山崎 泰広

Herein, fatigue and crack propagation tests were conducted using the small-punch-fatigue (SPF) test to verify its applicability to assess fatigue strength and fatigue crack propagation behavior. During the SPF test, a fatigue crack first initiated at the specimen center and then propagated in the radial direction. In most cases, three fatigue cracks propagated from the specimen center to the radial direction in point symmetry. X-ray computed tomography revealed that the crack initiated by the SPF test had a quarter ellipse shape. Elastic finite element (FE) analysis was conducted to estimate the stress state and stress intensity factor (SIF) of the specimen, and an approximate equation was obtained as a function of the applied load. The experimental results comprising fatigue lives and crack growth rates obtained via the SPF test were compared with the conventional uniaxial test data as functions of the stress amplitude and SIF determined via elastic FE analysis, respectively. This comparison showed that the SPF test can be applied to the fatigue test and fatigue crack propagation test.

The suspension plasma spray (SPS) method is expected to become a novel coating method because it can achieve various microstructures using a suspension with submicron spray particles. Thermal barrier coatings (TBCs) with a columnar structure, which might achieve high strain tolerance, can be obtained using the SPS technique. This study evaluated the internal stress distribution of the suspension-plasma-sprayed thermal barrier coating (SPS-TBC) with different columnar structures using hybrid measurement using high-energy synchrotron X-ray diffraction analysis and laboratory low-energy X-rays. The relationship between the microstructure and the internal stress distribution of the SPS-TBC was discussed on the basis of the experimental results. In addition, the in-plane internal stress was decreased by decreasing the column diameter. The thin columnar microstructure of the SPS-TBC has superior strain tolerance. The internal stresses in the SPS-TBC are periodic decrements caused by stress relaxation in porous layers in its column.

The suspension plasma spray (SPS) technique has recently attracted attention due to the variety of microstructures achievable using submicron spray particles mixed with a solvent to form a suspension. Thermal barrier coatings (TBCs) with a columnar microstructure can be obtained using SPS. Because of its unique columnar microstructure achieving high strain tolerance, a top coat sprayed using this technique can efficiently suppress thermal cycling damage in SPS–TBCs. However, there are few reports on the effect of microstructure on the damage behavior of SPS–TBCs with a columnar structure. In this study, the effect of the top coat microstructure on the thermal cycling fatigue of SPS–TBCs was investigated. The size of the complex oxide formed on the top coat–bond coat interface and the length of the cracks at the interface (formed as a result of thermal cycling) increase with the column diameter in the top coat of SPS–TBCs. The thermal spray conditions affect the thermal cycling fatigue life of SPS–TBCs. The SPS–TBC with fine columnar structure exhibits superior thermal cycling fatigue life.

The thermal stresses of the thermal barrier coatings (TBCs) under cyclic heating were measured using hard synchrotron X-rays of 70 keV at the beam line BL02B1 in SPring-8. We were able to quickly measure the unsteady stresses using the CdTe pixel detector which is an area detector. The two kinds of the TBC specimens were prepared. One was made by atmosphere plasma spraying (APS) and the other was made by suspension plasma spraying (SPS). The steady thermal stresses in both the APS and SPS specimens showed the same value during the heating and cooling processes. Therefore, the steady thermal stresses were caused by the miss match of the thermal expansions. The unsteady thermal stresses were larger than the steady stresses in both the APS and the SPS specimens. The unsteady thermal stresses also include the stress caused by the temperature gradient. According to the behavior of the unsteady thermal stresses measured, the thermal stress of the APS top coat did not exceed 100 MPa due to non-elastic deformation an sliding effect of lamella structure. For the SPS specimen, it was found that the tensile and compressive stresses were relaxed due to the columnar structure.

To investigate the mechanical properties of a suspension plasma-sprayed (SPS) thermal barrier coating (TBC) with a unique cauliflower-like columnar structure, shear and cantilever bending tests were conducted on its single submillimeter-sized columns. In the shear test, the fracture of the single columns occurred at the porous layer near the top-coating/bond-coating interface. Compared with conventional atmospheric plasma-sprayed (APS) TBCs having a lamellar microstructure, the cantilever bending test revealed a significantly low Young's modulus along the out-of-plane direction and strong anisotropy in the elastic modulus of the SPS TBC. The shear strength and out-of-plane Young's modulus of the single columns increased due to the sintering resulting from thermal aging; however, the increasing ratio of the shear strength was higher than that of the Young's modulus. Both experimental and finite element analysis results indicated that the mechanical properties of the SPS TBC, as well as their variation with thermal aging, are dominated by those of the porous layer within the single column undergoing sintering. Moreover, the experimental results suggested that SPS TBCs having a cauliflower-like columnar microstructure have superior durability to thermal cycles compared with APS TBCs.

Low cycle fatigue (LCF) life at ambient temperature of Ti alloys is well known to decrease with stress dwell. This phenomenon, called cold dwell fatigue (CDF), is influenced by the peak stress, dwell time, and microstructure. For this study, the CDF life was evaluated by the linear cumulative damage rule. The influence of test conditions and microstructure on the linear cumulative damage rule was also verified. By the linear cumulative damage rule, when creep damage is calculated using the time exhaustion rule, theCDF damage was evaluated by the inequality of DTotal = (DF, DC) ≤ (0.01, 10−6). However, the CDF damage can be evaluated in the range of DTotal = 0.6–1.2 when creep damage was calculated using the ductile exhaustion rule. Results indicate that the evaluation was almost independent of the dwell time, peak stress, and microstructure, so it is also a versatile method for evaluating CDF responses.

© 2021 American Society of Mechanical Engineers (ASME). All rights reserved. Low-alloy steels are extensively used in pressure boundary components of nuclear power plants. The structural integrity of the components made of low-alloy steels can be evaluated by the flaw evaluation procedure provided by Section XI of the ASME Boiler and Pressure Vessel Code. According to the Code, the stress intensity factor range K can be used to determine the fatigue crack growth rates of the material. However, it has been reported that the fatigue crack growth rate under severe reversing loads is also strongly influenced by crack closure behavior. This paper discusses the relation between applied stresses and the fatigue crack growth rate for cracks in low-alloy steels exposed to air. Compressive-tensile cyclic loadings are applied to center-notched plates to obtain the fatigue crack growth curves. The test data demonstrate that effective stress intensity factor range predicted by our closure model described the crack growth property more accurately. A comparison among crack closure models indicates that our crack closure model is suitable to predict the crack growth rates when low constraint conditions are assumed at the crack tip due to severe reversing loads.

The thermal cycle fatigue behavior of a suspension plasma-sprayed thermal barrier coating (SPS-TBC) with cauliflower-like columnar microstructure was investigated. The damage behavior of SPS-TBC was investigated compared with a conventional atmospheric plasma-sprayed (APS-) TBC. The thermally grown oxide (TGO) grew at the interface between the top- and bond-coats during not only the isothermal oxidation but also the thermal cycle fatigue. In the SPS-TBC, the growth of mixed oxide-type TGO can be suppressed. Additionally, the columnar structure of the SPS-TBC can prevent the propagation of delam-ination crack from the specimen edge promoted by thermal cycles. These results suggested that the SPS-TBC had superior thermal fatigue resistance because of its columnar structure. (C) 2020 Elsevier B.V. All rights reserved.

The mechanical properties and fatigue strength of titanium-aluminide (TiAl) alloys are sensitive to the environmental conditions, such as temperature, and their microstructures can be controlled by thermomechanical processing. In this study, two samples of a forged TiAl alloy were manufactured through high-temperature forging followed by different heat treatments to obtain a near-lamellar microstructure and a triplex microstructure, which contains lamellar and equiaxed gamma and beta grains. The fatigue crack propagation tests were conducted under isothermal low-cycle fatigue (LCF) and the out-of-phase type thermomechanical fatigue (OP-TMF) conditions. The experimental results indicated that the microstructure strongly affects the crack propagation behavior because the near-lamellar microstructure had a higher resistance to fatigue crack propagation compared to the triplex microstructure. This also revealed that the fatigue crack was remarkably accelerated by the OP-TMF conditions compared to the LCF conditions. The oxygen diffusion into the beta phase occurred at the crack tip and lead to the transformation of the beta phase into the brittle alpha phase. The results of the scanning electron microscope (SEM), energy dispersive X-ray (EDX), and electron backscatter diffraction (EBSD) analyses indicated that this transformation induced the acceleration of crack propagation under the OP-TMF loading conditions.

The fatigue crack propagation tests for low alloy steel were carried out under the elastoplastic fatigue condition which doesn't satisfy the small scale yielding condition. The effects of the stress amplitude and the R-ratio of stress on the crack closure behaviour were discussed based on the results of the experimental results. The experimental results indicated that the effective stress intensity factor range which is the linear fracture mechanics parameter taking into account the crack closure concept more accurately described the fatigue crack growth property due to plasticity induced crack closure. The crack closure stress was affected by not only the maximum tensile stress but also the minimum (compressive) stress. Based on the experimental results, the crack opening stress equation for the elastoplastic fatigue condition was proposed.

In this work, the effect of superimposing of isothermal Low Cycle Fatigue (LCF) loading to the thermomechanical (TMF) fatigue loading on the short crack propagation behavior of low-carbon nitrogen-controlled 316 stainless steel is investigated. The experimental results indicate that the crack propagation path depends on the loading condition; cracks initiate and propagate at grain boundary perpendicular to the loading axis (intergranular mode), which is a relatively weak region, under the in-phase TMF loading and the LCF loading at high temperature. On the other hand, cracks initiate by the transgranular mode under the out-of-phase TMF loading and the LCF loading at middle temperature. The crack growth rate is also affected by the microstructure, i.e., the intergranular crack exhibits higher crack growth rate compared with the transgranular crack. In addition, the crack growth rate is accelerated by the superimposing of the isothermal LCF loading to the TMF loading. The crack growth rate can be predicted according to the summation law of crack growth behavior based on the fatigue J-integral approach taking into account the crack propagation path.

This study investigated the behaviors of short cracks in a single crystal Ni-base superalloy under the thermo-mechanical loading. The effect of temperature condition on the propagation behavior of short crack was discussed taking into account of crack closure. The experimental results indicated that the short crack growth rates under the higher temperature condition are remarkably higher than those under the lower temperature condition even if the crack closure and the temperature dependence on the deformation resistance take into consideration. Based on the crack propagation mechanism, the summation law of the crack propagation rates controlled with the mechanical and environmental factors was proposed to predict the fatigue short crack propagation rate. It was found that the experimental results can be predicted with good accuracy by the proposed method. 2019 The Authors. Published by Elsevier B.V.

Copyright © 2018 ASME. Low alloy steels are extensively used in pressure boundary components of nuclear power plants. The structural integrity of the components made of low alloy steels can be evaluated by the procedure of flaw evaluation provided by Section XI of the ASME Boiler and Pressure Vessel Code. According to the Code, the range of stress intensity factor ?K can be used to determine the fatigue crack growth rates of the material. However, it has been reported that crack closure behavior also strongly influence the fatigue crack growth rate under strong compressive load cycles. This paper discusses the relation between ?K and the fatigue crack growth rate for cracks in low alloy steels exposed to air. Compressive-tensile cyclic loadings were applied to center-notched plates to obtain the fatigue crack growth curves. The test data demonstrated that effective SIF range ?Keff more accurately described the crack growth property due to plasticity induced crack closure. Comparing the test results with the reference crack growth curves in the ASME Code Section XI, it may seem that the crack growth prediction based on the Code underestimates the crack growth rates for compressive-tensile cyclic loadings under high stress level.

© 2018 Trans Tech Publications, Switzerland. Metal/resin joints have been widely used for automotive, electrical device and others. The degradation of interfacial strength of the joints through the effects of moisture is one of the important deterioration mechanisms in their structure applications. In this study, the interfacial strength of an aluminum-alloy/epoxy-resin joint was evaluated by the indentation test using of the instrumented indentation machine developed by ourselves. The in-situ observations of delamination cracking were carried out during the indentation test. The interfacial fracture toughness of the joint was evaluated from the relationship between the indentation load and the crack length. The effect of immersion into service water on the evaluated interfacial strength of the joint was discussed.

© 2017 Trans Tech Publications, Switzerland. In this work, the initiation and propagation behaviors of naturally initiated small crack in a polycrystalline Ni-base superalloy under the several kinds of thermos-mechanical fatigue (TMF) conditions. The effect of phase angle between temperature and strain cycles in TMF loading on the initiation and propagation behavior of small crack was discussed. The experimental results revealed that the initiation and propagation morphologies of fatigue small crack were affected on the phase angle; the crack initiation mode was changed from the intergranular mode to the transgranular mode with the increasing of the phase angle in this study. On the other hand, when the crack growth rates of naturally initiated small crack are correlated with fatigue J-integral range, the effect of the phase angle on the small crack growth rates became negligible even if the crack propagation mode depended on it.

© 2017 Trans Tech Publications, Switzerland. Metal/polymer joints as well as metal/metal joints bonded by epoxy adhesive have been widely used for automotive, electrical device and others. In the present study, the adhesion strength of a stainless-steel/epoxy joint was evaluated by using of the shear test and the 4-point bending test. Three kind of the surface finishing condition of the stainless steel were selected, and the effect of the surface morphologies of the interface on the adhesion strength in the metal/epoxy joint was investigated. The relationship between the interface morphology and the loading mode in the metal/epoxy joint was discussed.

<p>The propagation behavers of naturally initiated small crack in a conventional cast polycrystalline Ni-base superalloy were studied under the thermomechanical fatigue conditions. The small crack propagation tests were carried out under the in-phase (IP) and the out-of-phase (OP) type thermomechanical fatigue (TMF) conditions as well as the isothermal low-cycle fatigue (LCF) condition at the maximum temperature of the TMF conditions. The experimental results revealed that the initiation and propagation morphologies of the naturally initiated small crack were affected by the thermomechanical loading condition. Under the LCF and IP-TMF conditions, the small cracks were initiated and propagated at the grain boundaries perpendicular to the loading axis. On the other hand, under the OP-TMF condition, the small cracks were initiated and propagated with the transgranular modes. When the crack growth rates of naturally initiated small crack were correlated with fatigue J-integral range, which is the continuum mechanics parameters for homogeneous and isotropic materials, the crack growth rates under the TMF conditions were almost similar to those under LCF condition even if the propagation morphologies of small cracks were varied with the thermomechanical fatigue conditions. In addition, it could be considered that the crack growth morphologies, grain structure and grain orientation affect the scatter of the small crack growth rate. The prediction method for TMF and LCF lives was proposed based on the small crack propagation considered with the crack opening-closing behavior. The TMF and LCF lives of Ni-base superalloys could be predicted with higher accuracy by the proposed method.</p>

For long life-time durability of advanced gas turbines, protective coatings, which shield the underlying substrate superalloys from oxidation and corrosion attacks, are essential requirements for the hot section components. Conventionally, MCrAlY (where M indicates Co, Ni or their combinations) is deposited on the substrate as the protective coating, as well as the bond coat of thermal barrier coating system, by using low pressure plasma spray (LPPS) and high velocity oxygen fuel (HVOF) processes. Recently, Cold spray (CS) technique has been investigated as the coating process for those coating. These coatings induce additional thermal stress due to the mismatch of thermal expansion coefficient between the substrate and the coating, which impel the substrate to be exposed to more severe conditions. Thus, thermo-mechanical fatigue strength is one of critical issue to determine the life. In this work, the damage behaviors by thermo-mechanical fatigue loading were investigated in a CS protective coated Ni-base superalloy IN738LC. The high temperature fatigue strength of protective coatings was affected by the coating process due to the residual stress induced by coating process.

Damage behaviors of a thermal barrier coated (TBCed) Ni-base superalloy IN738LC were studied under high temperature low-cycle fatigue (LCF) condition. In this work, the HVOF process was selected as the spray process for bond coat. The CoNiCrAlY alloy was coated by 100 μm in thickness as the bond-coat on the Ni-base superalloy IN738LC substrate. And the 8YSZ was coated by 250 μm in thickness as the top-coat on the bond-coat. The LCF tests of TBCed IN738LC were carried out at 900℃ in air. The LCF lives of the TBCed IN738LC were compared with those of bare IN738LC and metal protective coated IN738LC specimen. The results of the LCF tests at 900℃ indicated that the LCF strength of the TBCed IN738LC is better than those of bare and metal protective coated IN738LC. The effect of thermal oxidation on the LCF life of the TBCed IN738LC was also investigated. The effect of thermal oxidation on the LCF life of the TBCed IN738LC was not significant. It was revealed from an observation of the damage morphologies that the cracking morphologies by LCF loadings were changed by the thermal oxidation.

In this paper, an in-situ measurement of crack size as a function of applied indentation load during indentation test was conducted. To perform the in-situ measurement, an instrumented indentation test machine with the in-situ observation system was developed and used. The joints of transparent ceramics by diffusion bonding were prepared as the specimen used in this study. The indentations were performed at the interface of the joints, and in the monolithic transparent ceramics by means of the instrumented indenter with the in-situ observation system. The relationship between crack shape and indentation load, as well as, the effect of the indenter shape on it were discussed.

A new test method is proposed in this study to evaluate the adhesion strength of thermal spray coatings.The proposed method can evaluate the adhesion strength of coatings against to the shear loading acted at the interface by the compressive external loading.In order to verify the validity of the proposed method,the influence of the gage length on the evaluated shear adhesion strength was investigated.With the decreasing of the gage length of the specimen,the evaluated shear adhesion strength decreased and the dispersion of it increased.However,the dependence of gage length on the evaluated shear adhesion strength and the dispersion of it decreased with the increasing of the gage length.It was revealed from these experimental results that there are limits of the specimen gage length and the specimen width for the proposed test method.And to obtain the valid shear adhesion strength the specimen should have the larger gage section than a certain length and width which depend on the mechanical properties of coating and substrate.It was also discussed to expand the proposed test method to evaluate the interfacial fracture toughness and the resistance to the fatigue crack propagation at the interface.

The purpose of the present study is to develop a novel implant material which has a superior biocompatibilities and high strength by means of a coating technology.In this study,the cold spray technique was selected and used as surface modification method of a titanium alloy for the development of implant material;the porous pure titanium coating was sprayed on a titanium alloy,Ti-6Al-4V,by using the cold spray technique.The influences of spray conditions and post-sprayed heat treatments on the mechanical properties of the cold sprayed biomedical coatings were investigated.It was revealed from the experimental results that the developed biomedical titanium coatings had a low elastic modulus,high adhesion and enough tensile strength.Most important factor to get the superior biomedical coatings was the post-spray heat treatment because both the adhesion and cohesion strengths of the coatings could be improved significantly due to the post-spray heat treatment.It was also revealed that both the adhesion and cohesion strengths were improved by the recrystallization of nanoparticles generated by the high speed collision of particle during the cold spray process.The cold sprayed biomedical coatings developed in this work have enough fatigue properties.

Collaborative research has been conducted by the Japan Thermal Spray Society (JTSS) to establish a standard test method for evaluating the interfacial fracture toughness of thermal sprayed coatings, including thermal barrier coatings. The test method is based upon the indentation test method utilizing a conventional Vickers hardness test machine. In this committee, round robin tests were performed to check differences in the evaluated results among collaborators. This paper reports on the progress of such activity in Japan. © (2014) Trans Tech Publications.

In this paper, the crack propagation behavior of naturally initiated small crack in in low-carbon/medium-nitrogen 316 stainless steel under thermo-mechanical fatigue loading was investigated. The experimental results indicated that the importance of the investigation of small crack propagation behavior because the information on the basis of physically long crack growth rate provides a dangerous evaluation on reliability to actual components. The small crack exhibits high growth rate under the In-phase TMF loading because of irreversible creep and plastic strains. However, the growth rates of small crack under the Out-of-phase TMF loading were lower because the effect of creep deformation became negligible in such condition.

Adhesion is an important and basic property for thermal spray coatings. The standard tensile test method "ISO 14916" is usually used to evaluate the adhesive strength of coatings. On the other hand, the indentation test method has some advantages to evaluate the interfacial fracture toughness as the adhesive strength, arising from the following reasons: the test procedure and the specimen preparation are easy in comparison with the typical testing method. Collaborative research has been conducted by "Committee on Standard Development" in the Japan Thermal Spray Society to establish a standard test method for evaluating interfacial fracture toughness of thermal spray coatings using a conventional Vickers indenter. This article reports the differences among collaborators in round-robin tests performed in this committee and discusses the validity of the test method and test conditions with respect to the test results and finite element analyses. Comparison among collaborators reveals that interfacial fracture toughness can be obtained with a small scattering from the indentation test under constraints found on the basis of the results.

© 2012 Elsevier Ltd. All rights reserved. This paper deals with the adhesion strength of ceramic top coat in thermal barrier coatings (TBCs) subjected to thermal cycles under several different test conditions. Here the TBC specimens consisting of 8% yttria stabilized zirconia, CoNiCrAlY alloy bond coat and Ni-base superalloy were prepared by plasma spraying. The isothermal exposure and the thermal cycles were applied to the TBC specimens by several conditions at high temperatures. A series of the test results clearly demonstrated that the adhesion strength of the top coat was significantly changed by the application of thermal cycles and by the isothermal exposure. It was also found that the thermal fatigue damage might be evolved depending on of the testing method by which the thermal cycles are applied. Some background of these findings were discussed, based on the measurements of elastic modulus, tensile strength, and thermal conductivity of the ceramic top coat, as well as both the thermally grown oxide at the top coat/bond coat interface and the residual stress in the TBC specimens.