Recently, microfracture in rocks, which is a cracking phenomenon occurring during deformation of rocks under compression, is studied by detecting and analyzing radiating microelastic waves (i. e. Acoustic Emission). In this study, in order to discuss the source parameters concerning microfracture in rocks, the wave analysis of the Acoustic Emission was performed by using the dislocation model. The results obtained show that the dislocation model can be applied to analyze the microfracturing process and to obtain the source parameters concerning the microfracture. The parameter thus obtained was used to estimate the energy emitted from a microfracture and the dilatancy resulting from a microfracture.
Stress and strain distributions in the plastic zone of notched thin sheet specimens were measured by an X-ray diffractometer and strain gauges, respectively. The results obtained are as follows. (1) The stress and strain distributions obtained under monotonic load were similar to those estimated with the plasticity assumption. They are expressed as σ∝r-0.1 and εT∝r-0.88, where r=x+ρ/4, ρ is the notch radius and x is the distance from the notch root. In this case, the plastic zone boundary was clearly identified in the stress distribution diagram. On the other hand, under repeated tensile load, the stress-range and strain-range distributions were similar to those estimated with the elasticity assumption. They are expressed as Δσ∝r-0.4 and ΔεT∝r-0.6. In this case, no clear plastic zone boundary was observed in the stress range distribution diagram. This must have resulted from the fact that the elastic strain is much larger than the plastic strain in the stress-strain hysteresis loop, even at the notch root. (2) The stress and stress range distributions measured by X-ray method can be estimated accurately from the strain and strain range distributions by using the stress-strain curves. (3) The relationship between the crack propagation rate and the stress intensity factor in the case of repeated zero-tension load in the range of da/dN=10-3∼10-1 (mm/cycle) was similar to that in the range of da/dN=10-5∼10-4 (mm/cycle), even though the former range did not satisfy the small scale yielding condition. This should be considered as a circumstantial evidence for the model obtained in (1), i. e. the stress (or strain)-range distribution in the plastic zone is similar to that estimated with the elasticity assumption.
The behavior of fatigue crack initiation and propagation from the notch tip with the notch radius of 0.10mm was examined in plate test pieces of HY140 and HY180 steels. Fatigue tests were done in air at the tension-to-tension, tension-to-compression, and compression-to-compression, respectively. The results are as follows; (1) The macroscopic crack growth rate da/dN does not change extremely with the crack length from the notch tip ai when ai is smaller than the cyclic plastic deformation zone size rN.P.C. (2) Cycles and da/dN at the crack length of 0.030mm ((N)0.03 and (da/dN)0.03) are determined by rN.P.C. (3) The da/dN decreases with an increase of ai due to residual stress in the monotonic plastic deformation zone at the notch tip which exists prior to crack initiation, when R<-1. (4) The da/dN increases rapidly with ai due to residual stress at the notch tip, when R>-1. (5) The da/dN does not correspond to the striation spacing (da/dN)S.P in the area in which the behavior of crack growth is controlled by the plastic deformation zone at the notch tip. (6) The da/dN coincides with the (da/dN)S.P and is a function of ΔK2 in the area in which the behavior of crack growth is not controlled by the plastic deformation zone at the notch tip.
Rotating bending fatigue tests were performed on SCM 21 casecarburized and hardened steel, and the effect of notch depth on the fatigue properties was examined by six types of notched specimens (the notch depth; t=0.2∼6.4mm, the notch root radius; ρ=0.08mm). Furthermore, the effect of notch root radius on the fatigue limit of carburized steel was discussed using the author's data as well as others'. SCM 21 test pieces were carburized at 920°C, under 0.9% carbon potential atmosphere for 4hrs. and oil-quenched from 820°C. Tempering was carried out at 180°C for 2hrs. The results obtained are as follows. (1) The crack initiating fatigue limit is not influenced by the notch depth under the condition of t/ρ>10, but is influenced under the condition of t/ρ<10. (2) The fatigue crack propagating stress (N=107 cycles) is nearly constant (31∼34kg/mm2), when the effective crack length (lnpc+t) is 0.8∼6.8mm. This tendency reveals that the compressive residual stress in the surface layer increases with notch depth. (3) The non-propagating crack is not in existence under the condition of t/ρ<2.5 and its properties are not influenced by the notch depth under the condition of t/ρ>5. (4) The crack initiating fatigue limit is influenced by the notch root radius, and the notch factor is smaller than one-half of the theoretical stress concentration factor.
In order to reveal the role of cross slip in fatigue crack nucleation and growth in the early stage of fatigue, a study on α-brass was made in succession to the preceding study on copper. As in the previous case, observations on the surfaces, longitudinal sections and fracture surfaces of the specimens fatigued at stress levels corresponding to fatigue lives 1.2×105∼8.0×106 cycles were made using optical and scanning electron microscopy. It was found that crack initiation and growth up to a few grain size occurred predominantly along the slip band. After this stage of growth, a crack route along the grain boundary was sometimes observed, especially in the specimens fatigued at high stress. Flat fracture surfaces accompanied with steps were observed just beneath the slip bands on the specimen surface. Transcrystalline fracture surfaces containing the crystallographic morphology such as parallel lamelae, however, were observed inside the specimen. Measurement of the angle between the facets suggested that the surfaces consisted of non-slip planes. Smooth surfaces containing clear and linear slip traces were found corresponding to the cracks growing along the grain boundaries just beneath the surface as well as inside the specimen. These results were not essentially different from those found in copper, and therefore, the mechanism of crack nucleation and growth in this material appeared not to be different from that in copper.
Non-martensitic structure is formed in the surface zone of carburized steel as a result of the local reduction of hardenability caused by internal oxidation of Cr and Mn during the gas carburizing process. Plane bending fatigue tests were carried out to investigate the effect of non-martensitic structure on the fatigue behavior in relation with the mean stress and residual stress. The results obtained are as follows; (1) Non-martensitic structures formed during the gas-carburizing process lower the hardness and compressive residual stress at the surface of carburized steel. (2) Both the residual stress and mean stress give the same effect on the fatigue strength of carburized steel, whether non-martensitic structures exist or not in the surface zone. (3) The increase of fatigue strength resulting from the removal of non-martensitic structures by electro-polish is mainly caused by the increase of compressive residual stress at the surface. (4) The S-N curves for the specimens tested under high tensile mean stress and low compressive residual stress show one discontinuity but those for the specimens tested under low tensile mean stress and high compressive residual stress show no discontinuity. (5) A semicircular crack propagation region is observed on the fracture surface by means of scanning electron microscope.
Recently, several studies have been performed to clarify the correlation between fatigue crack growth rate and toughness of materials. The results so far indicate that the effect of toughness on fatigue behavior is fairly complicated. In the present study, constant amplitude fatigue crack growth rate experiments were made on 13Cr steel, which is being used mainly for rotating parts of pumps, turbines and compressors, at the following three temperature levels: -25°C (low toughness), 100°C (ductile-brittle transition temperature), and 200°C (high toughness), to examine the relationship between the crack initiation time, the crack growth rate and the toughness of the material. The results obtained are as follows: (1) The material toughness has little effect on the fatigue strength provided the ultimate tensile strength remains constant. (2) At -25°C the initiation time of the fatigue crack is later than at 200°C, but the stable crack growth rate at -25°C is approximately 50% higher than at 200°C. This difference in crack growth rate is probably related to the fact that for the material with low toughness, the region of fatigue crack growth is apt to contain a number of intergranular, cleavage and quasi-cleavage facets, and they decrease as the toughness increases. (3) For the material with low toughness, rapid fracture initiates when the maximum value of the stress intensity factor, Kmax reaches the fracture toughness. And the rapid fracture occurs in such a brittle manner that the evidence of predominant cleavage or intergranular failure can be seen. On the other hand, when the testing temperature is higher than the ductile-brittle transition temperature, the fatigue fracture toughness tends to exceed the fracture toughness and the rapid fracture region shows ductile dimpled fracture.
The inhomogeneity and the residual stress are known to be important factors in determining the strength and the fracture of laminated inhomogeneous metals such as induction hardened materials, carburized hardened materials, superficially worked materials and other surface treated materials. This paper is concerned with the inhomogeneity and the residual stress after some heat treatments of a clad plate composed of low carbon steel and middle carbon steel, which can be regarded as an example of the above mentioned materials. The results obtained are as follows: (1) The microstructure and the hardness change continuously at the boundary between two materials of the clad plate. The hardness distribution was explained quantitatively in terms of carbon diffusion at the boundary in the oil quenched clad plate. (2) Tensile residual stress and compressive residual stress arise in the low carbon steel and in the middle carbon steel, respectively, of the oil quenched clad plate. The residual stresses become very small in the clad plate tempered at 400°C and disappear in the one tempered at 600°C.
This paper presents some experimental data concerning the relation between the shearing strength and roughness of base metal (mild steel) surface soldered with Pb-Sn eutectic alloy. The specimens used in this experiment were made according to the configuration of JIS Z3192, and had the same shape as designed by F.M. Miller and R.L. Peaslee. To make various degrees of roughness on the base metal, the specimens were finished by means of abrasive paper (60, 120, 240, 400 and 600 mesh) polishing, milling cutting or WA 60 and WA 80 wheel grinding. The soldering operation of these specimens was conducted using a 0.3kW resistance solder bath, and the joint clearance of each specimen was adjusted at the cementing fixture. To obtain the tensile-shearing strength of these specimens, a 50tons tensile testing machine was used in this experiment. The results were as follows: (1) As regards to the relation between the shearing strength of soldered joint and the roughness of soldering surface of base metal, it was clear that the shearing strength of a soldered specimen having a small roughness value was greater than that of the one having a large roughness value. (2) The maximum shearing strength for the soldered specimens was obtained at the joint clearance of about 0.2∼0.3mm. (3) It was clear that the shearing strength of a specimen having a long brazed joint length was lower than that of the one having a short soldered joint length. (4) Most of the specimens fractured at the weak part near by the interface of the soldered joint which had a different composition than the original Pb-Sn eutectic composition, but no specimen ruptured in the base metal.
The thermal conductivity of powder was studied by measuring the temperature rise of a linear heat source burned in samples. The materials tested were several kinds of sand for casting and five kinds of glass beads having slightly different diameters. The results obtained are as follows. (1) The thermal conductivity of powder becomes smaller as the particle size becomes smaller and as the porosity of the sample becomes larger. (2) The thermal conductivity of powder decreases with decreasing air pressure. The thermal conductivity vs. air pressure curve can be divided into three parts. In the pressure ranges lower than a few Torr and in the neighbour of atmospheric pressure, the thermal conductivity of powder doesn't vary so sensitively with the change of air pressure. But in the intermediate region of the above two regions, the thermal conductivity k varies with air pressure p as given by k=Alogp+B where A and B are the constants depending on a particle diameter, powder materials and porosity. (3) When the packing density is constant, the smaller the particle diameter is, the smaller the thermal conductivity is. If the particle diameter is extremely small, it seems that the thermal conductivity becomes independent of the powder materials. (4) The smaller the pore spaces in powder are, the larger the thermal insulation effect becomes. Therefore, as the powders for vacuum thermal insulation, it is necessary that their particle diameters and pore spaces are small. (5) To explain the reason why the thermal conductivity of porous materials varies with pressure, the following model was proposed. Solid parts and pore spaces are arranged layer by layer and the intervals of pore spaces are distributed in a wide range. In the pore spaces where their intervals are smaller than the mean free path of gas molecules, the behavior of gas molecules is controlled by the molecular flow, but in the other pore spaces it is by the viscous flow. These two regions controlled by the molecular and viscous flows, change depending on the environmental pressure, and thus the thermal conductivity of porous materials varies with pressure.
A remarkable improvement in abrasive wear resistance of polyvinylchloride (PVC) has been observed by blending silicon carbide (SiC) particles into the resin. This paper describes its abrasive wear properties against rock and coral determined by a Robin type abrasion tester in flowing water. The results obtained are as follows. (1) The wear rate of the PVC decreased rapidly with increasing the content of SiC particles. The wear rate of the PVC filled with 20 wt% SiC became one-tenth of that of the unfilled. (2) The increase of the abrasion resistance of the PVC seems to be mainly caused by the abrasion resistance of SiC, from the observation of the wear surface with an electron microscope. (3) Surface treatment of SiC particles seems to be needed to obtain the stable abrasion resistance of the PVC. (4) In the case of abrasive wear with coral, the PVC filled with proper SiC particles shows good abrasion resistance.
A promising, practical method of poly-chromatic X-ray (or white X-ray) stress measurement has been proposed and tested experimentally. The followings are its characteristic features when compared with the mono-chromatic X-ray stress measurement. (1) No mechanical scanning of the detector is necessary. (2) The accuracy is independent of diffraction angle. (3) The diffraction angle is better to be kept constant for all the sample inclination angles in practical stress measurements. (4) The energy difference appearing as the difference in angle is proportional to stress. (5) A fairly large sample mis-setting is permitted if the parallel beam slits with wide windows are employed. (6) The residual stress distribution in the sub-surface (up to 30 microns depth) of the lathe-machined and sand paper polished stainless steel (SUS 304L) can be measured non-destructively.
In order to obtain the influence of the friction between a tobacco shreds' sample and its container wall on the determination of filling capacity, we have constructed a novel device capable of measuring both the friction index (FI) and the filling capacity (FC) of tobacco shreds simultaneously. With the device, the relationship among the “FI” or the “FC” and the levels of four factors were studied experimentally. The factors chosen were the variety of tobacco (Flue-cured V1 and Domestic V2 as typical ones in Japan), the leaf position on a stalk (p), the length of shreds (S) and the moisture content of tobacco (w). The results obtained were as follows: (1) When the “w” increased, the value of “FI” increased, and the “FI” of “V2” was greater than that of “V1”. As compared with these variations of “FI” with “w”, the variation of “FI” with “p” or “S” was still small. (2) The coefficient of variation of “FC” increased, if there was a friction, at every level of any of factors of“p”, “S” and “w” for both “V1” and “V2”. (3) The bias of “FC” caused by the friction changed diversely depending upon the level of every factor of “p”, “S” and “w”, as well as upon the variety of “V1” or “V2”. (4) In conclusion. the “FC” should be measured under the state without any friction between tobacco shreds and its container.