The present study investigated the relation between the interatomic potentials and thermal properties. Nineteen potential functions for tantalum, which was a high melting point material, were proposed based on the concept of FS potential with some modification on the functional forms, and their melting points and thermal expansion characteristics were investigated by using molecular dynamics simulations. The melting point was determined by starting NPT ensemble simulations with the initial configurations where the solid and liquid phases coexisted in a basic cell. The thermal expansion characteristic could be estimated without MD simulations by calculating the change in potential energy with isotropic expansions and compressions. The melting point was influenced by amplitude of thermal vibrations of atoms, and was decreased with decreasing of the modified elastic moduli. An empirical potential function for tantalum was derived, which satisfied some properties near 0 K, the lattice constant at 2500 K and the melting point.
A method that gives the parameters of advanced Tersoff interatomic potentials for describing nonequilibrium atomic structures has been developed. This method uses a genetic algorithm to optimize the Tersoff potential parameters fitted to first-principles-calculated cohesive energies of various carbon systems, including bulk systems with atomic defects and amorphous, surface, or cluster systems under stress. These optimized parameters converge towards a set of Tersoff potential parameters that well describes not only crystals but also amorphous systems.
A material system of Al sputtered on crystalline Si was dealt with as one of the simple material models for semiconductor material systems. To investigate qualitative properties of sputtered films on an atomic scale, simulations were conducted by a molecular dynamics (MD) method using two film models; i.e. a deposition model based on MD simulations of sputtering process and a crystal model using a crystalline Al film instead of a deposited one. The surface roughness and porosity, which are defined in this work, were found to decrease with an increase in the incident energy of atoms. Relationships between tensile deformation properties and porosities in simulated thin films were also investigated. Although the porosity was found to affect the tensile strength in the direction parallel to the substrate surface, it was revealed that the tensile strength in the direction perpendicular to the substrate surface was hardly influenced by the difference in the porosity.
This paper is concerned with the theoretical treatment of transient thermal stress problem involving an angle-ply laminated strip consisting of an oblique pile of layers having orthotropic material properties due to nonuniform heat supply in the width direction. We obtain the exact solution for the two-dimensional temperature change in a transient state, and thermal stresses of a simple supported strip under the state of generalized plane deformation. As an example, numerical calculations are carried out for a 2-layered angle-ply laminated strip, and the numerical results for the temperature change, the displacement and the stress distributions are shown in figures. Furthermore, the influence of heat function on temperature change and thermal stress distributions are investigated.
A method is proposed for calculating the large deflection of beams on an elastic foundation using the boundary integral equation method. The elastic foundation is assumed to be of the Pasternak type. In the analysis, the governing equation is transformed into a more convenient form such that the iterative scheme can easily be applied to large deflection problems. A fundamental solution required in the formulation is derived by solving the singular differential equation. It is found that the numerical results obtained by the present method agree well with the analytical solutions. To show the usefulness of the method, several numerical results based on the formulation are given for various types of boundary conditions.
Prediction methods of macroscopic and local creep behavior of perforated plates are examined in order to apply these methods to the structural design of perforated structures such as heat exchangers used in elevated temperatures. Both primary and secondary creep are considered for predicting macroscopic and local creep behavior of perforated plates which are made of actual structural materials. Both uniaxial and multiaxial loading of perforated plates are taken into consideration. The concept of effective stress is applied to the prediction of macroscopic creep behavior of perforated plates, and the predicted results are compared with the numerical results by FEM for the unit section of perforated plates under creep, in order to confirm the propriety of the proposed method. Based on the idea that stress exponents in creep equations govern the stress distribution of perforated plates, a modified Neuber’s rule is used for predicting local stress and strain concentrations. The propriety of this prediction method is shown through a comparison of the prediction with both the numerical results by FEM for the unit section of perforated plates under creep, and experimental results by the Moire method.
In this paper, we propose a new method to produce the functional continuum. It is a topological optimization technique based on a function of the continuum and composed of three steps. In the first step, an initial mechanism model, which conforms the given function is determined, using only some levers and supports. The mechanism model is changed to a framed structure as the Rahmen model in the second step. The cross section and length of members in the framed structure are optimized to maintain the original design requirement. In the final step, the framed structure is changed to the functional continuum using some well known shape optimization techniques. Two examples of application of this technique for the functional continuum are presented.
Plate impact experiments with PVDF (PolyVinylidene DiFluoride) stress gauges to determine the dynamic mechanical properties of the ceramics Si3N4, SiC and TiB2 were conducted using a shock gun system. From the experiments, we obtained the Hugoniot curves and the Hugoniot elastic limits of the ceramics.
Complete pole figure measurement using only a back-reflection method with an imaging plate (IP) was attempted. In our method, complete pole figures were obtained using only the back-reflection method without the use of a transmission method. In particular, the complete pole figure theoretically is obtained when its diffraction angle 2θ is 90°. In the cases of other 2θ values, the complete pole figure is obtained by measuring two incomplete pole figures by the back-reflection method and combining them. Such incomplete pole figures could be simultaneously measured by means of the two-dimensional detection ability of IPs. In addition, the correction of X-ray diffraction intensity to compensate the deviation from the pseudofocusing condition was not needed in our method, unlike Schulz’s back-reflection method. Pole density data obtained from our complete pole figures were effectively applied to three-dimensional analysis of the texture of a dual-phase stainless steel sheet. Crystallite orientation distribution functions (ODFs) of each phase were obtained.
Recently, acoustic emission during hydroproof (AE/H) testing has been adopted for the quality assurance of filament-wound composite rocket motor (FCRM) cases. For the proper performance of this testing, it is crucial to understand the characteristics of elastic wave propagation in the FCRM case, since AE signals captured in this testing can very significantly according to various factors including its geometry and material properties. Furthermore, the optimization of the AE/H testing parameters is strongly desired based on this understanding for the suitable interpretation of the test results. To address such a need, we have conducted an experimental study on the analysis of the elastic wave propagation in the FCRM case. In the experiments, broadband ultrasonic waves radiated at a fixed point were received at many different locations after propagating through the FCRM case with different distances and/or directions. From the received signals, the characteristics of elastic wave propagation such as frequency components, the maximum propagation distances and velocity curves were investigated in two separate conditions: one with the case empty and the other with the case hydraulically pressurized. Based on this systematic investigation, the optimal frequency component of the elastic wave to be monitored in the AE/H testing of the FCRM case was determined successfully.
The purpose of this study was to investigate the effect of cyclic loading, which produced the condition of ACLs during sports activities, on tensile properties of femur-ACL-tibia complexes (FATCs). Paired FATCs of 40 New Zealand white rabbits were tested on a materials testing machine. One specimen of each pair was designated as a control and loaded until failure. The contralateral specimen was loaded cyclically (1.4 Hz, 1 hr.) with 20%, 30%, 40%, or 50% of ultimate tensile strength (UTS) of the control and then loaded until failure. The UTS and mode of failure were recorded after each test. Five specimens ruptured during cyclic loading in the 50% group. In the 40% group, the mean value of UTS of cycled specimens was significantly lower than that of controls. There was no statistically significant difference in UTS values between control and cycled specimens in the 20% and 30% groups. Cycled specimens had a significantly higher incidence of substance failure than controls. Our results demonstrated that FATCs have the strength to withstand cyclic loading within normal sports activity levels. However, FACTs can be damaged by cyclic loading under strenuous sports activity levels. We speculate that cyclic loading makes the ACL substance weaker than the insertion site.
This paper deals with a theory of ply-cracking damage on 90° and 0° plies in cross-ply laminate and its application to finite element method. An energy approach is extended to predict the initiation and evolution of ply-crcaking damage on 90° and 0° plies and the corresponding nonlinear stress-strain behavior of the cross-ply laminate under multiaxial in-plane loading. In this appraoch, the stress and strain condition for the progressive damage are determined by equating the decrease in potential energy to the released energy, where the former and latter are estimated from the stiffness reduction due to ply-cracking damage and from the mixed-mode critical energy release rate for cracking of unidirectional ply, respectively. This approach provides us with the constitutive relation of cross-ply laminates including the progressive ply-cracking damage. This theory is applied to the finite element method in order to analyze the ply-cracking damage and stress/strain distributions of the structures made of cross-ply laminates. As an example, finite element analysis is carried out on the seven kinds of CFRP cross-ply laminates with a circular hole. The numerical results show that the transverse cracking damage in 90° plies is widely spread out on the ligament of the plate, and the splitting damage in 0° plies extends in the longitudinal direction from the edge of the hole. The details of the damage evolution around a circular hole in CFRP cross-ply laminate depend on the stacking lay-up.
Integral equations are derived for a planar crack of arbitrary shape, subjected to both normal and shear tractions as well as fluid pressure, in an infinite, fluid-saturated, poroelastic, isotropic solid. The obtained equations relate the surface tractions and fluid pressure on crack surfaces to relative crack-surface displacements and leak-off flux into the porous medium. It is intended to implement the equations in a 3D simulator of hydraulic fracturing in oil reservoirs. Some discussions are given on reduction of the present solutions to the known ones as special cases and to pure shear mode solutions; it is found that the poroelastic effect does not disappear even for the pure shear mode.
Rotating bending fatigue tests were carried out on an 18%Ni maraging steel to investigate the effect of shot peening on fatigue strength. Fatigue strength was increased markedly by shot peening. The increase in fatigue strength due to shot peening depended on shot size and stress level. Fracture mode changed from surface fracture in the short-life region to interior fracture, i.e., fish-eye fracture, in the long-life region. In the middle-life region, however, cracks initiated from both the surface and the inside of the material individually coalesced and led to the final fracture. These results were discussed on the basis of the effects of shot size on surface roughness, the depth and magnitude of compressive residual stress and work hardening.
Crack tip opening displacement (CTOD, δ) tests were carried out for line pipe steels in buffer solutions, sand, and clay to evaluate initiation of hydrogen stress cracking (HSC) at surface defects in buried pipelines under cathodic protection. Four series of line pipe steels and two series of seam welds showed a similar tendency in cathodic current density (i) versus the critical CTOD (δc) curves, irrespective of types, pH and water content of the soils; δc showed a minimum (δHSC) when i>ith (ith=1mA/cm2) in all the testing conditions. δHSC increased with the increasing fracture toughness of the steel. Fluctuation of cathodic current density influenced δc when the maximum value of cathodic current density (imax) was larger than ith. HSC could be initiated at surface defects in pipelines only when imax>ith and δ≥δHSC.