Microcrack Density Research Articles

Mechanical behaviour of Lac du Bonnet granite after high-temperature treatment using bonded-particle model and moment tensor

The thermal cracking mechanism has been increasingly investigated to analyse and solve the mechanical behaviour of rocks in deep underground engineering. Classic laboratory tests bear the intrinsic limitation of non-repeatability and lack of direct observation of the interaction effect between thermal stresses and thermal micro-cracks. In this paper, the bonded-particle model and moment tensor is used to simulate the process of thermally induced micro-and macro-cracks in Lac du Bonnet granite to help provide insight into the detailed influence temperature on mechanical properties and acoustic emission characteristic of granite. During modelling, macro- and microscopic responses are quantified and compared against other laboratory data in the literature. The results indicate thermal stresses and thermally induced micro-cracks increase with increasing temperatures during heating, and cooling generally reduces the thermal stresses and increases the micro-cracks slightly. Grain-size distribution has a significant influence on thermal micro-cracks, thermal stress and the mechanical behaviours of granite. The increase in thermally induced tensile intergranular micro-cracks density mainly contributes to reduction of mechanical properties in granite samples subjected to heating–cooling cycles. It is also observed that the nature of sources and the b-value are associated with temperature, which gives some light on underground engineering in high temperature environments.

Distribution of microcrack surface density in the human otic capsule

Background The bony otic capsule is comprised of highly mineralized and dense compact bone. It is rarely remodelled and degenerative changes, therefore, accumulate around the inner ear. It is also a predilection site for the pathological remodelling seen in otosclerosis. Morphometric studies have documented increased numbers of dead osteocytes and microcracks in the human otic capsule. Microcracks may disrupt the lacuno-canalicular network and cause osteocyte apoptosis ultimately breaking up the perilabyrinthine bone signalling pathways and dynamics. This may be important to understand the pathogenesis of remodelling diseases like otosclerosis. Aims/Objectives This study describes the spatial and regional distribution of microcrack surface density in relation to the inner ear and compares it to that previously recorded for otosclerosis. Material and methods Forty-two temporal bones and five ribs were used. All samples were undecalcified, bulk stained in basic fuchsin and plastic embedded. Unbiased stereology was used to estimate the true surface density of microcracks (mm2/mm3) in perilabyrinthine bone. Results The surface density of microcracks accumulates around the inner ear spaces, particularly in the lateral window regions, and increases with age. Conclusions and significance This study documents the spatial and temporal association between microfractures and otosclerosis in the otic capsule.

Gas permeability of three-dimensional stitched carbon/epoxy composites for cryogenic applications

3D through-the-thickness stitching can be used to improve interlaminar properties of polymer matrix composites (PMCs) and produce lightweight, unitized structures for cryogenic storage tanks. To fully utilize stitched composite structures for these applications, their inherent gas permeability challenges must be understood. Therefore, the effects of cryogenic cycling on the gas permeation of three-dimensional stitched PMCs are investigated. Stitched and unstitched carbon/epoxy composites of standard modulus (237 GPa) and intermediate modulus (290 GPa) carbon fibers were thermally cycled and the helium gas permeability was measured at selected cryogenic cycles. Results show that stitching increases gas permeability in PMCs, with the standard modulus stitched composites having approximately twice the permeability of the intermediate modulus stitched specimens. Optical microscopy images show that cracks develop in the resin rich areas around the stitching seams. These internal cracks produce the increase in the midspan microcrack density that leads to the increase in permeability of stitched composites.

Experimental and Numerical Investigation of the Micro-Crack Damage in Elastic Solids by Two-Way Collinear Mixing Method.

This study experimentally and numerically investigated the nonlinear behavior of the resonant bulk waves generated by the two-way collinear mixing method in 5052 aluminum alloy with micro-crack damage. When the primary longitudinal and transverse waves mixed in the micro-crack damage region, numerical and experimental results both verified the generation of resonant waves if the resonant condition was satisfied. Meanwhile, we found that the acoustic nonlinearity parameter (ANP) increases monotonously with increases in micro-crack density, the size of the micro-crack region, the frequency of resonant waves and friction coefficient of micro-crack surfaces. Furthermore, the micro-crack damage in a specimen generated by low-temperature fatigue experiment was employed. It was found that the micro-crack damage region can be located by scanning the specimen based on the two-way collinear mixing method.

Analysis of autogenous shrinkage-induced microcracks in concrete from 3D images

A new image analysis procedure for quantifying microcracks from three-dimensional (3D) X-ray microCT images of concrete is presented. The method separates microcracks from air voids and aggregates by combining filtering and morphological operations. It was applied to study the effects of supplementary cementitious materials (SCMs) and curing age on autogenous shrinkage-induced microcracks in low w/b ratio concretes, and to determine the representative elementary volume (REV) for various properties of microcracks and air voids. Results showed that slag and silica fume significantly increased autogenous shrinkage and related microcracking. These SCMs increased volume fraction, width, length, dendritic density, anisotropy, and connectivity of microcracks, but decreased specific surface and tortuosity. Similar trends were observed with age. Comparison between 3D and 2D measurements was made. REV analysis showed that a sampling volume of ~20 × 20 × 25 mm3 is sufficient for characterising most parameters of autogenous shrinkage microcracks and air voids in concrete.

Adiabatic Shearing Behavior of Shock-Prestrained Ti-2Al-9.2Mo-2Fe Alloy at Different Reloading Strain Rates

The effect of shock-prestrain on the adiabatic shearing behavior of a metastable β titanium alloy, Ti-2Al-9.2Mo-2Fe (2A2F), is systematically investigated. 2A2F alloy blocks with average grain size of 67 μm were first shock-prestrained at 6 and 12 GPa, and then reloaded at strain rates of 3000 and 4000 s−1. The results show that, the dynamic strength of 2A2F hardly increases after the shock prestrain. Adiabatic shear bands (ASBs) are more likely to bifurcate and interact, and the density of micro-damages and micro cracks in ASBs increases in the shock-prestrained materials. This trend becomes more significant with the increased preshock stress amplitude. However, in general, the amount of ASBs is low regardless of shock stress amplitude. The dynamic ductility is also hardly affected by shock-prestrain. These suggest that 2A2F alloy has good resistance to multiple impacts.

Microcracking Behavior of Gabbro During Monotonic and Cyclic Loading

Microcracks are an integral part of intact rocks that can develop when the rock is exposed to static or dynamic loads. The loading may not fail the rock. It, however, may cause permanent damage to the sample by expanding the existing microcracks or developing new microcracks. Investigating the development of microcracks in intact rocks is significant to investigate the damage evolution, which is not visible in macro-scale. In this study, the evolution of microcracks in gabbro is investigated under static and dynamic loading. Gabbro samples were subjected to monotonic and cyclic loading until a crack stress threshold (i.e., crack closure, crack initiation, crack damage). After each test, microcracking properties of the sample such as Linear Microcrack Density (LMD), crack length and crack types were defined using fluorescence thin section and combining polarized and fluorescence light optical microscopy. The results demonstrated that samples that are subjected to cyclic loading experience microcracks propagation earlier than those subjected to monotonic loading. In addition, for the samples under the same stress level, higher LMD was observed for the samples that were subjected to cyclic loading. This trend becomes more significant at higher stress levels. Except for the samples that are loaded until the peak strength, the length of the microcracks is higher for those that are subjected to cyclic loading. The trend of LMD changes during cyclic loading followed the sample displacement changes trend. In general, microcracks are propagated usually followed the weakness planes; however, in samples that are failed during cyclic loading, new microcracks formed which do not follow the weakness planes and show a random orientation. In a gabbro sample, it was observed that microcracks first propagate in biotite and then in plagioclase, amphibole, and pyroxene. Pyroxene and amphibole showed more resistance to the generation of new microcracks. This was partially due to high stiffness of minerals and partly due to alteration. Plagioclase offers the highest LMD among the gabbro component minerals. It was because it has two cleavage weakness planes and has a lower elastic modulus in comparison to the amphibole and pyroxene.

Surface Characteristics and Corrosion Behavior of Wire Electrical Discharge Machining Processed Mg-4Zn Alloy

In the present work, wire electrical discharge machining (WEDM) has been investigated for surface characteristics and corrosion behavior of Mg-4Zn alloy. Using Taguchi’s design of experiment technique, the influence of WEDM parameters, namely spark-on time, spark-off time, and servo voltage, has been studied on surface roughness (SR) and corrosion rate (CR). SEM observations showed that WEDM processing has a significant influence on the surface modification of Mg-4Zn alloy. Increasing discharge energy in the WEDM process has resulted in large surface craters, a higher density of micro-cracks, and micropores resulting in higher SR and accelerated CR in Mg-4Zn alloy. Corresponding to minimum SR and CR, the optimal set of WEDM parameters has been selected to prepare the WEDMed Mg-4Zn samples that were further analyzed for electrochemical polarization, apatite formation, and weight loss in simulated body fluid (SBF) and were additionally compared to polished samples of Mg-4Zn over a period of 21 days. Electrochemical polarization tests demonstrate that the CR increases rapidly upon the exposure time of 7 days; beyond that, the CR decreases with further exposure time up to 21 days. Initially, up to 24 hours, WEDMed samples showed better electrochemical corrosion resistance, whereas, later on, polished samples demonstrated an increased corrosion resistance for up to 21 days. Higher mineralization (or apatite formation) has been observed on WEDMed samples, which directly affects the amount of Mg ions released into the SBF solution; thus, weight loss was higher in the WEDMed sample than the polished sample. The weight loss for polished and WEDMed Mg-alloy samples after 21 days of immersion test was 9.3% and 13.5%, respectively. SEM analysis revealed the distinct corrosion morphology among the corroded surface of polished and WEDMed samples.

A Statistical Evolution Model of Concrete Damage Induced by Seawater Corrosion

The transmission of sulfate ions in concrete results in formation of calcium sulfoaluminate crystals due to chemical reactions. The expansion of calcium sulfoaluminate crystals is the main cause of concrete corrosion damage. In this study, ultrasonic analysis was used to detect the modulus change of concrete due to sulfate corrosion to obtain the basic law of corrosion damage evolution. An exponential growth model was developed for the internal expansion force based on the chemical reaction rate of calcium sulfoaluminate crystallization. Then, the evolution equation of the number density of microcracks was derived based on their initiation and balance conditions. Finally, a statistical model was developed for the concrete damage evolution by integrating the volume of microcracks. It is shown that the statistical evolution model can well characterize the evolution of concrete corrosion damage.

The influence of additive powder on machinability and surface integrity of SKD61 steel by EDM process

ABSTRACT The powder mixed electrical discharge machining, also called PMEDM, is gaining much attention, because it is demonstrated as a good solution for enhancing productivity and surface integrity. In this research, the influence of main process parameters, including peak current, pulse on time, and powder concentration in the PMEDM with tungsten carbide powder on the machinability of SKD61 steel – i.e., material removal rate (MRR) and tool wear rate (TWR), was firstly investigated. Subsequently, the surface integrity of the recast layer, including the chemical composition, the recast layer thickness (RLT), and the percentage of the surface micro-crack density acreage (PSCDA) was analyzed and discussed. The results show that peak current, pulse on time, and powder concentration have influence on the machinability and surface integrity. MRR and TWR have changed in an uptrend when peak current, pulse on time, and powder concentration increase. At Ip = 3A; Ton = 200 µs; Cp = 60 g/l, the largest change in MRR and TWR are 165.714% and 163.830% respectively as compared with the EDM method. The chemical composition of machined surfaces was also transformed. In comparison to the EDM method, PSCDA and RLT generated by the PMEDM method were significantly reduced, up to 52.558% and 63.366%, respectively.

Evolution of Damage in All-Oxide Ceramic Matrix Composite After Cyclic Loading

While structural ceramics usually display a brittle mechanical behavior, their composites may show non-linearities, mostly due to microcracking. In this work, we studied the evolution of the stiffness of a sandwich-like laminate consisting of an Al2O3-15%vol.ZrO2 matrix reinforced with Nextel™ 610 long fibers as a function of number of cycles N in uniaxial testing mode. We found that the stiffness of the composite degrades with increasing N, indicating increasing microcracking. However, synchrotron X-ray refraction radiography showed that the internal specific surface of inhomogeneities (mainly cracks and pores) decreases with N. A modeling strategy was developed, based on the combination of Voigt and Reus schemes for the calculation of equivalent stiffness of mixtures with the Bruno-Kachanov model for the calculation of equivalent stiffness in microcracked and porous materials. Through modeling we could estimate the initial microcrack density in the matrix (due to the simple thermal expansion mismatch between the two constituents) and the amount of microcracking increase upon cyclic load. We found that the stiffness in such a composite degrades dramatically already after as few as 20000 cycles, but then remains nearly constant. The combination of the mechanical testing, the quantitative imaging analysis, and the modeling provided insights into the damage mechanisms acting: microcrack propagation is far more active than microcrack initiation upon cyclic loading. This scenario corresponds to previous results obtained on porous and microcracked ceramics.

Changes in thermomechanical properties due to air and water cooling of hot dry granite rocks under unconfined compression

Water has been used as a working fluid injected into the hot reservoirs during the exploitation of deep geothermal energy, therefore, understanding the thermomechanical properties of reservoir rocks after water cooling is essential. For that reason, we have conducted a series of laboratory tests on air and water cooled granites from normal temperature to 600 °C, to reveal the changes in their thermomechanical properties. At 600 °C, the average values of uniaxial compressive strength, elastic modulus and P-wave velocity of water cooled granite decrease by 84.9%, 73.1% and 66.2%, which are 11.0%, 17.0% and 17.7% larger than those of air cooled granite. Through optical microscopic analysis, the microcrack density and average width of water cooled granite increase with thermal temperature and are 4.18 mm/mm2 and 54.62 μm at 600 °C, while the values of air cooled granite are only 1.97 mm/mm2 and 25.16 μm. We thus combined the deterioration of the macroscopic mechanical characteristics of air and water cooled granites with the propagation and development of microcracks. Supported by data from international literature, the changes in the thermomechanical characteristics of granite has been systematically compared to international literature, which is hoped to provide technical support for the geothermal energy exploitation.

Variations in ultrasonic wave velocities of Miocene carbonate and clastic sedimentary rocks under dry and fully water saturated conditions

This study investigates the variations in P and S wave velocities (Vp, Vs) under dry and saturated conditions and attempts to develop empirical equations estimating Vs based on Vp. In this context, 48 intact core specimens extracted from the Miocene sedimentary rocks were examined. A significant decrease was observed in Vp under saturated conditions (Vp-sat) for several specimens. Thus, selected samples were further examined by thin section, X-ray diffraction, and scanning electron microscopy equipped with an energy dispersive spectrometer. Correlation analyses were performed to reveal the causes of reduction in Vp-sat in terms of porosity, microcrack density, and mineralogical content. Vs was also observed to have a decreasing trend under saturated conditions. Strength and elasticity loss after saturation were not well correlated with the reduction in Vp-sat. The measured velocity values were also compared with the ones derived from Gassmann’s theory. Despite the discrepancy in the literature, a new parameter explaining the negative difference between the values of saturated and dry Vp was suggested. The mudstone texture and the abundant microporosity were firstly declared to be indicative of a negative velocity difference among the same rock group having similar mineralogy. Afterward, two empirical equations estimating Vs were derived from the ultrasonic wave velocities measured in this study and the data compiled from published literature. These equations were applied to a different dataset and compared with the ones suggested by various researchers. Proposed equations were determined to be more widely applicable and valid for limestone, mudstone, and sandstone samples according to several accuracy metrics.

INTEGRATED COMPUTATIONAL MATERIAL SCIENCE ENGINEERING LIFING MODEL OF CMC COUPONS USING NANO-MICROMECHANICS BASED MULTISCALE PROGRESSIVE FAILURE ANALYSIS

The overall objective of this effort was to provide theoretical prediction for damage development for a set of laminated composites using AlphaSTAR Technology Solutions' (ATS's) commercial code GENOA (GENeral Optimization Analyzer) for the Air Force Research Laboratory (AFRL) program entitled "Assessment of Damage Progression Models for SiC/SiC Ceramic Matrix Composites (CMC Lifing Program)." Damage progression and prediction for advance ceramic matrix composite (CMC) benchmarks were done under static, fatigue, and creep service loading using test data from AFRL. Emerging and innovative multiscale (MS) modeling using computational structural mechanics (CSM) and progressive failure analysis (PFA) were proven to address the Air Force's vision to perform predictive evaluation of CMC materials using a building block validation strategy and certification process. Two different material systems and multiple layups were tested for both unnotched and notched configurations and at elevated temperatures. Calibration of the fiber and matrix properties was performed using in plane test data. The static, fatigue, and creep recalibration simulations of strength showed an average error of less than 10% between the simulation and test data. For stiffness the percent difference was found to be within 10% on average as well. All simulations used the same set of inputs (constituents, voids, fiber/matrix interphase, microcrack density, etc.) except for the noted analysis setting differences between blind and recalibration simulations. The method is consistent and follows a building block simulation approach that has an advanced yet simplistic theoretical multiscale progressive failure analysis (MS-PFA) approach.

Temperature Effect on the Velocity‐Porosity Relationship in Rocks

AbstractUnderstanding the effect of temperature on physical properties of rocks is important to develop deep oil, gas, and geothermal resources. Velocity‐porosity relations are essential ingredients for a seismic rock physics model. However, relatively little effort has thus far been made to capture the temperature effect on velocity‐porosity relations. To overcome this limitation, we develop a double‐porosity model based on Biot's theory. This model is complemented with the Batzle‐Wang empirical equations for the pressure and temperature dependence of the pore fluid, as well as the David‐Zimmerman model to quantify temperature‐dependent microcrack density. The combined model is tested on carbonate and hyaloclastite samples, for which the variation of ultrasonic P‐wave velocity with temperature has been measured in the laboratory. Comparison of the predicted P‐wave velocity with the measured velocity shows that the proposed model is capable of quantitatively describing the rock velocity‐temperature and velocity‐porosity relations. We found that, for our rock samples, the temperature‐dependent change of microcrack porosity was not the dominant factor affecting the velocity‐porosity relation and that the temperature‐dependent fluid properties had a greater effect. The proposed model also predicts that the velocity dispersion and the attenuation peak of P‐waves in carbonate rock samples become less pronounced with increasing temperature.

Validating a nonlinear elastic model for predicting the nonlinear mechanical behaviour of plasma sprayed YSZ coatings under tension

Plasma sprayed ceramics exhibit nonlinear stress-strain behaviour under tension and compression due to porosity and microcracks existing in their microstructure. The modelling done in the previous works, was reported under compression and tensile loading of YSZ. In the present study, it was extended to the effect of microstructural parameters on tensile behaviour of YSZ and validates the experimental results and introduces the variation of stress with microstructural parameters. In this paper, the nonlinear tensile behaviour of YSZ plasma sprayed material is modelled phenomenologically by obtaining a relationship between microcrack density and tensile strength . The governing equations in this model are used from the previous works done on compression behaviour of plasma sprayed materials. In the present work, an empirical relationship between tensile stress and crack density is proposed based on the experimental stress strain curves obtained from the uniaxial tensile tests on freestanding YSZ coating. The variations in maximum strength corresponding to a strain and elastic modulus were also studied for different microcrack densities and porosity. Further, the effect of porosity and crack density on mechanical behaviour of YSZ coatings is extrapolated at different combinations of porosity and crack densities ranging from 0.1–0.5 per unit volume. The result obtained using the proposed numerical model was found to be in good agreement with the obtained experimental results and the non-linear tensile behaviour of YSZ could be modelled phenomenologically. • A phenomenological model replicating the experimental behaviour of YSZ coatings in tension. • An empirical relation between the microcrack density and the tensile stress based on tensile experimental results. • Effect of tensile properties on microstructural parameters of YSZ proposed numerically.

Comparative study on fracture characteristics of coal and rock samples based on acoustic emission technology

Monitoring on the damage evolution process of the coal and rock mass in coal resource mining play a vital role in safe production and avoiding economic losses. This study aims at further improving effectiveness and accuracy of acoustic monitoring method in the process of coal resource mining. For this purpose, a rock mechanics testing system and an acoustic emission (AE) test system are employed to conduct AE event location experiments on the coal and rock samples under uniaxial loading. According to the basic theory of b-value and single-link cluster (SLC) method, changing trend of related parameters with stress is studied, including b-value, spatial correlation length ξ and information entropy H. Then, the reason for variation of these three parameters is revealed. The analysis results show that b-value has not obvious change at the initial stage of loading, but spatial correlation length ξ presents upward trend during this period. Compared with b-value and spatial correlation length ξ, information entropy H is not sensitive to damage state of the coal and rock sample during the most time of loading process. However, it is worth noting that that these three parameters show significant change trend before the buckling failure of the coal and rock samples. Based on three-dimensional crack growth theory and criterion of microcrack density, the research results show that the change of the parameters are the macro characterization of transformation process from small-scale microfracture to large-scale microfracture and from local damage to overall damage for the coal and rock samples subjected to uniaxial loading. This research provides a new method to evaluate the damage state of the coal and rock mass under high stress using AE source location technology, and it has important guiding significance and reference value for ensuring the safety of life and property in the process of coal resource mining.

Controllable configuration of conductive pathway by tailoring the fiber alignment for ultrasensitive strain monitoring

Fiber-based architectures exhibit remarkable advantages in wearable electronics. Despite the various techniques to achieve strain sensitive fibers, important factors concerning the sensing performance, e.g. fiber arrangement, have been rarely explored. Herein, various fiber sensing patterns with anisotropic and isotropic microstructures were fabricated, and their localized strain distribution and conductive pathway configuration were tailored through fiber alignment. The sensing performance and mechanisms were investigated based on experimental characterization and theoretical modeling. We find out that the fibers arrangement strongly determines their strain sensitive behavior including sensitivity and sensing range. Additionally, by introducing a mechanical-guided step to realize a constant microcrack density in the fiber patterns, strain sensitivity and stability can be further improved. As a proof of concept, we integrated two fiber array sensors into a strain sensing system to simultaneously achieve an ultrahigh sensitivity (gauge factor up to 9540) and a wide sensing range (0.01–110%) as well as fast response.

Study on the Quantitative Measurement Technique of the Micro Fracture Surface

The quantitative study on the distribution of cracks and micro cracks is a major problem in the study of reservoir properties of tight and lithologic regions In view of this problem, the present study is to evaluate the cracks by indirect method, and the direct evaluation method is lack of micro cracks This paper tries to combine the results of electronic scanning electron microscopy and image data processing method, which can directly test the density of micro cracks The test results prove that the method is convenient and feasible, and has a strong reference value for engineering practice and scientific research.

Effects of Cryogenic Milling on Stress Corrosion Cracking Resistance of AISI 316L Austenitic Stainless Steel

The stress corrosion cracking (SCC) susceptibility of 316L austenitic stainless steel milled with cryogenic cooling and conventional cutting fluid cooling is investigated and compared. The milled subsurface properties are characterized in terms of microstructure, residual stress and microhardness. The SCC susceptibility of the milled surfaces is evaluated in boiling MgCl2 by examining the density and depth of the SCC microcracks. Results show that the cryogenic milling increases the SCC susceptibility of 316L austenitic stainless steel. The increase of SCC susceptibility is attributed to the higher residual tensile stress level and the larger volume fraction of slip band produced in the cryogenic milling process in comparison with the conventional cooling milling.