Microcrack Morphology Research Articles

The Phenomenon of Cracking in Cement Concretes and Reinforced Concrete Structures: The Mechanism of Cracks Formation, Causes of Their Initiation, Types and Places of Occurrence, and Methods of Detection—A Review

Cracks and cavities belong to two basic forms of damage to the concrete structure, which may reduce the load-bearing capacity and tightness of the structure and lead to failures and catastrophes in construction structures. Excessive and uncontrolled cracking of the structural element may cause both corrosion and weakening of the adhesion of the reinforcement present in it. Moreover, cracking in the structure negatively affects its aesthetics and in extreme cases may cause discomfort to people staying in such a building. Therefore, the following article provides an in-depth review of issues related to the formation and development of damage and cracking in the structure of concrete composites. It focuses on the causes of crack initiation and characterizes their basic types. An overview of the most commonly used methods for detecting and analyzing the shape of microcracks and diagnosing the trajectory of their propagation is also presented. The types of cracks occurring in concrete composites can be divided according to eight specific criteria. In reinforced concrete elements, macrocracks depend on the type of prevailing loads, whereas microcracks are correlated with their specific case. The analyses conducted show that microcracks are usually rectilinear in shape in tensioned elements; in shear elements there are wing microcracks with straight wings; and torsional stresses cause changes in wing microcrack morphology in that the tips of the wings are twisted. It should be noted that the subject matter of microcracks and cracks in concrete and structures made of this material is important in many respects as it concerns, in a holistic approach, the durability of buildings, the safety of people staying in the buildings, and costs related to possible repairs to damaged structural elements. Therefore, this problem should be further investigated in the field of evaluation of the cracking and fracture processes, both in concrete composites and reinforced concrete structures.

Effect of Interfacial Strength on Mechanical Behavior of Be/2024Al Composites by Pressure Infiltration.

In this paper, two kinds of Be/2024Al composites were prepared by the pressure infiltration method using two different beryllium powders as reinforcements and 2024Al as a matrix. The effect of interfacial strength on the mechanical behavior of Be/2024Al composites was studied. Firstly, the microstructure and mechanical properties of the two composites were characterized, and then the finite element analysis (FEA) simulation was used to further illustrate the influence of interfacial strength on the mechanical properties of the two Be/2024Al composites. The mechanical tensile test results show that the tensile strength and elongation of the beryllium/2024Al composite prepared by the blocky impact grinding beryllium powder (blocky-Be/2024Al composite) are 405 MPa and 1.58%, respectively, which is superior to that of the beryllium/2024Al composite prepared by the spherical atomization beryllium powder (spherical-Be/2024Al composite), as its strength and elongation are 331 MPa and 0.38%, respectively. Meanwhile, the fracture of the former shows brittle fracture of beryllium particles and ductile fracture of aluminum, while the latter shows interface debonding. Further FEA simulation illustrates that the interfacial strength of the blocky-Be/2024Al composite is 600 MPa, which is higher than that of the spherical-Be/2024Al composite (330 MPa). Therefore, it can be concluded that the better mechanical properties of the blocky-Be/2024Al composite contribute to its stronger beryllium/aluminum interfacial strength, and the better interfacial strength might be due to the rough surface and microcrack morphology of blocky beryllium particles. These research results provide effective experimental and simulation support for the selection of beryllium powder and the design and preparation of high-performance beryllium/aluminum composites.

Microfracture behavior and energy evolution of heterogeneous mudstone subjected to moisture diffusion

Moisture has a great effect on the microstructure of mudstone and the stability of muddy strata. Due to many clay minerals in mudstone, swelling deformation and even disintegration occur during moisture diffusion, accompanied by energy storage and release. To reveal the deterioration mechanism and energy characteristics of mudstone induced by moisture diffusion, firstly, the Weibull distribution function is introduced into the FDEM-based moisture diffusion-fracture coupling model to consider the heterogeneity of swelling deformation and stress of mudstone minerals. Then, the microfracture behavior and energy evolution of heterogeneous mudstone during moisture diffusion are studied using the coupling model. Results indicate that the nonuniform swelling deformation between particles is the main reason for the microfracture, both tensile and shear failures occur in mudstone. The evolution of kinetic energy can be divided into three stages, while that of strain energy depends on the storage rate by swelling deformation and the release rate by microfracture in mudstone during moisture diffusion. When the storage rate is greater than the release rate, the evolution of strain energy is divided into two stages. Otherwise, it is divided into three stages. Besides, the effects of homogeneity index, swelling coefficient, and moisture content on the microcrack morphology and energy evolution are also discussed. Finally, the microcrack morphology and energy evolution of mudstone under different moisture loading conditions are investigated, where the microcrack morphology under the surrounding moisture condition agrees well with the experimental results. The results in this paper provide theoretical value for studying the swelling and microfracture mechanism of mudstone during moisture diffusion.

Microdanos ósseos gerados na preparação do sítio de instalação de implantes: uma alternativa para análise ex vivo de diferentes protocolos de velocidade de perfuração

ABSTRACT Objective: The aim of this study was to evaluate bone microdamage in sites prepared for implant placement by using an ex vivo model with three drilling rotation speeds. Methods: Bovine bone ribs were used for the creation of 18 osteotomy sites at different rotation speeds: 1200 rpm, 800 rpm, and 400 rpm. Specimens were stained with xylenol orange and prepared for histological analysis by using fluorescence and polarized light microscopies. Bone microdamage was evaluated by number and based on total bone area, as follows: microfracture density (Fr.D = n/mm2), microcrack morphology (diffuse or linear), and density (Cr.D = n/mm2), and presence of bone chips. To complement the analysis, linear microcracks were assessed by using confocal microscopy for three-dimensional visualization. Results: Bone microdamage on the osteotomy sites included microcracks, diffuse damages, microfracture, and bone chip formation. There was an association between bone microdamage and cancellous bone (p 0.0016), as well as a positive correlation between Fr.D and Cr.D (p 0.05, r 0.54). BM occurrence was not different between the three rotation speeds. In 3D, the height of the microcrack depth was 60.81 µm. Conclusion: Bone microdamage occurs during osteotomy, and the ex vivo model used was effective for the assessment of these biomechanical parameters. In addition, microdamage was not influenced by the drilling rotation speed in this experimental condition.

Multiscale modeling of distributed microcracking in concrete

AbstractConcrete is a quasi‐brittle material with a heterogeneous morphology across multiple scales, which also influences the evolution of damage in case of loading. Damage in concrete structures is initiated by growth of diffuse microcracks that is followed by damage localization eventually leading to structural failure. Thus, diffuse microcracking processes can be considered as failure precursors. Identification and characterization of these precursors can be applied in conjunction with structural health monitoring based on diffuse ultrasonic waves to develop early warning systems against catastrophic failure of concrete structures. To this end, a multiscale model for a simulation‐based description of diffuse damage in concrete is proposed. At the scale of the cement paste, microcrack growth is modeled by a combination of continuum micromechanics and linear elastic fracture mechanics [1]. The micromechanics model is incorporated into a reduced‐order Lippmann‐Schwinger based meso‐model for concrete [2], [3]. The mesoscale model is generated using Random Sequential Adsorption of realistic aggregates based on concave slicing of randomly cut polyhedrons. The influence of the microcrack morphology at the microscale on the diffuse microcracking and overall concrete stiffness degradation is presented.

High‐Transconductance Stretchable Transistors Achieved by Controlled Gold Microcrack Morphology

AbstractHigh‐transconductance stretchable transistors are important for conformable and sensitive sensors for wearables and soft robotics. Remarkably high transconductance, which enables large amplification of signals, has been achieved through the use of organic electrochemical transistors (OECTs). However, the stretchability of such systems has been tempered by the lack of stretchable conductors with high stability in electrolytes, high conductance at high strain (100%), and process compatibility with active layers. Highly stretchable and strain‐resistant Au conductors employed to fabricate intrinsically stretchable OECTs are demonstrated. Notably, the conductors exhibit a sheet resistance of 33.3 Ω Sq.−1 at 120% strain, the lowest reported value to date among stretchable Au thin film conductors. High‐performance stretchable Au is realized by suppressing strain‐induced microcrack propagation through control of the microcracks formed in deposited Au thin films. Then, the highly stretchable Au conductors are utilized to fabricate intrinsically stretchable OECTs with a high transconductance both at 0% strain (0.54 mS) and 140% strain (0.14 mS). Among previously reported systems, these OECTs show the highest transconductance at high strain (>50%). Finally, the high‐performance OECTs are utilized in stretchable synaptic transistors, which are critically important for the development of soft neuromorphic computing systems to provide artificial intelligence for future soft robotics.

Effects of polymeric filler on polarization fatigue of 1-3 0.74Pb(Mg1/3Nb2/3)O3-0.26PbTiO3 single crystal/polymer composites

Polarization fatigue tests have been conducted on both bulk Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals and two 1-3 PMN-xPT/polymer composites with both “soft” and “hard” epoxy fillers, respectively. The variations of remnant polarization, coercive electric field, and micro-crack morphology under 10 Hz cyclic electric loading were studied. It was found that the magnitude of coercive field for both composites was higher compared to that of single crystals. However, as the number of cycles increases, the degradation ratio of remnant polarization and the increased portion of coercive field for both composites were lower than that of single crystals; the sequence is PMN-0.26PT bulk single crystal > 1-3 PMN-0.26PT/soft epoxy composites > 1-3 PMN-0.26PT/hard epoxy composites. In addition, the observed micro-crack propagation morphology shows that 1-3 PMN-0.26PT/hard epoxy composites can substantially improve fatigue-resistance owing to the lower stretching strain benefiting from the polymeric filler. Our results indicated that 1-3 PMN-0.26PT/polymer composites present much better electric polarization stability.

Cascade continuum micromechanics model for the effective permeability of solids with distributed microcracks: Self-similar mean-field homogenization and image analysis

The transport and fluid flow in heterogeneous materials such as rocks, ceramics and concrete with a distributed random microcrack network is strongly influenced by the density and the topology (distribution and connectivity) of microcracks. The overall fluid flow characteristics of such microcracked solids can be quantified in terms of an effective permeability. In the paper, a semi-analytical formulation for the effective permeability is proposed within the framework of the mean-field homogenization method using the cascade continuum micromechanics model considering long range and short range interactions. We compare model predictions of the percolation threshold i.e. critical volume fraction of microcracks beyond which a solid with distributed microcracks becomes permeable, using results from numerical simulations. The model reveals a new perspective into the self-similar characteristics of the microcrack morphology near the threshold volume fraction of microcracks at which the microcrack structure changes from multiple disconnected microcracks to a connected self-similar microcracked structure.

Experimental and FEM study of thermal cycling induced microcracking in carbon/epoxy triaxial braided composites

The microcrack distribution and mass change in T700s/PR520 and T700s/3502 carbon/epoxy braided composites exposed to thermal cycling was evaluated experimentally. Acoustic emission was utilized to record the crack initiation and propagation under cyclic thermal loading between −55°C and 120°C. Transverse microcrack morphology was investigated using X-ray computed tomography. The differing performance of two kinds of composites was discovered and analyzed. Based on the observations of microcrack formation, a meso-mechanical finite element model was developed to obtain the resultant mechanical properties. The simulation results exhibited a decrease in strength and stiffness with increasing crack density. Strength and stiffness reduction versus crack densities in different orientations were compared. The changes of global mechanical behavior in both axial and transverse loading conditions were studied. By accounting for the obtained reduction of mechanical properties, a macro-mechanical finite element model was utilized to investigate the influence of microcracking on the high-speed impact behavior.

Morphology and growth of porous anodic oxide films on Ti-10V-2Fe-3Al in neutral tartrate solution

Porous anodic oxide films were fabricated galvanostatically on titanium alloy Ti-10V-2Fe-3Al in ammonium tartrate solution with different anodizing time. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) were used to investigate the morphology evolution of the anodic oxide film. It is shown that above the breakdown voltage, oxygen is generated with the occurrence of drums morphology. These drums grow and extrude, which yields the compression stress. Subsequently, microcracks are generated. With continuous anodizing, porous oxides form at the microcracks. Those oxides grow and connect to each other, finally replace the microcrack morphology. The depth profile of the anodic oxide film formed at 1 800 s was examined by Auger electron spectroscopy (AES). It is found that the film is divided into three layers according to the molar fractions of elements. The outer layer is incorporated by carbon, which may come from electrolyte solution. The thickness of the outer layer is approximately 0.2–0.3 μm. The molar fractions of elements in the intermediate layer are extraordinarily stable, while those in the inner layer vary significantly with sputtering depth. The thicknesses of the intermediate layer and the inner layer are 2 μm and 1.0–1.5 μm, respectively. Moreover, the growth mechanism of porous anodic oxide films in neutral tartrate solution was proposed.

Effect of Microstructure on Torsional Fatigue Endurance of Martensitic Carbon Steel

The microstructural influence of martensitic carbon steel on torsional fatigue endurance was investigated, taking into consideration the application of high strength steel electric resistance welded (ERW) tubes to automotive structural parts. The chemical composition of the base steel alloy was 0.1-0.2%C-0.2-1.5%Si-1.3-1.9%Mn-0.01%P-0.001%S-(Cr, Mo, Ti, Nb, B). Laboratory vacuum-fused ingots were hot-rolled, heated to 1023 or 1223 K in a salt bath, and then water-quenched and tempered at 473 K. Consequently, three types of microstructure, martensite (M), martensite and ferrite (M+F), and ferrite and pearlite (F+P), were prepared. Fully reversed torsional fatigue testing was conducted with 6 mm diameter round bar specimens. Torsional fatigue endurance was found to monotonously increase with increases in the tensile strength of the specimen from 540 to 1380 MPa. The martensitic single structure and the M+F dual-phase structure showed a similar level of fatigue endurance at a tensile strength of approximately 950 MPa. However, fatigue micro-crack morphology varied slightly between them. At the surface of the M+F specimen, many small cracks were observed in addition to the main crack. Conversely, in the martensitic specimen, these small cracks were rarely observed. ΔK decreasing/increasing crack growth testing with compact tension (CT)-type specimens was also conducted. Based on these experimental results, the effect of microstructure and stress level on the initiation/propagation cycle ratio is discussed. In addition to fatigue properties, some practical properties, such as low-temperature toughness and hydrogen embrittlement resistance, were also evaluated in view of actual applications for automotive structural parts.

Microcrack morphology of an adherent coating on a thermally mismatched substrate

—It is well known that thermal expansion mismatch between a coating and a substrate often results in a microcracked coating. Depending on the degree of adhesion to the substrate, the coating can fracture at the interface (leading to coating spalling) or the coating can fracture normal to the plane of the interface. The latter is given attention in this paper. A better understanding of the mechanics and the performance of coatings requires an improvement in the quantification of the coating crack morphology. This work proposes and implements a variety of coating crack morphology measurements. Characterization has been accomplished via section and plan views. Specimen preparation techniques and digital image processing approaches have been developed to make measurements feasible. A data set from a chemically vapor deposited SiC-coated carbon-carbon composite substrate is given. The data obtained are used to propose two simplified models of the coating crack morphology.

Residual stress-induced spontaneous microcracking in α-SiC platelet Al2O3 composites

The micromechanical stresses associated with hexagonal (6H) α-SiC platelets within a fine-grained alumina matrix were calculated using an Eshelby approach. The stresses within and around the interface of SiC platelets were determined. Both stresses were found to be strongly dependent on the morphology and the volume fraction of the SiC particles. The morphology effect, however, tended to be limited at aspect ratios ⩾ 10. Owing to anisotropy in the thermal and elastic properties of α-SiC, the residual stresses just outside the inclusion also depended on the position along the SiC/Al2O3 interfaces. The maximum tensile stress was found at the edges of SiC platelets. There were two principal tangential tensile stresses which differed greatly at the edges of disc-shaped inclusions. The results of the stress analysis were consistent with observed differences in microcrack morphology and the resultant reduction of the Young's modulus of the composites.

Twinning and Microcracking Associated with Monoclinic Zirconia in the Eutectic System Zirconia–Mullite

Crystallography and morphology of twins and microcracks in the eutectic system mullite–zirconia are discussed in view of the tetragonal‐to‐monoclinic transformation and associated toughening mechanisms. Specific twin relationships were observed in monoclinic ZrO2. Highly symmetric and crystallographically well‐defined microcracks were observed at the mullite–ZrO2 interface. Microdiffraction revealed closely related crystallography of monnoclinic twins and microcracks. The number of twin variants depend on the monoclinic ZrO2 particle size. A method to calculate twinning shear strain using microcrack morphology is suggested. This parameter is essential in several fracture‐mechanics calculations.

Effect of Microcracking on the Thermal Diffusivity of Fe2TiO5

The effect of microcracking on the thermal diffusivity of polycrystalline Fe2TiO5 subjected to a range of annealing treatments was investigated. At fine grain size (∼1 μm), the thermal diffusivity exhibited the decrease with increasing temperature common for dielectrics. Extensive microcracking in the larger‐grain‐sized materials significantly decreased their thermal diffusivity. On heating, the microcracked materials exhibited increased thermal diffusivity at elevated temperatures which can be attributed primarily to microcrack closure and healing; on cooling, they exhibited a pronounced hysteresis, attributable to irreversible crack opening and closing. Thermal cycling closed the hysteresis curves, which suggests permanent changes in microcrack morphology. It appears that microcracking is a promising technique for tailoring ceramic materials to a combination of high thermal shock resistance and good insulating capability.