This review article summarizes recent activities within the SAFE Research Center, which aim to open a new horizon in the field of structural integrity. These activities include (i) enhancement of fracture assessment methodologies, (ii) development of sensing technologies and (iii) development of IT (Information Technology)-based intelligent plant management system. The first issue is to describe the more reliable fracture assessment methodology, recently proposed by authors, and its application to non-linear fracture mechanics problems. The second issue is to describe how to implement the current advances in fiber-optic technology to condition monitoring systems. 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 and 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 advances in structural integrity assessment methodologies and fast-developing information technology in the future, and thus will become a key element in safe and informative plant management.
Intergranular stress corrosion cracking (IGSCC) of alloy 600 was reproduced in borated and lithiated high-temperature water at 340°C by slow strain rate test (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 the importance of a double-layer capacitance in the electrochemical events, which is associated with cracking.
In establishing a long life fatigue design criterion and estimating the reliability of spot-welded thin sheet structure such as the automobile and train, it is necessary to estimate fatigue strength of spot-welded joint. So far, many investigators have numerically and experimentally studied on the method of systematic fatigue strength estimation for various spot-welded joint. Also considerable amount of data on the fatigue strength of spot-welded joint has been accumulated. But, it is difficult to determine the fatigue design criterion based on the existing data when fatigue design is modified. Therefore, many tests were required to determine a new criterion for modified design. Then it takes a lot of money and time. Thus, an expert system was developed to predict the fatigue design criterion including fatigue strength and fatigue life of spot-welded joint. The expert system consists of the material destination database, the spot-welding condition database and the fatigue design module. And this expert system was developed based on the knowledge base, the numerical calculation module, interface engine and user interface. It is expected that the expert system can economically provide the designer and the manufacturer with useful information on the material properties and the fatigue design criterion for a modified design.
Cyclic loading tests were performed for the through-thickness center cracked specimen of annealed copper under load- and displacement-controlled conditions. The crack opening/closing behaviors were experimentally examined, and then calculated by means of elastic-plastic finite element method, employing the constitutive equation, which was proposed by the authors in their earlier papers. The calculated load-displacement responses were in quite good agreement with the measured ones under both the test conditions. The large crack opening generated on the first tensile load excursion decreases on the following unloading and compressive loading excursions under the load-controlled condition at the load ratio RL=−1.0. The crack opening displacement reaches the smaller value at the second maximum tensile load point than at the first one. Meanwhile, the greatly opened crack on the first tensile load excursion reduces its opening displacement at the successive unloaded point, and the crack opening displacements at the tensile peak load and the unloaded point tend to reach the corresponding stable states under the displacement-controlled cyclic loading at the displacement ratio RD=0. The calculation describes quite well such crack opening/closing behaviors under both the cyclic loading conditions.
The initiation and growth behavior of transverse crack in rail steel was experimentally investigated using the notched keyhole specimen under constant amplitude and variable amplitude loads. Fatigue limits of smooth and keyhole specimen in rail steel at R=0 were about 198 and 59MPa, respectively. The fatigue crack initiation length at notch root was about 1.3mm. The fatigue lives under variable amplitude load predicted by introducing the modified equivalent stress (σeq´) agreed closer to the experimental results than those predicted by using the conventional equivalent stress (σeq). As the characteristic stress intensity factor range (ΔKrms) increases, the fatigue crack propagation rate (da/dN) under variable amplitude load derived from the actual load history is faster than that under constant amplitude load. Through the examination of fracture surfaces in each load condition, it was found that this behavior results from the transition of fracture appearance.
Using two types of specimen that rolling direction was parallel and perpendicular to loading direction in film fatigue testing, effects of rolling texture on fatigue crack propagation properties in iron films of 100µm were discussed in relation with the grain size. A crystallographic analysis by EBSD (Electron Backscatter Diffraction) method showed that the anisotropy of rolling texture remained even in the films annealed at 873K and 1173K. The cracks in the film with the smaller grain size annealed at 873K propagated along the grain boundary and the crack propagation rate is approximately the same for both loading direction parallel and perpendicular to the rolling direction. On the other hand, the cracks in the film with the larger grain size annealed at 1173K propagated across the grain and the crack propagated slower toward the perpendicular direction to the rolling direction than toward the parallel direction. This is probably because the slip system of the rolling texture in a grain of the iron film annealed at 1173K is different in accordance with the relationship between rolling direction and crack propagation direction. Also, the rolling texture was outstandingly turned around the fatigue crack propagated perpendicular to the rolling direction in the film annealed at 1173K.
The stress field near a contact edge in fretting fatigue tests depends on the deformation type at the contact edge. During one cycle of loading in a fretting fatigue test, nine types of the stress field, among which six are independent, can possibly exist. In this study, the detailed forms of the singular stress variation range near the contact edge, i. e., the differences of singular stress fields corresponding to the maximum and the minimum loads, have been theoretically analyzed. The theoretical results were compared with numerical results obtained by the boundary element analysis. It was found that the singular order of the stress field obtained by the theoretical solution agrees well with the numerical results. It should be noted that the stress variation range presents different singular behavior for different deformation types. Therefore, the fatigue strength evaluation should be carried out in a manner corresponding to the stress variation range type. It was found that a fretting fatigue crack was initiated in the maximum shear stress range direction, and then kinked into the maximum tangential stress range direction following the initiation stage of the crack.
Long term cantilever-type rotational bending fatigue tests of up to 109 cycles were carried out on high carbon chromium bearing steel, SUJ2. The fatigue fracture behavior of SUJ2 in the super long life range was discussed based on scanning electron microscope observations and fracture mechanics. Fatigue failure occurred when the number of cycles exceeded 107. In the super long life range, the fish-eye-type fracture and the subsurface-type fracture were observed. In the fish-eye-type fracture, the stress intensity factor calculated from the area of the facet region was independent of the number of cycles to failure and was almost constant at 5.4MPa• m1/2. In the subsurface-type fracture, high carbon segregation was observed at the crack initiation area. The stress intensity factor for the carbon segregation area was close to 5.0MPam1/2. Pure fatigue crack was initiated from the area outside the facet region or the high carbon segregation area.
To the best of the authors' knowledge, any fracture parameter such as the dynamic J integral for a naturally and dynamically propagating crack front has not been evaluated. In our previous experimental study, high-speed photographs of dynamically propagating crack fronts in DCB specimens were firstly recorded. In this paper, first, to overcome difficulties in three-dimensional dynamic fracture simulation, a three-dimensional moving finite element method together with an automatic element control method is developed using a mapping technique. Next, to make it possible to evaluate the dynamic J integral along the dynamically propagating curved crack front, an equivalent domain integral method of the dynamic J integral is developed. Furthermore, to accurately evaluate dynamic stress intensity factors along the curved crack front, the component separation method of the dynamic J integral is also developed. Based on these simulation technologies, the generation-phase simulations are carried out, using the experimentally recorded histories of three-dimensional dynamic fracture events in 20mm thick DCB specimens. The distributions of the dynamic J integral and stress intensity factor along the actual dynamic fracture fronts are firstly elucidated. Based on these results, pertinent mechanism of three-dimensional fracture is also discussed.
In this work, the homogenized elastic-viscoplastic behavior of long fiber-reinforced laminates under in-plane loading is predicted by taking into account the microscopic structure and stacking sequence of laminae. A homogenization theory of nonlinear time-dependent composites is applied to such laminates, leading to the macroscopic rate-type constitutive equation of laminates and the evolution equations of microscopic and average stresses in each lamina. The macroscopic constitutive equation is shown to have a stiffness tensor and a stress relaxation function which are evaluated explicitly in terms of the microscopic structure and stacking sequence of laminae. The established theory is then verified by performing in-plane uniaxial tensile tests of unidirectional, cross-ply, and quasi-isotropic carbon fiber/epoxy laminates. It is thus shown that the theory predicts successfully the anisotropic viscoplasticity of unidirectional and cross-ply laminates and the negligible viscoplasticity of quasi-isotropic laminates.
This paper deals with progressive ply-cracking damage and nonlinear deformation of CFRP cross-ply laminate. Tensile tests in 0°-axis direction and in off-axis direction are carried out on five kinds of CFRP cross-ply laminates which are different in stacking lay-up. Under 0°-axis tension, initiation and evolution of the ply-cracking damage in 90° plies are observed, and corresponding stress-strain relation is slightly nonlinear after the damage. On the other hand, under off-axis tension, the stress-strain response exhibits nonlinear deformation, while the ply-cracking damage is not remarkable except for the specimen with large fracture strain. Therefore, it is concluded that the nonlinear deformation of the laminates under off-axis tension is mainly attributed to the nonelastic deformation of the matrix resin. Numerical analyses for the progressive ply-cracking damage and the nonlinear deformation of the CFRP cross-ply laminates are conducted based on a ply-cracking damage theory and a nonlinear lamination theory, respectively, and applicability of these theories are discussed based on the experimental and numerical results.
Composite patch repair of aircraft structural panels has recently received wide attention. In designing reliable composite patches enough to operate in severe environmental conditions, the accurate stress fields in the repaired structural panels and the bonded composite patches should be analyzed. If the conventional three-dimensional finite element method (FEM) is used to obtain the accurate stress fields, it may require a long computational time for iterative stress analysis. In this paper, we develop an effective three-dimensional FEM based on the concept of domain decomposition with independent interfaces. Using this method, we carry out several parametric studies to examine the effect of patch shape and size on the stress fields in the panel and the patches. The method is also used to determine the optimum patch shape and size so that the repaired structure can endure maximum applied tensile stress by a mathematical programming when the patches are of  unidirectional and [0/90]s cross-ply composites.
We verify that ultrasonic atomic force microscopy (UAFM) can detect and evaluate subsurface objects with a resolution of around 10nm. We first show that the resonance frequency of UAFM cantilever shows a measurable change due to subsurface low-elasticity layer by a finite element analysis. We then proved the ability of subsurface imaging in a highly oriented pyrolytic graphite (HOPG) specimen. We found a new type of dislocation motion. As a load was applied to the tip, apparent edge-type dislocations (Frank partial dislocations) moved to the direction of climb over distances of 47nm and returned to the original position as the load was removed. To explain this motion, we propose a possible model where the extra half-plane of the dislocation is elastically compressed to shorten its length due to the normal load applied by the tip.
In dynamic atomic force microscopy (AFM) for detecting local elastic properties of samples, it is desirable that the contact resonance of an AFM cantilever is sensitive to tip-sample contact stiffness. This paper presents a unique cantilever with its mass concentrated as a way of enhancing the sensitivity. The cantilever is made up of a commercially available normal cantilever and a tungsten particle adhesively attached to the free end. Spectra of the contact vibrations are measured for three sample materials. The mass-concentrating cantilever whose tip is in contact with a sample exhibits a distinguishing spectrum containing a meaningful resonant peak affected significantly by contact stiffness. If the attached mass is about four times larger than the distributed (cantilever) mass, the resonant peak obeys a spring-mass model. This means that the cantilever with its mass concentrated can provide the maximum sensitivity even for stiff materials.
To evaluate the depth of small fatigue cracks under conditions of no contact and without using any coupling medium, a novel microwave technique was demonstrated. An open-ended coaxial line sensor was used to increase the spatial resolution and the ratio of signal to noise. Closed fatigue cracks were detected successfully and a W-shaped characteristic signal was obtained. A dual frequency technique was proposed to evaluate the depth of small fatigue cracks for which crack closure characteristics were unknown. The evaluated results are shown to agree well with the actual values.
Acrylamide-based amphoteric gel containing -COOH and -NH2 can be hardened through the formation of salt-linkages, -COO-···+H3N-, which serve as columns to reinforce the gel matrix. These salt-linkages can be formed and disrupted reversibly by the control of ionic strength of bathing solution, resulting in the gel hardness variance. The same kind of salt-linkages were also found to form between immobile -SO3H and -NH2 both imported in acrylamide-based gels, in the form of -SO3-···+H3N-. It was also observed a reversible formation and disruption of -SO3-···+H3N- according to the ionic strength of bathing solution. Thus the import of both -SO3H and -NH2 groups in a single gel is expected to induce a reversible hardening and softening of gel matrix through the formation of salt-linkages. From this observation, it was strongly speculated that the reversible hardness variance has to be a quite common phenomenon for any type of amphoteric gel.
By computer simulation, we estimated macroscopic elastic moduli of sintered equal-sized spherical particles. The simulation is composed of sequential accumulation of spheres and structural analysis of a “random network of 6-degree-freedom springs”, which is a mechanical model of “sintered particles”. From the examination of statistical characteristics of the random packings of spheres, we discovered that their packing structure is affected by gravity; more precisely, line segments connecting the centers of spheres in contact lie more frequently around the direction of 45° from the vertical (gravity) line, although they are uniformly distributed about the vertical line. This non-uniform zenithal frequency-distribution of segments makes, in turn, the sintered aggregates transversely isotropic in elasticity: Young's modulus in the vertical direction is roughly 17% larger than that in the horizontal direction. Our additional experiments using sintered glass-beads saturated with water support the simulated anisotropy.
The framework of a homogenization method with characteristic deformation mode superposition is presented in this paper for nearly incompressible problems. Incompressibility is treated by the mixed method based on Simo-Taylor-Pister variational principle.
In this study, an ultrasonic technique for quantitative nondestructive evaluation of a small surface fatigue crack was developed. The use of an oblique longitudinal wave with large angle (over the critical angle) of incidence upon a specimen surface was emphasized. Ultrasonic testing was performed for a specimen with small surface fatigue cracks and artificial defects. We have used maximum amplitude of reflection echo due to these defects as a parameter, to evaluate them quantitatively. From such measurements, we have concluded that with the use of this parameter, surface fatigue cracks with more than 115µm of surface length can be detected. Simultaneously, crack depth and deflection can also be detected. Based on these experimental results, we have proposed a set of experimental equations to evaluate the surface length and depth of crack. Finally, a model based on three-dimensional elastodynamics was proposed to simulate this ultrasonic technique, and numerical analysis was performed for the surface crack reflection problem.
In order to evaluate the reliability of ceramics under transient thermal stress, thermal shock test equipment was designed and fabricated. Cylindrical specimens made of Si3N4 and SiC were tested by the test equipment. Each specimen was preheated uniformly at 1573K in an electric furnace and then pulled down rapidly into a cooling chamber beneath the furnace. At the same time, it was cooled locally by high-velocity helium gas that was passed through a narrow slit. The transient temperature distributions were measured by optical pyrometers and thermocouples. The moment of fracture was confirmed by the sound of the fracture itself. Fracture originated at the outer surface immediately under the cooling nozzle. Consequently, the average value of nondimensional fracture stress of Si3N4 specimens was about two times that of SiC specimens.
In this paper, the dynamic behavior of two collinear anti-plane shear cracks in a piezoelectric layer bonded to dissimilar half spaces was investigated for the impermeable crack face conditions. The cracks are vertical to the interfaces of the piezoelectric layer. By means of the Fourier transform, the problem can be solved with two pairs of triple integral equations. These equations are solved by use of the Schmidt method. This process is quite different from that adopted in previously. Numerical examples are provided to show the effect of the geometry of the cracks, the piezoelectric constants of the material and the frequency of the incident wave upon the dynamic stress intensity factor of the cracks.