Size Of Microcracks Research Articles

AE based crack size estimates from delamination propagation in fiber reinforced thermoset composites

In previous research it has been shown that micro-crack sizes estimated from acoustic emission (AE) data recorded during Mode I interlaminar delamination propagation in a carbon fiber reinforced PEEK and micro-crack sizes seen in video recordings from simultaneously performed X-ray projection radiography with a contrast agent indicate reasonable agreement. Now the approach of estimating crack sizes from AE data is extended to a carbon and a glass fiber reinforced epoxy laminate, using the same matrix. In a first step sensitivity of the AE measurement is estimated based on recorded AE data and estimated delamination area. Different signal filtering approaches are applied to AE data resulting in different estimated sensitivities. Furthermore, assumed delamination area affects sensitivity. Optical microscopy is used to account for roughness of the delamination area. Depending on the assumed sensitivity, average crack size diameters are either slightly below or above 100 µm in both investigated laminates.

Effects of multiple overloads and high-low sequence loading on the fatigue crack propagation of AZ31B magnesium alloy

An experimental study was conducted to examine the initiation of cracks in the AZ31B magnesium (Mg) alloy for tensile overload effects, exploring the influence of overload cycle number, magnitude, and multiplicity of the applied overloads (OL). The number of cycles to failure increased from 8 % to 15 % for overload ratio (OLR) of 1.5 and 2.0 respectively applied as first cycle when compared with constant amplitude loading (CAL). However, the number of cycles to failure increased from 25 % to 43 % for OLR of 1.5 and 2.0 respectively when overload was applied at 5000th cycle. The onset of cracks was delayed when two overloads were applied as the first and 5000th loading cycles. The longitudinal-transverse (L-T) specimens were tested using a two-step high-low sequence loading procedure by varying maximum and minimum load. The fractography results also depict variations in the initial crack formations under different overload conditions. The effects of different overloads on the fracture surfaces of Compact Tension specimens were examined using the Field Emission Scanning Electron Microscopy, indicating substantial reduction in both the number and size of microcracks. 3D profilometer tests were performed, indicating a surface roughness of 0.20 μm to 0.30 μm.

Research on microcracks detection of metal plate based on nonlinear ultrasound

Abstract In recent years, phased array ultrasonic non-destructive testing technology has been increasingly applied in the field of non-destructive testing. However, current phased array ultrasonic testing is based on traditional linear ultrasound and is not sensitive to defects such as microcracks in materials. This article proposes a nonlinear ultrasonic phased array detection technology—fundamental amplitude difference. Nonlinear components are obtained by subtracting the transmission of even and odd elements from the transmission of all elements in a phased array. First, to improve the detection effect, MATLAB software is used to simulate and study probes with different parameters. Under the premise of meeting the probe design standards, the various parameters of the probes are determined. Then, an experimental model was established using the finite element tool Abaqus. The relationship between microcracks of different lengths and nonlinear components was calculated by constructing multiple microcrack damage models. The final simulation results show that as the size of microcracks increases, the corresponding nonlinear components also gradually increase, which can more accurately detect nonlinear components and improve detection accuracy.

Advantages of hydrogen nanobubble-rich concrete in workability and mechanical properties

This study investigates the potential benefits of using the 150 nm level Hydrogen Nanobubble Water (HNBW) in cement concrete. Given the significant greenhouse gas emissions associated with the cement and concrete industry, enhancing building materials performance and environmental friendliness becomes crucial. In this work, experiments were conducted to evaluate the various properties of this particular concrete formulation such as hydration heat, compressive strength, slump, air content, water absorption, electrical resistivity, rapid chloride permeability, and SEM. The results of the analysis showed that the HNBW improved the workability of the concrete by 50%. The HNBW improved the mechanical properties and durability of the concrete, resulting in an increase in compressive strength of 18.03% at 3 days and over 6% at 7 and 28 days. It reduces water absorption while improving electrical resistivity and chloride permeability resistance. Additionally, SEM imaging confirms that HNBW contributes to a more compact structure with pore and micro-crack size, thereby improving the overall performance of the concrete. Therefore, hydrogen nanobubbles, as a nanomaterial, exhibit favorable effects in meeting the demands of concrete.

Development of a Laboratory Test Apparatus for Simulating Core Drilling under High In Situ Stress

Abstract Rock cores drilled under high in situ stress environments are inevitably permanently damaged due to the stress relief, which significantly affects the evaluation of the physical and mechanical properties of deep rocks. Quantitatively correlating this sampling damage with in situ stress distribution is challenging because of the complexity of the geological environment and the diversity of rock materials. To address this challenge, a laboratory test apparatus has been developed that can apply specific pressure to shallow rock materials to simulate high in situ stress environments and drilling–coring to simulate the sampling process of deep rock masses. The results of verification tests demonstrate that the integrity of cores gradually deteriorated as the sampling confining pressure increased. Moreover, simultaneous increases in the density and size of microcracks were observed within the cores, where these microcracks propagate through grain boundaries, generate intragrain and transgranular microcracks, extend outwardly to form macroscopic cracks, and enhance porosity. These findings confirm the applicability of the apparatus for conducting quantitative sampling damage studies. In the future, further investigations using the apparatus will be conducted to explore the mechanical attenuation patterns of rock cores, thereby providing a reference to evaluate rock mechanics parameters for engineering practices.

Revealing damage evolution and ablation behavior of SiC/SiC ceramic matrix composites by picosecond laser high-efficiency ablation

The picosecond laser was utilized to fabricate the slanting microholes efficiently on SiC/SiC ceramic matrix composites. The machining damage, material oxidation, and temperature field were mainly analyzed through static and in-situ detections. The damage evolution and ablation behavior of SiC/SiC composites were qualitatively and quantitatively revealed. The results showed that the lowest peak temperature of the slanting microhole was 282.6 °C. The heat accumulation effect and thermal stress that was tensile stress in nature present caused recasts, microcracks, fiber fracture, fiber debonding, and fiber stripping. Fibers and matrix underwent slight oxidation, forming SiO2 recasts through the phase transition. The pulse energy, frequency, and inclination angle regularly affected the quantity, size, and distribution of microcracks, material removal rate (MRR), and hole wall roughness. The maximum MRR was about 2.5 mm3/s, corresponding to the minimum time was 0.9 s. The circularly polarized picosecond laser could process parallel striped laser-induced periodic surface structures (LIPSS) on the hole wall.

Effect of White-Portland cement containing micro and nano silica on the mechanical and freeze-thaw properties of self compacting mortars

This study concentrated to investigate the influences of white-Portland cement (W-PC) containing silica fume (SF: micro-silica (µSi) and nano-silica (nSi)) as replacement (2–4 wt%) on mechanical (strength and durability), physical and microstructural behaviors of self-compacting mortars (SCMs) fabricated at 0.48–0.63 w/b ratios. In keeping with these purposes, besides of carrying out the workability (mini-slump flow: M−SF and V-funnel: V-F) experiments of SCMs, the mechanical and microstructural tests implemented were: unconfined compressive strength (UCS), flexural strength (FS), freeze–thaw (F-T) and scanning electron microscope (SEM) analysis. The outcomes displayed that, in comparison with the control, replacing W-PC by SF (particularly nSi) enhanced the UCS and FS, resisted against F-T durability by lowering the strength losses in UCS and FS. The ameliorated microstructure resulting in a reduction in the number and/or a narrowing in the size of micro-cracks and pores contributed to generate a more compact and consolidated SCM structure for the improving of strength and durability due of pore-filling effect and superior pozzolanic characteristics of µSi and especially nSi, this in turn reduced the permeability of SCM, which has a significant effect on the strength and durability behaviors of the SCM.

APPLICATION OF BENTONITE IN OBTAINING SULFUR-CONTAINING UREA BASED ON A FLUSH OF UREA AND SULFUR

This paper presents the results of a study of the processes for obtaining granular sulfur-containing carbamide from sulfur and a liquid carbamide melt using bentonite to influence the surface tension between two phases of sulfur and urea to obtain a mixed homogeneous phase. It was revealed that the addition of bentonite to sulfur before adding the latter to the melt of urea, which has a temperature of 135-145 °С at mass ratios (NH2)2CO : bentonite : sulfur = 100 : (1-7) : (1-15) prevents the stratification of melts sulfur and urea. The optimal conditions for obtaining, the composition and properties of granular sulfur-containing urea with a balanced content of nitrogen and sulfur are determined by adding a mixture of sulfur with bentonite to the urea melt before granulation. In fertilizers obtained with the addition of a mixture of sulfur with bentonite to the melt of carbamide at the studied ratios, the strength of the granules increases from the initial 2.65 to 4.76 MPa, the gyroscopic point increases from 58.4% to 62.3%, pH and porosity decrease from 8 02 to 5.32 and from 5.60 to 4.55, respectively, and the dissolution rate of the granules decreases 3.36 times. It was also found that when adding a mixture of sulfur with bentonite to the melt of carbamide, the density and viscosity of the melt increase with an increase in the amount of added mixture of sulfur with bentonite. An increase in the amount of addition of a mixture of sulfur and bentonite from 1 to 15 and from 1 to 7 g per 100 г melt of urea at a temperature of 135 °C leads to an increase in the density and viscosity of the melts from 1.16 to 1.33 g/cm3 and from 2.56 to 3.06 cPz. Microscopic examination showed that adding a mixture of sulfur with bentonite to the carbamide melt leads to a decrease in the size of micropores and microcracks up to 0.93 microns wide. On the basis of the results obtained, optimal technological parameters and a process flow diagram for the production of sulfur-containing granulated carbamide are proposed.

Automatic detection and pixel-level quantification of surface microcracks in ceramics grinding: An exploration with Mask R-CNN and TransUNet

Microcrack damage on the grinding surfaces of engineering ceramics is inevitably incurred. To accurately evaluate the microcrack damage, an automatic detection and pixel-level quantification model based on the joint Mask R-CNN and TransUNet is developed. In addition, a joint training strategy is employed and the model is trained effectively on the image dataset of microcrack damage derived from the Si3N4 grinding, as captured by SEM. The Mask R-CNN demonstrates reliable automatic detection of microcracks, achieving an Average Precision of AP50 = 0.989 and AP75 = 0.864. Meanwhile, the TransUNet achieves fine segmentation of microcracks with complex characteristics, with an F1 score of 0.914 and an IoU value of 0.785. A skeleton-based quantification model of microcrack size is proposed, which delivers comprehensive and precise measurements of area, length, and notably, the width. The proposed quantification model provides a technical reference for the automatic evaluation of grinding surface quality.

A novel fractal-statistical scaling model of rocks considering strain rate

The scaling-dependent behaviors of rocks are significant to the stability and safe operation of the structures built in or on rock masses for practical engineering. Currently, many size effect models are employed to connect laboratory measurements at small scales and engineering applications at large scales. However, limited works consider the strain rate effect. In this study, an fractal-statistical scaling model incorporating strain rate is proposed based on a weakest-link approach, fractal theory and dynamic fracture mechanics. The proposed scaling model consists of 8 model parameters with physical meaning, i.e. rate-dependent parameter, intrinsic material parameter, dynamic strain rate, quasi-static strain rate, quasi-static fracture toughness, micro-crack size, micro-crack intensity and fractal dimension, enabling the proposed scaling model to model the scaling behaviors under different external conditions. Theoretical predictions are consistent with experimental data on red sandstone, proving the reliability and effectiveness of the proposed scaling model. Thus, the scaling behaviors of rocks under dynamic loading conditions can be captured by the proposed fractal-statistical scaling model. The sensitivity analysis indicates that the size effect becomes more obvious with a higher strain rate, larger fractal dimension, smaller micro-crack size or lower micro-crack intensity. Therefore, the proposed scaling model has the potential to capture the scaling behaviors considering the thermal effect, weathering effect, anisotropic characteristic etc., as the proposed scaling model incorporated model parameters with physical meaning. The findings of this study are of fundamental importance to understand the scaling behaviors of rock under dynamic loading condition, and thus would facilitate the appropriate design of rock engineering.

Engineering microcracks in MWCNT/elastomer bilayers for high-performance stretchable sensor development

Stretchable strain sensors in motion detection, health monitoring, and human-machine interfaces are limited by device sensitivity, linearity, hysteresis, stability, and reproducibility in addition to stretchability. Engineering defect structures in sensing material is an effective approach in modulating the material's physical properties, particularly those associated with mechanical responses. Here, we demonstrate that bilayers of carbon nanotubes deposited on an elastomer substrate are mechanically coupled. The microcrack size, density, and distribution in the nanotube thin film can be engineered through uniaxial tensile training to exhibit highly tunable and stable piezoresistive responses with sensitivity, linearity, range, and reproducibility. These responses far exceeding those in uniform metallic films, patterned structures, and composites. In addition, numerical analyses performed on a two-dimensional network model of the cracked nanotube film provide quantitative explanations of how crack configuration, and evolvement under strain, lead to the significant enhancements in stretchable sensor performance using current bilayer structures.

Subduction as a Smoothing Machine: How Multiscale Dissipation Relates Precursor Signals to Fault Geometry

Understanding the process of earthquake preparation is of utmost importance in mitigating the potential damage caused by seismic events. That is why the study of seismic precursors is fundamental. However, the community studying non-seismic precursors relies on measurements, methods, and theories that lack a causal relationship with the earthquakes they claim to predict, generating skepticism among classical seismologists. Nonetheless, in recent years, a group has emerged that seeks to bridge the gap between these communities by applying fundamental laws of physics, such as the application of the second law of thermodynamics in multiscale systems. These systems, characterized by describing irreversible processes, are described by a global parameter called thermodynamic fractal dimension, denoted as D. A decrease in D indicates that the system starts seeking to release excess energy on a macroscopic scale, increasing entropy. It has been found that the decrease in D prior to major earthquakes is related to the increase in the size of microcracks and the emission of electromagnetic signals in localized zones, as well as the decrease in the ratio of large to small earthquakes known as the b-value. However, it is still necessary to elucidate how D, which is also associated with the roughness of surfaces, relates to other rupture parameters such as residual energy, magnitude, or fracture energy. Hence, this work establishes analytical relationships among them. Particularly, it is found that larger magnitude earthquakes with higher residual energy are associated with smoother faults. This indicates that the pre-seismic processes, which give rise to both seismic and non-seismic precursor signals, must also be accompanied by changes in the geometric properties of faults. Therefore, it can be concluded that all types of precursors (seismic or non-seismic), changes in fault smoothness, and the occurrence of earthquakes are different manifestations of the same multiscale dissipative system.

Structure, mechanical properties and tribocorrosion behaviours of superhard TiSiCN nanocomposite coatings

Superhard coatings are no longer very difficult to obtain, but the direct observation and prediction of their failure are still a challenge. 3D tomography of the microcracks in coating failure offers lots of new information on the geometric shape, size, and spatial structure relationship of microcracks which was not available up to now. Hence, in this work, cross-scale visualization analysis of the microcrack network in superhard yet toughness TiSiCN coatings around indentations was achieved through advanced 3D tomography combined with HRTEM characterization. TiSiCN nanocomposite coatings were deposited by high-power pulsed magnetron sputtering. Superhard TiSiCN coatings exhibit a simultaneous superhardness of 48 ± 6.99 GPa and favorable toughness due to fully dense microstructure and smooth surface morphology. Due to the excellent crack initiation resistance and super-high hardness, superhard TiSiCN coatings show enhanced tribocorrosion performance in 3.5 wt% NaCl aqueous solution, with a low coefficient of friction of 0.11 and low specific wear rates of 3.23 × 10−6 mm−3N−1m−1.

The influence of phase morphology of polycarbonate/polyethersulfone blends on the failure behavior between the blends and polyurethane in the peel test

AbstractIn peel, tests between polycarbonate/polyethersulfone (PC/PES) blends with different compositions and thermoplastic polyurethane (TPU) films with different thicknesses a large decrease in energy release rate is observed when the PC/PES blends formed a co‐continuous structure. With SEM analysis, XRD measurements, selective PC etching of failed samples, and finite element modeling of the stress distributions in the co‐continuous region this fracture behavior can be explained. It is caused by the interplay between the formation of the main crack in the process zone and the size and density of microcracks in the energy dissipation zone. While the pure components PC and PES exhibit good adhesion to the TPU, the immiscibility of PC and PES causes the steady decrease of with increasing volume fraction of the minor phase.

Concrete Composites Based on Quaternary Blended Cements with a Reduced Width of Initial Microcracks

This article is devoted to the study of the combined effect of siliceous fly ash (FA), silica fume (SF), and nanosilica (nS) on the cement matrix morphology and size of microcracks occurring in the Interfacial Transition Zone (ITZ) between the coarse aggregate and the cement paste of concrete composites based on ordinary Portland cement (OPC). The manuscript contains analyses of width of microcracks (Wc) occurring in the ITZ area of concretes based on quaternary blended cements and changes in ITZ morphology in the concretes in question. Experiments were planned for four types of concrete. Three of them were composites based on quaternary blended cements (QBC), while the fourth was reference concrete (REF). Based on the observations of the matrices of individual composites, it was found that the REF concrete was characterized by the most heterogeneous structure. However, substitution of part of the cement binder with active pozzolanic additives resulted in a more compact and homogenous structure of the cement matrix in each of the QBC series concretes. Moreover, when analyzing the average Wc values, it should be stated that the modification of the basic structure of the cement matrix present in the REF concrete resulted in a significant reduction of the analyzed parameter in all concretes of the QBC series. For QBC-1, QBC-2, and QBC-3, the Wc values were 0.70 μm, 0.59 μm, and 0.79 μm, respectively, indicating a decrease of 38%, almost 48%, and 30%, respectively, compared with the working condition of concrete without additives. On the basis of the above results, it can therefore be concluded that the proposed modification of the binder composition in the analyzed materials clearly leads to homogenization of the composite structure and limitation of initial internal damages in concrete.

Laboratorial Study of the Combined Effect of SO2 and High-Temperature Ageing on the Physical and Mechanical Properties of Encostinha Marble, a Portuguese Stone

ABSTRACT The effects of SO2, high temperature (600°C) and cooling type (slow and rapid) were investigated in this study as decay factors of a building stone (Encostinha Marble). Changes in colour, gloss, apparent density, open porosity, capillary water absorption and ultrasonic pulse velocity were assessed, complemented with SEM observations and XRD analysis. SO2 did not significantly react with the marble; no neoformed products were identified and only superficial changes were registered. Specimens became darker and duller. The increase in size of pre-existing microcracks and the development of new ones caused by high temperature, with a consequent increase in open porosity, was the main cause of the marked degradation of physical and mechanical properties. The cooling type was also shown to influence the physical and mechanical characteristics of the marble. A more pronounced increase in open porosity and capillary water absorption in specimens cooled by total immersion in tap water was observed. This increase was not in line with the corresponding decrease in ultrasonic parameter values.

3D DEM simulations of the variability of rock mechanical behaviour based on random rock microcracks

In this study, to investigate the variability of mechanical parameters of rock, a three-dimensional (3D) discrete element model (DEM) with the consideration of random microcracks was proposed. The random microcracks follow the negative power-law distribution with upper and lower limits of microcrack size. The results showed that coefficient of variations (COVs) and average values of elastic modulus, Poisson's ratio, unconfined compression strength and tensile strength of Lac du Bonnet granite are well matched through the present model. The present simulations indicate that the large COVs were induced by the interconnected large microcracks. The parametric study clearly indicates that the random microcracks significantly affect COVs, while the effect of grain size variation can be ignored. The size of microcracks determines the variability of mechanical parameters, and the volume density of microcracks can significantly influence the COVs. The correlation between the fractal dimension of microcrack size distribution and COVs is not linear. The increasing variability of microcrack density can result in the increase of COVs. Finally, a calibration procedure was proposed to determine the present model's parameters.

Cleavage Micromechanisms Study in a Ship Plate Steel through Four-Point Double-Notch Bend and Charpy Tests

In this work instrumented impact Charpy tests were performed to analyze the ductile-brittle transition of Grade A ship plate steels, which are composed of ferrite matrix with pearlite bands in the rolling direction (RD) and pearlite lands in the transverse direction (TD). Observations on fracture surfaces of Charpy specimens showed regions of cleavage from room to down temperatures, therefore blunt four-point double–notch bend specimens (4PBT), were also tested at 25°C, 0°C, –60°C and –196°C in RD and TD, to study cleavage nucleation and propagation mechanisms. In the RD the following four different cleavage microfeatures nucleated microcracks in the notch region of 4PBT in all test temperatures: lamellar-pearlite microstructure, pearlite-boundary, ferrite-grains boundary and grains ferrite-inclusions. However, in the TD at –60°C, close to the lower shelf, only lamellar-pearlite and pearlite-boundary microfeatures were found nucleating microcracks. Nevertheless, the higher density of microcracks was found in the RD. The biggest microcracks were developed in TD in pearlite lands of critical size, in all test temperature, which are the link for the development of microcracks of critical size, which can propagate and fracture steel plates. In RD most microcracks nucleated in a critical region of about 1mm from notch root for all test temperatures, however for the TD the critical region decreases with test temperature, from about 450 µm (+25°C), 370 µm (0°C) and 225µm (–60°C). The analysis also showed the effect of microstructure orientation on the number and size of microcracks, and microcracks arrest.

Critical Roles of Impurities and Imperfections in Various Phases of Materials.

In many materials, impurities and imperfections play a critical role on the physical and chemical properties. In the present review, some examples of such materials are discussed. A bulk nanobubble (an ultrafine bubble) is stabilized against dissolution by hydrophobic impurities attached to the bubble surface. An acoustic cavitation threshold in various liquids decreases significantly by the presence of impurities such as solid particles, etc. The strength of brittle ceramics is determined by the size and number of pre-existing microcracks (imperfections) in the specimen. The size effect of a BaTiO3 nanocrystal is influenced by the amount and species of adsorbates (impurities) on its surface as adsorbate-induced charge-screening changes the free energy. The dielectric constant of an assembly of BaTiO3 nanocubes is influenced by a small tilt angle (imperfection) between two attached nanocubes, which induces strain inside a nanocube, and is also influenced by the spatial strain-relaxation due to defects and dislocations (imperfections), resulting in flexoelectric polarization.

Fiber Optic Ice Sensor for Measuring Ice Thickness, Type and the Freezing Fraction on Aircraft Wings

Ice accretion on an aircraft affects the aerodynamic performance of the wings by disrupting the airflow, increasing drag, and altering its flight characteristics, leading to a main or tail wing-stall and altimetry to aircraft loss. The current generation of ice-detection systems relies on environmental parameters to determine icing conditions, with the sensors usually located on the nose of the aircraft, giving no information on the ice accreted on the wings. This work focuses on modeling and developing a fiber-optic-array ice sensor, which illuminates and detects the reflected and scattered light directly from the ice surface and volume, and measures the accretion rate and type of ice on the wings. The ice morphology is influenced by the rate of freezing of the super-cooled droplets impacting the wings, and partially or totally trapping the dissolved gasses. This leads to the formation of rugged surfaces and ice shapes, which can be transparent or opaque, a process which is dependent on the local Freezing Fraction (FF) of the impinging super cooled water. The detection method relies on the optical characteristics of ice, affected by density and the size of micro-cracks and micro-bubbles formed during freezing. By using high Numerical Aperture (NA) fibers, it was possible to accurately measure the ice thickness, and to investigate a proof-of-concept experiment, correlating the optical diffusion to the FF of the ice on a wing.