In design of a structure in consideration of the fatigue life, it is very important to evaluate not only a fatigue life under the constant loading but also a fatigue life under the service loading. Generally, a fatigue life prediction by the rainflow method is used when a service loading condition is given as a wave form (we call this as time domain information). However, recently we sometimes experienced that the service loading condition is given only as the frequency domain information such as power spectrum density (PSD) of dynamic load. In this paper, firstly we present the investigated results of the fatigue life estimation by using service loading with frequency domain information. Then fatigue tests are also carried out by using the round bar specimens under several service loading conditions. Finaly, these fatigue lives estimated by using frequency domain information are compared with the estimated results by using time domain information. We can confirmed that These both estimated results coincided well with the experimental results, and this fatigue life estimation method using frequency domain information is effective in the standardized fatigue life evaluation of structures.
A Monte Carlo simulation model for the process of stress corrosion cracking (SCC) in structural metal materials under non-uniform stress condition has been proposed. The possible number of crack initiations is set for a given space and initiation times for all cracks are assigned by random numbers based on exponential distributions depending on stress level. Sites and lengths of the cracks are assigned by random numbers based on uniform distribution and normal distribution, respectively. Coalescence of cracks and subcritical crack growth are determined based on the fracture mechanics concept. Through the SCC process in the model, the influence of semi-elliptical surface cracks is taken into consideration. SCC simulations were carried out on a smooth surface under two kinds of non-uniform stress conditions such as residual stress distribution around a weld line. Multiple parallel cracks and multiple cracks along narrow high stress region were obtained depending on the stress distributions, respectively. Simulation results exhibit the applicability of the model to describe the SCC behavior observed in real structures.
Thermal barrier coating (TBC) was applied to Ni based superalloy specimens, where bond coat was NiCoCrAlY deposited by high velocity oxygen fuel (HVOF) spraying and top coat was 8mass% yittria stabilized zirconia (YSZ) by atmospheric plasma spraying. The creep tests of the specimens were carried out at 1273K, 1373K and 1423K by three-point bending until the delamination of coating. The results showed that the creep strain increased with increasing loading time and heating temperature, and the strain at delamination decreased both with increasing delamination time and decreasing temperature. An equation to evaluate the delamination life of TBC was introduced by combining the damage of interface due to heat cycle, holding time and creep deformation using the strain parallel to interface. The predicted delamination life coincided relatively well with experimental one.
Fiber/matrix interfacial crack propagation behavior was investigated by using a model composite specimen. The model composite specimen used in the present study consists of two fibers bonded by matrix resin, and an interfacial crack can propagate without resin fracture in this specimen. Model composite specimens with an initial crack were successfully fabricated from glass fibers of about 200∼400μm in diameter and vinylester resin. Crack propagation tests were carried out by the DCB method for mode I loading and the ELS method for mode II. The average value of interfacial fracture toughness obtained from model composites with silane-treated fibers was much higher than that with untreated fibers for both mode I and II propagation. Thus, the characteristics of fiber/matrix interfacial crack propagation are successfully evaluated by the present test method. The crack propagation tests were also conducted by using another model composite specimen with four parallel fibers distributed in a square array in the cross section. As expected, fracture toughness of four-fibers model composites in which the crack propagation is a combination of interfacial debonding and matrix fracture was higher than interfacial fracture toughness because of large energy dissipation by matrix fracture. Using the interfacial fracture toughness, Gi, the fracture toughness of four-fibers model composites, Gc, and the fracture surface ratio of fiber/matrix debonding in four-fibers model composites, γi, the fracture toughness of matrix resin, Gm, was estimated on the basis of the rule of mixture, Gc = Giγi + Gm(1 + γi).
The mechanical properties of rubber that is the main member of a seismic isolation rubber bearing is evaluated for the cyclic tension, shear and torsion loading. The dimension of a specimen is a small size disk that imitates the bearing. The size and distribution of which the voids are nucleated inside rubbers applying tensile load depend on the rubber thickness. The preserving maximum shear stress rate (PMSSR) is defined as the shear stress at reaching the applying strain, 2 or 4 on each cycle divided by the shear stress on first cycle. The PMSSR on applying strain = 4 reduces with the increase of cycle numbers and more than on strain = 2. The tendency of reducing PMSSR depends on the thickness of rubber and the reduction attains about 20% at 30 cycles with more thickness than three millimeter and the reduction is enhanced by the voids inside rubber made by pre-tension. These results indicate that the assessment of durability will be needed for a post-seismic bearing.
Deformation mechanism of a shape memory alloy is investigated by molecular dynamics simulations using several multi-grain models. An EAM potential for Ni-Al alloy is applied, and a set of conditions with loading, unloading, heating and cooling is imposed. As a result, the stress-strain relation obtained by a multi-grain model revealed to draw a loop consisting of smooth curves which resembles experimentally obtained one, while previously-simulated single-crystal model produced a zigzag diagram. From the observation of the configuration of atoms and local structure during the deformation, it revealed that local deformation of martensite variants is initiated at grain boundaries, and also that the propagation of the deformed region is obstructed by the boundaries. The influence of the grain shape and distribution, as well as the crystal orientation of each grain, on the deformation behavior is then discussed : Qualitatively common features in deformation mechanism and stress-strain relation are observed despite different grain distributions, while quantitative values such as critical stresses are dependent on the crystal orientations of each grain and of the neighbors.
A new fracture criterion of the shear failure for the geomaterials is presented which enables to predict a straight extension of a crack in the elastic plastic materials under the compressive loads. It is an application of the Coulomb fracture criterion to a crack extension criterion called “The maximum frictional shear stress criterion”. We examine the criterion by using both the singular and the constant terms in the asymptotic expansion of the crack tip stress fields for a linear hardening Drucker-Prager elastic plastic material. As a result, we find that the confining pressure, the frictional coefficient of the crack surface, the small hardening parameter, and the dilatancy effect is the lowering of the extensive hoop stress causing the crack kink, which contribute the frictional shear stress extending a crack straight to maximum.
Recently the relation between the fatigue crack propagation and slip behavior around the crack tip has been researched by means of an Atomic Force Microscope (AFM). In the previous paper, we explained Mode I fatigue crack propagation using a stochastic damage accumulation model for crack propagation based the observation results by AFM. In this paper, the inclined fatigue crack growth process from the notch is analyzed by the use of the method introduced in above model. As the result, the following were found. (1) The inclined crack grows in mixed mode of Mode I and II by continued destruction owing to the equal release rate g. (2) When the crack changes its direction, it continues to propagate in mode I, because ΔKeff in mode I obtained from above g is beyond ΔKeff th.
The dynamic viscoelastic behavior during isothermal crystallization of poly(butylene terephthalate) (PBT) has been investigated as a function of crystallization temperature (Tc). The enthalpy of crystal formation during the isothermal crystallization is also examined by differential scanning calorimetry (DSC). The equilibrium degree of crystallization in the long time limit is about 30%, almost independently of Tc. Correspondingly, the viscoelastic parameters at the equilibrium of crystallization are also independent of Tc. The crystallization rate decreases with increasing Tc, and the Tc dependence is well approximated by the Arrhenius type equation. The effect of crystallization on the viscoelasticity is considerably small in the initial stage of crystallization whereas it becomes pronounced in the later stage.
Remediation method using zero-valent granular iron is one of the most popular in-situ remediation technologies, and is designed to eliminate contaminant migration beyond the boundaries of the contaminated site by constructing a Permeable Reactive Barrier (PRB) using an iron sand mixture. In contrast, direct in-situ mixing of zero-valent iron with the actual contaminated soil using a higher mixing ratio has been proposed to effectively clean up in a short period of time. For this direct mixing method a proper design methodology should be devised. To propose this design methodology, we conducted a batch test, a column test, a three-dimension advection-dispersion-attenuation analysis, and a field test. The results were as follows. The design based on a batch test was confirmed that it coincided with a column test and the clean-up result in a field test. The safety factor of a granular iron mixture ratio was evaluated from the numerical analysis, in which the heterogeneity of granular iron mixture was taken into consideration. Example of evaluating the heterogeneity of granular iron mixture was shown in the field test.
Single-phase (β-SiAlON (Si4Al2O2N6, z = 2) powder has been prepared by the combustion synthesis method. The raw materials (Si, Al and SiO2) were combusted with different ratios of (β-SiAlON (z = 1) diluent (0, 10, 20, 30, 40, 50wt%) in 1MPa of N2 pressure. Without diluent, the reaction temperature was very high (> 2000°C) and the product contained metal Si residue besides (β-SiAlON product. Both the reaction temperature and amount of residual Si decreased gradually with the increase of diluent content up to 50wt%. At 50wt% dilution, the combustion temperature was 1859°C and the XRD analysis showed complete conversion to pure (β-SiAlON (z = 2). The individual peaks of (β-SiAlON (z = 1) diluent were also detected in the XRD patterns and disappeared after using the product itself as a diluent five time repeats of combustion at which single-phase (β-SiAlON (z = 2) was produced. The sinterability of the best synthesized powder was further studied by spark plasma sintering (SPS) with 5wt% Y2O3 as a sintering aid. High densification (∼97.3%) was obtained after sintering at 1600°C for 5 min under 30MPa pressure with heating and cooling rates of 100°C/min. The Vickers hardness and fracture toughness of the sintered SiAlON were 14.8GPa and 4.4MPam1/2, respectively.
The tensile mechanical properties of PP/SEBS blend syntactic foams with the relative densities from 0.5 to 1.0 are characterized at the nominal strain rates of 1, 10 and 100 s–1. Moreover, the effects of the mechanical properties of the matrix materials on the macroscopic mechanical properties are investigated by comparing the experimental results of two types of the PP/SEBS syntactic foams. The apparent elastic moduli of the syntactic foams are similar without depending on the matrix's elastic moduli once the small content of the microballoons is blended. Their material ductility decreases drastically once the microballoons are blended in the matrix materials regardless of the type of the matrix. The strain rate dependency of the elastic modulus in the syntactic foams is dependent on that of the matrix material. In addition, the drastic increase of the elastic moduli in the syntactic foams is delayed by adding the polymer microballoons.