Flaking failure of Si3N4 from a pre-indentation was investigated under lubricated ball-on-plate rolling contact. SEM observations were made both on the contact surface and on the longitudinal section along the rolling direction before and after the flaking failure. Multiple semicircular arc cracks were observed on the contact surface before the failure. A growing main crack with branching cracks was observed on the longitudinal section. It was found that the subsurface main crack branches upward and downward at the crack tip, and the upward branching cracks grow faster than the downward branching cracks. The flaking layer was never separated by the upward branching cracks until the main crack grew into a threshold length.
In order to investigate high cycle long life fatigue characteristics, cantilever-type rotary bending fatigue tests were conducted in an open environment at room temperature using hourglass shaped specimens of high carbonchromium bearing steel. The S-N curve obtained was clearly classified fracture modes into two groups of different crack origin. One was governed by a crystal slip on the specimen surface which occurred in the region of short fatigue life and high stress amplitude level. The other was governed by nonmetallic inclusion at subsurface which occurred in the region of long fatigue life and low stress amplitude. The inclusion governed fish-eye fracture mode was distributed in a wide range of stress amplitude not only below the fatigue limit defined as the threshold of fracture due to the slip governed mode but also above the fatigue limit. This remarkable shape of the S-N curve was different from a step-wise one reported in previous literature and was characterized as duplex S-N curves which composed of two different S-N curves corresponding to the respective fracture modes. An examination of the initial stress intensity factor range of fracture origin, ΔKini, showed that the ΔKini of the slip governed fracture mode was larger than the threshold stress intensity factor range of the surface crack growth, ΔKth. On the other hand, cracks originate and propagate from inclusion at subsurface with smaller ΔKini than the slip-originating crack and the ΔKth.
Fatigue properties of two kinds of ferrous powder metals, which were produced from partially prealloyed iron powders and whose densities are 7.10g/cm3 and 7.35g/cm3, were investigated by tension-compression fatigue test. Main fatigue cracks did not initiate from pores but from slip bands in soft Fe particles in both these materials. The crack nucleated at the maximum Fe particle behaved as a defect larger than the maximum pore and accordingly the maximum size of the Fe particle existing on specimen surface was the crucial factor determining the fatigue strength. In order to confirm this fact more exactly, the maximum Fe particle size was evaluated by the statistics of extremes, and on the other hand an artificial hole with various sizes was introduced onto the specimen surface and the influence of the hole was investigated. The prediction of the maximum size of Fe particle by the statistics of extremes was in good agreement with the critical size of the artificial hole which was detrimental to fatigue strength. These facts explain that the difference in the fatigue strength between two materials was not caused by the difference in densities, i.e. pores, but by the strength properties of the microstructures such as Vickers hardness which was measured in details from the core to boundaries of grains.
In order to examine the mechanism of so-called fish eye-pattern fatigue failure, usually being caused by initiation and growth of internal small crack, uniaxial push-pull fatigue tests were carried out on a high strength low alloy steel in vacuum, for modifying the environment during internal crack initiation and growth on specimen surface. The test results showed that the fatigue life was elongated and the crack growth rate was delayed in vacuum. Furthermore, the results of fractographic observation of crack initiation at inclusions revealed that the surface type crack initiation in vacuum resembles the internal fish eye-type crack initiation in atmosphere. Finally the fish eye-type fatigue life was well evaluated in terms of small crack growth law in vacuum.
Since in superlong fatigue failure with Nf≥108, the average fatigue crack growth rate is much less than lattice spacing (-0.1Å or 0.01nm or 10-11--12m/cycle), we cannot assume that crack growth occurs cycle by cycle in the early stage of fatigue process. In this paper, possible mechanisms for extremely high cycle fatigue are discussed. A special attention was paid to a newly found particular fatigue fracture morphology in the vicinity of fracture origin (nonmetallic inclusions) of a heat treated alloy steel, SCM435. The particular morphology looks a dark area inside fish-eye mark by optical microscopic observation. Specimens with short fatigue life of N=-105 do not have such dark area in fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from usual fatigue fracture surface in martensite lath structure. The predictions of fatigue limit by the √area parameter model are -10% unconservative for fatigue limit defined for N=107. Thus, the fatigue failure for N≥108 is presumed to be caused by a mechanism which induces breaking or releasing of fatigue crack closure phenomenon in small cracks. The breaking or releasing crack closure mechanism is presumed to be caused by environmental effects such as hydrogen embrittlement coupled with extremely high cycle fatigue. Some indirect evidences to support this hypothesis are shown.
X-ray fractography is a technique for analyzing the cause of fractures through the information obtained by irradiating X-ray on the fractured surface. The residual stresses or full widths at half maximums are usually used as the information. This technique is a useful tool for fractrure analysis, since the quantitative loading condition etc. can be estimated. X-ray fractography is used in various countries alongside the electron-microscope fractography. In the previous paper, X-ray fractography has been applied to actual size four “Shinkansen (bullet train)” axles tested at the railway Technology Research Institute. The axles are about 200mm in diameters, induction hardened 0.38% carbon steel with semi-elliptical artificial flaw. X-ray stress measurements were carried out on the fractured surface along the line from the bottom of the artificial flaws to the axle center at intervals of 5mm. All the results showed that the residual stress on the fractured surface were tensile at the initial crack propagation region, increased to tensile maximums, decreased into compression reaching at the compressive maximums then went up toward zero stress near the final fractured surface. This paper describes the procedure estimating the crack propagation direction through rosette analyses of residual stresses. A CCT specimen, which was tested under similar stress condition as test axles, was analyzed and found out that crack propagation directions are coincide with the directions of residual principal stresses. This relationship was utilized in rosette analyses on fractured surface of a test axle, and crack propagation lines were estimated and also the crack closure behavior was discussed.
Hard shot peening (HSP) treatment was very successful for the improvement of the high cycle fatigue strength of SUS316L steel. The improvement of the fatigue strength was caused by a hardening (a strain hardening) and a compressive residual stress of surface hardened layer which have a thickness of about 500μm, and was caused by change from surface crack origin fracture to subsurface crack origin fracture. In this study, the estimation of fatigue limit at about 108 cycles was attempted by using the hardness distribution, the HVB distribution and the residual stress distribution. By regarding the residual stress as the mean stress and by transforming from the HVB distribution to the fatigue strength or the tensile strength, the local fatigue strength distribution of HSP-treated SUS316L steel was calculated using the endurance fatigue limit diagram and the modified Goodman's diagram (the general method). As the result, fatigue limit at about 108 cycles was not able to estimate by these general method. But, from the proposal method with considering the stress intensity factor, the estimation of fatigue limit was fairly good coincident with the experimental fatigue limit.
A fatigue damage rule of a structural steel based on plastic strain ranging from high-cycle to low-cycle fatigue regime is proposed, especially paying attention to load sequence effect. Four types of fatigue tests including i) constant strain amplitudes, ii) mean strain amplitudes, iii) surplus and iv) reversible strains against the foregoing half cycle were conducted. The fatigue damage was evaluated at each half cycle, so that a loading sequence may be evaluated in a unique manner. The fatigue damage rule was optimized for these fatigue tests and also compared to rules by other researchers. The load sequence effect (Hi-Lo or Lo-Hi) was found to exist but small in strain criterion compared to that in stress criterion. A step-wise strain increasing test was compared to a step-wise stress increasing test and resulted that coaxing effect was not big under strain controlled condition.
Two functionalization technichques are applied to the fatigue data base edited by the Society of Materials Science, Japan. Among many data, 273 sets of S-N curves of rotary bending test data of steels in S-N forms, were chosen for functionalization, expecting better usage of a data base on three points; i) to estimate a S-N curve of a material whose test conditions or chemical composition is not included in the data base, ii) to express the reliability of data in terms of standard deviation, iii) to detect miss-inputted data. A S-N curve is expressed in three functions with 16 variables (twelve chemical components, radius of specimen, stress concentration factor, testing frequency and tensile strength). One of the functionalization technique is a statistical binary regression analysis, and the other is an interpolating technique in a higher dimensional space. The standard deviation of data points from the regression funtion was bigger than that from the interpolated function. The regression function showed divergent tendencies within the range of analysis. Some examples of the interpolated function projected to lower dimensional spaces were illustrated to show reasonable results. The shape of interpolated curved surface is analyzed at a structural steel point, to detect effective or non-effective factors to shift the S-N curve among 16 variables. The interpolation showed a good technique to functionalize a data base.
In order to investigate Mode II and III besides Mode I fatigue crack propagation in grain oriented silicon iron thin plates with coarse grains, some experiments were carried out, selecting favorable directions of loading and starting notches for these modes. Mode II crack growth was expected by loading parallel to rolling direction of specimens with starting notches along a preferential slip direction, however, Mode II crack growth along this direction could not continue beyond about one mm in any case and the crack suddenly bent 110° to conjugate preferential slip direction. Striation-like patterns with a few μm spacing, which corresponded to crack growth during hundreds cycles, were observed in these fractographs. These results suggest existence of some substructures caused by cycling slips on one direction during hundreds cycles which may arrest continuous crack growth on one plane. The tensile test loaded perpendicular to rolling direction showed enough out of plane shear deformation and fracture ductility. Mode III crack growth was expected by loading perpendicular to rolling direction of specimens, but Mode I crack growth with brittle striation occurred and any shear lips could not be observed.
Several concepts concerning with fracture mechanics of materials having microstructures have been considered in last three decades. Especially, Eshelby's proposal of the elegant treatment of the generalized forces which act on elastic singularities made great contribution of this research field. Along with this conception, it is expected that the energy release rate for the material having microstructures can be described by using the generalized forces which act on singular stress fields at crack tip and around lattice defects at crack tip vicinity. This conception was first proposed by Ohnami. Through the series of studies, Ohnami and coworkers suggested that the stress singularities due to lattice defects at crack tip vicinity can be described by the dislocation density tensor αij and the Rieman-Christoffel curvature tensor Rijkl which is equivalent to the disclination density tensor θij. On the other hand, Klüge and Günther introduced the tensors αij and θij into Cosserat continuum to develop the lattice defect theory in it. The objective of this study is to derive the mathematical formulation for Ohnami's conception with the aid of lattice defect theory developed by Klüge and Günther. The energy release rate was described as the sum of resultant generalized forces which act on singular stress fields at crack tip and around lattice defects at crack tip vicinity in Cosserat continuum.
Poly [trioctyl (4-vinylbenzyl) phosphoniumchloride] (P-8CMS) is an amorphous polycation which has a superior antibacterial activity. However P-8CMS easily absorbs moisture due to the cationic character, resulting in the reduction of Tg to temperatures lower than room temperature. Because of this reduction of Tg, P-8CMS shows poor mechanical properties at room temperature. To solve this problem, fibers of P-8CMS/high density polyethylene (HDPE) blends were prepared. Morphology of the blend fibers was strongly related to the viscosity ratio of HDPE to P-8CMS, compatibility between HDPE and P-8CMS, and the spinning condition. When HDPE has a melt viscosity higher than that of P-8CMS, P-8CMS component covers the surface of the fiber, while P-8CMS is dispersed in HDPE matrix when HDPE has a lower viscosity. P-8CMS tended to disperse finely in Maleic anhydride-grafted-HDPE. These morphologies reflected the antibacterial activity of the blend fibers.
The transmission rate of impact stress for the jointed two plates were investigated by strain gage method. Impact force was applied by a pendulum to the PMMA (polymethylmethacrylate) plates jointed as simple butt contact, butt bonding, uniformed lap bonding, terraced lap bonding and lap bolt joints. The transmission rate, Tr, is defined as Tr=εt /εi, where εi and εt were the strain amplitude measured in the incident plate and the transmission plate, respectively, and both strains were measured just after the arrival of reflected wave at these measuring points, respectively. The transmission rate in the case of lap bonding joints was lager than that of simple butt contact, however the transmission rate did not always increase with increase of bonded width. The transmission rate in the case of bolt joint was lower than that of lap bonding one for the same lap width. These results were discussed based on numerical analysis. Influence of anti-plane bending was also discussed.
Mullite/SiC composite ceramics was sintered. Three point bend specimen was made according to JIS standard. Semi-elliptical surface crack of 100μm in diameter was made on the specimen. By using four kind of specimens (as-received, heal treated as-received, pre-cracked and pre-crack healed), cyclic fatigue and static fatigue strength of the specimen were tested systematically at room temperature. The main conclusions obtained are following: (a) Mullite/SiC composite ceramics has ability to heal crack. (b) Crack healed specimen (heal treated as-received and pre-crack healed) showed higher cyclic and static fatigue strength than as-received specimen, and this fact was caused by crack healing. (c) Crack healed part has enough fatigue strength and most fracture occurred outside the pre-cracked zone in pre-crack healed specimen.
The Great Hanshin Earthquake had affected seriously on many highway bridges in Hanshin area. From a lot of investigations about the failure mechanisms on the highway bridges during strong earthquake, it was found that the consideration as a highway bridge system which consists of many structural elements such as piles, footings, piers, bearings, restrainers, super structures, etc. is very important to improve the practical design procedure. In this paper, an evaluation method for multiple damage states of bridge structural system due to strong earthquake is proposed.Then, the bridge structures were divided into each structural element from a systematic point of view, and these elements are interacted each other. Therefore, probability of multiple damage states of structural elements are evaluated by Marcov-chain model based on a damage transition probability matrix which can include damage interaction between the elements. The probability of multi-damage-states for the bridges with countermeasures against earthquakes such as strengthening work for pier, base isolation and fuse type bearing is calculated, and improvement in earthquake resistance of these bridges was estimated quantitatively.
The paper reports the fundamental performances of light-weight concrete with density of 1.4, based on an experimental study on the properties of composite slabs with trussed steel decks using superlightweight concrete under bending moment. The test results by full scale specimens showed that the composite slab using trussed steel decks and the super-lightweight concrete had the same capacity with the composite slab using normal concrete in strength, deformation, and crack propagation, It was concluded that the calculated values by the conventional theoretical method showed good agreement with the experimental results.
In 1940, Kennedy found that the workability of concrete depends on the thickness of the excess cement paste, which is defined as the difference between the volume of the cement paste and the volume of the voids in the dry rodded mixed aggregate. It is thought that the mix design of concrete in fluid can be established by investigating how each factor of the mix design affects the consistency of concrete. The purpose of our research is to get a fundamental knowledge for the establishment design of high-fluidity concrete based on rheology. In this study, fresh concrete is considered to be in a two-phase flow consisted of aggregates and cement paste. As a result of experiment, the rheological constants, plastic viscosity and yield value, of high-fluidity concrete could be estimated by using the relative thickness of excess paste.
The duplex stainless prestressing steel has been developed for durable prestressed concrete members. This steel is cold drawn and has microstructure with two phases composed of ferrite (α) phase and austenite (γ) phase. The duplex stainless steel is excellent in durability. But there are few studies about cold drawn duplex stainless steel compared with those for annealed duplex stainless steel. So we have made research on both the basic properties of the duplex stainless prestressing steel and flexural behaviors of prestressed concrete beams using this tendon. This paper presents the results of tensile test, fatigue test, stress relaxation test, corrosion test and pull-out test, and further flexural test of the prestressed concrete beams using the tendon. The outlines of these results are as follows. The properties of fatigue and stress relaxation of the duplex stainless prestressing steel were roughly equal to those ordinary high carbon prestressing steel, and had satisfactory resistance to stress corrosion, chloride corrosion and satisfactory bond strength to concrete. The prestressed concrete beams using this tendon showed similar flexural behaviors to the concrete beams using the ordinary high carbon prestressing steel tendon. The duplex stainless prestressing steel is small in the elongation at ultimate strength and excellent in durability. Therefore, the tendon is suitable for the external cable system in which the cable tension is not expected to increase so large as in the internal cable system and durability is required. However, the behaviors of the cable at the saddle must also be investigated. A defect of the duplex stainless prestressing steel compared with high carbon prestressing steel is that the tensile strength is low.