The maximum load condition and Hart's instability criterion for the growth of area imperfection have been analytically studied in uniaxial tensile tests under various impact loadings. Three kind of typical loading pulses (a conventional constant incident pulse, diminishing incident (deceleration) pulse and ramp loading (acceleration) pulse) were selected for the study. It has been clarified by the analysis that a strain amount, where load reaches a maximum value, becomes larger with an increase in acceleration rate of the loading pulse, and therefore the maximum strength becomes higher with increasing acceleration rate. Hart's criterion is reformulated so as to be directly applied to investigate an instability point on stress-strain curves obtained by the tests. The correlation between the maximum load point and Hart's instability point have been clarified under impact loading conditions.
Compressive strength of β-Titanium alloy (Ti-15V-3Cr-3Sn-3Al) aged at 723K for 6 hours, as (α+β) two-phase alloy, was clarified in a wide range of temperature from 77K to 673K and at strain rates from 10-4 to 103S-1. Thermally activated strain rate analysis (TASRA) was applied to characterize the deformation mechanism. Adiabatic heating and strain rate change effect were taken into account to predict the stress-strain relation at high strain rate, and the experimental results were well convincing. The strain rate sensitivity of stress under constant mobile dislocation density predicts the temperature and strain rate dependences of flow stress in the high strain rate range, while the deformation behaviour in the relatively low strain rate range is understood with due regard to the change of mobile dislocation density. It is found that a single thermally activated process corresponding to α or β phase predominates the deformation behaviour at low and high temperatures, respectively, and may characterize the two-phase alloy as a whole.
A model to predict the process of embrittlement, deformation and fracture of reactor pressure vessel steels subject to neutron irradiation is proposed. In order to describe the fracture process under unirradiated condition, the notion of effective stress and the hypothesis of strain equivalence of damage mechanics are first incorporated into a viscoplastic constitutive equation with isotropic hardening. The damage state of the material is represented by a scalar damage variable D, and the evolution equation of D is modeled by postulating that the damage rate D is specified as a function of plastic strain εp and its rate εp. The material is assumed to fracture when D attains to its critical value Dc. The effect of neutron irradiation is represented by expressing the material constants of the constitutive and evolution equations as functions of neutron fluence Φ. The resulting constitutive equations of elastic-plastic damage under neutron irradiation are applied to describe the irradiation embrittlement of a low alloy steel ASTM A533B cl. 1 for the pressure vessel use of light-water reactors. The increase of yield stress and tensile stress as well as the decrease of fracture strain and fracture energy due to neutron irradiation can be described by the proposed model.
There are very few reports concerning X-ray residual stress measurements near the fatigue crack for Ti-6Al-4V alloys. The residual stress near the crack was measured by an X-ray method for the annealed materials of Ti-6Al-4V alloys and their aging treatment materials, in which the X-ray residual stress was confirmed to exist by comparing with loading stresses under tension. The compressive residual stress, measured in the wake of the propagating fatigue crack, was larger in the aging treatment material. In addition, the crack opening displacement was measured along the real fatigue crack, which was different from the theoretical one. This made it possible to evaluate the effective stress intensity factors Keff depending on the residual stress near the fatigue crack. The fatigue crack propagation rate under the same Keff-value was smaller in the annealed than in the aging treatment material. This was probably related not only to the zig-zag crack path in the annealed material, but also to the heavier plastic deformation accompanying the large compressive residual stress in the aging treatment material.
It is well known that the crack opening/closure phenomenon found by Elber plays an important role in the fatigue crack propagation behavior under any kind of loading condition. On the other hand, the retensile plastic zone's generated load (RPG Load) was newly proposed by Toyosada as for a fatigue crack propagation parameter in place of the crack opening/closure load. However, the accuracy of usual measurements seems to be not sufficient to discuss the fatigue crack opening/closure phenomenon quantitatively. In this paper, for the purpose of proper understanding of the fatigue crack opening/closure phenomenon, a detailed observation of change in strain near the fatigue crack tip was carried out. And successful measurement of the hysteresis curve of load vs. strain (ε') with high accuracy was made. By introducing the curve of compliance (dε'/dp) and the change rate of compliance (d2ε'/dp2) during one cycle, the following conclusions have been obtained. 1) The unloading part of the hysteresis curve is found to be “S” shape, and the point of inflection of the unloading part is considered to be the crack closure point as Nisitani pointed out. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of compliance, the crack closure point can be measured automatically as the maximum point of the unloading part of this curve. 2) The loading part of the curve of compliance is found to be “S” shape, and the point of inflection of the loading part is considered to be the RPG point. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of the change rate of compliance, the RPG point can be measured automatically as the minimum point of the loading part of this curve. 3) Even though the accuracy of measurement is high, it seems to be difficult to measure automatically the crack opening point, because it shows no distinguished feature.
The effect of the presence of liquid phase on cavitation and fracture during superplastic flow for high strain-rate superplastic Si3N4P/2124 Al composite has been investigated. The conclusions obtained are as follows: (1) At 723K, which is 60K lower than the partial melting point measured by DSC, a small elongation to fracture of 24% was obtained. This result is due to the development of microcracks at the interface between matrix and reinforcement which would be generated by high stress concentration at the interface. (2) At 783K, which is very close to the partial melting point, a large elongation to fracture of 550% was obtained. The large elongation resulted from the disappearance of microcracks. It is supposed that the nucleation of cracks is suppressed by low stress concentration at the interface due to the presence of liquid phase. (3) At 843K, which is 60K higher than the partial melting point, a small elongation to fracture of 18% was obtained. The small elongation could be explained by the excessive development of cavities due to too much liquid phase at the interface and grain boundaries. (4) It is suggested that the presence of adequate amount of liquid phase gives rise to the effective accommodation required for grain boundary sliding for the composite.
A method for measuring the effective crack tip fracture toughness, (Ktip)c, for bridging materials such as polycrystalline ceramics or fiber-reinforced ceramics is proposed in this paper. The value of (Ktip)c approximately coincides with the apparent fracture toughness, Kc, when using a bending specimen with an adequately deep crack. The stress intensity factor for the bending specimen with a deep crack increases very rapidly with crack extension. Therefore, the bending specimen with a deep crack fractures without stable crack propagation which causes the R-curve behavior and the deference between Kc and (Ktip)c. The volue of (Ktip)c for polycrystalline alumina is evaluated according to the present proposed method. The measured (Ktip)c is almost identical with that determined by numerical analysis.
High velocity impact tests are conducted by launching spherical stainless steel and PMMA projectiles into thin PMMA plate targets at various velocities. Ballistic phase diagrams are obtained which indicate the fracture behavior in targets and projectiles. Quasi-static perforation tests are also performed. The fracture surfaces in targets are examined microscopically.
Aramid fiber reinforced plastics (ArFRP) is being applied in severe service conditions, such as aeronautical or space environments. Therefore, it is necessary to elucidate its strength in various environments. In the present study, static tensile and fatigue tests of dry and wet specimens of ArFRP were carried out at room temperature and at cryogenic temperature, 77K. Two types of aramid fibers, Dupont's Kevlar49® and Teijin's Technora® were used for the reinforcement. The fractured specimens were inspected under a Scanning Electron Microscope (SEM) after the tests and the fracture mechanisms were analyzed. The tensile strength of aramid fibers was also measured at the same temperatures. The effects of cryogenic temperature and water absorption on the strength of ArFRP and aramid fibers were made clear.
Optimization of interface by changing the chemical properties of fiber surface is essential for the improvement of composite properties. Four kinds of carbon fibers with different surface treatments were produced specially at an industrial manufacturing plant in order to study the relation between the interfacial properties and mechanical properties of carbon fiber reinforced composites. In this report, the influence of the surface oxygen content of carbon fiber on the interfacial adhesion strength was examined by using the single fiber fragmentation technique. The interfacial shear strength (τ) and the interfacial transmissibility (κ) of four carbon fibers were investigated on the single filament embedded test specimens. Both τ and κ increased with increasing surface oxygen content, but τ showed different tendency in the case of sized and non-sized fibers. The distribution of the fractured fiber length was wide when the surface oxygen content was low, but it became narrow with increasing oxygen content. These results suggest the possibility that the adhesion strength at the interface is not uniform, and the unevenness is increasing with decreasing surface oxygen content.
A new boundary element method for analyzing the galvanic corrosion of a long underground structure such as a pipeline and an oil well casing was developed. A long underground structure is often buried in soil with non-uniform electric conductivity. In case where the conductivity change is distinct, the conventional multi-region method is useful, while it gives an unrealistic solution with discontinuous jumps for gradual change of conductivity. To overcome the difficulty, the fundamental solution for a field with linear change of conductivity was introduced by using the Fourier transform technique, and the polarization characteristics were formulated to express its continuous change. A few example problems were solved with this method to demonstrate its applicability and usefulness.
A method for improving accuracy of measuring polarization characteristics is proposed. In this method, a boundary element inverse analysis is used to eliminate the error due to the non-uniform distribution of current density on a specimen. In the inverse analysis an appropriate objective function, which is to be minimized, is proposed. A measurement of polarization characteristics of a steel on soil is made and the true characteristics is obtained by using this proposed method.
In the analysis of the scanning vibrating electrode technique (SVET), the electrolyte solution near the sample has been assumed to have uniform conductance for calculating the current density. In this study, however, it is found that the non-uniformity due to electrolysis can not be neglected in a dilute solution. To describe the ion concentration and electric field change in the vicinity of sample surface, the Nernst-Planck equation and electro-neutral condition are employed. This model is verified with the experiment. A time dependent axi-symmetric problem is solved, and it is found that the applied electric voltage becomes 10 times smaller than the conventional solution to keep constant current. A comparison of the electric field change between this numerical analysis and the experimental result observed by SVET is made.
Dynamic viscoelasticity during isothermal crystallization for polybutene-1 was investigated at temperatures between 90.9 and 97.3°C. Crystallization increased storage and loss moduli intensively. The crystallinity defined by dynamic measurements (xG) as (Gt-G0)/(G∞-G0) was discussed. At the same crystallizing temperature (Tc), the xG curves determined from G' coincided with those from G". The xG curves did not depend on the measured frequencies. The xG curves at different Tc could be superposed by horizontal shifts. The shift factors were the same as those for the data obtained by DSC. By comparing the data obtained by the viscoelastic measurements, it was found that the crystallinity from dynamic measurements increases more rapidly in short time.
Vinyl wallpapers were tested to clarify the discoloration mechanism. The samples were investigated by means of electromicroscopy, X-ray fluorescence and infrared spectroscopy. It was assumed that the cause of discoloration was the migrations of water and cobalt ions existing in putty from undercoat to wallpaper. Three methods of prevention of the discoloration were proposed: (1) application of wallpaper on dried undercoat, (2) avoidance of material which includes water-soluble colored components and (3) application of waterproof wallpaper.
In this paper, a method to evaluate the crack size in concrete structures by infrared thermography was investigated. This method is based on the relation between the crack depth or width and thermal distribution at the surface of concrete. In the beginning, the influence of crack depth and width to thermal distribution was examined. Next, the influence of the distance between cracks and heater to thermal distribution was investigated. Moreover, the efficacy of this method was confirmed by the experimental result of flexural cracks in a reinforced concrete beam. From these results, it was clarified that the thermal distribution measured at the surface of concrete by infrared thermography was effective to evaluate the crack size in concrete structures.