In order to investigate the relationship between creep-fatigue interaction and creep damage mode of SUS 321 fine- and coarse-grained steels, the combined creep-fatigue loading tests were conducted. The creep damage, φc, and the fatigue damage, φf, which were accumulated till rupture in these tests were calculated using a linear life fraction damage rule. The φcvs. φf relation obtained was described by the following equation: (φc-φc0)+α[(φc-φc0)(φf-φf0)]0.5+(φf-φf0)=1 where the values of φc0, φf0 and α depend on the creep damage mode corresponding to the creep loading condition in the combined creep-fatigue loading test. For the creep loading condition corresponding to transgranular fracture mode in creep test, the φcvs. φf relations of fine- and coarse-grained materials showed a convex form, but the value of φc0 was different between these two materials. The value of φc0 was dependent on cyclic hardening and softening behaviours of the materials under fatigue loading and on the sensibility of intergranular fracture of the materials tested. The φcvs. φf relation became φc+φf-1 under the fracture mode condition where the micro cracks were formed along the grain boundaries and the surface crack propagated through the micro cracks.
In order to predict the deformation characteristics from the results of soundings and convenient soil tests, relationships between the deformation characteristics and physical-mechanical properties were studied by using multiple regression analysis in which the soil test results for the samples obtained from clay deposit in Osaka and Hyogo prefectures were used. This analysis is based on the concept that the mechanical properties of soils are characterized by their kinds and states. The conclusions are summarized as follows. (1) The validity of the concept employed here can be quantitatively proved by the multiple regression analysis. (2) The deformation characteristics of soils, which are the modulus of deformation, consolidation yield stress and compression index, are predicted more accurately by using both the shear strength and physical properties of soils. (3) The characteristics of e-log p curves are explained reasonably by the results of the multiple regression analysis in this study.
A support post, which is used as a parmanent core-support component in the High Temperature Engineering Test Reactor (HTTR), is a cylindrical graphite component with spherical contact and one of the most essential elements for the integrity of the core. Therefore, the support post was examined to verify its structural integrity using full-scale and 1/2-scale models. Moreover the elastic-plastic stress was calculated by the FEM code “ABAQUS” under a two-dimensional axissymmetric condition. The following conclusions were mainly obtained; (1) The inclination angle of support post has little effect on the fracture load in the range from 0 to 1 degree. (2) The strength of support post was not affected by the cyclic loads which take account of the seismic load. (3) The fracture load of the support post which was oxidized to simulate the air ingress accident in the HTTR is sufficiently larger than the maximum design load. (4) It is confirmed from these results that the support post has a sufficient structural integrity.
The effect of mean stress on growing surface fatigue cracks in plates of annealed 0.15%C steel was studied under cyclic tension and compression loading and the crack tip opening loads at local area along the crack front were measured by an ultrasonic angle beam method (UAM). The crack was semielliptic and had a smaller aspect ratio under compressive mean stress than under tensile mean stress. The crack growth rate in the depth direction was higher than in the surface direction for the same ΔK-value, when the mean stress was tensile (R=0). When the mean stress was compresive (R=-2), the former was lower than the latter. For the same ΔKeff-value assessed on the crack tip opening load measured by UAM, not only the dependence of crack propagation rate on the mean stress but also the difference between the crack propagation rates in the surface and the depth directions almost disappeared. Prediction of aspect ratio variation of growing surface fatigue cracks in tension-compression loaded plates was made with the local ΔKeff-values and with the averaged ΔKeff-values obtained by a weighted averaging along the crack front. The averaged ΔKeff-value method overestimated the crack aspect ratio when the mean stress was tensile (R=0) and zero (R=-1). Estimation by the local ΔKeff-value method was fairly accurate irrespective of mean stress value.
A procedure to evaluate residual life under cyclic combined loadings was established for a given initial state of distributed small cracks. Distributed cracks at the initial stage were modeled as straight-line cracks by using an image-processing technique. The algorithm for the analysis of the crack growth after the initial stage was constructed by taking account of both modes of the propagation as a single crack and the coalescence between propagating cracks. Fatigue tests under combined axialtorsional loadings with constant and variable amplitudes were also conducted using cyclindrical specimens of pure copper with circumferential blunt notches. The fatigue life was correlated with the equivalent plastic strain range. When Compared for the same value of the parameter, the fatigue life became longer with increasing shear component in the stress state at the notch root, while no significant difference was observed between two loading modes of constant and variable stress amplitudes. The fatigue life defined by the formation of crack with a specific length was evaluated based on the proposed procedure. The predicted life almost coincided with the experimental data. Cracking morphology was also simulated by using the present model to show good correspondence with experimental observations.
The break-down welded structure is usually caused by fatigue fracture, starting from the toe of weld. This happens at the welded portion of truck chassis frame and Rear Axle Case. To improve their fatigue life, the following measures are usually taken: grinding, remelting, shot blast and shot peaning. However, these measures have some problems from the viewpoint of working conditions. It is known that crack propagation rate is lesser in vaccum than in air. It is also considered that this fatigue life improvement is larger when stress consentration factor is larger. Epoxy resin may be used to examine this phenomenon by coating it on the welded portion, but the coated resin also affects the deformation of crack generation area. So, the effect of epoxy resin coating on fatigue fracture strength was investigated in this paper.
Epoxy resins for encapsulating integrated circuit (IC) devices are filled with silica particles to reduce thermal expansion coefficient and to improve thermal conductivity. Since these particles occupy roughly 60-70% of the total volume, they have a significant effect on the fracture properties of the resins. This study reports the effects of filler particle shape and size on the static strength of smooth specimens and crack propagation of highly silica particulate-filled epoxy resins for IC encapsulation. The smooth specimen strength, fracture toughness and fatigue crack propagation rate were measured, respectively, by the three-point bending test, the double torsion test and the cyclic tensile test of singleedge notched specimens. The effect of particle shape was examined using one type of irregularly shaped and two types of spherical particles. It was found that large particles lower the static strength, while large and also irregularly shaped particles improve resistance to crack propagation. Although large filler particles cause cracks to originate at the smooth specimen surface, they help to resist crack propagation.
Fatigue crack growth behavior of continuous fiber reinforced composite materials was studied under variable stress conditions. Two kinds of laminates with different ductility were employed: a hybrid laminate of glass fiber-reinforced epoxy and 7075-T6 aluminum alloy (GLARE1) and a plain-weave roving fabric laminate of glass fiber-reinforced epoxy (GFRP). Fatigue tests of center-notched (GLARE1) and double-edge-notched (GFRP) plate specimens were performed for the cases of stress-increase and stress-decrease, under repeated tension-tension and alternating tension-compression loading conditions. For GLARE1, acceleration and retardation of crack growth were observed under repeated tension-tension loading condition, while they were not under alternating tension-compression loading condition. The fatigue crack of this material propagated only in the aluminum layer, and its growth rate was not adequately correlated with the stress intensity factor range ΔK. It was found that the crack growth rate was successfully correlated with an effective stress intensity factor range ΔKeff defined taking into account both the crack-closure and the fiber-bridging in the crack wake so that ΔKeff=ΔKbr-ΔKcl. The acceleration and retardation were explained using this parameter. For GFRP, on the other hand, no appreciable effect of the stress change on fatigue crack growth was detected. This was attributed to the brittleness of this laminate. The crack growth rate, in this case, was correlated well with the stress intensity factor range.
The effects of stress ratio on tension-tension fatigue behavior and mode I delamination fatigue crack propagation behavior were investigated on two kinds of unidirectional CF/epoxy laminates (Toray T300/#3601 and T800/#3631). The analysis of the equivalent stress intensity range proposed by the authors showed that the contribution of maximum stress was large in delamination fatigue crack propagation. A similar analysis was applied to the results of the tension fatigue tests. The contribution of maximum stress was also large in tension fatigue fracture. The tension fatigue fracture was mainly caused by longitudinal fracture. The similarity in fracture surface between tension fatigue fracture and delamination fatigue fracture for the T300/#3601 laminates was indicated by microscopic observations. For the T800/#3631 laminates, there was a difference between the fracture mechanism of tension fatigue fracture and that of delamination fatigue fracture. The degree of the contribution of maximum stress was correlated to the fracture mechanisms of tension fatigue fracture and delamination fatigue crack growth.
Behavior of delamination crack growth in a carbon fiber reinforced plastic (CFRP) was investigated under static creep, two step creep and high temperature fatigue at 473K (200°C) using DCB specimens. The material was a unidirectional laminate, APC-2, which consists of carbon fibers, AS4, and a thermoplastic polymer, poly-ether-ether-ketone (PEEK). In the static and two step creep tests, the crack propagation rate against time, dl/dt, was governed by the energy release rate, G, regardless of the stress change. The crack propagation in high temperature fatigue was classified into time-dependent and cycle-dependent ones. In the former, the propagation rate against time, dl/dt, was correlated well with the energy release rate, G, and the relationship coincided with that in static creep (dl/dt=CcGmc; Cc and mc are material constants.). In the latter, the rate against load cycles, dl/dN, was controlled by the energy release rate range, ΔG(dl/dN=CfΔGmf; Cf and mf are material constants.). These imply that the crack grew under the small scale creep and small scale yielding condition, respectively. The condition of transition from the time-dependent crack propagation to the cycle-dependent one was given by a relationship, Cc∫t10Gmcdt=CfΔGmf, where t1 is the cycle period. The fracture surface in the time-dependent fatigue was characterized by the interface cracking between fibers and matrices. On the other hand, the matrix cracking near the fiber dominated in the cycle-dependent fatigue.
This paper examines the strain controlled fatigue-creep interaction (FCI) life of directionally solidified (DS) superalloy René 80H. The author previously proposed a frequency-modified total strain energy parameter, obtained from the total tensile strain energy and tensile loading time, for use in evaluating the failure life of equiaxial Ni and Co based superalloys under FCI conditions. The present paper examines the applicability of this parameter for predicting the FCI life of DS René 80H. The FCI life prediction by the total strain range was first evaluated. It was found that the FCI life of the specimens cut in parallel to the DS direction of a directionally solidified plate was longer than that of the specimens cut in transverse to the DS direction. In addition, the specimen life under tensile hold time conditions was longer than under compressive hold time conditions. The FCI life prediction by the frequency-modified total strain energy parameter was evaluated next. For the specimens in both directions, there is a linear correlation between this parameter and the cycles to failure. Therefore, it is concluded that the FCI life prediction for DS René 80H can be made by using this parameter.
In order to establish a multiaxial low cycle fatigue criterion which can evaluate the fatigue life of high temperature components, biaxial low cycle fatigue tests of 304 stainless steel have been carried out with the cruciform specimens at 550°C under proportional and nonproportional loading conditions. A multiaxial low cycle fatigue criterion and its application to fatigue damage evaluation in design have been discussed based on the present test results and the previous results which were produced by tension-compression and torsion tests at elevated temperatures with the tubular specimens. Configuration of iso-fatigue life contours of the Case B loading on the Γ-plane in which the maximum shear planes are on planes inclined 45° from the surface was different from that of the Case A loading in which they are on planes 90° from the surface, because of difference in fracture mechanism. The influence of normal strain in the maximum shear plane, εn, on fatigue life was not observed in the Case B loading, while the fatigue life was strongly affected by εn in the Case A loading. From iso-fatigue contours on the Γ-plane, the equivalent shear strain range, Δγ for the Case B loading was newly defined. It was confirmed that the biaxial fatigue life data of proportional and nonproportional loadings could be correlated well with Δγ and a fatigue damage evaluation procedure by applying Δγ in design was proposed.
The fretting fatigue tests under various levels of contact pressure were carried out at both room and elevated temperatures using 12Cr-Mo-W-V steam turbine steel in order to investigate the effect of contact pressure on fretting fatigue properties such as fatigue life and fatigue strength. Two step tests, where fretting action is removed at certain cycles, were also carried out at elevated temperature to investigate the effects of contact pressure on fretting fatigue behavior and propagation rate of fretting fatigue crack. Although the fretting fatigue strength decreased with increasing contact pressure, it attained a constant value when the contact pressure was higher than 100MPa. The fretting fatigue crack initiated at the early stage of life and its growth rate was accelerated by fretting action. This acceleration of fatigue crack was significant in the case of the higher contact pressure of 300MPa. The effect of fretting on fatigue life seemed to appear within 30% or 40% of fretting fatigue life regardless the contact pressure level and test temperature. This result was explained by comparing two fatigue crack growth curves with and without fretting.
Poly (methyl methacrylate) (PMMA)/carbon black (CB) composites were prepared by thermal molding after mixing of PMMA and CB by a vibration ball mill. The electrical conductivity, dielectric constant and mechanical tanδ of these composites were investigated as functions of CB content, temperature and degree of distribution of PMMA particle size. The observation using a microscope revealed that the increase of CB particles on the surface of PMMA particles resulted in the formation of conductive chains from the aggregation state, which leads to the formation of a network structure in the composite film. The critical volume fraction of CB (Vc) corresponding to the jumping of conductivity increased with the decrease of the degree of distribution of PMMA particle size. At a small CB content, the mobility of the matrix increased inspite of mixing, which was assumed to be due to the action of CB as a plasticizer. Besides gaining about 1010S·cm-1 of conductivity, PMMA/CB composites appear to retain the mechanical characteristics of PMMA owing to the composite effect with CB particles.
Crystal structure and residual stresses in AlN films deposited on BLC glass substrates by a magnetron sputtering method were measured by X-ray diffraction method. Deposited AlN films had a columnar structure with its ‹00·1› orientation being perpendicular to the glass substrate. From this structure, the intensity of hk·l diffraction could be measured just only at a particular ψ-angle and, therefore, the so-called sin2ψ method could not be applied for measuring their residual stresses. A new stress analyzing method was proposed in the present investigation instead of the sin2ψ method. Compressive residual stresses were found in the AlN films deposited under the condition of fairly high nitrogen gas pressure, while tensile residual stresses were found under the condition of low gas pressure.
The X-ray diffraction method was applied to measure the applied and residual stress in WC-Co alloys with various cobalt contents. The X-ray diffractions of WC 112 and Co 311 by Fe-Kα radiation were used for stress determination. The mean stress in each phase increased linearly with the applied stress; the stress in WC phase was larger than that in Co phase at the same strain. The macrostress calculated from the phase stresses by using the rule of mixture was nearly equal to the applied stress. The theoretical analysis based on the self-consistent model of elastic deformation of composites agreed fairly well with the experiments. The compressive residual stress measured on the ground surface was larger than that on the lapped surface, and decreased with increasing Co volume fraction. Annealing at 800 and 1000°C greatly reduced the compressive residual stress. The residual microstress after annealing was compression in WC phase and tension in Co phase.