The X-ray fractography was applied to the full thickness, ESSO specimens made of a 250mm thick steel for nuclear reactor presssure vessels SA533B Cl. 1 to study the phenomena of brittle crack propagation and arrest. The stress intensity factor KIdyn obtained through the half value breadth of the X-ray diffraction intensity curve from the fracture surface increased strongly as the crack propagated except in the very early stage in which KIdyn decreased significantly. It had an upper limit around the half of the crack propagation length before the first arrest, decreased thereafter and then gradually increased with oscillations until the crack was finally arrested. While the crack propagated at a nearly constant velocity, KIdyn increased monotonically. When the crack velocity decreased sharply, the increase in KIdyn discontinued and oscillations started. The relation between the stress intensity factor at crack arrest KIdyn (Arrest) and temperature did not depend upon the testing method. The stress intensity factor at crack arrest Kca which was obtained by the static analysis of the ESSO test tended to underestimate the crack arrest toughness.
The distributions of residual stress in the vicinities of fatigue crack tips and behind them were measured in various conditions of crack length, stress amplitude and stress ratio by the counter method for X-ray microbeam stress measurement. On the basis of the distributions of residual stress, the crack closure behavior was examined analytically by the finite element method and the correlation between the crack growth and the distribution of residual stress was discussed. It was found that the distribution of residual stress in the vicinity of the crack tip was affected by the crack length and the loading conditions. The plastic zone size ahead of the crack tip was related to the maximum applied stress and the crack length, and it was evaluated from the maximum stress intensity factor Kmax. The crack growth rate was expressed in terms of the effective stress intensity range ΔKeff that was calculated by the finite element method based on the distribution of residual stress. It is considered that the crack growth behavior is closely related to the distribution of residual stress, and the crack growth rates can be predicted from the effective stress intensity range ΔKeff calculated by taking account of the distributions.
The residual stress left on fracture surface is a useful information to analyse local plastic deformation near the crack tip. In the present study, X-ray residual stress measurements were conducted on the fatigue fracture surface of a low-alloy high strength steel (JIS SNCM 8) and a structural-low-carbon steel (JIS SM 41 B), and the results were analysed based on fracture mechanics. The followings are the main results obtained. (1) The residual stress on the fatigue fracture surface was tension. The residual tensile stress increased with the maximum stress intensity factor Kmax in the case of the material tempered at 200°C, While it has a maximum value at about Kmax=30MPa√m in the case of the material tempered at 600°C. On the fracture surface of mild steel, the residual stress tended to decrease with increasing Kmax. (2) The distribution of the residual stress beneath the fatigue fracture surface of the high strength steel was able to be decomposed into three parts: the distribution caused by monotonic plastic deformation near the crack tip, that relieved by surface roughness, and that resulting from the compressive deformation due to crack closure. (3) The maximum depth of the plastic zone ω determined from the residual stress distribution was correlated to Kmax and the yield stress σY as follows. ω=0.21(Kmax/σY)2 This equation holded for the cases of high strength steels tempered at 200 and 600°C. On the other hand, in the mild steel, the width of X-ray diffraction profiles was suggested to be a more appropriate parameter for plastic zone determination than the residual stress.
The technique analysing the cause and the mechanism of fracture from the information obtained by X-ray irradiation on the fractured surface is called X-ray fractography. As a basic study of X-ray fractography, the relation between the residual stress along the crack propagation direction on the fatigue fractured surface and the range of stress intensity factor (ΔK) was investigated. The crack closure was also measured and was correlated with the residual stress. The material used was Boron steel 10 B 35 tempered at 600°C after quenching. The specimens were 12mm thick 1 CT by ASTM standard. The fatigue tests were carried out under the constant range of stress intensity factor. The constant load fatigue test was also carried out as a reference. The main results obtained from the present experiments were as follows. (1) In the constant ΔK fatigue tests, the crack length was linearly proportional to the number of load cycles, and the crack propagation rate and the crack opening ratio were constant. The crack propagation rate was about the same as that at the equal ΔK value under the constant load. (2) In the case of crack opening ratio U=1.0, the residual stress on the fractured surface by the constant ΔK test was almost constant and independent of the crack length. (3) In the case of crack opening ratio U=0.6, however, the residual stress slightly increased with increasing crack length.
The ψ-splitting behaviors were investigated for the ground and the milled surface layers of both iron and high speed steel in order to find out the relation among microscopic residual shear stresses. For the high speed steel, the X-ray elastic constants and the residual strains were measured on the carbide phase as well as on the matrix phase. It was clarified that the ψ-splitting was caused by a combination of the selective nature of X-ray diffractions and the microscopic residual shear stresses within the interior of cells and the carbide particles. The volume fraction occupied by the cell walls and the residual shear stresses sustained by them were estimated from the equilibrium condition of the microscopic residual shear stresses. The distributions of residual stresses over the deformed layers indicate that the thermal effect is dominant in grinding and the mechanical effect is dominant in milling for forming residual stresses.
It is well known that the X-ray stress measuring data, 2θ vs. sin2Ψ curves, of the surface layer of the uni-directionally machined metallic materials are often curved and splitted. This phenomenon makes it difficult to estimate the stress state by means of the 2θ vs. sin2Ψ method. In order to solve this problem, Dolle and Cohen proposed a new method which provides more detailed information on the rather complicated stress states. This study was carried out to make clear the effect of experimental conditions on the accuracy of stress measurement of ground and cut layers of steel using the Dolle-Cohen's method, and confirmed that this method is an effective one for this purpose.
The acoustoelasticity method which makes use of the birefringent effect of polarized shear wave is a promising technique not only for stress analysis but also for measuring texture-induced acoustic anisotropy in metallic materials. In this study the texture-induced acoustic anisotropy originated by plastic deformation under compression was measured by the acoustoelasticity method in order to investigate the effects of strain-rate and temperature during the tests. Alminum and copper polycrystalline materials were used as specimens and static compression tests under room temperature to -170°C and dynamic compression tests using a split Hopkinson pressure bar were done. After the compression tests, the texture-induced acoustic anisotropy was measured by a sing-around method. The variation in texture-induced acoustic anisotropy appeared to be evidently influenced by both strain-rate and temperature for the case of copper, but it did not appear definitely for aluminum.
The random heat conduction and its associated thermal stress problems were discussed in this report. Several problems were solved by using the theories of heat conduction and elasticity in flat plates, solid spheres and solid cylinders in which the surrounding temperature Ta was varied as a stochastic function of time. For the case where the surrounding temperature Ta followed a weakly stationary process, the autocorrelation function, the mean square and the power spectral density were derived. As an example, the problem in which the surrounding random temperature is regarded as a white noise process was solved. The form of the autocorrelation function corresponding to white noise was RTa(τ)=Tsδ(τ) where δ is Dirac's delta function and Ts is a constant. The numerical computation was carried out for the mean square of the temperature and thermal stress distribution. The numerical results showed that the large variations of temperature and thermal stress were confined in the thin surface layer and very small or almost zero in the rest. The thickness of the layer was 14 to 15% of the plate thickness or the outer radius of the sphere and cylinder.
Pure copper wires with grain size of 7.5, 23.5, and 123.0μm were cyclically strained in torsion with constant strain amplitudes of 0.025 and 0.05 at 77K. The effect of grain size on the behavior of lattice defects was studied by measuring the change in resistivity during fatigue as a function of number of cycles at 77K. The dislocation density in a fatigued specimem is given by ρ1/2=α'/β'(1-e-(β'/γ)2) β'=β+K3/Δγ, β=a+b/d where ρ is the dislocation density, α'and K3 are constants depending on the grain size of the specimen, γ is the cumulative plastic strain, Δγ is the plastic strain amplitude, d is the grain size, and a and b are constants. The increase in dislocation density was much smaller in the coarser grain size specimen during cyclic straining, and, therefore, a smaller peak torque was attained in this material. The change in concentration of point defect Ci in the fatigued specimen is represented as follows. Ci=Cs[1-1/β'/2-K4(β'/2e-k4γ-K4e-(β'/2)γ)] Cs=K5α'2/K4β' where K4 and K5 are constants. The concentration of vacancies determined experimentally by isothermal annealing was in agreement with the result calculated from the above equation.
The effect of grain size on fracture toughness of high hardness steels was studied in air at room temperature. By appropriate heat treatments four different grain sizes for the cold working roll steel and two different grain sizes for cold working die steel were obtained. The fracture toughness value (KQ) increased significantly with coarsening grain size when delayed crack growth was observed prior to unstable fracture. Fractographic observation of crack surface showed that delayed crack propagated along grain boundaries. The crack path was significantly distoreted because of kinking and branching especially in the case of large grain size. This kinking and branching of crack reduced the effective value of stress intensity factor at the crack tip and incresed the resistivity against delayed crack propagation. Therefore, large grain size leads to the improvement of fracture toughness of high hardness steels.
Tensile tests were carried out in molten zinc by using both welded and machined specimens with a modified reinforcement in order to investigate the factors which influence the“Liquid Metal Embrittlement”(LME) crack initiation of steel by molten zinc, and the relations between the stress-concentration factor (Kt) and the nominal tensile stress levels (σc), at which the LME cracking initiates, were examined. The steel pipe welded with several pieces of steel plates was dipped into the molten zinc and its thermal stress distribution was analysed by using the FEM. It was shown that the thermal stresses produced increased as the dipping speed decreased. By using the results of the FEM analysis, the dipping speed at which a crack would occur at the toe of the welded bead was decided, and the dipping experiment was carried out at this speed. The main results obtained are as follows: (1) The stress-concentration resulting from the shape of specimens was more influential than the metallurgical effect for the crack initiation of welded joints in molten zinc. (2) The thermal stress occurred when the steel structural members were dipped into molten zinc, and it increased as the dipping speed decreased. Thus, the increase in dipping speed was effective to prevent crack initiation.
The effect of the interaction between fatigue and creep damages on rupture life and crack propagation has been investigated on SUS 304 and SUS 316 stainless steels at 650°C. It was shown that under higher nominal stress and at intermediate frequencies, the rupture life under cyclic load increased in total time to failure as compared with the creep rupture life under static load, and the sum of fatigue and creep damage fractions is more than one. These results showed that a linear superposition model was not applicable. But under lower nominal stress, the behavior of rupture life could be interpreted on the basis of a linear superposition model. From an engineering design viewpoint, the model was in the safety side within the range of these experiments. In order to account for the behavior above mentioned under higher stress and at intermediate frequencies, crack propagation tests and observations of fractograph were made. These tests showed that the rate of crack propagation under cyclic load became slower than that under static load from the halfway through crack propagation. Fractographic studies indicated the fracture mode transition from intergranular creep fracture to transgranular fatigue fracture, corresponding to the change of crack propagation rate. The reason of this behavior of lower crack propagation or fracture mode transition is that creep strain recovery decreases net deformation in one cycle, causing to depress creep fracture and promote fatigue fracture. After all, it was found that creep strain recovery as well as fatigue and creep damages should be considered as the factors controlling the combined fatigue and creep fractures.
In the relation between OSD's parameter and logσ, two different linear relations were fourd to exist in the low and high stress regions. This behavior is due to two different kinds of mechanisms, intergranular fracture and transgranual fracture. The numerical constants calculated statistically from the short-time data can be used to extrapolate the long-time data. Based upon this relationship, a new calculable extrapolation method for creep rupture time under arbitrary stress and temperature was proposed. The difference between the predicted values and experimental values was also compared with the other time-temperature parameter methods for eight metallic materials. It was shown that the error of the present method was much smaller than those of the previously proposed methods viz. LM, OSD, MH, and MS methods.
The pitting corrosion, which affects the fracture of stainless steel in corrosive environments, is the electrochemical phenomenon, occurring in the specified electrode potential region. When halogen ions, especially chloride ions in a corrosive solution exceed a critical amount, the pitting corrosion occurs inevitably. In this study aimed to elucidata the relation between the pitting phenomena and mechanical properties, the fundamental experiments were carried out in deaerated 3% NaCl solution. For this purpose, the changes in the creep curve and the time to fracture under various potentials controlled potentiostatically above the critical potential for pitting corrosion as well as the magnitude of the applied stress that did not lead to any fracture of the material by the pitting during a specified period (104sec.) were determined. Observations of the specimen surface were also performed. The results obtained were as follows. (1) The controlled potentials within the pitting corrosion region influenced the creep fracture time, the applied stress that could not cause fracture in the material by pitting corrosion in 104sec., and the fracture process. Even though the applied stress was 352.8MPa (plastic region), the controlled potential below+0.1V could not cause fracture in SUS 430. When the controlled potential was raised to a higher value such as +0.2V, the applied stress of 196.0MPa (elastic region) was enough to cause fracture in SUS 430. On the other hand, for SUS 316, the applied stress of 450.8MPa was not enough to cause fracture at the low controlled potential below +0.5V. When the pitting potential was controlled at a high value such as +1.0V, a lower applied stress caused fracture in the material. (2) The change in the appearance of pitting corrosion caused by applied stress was also reflected in the change in the current. The current observed on SUS 316 was higher than that on SUS 430. This difference was remarkable at higher controlled potentials. When the applied stress was in the range from 548.8MPa to 588.0MPa and the controlled pitting corrosion potential was such a higher value as +1.0V, the difference was considerably large, and the creep strain dropped remarkably. Therefore, this difference was thought to be caused by the fast rate of pitting corrosion compared with the creep strain rate. (3) The appearance of pitting corrosion on SUS 430 was different from that on SUS 316. The pits on SUS 430 were localized in a certain area of the specimen surface, and they were small in number and deep. But the pits on SUS 316 were distributed all over the specimen surface, and they were numerous and shallow. This difference influenced their creep fracture properties. In SUS 430, the crack began to form from the large and deep pits grown at the side of the specimen and propagated very slowly. Such a phenomenon could not be observed on SUS 316.
Recently, hydraulic fracturing technique is being used worldwidely to determine the in-situ stress in the earth's crust. Most of the works deal with hard impermeable rock masses and therefore the theories are developed to suit for such kind of rock. However, there are so many cases where the rock masses are saturated and permeable. They are usually soft and weak. The in-situ stress determination in such soft rock mass has eagerly been searched for, but an effective method has not been invented yet. In this paper, the result of the feasibility study for the hydraulic fracturing technique on the soft rock (mudstone) was reported. The mechanical properties and the behavior of soft rock during injection of pressurized water were investigated. Vertical and horizontal fractures were created in a modified triaxial cell under various stress conditions. The flow rate and the pressure-increase rate in a drilled hole were found to be very influential to the fracture orientation. Also, the application of this hydraulic fracturing technique to the field problems was briefly discussed.