Details of a newly developed system for fracture surface contour measurement were described. A commercially available laser displacement meter based on the triangulation principle was used as a main hardware in the first system. Another laser displacement meter with higher resolution, which is based on the principle of critical angle, was utilized in the second system developed in the present study. An emphasis was put on the software development for widening a dynamic range of height measurement with keeping high resolution in both systems, in order to find a fracture surface zig-zag contour superposed on the long-range undulation. Finally, the successful use of the contour measurements by these systems was demonstrated for the topographical analysis of the fracture surfaces of ductile and brittle fracture samples.
Synthesis of AlN whiskers from Al powders obtained by a spark discharged process and their morphology were studied. AlN whiskers were synthesized by nitridation of the compositions in the Al-C system in N2-gas flow at 1450°C for 24 hours. Morphological aspects of the whiskers were observed by a scanning and a transmission electron microscopes. Al powders obtained by the spark discharged process were covered with a thin layer of hydroxide and did not melt nor coalesce up to 1000°C in air. Morphologies of the synthesized whiskers were divided into three types; two single filament types and one bicrystal type. Both of the single filament whiskers had ‹0001› growth direction. The growth direction of the latter type deviated about 13 degrees from ‹0001› axis. All the whiskers were thin plate-like. The internal defect of whiskers was also described.
A constitutive equation of irradiation creep for irradiated materials applicable to structural analyses in a multiaxial state of stress was developed. After reviewing microscopic mechanisms of irradiation creep and swelling, the relevant theories proposed from the view point of metallurgical physics and their applicability were discussed first. Then a constitutive model was developed by assuming that irradiation creep can be decomposed into irradiation-enhanced creep and irradiation-induced creep. By taking account of the SIPA (Stress Induced Preferential Absorption) mechanism, the irradiation-induced creep was represented by an isotropic tensor function of order one and zero with respect to stress, which is, at the same time, the function of neutron flux and neutron fluence. The volumetric part of the irradiation-induced creep was identified with swelling. The irradiation-enhanced creep was described by modifying Kachanov-Rabotnov creep damage theory by incorporating the effect of irradiation. Finally the irradiation creep and swelling of type 316 stainless steel at elevated temperatures were predicted by the proposed constitutive equation, and the numerical results were compared with the corresponding experimental results.
Nickel-foam reinforced aluminum alloy (NFRA) has recently attracted special interest as one of the metal matrix composites. Nickel foam is easily integrated with aluminum alloy because of high porosity, and its specific surface area is large enough to improve the wetability and adhesion and to form a Ni-Al intermetallic compound which is helpful in improving wear resistance. The aim of this investigation was to evaluate the strength of NFRA and to clarify the fatigue behavior of this material. Cantilever-type rotating bending fatigue tests and tensile tests were carried out on two types of NFRA and AC8A aluminum alloy. Two types of NFRA of which nickel-foam is squeeze-casted with AC8A aluminum alloy were prepared and heat treated in accordance with T6 processing; one has 2mm pore diameter of nickel foam, another has 0.4mm. It was observed by a microscope that two types of intermetallic compounds were formed at the interface between nickel foam and base metal. These hardness were about ten times harder than that of base metal. The fatigue life, and crack initiation and propagation behavior of NFRA were discussed in detail. From the experimental results, the fatigue life and tensile strength of NFRA decreased significantly as compared with those of AC8A-T6 aluminum alloy. Many cracks were initiated at the interface between intermetallic compound and base metal during the fatigue process, and crack initiation behavior was related to the shape and size of nickel foam. Fatigue cracks of NFRA propagated with coalescence of many microcracks, of which mechanism was very different from a homogeneous material such as AC8A aluminum alloy.
In a previous paper, mechanical properties of the injection moulded and sintered 304 stainless steels having different Mn/Si ratio were studied. As the result, the tensile strength was hardly affected by the Mn/Si ratio, but the fatigue strength changed considerably by the Mn/Si ratio. It was suggested that the shape and distribution of pores influenced the fatigue behavior. In this paper, two kinds of sintered specimens which have unlike shape of pores were produced by the same process as mentioned previously, and the effects of pore shape and distribution on fatigue crack initiation and crack propagation were studied in detail. The main results obtained are summarized as follows; (1) The pore shape of sintered stainless steel changed with the Mn/Si ratio of SUS304 powder. The steel with a high Mn/Si ratio contained a lot of sharpened pores which have a large aspect ratio. However, the steel with a low Mn/Si ratio contained many spherical pores. On the other hand, the pore distribution was independent of the Mn/Si ratio. (2) Fatigue crack propagation behaviors of injection moulded and sintered stainless steels were hardly affected by the Mn/Si ratio. (3) Although the chemical compositions of the steels were almost the same except Mn, the smooth specimen made of steel with a high Mn/Si ratio showed the fatigue limit lower than that of the steel with a low Mn/Si ratio, because the presence of sharpened pores in the former accelerate the fatigue crack initiation.
In order to develop and validate a procedure of creep rupture tests using miniature specimens in estimating the remaining life of fossil power plant parts, a method to make the miniature specimens was investigated first. Then the accelerated stress-rupture tests were performed on the miniature specimens to predict the remaining creep life of Cr-Mo-V, 21/4Cr-Mo and 11/4-1/2Mo steels. The results obtained are summarized as follows: (1) The miniature specimens could be produced by either only machining, or welding the grips at the ends of specimen and then machining them. The miniature specimen sized into 2mm dia.×30mm was suitable as a reduced size specimen. (2) The creep curve and rupture life of the miniature specimens tested in argon were similar to those of the conventional size specimens in air. The fracture mode of the miniature specimens was also consistent with that of the conventional size specimens. (3) On a graph of temperature vs. logtr, the creep rupture data of Cr-Mo-V, 21/4Cr-Mo and 11/4Cr-Mo steels were represented with an iso-stress line, respectively. Each iso-stress line moved parallel with the stress level. It was found that the miniature specimens could be used in the short time iso-stress creep line extrapolation method to predict the remaining creep life. (4) The present method was found to be applicable to the parts subjected to long-term service. It is considered that the creep life evaluation based on this method is very useful because of its convenience and reliability.
Thermal shock fatigue experiments were carried out on notched Al2O3 ceramic specimens by using a water quench technique. The crack growth behavior from initial cracks, which were introduced by thermal shock at the notch root or Vicker's indentation or on the surface of specimens, was studied. Each crack grew and finally branched under a repetition of thermal shock. In this study, the crack growth behavior before the branching was mainly investigated. The results showed that the crack growth rate was reduced with an increase of the number of thermal shock regardless of initial crack size and thermal conditions. When a relatively higher temperature difference was repeated, a crack grew discontinuously and finally stopped. Such a crack growth behavior suggests that some damage occured at a crack tip under a repetation of thermal shock. In order to consider these behaviors, steady-state thermal stress was analyzed by FEM. In this result, the crack opening stress at a crack tip near the surface increased at first and decreased with an increase of crack length after the maximum stress value. In order to show the damage produced at a crack tip by thermal shock fatigue, the bending strength was evaluated after the repetition of thermal shock. After the repetition of a smaller thermal shock, no crack was observed but the bending strength was reduced.
The rotary bending and cyclic torsion tests were performed on sintered Al2O3 cylindrical specimens having the same diameter. The tests were performed at room temperature within the range of 104 to 109 stress-cycles by using an Ono's rotary bending fatigue testing machine (147Nm, 3420rpm) and a Schenck-type torsional fatigue testing machine (40Nm, 3450rpm), respectively. The fractured surfaces after the both tests were also examined by SEM. The test results under rotary bending were compared with those under cyclic torsion. It became clear that the fatigue behavior under rotary bending was similar to that under cyclic torsion. The life of the material increased remarkably as the stress amplitude decreased. Although the material did not have a distinct knee at cyclic numbers less than 107, the knee seemed to exist at cyclic numbers more than 108. The fatigue limit under rotary bending was about equal to that under cyclic torsion. Each S-N curve could be represented by a straight line up to about 108 stress-cycles and could be expressed by the formulas, σnN=constant and τnN=constant, respectively. The exponent n under rotary bending was about 19 and n under cyclic torsion was about 16. These values were small in comparison with those in literature. It was considered that the similarity of fatigue behaviors under rotary bending and cyclic torsion was due to the crack propagation from the initial flaw subjected to the tensile and compressive principal stresses. It was known by macroscopic observation that the fractured surfaces did not have clear mirrors. The fatigue fractured morphology could not be found under microscopic observation.
It is important to predict the S-N curves of ceramics under cyclic loading in their application to machine components. In order to study the cyclic fatigue characteristics of ceramics, the cyclic fatigue tests under 3-point bending were carried out at room temperature in air on the smooth specimens of sintered silicon nitride, partially stabilized zirconia and alumina as well as the precracked specimens of sintered silicon nitride with surface cracks introduced by Vickers hardness indentation. The effect of specimen geometry on the fatigue-life was also investigated by conducting ring compression tests under cyclic loading on the silicon nitride specimens at room temperature in air. The effect of specimen geometry on the fatigue-life was found to be small for the endurance limit at 107 stress-cycles of the case investigated in this study. A large scatter was observed in the fatigue-life of the smooth specimens as compared with that of the pre-cracked specimens. The scatter in the fatigue-life on the smooth specimens may be explained from the scatter of the inherent flaw size in the specimens. From the slow crack growth rate determined on the pre-cracked specimens, the S-N curves for the smooth specimens with different sizes of initial cracks were predicted. Furthermore, the S-N curves for the smooth specimens of the tested materials were predicted on the assumption that the distribution of flaw size of the specimens used for the fatigue tests were equal to that of the specimens for the static fracture tests.
The wear rate of a low carbon steel as a function of contact load was obtained from the wear experiments performed in wet or dry air by using a wear test rig of pin-on-disk type, and the T1-transition from severe to mild wear was mainly investigated. Both the severe wear rate and the transition load in wet air were higher than those in dry air. The condition for the T1-transition to occur was then analyzed on the basis of a newly proposed wear model involving the effects of adsorption, oxidation, work-hardening, and flat-surface formation. The chemical condition of the asperity surface of the disk affected the T1-transition more than that of the pin. The analytical results on the probability for the occurrence of adhesive wear proposed in the present study explained the above-mentioned experimental results well.
The stress states of partially bonded bi-material half planes with an edge crack under couples at infinity were analyzed by using a rational mapping function and complex stress functions. Stress distribution and stress intensity factors were obtained. Stress intensity of debonding was related to the energy release rate, and re-extension of debonding after crack initiation was discussed.
It is desirable to design a structure and its members with a reasonable reliability. Here, a reduction factor for obtaining the design strength of a double reinforced concrete member under an axial load at a given eccentricity with a required reliability is proposed for the JSCE Code by using the probabilistic method of estimating the reliability (see ref. 1)), considering the variations in stength of both reinforcing bars and concrete, because the reliability of the strength provided in the JSCE Code has been found to be considerably lower than that specified in some cases. The validity of the factor is examined by comparing the obtained reliability with that of the strength provided in the ACI Code which was determined on the safer side, based on a large number of test data.
In order to study an effect of loading rate on fracture toughness of steels for pressure vessels, static and dynamic fracture toughness tests and brittle crack arrest test were carried out. The steels used were two kinds of nuclear pressure vessel steel, A508 Cl.3 and A533B Cl.1, and an Al-killed steel. The results obtained are as follows: (1) The plane-strain fracture toughness was independent of the increase rate of stress intensity factor, K. (2) The values of plane-strain fracture toughness obtained by static and dynamic fracture toughness tests and brittle crack arrest test were the same. (3) Fracture toughness became lower as K increased in the transition region in which plastic deformation cannot be ignored. (4) The fracture toughness during brittle crack propagation, KID, became higher as the crack propagation rate increased, owing to the rise of temperature caused by the fracture of material near the crack tip.
Tensile and fracture toughness tests of tungsten specimens were carried out at elevated temperatures to investigate the temperature dependences of mechanical properties and fracture toughness. Fracture surfaces were also observed in detail. Fracture toughness of tungsten was low in the temperature range lower than 1200K, at which the brittle-ductile transition was found. Fracture toughness was high in the temperature range higher than 1200K. However, the ductile-brittle transition was observed around 1500K, at which recrystallization occurred, and fracture toughness significantly decreased. On the basis of fractographic observations, quasi-cleavage fracture was dominant in the temperature range lower than the brittle-ductile transition temperature. Dimple fracture was dominant in the temperature range higher than the brittle-ductile transition temperature. However, intergranular fracture was dominant in the higher temperature range where recrystallization occurred. No significant difference in the above mentioned fracture behaviors between the recrystallized and the as-received specimens was found.
In order to determine the requisite value of toughness for friction welded joints on the basis of fracture mechanics, Charpy impact test and fracture toughness test were carried out at various temperatures on the friction welded butt joints composed of carbon steel JIS. S45C and the base metal. The experimental results indicated that the Charpy absorbed energy of the friction welded joints was almost equal to that of the base metal in the temperature range below room temperature, while the absorbed energy of the joints was about a half of those for the base metal above room temperature. The fracture toughness of the friction welded joints increased with an increase in temperature, but the toughness was slightly lower than those of the base metal over the whole temperature range. Particularly, in the temperature range above 185K, the value of the net section stress σnet for fracture of the joint was higher than the 0.2% proof stress of the base metal. When the transition temperature for the Charpy absorbed energy was compared with that for the fracture toughness, the former was much higher than the latter. This suggests that the fracture toughness for the joints is high enough, even when the Charpy absorbed energy is low. Furthermore, the usable critical stress for the friction joints with circumferential or penny-shaped cracks was estimated for various kinds of specimen size and temperature based on the experimental results of fracture toughness.
Carbon fiber reinforced plastics (CFRP) are expected to be used in many applications, because they have outstanding mechanical properties. However, the fracture behavior of CFRP has a special feature such as an interlaminar shear one. In this paper, the effects of heat treatment and sizing on interlaminar shear strength (ILSS) was investigated. ILSS was improved by 30% under such an optimum condition as heat treating carbon fibers at 150°C-250°C for 10min-15min.
A new fatigue testing method was developed to study the fracture mechanical fatigue properties of brittle hard materials such as high performance ceramics at room and elevated temperatures. Several computer-simulated experiments were first conducted to get the optimum specimen configuration and loading condition. Chevron-notched specimens made of PSZ (Partially Stabilized Zirconia) and glass were used to examine the performance of testing technique including the stability of newly designed eccentric compression loading device and the accuracy of the measuring method based on the extended unloading elastic compliance method. Calibration of crack length was made by using both of the initial equivalent crack length of chevron notch and the final fatigue crack length observed on fracture surface, and it was confirmed that the fatigue crack length detected by using the extended unloading elastic compliance method was quite satisfactory from the view point of accuracy.
The acoustic emission responses (AE) of the multi-layered hybrid composite made of many thin aluminum sheets and CFRP sheets were studied under three conditions of uniaxial tensile, unloading -reloading and loading-holding tests. The failure processes were monitored and identified by the AE. The mechanisms of hybrid effect which had been found in the previous paper were clarified on the basis of AE data obtained under the condition of uniaxial tensile test. By analysing AE data obtained under each condition it was also found that there was a stress range in which a few cracks were generated in the matrix but did not propagate. This fact made it possible to decide an allowable stress for the tested material by the AE technique. As the stress in the specimen increased more than the allowable stress during loading-holding testing, a large amount of AE events were generated suddenly. It was thought that they resulted from delaminating of aluminum and CFRP laminae and that the failure mechanism controlling the uniaxial tensile testing changed into the other one which includes a delamination mechanism.
An impact testing equipment employing a long column weight with semi-conductor strain gages was developed to evaluate the impact strength of structural ceramics. The advantages of the method are that the one-dimentional wave propagation in the column makes it possible to measure the impact load accurately, and that the impact resistance of material can be expressed in term of stress. Three -point bending impact strength was obtained on such structural ceramics as alumina and silicon carbide to know the relation between impact strength and strain rate. The results obtained are as follows. When the strain rate was less than 9S-1, the impact strength was independent of the strain rate and its value was almost the same as the static strength. But, when the strain rate was greater than 9S-1, the impact strength increased with strain rate.
Impact three point bending tests on CFRP and GFRP were carried out by using a split-Hopkinson pressure bar technique. Ramped incident waves were produced by plastic deformation of zinc and were applied to the specimens. Smooth transmitted stress waves without high frequency fluctuations were obtained and it was possible to evaluate the stress-strain relation precisely. Bending moduli for CFRP and GFRP were found to be almost independent of the strain rate investigated. The maximum bending stresses of the both materials were slightly rised with an increase of strain rate in the relatively low strain rate range, while their strain rate dependence was markedly increased in the high strain rate range. Non-linear deformation behaviour was initiated by shearing fracture in a local compression region of specimen adjacent to the central loading point, followed by the delamination, and was finally terminated by tensile fracturing in the tension region.