The impact-echo method is a nondestructive evaluation to detect the presence of voids within tendon ducts in prestressed concrete structure. The presence of voids in concrete is estimated by identifying peak frequencies in the spectrum. However, the detection for ungrouted ducts is not always successful because many peaks are often observed in the spectrum due to reflection at other reflection sources. In order to solve this problem, a new procedure is developed by applying an imaging procedure to the impact-echo data, known as stack imaging of spectral amplitudes based on impact-echo (SIBIE). In this study, impact-echo method is performed on the webs of post-tensioned girders on existing PC bridges to survey the presence of ungrouted ducts. To detect the voids within tendon ducts, the SIBIE procedure is applied. Applicability of SIBIE was verified at selected locations invasively by drilling and observing the interior of the tendon duct. Results show many peaks caused by reflection and so forth are observed in frequency spectra by impact-echo tests. Thus, it is difficult to extract the particular peaks associated with only the locations of void. In contrast, it is confirmed that the presence and location of ungrouted ducts can be visually identified by SIBIE analysis. After the SIBIE analysis is completed, the analytical results are compared with those obtained by invasive observation. Thus, the feasibility of SIBIE on detection of voids within tendon ducts in existing PC bridges is demonstrated.

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The impact-eco method is a nondestructive evaluation for defects in concrete structures analyzing elastic wave due to a mechanical impact. Cracks and voids are commonly repaired by injection of grout material. Impact-echo method might provide information of repair effectiveness of injection of grout material. In order to evaluate the effect of grout material on reflected waves and frequency spectra, impact-echo method is applied to repaired concrete structure and specimen. As a result, the followings are found: spectral peak of defects disappeared after injection of grout material; and spectral amplitude of defects decreased and disappeared with increasing of acoustic impedance of grout material.

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Diagonal shear failure could lead to the serious damage in concrete structures due to earthquakes. However, mechanisms of diagonal shear failure in reinforced concrete (RC) beams have not been completely clarified yet. Generally, the diagonal shear failure occurs, depending on the ratio of the shear span to the beam depth. In the case that the ratio is smaller than or equal to 2.7, the shear failure occurs as the diagonal tensile failure. In this study, diagonal shear failure in RC beams is investigated, applying acoustic emission (AE) method. AE method is one of nondestructive testings for concrete structures with a high capacity for diagnostics and health monitoring. Presently, it is known that fracture mechanisms are identified by AE waveform analysis. The moment tensor analysis of recorded AE waveforms can identify cracking kinematics of location, crack-type and crack orientation. The analysis is implemented as SiGMA (simplified Green's functions for moment tensor analysis) procedure. Prior to testing RC beams, theoretical waveforms were calculated in order to determine location of AE sensors. Theoretical waveforms were synthesized by introducing the dislocation model and Green's functions in a half space. Then, cracking mechanisms in the experiments are investigated by applying the SiGMA procedure to detected AE waveforms. Since classification of crack mechanisms can be analyzed by AE parameter, the results of AE parameter analysis and those of SiGMA analysis are compared and discussed.

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Successive observation of transgranular small fatigue crack growth behavior of alpha-brass was performed by means of an atomic force microscope (AFM) equipped with small in-plane bending fatigue testing machine. The fatigue crack deflection behavior, which was observed frequently in the low growth rate region, was investigated by the crystallographic orientation analysis based on the Electron Back Scatter Diffraction (EBSD) technique. The slip factor considering the slip system and singular stress field at the crack tip was introduced in order to evaluate the easiness of slip deformation instead of Schmid factor. The direction of crack deflection was found to be explained well by the slip factor and the geometric relative location between the preferential slip plane and crack front.

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The potential of fatigue crack growth experiments to sensitively analyze the anisotropic mechanical behavior of injection-molded, multiphase polymer blends is presented. The properties of immiscible blends based on poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(styrene-co-acrylonitrile) is investigated, both in perpendicular and in parallel to the injection-direction. As a result of the limited interfacial adhesion and the orientation of the blend phases, the direction in parallel to flow revealed to be the weaker link. In order to enhance the toughness behavior, the PPE/SAN blends were systematically modified by the addition of different polystyrene-b-polybutadiene-b-poly(methyl methacrylate) triblock terpolymers (SBM), potentially acting as a compatibilizing and toughening agent. Such a compatibilization step of PPE/SAN blend leads to the formation of nanostructured morphologies. Depending on the composition of the triblock terpolymer, the resistance to crack growth was either further degraded or, in the case of high interfacial presence and compatibilization efficiency, significantly improved. The results were correlated to the fracture mechanisms of the blends as analyzed by scanning electron microscopy of the fracture surfaces. In contrast to poorly-compatibilized blends, a significant plastic deformation, an enhanced phase adhesion and a strongly reduced anisotropy could be detected in case of well-compatibilized blends. In summary, the presented study of the fatigue crack growth behavior revealed a detailed insight into the mechanical property profile of the selected multiphase blends and, moreover, demonstrated the potential of this method to sensitively analyze the anisotropy of such materials.

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A modified linear-elastic method of analysis was proposed for the estimation of fatigue lifetimes of biaxially loaded components containing cracks. The following constitutive relation was used as a basis for the analysis: (da)/(dN)=A(ΔK_<eff>-ΔK_<effth>)^2 where a is the half crack length, N is the number of cycles, A is a material constant, ΔK_<eff> is the effective range of the stress intensity factor (SIF) given by K_<max>-K_<op>, where K_<max> is the maximum value of SIF and K_<op> is the SIF at the crack opening level; and ΔK_<effth> is the effective range of the SIF at the threshold level. A modified LEFM approach was used in the analysis. The modifications included a correction for elastic-plastic behavior, consideration of Kitagawa effect, and consideration of the development of crack closure in the wake of a newly formed crack. An analysis was made using the proposed method of the da/dN data obtained by Brown and Miller for different biaxial stress states. The effects of stress ratio were also investigated. It was found that the difference in elastic-plastic behavior and effects of stress ratio accounted for the effect of biaxial stress on da/dN.

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Laser peening without coating (LPwC : LP) is used practically as a preventing method for stress corrosion cracking (SCC) of nuclear reactor structure. Therefore it is known that the LP treatment improve the fatigue properties of cast aluminum alloy. However, there is no report about the retardation of fatigue crack growth on casting aluminum alloy by the LP treatment. In the present study, to evaluate the effects of the LP treatment for fatigue crack propagation behaviors on AC4CH cast aluminum alloy, some pre-crack specimens which had a surface fatigue crack length as about 2.5mm at center part of cylindrical test specimen were prepared and rotating bending fatigue tests were conducted on those specimens. As the results, fatigue crack propagation was prevented by LP treatment and the results obtained are summarized as the followings, (1) If a maximum stress intensity factor of fatigue crack tip at surface of specimen is lower than about 5 MPam^<1/2>, the fatigue crack growth can be stopped by the LP treatment. (2) Apparent maximum stress intensity factor of fatigue crack tip at surface on LP treatment specimen was nearly equivalent to about 4 times of threshold stress intensity factor of long fatigue crack at AC4CH when the surface crack began propagation. It was clear that the cause of this threshold stress intensity factor increasing was effect of compressive residual stress with LP treatment by calculation of the stress intensity factor.

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In the failure analysis of fatigue fracture accident of industrial machinery, the estimation of applied stress on the fatigue crack is an important issue in order to determine the cause of failure and to prevent another accident. The estimation of applied stress using fractographic technique has been already established. However, the estimation can be applicable in relatively high stress intensity factor region. Low stress intensity factor region, however, is much more important in the real engineering. The objective of this study is the development of estimation method of applied stress, which can be used in the near threshold region of crack propagation. High frequency current impedance method and fracture surface hardness method were examined. In the impedance method, there was a correlation between the high frequency current impedance of fatigue fracture surface and stress intensity factor range. In the hardness method, micro Vickers hardness was measured on the fatigue fracture surface. The hardness parameter of fatigue fracture surface was defined as the area of indentation mark in order to reduce the effect of surface roughness of the fatigue fracture surface. The hardness parameter had a correlation with the stress intensity factor range in the near threshold region. Each method can be used in the stress estimation in low stress intensity factor region.

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In order to reduce the weight of suspension coil springs used automobiles, high strength spring steel has been developed. The suspension coil springs are subject to torsional stress in a corrosive environment. So, torsional fatigue tests were conducted for spring steels whose Vickers hardness values were 430HV, 480 HV, 550 HV and 620HV. The effects of the corrosion environment and surface artificial pits on the fatigue strength were studied. It was found that the fatigue strength of 550HV smooth specimens was higher than those of other specimens in air, but in the case of corrosion fatigue the fatigue strength of 480HV specimens were the highest of all specimens. It is important to note that the explanation for this is that the fatigue was caused by the intergranular fracture. The fatigue fracture of smooth specimens grew from shear cracks in the case of fatigue in air and from pits in corrosion fatigue. The stress intensity factor range for crack initiation form corrosion pits was nearly the same for all materials. The fatigue strength reduction rate was constant under fatigue loading in air, but it decreased with increasing fatigue life and hardness in the case of corrosion fatigue.

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The cyclic stress-strain behavior and the crack closure behavior were investigated to study the mechanism of fatigue strength reduction caused by absorbed hydrogen using SUS304 and SUS316L. Solution heat-treated and pre-strained austenitic stainless steels were used. Strain measurement test to study the bulk behavior of cyclic plasticity and fatigue crack propagation test were done. There was no significant effect of absorbed hydrogen on the bulk behavior of cyclic plasticity. Fatigue crack propagation rate was accelerated by hydrogen especially in the case of R = -1. The crack opening stress of hydrogen charged material was lowered than that of uncharged material. The acceleration could be explained by the increase of ΔK_<eff>. The enhancement of fatigue damage by absorbed hydrogen is not caused by changing the macroscopic stress-stain behavior but by a microscopic phenomenon such as crack closure.

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This paper is concerned with the applicability of statistical linear cumulative damage (SLCD) rule for predicting the fatigue life under variable cyclic loading for CFRP laminates. The three-point bending fatigue tests for plain-woven CFRP laminate were carried out under various cyclic loadings with constant and variable load amplitude, load frequency and temperature. As results, the applicability of SLCD rule to the flexural fatigue life of this CFRP laminate was confirmed for the cases of variable load frequency and temperature as well as variable load amplitude from the viewpoint of statistics.

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The flexural rigidity evolvement of RC beams strengthened with carbon fiber laminate under cyclic bending loads is investigated in this paper. In the fatigue experiment, 14 reinforced concrete beams strengthened with carbon fiber laminates (CFL) are divided into three groups according to the peak stress applied on the CFL, and tested under bending loads with constant amplitude. The results show that: (1) the flexural rigidity development follows a rapid decrease phase, a uniform decrease phase, and a rapid decrease until failure phase; (2) there is linear relationship between flexural rigidity and normalizing fatigue cycles in the uniform decrease phase. Moreover, if the stress level is not beyond the main reinforcement yield stress, the flexural rigidity evolvement rules are independent on the stress levels.

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The fatigue behavior of RC beams strengthened with fiber reinforced polymer (FRP) is an important subject in civil engineering. In this paper, 18 RC beams (1850×100×200 mm) strengthened with carbon fiber laminate (CFL) are divided into 4 groups and tested to investigate their fatigue behavior under bending loads. Based on the above fatigue testing data and theoretic analyses, following results are obtained: 1) three conceptions of cracking fatigue life, permissive fatigue life and extreme fatigue life are proposed; 2) two kinds of S-N curves and P-S-N curves of RC beams strengthened with CFL are obtained, according to the new conceptions of permissive fatigue lives and extreme fatigue lives; 3) the ratios of permissive fatigue strength and extreme fatigue strength to the ultimate static strength are 65% and 67% respectively.

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Fracture behaviors of hybrid composite joint for tilting car body were evaluated under static and cyclic loadings. Two species of specimens were fabricated; a hybrid bolted joint specimen for the shear test and a hybrid joint beam specimen applied in the real tilting car body for the bending test. Characteristic fracture behaviors of those specimens under cyclic loads were obviously different from the fracture under static loads. For the hybrid bolted joint specimen static shear loading caused the fracture of the bolt body itself to be a pure shear mode, whereas cyclic shear loading brought about the fracture at the site of local tensile stress concentration. For the hybrid joint beam specimen static bend loading caused the shear deformation and fracture in the honeycomb core region, while cyclic bend loading did the delamination along the interface between composite skin and honeycomb core layers as well as the fracture of welded joint part. Those fracture behaviors were reflected in design parameter improvement for the hybrid joint structure in the real tilting car body

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In order to investigate the influence of grain size on the resistance to crack growth, fatigue properties of commercial fine-grained carbon steels with grain sizes of 6.5 and 20 μm were investigated based on the crack initiation and propagation behavior through the successive observation of surface state of smooth specimen under rotating bending. A crack initiated at the early stage of stress repetitions and most of fatigue life was occupied by the growth life of a crack smaller than 1〜2 mm. The crack growth rate determined by the term σ_a^nl, uniquely in both steels, where σ_a and l are the stress amplitude and the crack length and n is constant. The fine grained- steel has an excellent resistance to crack growth in comparison with many annealed carbon steels.

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Since a starting point of fatigue failure is usually in surfaces, clarifying the mechanism of surface damage formation due to cyclic stresses should lead to a better understanding of the fatigue characteristics of ultrafine grained (UFG) metals. With regard to surface damage, shear bands (SBs) in cyclically deformed UFG copper have been reported. The morphology of the SBs was found to be similar to the extrusions formed by persistent slip bands (PSBs) in large grain metals. An attempted to explain the SB formation with relation to the oriented distribution of defects along the shear direction of the last pressing has been performed. In many studies, observations of SBs have been performed on surfaces of post-fatigued specimens or surfaces after SB formation. Therefore, in order to study the formation mechanism of surface damage, the formation process of surface morphologies should be clarified. However, no such experiment has yet to be conducted. In the present study, fatigue tests of ECAP processed copper with grain size of about 250 nm were made. The successive observation of surface was made by optical microscopy and scanning electron microscopy. The formation mechanism of the surface damage of UFG copper was discussed through the observation of changes in surface morphological features.

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Fatigue crack initiation and propagation behavior was observed in ultrafine-grained Cu processed by ECAP. The integration of EBSD, AFM and SEM measurement revealed that grain coarsening occurs on smooth specimens accompanied by large slips in the scale of more than 10 μm in spite of the average grain size of 300 nm. The slips are introduced in the direction of the primary slip plane with the maximum Schmid factor. The grain coarsening with large scale slips was also observed at the tip of long fatigue cracks at the threshold of the stress intensity factor. Fatigue damage at grain boundaries without slips was dominant at higher stress intensity factor in the Paris's law region. The grain coarsening behavior is dependent on stress intensity factor.

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Axial fatigue tests were conducted for plane bar specimens of SKD61 steel and 0.65 mass% carbon matrix high speed steel, YXR3 in order to investigate ion nitride effect. In higher stress amplitude of S-N diagrams ion nitride effect was not observed for both steels. Ion nitride effect was observed in stress amplitude lower than 71% of ultimate tensile strength for SKD61 steel and in that lower than 56 % of ultimate tensile strength for YKR3 steel. The stress amplitude expected for ion nitride effect decreased with an increase in the Vickers hardness number of hot forging die steel. In higher stress amplitude fatigue cracks initiated from ion nitride layer at specimen surface, while in lower stress amplitude fatigue cracks initiated from non metallic inclusions located in sub-surface. Ultrasonic fatigue test with frequency of 20 kHz was also conducted to investigate ion nitride effect in higher fatigue life up to 10^7 cycles. Ion nitride effect was not observed for all tested specimens failed in ultrasonic frequencies because of sub-surface crack initiation from non metallic inclusions located at sub surface.

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In this investigation, mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed to characterize the fatigue behavior and to calculate the lifetime of metallic materials under stress-controlled single step and random loading. Single step sequences are periodically inserted in random loading tests to measure the plastic strain amplitude as well as the deformation-induced changes of the temperature and the electrical resistance. These fatigue data are plotted versus the number of cycles in so-called cyclic 'deformation' curves to describe and evaluate the fatigue behavior under random loading, similar as commonly practiced under single step loading. On the basis of Morrow and Basquin equations, which are generalized to be applicable for the applied measuring techniques, a physically based fatigue life calculation "PHYBAL" was developed. This new short-time procedure requires data of only three fatigue tests for a rapid and nevertheless very precise determination of Woehler curves for single step loading or fatigue life curves for random loading, leading to an enormous saving of experimental time and costs.

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Cold flakes are one of irregular structures with fine grains having oxide layer, and when they are included in aluminum die-casts, unexpected failure of products appears under repeated loading and thermal cycling. The strength of the die-cast product is largely reduced due to the casting defect. The present work was carried out on the influence of the cold flake on fatigue properties of die-casts. The extended aim of this work is to verify the appearance of cracks initiation and propagation from the cold flakes of different size that influence the fatigue strength. For this purpose, at first, cold flakes were detected by acoustic microscopy and then specimens were prepared with positioning the cold flake in a center position of the specimen. Then the fatigue testing was carried out with specimens containing the cold flake of various sizes. The result shows that not only the size of the cold flake but also the position of the cold flake drastically reduces the fatigue life; the exposed cold flakes reduce the fatigue life greatly.

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Fatigue fracture behavior of magnesium single crystals containing cracks growing in different crystallographic orientations has been investigated. To investigate fatigue fracture behavior of small single crystals, plain bending fatigue test method for thin sheet specimen was developed. In A-specimen, the surface plane and the loading direction are (0001) and [1120], respectively, a fatigue crack propagates along [1100] at higher stress level while a crack propagates [1210] at lower stress level. In C-specimen, the surface and the loading direction are (1120) and [1100], respectively, a crack propagates partially along {1012} twin interface. In D-specimen, the surface and the loading direction are (1100) and [1120], respectively, a crack propagates along {1012} twin interface at higher stress level while a crack propagates along [0001] at lower stress level. As a result, S-N curves of each specimen showed orientation dependence. Fatigue strength of C-specimen is higher than that of A- and D-specimen. We also discuss the fatigue behavior mechanism of magnesium single crystals.

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Rotating bending fatigue tests were performed at frequency of 30 Hz using four kinds of the extruded AZ31 magnesium alloys, which were processed under different extrusion ratios (10 and 100) and different extrusion temperatures (623 K and 696 K), to determine S-N curves and crack propagation behavior of those materials. Effect of the extrusion ratios and extrusion temperatures on the fatigue lives and crack propagation behavior were discussed in detail. Fatigue processes of those materials were observed by the replica method. It was clarified that cracks initiated early in the fatigue process, and total of fatigue life can be approximated as the crack propagation life. Crack propagation behavior observed in those materials was analyzed using a modified linear elastic fracture mechanics parameter, M. The relation crack propagation rate vs. M parameter was clarified to be useful in predicting fatigue lives and crack propagation curves in those materials.

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Machines and structures, such as automobiles are usually subjected to biaxial or three-axial stresses instead of uniaxial stress. However, research on a fatigue failure under multi-axial stress has not been fully presented because such experiments are difficult. To solve this problem, we developed the servo biaxial fatigue-testing machine. In this research, fatigue crack propagation tests of magnesium alloy AZ31B and pure titanium TP340C were conducted under conditions of biaxial and uniaxial loading by using a cruciform specimen in a biaxial fatigue machine, in order to investigate the effect of non-singular stress cycling on the fatigue crack growth properties ΔK-da/dN. From these comprehensive experiments, in the magnesium alloy, the remarkable effect was found in the specific biaxial load stress ratio on ΔK-da/dN relation. When biaxial load stress ratio was 0.5, it turned out that the fatigue crack propagation rate of a magnesium alloy becomes very slow. On the other hand, in the titanium, it was confirmed that there is a little influence of a biaxial load stress ratio on ΔK_1-da/dN relation.

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The ignition point of non-combustible Mg alloy is about 300K higher than that of normal magnesium alloy. It is expected that the non-combustible Mg alloy is used in substitution for Al alloy for structures. In this paper, the fundamentals, those are a notch effect, and a long or small crack effect on fatigue strength of a non-combustible Mg alloy, are investigated. Moreover, the fatigue strength characteristics of the non-combustible Mg alloy are compared with that of Al alloy to clarify differences of fatigue strength characteristics between Mg alloy and Al alloy.

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Results of thermal fatigue tests of the magnesium base alloy AZ31 for the case of out-of-phase-loading in a temperature range between -50℃ and +290℃ are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stress. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that rolled AZ31 shows different stress-strain behavior during tensile and compressive loading resp. at lower temperatures due to the fact that mechanical twinning occurs preferentially during the compression loading phase and not during tension. However untwinning processes may occur during the tensile phase of a loading cycle. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins can be observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.

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The effect of hydrogen on fatigue crack growth behavior of three austenitic stainless steels has been investigated from the viewpoint of martensitic transformation. The crack growth rates in the hydrogen- charged SUS304 and SUS316 were accelerated. The crack growth rate in the hydrogen-charged SUS316L was slightly higher than that in the uncharged SUS316L. The martensitic transformation on fatigue fracture surface was detected by the X-ray diffraction method in both the hydrogen-charged and uncharged specimens of SUS304, SUS316 and even SUS316L. However, SUS316L showed less martensitic transformation than SUS304 and SUS316. It is presumed that the martensitic transformation in the vicinity of fatigue crack tip contributed to the effect of hydrogen on crack growth rate. The fracture surface was examined by SEM in SUS304 and SUS316L. Clear striations were observed on the fatigue fracture surfaces of the uncharged specimens of SUS304 and SUS316L. Striations were also observed in the hydrogen-charged SUS304 and SUS316L. However, especially in the hydrogen-charged SUS304, striations were less distinct than those in the uncharged SUS304.

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In order to investigate the influence of hydrogen on the fatigue strength of AISI type 304 meta-stable austenitic stainless steel, specimens were cathodically charged with hydrogen. A small hole with a diameter d =100 μm and a depth h =100 μm, was introduced into the specimen surface after hydrogen-charging. Fatigue crack growth rates of the hydrogen-charged specimens were compared with those of uncharged specimens in tension-compression fatigue tests under a constant stress amplitude, σ_a = 320MPa, at a stress ratio, R = -1, and a test frequency, f = 1.0 Hz. Hydrogen-charging led to a marked increase in fatigue crack growth rate. The crack growth path in the hydrogen-charged specimens was relatively linear, whereas the crack growth path in the uncharged specimens was more zigzag. In the uncharged specimens, a large number of slip bands were observed in the vicinity of the fatigue crack, whereas in the charged specimens relatively few slip bands were observed. To elucidate the behavior of hydrogen during fatigue process, the surfaces of both uncharged and charged specimens were examined by the hydrogen microprint technique (HMT). In the uncharged specimens, no hydrogen emission was observed. On the other hand, in the hydrogen-charged specimens, a hydrogen emission was observed, especially in the vicinity of fatigue crack. A comparison between HMT image and etched microstructure revealed that hydrogen was mainly emitted from slip bands, which were supposed to be a pathway of hydrogen. An analysis of the electron backscatter diffraction pattern (EBSD) showed that both α' and ε martensites were induced along the slip bands.

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Magnesium alloys are now being increasingly used as a structural material wherever the 'lightness' is the major criterion. This is attributed to their inherent superior properties, such as very low density, high specific strength, excellent castability & machinability, good damping capacity, high dent resistance and so on. But due to low corrosion resistance, structural applications of magnesium alloys are still limited, since many mechanically-loaded parts are often subjected to prolonged cyclic stresses in an active medium. In the present work, fatigue behavior of extruded AZ61 magnesium alloy has been evaluated under three different kinds of corrosive environments: (a) high humidity environment (80% relative humidity) (b) 5 wt. % NaCl environment, and (c) 5 wt. % CaCl_2 environment. Fatigue tests were conducted under axial loading at stress ratio of -1 and at a frequency of 20 Hz. Extruded AZ61 indicated a fatigue limit of 135 MPa under low humidity of 35-40 %RH, while a clear effect of humidity was seen under high humidity of 80 %RH at stress amplitudes below the fatigue limit. In NaCl and CaCl_2 environments, the fatigue strengths were drastically reduced. The reduction rate of fatigue strength due to corrosive environment was 0.22 under high humidity condition, 0.85 under NaCl environment, and 0.77 under CaCl_2 environment. This remarkable reduction of fatigue strength in corrosive environments was attributed to the formation of corrosion pit, which was enhanced under cyclic loading.

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Structural failure by fatigue of automotive engine cover is discussed. Fatigue properties of mineral/glass reinforced polypropylene compounds, as an alternative material for engine cover, were measured by experiments. The use of polypropylene compounds offers weight as well as cost reduction. Finite element analysis (FEA) has been performed to investigate the fatigue behavior of the engine cover. Assembly load due to the tightening of the bolts as well as the vibration load was considered to describe the actual loading conditions in durability test. Natural frequency of the system was extracted considering the damping effect of the structure. Dynamic time history of engine cover under vibration load was calculated based on the extracted natural frequency of the system and stress distributions at each load condition were predicted. Experimental modal analysis (EMA) was performed to identify the modal parameters and measurements of strains were performed to verify the results of the analysis. Based on the S-N curve data for engine cover material, distribution of safety factor was predicted by using the FE analysis results. The calculated results were verified by experimental durability tests. Analysis results showed that contribution of the assembly load is not negligible to predict the fatigue failure of the engine cover.

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During its operational life automotive junction box fuse faces various challenges such as severe vibration, impact, temperature, humidity, etc. Accordingly, it demands high durability, thermal resistance, and humidity resistance. Therefore, to evaluate its life cycle establishment of measurement method is needed. To evaluate life cycle of junction box fuse, generally two exciters are needed to excite each side of the fuse separately. However, there are some difficulties such as high equipment cost, high maintenance and repairing cost, setting difficulty, etc., that limit its use. In this experimental paper we propose a durability measurement method that uses only one exciter. Also, to validate our method we performed fatigue life cycle experiment on fuse. The acceleration difference between ends of fuse which occurs due to interior design of fuse box is a primary factor behind its life cycle reduction. Also, the low frequency vibration that occurs in the engine seriously affects its durability. The stress analysis of the fuse is performed for finding maximum stress concentration, and its natural frequencies and mode shapes are calculated by finite element analysis. Fatigue life cycle is evaluated from the data measured in accordance with acceleration differences between left and right ends of the fuse. Excitation range and acceleration amplitude is set, and failure position of fuse is predicted and verified by simulation results. Hence, the experimental and simulation results validate our proposed measurement method and the principle of one-side excitation measurement.

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Rubber hose assembly for automotive hydraulic brake during operation is subject to combined stresses of cyclic pressure, cyclic bending and torsion as well as thermal load. The rubber hose in assembly is composed of ethylene-propylene diene monomer(EPDM) rubber layers reinforced by polyvinyl acetate(PVA) braided fabrics. A durability tester with loading rigs for applying the above cyclic stresses was used to investigate failure mechanisms in the rubber hose assembly. Failure examination was performed at every 100 thousands cycles of bending and torsion. Hose assembly samples were sectioned with a diamond-wheel cutter and then polished. The polished surface was observed by optical microscope and scanning electron microscope (SEM). Some interfacial delamination with a length of about 1mm between EPDM rubber and PVA fabrics appeared at the test cycles of 400,000. Such initial delamination grew to the rubber skin layer causing the final rupture of the hose.

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This study investigated the effect of weld geometry on the fatigue strength of non-load- carrying fillet welded cruciform joints. The weld geometry of the cruciform specimens was intentionally varied. Fatigue tests were carried out on various weld geometry configurations. The configurations included weld flank angle, weld toe radius and weld throat thickness. Fatigue tests were conducted using a servo hydraulic controlled 250 kN capacity UTM with a frequency of 5 Hz under constant amplitude loading. From the experimental results, relationship between the weld geometry and the fatigue strength of non-load-carrying fillet welded cruciform joints was analyzed in detail.

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Transient regimes arising during start-ups, shut-downs and change of operation regime in steam power plants cause unsteady thermal and mechanical loading with time in turbine components resulting in non-uniform strain and stress distribution. Thus accurate knowledge of stresses, caused by various loading conditions, is required for the integrity and life assessment for the turbine components. Although materials of steam turbine components deform inelastically at high temperature, only elastic calculations are currently performed from a conservative point of view for safety and simplicity. Although numerous models have been proposed to describe the viscoplastic (time-dependent) behavior, these models are rather elaborate and difficult to incorporate into finite element code to simulate the loading of complex structures. In this paper, the total lifetime for steam turbine components was calculated by combining the viscoplastic constitutive equation and the ABAQUS finite element code. The viscoplasitc analysis was particularly focused on simplified constitutive equations with linear kinematic hardening that is simple enough to be used effectively in computer simulation. The von Mises stress distribution of HIP turbine rotor was calculated during cold start-up rotor and a reasonable number of cycle was obtained from the equation of Langer.

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Like a diagnosis of human body, it is important to monitor conditions of electronic packages in use. Even if electronic packages are sound in the beginning, thermal cycles during operations can produce fatigue cracks that lead to the system failure eventually. If we know the crack length in the package in real-time, a fatal damage can be prevented before the system failure. Currently, there are no simple methods for real-time monitoring of electronic packages in use. In this research, we propose a non-destructive monitoring method that detects the crack length in electronic packages. The electronic package in use is heated continuously by itself. When the crack at a weak site of the electronic package occurs, thermal deformation on the chip side is changed. Therefore, we can measure these micro deformations by using Moire interferometry and find out the crack length. The proposed monitoring method is applied to ACF type package specimens. Experimental results show that the crack length of the electronic package affects the thermal deformation of the chip. If we have limitations of thermal strain at a specific temperature, we can monitor the crack length in real-time. Since the failure of a super-computer or network-server system affect the tremendous damage, we must know the health conditions of electronic packages in real-time. This research focuses the health monitoring of the electronic package of these system.

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The bolts and nuts are widely used in various fields as important joining elements with long history. However, loosening induced by the vibration and by the external loads is still a big problem. And the loosening sometimes causes very serious accident without notice. In this paper, two types of special bolt and nut are considered, named "Super Stud Bolt (SSB)" and "Super Lock Nut (SLN)" which can prevent loosening effectively. In this kind of bolt and nut, the thin walled tube between the upper threads and the lower threads can be deformed along the axial direction so that the phase difference is produced. This phase difference induces the contrary forces on the surfaces of the upper threads and the lower threads, which bring out the anti-loosening performance. In this study, the anti-loosening performance is analyzed and realized with the finite element method. Moreover, the anti-loosening performances under various phase differences are compared and finally best dimensions for SSB and SLN are examined.

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Bolts and nuts are widely used in various fields as an important joining machine element. In this mechanism, some problems always exist that loosening is induced by vibrations and external loads; screws are broken due to fatigue and so on. Previously several kinds of anti-loosening bolts and nuts were invented; however, they usually need a certain amount of prevailing torque even before the nut touches a fastened plate. A new outer cap nut named "Super loose proof (SPR)" has been developed to overcome such inconvenience. At first this outer cap nut can be rotated smoothly by hand until the nut touching the fastened plate; however, after fastening the outer cap nut, anti-loosening performance can be realized by deforming the outer cap and producing thread contact force at the outer cap region. In this study, stress concentration and tightening-loosening behavior are analyzed by axi-symmetric and three-dimensional finite element methods. Under a certain bolt-axial force, the load distribution of the first pitch decreases more than 12% with increasing initial clearance of outer cap nut. Stress concentration appearing at the first pitch of the bolt can be about 10% smaller than that of conventional nut, reflecting the increase of the thread contact force at the outer cap region. On the other hands, it is found that anti-loosening performance of SPR can be realized when the outer cap has high yield stress.

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Stress relief groove has been used to improve the fretting fatigue strength at fitted part between mechanical components. However, in the limited mechanical components such as railway wheelsets, applicability of the groove has not been fully evaluated and there are insufficient investigations to determine optimal shape of the groove. In this study, fretting fatigue tests were done using several kinds of grooved specimen. The shape of groove was systematically changed with parameters groove radius R and tangential angle θ. FEM stress analyses were also done to investigate stress conditions and to evaluate the fretting fatigue limit of the grooved specimen. In the fretting fatigue test, fatigue limit of a grooved specimen had a peak with increase of θ. The maximum improvement of fatigue limit was achieved at the transition point of the failure mode from the fretting fatigue at contact part to the fatigue at groove root. The radius of groove has an influence on the improvement of fatigue limit, since stress conditions at groove root become severe when the size of groove is too small. In the simple elastic FEM analysis, a highly compressive stress field was generated near the contact edge, where small cracks could never propagate. Thus, as an assumption to relieve the contact pressure concentration, the local plastic deformation at the contact edge was simulated by elasto-plastic FEM deformation analysis. As the results, it was found that fretting fatigue limit of grooved specimen could be evaluated using the maximum axial stress near the contact edge. The estimation of fretting fatigue limit using a relationship between K_t /K_<t0> and θd gave a good correlation with the experimental results and it would be a useful method to select the optimal groove shape.

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Brazing is a method of permanently joining a wide range of materials and has a wide application in fabricating components of commercial value. It is distinguished from welding in that the process takes at temperatures below the melting points of the materials to be joined and from soldering in that the temperature of processing is above 450℃. This method is commonly used in electronics or refrigerating machines due to its advantage of diminution of heat influence and easy manufacturing. Unfortunately, the brazing method for copper alloy used in household air conditioners has some demerits in that the copper alloy is susceptible to heat and the resulting joints may have various flaws. Therefore, deciding on the optimal joint conditions is an effective method to use based on the experimental data. However, using a trial-and-error method from the beginning in such a wide area in order to decide on the optimal conditions requires too much experience to overcome these problems and to decide on the optimal conditions for a procedure using response surface methodology. A response surface methodology, consisting of experimental design, statistical modeling and optimization is used to decide on the optimal joint parameter with process conditions.

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In the previous study, equivalent stress ratios were proposed as parameters for correspondence between fatigue test conditions of notched and unnotched specimens. Diagraming the relations between equivalent stress ratios and notch root stress ranges for fatigue limits and fatigue life strength, the relations proper to materials could be represented by consistent curves in spite of the difference of specimen types (plates and round-bars), loading types (axial, bending, torsional and their combined loading) and stress concentration factors of notches. In other words, it can be said that the relations diagramed by the equivalent stress ratios vs. notch root stress ranges for fatigue limits and fatigue life strength represent the characteristics on fatigue limits and fatigue life strength of unnotched specimens just as they are. In the present study, experimental data on fatigue strength of notched and unnotched specimens are widely picked up from literature and rearranged on the diagrams of equivalent stress ratios vs. notch root stress ranges. As a result, fatigue strength diagrams are categorized into three groups in accordance with fatigue processes and their characteristics are discussed.

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Compressive residual stress is introduced into various components by surface modification methods in order to mitigate stress corrosion cracking susceptibility and to improve fatigue strength. There are also some nondestructive evaluation methods of residual stress including X-ray diffraction, neutron diffraction and Barkhausen noise. However, these methods are not suitable for small areas of austenitic stainless steel because it has large grain size and nonmagnetic property. A more reliable method is therefore required to evaluate residual stress in austenitic stainless steel. Recently, much research has focused on an indentation method for the metrology of residual stress. The indentation load-depth curve and impression size obtained from the indentation process can be used to determine residual stress and various mechanical properties such as hardness, the Young's modulus, fracture toughness, yield stress and work hardening exponent of a material. However, these values affect each other and it is necessary to clarify the relations between residual stress and each mechanical property. This study investigates the possibility of separating stress and mechanical properties both experimentally and by numerical calculation using the micro indentation method. For the indentation test using a sharp indenter, it was possible to evaluate stress by comparison of the maximum depth hmax or residual depth hf obtained from the load-depth curve. Furthermore, the 2-dimensional X-ray diffraction method is examined comparatively with the indentation method.

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Conventional nanoindentation testing generally uses a peak penetration depth of less than 10 % of thin-film thickness in order to measure film-only mechanical properties, without considering the critical thickness for a given thin film-substrate system. The uncertainties in this testing condition make the measurement of film mechanical property more difficult. Thus, we estimated the critical relative thickness of Au thin film by adopting the mechanism-based strain gradient plasticity theory. Au film's critical relative thickness was determined by 0.17 and subsequent strength evaluations were carried out for nanoindentations shallower than 204 nm. The yield strength of thin film was estimated by considering the force equilibrium around an indentation-induced plastic zone; the plastic zone dimension is a key information for this analysis. However, conventional approach has analogized the plastic zone radius from few cross-sectional line profiles of the remnant indent and yielded erroneous strength result. Thus, we tried to extract a two-dimensional closed boundary of the plastic zone in this study by plotting a contour graph for the surface pile-up. This image processing was applied to the strength characterization of 1.2 μm-thick Au film on Si substrate. However, the analyzed indentation data produced strength overestimations compared to the microtensile results. This discrepancy in the yield strength was discussed from a viewpoint of microstructural anisotropy in the Au thin film.

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Strength and plastic deformation behavior of Cu/Au multilayers with different individual layer thicknesses ranging from submicron to nanometer scale have been studied by using instrumented-indentation. The deformation morphologies around the indents at the sample surfaces and the cross-sections of the multilayers were characterized by scanning electron microscopy (SEM) and focus ion beam. It is found that the hardness increases with decreasing individual layer thickness, and begins to deviate from the Hall-Petch strengthening relationship when the individual layer thickness is less than a certain length scale. Furthermore, significant layer thickness dependent shear banding behavior was observed. Quantitative measurement of the shear band dimensions exhibits that the region of the plastic deformation instability becomes more localized with decreasing the length scale of the multilayers. The mechanisms of strengthening and plastic instability of the material at the nanoscale are discussed.

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The aim of this paper is to evaluate the interfacial strength between thin film and its substrate by nano-indentation tests. From the indentation load and displacement curve, we proposed an evaluation method for the interfacial strength. The numerical simulation with damage-based cohesive zone model shows that the proposed method can evaluate the interfacial strength. Nano-indentation tests for PET substrate/ITO coatings layered specimens were carried out and we evaluated the interfacial strength of those specimens. The results are good agreement with the interfacial strength evaluated by peel tests.

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The aim of this investigation is to extract copper foil only fatigue properties at the micrometer scale and observe any mean stress effects on the fatigue life as well as the corresponding surface morphology. Tensile tests are preceded to obtain the baseline material properties of proportional limit and ultimate tensile strength. Constant amplitude fatigue tests are carried out by using a force-controlled fatigue testing machine. The mean stress as well as the stress amplitude is changed to get complete nominal stress-life (S-N) curves. An atomic force microscope (AFM) is utilized to observe the relationship between fatigue damage and the corresponding surface morphology change. The Basquin's exponent of -0.071 is obtained through the fatigue tests. The endurance limit of 87.5 MPa is inferred from the Haigh diagram. The specimen surface is roughened with fatigue cycles. An as-received specimen surface is too rough to evaluate the roughness change quantitatively. However, it is certain that there's a clear relationship between the fatigue damage and the surface roughness evolution.

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Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover an elliptical through-hole in a base plate subjected to push-pull cyclic loads, annealed copper films with the thickness of 100μm and those reduced the thickness from the 100μm to 50μm by an electro-polishing were fatigued under a constant stress amplitude with a stress ratio of zero. The crystal rotation behavior with the fatigue crack propagation was investigated by measuring the crystal orientation around the fatigue crack initiated from the notch root before and after fatigue testing, using EBSD (Electron Back-scatter Diffraction) method. Then, the change of crystal orientation with fatigue testing was evaluated quantitatively from the misorientation between the crystal orientation matrix on the same point obtained before and after fatigue testing. As a result, the angle of the crystal rotation obtained from the region showing the high fatigue crack propagation rate was larger than that obtained from the region showing the low fatigue crack propagation rate for the film with the thickness of 100μm, while the fatigue crack propagated faster in the film with the thickness of 50μm than that with the thickness of 100μm regardless of the small crystal rotation angles with the fatigue testing for the film with the thickness of 50μm.

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This paper describes a simple evaluation method for in-plane Poisson's ratio of thin film materials. We designed an on-chip bending test chip that generates bending of film specimen via torsion bars, when applying normal load to loading levers. Using micro machining technologies, the test chip has been fabricated from SOI wafer. Average value of in-plane Poisson's ratio was 0.063, which deviates only 2% from the ideal value of single crystal Si. Therefore, this technique is useful for Poisson's ratio evaluation of thin film materials.

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Biomaterials including micro and nanoscale polymer fibers have been extensively used in biomedical applications such as tissue engineering and controlled drug delivery. The stiffness of these individual polymer fibers that make up the entire 3D scaffold not only determines the growth, motility and differentiation of cells, but also the structural integrity of the scaffold. Due to the physical size of these fibers, it is important to know if these individual micro or nanofibers are strong enough to withstand the forces exerted by the cells as they grow and migrate on the scaffold. Thus, there is a need to perform the nanomechanical testing of a single micro and nanofiber to determine their structural integrity. Here, various techniques to perform the nanomechanical testing of a single micro or nanofiber will be presented. These techniques include tensile test, 3-point bend test and nanoindentation test performed at the nanoscale.

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Mechanical behavior of film material in micron and nano scales has gained much consideration as it has been extensively used in microelectronics and Microelectromechanical Systems (MEMS). Many works have been carried out to investigate the mechanical properties of film material by undergoing uniaxial micro-tension testing where the micro-tension setups used by many authors are mainly the uniaxial tension in a unique direction. But when these setups are used under a microscopic system, the micro regions being detected often go beyond the scanning scope during the test. This brings in difficulty to accurately calculate the deformations of the detected regions. In this paper, a delicate setup which can accomplish uniaxial tension in the opposing directions and micro force detection is designed to investigate the in-situ microstructure evolution of specimens under loading. Two identical piezoelectric actuators moving in the opposing direction are used to exert forces on the specimen synchronously controlled by a computer. Two symmetrical double-cantilevered force sensors respectively connected to the two PZT actuators are devised to detect the tensile loads applied on the specimen. The force sensors are calibrated using Electronic Speckle Pattern Interferometry (ESPI). The relative gripping and adjusting units located between the double-cantilevered force sensors are employed to ensure that the specimen is under uniaxial load. The proposed tensile setup can be used under an optical microscopic or an AFM stage for studying the morphology changes and deformations of material on grain scale. The elaboration of the system is presented in the paper and an experiment of polycrystalline aluminum thin film deformations based on grain scale is performed to demonstrate its performance.

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In order to evaluate strength reliability of micron size polycrystalline silicon (poly-Si) structure for MEMS, bending strength tests of cantilever beam, Weibull analysis of the strength and fracture surface analysis are performed. Recently, the importance of microelectromechanical systems (MEMS) in society is increasing, and the number of production is also increasing. The MEMS devices, which contain mechanical movement, have to maintain their reliability in face of external shock, thermal stress and residual stress from manufacturing processes. In greeting the mass production era of the MEMS, in case the material strength design of MEMS is performed, required strength data is not average value but the variation, especially minimum value assumed of the structure and material. Micron size poly-Si structure is widely employed in the MEMS such as microsensor, switching device and so on. Then, in order to evaluate strength reliability of micron size poly-Si structure, tests and analysis are performed. The specimen is made by chemical vapor deposition (CVD) process and the thickness is 3.5, 6.4 and 8.3 micrometer and the specimen has notch (stress concentration). The test specimen used for the test changed characteristics of (1)film thickness (2) stress concentration, and investigation about the influence each effect of the variation in a bending strength with fracture surface analysis are discussed.

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