Crack Orientation Research Articles

Very low-cycle fatigue failure behaviours of pipe elbows under displacement-controlled cyclic loading

In this study, quasi-static very low-cycle fatigue tests were conducted on pipe elbows at room temperature to investigate the effects of the pipe thickness, internal pressure, and material on the failure behaviours under large-amplitude cyclic loads. Three types of elbow specimens comprising two different materials and two different pipe thicknesses were used in the tests. Elbow specimens were subjected to displacement-controlled, fully reversed, in-plane mode cyclic bending with and without internal pressure. All specimens failed due to through-wall cracking of which the location and orientation depend only on the pipe thickness regardless of the internal pressure and elbow material. For thin-walled elbows of Schedule 40, the axial crack at flank was developed, whereas for thick-walled elbows of Schedule 160, the circumferential crack at intrados. The number of cycles to failure decreased with increasing internal pressure in thin-walled elbows. However, the effect of internal pressure on the number of cycles to failure was negligible in thick-walled elbows. Regardless of internal pressure level, stainless steel elbows were more resistant to failure, compared with carbon steel elbows of identical dimensions. Finally, the strain-based evaluation model considering the void growth and shrinkage effect was applied to predict fatigue failure location and cycles. Two parameters in the model, the multi-axial fracture strain locus and void shrinkage ratio, were determined from monotonic tensile test data and the fatigue life curve of the material. The predicted crack initiation site and orientation were consistent with experimental finding. For all cases, the predicted failure cycles were less but overall close to experimental data.

Numerical Simulation Analysis of Fracture Propagation in Rock Based on Smooth Particle Hydrodynamics.

The mechanical properties of fractured rock have always been a focal point in the rock mechanics field. Based on previous research, this paper proposes improvements to the SPH method and applies it to the study of crack propagation in fractured rocks. By conducting uniaxial compression tests and simulating crack propagation on various specimens with different crack shapes, the characteristics of crack propagation were obtained. The comparison between the simulated results in this study and existing experimental and numerical simulation results confirms the validity of the SPH method employed in this paper. The present study utilizes the proposed methodology to analyze the influence of the crack angle, width, and orientation on crack propagation. The SPH method employed in this study effectively demonstrates the expansion process of fractured rock under uniaxial compression, providing valuable insights for the engineering applications of SPH.

Meso-mechanics-based microstructural modelling approach to predict low cycle fatigue properties in additively manufactured alloys

Grain, grain boundaries, voids, alloying elements and other anomalies have an important impact on the stress distribution at the sub-grain meso level which consequently affect the strain controlled low cycle fatigue (LCF) in additively manufactured (AM) alloys. In alloys AM materials yielding with large stress–strain gradients tends to form around voids and other defects, thus inducing preferential crack nucleation sites and accelerating fatigue failure of the substrate. In order to investigate the effects of these microscopic features on the LCF performance in AM alloys, this paper proposes an idealized grain/grain boundary model using Voronoi tessellation technique and to quantify the effects of different features such as randomly distributed internal pores and hard interstitial on the LCF performance. The proposed method is an important meso-scale continuum damage modelling approach that can be developed to study and analyze the LCF properties with the consideration of microscopic features.

Effects of Stress State, Crack—γ/γ′ Phase Interface Relative Locations and Orientations on the Deformation and Crack Propagation Behaviors of the Ni-Based Superalloy—A Molecular Dynamics Study

In this study, we systematically investigate the influence of stress states, relative locations, and orientations of crack—γ/γ′ phase interfaces on the deformation and crack propagation behaviors of the Ni-based superalloy through molecular dynamics simulations. The stress state with high stress triaxiality will impede the plastic deformation process of the system, thereby promoting brittle crack propagation within the system. But the stress state of low stress triaxiality results in obvious plastic deformation and plastic crack propagation behaviors of the system. The deformation system with cracks located in both the γ and γ′ phase exhibits the slowest growth rate, regardless of applied stress states. Additionally, the deformation process demonstrates prominent plastic behavior. For the deformation system with cracks perpendicular to the γ/γ′ phase interface, the γ/γ′ phase interface will hinder the crack propagation. Our research provides interesting observations on deformation and crack propagation behaviors at an atomic level and at a nano-scale which are important for understanding deformation and fracture behaviors at a macroscopic scale for the Ni-based superalloy.

An experimental analysis of crack terminating perpendicular to the bimaterial interface under varying mode mixities

SIFs for crack terminating perpendicular to the interface of a bimaterial subjected to varying mode mixities are evaluated with a novel bimaterial specimen using the experimental technique of photoelasticity. A simple form of multiparameter equation defining the stress field for the crack terminating at the interface at an arbitrary angle along with the details on interfacial SIF and combined SIF for a bimaterial problem are brought out. The stress field equation is incorporated in an in-house developed PSIF software as a separate problem module for the determination of crack tip stress field parameters using a non-linear least squares methodology. A compact bimaterial Brazilian disc specimen with crack in one of the material sides terminating perpendicular to the interface and at an arbitrary angle to the point of loading, is considered for the study. SIFs of two such specimens, a polymer–polymer pair, and a metal-polymer pair with two different bimaterial constants are determined. The normalized SIF values are evaluated as a function of crack orientation angles. The SIF values for the metal-polymer pair were found to be higher compared to the polymer–polymer interface. The peak values of SIFs are observed at a crack orientation angle of 30°. At orientation angles close to 0° and 90°, the SIF values were less pronounced.

Determination of the Rate Dependence of Damage Formation in Metallic‐Intermetallic Mg–Al–Ca Composites at Elevated Temperature using Panoramic Image Analysis

A cast Mg‐4.65Al‐2.82Ca alloy with a microstructure containing an α‐Mg matrix is studied, which is reinforced with a C36 Laves phase skeleton. Such ternary alloys are targeted for elevated temperature applications in automotive engines, since they possess excellent creep properties. However, in application, the alloy may be subjected to a wide range of strain rates, and thus accelerated testing is often essential. It is, therefore, crucial to understand the effect of such rate variations. Herein, their impact on damage formation is focused. The analysis is based on high‐resolution panoramic imaging using scanning electron microscopy, combined with automated damage analysis using deep learning for object detection and classification (YOLOv5). It is found that with decreasing strain rate the dominant damage mechanism for a given strain level changes: at a strain rate of 5 × 10−4 s−1, the evolution of microcracks in the C36 Laves phase dominates damage formation. However, when the strain rate is decreased to 5 × 10−6 s−1, interface decohesion, becomes equally important. A change in crack orientation is also observed, indicating an increasing influence of plastic codeformation of the α‐Mg matrix and Laves phase. This transition is attributed to the leading damage mechanism to thermally activated processes at the interface.

Influence of the expansive sites distribution on alkali-silica reaction expansion under applied stress: Crack orientation as a key factor for expansion transfer

The influence of the expansive site distribution of the alkali-silica reaction (ASR) on the expansion behavior under applied stresses was numerically investigated to verify the mechanical factor of expansion transfer. In the analysis, the aggregate and mortar phases were individually modelled by mesoscale discrete analysis. To reproduce expansive site distribution and time-dependent behavior, expansive phase model, pressure development model, and creep model were developed. As a result, expansion transfer was observed for the model based on the reaction rim assumption and the crack effect model under applied stress of less than 10 MPa. A detailed numerical investigation identified that the cracks under applied stress in the reaction rim model are modified more easily than those in the gel pocket model, despite crack generation being inhibited. Consequently, expansion transfer was found to be significantly influenced by the crack orientation because of the mechanical interactions at the expansive sites under applied stress.

An Investigation into the Free Vibration of Intact and Cracked FGM Plates

In this study, the linear free vibration of intact and cracked functionally graded material plates is investigated numerically and experimentally. The experimental work is limited to the isotropic materials. The numerical work is based on finite element, where a code is developed to obtain the natural frequencies of intact plates based on the first-order shear deformation theory (FSDT) using MATLAB software. Also, a model of through-cracked FGM plate is developed using ANSYS Workbench with the help of APDL coding. The material properties of the plates under study are graded in one, two, and three directions. The novelty of this study emerges through its examination of the synergistic impacts resulting from variations in FGM material properties, crack length, crack orientation, and crack location. These effects are comprehensively discussed in the results section. The result of the present model shows that the use of three-directional FGM reduces the natural frequency compared with the other cases of two-directional and unidirectional FGM. Also, the results show that the effect of FGM gradient on the frequency of intact and cracked plate is high when the gradient index n < 3 . The present paper results are useful for the design of FGM plates especially when cracks exist.

A novel application of recovered stresses in stress intensity factors extraction methods for the generalized/extended finite element method

SummaryAn efficient improvement to compute Stress Intensity Factors (SIFs) in two‐dimensional fracture mechanics problems solved by Generalized/eXtended Finite Element Method and Stable Generalized Finite Element Method is proposed. A stress field obtained by a recovery procedure based on a locally weighted projection of the approximate stresses, is used in the formulation of two energy‐based SIFs extraction methods: the Contour Integral Method (CIM) and the Interaction Integral strategy. The accuracy and the path independence feature of the proposed approach are investigated in three numerical examples, considering crack opening modes I and II, different crack orientations and the presence of holes. The results demonstrate that the recovered stresses significantly improve the accuracy of the CIM and ensure a notable decrease in SIFs variation for different integration paths. On the other hand, an equivalent impact is not observed for the Interaction Integral strategy.

Experimental and numerical investigation on meso-fracture behavior of Beishan granite subjected to grain size heterogeneity

Research on crack evolution mechanism and failure pattern of Beishan granite is vital for evaluation of the engineering stability of disposal repository. The mechanical properties, especially the strong heterogeneity, significantly affect the cracking behavior of granite. To investigate the fracture behavior of heterogeneous granite, mesoscopic three-point bending tests were conducted on Beishan granite using an in situ scanning electron microscope (SEM) with a loading device, and the entire cracking process was observed in real time. Additionally, the effect of grain size heterogeneity on strength and deformation behavior of granite was quantitatively investigated by introducing a statistically defined heterogeneity index into the grain-based model in two-dimensional Particle Flow Code (PFC-GBM), and the crack development was characterized through fractal theory. The results demonstrated that fracture mechanism of granite is mainly tensile and shear failure, and the fracture surface of tensile failure is rough and irregular, which is mainly caused by the tearing between various mineral crystals, whereas the fracture surface of shear failure is relatively smooth, which is mainly caused by the dislocation and slip between minerals. The crack edge can be regarded as the damage zone, and the development of the main crack is promoted by the continuous generation and combination of micro-damage at the crack tip. The crack propagation path from a mesoscale perspective is a process of continuous crack orientation, which depends on the competition mechanism between mineral properties and maximum tensile stress. Generally, the granite strength tends to increase with the increase of heterogeneity.

Sensitivity-matrix-based identification of anisotropic elastic constants and microstructure features in plasma sprayed coatings utilizing ultrasonic back-reflection

With the unique lamellar microstructure, macroscopic elasticity of plasma sprayed coatings shows either positive or inverse anisotropy due to complex microstructure of inter-layer pores and intra-layer cracks. It is extremely difficult to nondestructively evaluate anisotropic elastic constants of these coatings with thin thickness. Moreover, it is an absolute challenge to reveal the elastic anisotropy transitions between positive and inverse. It is also difficult to clarify the nonlinear relationship between macroscopic elasticity and microstructure, which is necessary for microstructural identification. In present work, a sensitivity-matrix-based method was developed to determine five independent elastic constants of plasma sprayed coating by ultrasonic back-reflection method, with the relative errors of inverted elastic constants less than 1.33 %. Then, an optimized physical-descriptor-based microstructure characterization and reconstruction method was proposed to model “pore and crack” coatings with different microstructural features, and relative errors of microstructural features between the reconstructed and preset values were less than 4.35 %. Based on above, the dependences between six microstructural features and elastic constants were established. The inverse elastic anisotropy was explained systematically as the increase of vertical crack content, the “roundness” of horizontal pore and its “weakening” preferred orientation. Finally, the sensitivity-matrix-based method was used to simultaneously identify the porosity, crack content, pore average aspect ratio, pore orientation factor, and crack orientation factor of plasma sprayed Al2O3 coating based on elastic constants inverted. The relative errors in these identifications were less than 14.85 %. This work provides a new sensitivity-matrix-based solving strategy for multi-parameter integrated quantitative identification of complex microstructural features in materials.

Thermal pyrolysis-induced shale micro-cracks: 3D characterization and implication for reservoir stimulation

The geometrical characteristics of thermal cracks at the micro-scale contain valuable information for subsurface enhancing the production of shale oil and gas via in-situ pyrolysis. In this study, six organic-rich shale samples from the Junggar Basin, western China, were thermally pyrolyzed to 250–500 °C, respectively, which were then imaged using X-ray micro-CT. A novel image-processing technique was employed to separate and identify individual cracks that intersect and form 3D networks. Unprecedentedly, over thousands of cracks in each of the volumes were individually characterized by their length, width, thickness, and orientation and then statistically analyzed. It is shown that crack orientations are predominantly parallel to the bedding structures of the shale, different crack networks of the samples represent the heterogeneity of the shale, and the fractal dimension of the crack length ranges from 1.50 to 1.85. All parameters, including porosity, specific surface area, percolation, and the fractal dimension of crack length, show a distinct increase from 350 °C, which is consistent with the threshold temperature for gas generation during thermal pyrolysis. The analysis quantifies the efficacy of pyrolytic stimulation methods and paves the way for developing pyrolysis stimulations for producing shale oil and gas.

Assessment of repaired reinforced concrete dapped-end beams exhibiting bond deterioration under static and cyclic loading

An experimental investigation has been conducted to scrutinize the behavior of reinforced concrete dapped end beams repaired with different configuration of CFRP sheet under static and cyclic loading due to induced cracks from previous examination. The static load was adopted to determine the amplitude of cyclic loading at an increment of 15% until failure. It was demonstrated that retrofitting increased load capacity and ductility significantly; approximately 28% increase in capacity and 1.5 times higher ductility over the un-retrofitted beam under static loading, and approximately 80% higher capacity with three times increase in ductility and a change in crack orientation under cyclic loading. Nonetheless, the reentrant corner's significant concrete spalling is a major concern. It is thus essential to focus on the dapped section for monitoring and maintenance of this beam.

Experimental Evaluation of KIEAC of a Carbon Steel Using the Pin-Loaded SENT Geometry

Abstract The experimental determination of the threshold stress intensity factor for environment-assisted cracking (KIEAC) is described by several standards, which allow the use of different specimen geometries and methodologies for crack length estimation. In some cases, the combination of structure, specimen size, or both, main loading direction, and crack orientations of the component that need to be characterized limit the use of standardized geometries. Consequently, alternative geometries must be used. In this study, fatigue pre-cracked pin-loaded single edge notched tension specimens as defined by BS 8571:2018, Method of Test for Determination of Fracture Toughness in Metallic Materials Using Single Edge Notched Tension (SENT) Specimens, were applied for the experimental evaluation of the KIEAC of a carbon steel. The specimens were tested in deaerated substitute ocean water solution saturated with carbon dioxide at 40°C and at 1 and 10 bar (100 and 1,000 kPa) under constant load conditions with incremental (step) loading. The crack length during the tests was monitored by direct current potential drop, which was helpful for defining the applied K corresponding to the onset of subcritical crack growth (KIEAC). Additionally, fractographic analysis of KEAC specimens and results from the fracture toughness evaluation of this material in air at room temperature have also been reported.

Microstructure sensitivity of the low cycle fatigue crack initiation mechanisms for the Al0.3CoCrFeNi high entropy alloy: in-situ SEM study and crystal plasticity simulation

In this study, in-situ fatigue tests under scanning electron microscopy (SEM) were performed to reveal the relationship between grain slip deformation and fatigue crack initiation of Al0.3CoCrFeNi high entropy alloy (HEA). It was found that the fine grains (FG) exhibited a higher fatigue crack initiation life than that of the coarse grains (CG), which is related to the inhibition effects of twins and grain boundaries on fatigue crack initiation. The initiation of fatigue cracks in both the CG and FG specimens occurred in the area of high density of slip lines. The fatigue cracks were observed to deviate from the single slip line, which is related to the activation of multiple slip systems inside the grain, leading to the alternation of fatigue crack propagation path. In addition, the intersection of different slip planes can be another preferential crack initiation site if two or more slip planes are activated inside one grain. By combining EBSD analysis and crystal plasticity finite element modeling (CPFEM), the influence of microstructures, such as grain orientation and grain boundaries, on the fatigue crack initiation mechanism was further analyzed. The CPFEM results confirmed that the CG specimen resulted in a higher cumulative shear strain (CSS) than that of the FG specimen under the same stress level, which led to higher fatigue damage.

Migration of Mechanical Perturbations Estimated by Seismic Coda Wave Interferometry During the 2018 Pre‐Eruptive Period at Kīlauea Volcano, Hawaii

AbstractWe use seismic ambient noise correlation and coda wave interferometry to estimate velocity variations at high temporal resolution, during the pre‐eruptive period and the onset of the 2018 eruption of Kīlauea volcano. A progressive velocity increase is observed from March to the end of April. It is followed by rapid decrease starting a few days before the onset of the East Rift Zone (ERZ) eruption and then by sharp velocity drop when the eruption started. The change of trend from velocity increase to decrease is progressively delayed by a few days from the summit caldera toward the ERZ. The location of the velocity perturbations shows a migration of the sources of velocity changes from the summit caldera toward the ERZ before the eruption. Using a model of pressure source, we show that the simultaneous caldera inflation and velocity increase probably result from an anisotropic distribution of fault and crack orientations. The velocity decrease could be due to damaging processes above the shallow reservoir and to plastic deformations around the caldera. We introduce a forward model of rock damage associated with the volcano‐tectonic seismicity to calculate the velocity decrease. The good agreement between the calculated and the observed velocity variations shows that a large part of the velocity decrease results from damage of the medium. The delayed onsets of velocity decrease and the source migration of velocity perturbations are probably related to progressive fault openings in the Southern and Eastern parts of the caldera and to magma transfer toward the ERZ.

New design of rotating eddy current probe for arbitrary orientation cracks detection

PurposeDetecting the orientation of cracks is a major challenge in the development of eddy current nondestructive testing probes. Eddy current-based techniques are limited in their ability to detect cracks that are not perpendicular to induced current flows. This study aims to investigate the application of the rotating electromagnetic field method to detect arbitrary orientation defects in conductive nonferrous parts. This method significantly improves the detection of cracks of any orientation.Design/methodology/approachA new rotating uniform eddy current (RUEC) probe is presented. Two exciting pairs consisting of similar square-shaped coils are arranged orthogonally at the same lifting point, thus avoiding further adjustment of the excitation system to generate a rotating electromagnetic field, eliminating any need for mechanical rotation and focusing this field with high density. A circular detection coil serving as a receiver is mounted in the middle of the excitation system.FindingsA simulation model of the rotating electromagnetic field system is performed to determine the rules and characteristics of the electromagnetic signal distribution in the defect area. Referring to the experimental results aimed to detect artificial cracks at arbitrary angles in underwater structures using the rotating alternating current field measurement (RACFM) system in Li et al. (2016), the model proposed in this paper is validated.Originality/valueCEDRAT FLUX 3D simulation results showed that the proposed probe can detect cracks with any orientation, maintaining the same sensitivity, which demonstrates its effectiveness. Furthermore, the proposed RUEC probe, associated with the exploitation procedure, allows us to provide a full characterization of the crack, namely, its length, depth and orientation in a one-pass scan, by analyzing the magnetic induction signal.

Fatigue crack growth behavior in an aluminum alloy Al–Mg–0.3Sc produced by wire based directed energy deposition process

AbstractAdditive manufacturing (AM) of Al–Mg–Sc alloys has received considerable interest from the aerospace industry owing to their high specific strength and suitability for AM processes. This study has investigated the fatigue crack growth behavior in an Al–Mg–0.3Sc alloy made by wire and arc additive manufacturing. Tests were conducted with two different crack orientations at cyclic load ratios of 0.1 and 0.5. At the lower load ratio, the horizontal crack showed a faster growth rate owing to the smaller grains and coarser second‐phase particles that the crack tip had encountered when it propagated along the material build direction. The anisotropy in crack growth rate was mainly caused by the grain size effect. When the applied stress intensity factor range exceeded the value of 10 MPa m1/2, an isotropic crack growth rate between the two crack orientations was measured. This is due to the microstructural influence being overcome by the governing parameter of fracture mechanics. At the higher load ratio of 0.5, crack growth rate is isotropic, and the threshold stress intensity factor range was much lower than that tested under load ratio 0.1. Finally, the modified Hartman–Schijve equation has been successfully employed to represent the crack growth rates in all three regions.

Finite Element Analysis Using the Crack Strain Separation Model for Reinforced Concrete Membrane

This paper presents a finite element analysis of the bi-directional orthogonal model, which incorporates individual crack strain separation and tracking. The objective of this research is to expand the current shear friction model to manage bi-directional cracking at any angle, allowing for a more universal model that can be applied to intricate structures and non-proportional loading cases. The proposed model was initially developed as a total strain-based model, with the assumption that crack strains are equivalent to total strains, but it was subsequently recalculated to improve accuracy by separating crack strains from total strains. Furthermore, a separate crack strain formulation was created to account for strains in the concrete’s uncracked portions and locked-in crack strains. The article then discusses the testing of various convergence methods and loading programs to achieve high convergence. Comparative analyses of the generalized shear friction model with other models for crack orientation, and loading cases similar to those of a reinforced concrete membrane, are also presented. The MATLAB program successfully applied the bi-directional cracking model for one finite element under a uniform cyclical strain state, using a secant stiffness formulation.

Experimental investigation of impacted multidirectional laminates under compressive static and cyclic loading

In this paper, the static and fatigue behaviors under compression of Multi-Directional (MD) Carbon Fiber Reinforced Polymer (CFRP) laminates with impact damage were investigated. After low-velocity impact performed on three 24-ply Quasi-Isotropic Quasi-Homogeneous (QIQH) laminates having different stacking sequences, the influence of the distribution of interfaces between two plies on the compression after impact and post-impact compression fatigue properties was evaluated. The damages on the surfaces and internal delamination were accessed via visual and optical observation and ultrasonic C-scan inspection. The thermographic approach, which has been developed and validated on numerous non-impacted composite materials, was applied for the rapid determination of the fatigue limit together with the S-N curves of post-impact fatigue specimens. Experimental results showed that with 5 J impact energy, the three stacking sequences gave similar impact results. The residual strengths measured by compression after impact tests were also similar to each other, so did the global damage modes. The determined fatigue limits of impacted specimens of different stacking sequences were nearly identical and the predicted S-N curves were also close to each other despite the exact crack propagation and orientation were not the same, under both compressive static and cyclic loads.