Showing papers in "Journal of The Society of Materials Science, Japan in 1986"

Effects of hardness and crack geometry on .DELTA.Kth of small cracks.

Abstract: The dependence of ΔKth on crack size and material properties under stress ratio R=-1 was studied on various materials and microstructures. The values of ΔKth for all the materials investigated were standardized with one geometrical and one material parameter.The geometrical parameter, √area, is the square root of the area which is occupied by projecting defects or cracks onto the plane normal to the maximum tensile stress. The relationship between ΔKth and √area is expressed as follows:ΔKth∝(√area)1/3 (a)The most relevant material parameter to standardize the data was the Vickers hardness, and the following relationship was obtained:ΔKth∝(HV+C) (b)The constant C in Eq. (b) reflects the difference of nonpropagation behavior of small cracks in soft and hard metals.By combining Eqs. (a) and (b), the following equations were derived for predicting ΔKth and the fatigue limit σω of cracked members.ΔKth=3.3×10-3(HV+120)(√area)1/3 (c)σω=1.43(HV+120)/(√area)1/6 (d)where the units are ΔKth: MPa·m1/2, σω: MPa, √area: μm and HV: (kgf/mm2). Equations (c) and (d) are applicable to a crack having √area approximately less than 1000μm.

Preparation and properties of binary oxide glasses containing rare earth oxides.

Abstract: Thermal image furnace melting and twin-roller quenching were used to prepare glasses containing large amounts of rare earth oxides in Ln2O3-MXOY systems (Ln2O3=La2O3, Nd2O3, Y2O3; MXOY=SiO2, Al2O3, TiO2, Nb2O5).The glass-forming regions were found at the following compositions close to the eutectic points of the phase diagrams: (20-30)Ln2O3-(70-80)SiO2, 60Ln2O3-40SiO2, 20Ln2O3-80Al2O3, 20Ln2O3-80 TiO2, (20-30)Ln2O3-(70-80)Nb2O5 and 60Ln2O3-40Nb2O5. The glass transition temperature Tg was determined by DTA. All the glasses showed high glass transition temperatures over 700°C. Although Tg increased or decreased with Ln2O3 content depending upon the system, the change was linear with Ln2O3 content around the eutectic compositions. This seems to indicate that there is no peculiar behavior of viscosity itself at the eutectic compositions. The liquidus viscosity becomes the maximum at the eutectic compositions due to low liquidus temperatures, which makes glass-formation easier at the eutectic compositions.A glass model consisting of four-coordinated aluminum units was proposed for Ln2O3-Al2O3 glasses based on the results of IR spectra and observed AlKα chemical shifts. On the other hand IR spectra of Ln2O3-TiO2 glasses showed a mixture of six- and four-coordinated titanium units.Visible and ultraviolet absoption spectra of Nd3+ were studied on Nd2O3-MXOY glasses. The peak positions of these absorption bands moved towards lower wave numbers linearly with increasing compositional basicity of the glasses. Their color changed from purple-like blue to green-like blue, as the absorption bands shifted towards lower wave numbers.

Analysis of temperature, stress and metallic structure in carburized-quenched gear considering transformation plasticity.

Abstract: A method for the simulation of carburized-quenching process is presented in this paper, which takes into account the coupling effect of temperature, stress and metallic structure. Here, emphasis is placed on the role of transformation plasticity.The constitutive equation of the transformation plastic strain was determined experimentally assuming that the strain rate is proportional to the applied stress for both pearlitic and martensitic transformations.The carburized-quenching process of a steel gear of 1.8%Ni-0.6%Cr-0.25%Mo with 0.2%C (SNCM420HK) was analyzed by use of the finite element method, and the calculated results of carbon content, temperature, metallic structure and stress were compared with the experimental data. The distribution of the residual stress revealed that the transformation plasticity should be introduced in the simulation.

Elastic Moduli of Crystalline Region of Polytrimethylene Terephthalate

Abstract: Elastic moduli of the crystalline region of polytrimethylene terephthalate (PTT) have been measured by the X-ray diffraction method. The elastic modulus parallel to the chain axis, El, was 2.59GPa, and this value of El is the smallest as compared with those already reported. This is reasonable, judging from the crystal structure of PTT which consists of a highly contracted conformation of molecular chain. The chain extension is considered to occur mainly by the internal rotation of methylene groups. The elastic modulus of the bulk specimen increased with increasing draw ratio and was found to reach to the value of El at the draw ratio over 3.3.On the other hand, the elastic modulus perpendicular to the chain axis, Et, was 3.8GPa. Since many other polymers have been found to exhibit larger El than Et, PTT is unique in this respect, and it has a characteristic crystal structure in which the intermolecular force has larger effect against deformation than the force holding the contracted molecular conformation.

Submicron Grinding of BaTiO3 by Ball Milling

Abstract: Submicron grinding of BaTiO3 powder was studied by a ball mill with small alumina balls of 0.75mmφ to 30mmφ. BaTiO3 clinker which was obtained by calcining equimolar of TiO2 and BaCO3 was preground to 1.8μm. Submicron grinding of the BaTiO3 powder was possible by ball milling with several mmφ balls. In the case of ball diameter of 2mm, the specific surface area was maximum.An attempt was made to represent the specific surface area in terms of volume per ball and total surface area of used balls, and the following experimental equation with a time term was obtained.ln(SS)=-0.247lnr3-0.738/r+0.429lnt+1.57 where SS is the specific surface area, r is the ball radius and t is time. The multiple correlation coefficient R was found to be 0.982 when calculated by a computer.

Evaluation of crack propagation in ceramics by double-torsion.

Abstract: In ceramics, delayed failure occurs due to slow growth of preexisting cracks by stress corrosion. Such subcritical crack growth in high performance ceramics should be characterized for their life prediction based on fracture mechanics. In the present study, the effect of environment on the stress intensity factor (K)-crack velocity (V) relationship was studied for various high performance ceramic materials. The double torsion (DT) method that utilizes the load relaxation technique was employed for the determination of K-V characteristics.Three regions were distinctly observed in the K-V diagrams not only for oxide ceramics, such as alumina and yttria-partially stabilized zirconia (PSZ), but also for non-oxide ceramics, sintered and hot-pressed silicon nitride. In water, straight lines corresponding to region 1 were obtained. The position of region 2, in which crack velocity is controlled by diffusion of corrosive species, depended on the amount of water in air and toluen. Stress corrosion of silicon nitride by water is believed to be caused by the presence of residual oxinitride glass phase at grain boundaries. The mechanism of stress corrosion by water was discussed on the basis of chemical bond rupture model.Acoustic emission caused by crack propagation was also studied for PSZ and sintered silicon nitride. The amplitude of acoustic emission event was dependent on crack velocity for sintered silicon nitride, but independent for PSZ.

低合金鋼の空気中および3.5%食塩水中での疲労破面のx線フラクトグラフィ

Abstract: Fatigue crack propagation tests of a low alloy steel (JIS SNCM 439) tempered at 200°C and 600°C were conducted both in air and in 3.5% NaCl solution. The residual stress near the fatigue fracture surface was measured by the X-ray diffraction method. The results obtained are summarized as follows:(1) The residual stress measured on the fracture surface was tension both in air-fatigue and corrosion fatigue. The tensile residual stress increased with the maximum stress intensity factor Kmax in the case of the material tempered at 200°C, while it had a maximum value at about Kmax=30MPa√m in the case of the material tempered at 600°C. When compared at the same Kmax value, the residual stress was lower for a lower stress ratio and in corrosion fatigue.(2) The distribution of the residual stress beneath the fatigue fracture surface was able to be decomposed into two components: the tensile residual stress in the vicinity of the fracture surface caused by monotonic tensile plastic deformation, and the compressive residual stress in the vicinity of the fracture surface caused by stress relief due to roughness and by compressive plastic deformation.(3) The maximum depth of the plastic zone was evaluated on the basis of the residual stress distribution. The depth ωy is related to Kmax by the following equation:ωy=α(Kmax/σY)2where σY is the yield strength obtained in tension tests. α is 0.19 for air fatigue and 0.06 for corrosion fatigue. The small value of α in corrosion fatigue suggests the hardening of the material in the plastic zone due to the environmental effect.

Effect of Weld-Induced Residual Stress on Fatigue Crack Growth in Compact Specimen

Abstract: In the general area of fatigue crack growth in the presence of residual stress, it is noted that the correction of stress intensity factor (K) to account for residual stress is important for the determination of both range and ratio (R) of K during a loading cycle. The superposition technique can be applied generally for the determination of K. For compact specimens, however, redistribution of residual stress occurs during crack growth and its effect is not negligible. In this study, experiments were done for the compact (CT) specimens which had such an orientation that the crack grew along the weld line in the weld metal of a butt weld joint of carbon steel. Fatigue crack growth characteristics of the weld joint were examined in comparison with that of the base metal and discussions for the evaluation of residual stress were made. The results obtained are as follows;(1) For the CT specimen, it is possible to predict the crack growth characteristics by using the initial residual stress distribution.(2) When the crack grows first through the compressive portion of a residual stress field, the effect is actually a result of the ratio (R) of K, although apparent high values of the crack opening stress intensity factor Kop were obtained experimentally.(3) The residual stress distribution through the thickness has no important effect on the average through-thickness crack growth rate, although it has some effect on the crack curvature.(4) Near the threshold region, the crack growth rate of the weld joint is higher than that of the base metal beyond the consideration of the residual stress effect.

A remaining life prediction method on the basis of micro-crack initiation and growth behavior under creep-fatigue in plain specimen of 316 stainless steel.

Abstract: To assure high-temperature component safety and reliability, a life prediction method has been developed. The method is based on the micro-crack growth rate and the microcrack distribution.Strain controlled low-cycle fatigue tests were carried out on the plain specimens of SUS 316 at 923K in air and the initiation and growth behavior of surface micro-cracks were observed.The results obtained are summarized below.(1) The creep-fatigue life is governed by micro-crack growth.(2) A linear relationship is observed between the logarithms of main surface crack length and the life ratio, N/Nf, for various strain loading conditions.(3) The distribution of micro-crack length obeys the Weibull distribution.Moreover, the maximum crack length in an area larger than the sample area can be estimated with a help of the statistics of extremes. Therefore, the remaining life of high-temperature components may be accurately predicted by measuring the distribution of surface micro-crack length in critical parts.

Tensile strength of engineering ceramics.

Abstract: Uniaxial tensile strength tests were carried out on several engineering ceramics in air at temperatures ranging from room temperature up to 1600°C. The materials chosen for the present study were silicon nitride, silicon carbide and partially stabilized zirconia ceramics, which are in the process of development for gas turbine applications.The tensile strength of silicon nitride rapidly decreased at temperatures above 800-1000°C. When the temperature rose as high as 1300°C, the strength decreased to about 10 percent of the room temperature strength. On the other hand, the strength of silicon carbide did not change appreciably with temperature. The strength of zirconia decreased with increasing temperature; at 1000°C the strength was reduced to 8 percent of the room temperature strength.

Small fatigue crack growth at notch root in elastic-plastic range.

Abstract: The growth and closure behavior of a small crack initiated at a notch root was investigated for SUS 304 and SM41A steels under the situation of enhanced notch plasticity. An acceleration of growth rate as well as an increase in crack-opening range were observed for a small crack which was embedded in the notch-plastic zone. All of the growth data for the small crack at the notch root were successfully correlated with those for long cracks by the effective stress intensity factor range, ΔKeff. ΔKeff was found to be a controlling parameter for crack growth up to higher nominal stress range, as far as the crack closure was observed and the notch-root plastic region was surrounded by the elastic region. A primary controlling factor for the closure of the small crack at the notch root was found to be notch-root plasticity. This was supported by the analytical study by using the finite element method.

モードi・モードii組合せ応力負荷の下での疲労き裂進展挙動とき裂の発生・破断寿命

Abstract: The mixed mode fatigue testing was carried out by applying the same phase and arbitrary ratio of torsional loading to tensile loading.The threshold stress intensity factor and fatigue fracture toughness decreased with an increase of ΔKII0 for initial crack.On the other hand, the stable fatigue crack growth region was found to be devided into IIa region of low growth rate and IIb region of accerated fatigue crack growth rate. Both regions were influenced by mixed mode loading.The crack initiation life Ni and propagation life Nf had similar characteristic in terms of (1+Δτ/Δσ)/√ΔKI02+3ΔKII02=(1+ΔKII0/ΔKI0)/√ΔKI02+3ΔKII02, where ΔKI0 and ΔKII0 are the stress intensity factor amplitude of initial crack. The value of Ni/Nf was found to be about 0.12.

Study of fracture toughness of columnar grained ice using large specimens.

Abstract: The effects of loading rate and specimen size on the fracture toughness KIc of columnar grained ice have been investigated at -10°C. Notched bend specimens with the section size of 50×50mm (medium size specimen) and 200×50mm (large size specimen) were used. The notch of specimen was made using a thin razor blade embedded in the mold. Several tests were carried out at each test condition, and probabilistic nature of fracture toghness was also investigated.The results obtained are as follows.(1) The value of KIc decreased as KI increased, and there was a transition in the range of KI≈10-100kPam1/2/s. The value of KIc was not substantially affected by KI in the high KI region beyond the transition.(2) Scatter of KIc for the large size specimens was considerably small compared with that for the medium size specimens.(3) The minimum value of KIc was not affected by the specimen size in the low KI range. However, in the high KI range, the minimum value of KIc of a large specimen showed a little higher value than that of a medium specimen.(4) The size effect was analyzed from the view point of the “weakest link theory”. It was observed that the experimental data did not agree with the theoretical prediction in the range of low values of KIc.(5) The relation between KI and KIc was compared with other experimental data on a similar kind of ice. Considerable difference in KIc, especially in the low loading rate range, was observed. The reason is not yet clear, and more studies are needed.

X-ray stress measurement of sintered alumina.

Abstract: The X-ray diffraction method was used to measure the residual stress in the surface layer of sintered alumina finished under various conditions. X-ray elastic constants were determined for the diffraction lines of (1.0.10) plane and (220) plane obtained by using CrKα radiation. Those values were nearly identical for the as-fired surface, ground surface and lapped surface. The measured residual stresses were all compressive for various surfaces. The residual stress was the maximum compression of about -140MPa for the surface ground with #300-diamond wheel. The surfaces finished by #600-diamond grinding and lapping were about -90MPa. The residual stress measured on the fracture surface was small compression. Both the full width at the half maximum of diffraction profiles and the feature of X-ray microbeam diffraction patterns changed depending on the amount of plastic deformation associated with surface finishing and fracture processes.

Tensile properties of microduplex stainless steel at low temperature.

Abstract: Low temperature tensile properties of microduplex stainless steel having different hardness of ferrite phase were investigated by means of transmission electron microscopy and X-ray diffraction. The hardness of ferrite in duplex stainless steel was changed by heat treatment.The main results obtained were as follows;(1) The austenite phase in the microduplex stainless steel was metastable. Therefore, martensite transformation was induced during tensile tests below room temperature.(2) The elongation to fracture showed a peak at 201K in the elongation-test temperature curves. This phenomenon is closely related to the amount of strain necessary to induce α' martensite, the amout of α' martensite and the ductility of the ferrite phase.(3) A discontinuity in flow stress-strain curves, namely the quasi-yield point phenomenon, was found buring tensile tests at 77K. This phenomenon seems to be associated with both necking arising from the formation of e martensite and strengthening of that portion due to the formation of α' martensite.(4) Md temperature was unchanged in spite of the variation of ferrite hardness in the duplex stainless steel. This fact may suggest that the deformation of ferrite and austenite phases follows the rule of mixture.

Effect of Cyclic Stress Change on Creep Crack Propagation

Abstract: In order to investigate the effect of cyclic stress change on creep crack propagation, deformation tests on smooth specimens and crack propagation tests on notched specimens of 0.16% carbon steel were carried out at 673K (400°C) under a time-dependent fatigue condition.The crack propagation rate in time-dependent fatigue was much faster than that in static creep. It was owing to the acceleration of creep strain rate in the vicinity of crack tip because the dynamic recovery took place in the compression period. In these tests, the effect of the transition from small scale creep to large scale creep was small. The crack propagation rate had fairly good correlation with the experimentally evaluated creep J-integral range, Delta;Jc, which was equivalent to the analytically evaluated creep J-integral range based on the deformation property in time-dependent fatigue.In detail, however, the crack propagation rate in time-dependent fatigue was rather slower than that in static creep for an equal Delta;Jc value. It was concluded to be attributable to the existence of the incubation time of crack propagation during the tensile stress hold time in time-dependent fatigue. The equivalent creep J-integral range, (Delta;Jc)eq, which was the effective component of Delta;Jc for the creep crack propagation rate under a fatigue condition could be evaluated by the following equation;(Delta;Jc)eq=Delta;Jc/γ, γ=1+C11-R/C2+τH, where C1 and C2 are constants, R is the stress ratio, τH is the nondimensional tensile stress hold time.

Detection of Configuration of Surface Fatigue Crack in Inside of Stainless Steel Pipe by DC Potential Drop Method

Abstract: The configuration of surface fatigue cracks in 12 inch stainless steel pipes was detected by Direct Current Potential Drop Method (PDM). In the process, a simplified method for determining the surface crack configuration was invented based on the analysis of electrical field by FEM. First, the calibration curves of potential ratio V/V0 vs. normalized crack depth a/t were made for every a/c=0.01 using the relation of potential ratio V/V0 and crack aspect ratio for various crack depths. The crack depth was derived by substituting the maximum potential ratio measured at the crack center into an appropriate calibration curve. The calculated crack aspect ratio was compared with the crack aspect ratio of the calibration curve. If the former differed from the latter, the crack depth was evaluated by the calibration curve with another crack aspect ratio until both of the crack aspect ratio coincided. The whole crack configuration was determined by substituting the potential ratio measured at each position into the coincident calibration curve. The method was applied to the fatigue crack and the crack shape was estimated with the accuracy of ±0.3mm.

Stress intensity factor for small surface cracks at the site of stress concentration.

Abstract: A method of the estimation of approximate stress intensity factor for a semi-elliptical crack at notch root was proposed. The stress distribution in the vicinity of notch root was divided into two components: uniform and linear components. The linear component was correlated with the notch root radius ρ. The stress intensity factors for these two stress components were calculated using the equations which were made on the basis of numerical results for a semi-elliptical crack at the surface of a semi-infinite body.First, the accuracy of the proposed method was checked by comparing the present values with the accurate numerical results for a semi-elliptical crack at the inner wall of a pressurized hollow cylinder with an infinite outer radius. And then, the method was applied to a semi-elliptical crack in a pressurized hollow cylinder with a finite outer radius and to a semi-elliptical crack at notch root of CT specimen.The following closed form formula is proposed for the estimation of stress intensity factors for a semi-elliptical crack at notch root in other problems.KI, D=KIT+KIBKIT=σ1√πbFIT(λ), λ=b/aFIT(λ)=1.122-0.230λ-0.901λ2+0.949λ3-0.280λ4KIB=(σ2-σ1)√πbFIB(λ)FIB(λ)=0.443-0.310λ-0.104λ2+0.206λ3-0.061λ4where σ2 is the maximum stress at notch root and σ1 is that at the point corresponding to the deepest point of the elliptical crack front.

Crack growth and phase transformation in partially stabilized zirconia.

Abstract: In order to clarify the dependence of crack growth on phase transformation and the effect of stress on phase transformation in partially stabilized zirconia, the hardness and fracture toughness of tetragonal zirconia polycrystalline specimens containing 3mol% Y2O3 (the average grain size of 0.3-0.4μm) were measured as a function of indenting load by using the Vickers indentation method. The fracture energy was obtained by the single edge notched beam method at various strain rates and by the work of fracture method. The same measurements were performed also on the specimens heat treated at different temperatures in which a part of tetragonal phase transformed to the monoclinic phase. Furthermore, the degree of phase transformation at 300°C was determined as a function of time and bending stress. The results obtained are summarized as follows:(1) All the specimens with or without heat treatment had almost the same hardness of about 12GPa, although a slight decreasing tendency with increasing load was observed. They also showed a similar tendency in fracture toughness except that the untreated specimen showed very high toughness when indented at low loads of less than 200N.(2) The KIc obtained by SENB method increased with increasing stress rate in the range above 10-2mm/min, but remained almost constant at the stress rate lower than 10-2mm/min. This seems to indicate that some threshold stress exists to start slow crack growth in tetragonal zirconia.(3) The fracture energy obtained by SENB method was several times larger than that of WOF method. Since the former reflects the energy needed to start the fracture rather than the energy needed to make a new fracture surface, a very large energy is spent to start crack growth, probably due to the phase transformation.(4) The phase transformation was enhanced by the residual stress around the Vickers indent when heat-treated at 300°C. This stress induced transformation becomes apparent when the tensile stress exceeds 450MPa.

Solid-state interfacial reaction in molybdenum-carbide systems at high temperature-pressure, and its application to bonding technique

Abstract: Diffusion couples of molybdenum with several carbides, i.e. SiC, B4C, TiC, ZrC, HfC and TaC, were heated at various temperatures ranging from 1500 to 1840°C under high pressures of 3GPa and 100MPa for up to 4hr. The couples were then examined for the composition of reaction products, the growth rate of reaction layers, interfacial structures, and tensile strength. In case of Mo-transition metal carbides, Mo2C layer was mainly formed, so that the carbides, which had supplied carbon, resulted in having the nonstoichiometric composition near the interface. The activation energy for the growth of Mo2C layer in Mo-TiC system was 332kJ/mol, and that in Mo-TaC system was 366kJ/mol. In Mo-SiC system, Mo2C layer, the mixed phase of Mo2C and Mo5Si3, and Mo5Si3C layer were formed in order from the Mo side. In Mo-B4C system, the mixed phase of Mo2B and MoB, and Mo2BC layer appeared. The decomposed graphite from B4C was also observed between B4C and Mo2BC phase. The activation energy for the growth of total reaction layer in Mo-SiC system was 531kJ/mol, and that in Mo-B4C system was 183kJ/mol. It can be said that the growth of reaction layers is controlled by diffusion. The orientation of crystals was observed in all reaction products except for Mo2BC phase in Mo-B4C system and (Mo, Ta)2C phase in Mo-TaC system. In HIPed couples, the magnitude of tensile strength was dependent on the difference in thermal expansion coefficient between Mo and carbides. HIPed Mo-TaC couple had the best weldability among the systems examined in the present investigation.

Fracture Behavior under Impact Lateral Compression in Filament-Wound Composites( Minor Spacial Issue on Fiber Reinforced Plastics)

Abstract: When a helical-wound composite cylinder is subjected to lateral compression load, delamination ocurres in the interlamina. This delamination causes a remarkable decrease in strength.In this paper the behavior of macroscopic failure of helical-wound composite cylinder under impact lateral compression load is observed experimentally and the elemental property is made clear. From the numerical analysis, it is made clear that stress wave propagation induces macroscopic fracture.The following are the main results.(1) The fracture behavior of helical-wound composite cylinder under impact lateral compression load is affected by the deformation velocity and the winding-angle of filament.(2) The impact absorbed energy of helical-wound composite cylinder under impact lateral compression load increases with increasing deformation velocity, and this effect becomes remarkable when the winding-angle was large.(3) The fracture of helical-wound composite cylinder ocurres in fiber boudle and interlamina.In paticular, interlamina fracture causes a remarkable decrease in strength, because the speed of stress wave propagation is affected by the different mechanical impedance of the fiber boudle-layer and the resin-layer.

Ductile crack initiation and void volume fraction. Investigation based on Gurson's yield function.

Abstract: An investigation on ductile crack initiation in structural steel has been made, based on the concept of Gurson's yield function for porous material.In the first part, the condition of ductile crack initiation in a uniform stress field was investigated. The condition of ductile crack initiation under various stress triaxiality obtained from the tests on axisymmetric notched tensile specimens was found to be well expressed by the condition of constant void-volume-fraction obtained analytically from the Gurson's model. This result means that the condition of constant void-volume-fraction may be used as the local criterion of ductile crack initiation.In the second part, the behaviors of void growth and ductile crack initiation in the area near the notch tip under mode I and mode II loading were investigated. Under mode I loading, the increase in void-volume-fraction around the notch with an increase in applied load agreed well with the behavior of porous material predicted by FEM analysis based on Gurson's yield function, and the ductile crack initiation could be predicted by the criterion of critical void-volume-fraction as in the case of uniform stress field given above. The same criterion was not applicable for the crack initiation under mode II loading and further study is needed.

Study of fracture toughness of coarse grained ice and the effect of specimen size.

Abstract: The effect of loading rate on fracture toughness of ice has been investigated using sharply edge-notched specimens by bending at -10°C. The loading rate KI ranged from 1 to about 103kPam1/2/s. The notch was made by molding a razor blade in the specimen in the ice growing process and removing it before the bending test. Two sizes of specimens, section sized 25×25mm (small size specimen) and 50×50mm (medium size specimen) respectively, were used. About thirty small size specimens or about ten medium size specimens were tested at each loading rate. The experimental results were as follows.(1) The values of KIc were lower in the higher KI range, and there was a transition in the range of KI=10-100kPam1/2/s.(2) The effect of KI was found to be very small in the range of KI larger than 100kPam1/2/s.(3) The effect of loading rate on the minimum value of fracture toughness KIc was small in the whole range of KI tested for both small and medium size specimens.(4) The minimum KIc values of small and medium size specimens seem to coinside each other approximately. The maximum and the median KIc values of medium size specimens were smaller than those of small size specimens.(5) It was found that a vinyl silicone impression material being used for dentistry was suitable to make replica of the fracture surface for macrofractography.

Evaluation on limitation of linear elastic fracture mechanics for small fatigue cracks under rotating bending.

Abstract: In the present investigation, the growth characteristics of small fatigue cracks were examined in two steels, and the critical crack length above which linear elastic fracture mechanics (LEFM) was applicable was evaluated.It is found that the critical crack length can be obtained as the sum of the microstructurally small crack length and the region of mechanically small cracks. The microstructurally small crack length and the region of the mechanically small cracks are also found to be an unique function of the microstructure (grain size) and yield strength of materials, respectively. Therefore, it is possible to evaluate the critical crack length for a given material.

A Model for Evaluating Strength and Fracture Toughness of Structural Ceramics

Abstract: In order to elucidate the notch width or notch root radius dependence of the critical stress intensity factor evaluated by SENB (Single Edge Notched Beam) specimens, 4-point bending tests were carried out on hot-pressed SiC (with 2wt% AlN) and sintered Si3N4. The Experimental results showed that the critical stress intensity factor Kc determined with SENB specimens coincides with a valid fracture toughness KIc value only when notch root radius ρ is smaller than about 10μm. For larger root radii, the variation of Kc with ρ was observed, and this can not be attributed only to the stress concentration effect of notch.In the present study, by introducing a micro-damage concept a method was proposed for evaluating KIc from Kc values given by SENB specimens having rather blunt notch.On the basis of the published strength data for SiC, the relation between strength and microstructure was examined. It was assumed that a micro-damage existed at the machine finished surface, and its size should be the sum of the machined damage depth hmax and the maximum grain diameter dmax. Here, hmax can be estimated from the average grit size of the diamond disk used. An analysis was made of the dependence of Kc on ρ by considering the micro-damage at the notch root, and a theoretical expression was formulated. The experimentally observed Kc-ρ relation agreed quite well with the theoretical prediction.

A measuring system for dynamic stress intensity factors using a FFT analyzer - Application to drop weight impact three point bend testing.

Abstract: Evaluation of the dynamic stress intensity factor KI(t) is especially important in impact fracture toughness testing, since KI(t) is influenced by the inertia force of a specimen and quasi-static estimation of KI(t) usually leads to erroneous results.In the present paper, a procedure is presented for measuring KI(t) in the impact three-point bend testing. The contact force between the impactor and the specimen is computed first by using the output signal of a strain gauge mounted on the impactor and the transfer function of impactor. The FFT (Fast Fourier Transform) analyzer is utilized for measuring the transfer function in the pre-experiment in which the impactor is struck by a steel sphere and the impact force is measured by a piezoelectric accelerometer pick-up attached to the sphere. The time variation of KI(t) is then determined by the computed contact force and a simple formula which has been derived previously by the present authors using Timoshenko's beam theory.A program to run on a microcomputer was developed and assessment tests were conducted for the case that a steel or aluminum alloy specimen was impacted by a falling cylinder. Good agreement was obtained between the computed KI(t) and the value determined from the strain gauge mounted near the crack-tip.