Residual stress development in steels having spherical cementite particles were investigated with a function of plastic strain and a volume fraction of cementite phase. Residual microstresses in both the matrix and the cementite phase were measured by means of X-ray diffraction. The results were compared with analytical prediction by the method of micromechanics and a finite element method. Compressive residual stress was observed in the matrix phase after tensile deformation, whereas tensile one in the cementite phase. Each residual microstress increased linearly with plastic strain up to about 1×10-2 for the specimen having a fine particle distribution. The increasing rate of residual microstress in the specimens depended on the volume fraction of the second-phase particles in agreement with theoretical prediction. The FEM calculation revealed inhomogeneous deformation in the matrix phase. This may be one of the reason why the increasing rate of residual microstress decreases with plastic elongation.
The X-ray diffraction method was used to measure the residual stress on and beneath the surfaces of sintered alumina and silicon nitride ground with diamond wheels of #200 and #80 grain-size numbers. The residual stress measured on the ground surface was the largest compression on the surface of silicon nitride ground with a #80 diamond wheel. The depth of the compression zone from the surface was between 20 and 30μm. The compressive residual stress due to grinding with a #200 diamond wheel improved the bending strength of ceramics. On the other hand, gringing with #80 diamond wheel often introduced flaws into ceramics, reducing the benefitical effect of the compressive residual stress. The effect of gringing residual stress on the relation between the bending strength and the defect size was discussed on the basis of fracture mechanics. A model was proposed to assess the effects of the residual stress and the material defect on the bending strength of ceramics.
Low cycle fatigue properties of two types of formable 60kgf/mm2 class hot-rolled high strength sheet steels for automobile use were studied by the X-ray diffraction and TEM methods. Si-Mn dual phase high strength steel sheets, with duplex microstructures of ferrite and martensite, showed initial cyclic hardening followed by cyclic softening on the stress response curves in the strain-controlled fatigue tests. Nb bearing precipitation hardened steel sheets, however, showed monotonuous cyclic softening. These cyclic properties are explained as follows: (1) The initial increase followed by a decrease in the X-ray half value breadth with increased cycles observed on Si-Mn dual phase steel results from fine cell structures in the ferrite matrix of the soft phase. (2) The monotonuous decrease in the X-ray half value breadth observed on Nb precipitation hardened steel results from subboundaries due to the rearrangement of the dislocations.
The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section with Fish eye of the heat treated high-speed steel was investigated by using an acoustic emission technique. Moreover, the property of crack propagation in the 1/2CT test piece was investigated. The results obtained are summerized as follows; (1) The number of stress cycles NAE for crack initiation and propagation and the number of stress cycles Nf for fracture were accurately estimated by measuring continuously AE total ringdown counts during the fatigue process. (2) The fatigue life was good accord with the value of Nf estimated by the acoustic emission technique. The life of fatigue crack propagation (Nf-NAE)Lab. was 25-50% of the fatigue life Nf and decreased with decreasing stress amplitude. (3) The life of fatigue crack propagation (Nf-NAE)cal. calculated from the property of crack propagation in the 1/2CT test piece showed good agreement with the value of (Nf-NAE)Lab. obtained by the acoustic emission technique. (4) The behavior of crack propagation in the complete reversed tensile and compressive fatigue test piece of circular section showed good corrrelation with the behavior of crack propagation in the 1/2CT test piece, so that it is suggested that the initiative mechanism of residual stress on the fracture surface with Fish eye can be discussed by using the 1/2CT test piece.
Rolling contact fatigue tests are performed by using cylindrical test pieces taken out of rail, under conditions of slip ratio 0%, Hertz' contact stress 1.2GPa, velocity 60km/hr and water lubrication. The behavior of plastic deformation below the contact surface of the test piece is analyzed with an X-ray device and a transmission electron microscope. The results indicate that the dislocation density increases, while a preferred orientation texture and a cell structure emerge in the contact surface layer with an increase in the number of cycles of rolling contact fatigue. Slip deformation of crystal is accelerated further when fatigue progresses to a number of cycles larger than a specific value and then so called “shelling” grows with a remarkable development of the texture in either the rail or the wheel.
A new method was developed for the quantitative X-ray fractographic analysis of fatigue fracture surface. The method was basically assumed that the residual stress under the fracture surface could be adjusted on one master curve in a plot of dimensionless residual stress, σ(Y)/σY, against a dimensionless depth, Y/ωp, where σY is the yield stress of the material and ωp is the depth of the plastic zone which is produced under the applied maximum stress intensity factor, Kmax. The experimental data for three steels of SM50A, HT80, SNCM815 were analysed in order to obtain such master curves. The residual stress was found to be adjusted perfectly on one master curve in SM50A, while it was found to be adjusted in a limitd range of the stress ratio, probably due to the cyclic softening effect, both in HT80 and SNCM815. The value of Kmax could be estimated succcesfully in the present method by using the multiple values of the residual stress at different depths, although it could be estimated in principle by a single value at a certain depth.
The residual stress and the integral breadth on the fracture surface of cold forging die set made from heat treated high-speed steel were investigated by using the X-ray microbeam stress measurement technique. The results obtained are summarized as follows; 1) In the region of crack initiation, the maximum compressive residual stress existed. The residual stress on the fracture surface changed from compressive to tensile according to the increase of crack length from the surface. The residual stress on the brittle fracture surface was tensile. 2) The X-ray microbeam fractographic technique gives us essential understanding of fatigue of high strength steel. This technique is applicable to the defect analysis of industrial products and thus useful in many areas.
When the X-ray stress analysis is performed by using a position sensitive proportional counter (PSPC), a parabolic (curved) background line is obtained on the X-ray diffraction profile because of geometrical problem of PSPC and absorption of X-rays. Since such phenomenon is especially remarkable in the broadened profile, it is necessary to correct it to a straight background for measuring a diffraction angle or a half-value breadth with high accuracy. This paper analyzes such curvature of background line of the profile and suggests a correction method to linearize the background line. The results obtained are summarized as follows: 1) The intensities at the both ends of effective length of the PSPC decreased because of the tilt incidence of X-ray beam. 2) The correcting factor for the linearization of the background line was essentially the inverse of background intensity. However, because the background is apt to scatter due to the statistical property of X-rays, such factor was determined by smoothing the inverse of the scattering X-ray intensity obtained from a glass plate. 3) By correcting the background line, the half-value breadth was able to be measured as a constant value wherever the starting points for determining the background line by digital calculation were positioned. 4) On the other hand, the correction was not necessary on the stress measurement.
Thermally induced stresses in the matrix and fibers have been studied by an X-ray diffraction technique during the thermal cycling of an α-Al2O3/Al composite with 50vol% and 20μm diam continuous FP fibers (FP is a tradename of Du Pont Company). The Al matrix contained 1.0wt% Li to wet the fibers. After annealing the composite at 800K for 3hr, the in-situ thermal stress measurements were carried out at temperatures up to 580K by the use of a vacuum resistance furnace mounted on the computer controlled Ω-diffractometer. The results obtained are summarized as follows: (1) When the composite undergoes a change in temperature, both the matrix and the fibers mechanically deform in such a way as to modify thermal expansion mismatch between two phases.The resultant thermal stress in each phase balanced with each other in the composite. (2) On cooling the composite from 580K to room temperature (RT), the matrix showed a tensile residual stress of 50MPa. Upon heating the composite from this stress state the matrix tensile deformation was reversed to compressive deformation. On this heating process the matrix showed a strong Bauschinger effect, that is, the matrix yielded during the unloading before being in compression. (3) On cooling the composite to liquid nitrogen temperature after cooling to RT from 580K, and heating back to RT a compressive residual stress of -30MPa was observed in the matrix. On the subsequent heating process the matrix plastically deformed in compression and a permanent softening due to the stress reversal had been observed in the matrix.
In this paper, the torsional stress and torsional rigidity of a two-layer beam coated with viscoelastic material were analyzed exactly, and an expression for the composite loss factor of the beam in torsion was presented based on the analytical results. The numerical results obtained are summerized as follows. (1) The maximum shear stress arose at the upper or lower interface between two layers, whichever side has low shear strength. The limitation of the beam composition in such case was obtained in conjunction with the thickness ratio and the shear modulus ratio of viscoelastic layer to elastic one. (2) The scale rule for the composite loss factor does not hold exactly in a torsional condition, but approximately hold for the limited ranges of the thickness of elastic layer, the thickness ratio and the shear modulus ratio. (3) Both of the composite loss factors in torsion and in bending for the two-layer beams having the same beam composition were nearly equal except that the thickness ratio was over unity.
Fracture toughness of GFRP composite materials has been studied mainly from the view point of the effects of test temperature and specimen size at temperatures ranging from 300K to 4K. The CT specimens with cut slits of prescribed depth and width were used. The fracture toughness of the GFRP specimens increased linearly with lowering test temperature from 300K to 77K. The effect of the slit width on fracture toughness was found to be the tendency of fracture toughness converging to a constant value below 0.2mm slit width. The plane stress state was attained when the ratio (B/W) of thickness (B) to width (W) of the specimens was below 0.2, accompanied with the incrase of fracture toughness. Also the whitening tendency of the tear crack portion of the GFRP specimens in the tear test became clearer as the test temperature decreased.
Static and cyclic fracture tests were performed on amorphous aluminum oxide thin films coated on Fe-42% Ni alloy base. The thin films were subjected to the static and cyclic tensile strains using a four-point bending test apparatus, and the initiation and propagation of cracks in the thin films were observed by an optical microscope. The effects of coating thickness and surface finishing of the metal, such as mirror polishing or machining, on the static and cyclic fracture strengths of Al2O3 thin coating films were investigated. Under cyclic loadings, the fracture of Al2O3 thin film was observed at a strain below the static fracture strain. The both static and cyclic fracture strengths were found to decrease with increasing coating thickness. It was also found that the surface finishing of base metal affected the fracture strength in such a way that the mirror polishing increased it.
Cracks often appear in the surface of the VC coated steel specimens subjected to rotating bending fatigue tests. In order to investigate the cause of formation of these cracks, detailed observation was made on the surfaces of the rotating bending specimens after the fatigue test and during the static test, and of the two dimensional model specimens during the plane bending fatigue test. The results obtained are as follows: (1) In the rotating bending fatigue test, wide aperture cracks were observed on the fracture origin side, while narrow cracks and sometimes netty cracks were observed on the opposite side. These cracks became more significant with increasing ductility of the substrate steels. (2) Even in the last stage of the fatigue process, no such surface crack appeared in the unbroken specimens. Therefore, it is considered that the surface cracks are generated by the deformation around the fracture point in the separation process of fracture. (3) The static bending test results suggest that the wide cracks on the fracture origin side are formed by compressive stress, while the narrow and netty cracks on the opposite side are formed by tensile stress. Consequently, it is considered that the cracks mentioned at (1) are formed through the following process: During the final fracture of the rotating bending fatigue specimens, separation occurs by tensile stress, which creates narrow and netty cracks. Then, the cracked parts of the origin side strike each other, producing compressive stress, which creates wide cracks.
The effects of a single thermal shock and its repetition on the initiation and growth of cracks were studied experimentally by using notched Al2O3 ceramics specimens. Various temperature differences were given by rapid cooling in water after heating at different temperatures in a furnace. The results obtained are summarized as follows; 1) The boiling point of water, instead of water temperature, seems to be appropriate as the low side temperature to determine the critical temperature difference for the initiation of a crack by a single thermal shock as well as for the increase of the crack density under the repetition of thermal shock of small temperature difference. 2) For the increase in the number of surface cracks including branched ones, the temperature of cooling water seems to be appropriate as the low side temperature under the repeated thermal shock conditions. 3) The threshold temperature difference to increase the crack density under the repetition of thermal shocks agreed with the critical temperature difference for crack initiation by a single thermal shock.
The static fatigue fracture behavior of glass in vacuum was investigated on the basis of a stochastic damage model. Our theoretical results were in good agreement with the experimental ones for 4 glasses and it was shown that the static fatigue crack velocity of glass was proportional to some power of the stress intensity factor. On these theoretical bases, it was concluded that subcritical crack growth was the result of a thermally activated growth process which depended on the crack-tip stress in glass.
In order to investigate the effect of hydrogen charging on the mechanical properties of 0.1%, 0.3% and 0.6% carbon steels with ferrite-pearlite and cementite-spheroidized structures, creep tests during cathodic polarization have been carried out. Cathodic polarization introduced hydrogen into the specimen, and produced hydrogen-induced cracks. Charging with hydrogen increased the creep rate. The creep rate changed in a complex manner, depending on the charging time, the structure and thickness of the specimen, the carbon content, and the external stress. During hydrogen charging, the application of external stress larger than a critical stress, which depended on the structure, carbon content and thickness of the specimen, caused creep fracture of the specimen. In general, the hydrogen-induced creep deformation and fracture process can be explained by the following three mechanisms; (i) Lüders deformation caused by the generation of hydrogen damage (crack and dislocation), (ii) the motion and multiplication of the dislocations caused by the internal stress due to hydrogen and the external stress, and (iii) the growth of cracks caused mainly by the application of external stress. The creep (i) is observed only in the case of the application of the stress just below the yield stress. In the case of small external stress, only the creep (ii) occurs.
Some difficulties inherent in the measurements by an instrumented Charpy impact testing apparatus have been studied by using aluminum alloy and mild carbon steel specimens in order to solve the disorderly oscillation of load and displacement observed during the measurement. The measurements by the instrumented apparatus showed a remarkable oscillation of the measured load in the Charpy impact test under the condition of the standard initial tapping velocity. The load oscillation was effectively mitigated and a successful smoothing of the load-time records was obtained when the initial tapping velocity was lowered to about 75% of the standard value. The load oscillation behavior was also effectively avoided by attaching a buffer material, such as a fat clay sheet, to the specimen tapping portion, and an effective smoothing of the load-time records was achieved simultaneously. The sensing error of the absorbed energy due to the attachment of the fat clay sheet was negligibly small. It was confirmed that the cause of the displacement oscillation observed during the course of the impact test was the vibration of hammer shank of the testing apparatus due to the elastic deformation generated by tapping the specimen. It was also proposed that the increase in rigidity of the hammer shank is an effective procedure to reduce the displacement oscillation.