This paper describes an experimental and analytical study on the static fatigue behavior of piezoelectric ceramics under electromechanical loading. Static fatigue tests were carried out in three-point bending with the single-edge precracked-beam specimens. The crack was created perpendicular to the poling direction. Time-to-failure under different mechanical loads and dc electric fields were obtained from the experiment. Microscopic examination of the fracture surface of the piezoelectric ceramics was performed as well. A finite element analysis was also made, and the applied energy release rate for the permeable crack model was calculated. The effect of applied dc electric fields on the energy release rate versus lifetime curve is examined. The most important conclusion we reach is that the lifetimes for the piezoelectric specimens under a positive electric field are much shorter than the failure times of specimens under a negative electric field for the same mechanical load level.
Accurate determination of friction at the die/billet interface in hydrostatic extrusion is a complex issue due to involvement of various operating parameters viz. billet velocity, die geometry, contact pressure, material parameter and the regime of lubrications prevailing at die/billet interface. Therefore, the objective of this paper is to investigate friction and friction stress at die/billet interface in hydrostatic extrusion process of aluminium based alloys. The friction stress at die/billet interface is numerically computed for three lubricants whose rheology is represented by Roelands' viscosity model. Investigations have been carried out for friction stress variations along the work zone for a wide range of extrusion ratios (A = 2 to 10), semi-die angles (θ = 100 to 200) and material parameters (G = 0.67 to 1.86). Moreover, the validation of the proposed model has been done with the published work available in the literature.
Green composites, such as natural fiber reinforced naturally-derived plastics attract much attention because of reducing CO2 emission. Many studies about green composites have enabled the materials substitutes as glass fiber reinforced composites. However, lack of investigations about fatigue properties restricts the actual usage of the composites. In the present study, we investigate fatigue properties of green composites. A hemp fiber yarn reinforced poly(lactic acid) composite was selected as a green composite. Unidirectional (UD) and textile (Textile) composites were fabricated using micro-braiding technique. Fatigue tests results indicated that fatigue damages in UD composites was splitting which occurred just before the final fracture, while matrix crack and debonding between matrix and fiber yarn occurred and accumulated stably in Textile composites. These results were consistent with modulus reduction and acoustic emission measurement during fatigue tests.
The present study aims to clarify the scale dependency of progressive adhesion behavior of work material on micro tools under the dry friction in microforming. Scaled progressive deep drawing test up to 300 times is conducted under dry condition in micro- and milli- scale. The process dimensions of 0.97mm and 5.82mm in drawn cup diameter are produced with the stainless steel foils of 0.05mm and 0.3mm in thickness, respectively. The experimental results show that the transition of maximum punch force has different tendencies in each scale. These tendencies are well corresponded to the transition in surface state of tools and drawn cups. While, the strong adhesive wear is observed for the milli-scale, there is a slight change in the surface state of the tools for micro-scale. To investigate this difference in the adhesion behavior of the work material in each scale, a finite element analysis considering surface asperities is conducted. The distribution of the adhesion volume on the die corner radius is evaluated with a semi-empirical wear model in which is calculated with a function of normal pressure and relative velocity between blank and die. The results show the low adhesion volume in micro-scale, due to the short sliding distance during the process. The progressive adhesion behavior of work material and the advantage of tool life in micro-scale metal forming are demonstrated.
This paper describes about the joint strength and their improvement of a type 5052 aluminum alloy (A5052) autocompleting friction welded joint, that welding method was developed by authors. When the joint was made at a friction pressure of 60 MPa, it had approximately 75% joint efficiency at a groove bottom thickness of 1.1 mm with an overhanging length of the weld faying surface part (overhanging length) for the fixed workpieces of 15 mm. Those joints had the flexural deformation of the fixed workpiece during the welding, although those had the circumferential shear fracture by the increasing insert thickness with reliability. To reduce the flexural deformation of the fixed workpiece, the joint was made with an overhanging length for the fixed workpieces of 5.0 mm. The joint had 100% joint efficiency with the base metal fracture at a groove bottom thickness of 1.4 mm which was made at a friction pressure of 80 MPa, although that did not achieve 100% joint efficiency at a friction pressure of 60 MPa. In addition, the joint with a groove bottom thickness of 1.3 mm or thin did not achieve 100% joint efficiency, and that with 1.5 mm or thick did not have the circumferential shear fracture. To obtain the generating of the circumferential shear fracture with reliability during the friction process and 100% joint efficiency with the base metal fracture, the joint should be made with thick insert piece, opportune groove bottom thickness of the insert piece, short overhanging length for the fixed workpieces, and relatively high friction pressure.
A structural health monitoring system using multiple fiber Bragg grating sensors (FBG sensors) was developed. The system was designed to measure a large and a fast strain change and also to measure acoustic emissions ( AE ) simultaneously. The strain up to 1% and up to 100 kHz was considered. A multiple fiber ring laser was adopted as a light source, which consists of a multiple erbium-doped fiber amplifier (EDFA), an optical circulator, optical couplers and FBG sensors. Multiple fiber ring lasing wavelengths depended on strains loaded to FBG sensors. A CFRP beam bending test was carried out to confirm the possibility of simultaneous measurement of both strain and AE signals from a single FBG sensor. In the test, signals from a conventional electric resistive strain gage and a piezo-electric AE sensor were revealed equivalent to those from the FBG sensor. The system will be applied to development of composite structures in aerospace field such as Epsilon Launch Vehicle.
Micro-patterning of nerve cells is important technical issue to clarify their function and/or apply cell-based biosensors. Self-assembled particles are candidate for simply micro-structured scaffold to enable cells to adhere selectively. In this present study, using silica (SiO2) particles modified by specific protein, effects of particle-structure and protein-modification are investigated in a case of cultured rat pheochromocytoma (PC12) cells. SiO2 particles are gradually covered with specific protein, fibronectin (FN), by using electrostatic force with pH of FN solution kept at 4.0-8.5. It is also confirmed that FN coverage is proportional to reaction time. Dip-coating to pre-patterned glass plates, on which straight lines of hydrophobic molecules are arranged, allows the FN-coated SiO2 particles to be autonomously structured in a line-and-space pattern. The width and thickness of assembled particles are 35 µm and a few micrometers, respectively. It is also demonstrated FN is successfully supported to the surface of SiO2 particles throughout the structuring process. PC12 cells are incubated on the glass plate at 100% humidity and 310 K. The structured particles work as a scaffold for PC12 cells. Most cells preferentially adhere to FN-coated SiO2 particles. The adhered cells, which are located at the center of structure, are aligned at regular intervals. Meanwhile, in the case of flat glass region and structured bare particles, the number of adhere cells are remarkably small as compared to that of FN-coated SiO2 particles. It is found that an increase of FN coverage raises the number of adhered cells on the particles. These experimental results demonstrate that the structure of protein-coated particles have a function to induce self-alignment of cells. This indicates subsequent differentiation enables a desired network of neuron on the structure.
One of the authors developed a new micro piercing process by combining press indenting and chemical etching. By using conical and V-shaped indenters, circular and rectangular holes having tens of micrometers in size were successfully produced for 0.1mm thick copper alloy sheets, respectively. In this paper, multiple holes were pierced by this method and the influential factors on the accuracy of holes were investigated. The hole was distorted when the indenting pitch was below a specific threshold. The threshold pitch for rectangular holes was considerably larger than that for circular holes, which was explained by metal flow during indenting. As far as an adjacent hole was not distorted by subsequent indenting, dimensional accuracy of holes was affected by the tip angle of indenters, but little affected by the shape of holes.
In the present study, the enriched finite element method (enriched FEM) is used to analyze the intensity of singularity in 2D-dissimilar material joints. Using the enriched FEM, the intensity of singularities can be directly evaluated and very refined mesh around the singular point is not necessary. An eigenvalue and eigenvector analysis by FEM is used to calculate the order of singularity and the asymptotic displacement fields on the enriched elements. The singular stress fields in three-material joints described by 2-real singularities are analyzed. In order to investigate the influence of material properties and boundary conditions on the singular stress field, models with various material combinations and loading styles (tensile and shear loadings) are considered. The relationships between the order of singularity, the intensity of singularity and the maximum stresses are discussed. Finally, the circumferential stresses, σθθ, and individual singular stress terms for the order of stress singularities, λ1 and λ2, are plotted, and the relationship between each singular term and asymptotic stresses is discussed.
For the austenitic stainless steel of SUS316L and SUS304L, the braze pressure welding (BPW) was applied with variation in the total heating time 60s-660s, and with the cleaning temperature of 1280°C and 1180°C. To evaluate the corrosion resistance of the BPW joint interface, the reactivation rate, R, of sensitization was measured using the EPR test. For SUS316L, the reactivation value increased proportionally with the total heating time. For each total heating time, its reactivation value was comparable with that of SUS304L. For enough total heating time longer than 360s, R value stayed constant. The R value was suppressed with lowering cleaning temperature. The proportional increase in the reactivation rate indicates the sensitization of the austenitic stainless steel with the total heating time. SUS316L's R value was comparable with SUS304L. Therefore, the sensitization of the BPW joint might occur by other than the precipitation of the chromium carbides. The corrosion degradation of BPW joint could considered to be due to the delta ferrite formation at higher temperature than 1200°C followed by the decomposition into the sigma phase. The results of this research suggest the sensitization of BPW joint could be suppressed by shortening the total heating time.
The analytical solution of an infinite plate with a circular hole is introduced for investigating algorithm errors in identification of material parameters instead of performing actual experiments. The simulated speckle image pairs, consisting of undeformed images and deformed images, are created from analytical function. Then, a new formulation of digital image correlation (DIC) based on optical flow and finite element methods is developed to estimate heterogeneous displacement fields from simulated speckle images. The compliance coefficients of testing materials are iteratively computed by comparing finite element strains to DIC strains. The isotropic and orthotropic models of material parameters are investigated for accuracy of the purposed algorithms. The interaction between algorithm errors is studied. The sources of errors are discussed and progressive improvement is suggested for these identification techniques.
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.
We propose a novel evaluation method to obtain precise stress-strain curves by spherical-tip nanoindentation with continuous multiple loading technique. The small-sized spherical-tip indenters have an imperfect shape and it causes miss-fitting with tensile test result. We adopted the variable radius method, where we defined the precise indenter radius as the function of contact depth to calculate the stress-strain response precisely. We calculated the representative stress and representative strain by combining Hollomon’s law, Hill’s model, proper strain model, and the variable radius and compared with the tensile test result. It is found that stress-strain curves obtained by using the variable radius method agree well with the tensile test result rather than using nominal radius. These results clearly indicate the effectiveness of our method to evaluate the stress-strain response by using the small-sized spherical-tip indenter.
This study presents a simple interatomic bonding model for disordered A2 and B2 crystal structure. The model consists of an atom-like sphere and eight polar bars attached to its surface. Every bar has a polarity and can only attract a bar of the opposite polarity. Each bar is oriented from the surface of the sphere to a vertex of the cube, of which the sphere is placed in the center. A motif is defined as a combination of polarities on the eight bars, the total of which is 256 although a number of them are identical after rotation and/or inversion. Only 13 motifs classified by point groups are found to be unique by point symmetrical operations. In addition, an A2-like crystal structure is formed by connecting the bars between the models and filling the space. Four bonding patterns are found to configure the space of models with unique motifs according to symmetrical operations between the motifs of the center and neighbor atoms. Each crystal structure includes several kinds of point symmetry, and each shape memory binary alloy (SMBA) can be connected to the motifs on the atomic model via crystal structure and point symmetry. The results reveal the potential structure of the electron population distribution of each element in an SMBA that could be compared with ab initio calculations.
Abstract Waveform simulation techniques are widely used for nondestructive inspections using elastic waves. They are also used for the acoustic emission (AE) technique. Source dynamics can be identified by AE waveform analysis. However, waveforms detected by AE sensors are deformed by the sensor response, and their characteristics depend on the frequency response and sensitivity; thus, accurate AE waveform simulation is difficult because of these problems. In this study, accurate AE waveform simulation involving a sensor response, using a finite-difference time-domain (FDTD) method was developed. The original waveform is calculated by the 2D FDTD method. The sensor response is determined by extraction of the input waves to the AE sensor by a pulse generator and detection by a laser interferometer. To include the sensor width and dispersion of the sensor response, some points of the sensor response on the sensor face are detected. These are convoluted, and the sensor responses of all points are added. After Lamb wave simulation, characteristics of the simulation waveforms were correlated to the artificial AE waveform produced by Hsu–Nielsen sources (pencil lead breaking). AE signals from mode I fracture were simulated next. The fracture was excited by exfoliation between a polyester plate and glass bar embedded in the plate. The characteristics of simulation waveform by developed technique corresponded to the waveform detected by the AE sensor.
The changes in the microstructures associated with plastic deformation and strain/stress were measured using the strain scanning method with hard synchrotron radiation. Aluminum single crystals tensile-deformed along the <111> direction exhibit a macroscopically uniform multiple-slip without deformation bands. Plastic deformation has an insignificant effect on lattice spacing, and is likely responsible for the relatively uniform distribution of internal stress with a small absolute value. With regard to the crystallographic orientation, the rocking curves in the as-annealed sample differed with position, strongly suggesting that mosaic structures exist in a single crystal. In the 8% tensile-deformed sample, mosaic blocks are divided into smaller size. The increase in the number of mosaic blocks with deformation causes broadening the rocking curve and FWHM of the profile.
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.
For high-sensitive micro bio-analysis, fabrication of micro/nano structure by synthesis of vertically aligned carbon nanotubes (VACNTs) and self-organization, and improvement of surface activation of the CNTs by Ar plasma irradiation were performed in this study. The influence of structure dimensions and plasma irradiation on the bio-analysis was investigated. CNTs structures with various feature size and with various treatment conditions was fabricated, and protein adsorption characteristics on the CNTs structures were evaluated by fluorometry. The results revealed that the adsorbability of proteins depends on structure and surface treatment condition significantly. Finer structures with proper treatment tend to adsorb more proteins. The adsorbability for the CNTs reaction fields with finest structure and with best treatment condition was improved more than 30 times.
This work presented new developments in the constitutive modeling of shape memory alloys (SMAs). As an increasing number of experimental results are being published, it is becoming increasingly difficult to describe complicated SMA behaviors using conventional models. To overcome the shortcomings of existing models, this research proposed two major improvements to the assumed phase transformation function. A more flexible function called logistic sigmoid function has been introduced into the phase transformation function. This improvement affords a better fit to the typical SMA stress-strain relationship. Moreover, a cyclic effect has been considered while developing the new model. The new model is proposed by connecting accumulated strain with the critical phase transformation constants of SMAs. Both improvements were first validated at the material level. Thereafter, structural level validation and application were conducted. Accuracy enhancement may be expected by adopting these new models in SMA-related simulations.
In this paper, a combined model was proposed for reflection from a discontinuity and for a wave field of a fundamental shear horizontal guided wave in a plate structure. The reflection coefficient of a guided wave from a rectangular discontinuity can be modeled in the same way as the total reflection coefficient of a bulk wave. For the discontinuity having a slightly changed cross-sectional area, the reflection coefficient from the whole discontinuity can be calculated by spatial integration of the waves reflected from the divided regions. Some experiments and analyses were performed for 4mm-thick stainless steel plate specimens with EDM notches using a 300kHz frequency shear horizontal angle beam sensor, and for a cylindrical shell specimen (4.2m inner diameter, 9mm wall thickness) with discontinuities using a 40kHz frequency shear horizontal inter-digital sensor. The experimental reflection amplitudes were in good agreement with the analytical reflection amplitudes derived by the proposed model.
