) has been widely used to quantify fiber strength and fiber-matrix interfacial properties of fiber-reinforced composites. Here, a numerical model with an embedded-process-zone model to permit both interface debonding and matrix cracking is used to predict the fragmentation process and the microscopic damage around fiber breaks in SFC tests as a function of the interface strength and toughness. For low interface strengths, interface debonding occurs. For intermediate interface strengths, matrix cracks occur and delay debonding. For high interface strengths, debonding does not occur and deformation is controlled by a matrix shear, with strain hardening playing an important role. Interface toughness plays a secondary role in determining the transitions in damage modes. Well-established models assuming a constant interfacial shear strength can fit SFC data for low interface strengths, but the interface strength parameter is unrelated to the actual shear strength. In the high-strength regime, a strain-hardening shear-lag model can fit the SFC data quite well. Overall, the fiber strength distribution can be obtained from SFC tests by fitting to the fragment length versus applied strain, but estimation of interfacial properties is difficult due to the transition in dominant deformation and damage mechanisms, including matrix cracking.

The present study has focused on achieving a micromechanical understanding of the microbond test, which involves pulling a fiber out of a bead of matrix (i.e. droplet) through a knife-edge, in order to quantify the interfacial fracture properties of fiber-reinforced composites. According to the microbond test results for carbon-fiber and epoxy-resin system, matrix cracking occurred during the fiber pullout, in addition to the debonding at the fiber–matrix interface. Therefore, in evaluating the fracture properties of the fiber–matrix interface, we should pay attention to the coupling effects of matrix failure and interfacial debonding on the test results. Then, we discuss how to best extract the interfacial properties while excluding the influence of matrix plasticity and cracking, using numerical simulations. The key mechanism demonstrated here is that the pullout force, in the cases where the influence of matrix cracking is negligible, appears as the upper limit among the experimental data of the pullout force for a constant initial embedded length of the fiber in the matrix. For this reason, the upper-limit data all over the range of embedded fiber length in experiments can be reasonably evaluated by the simulation focusing on the debonding process with matrix plasticity. This evaluation technique is effective as a way of extracting interfacial properties appropriately from microbond test results.

Cavitation peening (CP), which utilises the impact caused by cavitation bubbles collapsing, can improve the fatigue strength of metallic materials as well as the yield stress and tensile strength. The rotor cores in interior permanent magnet motors use electrical steel sheet. In order to increase the permissible rotating speed of the motor and achieve high efficiency, the mechanical strength of the stress concentration area in the steel sheet where the magnet is pressed needs to be enhanced. In the present paper, to improve the yield stress and tensile strength, the steel sheet was peened by CP. The peening effect on these mechanical properties was evaluated by tensile and indentation tests. The experimental results from the tensile tests revealed that CP can increase the yield stress by 22% compared to that before CP. In addition, the yield stress and tensile strength increased along with the CP processing time.