Microfracture Process Research Articles

Dynamic Finite Element Analysis of Failure in Alternating Phase-Shift Masks Caused by Megasonic Cleaning

The mechanical response of alternating phase-shift mask (APSM) microstructures subjected to dynamic pressure loadings relevant to those encountered in megasonic cleaning was analyzed with the finite element method (FEM). A parametric study of the effects of microstructure dimensions, pressure amplitude, and loading frequency on the mask structural integrity was performed for two typical chromium/quartz APSM patterns. Failure due to microfracture and plastic deformation processes which may occur during megasonic cleaning was examined for loading frequencies of 1, 5, and 10 MHz. The FEM results provide insight into possible failure modes and critical microstructure dimensions for instantaneous microstructure damage. Different failure scenarios revealed by the FEM results are in qualitative agreement with experimental observations. The results of this study have direct implications to the design of extreme ultraviolet lithography masks and the optimization of the megasonic cleaning process.

Direct observation of microfracture process in metallic-continuous-fiber-reinforced amorphous matrix composites fabricated by liquid pressing process

Zr-based amorphous matrix composites reinforced with metallic continuous fibers were fabricated by liquid pressing process, and their fracture property improvement was explained by directly observing microfracture processes. About 60vol.% of metallic fibers were homogeneously distributed inside the amorphous matrix. Apparent fracture toughness of the tungsten-fiber-reinforced composites was lower than that of monolithic amorphous alloy, while that of the tantalum-fiber-reinforced composite was higher. According to the microfracture observation, shear bands or cracks were initiated at the amorphous matrix, and the propagation of the initiated shear bands or cracks was effectively blocked by fibers, thereby resulting in stable crack growth which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length improved fracture properties of the fiber-reinforced composites, and could be explained by the formation of multiple shear bands or multiple cracks at the amorphous matrix, blocking of crack or shear band propagation, and multiple necking at metallic fibers.

<i>In Situ</i> Fracture Behavior of AZ51 and AZT513 Alloy

The fracture behavior of Mg-5Al-1Zn and Mg-5Al-1Zn-3Sn alloy was investigated by direct observation of microfracture process using an in-situ loading stage installed inside a scanning electron microscope chamber. Crack was initiated at the interface of Mg/second-phase particles or second-phase particles. Fracture of the alloys was predominantly dimple or/and quasi-cleavage failure. The improvement of what could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting and shear band branching.

In situ microfracture observation of Cu-based amorphous alloy matrix composites containing copper or brass particles

This study aims at investigating fracture mechanisms occurring in Cu-based amorphous alloy matrix composites containing ductile Cu or brass particles. Amorphous alloy powders were mixed with 20–40 vol.% of Cu or brass powders, and were consolidated at 460 °C for 1/2 min under 700 MPa by spark plasma sintering (SPS) equipment. The consolidated composites contained Cu or brass particles homogeneously distributed in the amorphous matrix, and showed a considerable plastic strain under a compressive loading condition, whereas their strength was lower than that of the monolithic amorphous alloy. Microfracture mechanisms were investigated by directly observing microfracture processes using an in situ loading stage. Cu or brass particles present in the composites acted as blocking sites of the propagation of cracks initiated at the amorphous matrix, and provided the stable crack growth. In front of some Cu or brass particles, the crack blunting, deflecting, and bridging were also observed, and the final crack propagation path showed frequent stops of cracks. These findings suggested that the composites consolidated by the SPS presented new possibilities of application to structural materials or parts satisfying excellent mechanical properties and large size requirements.

21311 サーメットの微視破壊過程における微視損傷のAE法による検出(材料強度と構造設(1),OS.5 材料強度と構造設計)

21311 サーメットの微視破壊過程における微視損傷のAE法による検出(材料強度と構造設(1),OS.5 材料強度と構造設計)

Dynamic experimental study on rock meso-cracks growth by digital image processing technique

A new meso-mechanical testing scheme based on SEM was developed to carry out the experiment of microfracturing process of rocks. The microfracturing process of the pre-crack marble sample on surrounding rock in the immerged Long-big tunnel in Jinping Cascade II Hydropower Station under uniaxial compression was recorded by using the testing scheme. According to the stereology theory, the propagation and coalescent of cracks at meso-scale were quantitatively investigated with digital technology. Therefore, the basic geometric information of rock microcracks such as area, angle, length, width, perimeter, was obtained from binary images after segmentation. The failure mechanism of specimen under uniaxial compression with the quantitative information was studied from macro and microscopic point of view. The results show that the image of microfracturing process of the specimen can be observed and recorded digitally. During the damage of the specimen, the distribution of microcracks in the specimen is still subjected to exponential distribution with some microcracks concentrated in certain regions. Finally, the change law of the fractal dimension of the local element in marble sample under different external load conditions is obtained by means of the statistical calculation of the fractal dimension.

Experimental investigation of three-dimensional propagation process from surface fault

Experimental study of the formation and propagation of three-dimensional (3D) faults is of great significance in the understanding of the propagation process developing from initial natural faults. In the study described in this paper, experimental investigations of 3D propagation processes of a type of surface fault are carried out under biaxial compression. The strain field near the surface fault is dynamically measured and fully analyzed with a high-density Multi-Channel Digital Strain Gauge (MCDSG) and Digital Speckle Correlation Method (DSCM) based on the white-light image analysis. Simultaneously the micro-fracture process involved in fault formation is observed by a 3D acoustic emission (AE) location system with a set of multi-channel whole-wave record equipment. The experimental results show that the 3D propagation process of surface fault differs greatly from that of the two-dimensional (2D) state and that a new more complicated type of 3D morphological characters and deformation mechanisms are produced. The 3D propagation process of surface faults may be divided into three stages: 1) the first stage of crack propagation initiated by wing cracks; 2) the conversion stage propagated by petal cracks; and 3) the second stage of crack propagation formed by shell-shaped fracture surface. The primary propagation patterns of the three stages are different. The corresponding deformation fields and micro-fracture distributions are likewise different. The fracture activities from petal cracks especially are of vital importance during surface fault propagation. This is also a key conversion state and marks an intrinsic difference between 2D-like and the 3D state in fault development.

Effects of Tempering on Microstructure, Hardness, Wear Resistance, and Fracture Toughness of Cr3C2/Steel Surface Composites Fabricated by High-Energy Electron-Beam Irradiation

This study aimed at improving hardness, wear resistance, and fracture toughness by tempering chromium carbide (Cr3C2)/carbon steel surface composites fabricated by high-energy electron-beam irradiation. The mixture of Cr3C2 powders and MgF2 flux was placed on a plain carbon steel substrate, and then electron beam was irradiated on this mixture using an electron-beam accelerator. In the specimens fabricated with flux powders, the surface composite layer of 1.9 mm in thickness was successfully formed without defects, and contained about 3 vol pct of Cr7C3 carbides in the matrix composed of plate-type martensite and austenite. When the Cr3C2/steel surface composite was tempered, the martensite was resolved, and Cr7C3 carbides were coarsely precipitated along cell boundaries while the austenite disappeared, thereby leading to the increase in hardness and wear resistance. Observation of the microfracture process of the 500 °C-tempered surface composite revealed that cracks initiated and propagated along intercellular Cr7C3 carbides and stopped propagating when they met the relatively ductile tempered martensite matrix. Accordingly, this composite showed improved fracture toughness and presented good application possibilities as hard and tough wear-resistant materials.

Effect of Non-Homogeneity on Limit Bearing Capacity of Rock Block

Rock is a typical non-homogeneous material. The behavior of a rock block under compression and the process of micro-fracture in that block are phenomena of considerable interest in understanding the strength characterization of brittle rock. In this study, the effect of the non-homogeneity on limit bearing capacity of rock block based on Rock Elasto-Plastic Failure Process Analysis code (REPFPA) are simulated and investigated. Numerical simulations find out that there exists a functional relation between the heterogeneity coefficient and the limit bearing capacity of rock block. For specimens with the same heterogeneity, however, the numerical simulations show that the failure modes depend greatly on the crack initiation location, which is found to be sensitive to the local disorder features within the specimen. In addition, the characteristics of acoustic emissions with the changing of heterogeneity coefficient were also found. These identifications are crucial for better understanding and interpreting the experimental results and consequently, improve our concepts in design or analysis of rock engineering structures.

Study on Crack Propagation Behavior of Asphalt Concrete under Uniaxial Compression Load by Using CT

Various asphalt pavement distresses, such as longitudinal cracking, thermal (transverse) cracking, and reflective cracking, are directly related to the fracture properties of the asphalt materials used in the pavement layers. The fracture resistance of asphalt materials significantly influences the service life of asphalt pavements and consequently affects the maintenance and rehabilitation costs of the pavement network. Therefore, there is significant interest in understanding the mechanism of fracture in asphalt pavements and in developing analysis tools that would lead to the selection of asphalt materials with increased fracture resistance. This paper reports a research effort of dynamic observation of the micro-fracture process of asphalt mixture under uniaxial compressed condition by using CT (computerized tomography) method. CT images were analyzed to investigate the process of micro-fracture of asphalt concrete. Research results show that the abnormal belt of density damage of asphalt concrete is the initiation belt of cracking and the turning point in the curve of stress and density distress increment is the point of cracking failure of asphalt concrete.

CT Real-Time Analysis of Meso-Damage Propagation Law of Asphalt Concrete

Cracking is one of the major distress types of asphalt pavements. Expected life of pavements, pavement condition and maintenance cost are directly related to pavement cracking. It is necessary to promote the understanding of cracking mechanism of hot mix asphalt (HMA) based on fracture mechanics. Simulation, surveying, observation, and measurement of cracking in pavement structures have been reported in literature in last three decades. However, cracking process in asphalt mixtures in a three dimensional scale is still a great challenge to road engineers. This paper reports a research effort of dynamic observation of the micro-fracture process of asphalt mixture under compressed condition using CT (computerized tomography) method. Through CT scanning, the clear CT images, which include the microcrack compressed, growth, bifurcation, development, the crack fracture, and the asphalt concrete sample failure were obtained. The CT numbers, CT images and the other data were analyzed. Analysis of the relationship between of CT number and volumetric strain suggests that damage value at zero volumetric strain can be used as threshold point in damage analysis. Corresponding stress value can be used as maximum allowable stress in design practice.

Influence of Heterogeneity on Direct Tensile Failure Process of Rocks and Associated Fractal Characteristic of AE

The investigation on the behavior of a specimen under uniaxial tension and the process of microfracture attracts considerable interest with a view to understanding strength characterization of brittle materials. Little attention has been given to the detailed investigation of influence of heterogeneity of rock on the progressive failure leading to collapse in uniaxial tension. In this paper, a numerical code RFPA3D (Realistic Failure Process Analysis), newly developed based on a three-dimensional model, to simulate the fracture process and associated fractal characteristic of heterogeneous rock specimen subjected to direct uniaxial tension. Specimens with different heterogeneity are prepared to study tension failure. In a relatively homogeneous specimen, the macrocrack nucleates abruptly at a point in the specimen soon after reaching peak stress. In more heterogeneous specimens, microfractures are found to appear diffusely throughout the specimen, and the specimens show more ductile failure behavior and a higher residual strength. Development of fractal theory may provide more realistic representations of rock fracture. The fractal dimension of distributed AE is computed during the fracture process. For all specimens, the fractal dimension increases as the loading proceeds, and it reaches the peak value when macrocrack nucleates abruptly. It is also found that fractures scatter more diffusely in relatively heterogeneous specimens, and the fractal dimension has a smaller value. The homogenous rock specimens have flat and smooth rupture faces which are consistent with the fractal results.

Microfracture Observation of Zr-Based Bulk Metallic Glasses

Microfracture mechanisms of Zr-based bulk metallic glass (BMG) alloy containing ductile crystalline particles were investigated by directly observing microfracture processes using an in situ loading stage. Strength of the BMG alloy containing crystalline particles was lower than that of the monolithic BMG alloy, while ductility was higher. According to the direct microfracture observation, crystalline particles initiated shear bands, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

In situ fracture observation and fracture toughness analysis of Zr-based bulk amorphous alloys

The fracture property improvement of Zr-based bulk amorphous alloy containing ductile crystalline particles was explained by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber. Strength and apparent fracture toughness measured from the in situ fracture test of the amorphous alloy containing crystalline particles were lower than those of the monolithic amorphous alloy, whereas its ductility was higher. According to the microfracture observation, shear bands were initiated from ductile crystalline particles, and the propagation of the shear bands or cracks was blocked by crystalline particles, thereby resulting in stable crack growth which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length improved fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

A numerical study of the influence of heterogeneity on the strength characterization of rock under uniaxial tension

The behavior of a specimen under uniaxial tension and the process of micro-fracture in that specimen are phenomena of considerable interest in understanding the strength characterization of brittle or quasi-brittle materials such as rock. In this study, numerical results based on a Rock Failure Process Analysis code (RFPA 2D) are presented. They show the nucleation and growth of macro-cracks in relatively homogeneous and heterogeneous specimens under uniaxial tension. Although the details of macro-crack formation vary from specimen to specimen, the numerical simulations consistently display a number of features. In relatively homogeneous specimens, the macro-crack nucleates abruptly at a point in the specimen soon after reaching the peak stress. Before macro-crack nucleation, acoustic emission (AE) events, or micro-fractures occur at locations that are randomly distributed throughout the specimen. It is very difficult to predict where the macro-crack will begin. The failed specimen has no residual strength. However, relatively heterogeneous specimens show a somewhat different response, with more randomly distributed AE events appearing in the early stage of loading. In contrast to the homogeneous specimens, macro-crack nucleation in heterogeneous specimens starts well before the peak stress is reached, and the crack propagation and coalescence can be traced, which can then be used to predict the macro-fracturing of the specimen. For specimens with the same heterogeneity, however, the numerical simulations show that the failure modes depend greatly on the crack initiation location, which is found to be sensitive to the local disorder features within the specimen.

Grain-scale deformation in the Palaeoproterozoic Dongargarh Supergroup, central India: implications for shallow crustal deformation mechanisms from microstructural analysis

Analysis of the grain-scale deformation mechanisms in folded rocks of the Dongargarh Supergroup, central India, reveals that deformation was accomplished by a combination of pressure solution, microfracturing and dislocation creep processes. The finite strain was assessed using the Rf/ϕ method (X/Z ≈ 2). Partitioning of strain into various deformation mechanisms revealed that dislocation creep and pressure solution were the major contributors to the finite strain, followed by microfracturing. Analyses of microstructures suggest a sequence of dislocation creep followed by pressure solution and microfracturing, that ultimately gave way to microfracturing and limited crystallization or recrystallization. Overall constancy in volume during deformation is suggested from the balance between fracture-related grain-scale dilatancy and solution-related volume loss. Observations on cleavage spacing within various lithologies in a specific structural setting suggest that lithology played a vital role in cleavage development. Cleavage development in sandstones of the Dongargarh Supergroup required thin shale interbeds (for competency contrast) and grain size <0.036 mm (4.75 ϕ).

Long-Term Reliability Assessment of Bioceramics for Artificial Joints Using Microdamage Monitoring by AE

Microfracture process during bending tests of alumina ceramics used for artificial joints was evaluated by acoustic emission (AE) technique. Four-point bending tests were carried out in air, refined water, physiological saline and simulated body fluid. AE behavior during bending test inhibited the rapid increasing point of AE events and energy prior to the final unstable fracture. It was understood that the bending stress at the increasing point corresponds to the critical stress for maincrack formation. The critical stress was affected by water in environments more strongly than fracture strength. Consequently, it was suggested that the characterization of maincrack formation is essential for the long-term reliability assessment of load-bearing bioceramics.

Avalanche Behaviour in Microfracturing Process of 3-D Brittle Disordered Material

Using a newly-developed Material Failure Process Analysis code (MFPA3D), the micro-fracturing process and the avalanche behavior characterization of brittle disordered materials such as rock or concrete is numerically studied under uniaxial compression and tension. It is found that, due to the heterogeneity of the disordered material, there is an avalanche behavior in the microcrack coalescence process. Meanwhile, a hierarchy of avalanche events also numerically observed though a study of numerically obtained acoustic emissions or seismic events. Numerical simulations indicate that macro-crack nucleation starts well before the peak stress is reached and the crack propagation and coalescence can be traced, which can be taken as a precursory to predict the macro-fracture of the brittle disordered materials. In addition, the numerically obtained results also reveal the presence of residual strength in the post-peak region and the resemblance in the stress-strain curves between uniaxial compression and tension.

Distinguished seismological and electromagnetic features of the impending global failure: Did the 7/9/1999 M5.9 Athens earthquake come with a warning?

AbstractClear VLF electromagnetic (EM) anomalies were detected prior to the Athens earthquake (EQ). We attempt to establish the hypothesis that these emissions were launched from the pre-focal area during micro-fracturing process. The spectral analysis in terms of fractal dynamics reveals that distinquished alterations in the associated scaling parameters emerge as the EQ is approached. These alterations suggests that the evolution of the Earth’s crust towards the “critical point” takes place not only in the seismological sense but also in the pre-fracture EM sense. VAN-signals and space-time TIR-signals were also detected prior to the Athens EQ. These anomalies, as well as the fault modeling of the Athens EQ obtained by interferometric combinations of ERS2 SAR images bring further support for the confidence in the reliability of our conclusions.

Fatigue as a process of cyclic brittle microfracture

ABSTRACTWhile fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack‐tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half‐cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near‐tip stresses, such as load history and constraint. BMF dominates near‐threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip‐driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress–strain hysteresis and load‐sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications.