Crack Orientation Research Articles

Atomistic study on crystal orientation-dependent crack propagation and resultant microstructure anisotropy in NiTi alloys

The primary objective of this study was to explore the crystal orientation-dependent fracture mechanism of nickel–titanium (NiTi) alloys by using molecular dynamics simulations. We discovered that the stress concentration ahead of the crack tip induced the formation of martensite with a herringbone twin or mixed HCP/OTHER atom morphology, while reverse martensite transformation occurred behind the crack tip due to stress relaxation. Interestingly, typical brittle fracture by cleaving along the high-energy domain boundary, quasi-brittle fracture as a result of the transformation-induced plasticity, as well as ductile fracture caused by a combination of phase transition, stacking fault, and atomic slip were observed in the samples with (1¯00) [010], (110) [11¯0], and (1¯1¯2) [111] crack orientations, respectively. The findings demonstrated that the crack growth behavior in each orientation was closely correlated to the microstructure evolution around the crack tip. Moreover, the relationship between the traction force and separation was achieved from the fracture process zone near the crack surface. The reported orientation dependency of the traction–separation curves was decisively driven by the anisotropy of cracking characteristics and microstructure morphologies. The results of this work offer comprehensive insights into the importance of crystal orientations on the atomic-scale fracture performance of NiTi alloys.

Finite element-based phase field simulation of complex branching crack propagation under different loads

The tensile failure of complex branching cracks will cause irreversible damage to the structures. In this paper, the effects of the crack orientations, the local branch lengths, and the different load forms on the crack propagation pattern and the bearing capacity of structures are investigated based on the phase field method and experimental research method. The results show that the bearing capacity and propagation patterns of the specimens change significantly with the change of the orientation of the Y-shaped crack under tensile load. The local branch length of the crack only affects the sequence in which the regional crack reaches the final failure and the bearing capacity of the specimen. Under the dynamic impact load, the bifurcation phenomenon occurs, and with the increase of the dynamic impact load, the number of bifurcations increases. The crack propagation pattern in the numerical simulation is completely consistent with the experiment, and the difference in the maximum reaction force is 2.5%.

Three-dimensional characterization of cracks in undisturbed Mile expansive soil using X-ray computed tomography

The distribution of cracks strongly affects the hydro-mechanical behavior and strength of soil. In order to characterize the distribution of cracks in undisturbed expansive soil, X-ray computed tomography (X-ray CT) tests were carried out for one type of undisturbed expansive soil from Mile City, China. Seven undisturbed specimens retrieved from depths between 1 and 4.7 m were used to study the evolution of cracks along the drying path. The characteristics of the cracks were quantitatively studied by 3D parameters extracted from CT images by commercial software. The test results suggested that, with an increase in suction, the total volume of the cracks gradually increased, and one large crack formed due to the merging of sub-cracks before the shrinkage limit. When the suction was larger than the suction corresponding to the shrinkage limit, degradation of the cracks caused by the collapse of the soil structure was observed. Meanwhile, due to the development of sub-cracks, the crack anisotropy caused by the typically oriented primary cracks decreased along the drying path. Finally, a conceptual model of crack development for undisturbed Mile clay was proposed based on the evolution of crack volume, number, shape, and orientation.

Fatigue crack growth behavior of a nanocrystalline low Young's modulus β-type Ti–Nb alloy

Low Young's modulus β-type titanium alloys, such as the Ti–45Nb alloy, with elastic properties close to that of human bone containing non-toxic elements are very desirable for load bearing implants. Their relatively low hardness and strength can be significantly increased by severe plastic deformation (SPD) methods, for example high pressure torsion (HPT). To gain a better understanding of the fatigue characteristics, the fatigue crack growth (FCG) behavior of Ti–45Nb was investigated in various microstructural states (as-received, HPT-deformed, HPT + cold-rolled, HPT + heat-treated). Compact tension (CT) specimens with different crack orientations were tested at different load ratios under tension-tension loading. The microstructure of the HPT and HPT + cold-rolled material shows highly elongated grains parallel to the principal deformation direction with grain sizes in the nanocrystalline regime. Compared to the majority of studies on FCG of SPD-processed materials this titanium alloy exhibits a remarkably weak anisotropy in the FCG-behavior in all material states but especially in the HPT and the cold-rolled state. This is mainly attributed to the transcrystalline fracture mode. The threshold of fatigue crack growth was found to decrease with increasing load ratio. This is related to a lower contribution of plasticity-induced crack closure at higher load ratios.

Short crack propagation near coherent twin boundaries in nickel-based superalloy

Short crack propagation near a coherent twin boundary in polycrystal nickel alloy is investigated with 3D crystal plasticity extended finite element modelling (CP-XFEM), utilising reconstructed experimentally characterised microstructures and crack path observations. Short 3D cracks at coherent twin boundaries are shown to grow on parallel (111) slip planes at very high rate so as to be an intrinsic part of the nucleation process. This is found to occur predominantly because of the stress states established by twin/parent constraint driven by the local elastic anisotropy. If elastic isotropy is modelled, crack growth is found to be non-planar and inclined to the twin boundary. The twin/parent twist angle is also found to influence local stress state, such that the actual 60° twist angle gives rise to the highest local stresses, preferential crack nucleation and rapid growth parallel to the twin boundary, and the deviations from 60° change the crack morphology and rate of growth.

A Crack Detection Method for Aero-engine Blade Based on Air-Flow Thermography

Aero-engine blade is one of the core components of aero-engine and its safe service is crucial to the normal operation of aero-engine. Thus, it is very important to localize and quantify cracks of aero-engine blades to prevent safety accidents. To guarantee the structural health of aero-engine blades, the air-flow thermography (AFT) method which has advantages of non-contact, high performance, and sensitivity, was proposed for detection of the blade cracks. Firstly, the theoretical thermal response model was established and the crack simulation models with different depths were constructed. Then a detection system was established to detect the artificial cracks with different depths and orientations. Results showed that the proposed method can effectively quantify the cracks with different depths from 0.2 mm to 1.0 mm and characterize the crack orientations. In addition, the natural crack on the blade was detected and the principal component analysis (PCA) method was used to enhance the crack contrast with the background.

Comparison of critical plane models based on multiaxial low-cycle fatigue tests of 316L steel

An endurance against multiaxial fatigue of 316L steel is studied. A fatigue life curve of torsional mode is substantially shifted to longer fatigue lives while tension/compression mode causes fracture the earliest. Fatigue life curves of both multiaxial modes are close to axial mode. Fourteen critical plane models for fatigue life predictions were applied and compared. Modified Smith-Watson-Topper models proved to be the most accurate. Criteria proposed by Fatemi and Socie, Li et al., Zhu et al. and by Wang and Brown provided satisfactory results as well. Smith-Watson-Topper model led to the most precise predictions of fatigue crack orientation.

Passivity Breakdown and Crack Propagation Mechanisms of Lean Duplex (UNS S32001) Stainless Steel Reinforcement in High Alkaline Solution Under Stress Corrosion Cracking

The passivity breakdown and subsequent stress corrosion cracking (SCC) of Type 2001 lean duplex stainless steel (UNS S32001) reinforcement were investigated in a highly alkaline environment containing chlorides at a low temperature. Electrochemical analysis and mechanical testing were utilized to characterize the passive film development. Fractographic analysis was performed, correlating microstructure and corrosion performance, to reveal preferential crack paths. A chloride threshold below 4 wt% Cl− for a high alkaline environment was elucidated, with pitting susceptibility factor values close to unity, having a threshold critical areal cation vacancy concentration for passivity breakdown close to the 1013 cm−2. Pit initiation leading to passivity breakdown and crack nucleation in 4 wt% Cl− was triggered for stresses above σy, developing a low-frequency peak (0.1 Hz to 0.01 Hz) of the cracking process. Current peak deconvolution demonstrated passivity breakdown was triggered by the intensification in the rate of Type II transient and exposure time, while an increase in transient amplitude was related to the crack propagation. The α phase served as a nucleation site for pits, whose propagation was arrested at the γ phase. Predominant intergranular-SCC morphology through the α/γ interface was developed following anodic dissolution given the more active nature of the α phase (most active path); minor transgranular-SCC propagated through γ phase when high-stress concentration was reached, corresponding to slip-step dissolution.

Fatigue damage criteria for antivibration systems with filled natural rubber under negative and positive R ratios

A shape change can initiate fatigue damage in rubber components, which has led to the development of the effective shear strain γt criterion in this investigation. Referring to the previous damage criterion, i.e., effective tensile strain εt, the relation between the two was revealed. To benchmark the effectiveness of different damage criteria, six damage criteria, including the commonly used maximum principal strain, stress, and strain energy density, were compared using cylindrical dumbbell samples in 30 fatigue cases under ± R ratios. A large scatter was generated using either the maximum principal strain or the stress criterion. The strain energy density was in excellent agreement in uniaxial tension loadings but failed in torsional loadings with R = −1. To save significant cost in producing a completed new S–N curve for each rubber material, the derived S–N base functions using both εt and γt have been successfully validated on three independent experiments, i.e., the cylindrical dumbbell specimens, the ring specimens, and the industrial mount MDS. To fully verify the applicability of both εt and γt, the prediction of the crack orientation was performed for the first time using the proposed criteria, to the knowledge of the author, and the results were consistent with the experimental observations. The proposed criteria are useful for antivibration product design.

Extended Isogeometric Analysis of Cracked Piezoelectric Materials in the Presence of Flexoelectricity

AbstractTo accurately analyze the fracture behavior of piezoceramics at small length scales, flexoelectricity must be considered along with piezoelectricity. Due to its dependence on size, flexoelectricity predominates at the micro‐ and nano‐length scales. Additionally, crack tips having the largest strain gradient state cause large flexoelectricity around them. Different approaches are employed in the past to model cracks computationally. However, extended isogeometric analysis (XIGA) is proven to be an accurate and efficient method. C1 continuity requirements for modeling gradients in flexoelectricity are met by non‐uniform rational B‐splines (NURBS) basis functions used in XIGA. In this work, XIGA‐based computational model is developed and implemented to study the fracture behavior of the piezoelectric‐flexoelectric domain. An in‐house MATLAB code is developed for the same. Several numerical examples are studied to ensure the efficacy and efficiency of the implemented model, and crack behavior is presented in the form of an electro‐mechanical J‐integral. The analysis is carried out to investigate how cracks behave for different flexoelectric coefficients under different electrical and mechanical loading combinations. J‐integral is also analyzed against crack parameters such as crack orientation and length. It is observed that boundary loads and flexoelectric material constants significantly influence J‐integral. Results also show a considerable amount of fracture toughening in the presence of flexoelectricity. The peak value of J‐integral is found to be reduced with an increase in the flexoelectric coefficient. A significant reduction in J‐integral, as much as 45%, is observed when the flexoelectric constant varied from 0.5 to 2 µCm−1.

Continuous crack detection using the combination of dynamic mode decomposition and connected component-based filtering method

Concrete surface cracks are one of the earliest indicators used to assess structural damage, serviceability and durability. This study proposes a novel method to detect concrete cracks and evaluate the crack features. The proposed approach introduces the dynamic mode decomposition (DMD) algorithm to identify individual cracks in each frame of a video based on the analysis of the DMD spectrum. Since cracks contain different types of noise, a connected component-based filtering method is proposed to discard the noise of the bubbles and blobs. The inspected cracks can be used to evaluate crack features, including area, orientation and perimeter. The proposed crack detection approach is then applied to inspect the crack development during a cubic concrete sample test under axial loading. The detected results demonstrate that the proposed method can correctly identify most cracks. Because video frames generate enormous data volumes that are not conducive to quick detection, dimensionality reduction based on normalized singular values is also discussed and compared with the proposed method. After the cracks are identified, the crack features of the area, orientation and perimeter are determined over time, which helps to assess crack properties and to understand structural performance. The accumulated database of crack characteristics will provide useful information of lifetime predictions.

Assessment of Convolutional Neural Network Pre-Trained Models for Detection and Orientation of Cracks.

Failure due to cracks is a major structural safety issue for engineering constructions. Human examination is the most common method for detecting crack failure, although it is subjective and time-consuming. Inspection of civil engineering structures must include crack detection and categorization as a key component of the process. Images can automatically be classified using convolutional neural networks (CNNs), a subtype of deep learning (DL). For image categorization, a variety of pre-trained CNN architectures are available. This study assesses seven pre-trained neural networks, including GoogLeNet, MobileNet-V2, Inception-V3, ResNet18, ResNet50, ResNet101, and ShuffleNet, for crack detection and categorization. Images are classified as diagonal crack (DC), horizontal crack (HC), uncracked (UC), and vertical crack (VC). Each architecture is trained with 32,000 images equally divided among each class. A total of 100 images from each category are used to test the trained models, and the results are compared. Inception-V3 outperforms all the other models with accuracies of 96%, 94%, 92%, and 96% for DC, HC, UC, and VC classifications, respectively. ResNet101 has the longest training time at 171 min, while ResNet18 has the lowest at 32 min. This research allows the best CNN architecture for automatic detection and orientation of cracks to be selected, based on the accuracy and time taken for the training of the model.

Conversion of lignite-derived volatiles into aromatics over Zn@MCM-41 and ZSM-5 tandem catalysts with a high stability

Direct conversion of lignite into light aromatics via catalytic fast pyrolysis is a great challenge because of severe catalyst deactivation and low aromatics yield. The tandem catalysis (1Zn@MCM-41 and ZSM-5) system was employed to promote the conversion of thermally-derived volatiles of lignite into aromatics at 600 °C. The mesoporous Zn-incorporated MCM-41 (xZn@MCM-41) with different doping amount of Zn was prepared with a one-pot hydrothermal procedure. The structure and the dispersion of Zn species of 1Zn@MCM-41 were systematically studied with XRD, SEM, BET, NH3-TPD and so on. The presence of 1Zn@MCM-41 significantly expands the lifetime of the microporous ZSM-5. Meanwhile, the yield of aromatics, especially benzene, is also increased to some extent. The content of the long-chain hydrocarbons and the phenolic compounds are decreased significantly after being cracked by the mesoporous 1Zn@MCM-41. More small molecules are able to contact with the active sites located in the micropores of ZSM-5 with the presence of Zn@MCM-41. In addition, the ZSM-5 in the tandem catalysts system presents an excellent stability. Therefore, this meso-microporous tandem catalysis system promotes the preferential cracking of macromolecular compounds, the cracked small molecules generated from which then contact with ZSM-5 to produce aromatics. This work provides a new perspective for the gradient catalyzing lignite-derived volatiles over a tandem catalysis system.

Nonlocal analysis of edge‐cracked nanobeams under Mode I and Mixed‐Mode (I + II) static loading

AbstractIn the present paper, the behavior of an edge‐cracked nanobeam under both Mode I and Mixed‐Mode (I + II) static bending loading is analyzed by using the stress‐driven nonlocal model (SDM), being the SDM applied for the first time in the case of Mixed‐Mode loading. More precisely, the cracked nanobeam is modeled by using a modification of the classical cracked‐beam theory, consisting in dividing the beam into two beam segments connected through a massless elastic rotational spring located at the cracked cross section. The bending stiffness of the cracked cross section is computed by exploiting: the Griffith energy criterion and the conventional linear elastic fracture mechanics. Cracks under both Mode I and Mixed‐Mode (I + II) loading are considered. Finally, an edge‐cracked cantilever nanobeam subjected to a transversal point load at the free end is analyzed. The static deflection is calculated and discussed for different values of relative crack depth, crack location, and crack orientation.

Fracture behavior of B2 phase matrix of Ti2AlNb-based alloy with microcracks of different orientations

The influence mechanism of crack orientation on B2 phase crack propagation behavior under cyclic loading is studied by using the meta-atom molecular dynamics (MAMD) method, and the simulation results are compared and verified based on the theory of linear elastic fracture mechanics. According to the different propagation speed of crack in B2 phase matrix, the crack propagation process can be divided into three stages: elastic stage, rapid propagation stage and slow propagation stage. The toughness or brittleness of the crack mainly depends on the competition between the cleavage fracture of the atomic bond and the plastic slip at the crack tip. The crack propagation behavior is obviously orientation dependent. Orientation I and orientation II expand in the form of ductile fracture. Orientation III shows obvious brittleness and expands in the form of cleavage fracture. Orientation IV (Orientation I-IV represent the four angles between the crack plane and the crystal orientation) shows both ductile fracture and brittle cleavage fracture. The defects and stress concentration at the crack tip provide a driving force for martensitic nucleation, and the hcp phase produced by martensitic transformation effectively increases the toughness of the crack. The simulation results of molecular dynamics are basically consistent with the theoretical prediction of linear elastic fracture mechanics. This suggests that the variation of the orientation could tune the crack tip plasticity and toughening in B2 phase matrix.

Influence of 3D printing on compressor impeller fatigue crack propagation life

Taking a compressor impeller with a pressure ratio less than 3 as the research object, the influence of 3D printing on the fatigue crack propagation life of the compressor impeller is analyzed under the combined action of 3D printing residual stress and external load. The impeller is printed by selective laser melting (SLM) technology. Finite Element Method (FEM) is used to simulate the 3D printing process of the impeller, and the residual stress, residual deformation and other mechanical quantities are comprehensively analyzed. The numerical simulation results are in good agreement with the crack location in 3D printing experiment. The heat treatment and numerical simulation of the printed impeller significantly reduce the residual stress. According to the experimental results and the magnitude and direction of the first principal stress, the position and orientation of the macroscopic crack are determined. The FEM of linear elastic fracture mechanics (LEFM) and the Forman-Newman-de Koning (FNK) model are used to give the propagation law and fatigue crack propagation life of the cracked compressor impeller under different working conditions. The results show that the crack initiation position is affected by the maximum residual stress of the printed impeller after heat treatment. When only the external load is applied, the stress intensity factor (SIF) of the initial crack front varies with the rotational speed as a quadratic power. When the external load and 3D printing residual stress act at the same time, the law will no longer be satisfied. The general trend of the crack propagation path is along the radial direction. The speed has a linear influence on the critical crack size and a nonlinear influence on the fatigue crack propagation life. The research work in this paper can provide a basis for damage tolerance analysis of 3D printing compressor impeller, and also provide a reference for maintenance work.

Analysis of cracked Reissner-Mindlin plate using an extended meshfree method

An extended meshfree method for analyzing cracked plates based on Reissner-Mindlin theory is presented in this paper. Among a variety of meshfree formulations, the radial point interpolation method (RPIM) is chosen in this study due to the satisfaction of the Kronecker delta property. The essential boundary conditions, therefore, are easily imposed in the RPIM. The shape function derived from RPIM is employed to interpolate the field variables. An extended RPIM formulation is used to model the crack segment without explicitly defining it in the discretized domain. The discontinuity due to the crack is defined by extrinsic enriched functions, particularly, the jump in the displacement field on two sides of the crack is modelled by the Heaviside function, and the stress singularity near the crack tip is described by the asymptotic enriched function. In this study, the stress resultant intensity factors (SRIFs) are evaluated through the interaction integral approach. The obtained SRIFs are shown in the paper through many numerical examples for comparison purposes. The trending variation of SRIFs is also observed from the numerical results. It can be remarked that the SRIFs depend on many factors: the number of cracks, crack orientation, load type and boundary conditions. The numerical examples show the accuracy of the present approach. The obtained results are compared with analytical solutions and other numerical methods.

Machine Learning Approach to RFID Enabled Health Monitoring of Coal Mine Conveyor Belt

The mining conveyor belt is an imperative component of the mine industry which serves the crucial role of transporting materials. Predicament such as conveyor belt cracks has to be avoided, because any potential cause of conveyor belt failure constitutes risk of fatalities and significant economic implications. Hence, a new crack detection method is developed which can duly alert prior to any inception of failure. The research incorporates an interdigital capacitor (IC) based Ultra High Frequency (UHF) radio frequency identification (RFID) tag as sensor and Machine Learning (ML) to monitor the formation of cracks. This is done by collecting data from the UHF sensor when it is placed on a conveyor belt. Two states of the conveyor belt are investigated -stationary and a travelling belt moving at speeds between 1-5 m/s. For the case of motion, it is successfully replicated with the help of an electrical treadmill. By using ML integration, the outcomes will be more effective in tracking crack features. The model is tested with different types of inputs, followed by classification based on experimental data. This method results in a very accurate determination of cracks, crack orientation and width. The experimental model is 97.2% accurate in identifying the presence of cracks and 93.9% accurate in detecting their orientation; it can also determine half a millimetre-wide crack width with 97% of accuracy. This detection system is extremely accurate, even when there is movement. The achieved detection rate of 99.4% depicts the effectiveness of the model performance by successfully combating the interferences caused by motion. This research is intended to be a stepping stone towards an efficient remote health monitoring (HM) system suitable for the smart future.

Crack initiation mechanism of laser powder bed fusion additive manufactured Al-Zn-Mg-Cu alloy

Crack is a serious defect limiting the additive manufacturing of metal materials. The morphologies and crystallographic orientation of cracks for Al-Zn-Mg-Cu alloy prepared by laser powder bed fusion (LPBF) were clarified. The mechanisms of crack initiation and propagation were expounded. Results show that the cracks were crisscrossed horizontally and propagated through multilayers along the building direction in the as-printed specimens. The cracks were classified as solidification cracks, which occurred in the final stage during solidification when the liquid fraction was <20%. Al-Zn-Mg-Cu alloy has a wide temperature range between the liquidus and solidus, leading to high cracking susceptibility. Solidification cracks were prone to initiate at high-angle grain boundaries (HAGBs) and terminate at low-angle grain boundaries (LAGBs) or within grains. LPBF process could affect crack initiation and propagation by refining the grain size and inhibiting the growth orientation. Solidification and thermal shrinkage of adjacent grains during solidification provided subjective factors for cracking initiation. Directional preferential growth of dendrites along the 〈001〉 crystal orientation and the untimely filling of the liquid phase provided secondary factors. This study can provide theoretical guidance for crack suppression in additive manufacturing of 7075 aluminum alloy and other metals with high cracking susceptibility. • Crack initiation and propagation mechanisms of Al-Zn-Mg-Cu alloy in LPBF were investigated. • Internal cracks of as-printed specimens are solidification cracks. • Cracks occur in the final stage during solidification. • Oriented crystallization and texture affect the initiation and propagation of cracks. • Cracks initiate at straight HAGBs and terminate at LAGBs or within grains.

DEM study on the macro- and micromechanical behaviours of breakable granular materials under cyclic loading

The effect of cyclic loading on the macro- and micromechanical behaviours of breakable granular materials was investigated via DEM. Breakable particle models composed of Voronoi cells with realistic shapes were constructed. A series of confined cyclic loading tests were performed at different loading amplitudes. The macroscopic deformation behaviours were investigated in terms of the cumulative plastic strain and dynamic resilience modulus. Then, particle breakage characteristics, including the extent of particle breakage, crack orientation distribution, breakage pattern and correlation between the particle breakage and energy input, were explored. Furthermore, micromechanical results, including the coordination number, sliding contact and contact orientation anisotropy, were examined. It is found that both particle breakage and cumulative plastic strain increase rapidly in the early stage of loading. In the stable state, both showed an S-shaped increase with increasing loading amplitude. This study highlights that particle breakage is one of the main sources of permanent deformation.