Cavitation impacts induced by a submerged water jet with cavitation, i. e. a cavitatiog jet, can introduce the compressive residual stress into metals as same way as shot peening. In case of peening by using cavitation impacts, shots are not required, then it is called cavitation shotless peening CSP. The blade/disk dovetail joint in turbine engine commonly fails due to fretting fatigue. The introduction of the compressive residual stress is the most significant factor in improvement of fretting fatigue behavior of metals. In order to investigate the possibility of CSP on reduction of fretting fatigue of titanium alloy, Ti6Al4V, the residual stress of the titanium alloy peened by CSP was measured by using an X-ray diffraction method. It was concluded that CSP can introduce the compressive residual stress into the titanium alloy. The compressive residual stress at the surface peened by CSP is larger than that of shot peening and the roughness of CSP is smoother than that of shot peening.
For bonded dissimilar materials, the free-edge singular stress fields usually prevail near the intersection of free-surface and the interface. When two materials are bonded by welding or by using an adhesive, an interlayer or an adhesive layer may develop between two bonded materials. An interlayer may be inserted between two materials to defuse the residual stress. Stress field near the intersection of the interface and free-surface in the presence of the interlayer is then very important to evaluate the strength of bonded dissimilar materials. In this study, to investigate the effect of the interlayer on the stress distribution on the interface, stress distributions on the interface of bonded dissimilar materials with an interlayer were calculated by using the boundary element method. Relation between the free-edge singular stress fields of bonded dissimilar materials with and without an interlayer were investigated numerically. It was found that the influence of the interlayer on the stress distribution was limited within a small area of the order of interlayer thickness around the intersection of the interface and the free-surface when the interlayer is very thin. Stress distribution near the intersection of the interface and free-surface was controlled by the free edge stress singularity of the bonded dissimilar materials without the interlayer. The interlayer in this condition can be called free-edge singularity controlled-interlayer.
While the structural density in packaged silicon integrated circuits (ICs) is increased, some reliability issues emerge. For example electromigration damage due to high current density and high temperature, and resistance capacitance (RC) delay times resulting from increases in the resistance and the capacitance between lines have been reported as key reliability issues. Recently, the employment of Cu material, which has lower resistivity than that of aluminum, is suggested to resolve these problems. In Cu-based interconnection, the barrier metals (TiN, TaN, etc.) are generally required underneath Cu line in order to defend Cu diffusion into adjacent dielectrics and underlying silicon. However, the poor adhesion of the interface between Cu and barrier metal causes the delamination and leads to deterioration of reliability. A quantitative evaluation of adhesion between Cu and barrier metals is urgently demanded from the reliability point of view. Recently, Kamiya et al. developed the evaluation method of adhesion of the thin film structure with energy release rate. In this study, we use Kamiya's method, and the toughness of the interface between Cu and barrier metal (TiN) is quantitatively evaluated with energy release rate, where Cu film is deposited onto Si covered with TiN film by sputter.
This paper aims at the evaluation of crack initiation along the interface between sub-micron films. The stress singularity field in the vicinity of the edge due to the mismatch of deformations of films governs the crack initiation. Especially, a crack initiates at the corner point where singularity filed of stresses appears strongly, but it is not easy to clarify its mechanism because of the difficulty of controlling the crack initiation and the complication of the structure of the corner. In this paper, the specimen and loading system for interface cracking at the corner point are proposed and the interface strength between Si3N4 and Cu films in multi-layered films on the silicon substrate for an advanced LSI is measured. By using a three-dimensional finite element analysis with sub-modeling method, the stress singularity field in the vicinity of the corner of interface where the crack initiates is revealed. The interface strength of crack initiation can be measured on the basis of fracture mechanics concept and the criterion of mixed-mode interface fracture at the edge or corner is examined.
This paper demonstrated Multi-stages peel test method to evaluate the interfacial adhesion strength between brittle thin film and polymer substrate for flexible optoelectronic devices. The mechanical properties of polymer substrate are strongly influenced by UV irradiation. Therefore, it is considered that the UV irradiation also affects the interfacial adhesion strength between ceramic thin film and polymer substrate. The interfacial adhesion strength was measured by Multi-stages peel test. The results show that the interfacial strength depends on the peeling angle and UV irradiation makes the interfacial adhesion weaker. In-situ observations under tensile test of PET/ITO film were also carried out to investigate the fracture process of brittle thin film under tensile loading. The results show that the crack initiation on brittle thin film decreased after UV irradiation. It is considered that since the interface was damaged due to UV irradiation and the rigidity of interface decreased, the shear stress at the interface between PET/ITO interfaces cannot be high enough to make new cracks.
Thermal spraying of WC-Co cermet was applied on annealed or quench-tempered tool steel substrates by a high-pressure high-velocity oxygen fuel method. Rolling contact fatigue tests were carried out under the loads of 2000, 3500 and 5000N, and delamination energy was obtained by an edge-indent test. After certain numbers of rolling contact cycles, pitting cracks appeared on the surface and preexisted cracks in the coating propagated in a direction parallel to the interface. For the annealed substrate, many cracks were formed in the coating vertical to the interface under the rolling load of 5000N. The delamination energy was larger for the annealed substrate than the quench-tempered substrate before tests, but it decreased with increasing number of rolling contact cycles for the annealed substrate than the quench-tempered substrate. The results of a finite element method reveal that large shear stress and tensile plastic strain exist near the interface, which means that the fatigue damage due to the repeating stress and strain have caused the decrease in delamination energy.
This paper deals with the influence of debonding damage between particles and matrix on elastic and elastic-plastic singular fields around a crack-tip in particle-reinforced-composites. Numerical analyses are carried out on a cracktip field in elastic-matrix and elastic-plastic-matrix composites reinforced with elastic particles using a finite element method developed based on an incremental damage theory of particle-reinforced composites. A particle volume fraction and interfacial strength between particles and matrix of the composites are parametrically changed. In the elastic-matrix composites, the complete damage zone, in which all particles are debonded, and the surrounding progressive damage zone develop around a crack-tip. A unique elastic singular field is created on the complete damage zone in addition to the conventional elastic singular field of the non-damaged zone. The macroscopic stress level around a crack-tip is reduced by the debonding damage while the microscopic stress level of the matrix remains no change. In the elastic-plastic-matrix composites, the damage zone develops in addition to the plastic zone due to matrix plasticity, and both the macroscopic and microscopic stress levels around a crack-tip are reduced by the debonding damage. It is concluded from the numerical results that the toughening due to damage could be expected in the elastic-plastic-matrix composites, while it is suspicious in the elastic-matrix composites.
Fracture toughness tests of Cr-Mo-V steels were conducted by the CT specimens of various thickness (B=b=a, B: specimen thickness, b: uncracked ligament, a: crack length), and 3-dimensional elastic-plastic finite element analyses of them have been performed. Based on the maximum tensile stress criterion for the cleavage fracture, specimen thickness effect on fracture toughness has been investigated analytically. It is proposed from the analyses that the fracture toughness is equivalent to the plane strain fracture toughness, KIc according to ASTM E399, provided that the following condition is met. B≥350(Jc/σflow) or B≥1.0(Kc(J)/σY)2 Where σflow is flow stress, and σY is 0.2% offset yield stress. The thickness required from the criterion is about 40% of that from the requirement of ASTM E399. Ikeda et al. proposed that Pmax=PQ as a validity criterion of KIc, which means that unstable fracture occurs before load-crack opening displacement curve intersects the 5% secant line. The present tests are same results as the proposal. It is demonstrated from the present analyses that the criterion of Pmax=PQ, which were obtained from only experimental results, satisfies the above requirement, B≥350(Jc/σflow).
Fatigue crack growth tests were conducted for SUS304 stainless steel in the near-threshold region using single-edge-cracked (SEC) specimens at 350°C, 550°C and 650°C in a vacuum. Crack growth behaviors of SUS304 in a vacuum were compared with those in air to investigate the effect of the environment and oxidation on the behavior of fatigue crack growth in the near-threshold region. It was found that threshold value, ΔKth, was higher than that in air, and that ΔKth at high temperatures reached a maximum value at about 550°C. When the value of Kmax was increased in a threshold condition obtained by using the conventional ΔK-decreasing test, crack growth was observed for ΔK lower than the conventional threshold. Crack growth was also observed after successive increase in Kmax in the newly established subthreshold region in a vacuum as well as in air. In the region below a certain value of ΔK, however, crack no longer grew even after the change of Kmax at 550°C. Discussion was made on the characteristics of the fatigue crack growth threshold.
In this study, the triaxial deformation experiments were conducted to investigate the temperature and pressure effects on fracture strength of a granite using slip-weakening model. The experiments were conducted at temperatures and pressures up to 600°C and 150MPa under water environment. experimental results show that the peak differential strength decreases with increasing temperature. In contrast, the slip-weakening behavior shows temperature insensitive within the temperature range of this study except 600°C conditions. We measured the fracture energy using slip-weakening curve at various temperature and pressure conditions. From our experimental results, under the same effective normal stress conditions, we could not observe the effect of temperature and pressure in our experimental conditions. Since the tectonic stresses applied to rock mass are constant at the geothermal field, these results suggest that it is available to apply the slip-weakening model to explain the crack propagation behavior in geothermal field.
This paper presents usefullness of the tension-softening model for an evaluation of Mode I fracture toughness in rocks. Rock fracture behavior is dominated by a fracture process zone. The fracture process zone in rock can be classfied into two types, such as planar fracture process zone and diffused microcrack zone. In this paper, the fracture process zone type for various rocks is examined based on microscopic strength distribution. The microscopic strength distribution is estimated from uniaxial tension test data using a statistical model of microcracking prior to damage localization. This examination demonstrates that the fracture process zone should be planar type in various rocks. Next, the fracture toughness of Iidate granite is evaluated using the tension-softening model. The fracture toughness value is compared with toughness values evaluated by means of other methods. The comparison results indicates that the tension-softening model can be utilized in a wide range of fracture ploblems in rock.
Crack is the most dangerous defect in machines and structures. Cracking often initiates from the surface, since the surface stress is often higher than the stress of inside when a metal is under load. Nondestructive evaluation of a small crack is always regarded as a difficult but an utmost important issue for the integrity assessment of structural components. In the present paper, crack closure stress was estimated based on the compliance technique, while crack depth was measured by means of the microwave dual frequency technique. The effect of crack closure stress on the detection and evaluation of 2-D closed cracks in stainless steel by using the microwave dual frequency technique was discussed. In addition, a method to evaluate the shape and size of small 3-D cracks by microwaves was demonstrated. By considering the interference phenomenon occurring in the case of 3-D cracks, a parameter reflecting microwave interference effect, which is a function of the position of the sensor on the crack length, was introduced into the usual dual frequency evaluation-equation. From the modified dual frequency equation, a new component named interference waveform, which is based on the amplitude of the reflection coefficient measured at two different frequencies was obtained. On the other hand, based on the interference model, a corresponding interference waveform, which can be calculated from the assumed shape and size of 3-D cracks, was also introduced. By comparing these two interference waveforms, the evaluation of the shape and size of 3-D cracks were carried out.
A stretched zone is formed as a result of blunting and stretching at the tip of the crack during the fracture process. Consequently, the fracture surface becomes fine and smooth. Paying attention to this characteristic, the authors proposed a new method to evaluate the stretched zone width quantitatively, based on the difference in the fracture surface roughness. To verify the validity of the proposed method, elastic-plastic fracture toughness tests were performed on compact tension specimens cut out of carbon steel pipes. After the fracture toughness tests, the stretched zone was observed with a high-resolution scanning electron microscope to measure the critical stretched zone width (SZWc) for stable crack initiation quantitatively. The critical stretched zone widths determined by the proposed method were compared with those reported in the references. The comparison led the conclusion that this method enabled us to measure SZWc quantitatively, which had been difficult to measure in the absence of skilled microscope operators. The method also showed the possibility to estimate the failure load from the fracture surface by using fracture mechanics parameters.
Fracture surface observations were enforced after cyclic bending fatigue test, three point bending test and impact test. It was confirmed that striation pattern, dimple pattern and cleavage was formed on each fracture surfaces. These patterns corresponded with each failure modes. Pearlite phase was only observed on fatigue fracture surface. These obtained data will be applicable to compare to the fracture surface, when the machine part made in gray cast iron was damaged. Because the crack occurred on gray cast iron is apt to propagate along it due to stress concentration and stress intensity factor increase in the graphite, that is why if some pattern was formed on the fracture surface of machine part, but it is difficult to judge whether actual failure mode is suggested due to its pattern or not.
The hybrid composites with non-woven tissue (NWT) are developed to improve the mechanical properties of conventional FRP composite materials. The hybrid prepreg with NWT consists of unidirectional FRP prepreg and NWT prepreg. The NWT prepreg consists of NWT and resin film. The NWT has short fibers, discretely distributed with in-plane random orientation fibers. A production technique to decrease voids in NWT is proposed in this paper. The mechanical properties of hybrid composites are measured, and then the validity of present technique is confirmed.
Ceramic coating was widely used as film coating technology, due to superior corrosion resistance, wear resistance and high hardness. However, various types of defect in nm or μm levels such as pinhole defect and crack were inevitably existed in thin film due to film formation process. Depending on these various factors, localized corrosion was generated and even developed into micro-cracks in some cases, when ceramics coating was applied in corrosive environments. And also, evaluation of corrosion resistance of ceramic thin film coating, the electrochemical potential sweep methods were employed due to superior reproducibility and simple experimental procedure. However, corrosion pit was not necessarily formed depending on the difference of corrosion resistance of coated thin film in case when the potential was swept up to relatively higher value. Therefore, the understanding of correlation between localized corrosion progress and defect morphology of thin film coating was needed. The defect morphology and nm or μm order's localized corrosion process generated on various types of defects in 3% NaCl aqueous solution was examined by Atomic Force Microscopy observation. And also, difference in localized corrosion behavior in relation to kinds of defects was investigated. As a result, the correlation between localized corrosion behavior and the kinds of defects in film that existed from the initial stage was recognized in TiN thin film.