The intrinsic threshold behavior of fatigue cracks and the disappearance of any cyclic plastic deformation below a threshold value can be understood by taking into account the discreteness of plasticity with recourse to discrete dislocation models. The aim of this paper is to document the progress in the discrete dislocation modelling within the past twenty years and the resulting increase in the understanding of fatigue cracks. The problems addressed are (1) the nature of the intrinsic fatigue threshold, (2) the influence of microstructure and/or of the mean stress level on the crack tip deformation and (3) the physical reason for the minimum striation spacing at small stress intensity ranges. A particular purpose of this paper is to compare the different dislocation models proposed in the literature in order to differentiate aspects of fatigue crack growth that do and do not depend on modelling and on microstructural details.

View full abstract

Experimental analysis of plastic zone growth in silicon have recently underlined the fact that two different populations of dislocations sources have to be considered. A small number of primary ones, linked to defects of the crack front, emit tens of dislocations in primary planes. A large number of secondary ones, resulting from cross-slip of primary dislocations coupled with the mechanism of stimulated emission, only emit a few dislocations. This paper shows that primary dislocations cannot account for observed brittle to ductile transition.

View full abstract

The influence of a plastic zone developed at a crack tip is analyzed under a micromechanical point of view. Mechanical interaction is described in terms of crack shielding caused by the stress intensity factor induced by a dislocation loop in the three opening modes. Numerical calculations and experimentally studied configurations quantify the total relaxation rate induced by this dislocation loop.

View full abstract

Emission of dislocations from a crack which propagated at the ductile-brittle transition temperature (DBTT) in Si was observed by combined use of focused-ion beam (FIB) technique and transmission electron microscopy (TEM). At the wake of a DBTT crack many dislocation lines and dislocation loops were observed, while a wake of a precrack introduced at room temperature, no dislocations were observed. In addition, those glide dislocation lines which are emitted at the DBTT crack are smoothly curved, indicating that they can overcome easily the Peierls stress.

View full abstract

Dislocation structures near the tip of a crack in ionic crystals with the NaCl-type structure were investigated by using an etch-pit technique, photoelasticity and high voltage electron microscopy (HVEM). The characteristics of those dislocation structures are reviewed and their effect on the local stress intensity factor is discussed. We focused on two kinds of plastic zones developed near a crack tip. Firstly, in bulk NaCl crystals, the so-called hinge-type structure of slip bands were formed along {110} planes ahead of a {100} crack. The effect of crack tip shielding due to dislocations was demonstrated using a photoelastic method visualizing the internal stress field around a crack tip. Secondly, dislocation configurations in MgO thin crystals were analyzed using HVEM, where dislocation arrays of the {110}⟨1\\bar10⟩ slip system corresponding to the plastic zone of the 45^°-shear-type were formed around a {100} crack. 3-D stress analyses for the crack-dislocation interaction indicates that the dislocations observed contribute to mainly the mode I shielding to suppress the crack extension.

View full abstract

The causal relation between cross-slip structure and brittle-to-ductile transition has been examined using NaCl single crystals with the [100] axis. At around room temperature, the crystals exhibit cleavage fracture after a few percent elongation, although their yield stress is lower than 2 MPa. When the temperature is raised, brittle-to-ductile transition takes place at around 400 and 500 K with the respective strain rates of 5.5×10⁻⁶ and 5.5×10⁻⁵ s⁻¹. The transition temperatures correspond well to those for the abundant operation of cross slip. The cross-slip lines are not parallel to a specific crystallographic plane, but are widely distributed in angles of 10^°∼40^° from the slip line of the primary plane (10\\bar1) on the (001) surface. Even in high magnification images observed using an UHV-AFM, cross-slip lines appear to be wavy, indicating the cross slip on indefinite planes. Dislocations emitted from a crack tip can extend sideways along a moving crack front by the cross slip on indefinite planes, and reduce local stress intensities to impede the crack advance by crack tip shielding due to dislocations.

View full abstract

Crack propagation and brittle fracture are simulated with a combined model of molecular dynamics with micromechanics. In the simulation of NaCl the material cleaves before it emits dislocations, whereas dislocation emissions are observed in experiments. In the simulations of tungsten we discuss the validity of interatomic potentials at first and simulate brittle fracture processes at the temperatures between 77 (K) and 225 (K). In the simulation using a pair potential, phase transformation, which is not likely to occur, is observed at the crack tip region, whereas it is not observed in the simulation using an EAM potential. In the simulation of brittle fracture processes using an EAM potential, cleavage along {121} planes is observed, while the pre-cracks are introduced on {110} planes. The cleavage along {121} planes is also observed in experiments. Fracture toughnesses obtained in the simulations show the clear temperature dependency. The values of fracture toughness, however, do not show good agreements with the experimental values. The critical stress intensity factor K_IE for dislocation emissions is discussed to investigate the thermal effect on the brittle fracture precesses. As the result, it is shown that the temperature dependency of fracture toughnesses are caused by the difference of dislocation mobilities.

View full abstract

This paper reviews specimen size effect which significantly affects fracture toughness and fracture behavior of materials mainly based on the present authors’ research works. After introducing controversial scale problems on fracture and fracture toughness, various actual examples of the scale problems are presented from a viewpoint of the effects of specimen size on dynamic fracture toughness values in various materials, such as steel for nuclear pressure vessel designated as A508cl.3, ductile cast iron, AC4CH aluminum casting alloy and silicon nitride ceramic. Then, the λ factor which has been introduced by Atkins et al. to interpret differences in fracture behaviors between a specimen (i.e. model) and the actual structure (i.e. prototype) and the Q factor which expresses the discrepancy between the HRR solution and the actual stress distribution at a crack-tip, are described for recognition of the scale problems.

View full abstract

Damage-tolerant design and life-prediction methodologies have been practiced for metallic structures for decades, although their application to brittle materials, such as ceramics and intermetallic alloys, still poses particular problems, primarily because of their extreme flaw-sensitivity. Moreover, like metals, they are susceptible to premature failure by cyclic fatigue, which provides a prominent mechanism for subcritical crack growth that further limits life. One specific problem involves the large dependency of growth rates on the applied stress intensity, which necessitates that design is based on the concept of a fatigue threshold, particularly in the presence of small cracks. In this paper, studies on the role of small cracks in influencing thresholds and near-threshold growth rates are described for both brittle and ductile materials. Examples are given from the military engine “High-Cycle Fatigue” initiative, which represents an important problem where information on the behavior of small fatigue cracks is critical.

View full abstract

Fatigue crack growth tests have been performed for micro-sized Ni–P amorphous alloy specimens to investigate the size effects on fatigue crack growth behavior of such micro-sized specimens. Two types of cantilever beam type micro-sized specimens with different breadth (B10×W12×L50 \\micron³ and B30×W12×L50 \\micron³) were prepared from an electroless plated Ni–P amorphous alloy thin film by focused ion beam machining. Notches with a depth of 3 \\micron were introduced in the specimens. Fatigue crack growth tests were performed using a newly developed fatigue testing machine for micro-sized specimens in air at room temperature under constant load range and stress ratios of 0.1, 0.3 and 0.5. Striations were observed on the fatigue fracture surfaces and fatigue crack propagation rates were estimated by a careful measurement of the striation spacings. The fatigue crack growth rates at stress ratios of 0.3 and 0.5 were higher than that at 0.1. This suggests that crack closure may occur even in such micro-sized specimens. The fatigue crack growth resistance is also dependent on the specimen breadth. Shear lips which correspond to plane stress dominated region and a flat fatigue surface which corresponds to plane strain region are clearly observed on the fatigue surfaces, and the size of the plane strain region is different between the specimens with different breadth. This difference in stress state ahead of the crack may affect the crack growth behavior. The results obtained in this investigation are the first measurements of fatigue crack growth properties for micro-sized specimens and provide important information on reliability of actual micro systems and microelectromecahinical devices.

View full abstract

Cyclic deformation of ultra-fine grain (UFG) materials processed by severe plastic deformation is reviewed in light of recent experimental results and common concepts of fatigue. High strength bulk metals with a characteristic structural element size of 200–300 nm were obtained through the so-called equal-channel angular pressing (ECAP) technology. Fatigue properties are discussed in terms of stress-controlled and strain-controlled fatigue. Enhancement of fatigue life under constant stress amplitude is emphasized in comparison with some shortening in fatigue life under constant plastic strain amplitude. Fine structure and surface morphology of post-fatigued materials are characterized on different scale levels to account for the fatigue behaviour observed. Mechanisms of fatigue in ECA-processed materials are discussed within frameworks of a simple one-parameter model of dislocation kinetics.

View full abstract

Experimental assessment of fracture toughness behavior was performed using CT- and precracked Charpy-V specimens of two reactor pressure vessel steels in different warm prestressed conditions. By means of X-ray analysis, a butterfly-like compressive residual stress field was evaluated at the prestressed crack tip. Fractographic features indicated an increase in the critical distance for cleavage initiation after prestressing. Based on the experimental results, the warm prestress effect may be seen as a complex phenomenon in which the loss of constraint due to compressive residual stresses and a microstructural predamaging at the blunted crack tip are interrelated to an apparent enhancement of fracture toughness.

View full abstract

This paper reviews fracture features of a variety of conventional and advanced materials which consist of more than two different phases in terms of composite material. To be concrete, the materials include ductile cast irons, carburized steels, aluminum alloys, and several kinds of discontinuously-reinforced composites, all of which have been studied by the present authors to date. Firstly, features on fracture of the above-mentioned three conventional materials are introduced. Those have constructions common to artificial composites; particle reinforced composite and layered composites. In the latter half of this paper, the detailed mechanisms of deformation and fracture in artificial composites and those analyses mainly on the basis of continuum mechanics and fracture mechanics are shown. Deformation and fracture behaviors of “natural” (or in-situ) and artificial composites are discussed in terms of their similarities and discrepancies both from metallurgical and mechanical points of view in order to bring systematic understanding of the materials having complex microstructures. In addition, it is shown that certain phenomena that already constitute common knowledge in the field of composite materials, should be strongly considered in the field of conventional materials.

View full abstract

The overall interfacial debonding in unidirectional multifilamentary composites is affected by many factors such as damage(broken components (fiber and matrix) and interfacial debonding)-induced mechanical interactions, residual stress and frictional shear stress at debonded interface. In the present work, the influences of such factors on debonding behavior were studied by applying the simulation method, in which the modified shear lag analysis was combined with the energy release rate criterion for debonding, to the various spatial distributions of cut-ends of components in a two-dimensional model composite. Main results are summarized as follows. (i) The progress of debonding is dependent on number, species and geometrical location of cut components. (ii) The overall debonding progresses more rapidly with increasing number of cut components due to the enhanced mechanical interactions. (iii) The debonding progresses intermittently, resulting in serrated stress-strain curves. (iv) When tensile and compressive residual stresses exist in matrix and fiber, respectively, along the fiber axis, they act to enhance and retard the debonding when the matrix and fiber are cut, respectively. (v) The frictional shear stresses at debonded interface act to retard the debonding and to raise the load carrying capacity of the composite in which debonding is saturated.

View full abstract

Particle dispersed glass matrix composites have been developed in order to increase the strength of glass, and microfracture before the final fracture during bend test has been observed in many ceramics and glass composites as acoustic emission (AE) signals. Stochastic process treatment for microfracture of these composites was performed to understand the mechanical properties of these materials. Bending strength of these materials was measured as various conditions in loading rate and atomoshere. AE behavior during these tests was also detected with two transducers and a two-channel waveform acquisition system to evaluate the microfracture location in the materials. Microfracture processes during bending tests were clealy classified into four major types from the results of source location of AE, that is, (i) unstable fracture type, (ii) crack propagation type, (iii) transition type from random microfracture to crack propagation and (iv) competition type between random microfracture and crack propagation. The effect of loading rate, atmosphere and volume fraction of reinforcing particles on microfracture process was discussed.

View full abstract

Direct observation methods were successfully applied to investigate the pore structure in green and sintered bodies of alumina ceramics made through a powder granule compaction process. Fracture strength was measured for alumina sintered bodies prepared under three different processing conditions, using granules made through the same procedure. The strength variation among the sintered bodies was quantitatively correlated to the difference in the size and concentration of large pores determined by the observation methods. These pores were developed during sintering.

View full abstract

Laminated nanostructures of nickel and copper were fabricated via electrodeposition and their microstructure, strength and fracture behavior were characterized using x-ray diffraction, tensile testing and electron microscopy techniques. The results of this study indicated that the formation of porous regions is responsible for the brittleness of electrodeposited Ni/Cu laminated structures. Microprobe analysis revealed that within these porous regions copper is deposited with a low efficiency. It is suggested that local depletion of copper ions and formation of hydrogen bubbles due to hydrodynamics effects are responsible for the low efficiency of copper deposition and formation of pores. The brittle fracture of nickel and copper layers is discussed in terms of cleavage and tearing mechanisms.

View full abstract

By using a vertical sectioning method (VSM) or secondary electron line scanning method (SELSM), the fractal dimension D_S for surface, D_L for scanning profile were measured quantitatively on some typical fracture surfaces, namely cleavage fractures, dimple fractures and fatigue fractures of composite materials. It was shown that the measured value of D_S relates differently to the impact energy values of materials for cleavage or dimple fractures. The microstructure of materials should be considered comprehensively when relating D_S to the mechanical properties of materials. It is found that the correlation between the fractal dimension and the impact energy obtained by SELSM and VSM methods appears to be quite similar. Moreover, the quantitative measurements on the fatigue fracture surfaces of SiC/Al composite materials showed that SiC volume fraction has a strong effect on fractal dimension D_S, and that there is an obvious difference in the D_S values for fatigue fractures which are due to different fracture mechanisms. These results show that it is possible to reflect the fracture mechanism using D_S and relate it to the fracture properties of materials.

View full abstract

The effect of hydrogen on the shear localization and associated crack nucleation has been investigated by means of a three point bending test of hydrogen-charged steels. The ductile crack growth resistance in terms of the slope of R-curve was lowered under the presence of hydrogen, the decrease being more pronounced in the steel with more abundant slip constraint phases along grain boundaries. Enlargement of size and reduction in depth/width ratio of primary dimples, occasionally associated with quasi-cleavage, were observed on the fracture surface of the hydrogen-charged steels. By means of a FEM calculation, the increase of the nucleation void volume fraction localized at the crack tip with strain localization as well was shown to take place in the hydrogen-charged steel in consistent with enhanced shear instablity. It was discussed that the evolution of vacancy-type defects, rather than void nucleation at second phase particles, in the course of plastic straining was enhanced under the presence of hydrogen, reducing the ductile crack growth resistance.

View full abstract

From optical microscopic observations a small drop of liquid gallium placed on an aluminum thin film has been found to form arachnoid rises on the film surface. In addition, metallic luster was lost in a bigger region concentrically extending from the position of the gallium drop. These must have occurred because of the penetration of gallium atoms into the film. The speed of gallium webbing was measured to be 2.3 \\micron/s to 9.1 \\micron/s, depending on the film thickness, the temperature and the amount of gallium drop. On the other hand, the dull region extended at the speed of approximately 0.83 \\micron/s, insensitively to the above factors. The embrittlement of aluminum by liquid gallium can be associated with this fast penetration of liquid gallium.

View full abstract

Superelastic Ni–Ti alloys are widely used as engineering and medical materials. However, the alloy is susceptible to environmental embrittlement in a corrosive atmosphere. Accelerated tests of hydrogen embrittlement of the Ni–Ti alloy were carried out. The effect of electrolytic charging hydrogen on the properties of the alloy was measured. The rupture process in tension of the alloy with charged hydrogen was discussed on the basis of the tensile stress-strain curve, hardness in a cross-sectional area and fractography. The results of the measurements suggested that hydrogen concentration on the surface of the alloy and immersion time in an electrolytic bath had different effects on embrittlement. This will be related with the distribution of the hydrogen concentration, which was determined from the diffusion equation and boundary conditions.

View full abstract

The microstructure of the grain boundaries and solute segregation are studied for 0.3 mol%CuO doped 3 mol% yttria-stabilized zirconia (3Y-TZP) using TEMs equipped with EDS and EELS . The observations revealed grain boundaries of various thickness ranging from a sharp boundary without any extra phase to those with amorphous-like phase, of up to 2 nm thickness in between. There were also pockets filled with amorphous-like phase at the triple junctions of grain boundaries. Segregation of Cu and Y were observed not only in these amorphous-like regions but also along sharp grain boundaries without any extra phase.

View full abstract

To get an insight into high cycle fatigue (HCF) properties of austenitic stainless steel under fusion reactor operation conditions, we carried out the ε_p-controlled HCF tests at 403 K and those under in situ thermal pulses (TP-tests) for a low-carbon and high-silicon 316L stainless steel, where ε_p is the plastic strain amplitude. The surface morphology of slip bands observed for the specimens subjected to HCF strains is similar to that of the persistent slip bands (PSBs) in copper. However, application of the traditional method employed for the confirmation of PSBs in copper to the present specimens indicates that the slip bands observed at the early quarter of the fatigue life, N_f, are composed of PSBs as the major fraction and suspended slip bands as the minor fraction. After the slip band observation, we surmise that the elongation in N_f reported for the TP-tests with a temperature jump ΔT of about 10 K at about 333 K is associated with revival of the suspended slip bands. On the other hand, we found here the shortening in N_f for the TP-tests with ΔT of 16 to 32 K at 403 K as well as that reported for the TP-tests with ΔT of about 100 K at about 333 K reported, suggesting that the constituent plastic strain per PSB increased after TPs. We surmise that fine fatigue-induced defects are formed in PSBs at around 403 K and work as obstacles against dislocation motions in PSB . The present work demonstrates that for the structural material subjected to the HCF strains, the shortening in N_f can be expected under in situ TP condition as well as under in situ irradiation condition reported.

View full abstract

Superplastic behavior was examined in fine-grained (∼2 \\micron) WE43 magnesium alloy, that contained, within its grains, spherical precipitates with a diameter of ∼200 nm. The material exhibited superplasticity with an elongation-to-failure of over 1000% at a temperature of 673 K and a strain rate of 1×10⁻⁴ s⁻¹. Large elongations were obtained in spite of the existence of particles. The dominant deformation mechanism was suggested to be grain boundary sliding accommodated by slip controlled by grain boundary diffusion. Data analysis based on the constitutive equation for superplastic flow revealed that the normalized strain rate for particle-strengthened WE43 alloy was about fifty times lower than that for conventional superplastic magnesium alloys. It was suggested that the existence of intragranular particles affects the superplastic flow.

View full abstract

Graphite was bonded to nickel under joining compressive stress of 3 to 33 MPa in a vacuum at temperatures within the range of 973 to 1273 K using an RF-induction furnace. The influence of joining conditions on the bending strength of the graphite/nickel joint, and changes in the microstructure and hardness of nickel near the joining interface, were investigated. Thermal stress induced in the joint was estimated using the finite element method. On the basis of these results, the influence of thermal stress on the bending strength of the joint was examined. Completion of the graphite/nickel joint depends on both joining compressive stress and joining temperature. At high joining temperature, good solid-state bonding under relatively low joining compressive stress becomes feasible. Axisymmetric thermoelastic finite element analysis suggests that the maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of graphite and is increased with increasing joining temperature. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. Part of the thermal stress in practical joints is relaxed and less than that calculated using the finite element method. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of the maximum tensile thermal stress depends on the amount of carbon which diffuse into nickel. This may be related to changes in plasticity of both nickel and graphite, that is, changes in the amount of carbon that dissolves in nickel to supersaturation and the point defects introduced in graphite.

View full abstract

Surface morphology changes of silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composite materials occurring after simultaneous triple-ion-beam irradiation were studied. Irradiation tests were performed with helium (He) ions, hydrogen (H) ions, and self-ions (carbon (C) ions or silicon (Si) ions). The peak displacement damage was 10 dpa (displacements per atom), and the irradiation temperatures were 600, 800 and 950°C. The concentrations of He and H at the damage peak region were 1000 atomic ppm and 385 atomic ppm, respectively. Observations of the irradiated surface and the measurement of morphology changes were performed. The shrinkage of SiC fibers and the apparent shrinkage of the interfacial material, carbon, between the matrix and the fibers at the irradiated surface were observed. These phenomena were mainly attributed to displacement damage caused by irradiation.

View full abstract

An amorphous Hf₇₃Pd₂₇ binary alloy was prepared by a single roller melt-spinning method. The crystallization process was studied using differential scanning calorimetry, X-ray diffraction and transmission electron microscope. The crystallization proceeds through two exothermic reactions, in which the low temperature one corresponds to the precipitation of an icosahedral quasicrystalline phase (I-phase). Further annealing causes the phase transformation from the I-phase to the Hf₂Pd crystal, indicating the metastable character of the I-phase. The present results imply that the precipitation of an I-phase in the Hf–Pd binary alloy is the basis for its formation in the Hf–Al–Ni–Cu–Pd and Hf–Al–Ni–Pd alloys reported previously.

View full abstract