Pre-existing Microcracks Research Articles

Effects of high temperature thermal treatment on the physical properties of clay

High temperature thermal treatment to clay mineral often leads to the development of new micro-cracks or extension/widening of pre-existing micro-cracks, which naturally would result in the variation of physical properties. Experiments aiming to understand the evolution of physical properties of clay have been carried out from 20 to 800 °C in a heating furnace, and the results were summarized as follows: two temperature ranges (20–200 °C and 300–500 °C) corresponding to the vaporization of moisture (i.e. adhered water or bound water) and oxidation/decomposition reactions of organic matters are obviously evident; clay specimens seem to exhibit the most pronounced physical property changes within the temperature range of 300–500 °C, which presumably is attributed to the transition of structural water, organic carbon and kaolin of clay. Moreover, within the temperature range of 400–600 °C, especially between 500 and 600 °C, kaolin structure of clay appears to undergo pronounced chemical changes, ultimately affording increased material porosity accompanied by altered wave velocity.

In situ Formed α-Al2O3 Nanocrystals Repaired the Preexisting Microcracks in Plasma-Sprayed Al2O3 Coating via Stress-Induced Phase Transformation

In the present study, the phase composition and generation mechanism of the nanocrystals located in the microcracks of plasma-sprayed Al2O3 coating were reevaluated. The Al2O3 coatings were investigated using transmission electron microscopy and x-ray diffraction. We supply the detailed explanations to support the new viewpoint that in situ formation of α-Al2O3 nanocrystals in the preexisting microcracks of the as-sprayed Al2O3 coating may be due to the stress-induced phase transformation. Owing to the partially coherent relationship, the phase interfaces between the α-Al2O3 nanocrystals with the preferred orientation and the γ-Al2O3 matrix may possess better bonding strength. The α-Al2O3 nanocrystals could repair the microcracks in the coating, which further strengthens grain boundaries. Grain boundary strengthening is beneficial to the coating fracture toughness enhancement.

Dynamic Constitutive Model for TiC-Particulate Reinforced Titanium Matrix Composites

A coupled model of damage and plasticity is presented to describe the dynamic behaviors of TiC-particulate reinforced titanium matrix composites (TiCp/TMCs) subjected to shock loadings. The TiCp/TMCs are assumed as homogeneous continuum with pre-existing micro-cracks and micro-voids. Damage to TiCp/TMCs is caused due to micro-crack nucleation, growth and coalescence, and defined as the probability of fracture at a given crack density. In terms of crack growth model, micro-cracks are activated, and begin to propagate gradually. When crack density reaches a critical value, the smashing destroy takes place. The model parameters for TiCp/TMCs are determined using plate impact experiments. Comparison with the test results shows that the proposed model can give consistent predictions of the dynamic behaviors of TiCp/TMCs subjected to impact loadings.

Quantitative assessment of alkali silica reaction potential of quartz-rich aggregates: comparison of chemical test and accelerated mortar bar test improved by SEM-PIA

The alkali silica reaction (ASR), which originates in highly alkaline conditions in concrete where reactive forms of silica are available, causes serious damage to concrete structures. The ASR potential of various quartz-rich aggregates (pegmatite quartz, quartzite, quartz meta-greywacke, and chert) was quantified using accelerated mortar bar test (AMBT), chemical test (CT), and scanning electron microscopy combined with petrographic image analysis (SEM-PIA). Only two samples (quartz meta-greywacke, and pegmatite quartz) were judged as deleterious according to the CT; the other aggregates were classified as innocuous. AMBT gave different results. Quartz meta-greywacke, chert, and quartzite exceeded the 0.100 % expansion limit after 14 days of testing. Pegmatite quartz indicated a lower value. The results of SEM-PIA confirmed the results of AMBT, indicating the most extensive ASR in those AMBs containing chert and quartz meta-greywacke. Parts of aggregates were leached out and massive deposits of alkali–silica gels were found filling air voids, microcracks, and the aggregate/cement paste interface. The medium or low degree of ASR was confirmed in AMBs containing quartzite or pegmatite quartz, respectively. ASR accentuated pre-existing microcracks and formed new dissolution gaps. In contrast, no correlation was found with the results of CT, which under-evaluated the ASR potential of chert and quartzite and over evaluated the ASR potential of pegmatite quartz. The variable ASR potential of the investigated aggregates was explained by the significant content of poorly crystalline matrix (in chert and quartz meta-greywacke), and by the presence of strained quartz typical with undulose extinction and the origin of quartz subgrains (in quartzite and pegmatite quartz).

Discrete element model for the analysis of fluid-saturated fractured poro-plastic medium based on sharp crack representation with embedded strong discontinuities

Abstract In this work, we present a discrete beam lattice model capable of simulating localized failure in a heterogeneous fluid-saturated poro-plastic solid. Coupling conditions between the solid and fluid phase are governed by the Biot’s porous media theory, enhanced with the fluid flow through cracks. The basis for development of discrete 2D plane strain model representation of heterogeneous material consisting of material grains, is an assembly of Voronoi cells that are kept together by cohesive links in terms of Timoshenko beams. Localized failure of saturated medium is enabled through embedded discontinuities positioned in cohesive links where Timoshenko beam’s longitudinal and transversal directions possess enhanced kinematics representing failure modes I and II. The model can also take into account the fracture process zone with pre-existing microcracks coalescence prior to the localized failure, which is described by the poro-plasticity formulation. Fluid flow is spread across the lattice network by Darcy’s law in terms of continuous pore pressures, with a special care taken in computing the lattice permeability parameters. This is brought by the proposed discrete model which can capture the initial material heterogeneities, the fracture process zone initiation and the localized failure mechanisms. The macro-cracks can be deflected throughout the medium by the heterogeneities and exhibit irregular geometries. The model theoretical formulation and computational procedure are presented in sufficient detail. Several numerical simulations are given to illustrate a very satisfying performance of the presented discrete model.

Experimental characterisation of thermo-mechanical coupling properties of Beishan granite

With the help of MTS815 rock mechanics test system and 3D acoustic emission monitoring system, a series of thermo-mechanical coupling tests at temperatures in the range of 23–90 °C and confining pressures from 5 to 30 MPa are carried out, to investigate the thermo-mechanical properties of Beishan granite. The granite is taken from Beishan site, the most potential area for the China’s high-level radioactive waste repository. The experimental results indicate that the mechanical behaviour and crack characteristics are essential related to the coupling effect of temperature and confining pressure. The mechanical characteristics such as elastic modulus, crack damage stress and rock failure strength are degraded slightly with the increase of temperature. In accordance with the degradation of mechanical properties, high temperature leads to the development of new microcracks or propagation of pre-existing microcracks within the rocks, which can be revealed by the increase of the AE activities. Rock specimen tends to become more stable under high confining pressure. A brittle–ductile mechanical behaviour transition of Beishan granite can be observed, especially under a high confining pressure.

Experimental and Numerical Simulation of the Microcrack Coalescence Mechanism in Rock-Like Materials

Rocks and rock-like materials frequently fail under compression due to the initiation, propagation and coalescence of the pre-existing microcracks. The mechanism of microcrack coalescence process in rock-like materials is experimentally and numerically investigated. The experimental study involves some uniaxial compression tests on rock-like specimens specially prepared from portland pozzolana cement, mica sheets and water. The microcrack coalescence is studied by scanning electron microscopy on some of the prepared thin specimens. It is assumed that the mica sheets play the role of microcracks within the specimens. Some analytical and numerical studies are also carried out to simulate the experimentally observed microcrack coalescence phenomena within the specimens. A higher-order indirect boundary element method known as higher-order displacement discontinuity method implementing special crack tip elements to treat the singularities of stress and displacement fields near the crack ends is used to estimate the Mode I and Mode II stress intensity factors at the microcrack tips. The maximum tangential stress fracture criterion is implemented in a sophisticated computer code using the linear elastic fracture mechanics theory and the propagation and coalescence of random microcracks within the rock-like specimens are numerically simulated based on an iterative algorithm. The proposed analyses are validated by comparing the corresponding experimental and numerical results of microcrack coalescence phenomena of rock-like materials.

Fracture dynamics in implanted silicon

Crack propagation in implanted silicon for thin layer transfer is experimentally studied. The crack propagation velocity as a function of split temperature is measured using a designed optical setup. Interferometric measurement of the gap opening is performed dynamically and shows an oscillatory crack “wake” with a typical wavelength in the centimetre range. The dynamics of this motion is modelled using beam elasticity and thermodynamics. The modelling demonstrates the key role of external atmospheric pressure during crack propagation. A quantification of the amount of gas trapped inside pre-existing microcracks and released during the fracture is made possible, with results consistent with previous studies.

An experimental study on the relationship between localised zones and borehole instability in poorly cemented sands

Abstract Poorly cemented sands are mainly located in areas where layers of unconsolidated formations exist. Drilling a borehole in the ground causes stress perturbation and induces tangential stresses on the borehole wall. If the cohesion between sand particles generated by existing cementation is not high enough, the tensile stress concentration may cause grain debonding and, consequently, borehole breakout. In this study a series of solid and thick-walled hollow cylinder (TWHC) laboratory tests was performed on synthetic poorly cemented sand specimens. The applied stresses were high enough to generate breakout on the borehole wall. Simultaneous real-time monitoring and deformation measurement identified the development of localised breakout zones and compaction bands at the borehole wall during the tests. The results from the video recording of the tests showed that a narrow localised zone develops in the direction of the horizontal stress, where stress concentration causes the full breakout in specimens. Dilation occurred at lower confining pressures in TWHC specimens and contracting behaviour was observed during the onset of shear bands at higher pressures. Scanning electron microscopy (SEM) studies showed that sand particles stayed intact under the applied stresses and micro- and macrocracks develops along their boundaries. The SEM imaging was also used to investigate and characterize pre-existing microcracks on the borehole wall developed due to the specimen preparation. It showed that boring the solid specimen in order to produce a TWHC specimen could generate microcracks on the borehole wall prior to testing which affects the process of borehole failure development during the test.

Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application on energy-saving kettle

Starting from Fe2O3, MnO2, Co2O3 and NiO powders, the ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance were successfully prepared on the surface of carbon steel by high velocity oxy-fuel spraying (HVOF). The coating thickness was about 120–150μm and presented a typical flat lamellar structure. The coating surface was rough and some submicron grade grains distributed on it. The infrared emissivity of the ferrites based coating by HVOF was over 0.74 in 3–20μm waveband at 800°C, which was obviously higher than that of the coating by brushing process in the short waveband. The bonding strength was 30.7MPa between the coating and substrate, which was five times more than that of conventional coatings by brushing process. The combined effect of the superior bonding strength, typical lamellar structure, pre-existing microcracks and newly generated pores made the cycle times reach 27 when the coating samples were quenched from 1000°C using water. Lastly, the infrared radiation coatings were applied on the underside of household kettle, and the energy-saving efficiency could reach 30.5%. The ferrites based infrared radiation coatings obtained in this work are good candidates for saving energy in the field of cookware and industrial high temperature furnace.

Thermal damage pattern and thresholds of granite

High temperature may lead to the development of new microcracks or growth of pre-existing microcracks within granite, varying its physical and mechanical properties. Experiments were conducted to study the evolution of the physical and mechanical properties of granite specimens from room temperature to 800 °C. The specimens were heated in heating furnace and uniaxial compression tests were done using MTS servo-controlled testing machine. The results indicate five phases in the variation of physical and mechanical properties with temperature: from room temperature to 100, 100–300, 300–400, 400–600, and 600–800 °C. The first phase corresponds to the vaporization-escaping interval of adhered water, bound water, and structural water. Larger changes of physical and mechanical parameters in the temperature range of 300–600 °C, mostly 400–600 °C, are probably caused by the transition from the brittle state to plasticity (or ductility) of granite, and 400 °C may be a critical threshold of its thermal damage. These results confirm the important link among physical and mechanical properties in response to thermal treatment.

A boundary element analysis of crack-propagation mechanism of micro-cracks in rock-like specimens under a uniform normal tension

In this work, the mechanism for fracture of brittle substances such as rocks under a uniform normal tension is considered. The oriented straight micro-cracks are mostly created in all the polycrystalline materials resulting from the stress concentrations. The present work focuses on the interactions of the pre-existing micro-cracks, which can grow and propagate within a rock-like specimen. The micro-crack initiation and propagation in rock-like specimens is investigated using the Fortran Code TDDCRACK 2D , which is a 2D displacement discontinuity method (DDM) for crack analysis, a boundary element computer code based on the linear elastic fracture mechanics (LEFM) theory. In the present work, a higher order DDM is used to implement special crack tip elements for estimation of the stress intensity factors (SIFs) and crack initiation angles for the wing-crack problems initiated at different angles from the original micro-crack tips in an infinite specimen under a uniform tension.

Neutron irradiation induced microstructural changes in NBG-18 and IG-110 nuclear graphites

This paper reports the neutron-irradiation-induced effects on the microstructure of NBG-18 and IG-110 nuclear graphites. The high-temperature neutron irradiation at two different irradiation conditions was carried out at the Advanced Test Reactor National User Facility at the Idaho National Laboratory. NBG-18 samples were irradiated to 1.54dpa and 6.78dpa at 430°C and 678°C respectively. IG-110 samples were irradiated to 1.91dpa and 6.70dpa at 451°C and 674°C respectively. Bright-field transmission electron microscopy imaging was used to study the changes in different microstructural components such as filler particles, microcracks, binder and quinoline-insoluble (QI) particles. Significant changes have been observed in samples irradiated to about 6.7dpa. The closing of pre-existing microcracks was observed in both the filler and the binder phases. The binder phase exhibited substantial densification with near complete elimination of the microcracks. The QI particles embedded in the binder phase exhibited a complete microstructural transformation from rosettes to highly crystalline solid spheres. The lattice images indicate the formation of edge dislocations as well as extended line defects bridging the adjacent basal planes. The positive climb of these dislocations has been identified as the main contributor to the irradiation-induced swelling of the graphite lattice.

Calibration of Rebound Hardness Numbers to Unconfined Compressive Strength in Shale Formations

Young Technology Showcase Today, unconventional-reservoir characterization is actively studied; however, rock-mechanics tests are still restricted to intact rocks. Intact intervals are necessary to acquire plug specimens (Fig. 1a). Anisotropy caused by the presence of various discontinuities affects rock-mechanical properties significantly. Shale formations consist largely of laminations, pre-existing microcracks, and fractures along bedding planes (Fig. 1b), and they inhibit sample preparation with the standard geometry (1:2 ratio of diameter/length). The primary purpose of this study is to develop a method that overcomes the restrictions of rock-mechanics tests with respect to unconventional shale formations. This study applies an Equotip Hardness Tester (EHT) and uses its rebound hardness numbers (RHNs) to estimate unconfined compressive strength (UCS) with the recovered cores from shale formations. The application enhances laboratory-testing results to calibrate petrophysical models that show higher dynamic rock strength generated by acoustic logging or ultrasonic-wave-velocity measurement. It can also support fracture designs by providing a continuous strength- contrast profile to identify where fracturable zones and fracture barriers exist. RHNs The EHT was originally developed by Dietmar Leeb (1978) to measure the hardness of metallic materials. Recently, its use has been extended to various intact rocks to estimate their UCS values (Meulenkamp 1997; Meulenkamp and Grima 1999; Papsworth 2014; Verwall and Mulder 2000). The EHT is a small, portable, electronic, battery- operated, spring-loaded device. A spherical tungsten carbide tip impacts the metal or rock surface driven by a constant spring force, then rebounds. The impact and rebound velocities of the tip are measured and converted into RHNs (Proceq 2014). Fig. 2 presents a schematic of the impact device.

103 AE信号処理による鋼に起こるき裂進展の初期検出に関する考察(セッション1 損傷検知・信号処理)

Early detection of crack initiation and propagation in structural steel is very important for avoiding enormous industrial and plant accidents due to the occurrence of permanent damage in the material. Acoustic emission (AE) technique is an influential tool for observing the transient modes of this damage characteristics. Elastic wave propagation for crack formation and propagation inside the material generates identifiable AE parameters. However, some dummy like AE features because of the rubbing operations among pre-existing microcracks in material and or unstable loading features of the material cause undue artifacts in early evaluation of crack formation and propagation. Careful evaluation of some frequency distribution characteristics along with other influential features comprise the AE technique in early-detectable of cracking. In the present study, the failure of steel plate caused by cracks and micro-cracks has been investigated sequentially by AE technique. AE event analysis along with microstructural evaluation show the reasonable results as well.

Quantitative evaluation of carbonation in concrete using nonlinear ultrasound

Abstract A new nonlinear ultrasonic technique for nondestructive evaluation of concrete components is developed and implemented to characterize the effects of carbonation on concrete. The physical principle of this method is the second harmonic generation (SHG) in propagating Rayleigh surface waves which are detected by a non-contact air-coupled transducer. The nonlinearity parameter, as an indicator of material properties, is experimentally obtained from measured Rayleigh wave signals and is used to quantitatively evaluate the progress of carbonation under accelerated conditions. The experimental results show that there is a significant decrease in the measured nonlinearity parameter, most likely originated from the deposit of the carbonation product, CaCO3, in pre-existing voids and microcracks. The sensitivity of the nonlinearity parameter is also verified by comparing with the measured Rayleigh wave velocity. The results in this paper demonstrate that the SHG technique using Rayleigh surface waves can be used to monitor carbonation in concrete.

Numerical analysis on micro-damage in bisphere model of granitic rock

Microcracking in contact with constituent minerals plays an important role in the nonlinear deformation process and micro-damage, leading to shear fracture of brittle materials such as granite. For damage propagation, the surface energy stored in the grain as a result of applied stresses thus tends to be relieved on the plane firstly, and pre-existing intracrystalline microcracks perpendicular to axial stress direction are closed. On the other hand, those parallel or subparallel to the axial stress direction are open. These stress-induced microcracks are predominantly tensile in nature, and the directions are parallel or subparallel to the axial stress direction. Numerical analysis provides some basis for a micromechanics of stress-induced micro-damage occurring at the major constituent mineral contact portions in feldspar. A homogenization theory was applied to analyze the damage development for the vicinity of pre-existing microcrack and stress-induced microcrack at grain contact. Numerical analysis with the bisphere model for damage development at micro-damage portions was discussed, assuming that feldspar grains surround quartz grain.

Numerical modeling of Fabric Reinforce Cementitious Matrix composites (FRCM) in tension

Existing masonry structures often need to be strengthened or repaired. In many cases, the intervention is realized using composite materials bonded to the surface of the structural element.In many masonry structures the use of fabric reinforced cementitious matrices (FRCM) is preferred to the fiber reinforced polymers (FRP).The typical experimental stress–strain behavior exhibited by a FRCM composite under a direct tensile test is a tri-linear curve with a first phase that increases linearly according to mortar Young’s modulus, a second phase where the cracks in the mortar start to grow, and a last phase in which the mortar is fully cracked and the curve assumes the same slope of the stiffness of the fabric. According to a wide experimental campaign conducted on the subject at the Politecnico di Milano, the curves exhibit a relatively wide scatter, especially in the second phase, making the standardization of the direct tensile test a rather difficult task.With the aim of having an insight into the observed experimental variability, a comprehensive FE numerical analysis was conducted and is presented in this paper. Two different FE codes were utilized. One with less sophisticated material models, the second with the possibility to deal with softening and damage in the post peak range. The use of commercial codes instead of home-made models was voluntary, with the precise final aim of enabling other researchers the reproduction of results with similar models and for analogous experiments. Three different variables that can affect the mechanical behavior in tension were examined (non-planarity of the composite grid, bending of the specimen and pre-existing micro-cracks), leading to three different sets of simulations.A final objective of the numerical simulation was to study and compare possible constitutive models for the cementitious matrix to simulate the experiments. At this aim, three different material models were used for mortar belonging to FRCM specimens in tension.The numerical results obtained satisfactory reproduce experimental evidences and provide a justification of the relative large scatter of the data.

Edge cracking mechanism in two dual-phase advanced high strength steels

Abstract One of the manufacturing issues restricting the applications of advanced high-strength steels is edge cracking during forming. Computer simulations using the conventional forming limit curve (FLC) as a failure criterion often fail to predict edge cracking. A new failure criterion is needed to assess the edge stretchability in simulations and applications, subsequently understanding the fracture mechanism in the microstructural scale is a prerequisite. In this study, the edge fracture mechanisms of two selected dual-phase steels (designated as DP980 and IBF980) with identical chemistry but different edge cracking behaviors are studied by controlled edge tension tests and scanning electron microscopy (SEM). The results show that the edge cracking behaviors are essentially fractures propagated from pre-existing microcracks introduced by the shearing process. The dominant edge fracture mechanism is decohesion between the martensite and ferrite phases. This study employs the interfacial strength between ferrite and martensite as an index to predict the material edge cracking resistance. The interfacial strength is calculated to be 1070 MPa for IBF980 and 854 MPa for DP980 using void-nucleation models. This result indicates that IBF980 has a higher resistance to edge cracking and subsequently a higher local formability, which is consistent with the higher hole expansion ratio values observed. Refining the martensite particle size and minimizing banded martensite structures are effective approaches to increase the local formability.

Effects of fatigue induced damage on the longitudinal fracture resistance of cortical bone

As a composite material, cortical bone accumulates fatigue microdamage through the repetitive loading of everyday activity (e.g. walking). The accumulation of fatigue microdamage is thought to contribute to the occurrence of fragility fractures in older people. Therefore it is beneficial to understand the relationship between microcrack accumulation and the fracture resistance of cortical bone. Twenty longitudinally orientated compact tension fracture specimens were machined from a single bovine femur, ten specimens were assigned to both the control and fatigue damaged groups. The damaged group underwent a fatigue loading protocol to induce microdamage which was assessed via fluorescent microscopy. Following fatigue loading, non-linear fracture resistance tests were undertaken on both the control and damaged groups using the J-integral method. The interaction of the crack path with the fatigue induced damage and inherent toughening mechanisms were then observed using fluorescent microscopy. The results of this study show that fatigue induced damage reduces the initiation toughness of cortical bone and the growth toughness within the damage zone by three distinct mechanisms of fatigue-fracture interaction. Further analysis of the J-integral fracture resistance showed both the elastic and plastic component were reduced in the damaged group. For the elastic component this was attributed to a decreased number of ligament bridges in the crack wake while for the plastic component this was attributed to the presence of pre-existing fatigue microcracks preventing energy absorption by the formation of new microcracks.