Surface breaking cracks in a structure can be ultrasonically detected by using Rayleigh or Lamb waves. Either reflected echoes or transmitted signals may be monitored in the pulse-echo or pitch-catch modes of operation. Pulse-echo and pitch-catch techniques can also be used with laser ultrasonics where a high-power pulsed laser is used to generate ultrasound thermoelastically. Laser-based ultrasonic (LBU) techniques provide a number of advantages over conventional ultrasonic methods. However, small flaws give rise to weak reflections or small changes in the amplitude of transmitted signals. These small variations are often too weak to be detected with existing laser detectors. In this paper we provide the theoretical background for an alternate approach for ultrasonicetection of small surface-breaking cracks using laser-based techniques - the Scanning Laser Source (SLS) technique. This approach does not monitor the interaction of a well-established ultrasonic surface wave with a flaw, but rather monitors the changes in the generated ultrasonic signal as the laser source passes over a defect. Changes in amplitude and frequency of the generated ultrasound are observed which result from the changed constraints under which the ultrasound is generated over uniform versus defective surface areas. These changes are quite readily detectable using existing laser detectors even for very small flaws. To analyze the ultrasound generated when the laser beam is right above the defect, the locally important effects of heat absorption and diffusion must be carefully considered.

View full abstract

Fe-Pd alloys containing around 30at.%Pd have face centered cubic (fcc) structure, which is ferromagnetic, and transforms to martensite with face centered tetragonal (fct) structure. The martensite can be induced from the austenite phase by stressing just above the Ms temperature. The magnetization vs. external magnetic field (M-H) curves and stress-strain curves of both austenite and martensite phases were measured. Based on the magnetic and mechanical properties, a new method to utilize the stress-induced transformation is proposed for the actuation mechanism of ferromagnetic shape memory alloy, and compared with the other mechanisms so far proposed, i.e. magnetic field induced transformation, and variant change mechanism. The effects of substituting iron or palladium by platinum, cobalt, or nickel on the mechanical and magnetic properties are examined with respect to their improvement.

View full abstract

Special observations on Al_2O_3 seeded P/M Rene95 superalloy were conducted at SEM in-situ tension loading stage to study the inclusion effect on crack initiation, propagation and fracture behavior of P/M nickel-base superalloy. Experimental results show that, during SEM in-situ tensile process cracks easily initiate at the ceramic inclusions even at very low stress level(lower than yield stress of P/M Rene95). Generally, the cracks nucleate at inclusion/matrix interfaces and easily propagate along the interfaces to the matrix. Sometimes cracks can also directly initiate inside inclusions and propagate in the inclusions till to break the cracked inclusions. Stress concentration field nearby inclusion and the effect of inclusion size and distribution on stress fields were calculated. A proposed model of the inclusion effect on crack and fracture behavior has been discussed in this paper.

View full abstract

Environmentally-assisted cracking has presented a significant structural integrity problem in various industries including marine, petrochemical, aerospace and power generation. This may be attributed to various factors, including inadequate data bases for engineering-based design criteria, and the complex interactions between the relevant material, stress and environmental conditions, which make it difficult to develop effective life prediction strategies. This situation is offset by the increase in quantitative understanding of the mechanisms of cracking over the last 20 years. This paper reviews the role that such developments have had in mitigating the problem of stress corrosion and corrosion fatigue of ductile structural alloys in boiling water reactors (BWRs). These reactors have experienced cracking problems in austenitic and ferritic alloys in, for instance, piping, pressure vessels and in irradiated core internals. Emphasis in this paper will be placed on describing, (a) the development and qualification of a mechanistically-based prediction methodology for the relevant degradation mode and,(b) the use of such a qualified methodology for proactive, cost-effective, life-management decisions.

View full abstract

T^*_ε integral, which includes J integral as a special case, is shown to be a material property suitable for characterizing crack propagation in thin 2024-T3 aluminum sheets. The T^*_ε resistance curve, which was generated through stable crack growth tests of single edge-notched (SEN) and central notched (CN) specimens, were used to predict curved crack growth and crack link-up of in-line multiple cracks. T^*_ε integral for rapid crack growth reached a maximum value at the terminal crack velocity.

View full abstract

In this paper, recent activities within the SAFE research center are summarised which aim to open a new horizon in the field of structural integrity. These activities include the enhancement of fracture assessment methodologies, sensing technologies and the development of IT(Information Technology) based intelligent plant management system for the 21^<st> century. One of the key elements in structural integrity analysis is a reliable fracture assessment methodology. These days finite element analyses for complex geometries, regarded as difficult tasks, can be easily performed, and accordingly more reliable method can be developed. In this respect, this paper describes the new method, and its application to non-linear fracture mechanics problems. Secondly, this paper describes how to implement the current advances in fiber-optic technology to the condition monitoring system. Two fiber-optic sensors for condition monitoring systems are introduced. Finally, an integrated intelligent system for the plant management is introduced by incorporating advances in fracture assessment methodologies, sensing technologies into a currently available IT based plant management system. The proposed intelligent plant management system is not only flexible to adopt advances in fracture assessment methodologies and sensing technologies, but also easily accessible to field engineers and experts. This system is expected to play a key role in combining fast-developing information technology and advances in structural integrity assessment methodologies in the future, and thus will become a key element in safe and informative plant management for the 21^<st> century.

View full abstract

SCC damage of the weldment for HT-60 steel under corrosion control was evaluated, and fracture process due to SCC was modeled by analyzing the fractography and AE signals produced in the process of SCC. In case of the PWHT specimen, fracture by SCC occurred through the three steps such as formation of pits at grain boundaries, coalescence of pits and production of micro-cracks before the maximum load and final fracture. However, fracture process by SCC of the weldment consists of four steps like formation of pits at grain boundaries and within the grain, coalescence of pits and production of micro-cracks before the maximum load, coalescence of the micro-cracks after the maximum load and final fracture. Cathodic potential like -0.8V for the parent can be recommended to lengthen the time to failure and PWHT procedure was effective in improving the resistance of SCC.

View full abstract

In order to clarify the significance of corrosion damage for risk assessment, bend tests were carried out on a roughened corroded annular plate prepared from the oil storage tank. The tank had been used in service for about 6 years. Corrosive attack occurred extensively on the outer surface of the annular plate just under the welded joint between the shell plate and the annular plate. It was found that a marked decrease of load carrying capacity was noted in the high deflection region depending upon the degree of corrosion damage. The reason for extensive loss of load carrying capacity was successfully explained from successive observation of crack growth behavior on the corroded surface of the annular plate. Photographic evidence showed that multiple cracks, initiated around pits, grew rapidly not only by crack growth by itself, but also by coalescence with other neighboring cracks under the influence of plastic constraint derived from the welded joint. Then, it should be emphasized that the linking up of multiple cracks leading to a catastrophic failure was considered to be a characteristic event of the roughened corroded component, and was expected to occur depending upon the degree of corrosion damage under a complex stress state.

View full abstract

Corrosion fatigue tests were carried out to clarify the influence of acid fog as environmental factor on the fatigue strength of SM55C using a rotary bending fatigue tester. The fatigue strength of acid fog specimen extremely decreased about 80% compared to that of distilled water specimen. In acid fog environment, a number of cracks commenced at corrosion pit and coalesced with the adjacent cracks during they propagate, and they formed a single circumferential crack on surface. However, there is a discontinuity in depth direction from surface, and the depth of crack is smaller than that of normal fatigue crack cracks. The reason of this peculiar crack growth characteristics is that the crack opening-closure behaviors are hindered by corrosion products on the surface crack faces, and hence it is thinkable that the strong corrosion action like anodic dissolution for crack growth in depth direction is weaker compared with surface, resulting from faint pumping action of crack during loading-shedding processes.

View full abstract

In this research, in situ microscopic Raman spectroscopy was used to measured and to analyze the local enrichment of chloride ions on the surface of 304L stainless steel in a sodium chloride solution where the potential of the sample was controlled by a potentiostat. The Raman Spectroscopy measurement was performed on the specimen under the potential control with an area of φ10μm. The local concentration of chloride ions was measured by the variation of the water Raman spectrum since the O-H stretching mode of water Raman scattering (at 3250 - 3400cm^<-1>) is very sensitive to changes in the electrolyte concentration. The parameter for a measurement of small bands fits by Raman difference spectroscopy (RDS) proposed in the recent literature, which can clearly indicates a small change in the Raman spectrum, was utilized in this investigation. Corrosion pits occurred on the specimen surface after 60 minutes under the condition of +200 mV(SCE) in 0.5 mol/l NaCl solution and, in this case, the parameter was monotonically increased with the exposure time. On the contrary, in the case where no corrosion pits occurred, no increase in the parameter was observed. These results demonstrate that the enrichment of chloride ion was observed only on the specimens where the corrosion pits were formed.

View full abstract

Three slow strain rate tests (SSRT) on one-inch thick-compact tension (1T-CT) specimens, which were fabricated from sensitized 304L stainless steel, were performed with different extension rates in a simulated boiling water reactor (BWR) environment, to derive the environmentally assisted cracking (EAC) growth kinetics. The crack growth kinetics, depending on the testing conditions, were also described theoretically by Shoji's model. Inverse analysis was performed by fitting the experimental data to the theoretical data, to determine the model parameters r_0, which represents a characteristic distance to define crack tip strain rate (CTSR), and m, which is a slope of the current decay curve and a function of crack tip water chemistry. Both r_0 and m can be determined using experimental crack growth data in the analysis. Theoretical CTSR of sensitized 304 stainless steel during constant load line displacement rate tests was calculated using Shoji's model and experimental data of P. Aaltonen et al. The calculation shows that dependences of experimental crack growth rate (CGR) on theoretical CTSR for tests under different loading conditions are quite consistent. The theoretical CTSR was also compared to that evaluated using Rice's model under the same experimental conditions. Differences between the two theoretical CTSRs were found and the causes are discussed.

View full abstract

Effect of yield strength on stress corrosion cracking (SCC) of stainless steels in high temperature water environments was investigated experimentally and theoretically. In the experiment, warm forging in 200-290℃ range was used to raise the yield strength of mill annealed Type 304 steel to different levels but keep the same phase constitute. The SCC crack growth in oxygenated pure water at 288℃ was examined by using disk-shaped compact tension specimens with trapezoidal wave loading. The specimen with higher yield strength exhibited higher crack growth rate. Theoretical approach based on Shoji's model showed quantitatively that higher yield strength caused higher strain rate at SCC crack tip and subsequently higher crack growth rate. The theoretical results were in good agreement with the experimental results. The theoretical approach also showed that the yield strength effect was more significant in the situation where the materials had higher repassivation ability such as sensitized steels in oxygenated environments or had lower strain hardening ability.

View full abstract

In this paper, Stress Corrosion Cracking (SCC) surface crack behavior of sensitized SUS304 in low K region in high temperature water was evaluated under triangular and trapezoidal wave forms, using surface cracked specimens. Besides, as a reference, through thickness crack behavior was also evaluated under trapezoidal wave forms. The experimental results indicated fairly fast crack growth rates both in lower K region below 10MPam^<0.5> and in higher K region above 15MPam^<0.5>. Based on these experimental results, differences and similarities between the surface crack behavior and the through-thickness crack behavior are discussed. Also the results obtained from this study are compared with the theoretical SCC prediction model proposed by Shoji et al.

View full abstract

Understanding short crack behavior is essential for predicting the lifetime of light water reactor components. However, it is said that crack growth rates of short cracks can vary owing to obstacles by microstructures such as grain boundaries. Statistical behaviors of short cracks can be deduced by crack size distributions. This study investigated the influences of grain boundary inclination on the short crack growth behavior of intergranular stress corrosion cracking (IGSCC). A crack growth simulation model, which considered the mechanical effects of the crack kink and bifurcation by grain boundaries, was developed. Crack depth distribution obtained by the simulation exhibited a kink in the distribution line as seen in the experimental results. This suggests that the grain boundary inclination plays an important role in the short crack growth behavior.

View full abstract

Two kinds of empirical learning methods based on artificial neural network (ANN), i.e., Double Layer Perceptron (DLP) model and Elman Feedback (EF) model, were used to analyze SCC data and predict the SCC susceptibility of austenitic stainless steels in high temperature water(HTW). DLP model could not converge after long training epochs while EF model could reach a steady value within limited training epochs for two sets of SCC data of 304SS and 316SS respectively. Threshold value (ThV) used in EF model had obvious effect on prediction ratios. In the processes of EF model, method I of including sample to be predicted generally had higher prediction ratio than that of method II of excluding the sample to be predicted. The data of the SCC susceptibility of 304SS and 316SS in HTW related to environmental factors such as temperature (T), dissolved O_2 content (DO), chloride ion content ([Cl^<-1>]), electrode potential (E) were used. In the case of ThV<0.6, the ranges of prediction ratio were ca.0.70〜0.95 with method I and ca.0.6〜0.85 with method II for 304SS, ca.0.75〜0.95 with method I and 0.75〜0.90 with method II for 316SS. Results showed that Elman Feedback model was a useful tool for qualitatively predicting the SCC behaviour of austenitic stainless steels in high temperature water.

View full abstract

One possible way of explaining the environmentally assisted cracking of steel construction materials is to assume that the transport of atomic-scale species such as cation vacancies and metallic ions through the oxide film contribute to the growth rate of cracks in stainless steels. A similar approach can be followed to investigate the process that leads to the initiation of cracks on a smooth surface. At the moment, to our knowledge, there is no data available in the literature relating to the evaluation of the effect of stress on the electronic properties of oxide films. This is mainly due to the difficulties of carrying out reliable experiments in high temperature and high-pressure water environments. This paper describes a novel technique that can be used to measure in-situ the electronic properties of oxide films in different parts of a specimen during the slow strain rate test in high temperature aqueous solutions. By measuring the resistances of oxide films on sensitised 304 stainless steel in high purity water at 288℃, we found that during the straining process the oxide film resistance in the gauge part of the specimen increased significantly for up to 5% strain, and then gradually decreased again. The oxide film resistance on the unstressed part of the test sample behaved in a similar way, except the gradual decrease in film resistance at higher strain conditions was not observed. Ex-situ analysis of the oxide film thickness in different parts of the SSRT specimen indicated that the oxide film thickness was highest in the stressed part, thus supporting the idea of increased ion movement through the film.

View full abstract

Intergranular stress corrosion cracking (IGSCC) of alloy 600 was reproduced in borated and lithiated high-temperature water at 340℃ by slow strain rate technique (SSRT). The electrochemical fluctuations both in current and in potential were monitored during the test to investigate localized and transient electrochemical events taking place on the straining specimen. Analysis of the electrochemical transients has been done to deliberate on the mechanism of IGSCC in alloy 600. The results also suggested importance of a double-layer capacitance in the electrochemical events, which is associated with cracking.

View full abstract

This paper introduces the concept of a general approach to the control of grain boundary fracture which is widely recognized as the primary source of the intrinsic brittleness in brittle materials and of extrinsically induced intergranular brittleness by grain boundary segregation, oxidation and wetting of liquid metal. The concept is based on the observations that intergranular brittleness in refractory metals, intermetallics and nanocrystalline materials can be controlled by engineering of grain boundaries, that is, by the elimination of high-energy weak random boundaries or by introduction of a higher fraction of low-energy strong grain boundaries. The transition from brittle to ductile fracture can occur by changing the operating fracture mode from intergranular to transgranular fracture, in both intrinsically and extrinsically brittle materials. Recent achievements of the control of intergranular brittleness by grain boundary engineering are shown to prove the general applicability of the concept for brittle materials with different origins of their brittleness.

View full abstract

High temperature deformation and fracture of Fe-Co and Fe-Cr alloys have been studied in connection with the magnetism. The flow stresses of both alloy systems increased with increasing the solute contents at ferromagnetic temperatures below the Curie temperature while they were independent of the solute contents at the paramagnetic temperatures. Therefore, the solid solution strengthening observed in this study is closely related to the ferromagnetism. Furthermore, the fracture strain in the ferromagnetic temperature region was about 10% larger than that in the paramagnetic temperature one. These findings indicate that the magnetic strengthening originating from the magnetism produces high strength, high ductility and high thermal stability.

View full abstract

In the surveillance program for reactor pressure vessel (RPV), Charpy impact properties are measured concerning neutron irradiation embrittlement. Fracture toughness values are estimated by assuming that radiation effect on fracture toughness is equivalent to that on Charpy properties. Therefore, it is necessary to establish the correlation between both properties especially on irradiation embrittlement. In this paper, we present the fracture toughness data obtained by applying the master curve approach that was adopted recently in the ASTM test method E1921. Materials used in this study are five ASTM A533B class 1 steels and one weld metal. The specimen size effect on the reference temperature in the ductile-brittle transition range was studied by comparing the data by Charpy-size, 25mm thick CT and larger size specimens. Neutron irradiation for Charpy-size specimens as well as standard Charpy-v specimens was carried out at the Japan Materials Testing Reactor (JMTR). The shifts of the reference temperature on fracture toughness due to neutron irradiation are compared with Charpy transition temperature shifts. Correlation between the reference temperature on fracture toughness and Charpy transition temperatures is established. Based on the correlation, the optimum test temperature for fracture toughness testing is suggested. The method to determine a lower bound fracture toughness curve is also discussed.

View full abstract

Welded joint generally has heterogeneity of strength, material, and fracture toughness and it is important to understand the characteristics of material strength and fracture in welded joint considering heterogeneous effect. Furthermore, the material behavior becomes more complicated when welded joint received dynamic loading like as earthquake. Characteristics of strength and fracture in undermatched joint with strength heterogeneity by dynamic loading was studied by round-bar tension test and thermal elastic-plastic analysis in this paper. The strength and fracture in undermatched joints were evaluated by considering the effects of strain rate and temperature including temperature rise by dynamic loading. The differences of fracture characteristics like as ductile-to-brittle transition behavior are well precisely explained from the stress-strain distribution obtained by numerical analysis.

View full abstract

The mesoscopic-structure related strength of graphite was investigated by using the microstructure-based fracture model. The model can treat mesoscopic grain and pore structures with the fracture mechanics as well as the statistical approach. In this study the model was applied to the prediction of tensile strength of the nuclear grade H-451 graphite. Input mesoscopic-structural parameters in the analysis were the grain size and pore size distribution. These parameters were determined by an image analysis with microstructural observation. From the analytical investigation following results are obtained: (1) The prediction by the microstructure-based fracture model had fairly good mean tensile strength as well as strength distribution. (2) Both experimental and predicted tensile strengths fitted in well with the Weibull statistical distribution. (3) The material with fine grain size had high tensile strength with large scatter, when the pore size is assumed to be constant. (4) The material with large pore deviation parameter had low tensile strength with large scatter, when the grain size is assumed to be constant.

View full abstract

It is well known that the tension-softening model can describe the macroscopic fracture behavior of quasi-brittle materials containing pre-existing microcracks such as rocks, ceramics and concretes. The problem studied in this paper is a tension-softening behavior in brittle materials based on the growth of a non-coplanar parallel crack array. Crack growth analyses are performed numerically using the body force method. The problem calculated is a non-coplanar crack array subjected to uniform tensile stress at infinity. The plane strain condition is assumed. The maximum principal stress criterion is employed to determine the each crack growth direction and the global stress required for crack propagation. The tension-softening behavior is studied from the remote tensile stress and deformation of the numerical result. The deduced tension-softening curve is consistent with experimental data obtained for granites. This evidence revealed that the overlapping of non-coplanar cracks, i.e. the ligamentary bridging, is the main source of the tension-softening behavior of granites.

View full abstract

Nondestructive evaluation (NDE) of a pipe-shaped specimen was performed by the use of the acoustic resonance of an open-pipe. A modulated laser beam irradiates an inner surface of the specimen. An acoustic wave was generated by the photoacoustic (PA) effect. Modulation frequency was tuned across a resonant frequency of the pipe. Since a defect of the inner surface of the pipe decreases a Q-factor of the acoustic resonance, tuning characteristic of the pipe showed the existence of the inner surface of the pipe. The PA imaging of the specimen was also demonstrated by attaching a PA cell on x-y slide-stages. Relation between acoustic resonance of the pipe-shaped specimen and that of a PA cell was also discussed. The applications of the present method to NDE of structured materials with a pipe-shaped structure was discussed with a special attention to that of a variable-orifice nozzle (VON) used for the injection of diesel engines.

View full abstract

A phased array transducer is a multi-element piezoelectric device which offers a method of dynamic ultrasonic beam focusing, in which both the focal length and the steering angle of the ultrasonic beam vary as the pulse propagates through the material. The phased array transducer provides an advanced ultrasonic testing such as fast scanning and evaluation for complex-shaped structures. There are various design parameters of the phased array transducer, playing a decisive role in the efficiency of the transducer, such as the number of elements, inter-element spacing, frequency bandwidth and element size etc. In this study, numerical simulation was conducted to investigate the influences of the design parameters of the linear phased array transducers on the beam focusing and the steering performance. The waves generated from piezoelectric elements are considered as simplified transient ultrasonic waves which are constructed with the cosine function enveloped with a Manning window. In this study, the design parameters were optimized for the performance of beam steering and focusing by a numerical simulation.

View full abstract

NDE of complicated surface defect which is composed of two defects with different depths crossing each other has been demonstrated using photoacoustic (PA) microscope. In the present study, simulated intersecting defects are fabricated on specimen surface by mechanical processing, the defect depths of which are ranging 0.5 - 2.0mm in increments of 0.5mm. The measured PA signal of the intersecting surface defect was processed and displayed as an image. Furthermore, the identification of each detected defect and the depth profile was examined with PA signal intensity distribution. The results show that PA method is useful to detect intersecting surface defect which is difficult to detect by the present NDE techniques, and a marked increase in the irradiated area and the edge effect caused by the presence of four corners at the crossing of the orthogonal defects was observed.

View full abstract

Natural frequency method (NFM)-a novel approach for the measurement of creep damage was proposed in this paper. The techniques with instrument configuration, sample cutting and experimental procedure were described in detail. The application of the approach has been made on material 25Cr20Ni that was widely used for furnace tubes. The measured results of creep damage show good agreement with theoretical predictions. The critical damage value ω_<cr> of 25Cr20Ni has been also obtained by the proposed approach. Compared with current density method and load-on and load-off method, NFM has many advantages, such as higher accuracy, simpler apparatus and better repeatability. On the basis above, an efficient approach was given for life prediction and extension of high temperature components.

View full abstract

It is essential to measure the residual stress of resin material to evaluate the reliability of a resin packaged electron device. The leaky surface skimming compressional wave (LSSCW) velocity is determined by the V(z) curve obtained by the acoustic microscope. The LSSCW velocity depends on the stress, therefore, it is possible to evaluate the localized residual stress using the LSSCW velocity. The purpose of this study is to establish a stress measurement method of the resin material of the electron device with an acoustic microscope for basic research. The effect of surface roughness on the LSSCW velocity is investigated using acrylic resin as the resin material. In addition, the relationship between stress and the LSSCW velocity of the resin material is examined. On the basis of these results, a stress evaluation method of resin material using the acoustic microscope is proposed.

View full abstract

A new technique for estimating in vivo bone mineral density and mechanical integrity has been proposed using the results of ultrasonic inspection for the diagnosis of osteoporosis. The technique firstly evaluates that two-dimensional solid area fraction of bone (S, percent bone area fraction between bone and bone marrow) calculated from the difference in the speed of ultrasonic wave propagation through cancellous bone. Secondly, mechanical integrity (Young's modulus and strength) is calculated using finite element analysis for the cancellous bone with the bone area fraction (S) evaluated by the ultrasonic testing. The periodic estimation of the bulk Young's modulus and strength applying the technique proposed here could be used to predict the fracture risk for in vivo bone. Since there is a good relationship between the BMD (bone mineral density) by DXA (dual energy x-ray absorptiometry) and the bone area fraction by ultrasonic testing, the technique also allows the estimation of in vivo BMD of the spine which has been traditionally used to diagnose osteoporosis.

View full abstract

Thermal images of real pitting corrosions generated in alcoholic aqueous solutions were obtained by photoacoustic microscope (PAM). Using immersion tests and electrochemical methods, two different types of pitting corrosions were generated. Immersion testing of pure aluminum plate was conducted in sodium chloride and magnesium chloride alcoholic aqueous solutions. Nondestructive measurement of pitting corrosions was carried out using photoacoustic microscopy. The amplitude and phase of the photoacoustic (PA) signal were measured at every point of the scan and PA amplitude and phase images were reconstructed. The PA amplitude signal intensity varied with both the shape and depth of the pitting corrosion.

View full abstract

This study was conducted with the aim to reduce the minimum detectable size of present ultrasonic technique for the quantitative nondestructive testing of small surface fatigue crack, in order to develop fatigue life management technique. The water immersion ultrasonic method, using an oblique longitudinal wave with large angle (over the critical angle) of incidence, was carried to measure small surface fatigue crack due to low-cycle loading. Surface fatigue crack initiation process, beginning from persistent slip band initiation, was measured using scanning ultrasonic microscope. To estimate the precision of measurement, four transducers with different frequency, they are 5, 10, 20 and 50MHz, respectively, were used to make clear the relation between the ultrasonic frequency and its minimum detectable size of surface fatigue crack.

View full abstract