Crack Origin Research Articles

Formation of fine granular area in a non‐defect matrix of austenitic stainless steel during very high cycle fatigue

AbstractA fine granular area, FGA, is a typical phenomenon observed at the very high cycle fatigue fracture crack origin with a subsurface defect in the material. The FGA has been widely investigated, and different mechanisms have been proposed. In this paper, the formation of FGA in a non‐defect matrix of one austenitic steel during very high cycle fatigue was studied using a progressive stepwise load‐increasing method and electron scanning microscopy/electron channeling contrast imaging (ECCI) technique. A nano rough surface area or FGA at the fatigue crack origin has been observed in the subsurface matrix without any defect. It is a new phenomenon. A mechanism was proposed using the dislocation plasticity theory. The formation of FGA in a non‐defect matrix is a localized plasticity exhausting process by strain localization, grain fragmentation, stress concentration and nano crack initiation and propagation along low‐angle grain boundaries.

Failure analysis of the conductive slide support rod of section insulator applied in urban rail transport system

The support rod of conductive slide of a section insulator which was applied in urban rail transport system fractured after operation for about 4 years. The failed section insulator was installed at the turning of the train route. Multiple failure causes had been analyzed utilizing a combination methods of chemical composition analysis, mechanical property test, metallographic examination, macro morphology and microstructure observation. The support rod is made of AISI-304/L/C stainless steel. No obvious microstructure defects are detected in crack origins or the matrix material. The mechanical properties and the chemical composition satisfy the requirement. The failure mechanism of the support rod is proved to be high cycle fatigue fracture (HCF). The main fatigue crack origin locates at the right-angle edge of the arc-shaped transition region of the support rod where stress concentration can be generated easily. In addition, arc discharge-induced cracks can be observed on the local burned surface of the support rod, and they contribute a lot to the fatigue crack initiation. Finally, the alternating load mainly results from the change of train operation direction at the turning position and this change further leads to the direction change of the contact force between pantograph and insulator section. Besides, the impact, vibration and unsmooth contact caused by pantograph also contribute to the alternating load.

High‐cycle and very‐high‐cycle fatigue behavior at two stress ratios of Ti‐6Al‐4V manufactured via laser powder bed fusion with different surface states

AbstractThe high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) behavior of Ti‐6Al‐4V manufactured by laser powder bed fusion (L‐PBF) was investigated to reveal the effects of surface roughness and stress ratio on fatigue performance. Fatigue tests were performed by an ultrasonic vibration machine with a frequency of 20 kHz. The fatigue strength of surface‐polished specimens is generally higher than that of as‐built specimens in HCF and VHCF regimes. Fatigue cracks initiated from the subsurface of as‐built and surface‐polished L‐PBF Ti‐6Al‐4V. The values of size and depth for the defects that acted as fatigue crack origins present large scattering for L‐PBF Ti‐6Al‐4V due to the interaction between surface roughness and subsurface defects. For surface‐polished L‐PBF Ti‐6Al‐4V, fatigue cracks have almost the same resistance to propagation at both stress ratios of R = −1 and 0.7.

Cracking of Gem Opals

The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.

Аналитическая модель квазихрупкого разрушения пластины с трещиной

The paper considers a rectangular plate with the edge crack of mode I of normal separation from the elastoplastic material with the ultimate strain. This class of materials includes, for example, the low-alloy steels used in structures operating at temperatures below the cold brittleness threshold. The strength of the plate was studied within the framework of the Neuber --- Novozhilov approach. The crack propagation criterion was formulated using the modified Leonov --- Panasyuk --- Dugdale model using an additional parameter, i.e., the plasticity zone diameter (pre-fracture zone width). Under conditions of small-scale yielding in the presence of the stress field singular feature in the vicinity of the crack tip, the two-parameter (dual) criterion for quasi-brittle fracture was formulated for mode I cracks in the elastoplastic material. The fracture dual criterion included deformation criterion at the crack tip, as well as the force criterion at the fictitious crack tip. The lengths of the original and fictitious cracks were differing by the length of the pre-fracture zone. Diagrams of the plate quasi-brittle fracture under conditions of plane deformation and plane stress were constructed. The parameters included in the proposed quasi-brittle fracture model were analyzed. It was proposed to select model parameters according to the approximation (σ--ε)-diagram of uniaxial tension and the KIc critical stress intensity factor

Tensile and axial fatigue properties of AISI 316 L stainless steel fabricated by materials extrusion additive manufacturing

Herein, the surface quality, physical, microstructural, tensile and axial fatigue properties of 316 L stainless steel fabricated by materials extrusion additive manufacturing (MEX) are investigated and compared to the well-established and similar fabrication process, metal injection moulding (MIM). The results reveal no discernible difference in microstructure, preferential orientation and grain size between MEX and MIM parts, except for voids between deposited paths found in MEX parts, and Si-O inclusions in MIM parts. MEX parts have slightly higher tensile properties than MIM parts due to the higher relative sintered density. However, the endurance limit of MEX parts is lower than MIM. This is attributed to differences in the surface characteristics, in which MEX parts have layer-by-layer characteristics, higher roughness and Al2O3 contamination from the separator. They sensitively act as crack origination sites and result in lower fatigue resistance when compared to MIM.

Study on Polypropylene Twisted Bundle Fiber Reinforced Lightweight Foamed Concrete

Recent industrial developments have focused more and more on the applications of lightweight foamed concrete (LFC) in the construction industry, having advantages over normal-strength concrete. LFC, however, has several drawbacks including brittleness, high porosity, excessive drying shrinkage, rapid cracking, and low deformation resistance. Practical engineering typically chooses steel fiber or polymer fiber to increase the tensile and fracture resistance of LFC. The polypropylene twisted bundle fiber (PTBF) was added to the LFC with varying weight fractions of 0.0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5%. Three low densities of LFC were prepared, specifically 500 kg/m3, 700 kg/m3 and 900 kg/m3. The mechanical and durability properties of PTBF-reinforced LFC were determined through compression, flexural, splitting tensile, flow table, porosity, and water absorption tests. The results show that the addition of PTBF in LFC significantly improves the strength properties (compressive, flexural, and splitting tensile strengths) and reduces the water absorption capacity and porosity. The optimal weight fraction of PTBF was between 1.5 and 2.0% for mechanical properties enhancement. The inclusion of PTBF increased the ductility of LFC, and the specimens remain intact from loading to failure. The PTBF reduces the original cracks of the LFC and inhibits the development of further cracks in the LFC.

A deeper generative adversarial network for grooved cement concrete pavement crack detection

Periodic grooved cement concrete pavement crack detection is of great importance for pavement condition monitoring and maintenance. The current state-of-the-art (SOTA) detection solutions highly depend on datasets. However, due to the limited access to crack images, more efficient methods are urgently needed to advance the detection of cracking on grooved cement concrete pavement. This study proposes an improved deeper Wasserstein generative adversarial network with gradient penalty (WGAN-GP) to generate datasets of pavement images with a size of 512 × 512 pixels 2. Poisson bleeding is adopted to create the synthesized grooved cement concrete pavement crack images based on the generated crack images and groove images. The robustness of the proposed improved deeper WGAN-GP model is validated by Faster R-CNN, YOLOv3, and YOLOv4 models trained on original crack images and generated crack images for region-level detection. U-Net and W-segnet are used to achieve pixel-level crack detection to evaluate the effectiveness of proposed model. Results show that the improved deeper WGAN-GP could generate more realistic transverse, longitudinal and oblique crack images. In addition, the Poisson bleeding algorithm contributes to synthesizing grooved cement concrete pavement crack images. Moreover, it is observed that YOLOv3 trained by the augmented dataset could achieve a mean average precision (MAP) of 81.98%, 6% MAP higher than the non-augmented dataset. U-Net and W-segnet benefit from augmented dataset with a better pixel-level segmentation result. Based on the results, it can be concluded that the improved deeper WAGN-GP image generation method can provide a straightforward way to fill the data shortage gap of grooved cement concrete pavement cracks, thus increasing the problem-solving capability of the SOTA crack detection models.

О ГЕНЕТИЧЕСКИХ КЛАССИФИКАЦИЯХ ТРЕЩИН ГОРНЫХ ПОРОД

Before authors have made attempts to classify cracks according to certain characteristics: geometric, geologic, tectonic, hydrogeological, and others. Knowledge of the genesis of cracks is very useful in solving applied problems, especially in studying the distribution of cracks in the medium. Information about the origin of cracks allows us to express certain hypotheses about the patterns of their distribution and plan further measurements on this basis. The article presents an analysis of the genetic classifications of cracks developed in rocks and their systematization.

Materialistic cracks & integral equations

Many engineering structures are made by bonding together two or more materials with different mechanical and electronic properties. The dissimilar material system needs to function as a unit in which loads are transmitted from one material to another through interfaces. The advent of fiber-reinforced composite material has resulted in the ever-increasing use of laminated fiber-reinforced plates and other structural shapes used in aerospace and electronics industries. These composite materials are treated as an orthotropic medium when layers are more than 6. The composite materials in laminated form are widely castoff in many industries due to its excellent mechanical possessions and lightweight. However, the involvedness of their assembly and the extensive range of material combinations require a thorough analysis of their mechanical response. Experimental testing is necessary to validate the numerical models used to predict the mechanical behavior of these materials. The results obtained from the tests can then be used to develop more accurate models and improve the design of laminated composite materials.Though in article not concentrate upon the origination of cracks, the orientation will provide an overview of the integral approach for solving the problem in bounded dissimilar materials having cracks and will include an examination of the displacement fields & stress in the vicinity of crack. Here article covered will include a review of existing theories on crack propagation, an overview of the integral approach, a discussion of the relevant equations and how they are used, and an examination done for displacement fields & stress in the locale of crack tips. Examples and case studies will be presented to illustrate the application of the integral approach in practice.

MATHEMATICAL AND MECHANICAL RELATIONSHIP OF CRACKS GRIFFITH AND A NEW TYPE IN SOME NANOTECHNOLOGIY

In this paper, for the first time, mathematically strictly, models of Griffiths cracks and cracks of a new type are constructed simultaneously. Accurate models are constructed as a result of the convergence of the ends of two semi-infinite deformable stamps located on a deformable multilayer base. The mechanism of destruction of the medium by cracks of a new type is radically different from the mechanism of destruction of the medium by Griffiths cracks, and has so far been poorly studied. Griffiths formed his cracks with a smooth border as a result of compression from the sides of an elliptical cavity in the plate. Cracks of a new type have a piecewise smooth border, resulting from the replacement of an ellipse with a rectangle compressed from the sides. The construction of models in the work is based precisely on these descriptions of the origin of cracks formed by the ends of approaching deformable stamps. When creating models, the results of previously performed studies on the construction of exact solutions to a number of integral equations of contact problems, published in this journal, are used. The paper found that the destruction of the medium, in the case of cracks of a new type, occurs earlier than in the case of Griffith cracks, which, according to the authors, explains the reason for the destruction of Griffith cracks in the experiment earlier than the theory predicts. Griffiths explained this by the presence of microcracks on the border of the main crack. The authors tend to believe that the cause is the transformation of one type of crack into another, under certain conditions. The results obtained make it possible to compare and quantify the behavior of the medium in the presence of both one and another type of cracks. It is applicable both for the study of mutual transitions of Griffith cracks and cracks of a new type, and for the construction of models of adhesion of wetted nanoparticles.

Fractographic analysis of fractured fixation screws in implant-supported restorations.

The aim of this study was to fractographically examine clinically fractured fixation screws used to fix implant supported restorations and to estimate stress at failure. 20 clinically fractured titanium implant fixation screws were collected and analyzed under scanning electron microscope in order to locate critical crack origin and calculate stress at fracture. Principles of fractographic analysis were applied using relative equations and cause and conditions of screw fracture were calculated. X-rays and patients' records were collected after approval of necessary consent forms. Fractographic analysis revealed location and dimensions of critical crack size for every fractured screw examined. Two patterns of screw fracture were identified; the first was related to improper seating of fixation screw leading to damage at screw threads and eventual fracture of screw body. The second was related to slow crack growth propagation causing final fracture of screw body. The calculated stress at failure ranged from 755 to 804MPa. Patients' records revealed that 75% of fractured screws were related to single unit screw retained restorations. Single unit screw retained restorations and improperly seated implant abutments impose high stress concentration on fixation screw which may result in fixation screw fracture.

Criterion of the limit state of composites materials

The work proposes and substantiates a type of phenomenal criterion of composite material destruction at the stage of macro crack origination. It considers damageability and two mechanisms of destruction: tearing and slice. It describes that as parameter of damageability it can be taken specific energy of extra stresses or specific energy of diffusion. For proportionate and complex processes of metallic materials loading they result equally in practice. Methodologies of respective energetic parameters search are described. It is set threshold value of stress in the element of construction during the reach of it it is necessary to consider an accumulation of damage. It is described the methodology that describes interconnection between anisotropy factors and components of damage tensor.

Bulk oxygen release inducing cyclic strain domains in Ni-rich ternary cathode materials

Nickel-rich layered transition metal oxide is limited by the poor structural stability during cycling as cathode materials for next-generation lithium-based automotive batteries. In the past, the poor electrochemical performance was mainly attributed to cracks and formation of rock-salt phase on the particle surface at high potentials. Rarely is the effect of bulk phase structure evolution on properties discussed. Here, we report a bulk oxygen release induced dynamic accumulative electrochemical–mechanical coupling failure mechanism. Domain-like rock salt phases are generated due to the oxygen release and transition metals migration in the bulk region of LiNi0.6Co0.2Mn0.2O2 (NCM622) particles at the first cycle high cutoff voltage. Then, reversible compressive/tensile lattice strain alternately dominate around the domain boundary and accumulate with cycling, leading to capacity fading and becoming the origin of intracrystalline cracks. The results suggest that, in addition to the side effects from the surface, the structural transformation of the bulk plays an important role in the capacity fading. The stabilization of lattice oxygen in bulk region is a feasible solution to suppress the structural transition and the inhomogeneous stress distribution.

Quasi‐static fracture characterization of fiber metal laminate with crack resistance curve based on δ5

AbstractFiber metal laminate (FML) is a hybrid composite typical in aircraft industry. It is necessary to effectively characterize the quasi‐static fracture performance of FML, considering the importance of damage tolerance design of aircraft structures and the complicated fracture mechanism of FML. In present investigation, glass fiber reinforced aluminum laminates (Glare) with middle cracks were tested under quasi‐static tensile load. The experimental crack resistance curve (R curve) based on the special crack opening displacement at original crack tip (δ5) was used for revealing the superior crack resistance of Glare laminate to aluminum sheet, which expressed in crack extension aspect was the extended critical crack length. This was thought to be benefit from the contributions of glass fiber layers and fiber/metal delamination, by performing the progressive damage analysis. Since δ5 can embody the elastic–plastic response of the crack but without need to interactively correct for crack tip plasticity. The δ5‐R curve‐based macro and meso fracture models were proposed to accurately predict the residual strength and stable crack extension of Glare laminate, which gave further evidence for the effectiveness of δ5‐R curve in characterizing the quasi‐static fracture performance of FML.

Crack identification of automobile steering knuckle fluorescent penetrant inspection based on deep convolutional generative adversarial networks data enhancement

As a key safety component of automobiles, automobile steering knuckles must be subjected to strict quality control. Currently, the identification of cracks in finished products primarily relies on manual identification of fluorescent penetrant detection. Owing to the complex shape of the workpiece, the interference of the displayed image and the small sample size, the accuracy of the automatic discrimination result of the fluorescent penetrant detection image is directly reduced. Therefore, this study proposed a data augmentation method based on deep convolutional generative adversarial networks (DCGAN) for crack identification in automotive steering knuckle fluorescent penetration inspection images. An image acquisition platform was built for fluorescence penetration detection of automobile steering knuckles, and fluorescence display images of various parts of the workpiece were collected. Based on the feature analysis of the displayed image, the image was preprocessed to suppress relevant interference and extract crack candidate regions. Further, using the original crack image to train DCGAN, several crack image samples were generated, the ResNet network was trained with the expanded dataset, and the extracted candidate regions were identified. Finally, the experimental results show that the recall rate of the crack recognition method used in this paper is 95.1%, and the accuracy rate is 90.8%, which can better identify the crack defects in the fluorescent penetrant inspection image, compared with the non-generative data enhancement method.

Determination of the critical defect and fatigue life of high-speed railway axles under variable amplitude loads

High-speed railway axles are suffered from extremely complex variable amplitude (VA) loads during operation. However, in the design codes and operation maintenance of railway axles, most of them are based on the equivalent constant amplitude (CA) load spectrum to perform the fatigue resistance design by using the traditional nominal stress method, and carry out the routine inspection based on running experiments and data. In this paper, an innovative time-domain stepwise fatigue assessment (TSFA) approach has been introduced to include the influence of VA loads on the accumulated damage and fatigue crack propagation. Based on this procedure, the fatigue load spectrum from the multi-body system (MBS) dynamics simulation was integrated into the fatigue damage and remaining lifetime assessment of the axles. The calculation indicates that the safe lifetime of the powered and non-powered axles under the time-domain loading is considerably larger than the design value, which shows that the axle could safely operate until they are retired. However, for an original crack with the morphology ratio of 0.6 propagating to 45.0 mm, the lifetime of the powered and non-powered axle is sharply reduced to 1.32, and 2.07 years, respectively. Compared with the classical nominal stress method, the newly-developed TSFA procedure could produce a more conservative but practical result.

The Fracture and Energy of Coal Evolution under Thermo-Mechanical Coupling via a Particle Flow Simulation

The increase in mining depth causes the temperature of the coal seam to rise. Studying the effect of temperature on the mechanical property of coal is very necessary. In this paper, based on the results of conventional triaxial compression experiments of coal samples, the discrete element program PFC2D was used to conduct a conventional triaxial compression simulation of coal samples to obtain microscopic parameters. On this basis, the conventional triaxial simulation study of coal samples at different temperatures was carried out to explore the influence of temperature on the physical and mechanical properties of coal. In this process, acoustic emission and energy monitoring were carried out. The damage and failure process of coal is divided into three stages: the undamaged stage, the stable damage stage, and the rapid damage stage. The relationship between acoustic emission characteristic law, energy transformation law, and crack evolution in the damage and failure process of coal was analyzed in detail. The results indicate that the fracture evolution is affected by the increase in cross-sectional area and the decrease in distance between particles due to the expansion of particles, which corresponds to the phenomenon that the temperature increase produces new microcracks and the particles expand to fill the original cracks in the experiment. This indicates that PFC2D can be used as an effective numerical method for a coal mesoscopic study, and the results of these numerical experiments are also helpful to deepen the understanding of the damage mechanism of coal failure under thermo-mechanical coupling.

Numerical investigation on origin and evolution of polygonal cracks on rock surfaces

We studied the formation and evolution mechanism of polygonal cracks on rock surfaces under cooling by modelling meso-damage mechanics, continuum mechanics and thermodynamics. Factors that affect rock surface damage include ambient temperature, lithology difference and boundary restrictions. We established and simulated a heterogeneous model with a surface weak layer for three types of boundaries. These were biaxial constraint, uniaxial constraint and free boundary. The initiation and propagation of polygonal cracks were reproduced for varying thickness and homogeneity of the weak layer. The results show that the boundary constraints strongly influence the polygonal cracking. Many polygonal or parallel cracks are generated on the rock surface under biaxial or uniaxial constraint. The unconstrained rock surface displays polygonal cracks at the center and parallel cracks in the surrounding areas. The thicker the surface weak layer, the larger the average area of formed blocks. Small blocks and short cracks are more numerous than large blocks and long cracks. As the heterogeneity index increases, the rock layer is more likely to produce blocks with relatively regular shapes. Quadrilateral, pentagonal and hexagonal blocks dominate regardless of changes in layer thickness and heterogeneity. However, the number of edges of the polygonal blocks is sensitive to rock heterogeneity. The polygons tend to become more complex with increasing inhomogeneity. This study contributes to understanding the complex formation mechanisms of polygonal cracks on rock surfaces in nature. Additionally, the simulations of three-dimensional fracture geometry provide a basis for developing algorithms to generate discrete fractures and blocks in discrete fracture network (DFN) analyses.

The origin of hierarchical cracks in floor-fractured craters on Mars and the Moon

Numerous craters showing floor fractures have been identified on the Moon and Mars. Whereas on the Moon, the floor morphology, crater geology and fracture characteristics suggest that the floor deformation is caused by underlying magmatic intrusions, the origin of fractures and floor modifications of Martian floor-fractured craters (FFCs) remains debated. Here, we focus on FFCs displaying hierarchical crack patterns. We use different quantitative fracture characteristics to investigate the physical processes at the origin of cracks and compare their variations to scaling laws derived theoretically and from desiccating laboratory experiments. We show that cracks on a large set of Martian FFCs close to Valles Marineris differ from that of Lunar FFCs: they may be explained by tensile stresses caused by the shrinkage of a material adhering on the crater floor, similar to desiccating cracks. Other Martian FFCs close to Syrtis Major show cracks more similar to those caused by a magmatic intrusion.