Crack Morphology Research Articles

Metallographic examinations and hydrogen measurements of high-burnup spent nuclear fuel cladding

In the US, commercial spent nuclear fuel (SNF) is transferred to interim dry storage casks where it will be stored for decades awaiting transport to a consolidated interim storage facility or a geologic repository. Because the fuel rod cladding is the first barrier against any radioactive material release, understanding the behavior of SNF cladding, particularly at high burnup (HBU), in dry storage conditions is crucial to safely store and transport the spent fuel. In this study, a series of metallographic examinations and cladding hydrogen measurements were conducted on HBU SNF cladding at Oak Ridge National Laboratory as a part of the High Burnup Spent Fuel Data Project, which is sponsored by the US Department of Energy (DOE) Office of Nuclear Energy (NE). To investigate the effect of simulated drying conditions on the cladding, three as-received fuel rods with different cladding materials—M5, ZIRLO, and Zircaloy-4—were heated to 400°C and then slow-cooled to room temperature. The pellet and cladding were then qualitatively and quantitatively analyzed and compared in terms of pellet crack morphology, HBU rim, waterside oxide, cladding H, and cladding hydride morphologies. This paper presents and discusses the results of these analyses in detail.

Self-healing and rebar corrosion performance of cracked engineered cementitious composites exposed to wet coral sand environment

Wet coral sand environment on islands and reefs features high humidity and abundance in aggressive fluids and gases, thus posing a great threat to the durability of reinforced concrete structures. In this study, engineered cementitious composites (ECCs) with strain-hardening and self-healing characteristics are proposed as a potential cover material in wet coral sand exposure. The autogenous self-healing capability of cracked ECCs was evaluated by the visual crack morphology as well as water permeability, and their rebar corrosion performance over a period of one and a half years was investigated based on electrochemical tests. The results show that cracks with initial width up to 70 μm are able to completely close within one day, probably due to the efficient precipitation of calcium carbonate and further hydration of cement. Compared with the uncracked control, the 0.4 %, 1.0 % and 1.6 % strained ECCs can be healed up to 100 %, 93.4 % and 60.5 % at 30 days, respectively. The water permeability coefficient of the strained ECC decreases rapidly in the first 30 days, and then stabilizes in the later stages. Moreover, the load-carrying capacity of preloaded ECC beams is recovered, exhibiting comparable performance to the uncracked controls after the same exposure periods. The corrosion current densities of steel bars suggest an active corrosion conditions. Both of the corrosion current densities and mass losses of rebar show lower values at decreased strain level, which is closely related to the width and number of cracks that reach the reinforcement.

Study on mechanical properties and microstructure of fly-ash-based geopolymer for solidifying waste mud

The chemical solidification of construction waste mud presents a strategy for mitigating environmental concerns as well as addressing problematic geotechnical engineering properties. The present study explores the efficacy of utilizing fly ash (FA)-based geopolymer, activated by calcium carbide residue (CCR) and sodium silicate solution (NS), for waste mud solidification. An unconfined compressive strength (UCS) test was conducted to ascertain the optimal CCR/NS ratio. Supplementary tests were executed to examine factors such as water content, pH level, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and natural dehydration. According to the UCS results, an optimal 28-day strength of 2.2 MPa was observed when employing a CCR/NS ratio of 6:4. A notable reduction in the water content of the mud was achieved in the short term, with the most effective results observed at a pH level of 12. Microstructural analyses indicated that calcium silicate hydrate (CSH) and aluminosilicate-Na hydrate (NASH) acted as the primary cementitious products, filling pores and creating bonds with particles to facilitate strength development. Variations in crack morphology and water content during the natural dehydration process were also analyzed for both solidified and untreated mud. The findings suggest that solidified mud holds promise for application in subgrade engineering projects, potentially diverting significant volumes of waste mud and industrial by-products such as CCR and FA away from landfills.

Experimental and numerical study of disk specimen with rough structural plane under splitting test

To study the effects of the anisotropic matrix and structural planes on the splitting strength and failure mode of rocks, Brazilian splitting tests were carried out with seven different loading angles on specimens of rock-like materials with rough structural planes. The surface strains of the samples during the failure process were monitored and analysed with the help of a high-speed camera and digital image correlation (DIC) technology. The test results showed that the Brazilian splitting strength (BSS) decreased gradually with an increased loading angle. According to the crack morphology, the samples showed three failure modes, and the structural plane and the loading angle (θ) had an important effect on the failure mode. When θ < 75°, the sample failure was mainly affected by the matrix, and when θ > 75°, the sample failure was mainly controlled by the structural plane. The numerical simulation of the sample with a structural plane was carried out by the PFC2D particle flow program, the micro parameters were calibrated using a back propagation (BP) neural network model. The internal cracks of the sample under a splitting load were mainly matrix tensile microcracks and structural plane shear microcracks, and the tensile microcracks in the side with the weak matrix appeared significantly earlier than those in the side with the strong matrix. With increasing loading angle, the proportion of tensile microcracks in the matrix increased, while the proportion of shear microcracks in the matrix decreased, especially in the weak matrix. The microcracks at the structural plane mainly changed from tensile microcracks to shear microcracks, and the development degree of microcracks along the structural plane was more significant than that on the weak matrix with increasing loading angle. The results of the study can provide a reference for rock stability evaluation and utilization.

Influence of rolling speed on fatigue crack initiation and growth of wheel steel under dry-wet rolling/sliding contact

In order to examine the effect of rolling speed on rolling contact fatigue (RCF) of wheel steel, dry-wet rolling/sliding tests were carried out at varied rolling speeds. Wear loss, friction coefficient, crack morphology, and crack size distribution of test wheel discs were analyzed. Finite element (FE) analysis was subsequently performed to evaluate the RCF crack growth according to the max ΔKeq criterion. By combining the findings from both experimental and FE analyses, potential influences of rolling speed on RCF crack initiation and growth were postulated. It was discovered that the crack initiation is governed by the ratcheting mechanism under dry condition. As the rolling speed increases, the initiated crack size first increases and then decreases, probably due to the concurrent rising of friction coefficient and yield strength. Furthermore, the predicted crack growth paths using FE model with and without considering the trapped fluid are consistent with test results at lower and higher rolling speeds, respectively. The driving force of crack growth in depth and branching direction decreases with an increase in rolling speed, probably owning to the reducing hydrostatic pressure in the crack cavity induced by the increasing air volume. This ultimately results in smaller crack size and reduced wear loss under wet condition.

Research on crack propagation behaviour of EB-PVD TBCs based on TGO evolution

Thermal Barrier Coatings (TBCs) are functional coatings used to protect high-temperature components that are prone to early damage and premature failure under the influence of complex working conditions. This paper examines the crack propagation behaviour of 8% yttria-stabilized zirconia (8YSZ) EB-PVD TBCs under different oxidation conditions at 1100 °C. The morphology of interfacial cracks after oxidation was summarized and the evolution of thermally grown oxide (TGO) was quantified. Based on the evolution of TGO, the causes of crack propagation were analyzed. For the specimens after oxidation experiment, the interfacial crack propagation behaviour was observed and analyzed by SEM, and the reason of lateral crack propagation was explained from the perspective of interfacial fracture toughness. The reason for crack deflection is analyzed from the perspective of energy release rate. The equivalent thickness, normalized rumpling index and two-dimensional roughness index were calculated, then the TGO growth behaviour was comprehensively analyzed and related to the crack propagation.

Proper orthogonal decomposition assisted eigendeformation-based mathematical homogenization method for modeling cracks in 3D polycrystalline microstructures

In this manuscript, a reduced order model (ROM) is proposed for full three-dimensional (3D) polycrystalline microstructures with tortuous short cracks. The model reformulates eigendeformation-based homogenization method (EHM) assisted with the proper orthogonal decomposition (POD) method to account for local plastic deformation and crack separation fields. A constrained optimization problem is formulated with reduced order objective functions and un-reduced constraints to solve for the separation. To make the optimization process more feasible, an integrated form of the constraint is introduced, enabling the use of only one constraint regardless of the crack morphology. The model is verified against direct numerical simulations (DNS) with crystal plasticity as the constitutive relation. The use of priori knowledge in building crack separation field basis functions as well as sampling strategies in the absence of prior knowledge are discussed. The performance and accuracy of the ROM are assessed under various loading conditions and different crack configurations.

Investigating cracking behavior of saline clayey soil under cyclic freezing-thawing effects

This study investigates the cyclic freeze-thaw (F-T) effect on crack patterns in saline clayey soil. Samples with different NaCl concentrations (0 M, 1 M, 2 M, and 3 M) are prepared and subjected to cyclic F-T processes. Each cycle involves freezing the samples at a constant temperature of −20 °C for 24 h and thawing at room temperature (21 °C) for 24 h. Additionally, a parallel experiment is conducted to observe the effects of desiccation at room temperature. The weight and surface crack patterns of the samples are continuously monitored. The image processing technique quantifies crack morphology and determines crack area, surface crack ratio, crack width, crack length, and fractal dimension. The results reveal that as the salt concentration increases, the F-T and desiccation samples exhibit decreased crack area and width. This decrease is attributed to the reduced saturation vapor pressure caused by higher salt concentrations, which slows down water evaporation and delays crack formation. The final crack pattern in the F-T samples is closely linked to the initial pattern of ice crystal formation. Lower salt concentrations result in the formation of larger ice crystals, leading to larger cracks during thawing. In the experiment, where the temperature was above the NaCl eutectic point, only ice crystals were formed, and the salt remained isolated as the salt concentration increased in the unfrozen solution. Conversely, during the freezing process, samples with higher salt concentrations with higher unfrozen solution content experience upward movement of the solution due to temperature gradients and water evaporation. This study provides new insights into understanding the soil water and salt movement and the phenomenon of surface clay soil cracking during the freezing-thawing process.

Using X-ray imaging for the study of crack development in solder reliability testing

The possibility of monitoring the progressive damage and cracking in solder joints is investigated through a non-destructive method. As X-ray 3D Computed Tomography is known to require small sample dimensions in order to ensure a good resolution, Computed Laminography was used to get a high-resolution top view of various types of solder joints located on a large printed circuit board. In a first step, the assemblies were cross-sectioned after X-ray imaging in order to verify the findings and confirm the crack morphology. Then, by avoiding a destructive sample preparation step, it was possible to identify in a single sample the effects of solder fatigue during a standard reliability test. Firstly, multiple crack initiation sites could be highlighted in a single X-ray analysis. Moreover, the study gave an interesting insight of the time evolution of voids in solder joints. X-ray laminography produces 3D-like images that contain more information than a conventional cross section. By improving the recognition of horizontal cracks in this unusual point of view of solder joints, the refinement of this imaging and monitoring method can result in a real progress in failure analysis time and efficiency.

Deep convolutional neural network ensemble for pavement crack detection using high elevation UAV images

Crack morphology is a major indicator of pavement distress and can indicate the extent of pavement rehabilitation required. Researchers have investigated the detection of cracks using images captured at close proximity. This is often time-consuming, labor-intensive, and inefficient. This research implemented the weighted ensemble technique for detecting pavement cracks on a pixel level using UAV images obtained at high elevations. The images were trained using five deep convolutional neural network architectures: UNet, Vgg-UNet, Resnet-UNet, Inception-UNet, and PaveNet. The pixel-level crack detection results are combined using the ensemble technique to maximize performance. The performance of the ensemble methodology was evaluated and compared with some of the state-of-the-art networks. The predictions obtained were used to estimate the area, length, and mean width of the cracks in the pavement images. It is worth noting that the proposed system can be applied to a specific road segment. A quantitative index is then proposed for quantifying the level of deterioration present in a pavement section.

Fatigue crack initiation in the presence of stratified surface layers on rail wheels

This work presents a study on the fatigue crack initiation in the presence of stratified surface layers (SSL) – a combination of white and brown etching layer – on rail wheels. Reproducible SSLs comparable to field samples are created by a defined mechanical load and two consecutive laser surface treatments on discs made from two wheel materials (ER7, ER9). Fatigue testing is done by a twin disc tribometer, where the wheel discs with SSLs are driven against discs made from R260 rail material. The results show significant fatigue crack initiation and propagation in the presence of the SSLs, exclusively on the leading edge of the SSL, more pronounced for the wheel grade ER9. Based on topographical, light optical and scanning electron microscopy analysis an explanatory model is presented, describing fatigue crack morphologies and outpointing the relevance of SSLs on rail wheels to fatigue crack initiation.

Cavitation impact damage of polymer: A multi-physics approach incorporating phase-field

The challenge of material surface damage and spalling, caused by high-frequency, high-pressure jets owing to cavitation, remains a substantial concern. To better understand the physical mechanisms of cavitation-induced cyclic impact, we developed a multi-field-coupling framework. This framework encapsulates polymer viscoelastic-viscoplastic deformation, thermal softening, strain softening, and damage evolution represented by phase fields, which are crucial processes in cyclic impact. Particularly, we achieved full coupling between these key physical processes by introducing appropriate functional forms. We emphasize the need to include both plasticity and viscosity in the energy-driving crack propagation, to accurately represent the accumulation of polymer fatigue damage. With this thermodynamically consistent, fully coupled model, we could simulate surface ring cracking, a phenomenon often observed experimentally under cyclical impact loading. In the context of this particular crack morphology, we also discerned distinct fatigue failure mechanisms when variables such as strength, the radial extent of the cavitation impact load, and others were altered. We propose that a key determinant of cavitation flow impact is the combined effect of thermal softening, strain softening, and phase-field degradation within the polymer. Understanding these mechanisms offers a deeper insight into cavitation damage and provides a theoretical basis for cavitation-resistant design.

Mechanical behavior and fracture mechanism of red-bed mudstone under varied dry-wet cycling and prefabricated fracture planes with different loading angles

Red-bed mudstone is a common geological material in engineering construction in the Southwest region in China. Its initial fractures are easily influenced by the combined effects of dry-wet cycling and external loads. Therefore, evaluating the mechanical behavior and fracture mechanism of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles is of great significance. In this study, the cracked straight through Brazilian disc (CSTBD) specimens with a prefabricated crack were tested using an RMT testing machine. Multiple-scale analysis methods, including digital image correlation (DIC) and acoustic emission (AE) techniques, 3D scanning, and scanning electron microscopy (SEM), were employed to comprehensively analyze the mechanical evolution and fracture damage mechanism of mudstone under dry-wet cycling. The experimental results indicate that as the loading angle increases, the peak load gradually increases, while an increase in the number of dry-wet cycles leads to a decrease in peak load. At the same time, dry-wet cycling causes the expansion and connection of initial microcracks, forming weak layers that affect the morphology of the main crack and the distribution of secondary cracks. Furthermore, with an increasing number of dry-wet cycles, the acoustic emission count significantly decreases, and the relative high value range of acoustic emission count expands. 3D scanning results show that an increase in the number of dry-wet cycles results in rougher fracture planes, while an increase in loading angle generally increases the roughness of the fracture planes. Additionally, SEM observations reveal that with an increasing number of dry-wet cycles, intragranular fracture patterns and microcrack distribution become more widespread, and tensile and shear fracture patterns are also observed in the center of the fracture plane. These research findings have important practical value for evaluating the stability and reliability of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles.

A non-local damage model-based FFT framework for elastic-plastic failure analysis of UD fiber-reinforced polymer composites

A novel computational framework combining fast Fourier transform (FFT) with the non-local damage model is proposed to simulate the nonlinear mechanical behaviors of unidirectional composites. Integral-type non-local damage model and gradient-regularized non-local damage model are integrated to analyze brittle materials and ductile materials, respectively. Under transverse tensile load, the failure of matrix in the composites is characterized by non-local maximum principal stress damage model or the elastic-plastic damage model obeying the parabolic yield criterion, respectively. It is verified that predicted stiffness, strength, and ultimate crack morphologies obtained by the proposed model are in good agreement with traditional finite element method (FEM) and phase field method (PFM) in the literature. The effects of microstructure features on the mechanical behaviors and the suitability of multiple crack propagation are further studied effectively and accurately.

Fracture process characteristic study during fracture propagation of a CO2 transport network distribution pipeline

The transportation of carbon dioxide (CO2) from the capture point to the pooling site through distribution tubes is a crucial link in the carbon capture, utilization, and storage (CCUS) chain. Existing experiments have not been conducted to study the full-scale fracture of the CO2-distributed pipelines, the pressure and temperature evolution inside the pipe during crack initiation, fracture extension, and stopping have not been revealed. The rate of fracture extension velocities has not been quantitatively described, and the crack morphology has not been analyzed. This paper conducted the first supercritical CO2 pipeline full-scale fracture experiment based on a novel experimental setup with a total length of 21.7 m and the inner diameter of 98.3 mm, 16.7 m of which is the main pipeline and 5 m is the sacrificial pipeline. After the sacrificial pipeline rupture, high-frequency transducers were used to measure the change in fluid pressure, and multilayer thermocouples monitored the temperature distribution in four cross-sections within the pipe. Moreover, the decompression wave propagation velocity of supercritical CO2 and sacrificial pipeline fracture velocity has been obtained and calculated, respectively. The microscopic morphology of the cracks obtained by scanning electron microscopy revealed the failure mechanism of the sacrificial pipeline with prefabricated defects. This work establishes a reliable experience for industry-scale CO2 pipelines and creates a pipeline design foundation for higher security transport in the future.

Failure analysis of the 304SS nut in rural atmosphere

An unexpected cracking of a 304SS nut exposed to a rural atmosphere caused gas leakage. Aiming at identifying the failure cause, this study has conducted crack morphology analysis and corrosion product analysis using X-ray computed tomography, electron backscatter diffraction, and scanning electron microscope. The results revealed that the primary failure is attributed to intergranular corrosion cracking, where corrosion has played a predominant role. Initial corrosion was induced by MnS inclusions and chromium carbides. The application of additional force during re-examination promoted crack propagation. MnS inclusions played an auxiliary role in crack propagation due to the high stress concentration around them. The main contributing factor to the failure is the unqualified material, which contains excessive amounts of carbon and sulfur, significantly reducing the corrosion resistance and toughness. Based on these findings, recommendations for preventing further failures were proposed. This study has provided a failure case in mild environment and demonstrate that failure of 304SS would also occur even if the environment is the rural atmosphere, as long as the material is sensitized.

Stress corrosion cracking behavior of AISI 316L stainless steel treated by surface mechanical grinding

The effects of surface mechanical grinding treatment (SMGT) on stress corrosion cracking (SCC) behavior of machined 316L stainless steel surface were investigated by comparing to those of turned and milled surfaces. The surface stress state, roughness, microstructure, and microhardness induced by the different surface machining processes were tested and analyzed. The SCC tests on the turned, milled and SMGT samples were conducted in boiling MgCl2 solution at 155 ℃, and the crack morphologies were analyzed in detail to establish the relationship between the surface integrities and the resulting SCC behavior. The findings indicated that the SMGT altered the SCC initiation behavior and improved SCC resistance of the turned and milled surfaces of 316L stainless steel, and the surface stress state, microstructure refinement and surface strengthening induced by SMGT contributed to the improvement of the SCC resistance.

A multi-task learning model for fast prediction of mechanical behavior of UD-CFRP composites under transverse tension

The weaker transverse mechanical response of unidirectional carbon fiber reinforced polymer (UD-CFRP) composites has raised a lot of attention and research. The single-task modeling strategies based on deep learning (DL) have achieved impressive results in terms of surrogating the finite element method to accelerate computation, but they do not sufficiently consider the data correlation between different tasks. In this study, a multi-task learning (MTL) model based on DL is developed for fast and accurate prediction of the transverse mechanical response of UD-CFRP, which includes crack morphology and stress–strain response. Different from traditional single-task learning (STL) models, MTL models can take into account the relevance of different tasks to achieve one model to predict multiple tasks simultaneously. The prediction results of the RVE model confirmed that the developed MTL model has better prediction accuracy and generalization compared to the STL model. Moreover, the model can perform inference for two subtasks simultaneously in milliseconds. The developed MTL model takes the image of the initial structure as input without any prior assumptions and simplifications. Therefore, it has good potential to be extended and transferred to other multitask scenarios in the field of composites.

The early-age shrinkage cracking and late-age fracture properties of fiber-reinforced cementitious composites

In this research, the shrinkage cracking and fracture properties of fiber-reinforced cementitious composites (FRCC) are investigated using eccentric ring test and three-point bending test. In the eccentric ring test, the concept of the equivalent crack width (weq) is proposed based on the morphology and number of cracks of FRCC. A function considering curing time and fiber volume fraction is established to calculate the equivalent crack width of FRCC. The initial and complete-through cracking times of FRCC were measured to explain the effect of the fibers. In the three-point bending test, the crack initiation in FRCC is determined based on the average transverse force. The initial and unstable fracture toughnesses, as well as the fracture energy, have been calculated. The experimental results indicate that an increase in polyvinyl alcohol (PVA) fiber content can reduce the crack width and prolong the initial and complete-through cracking times induced by shrinkage. The unstable fracture toughness and fracture energy increase with the increase in the PVA fiber content. The results of the eccentric ring test and three-point bending test demonstrate that the primary role of PVA fibers is to extend the intermediate stage of cementitious materials from initial cracking to unstable cracking. Additionally, PVA fibers delay the energy release rate during the failure process.

Full‐scale studies of stiffening walls made of autoclaved aerated concrete

AbstractAutoclaved aerated concrete is successfully produced as masonry units to construct single and multi‐family houses. Residential buildings are often erected as load‐bearing structures. Although designers are used to verifying the masonry structure for mainly vertical loads, some elements must act as a stiffening of the building. Horizontal loads of the building can be caused not only by wind pressure and suction but also by uneven subsidence of the ground or mining influences. The role of the stiffening walls is to ensure the geometric invariability of the building and the safe transfer of horizontal loads to the foundations. The Silesian University of Technology conducts extensive research on stiffening walls on full‐scale models of single‐storey buildings. Experimental analyzes enable the description of the propagation and morphology of cracks. Designation of the behavior phases of the stiffening walls is the basis for developing the calculation methodology for stiffening elements.