Central Crack Research Articles

Effect of heat preservation cover on solidification quality in large round bloom

Installing heat preservation covers in the air-cooling zone during continuous casting will affect the solidification quality of large blooms. In this study, a multi-stage anti-radiation heat preservation cover model was developed for heat transfer, and stress coupling calculations based on a vertical continuous casting process of a large round 600 mm diameter bloom. The accuracy of the model was verified by comparing the calculated surface temperature of the bloom against industry-standard surface temperature measurements. Results demonstrate that after applying heat preservation covers with the strongest insulation effect, the surface temperature of the bloom at 14 m from the meniscus increases by 19.42%, and the central porosity area decreases by 23.79%. Furthermore, the maximum stress at the point 43 mm from the centre of the bloom within the ZDT-LIT range decreased by 2.032 MPa. This study elucidates the mechanism underlying the effect of heat preservation covers on the formation of shrinkage and central cracks in the blooms, thereby providing theoretical support for the rational application of heat preservation covers in the vertical continuous casting process of large round blooms.

Weak interface-induced repeated kinking of a central crack in a brittle multilayered plate under remote shear loading

Understanding the repeated kinking pattern of cracks is essential for controlling the complex fracture trajectories in multilayered composites. While previous studies have explored many aspects of fractures in layered materials, the conditions governing the repeated kinking behavior under basic loading modes remain largely unrevealed. This research elucidates the continual kinking condition for a central crack in a brittle, multilayered plate with closely-parallel weak interfaces under remote shear stresses. Using the strain energy release rate (SERR) ratio criterion, we analytically derived a closed-form solution to the condition through the equivalent crack concept. The solution specifies a value domain for the intersection angle and fracture toughness ratio of weak interfaces, where the load-guided direction competes closely with the weak interface-guided direction to shape the crack trajectory into a repeated kinking pattern. Our theoretical results, validated by a series of finite element (FE) simulations, clarify how closely-parallel weak interfaces induce repeated crack kinking in multilayered materials under remote shear loading. This research paves the way for the deeper understanding of intricate crack trajectories under mixed loads in various practical applications.

Determination of an optimum patch configuration in the repair of a center crack in thin aluminum sheet using polymer composite patch

AbstractThe formation of cracks and their growth is an issue of concern in structural applications incorporating thin metal sheets. Adhering cracked region with a patch of composite material has proved effective in repairing cracks. Determining an optimum configuration of composite patch is still a challenge as a shorter patch may not be effective and a larger patch leads to wastage of resources. An appropriate methodology is required for specifying an optimum patch configuration in terms of its length, width, and thickness. The work presented prescribes a methodology for obtaining an appropriate combination of geometrical parameters of the composite patch. The computational tools like finite element analysis and response surface method have been incorporated. This is an extensive work consisting of a number of iterations of numerical analyses carried out for different combinations of the length and width of the patch. The results were also obtained for three different patch thicknesses in which the number of plies were increased from a single ply to three plies. The optimum size was obtained for each of the three patch thicknesses. Finally, a methodology for obtaining the most appropriate patch configuration based on the practical constraint of a small available area is prescribed.Highlights The crack repair in a thin structural sheet using fiber reinforced polymer composite patch separates at the interface. The optimum size of the patch needs to be used for an effective repair. The work consists of a combination of experiments and numerical simulations using finite element analysis. Response surface methodology is employed in this optimization study.

Study on fracture behaviour of pipeline girth welds based on curved wide plate specimens: Experimental and numerical analysis

Understanding the fracture characteristics and strain capacity of pipelines with girth welds under external loads is crucial for evaluating pipeline safety. The use of curved wide plate (CWP) specimens closely resembling full-scale (FS) pipelines in geometry provides a more realistic representation of crack tip constraint states compared to small-size fracture toughness test specimens in laboratory settings. This study aims to investigate the ductile fracture characteristics of center cracks on the outer surface of girth welds in high-grade steel pipelines under external loads through large-scale CWP tensile tests and advanced numerical simulations. Tensile tests were conducted on CWP specimens using a kiloton tensile testing machine, with double crack opening displacement (COD) gauge monitoring crack mouth opening displacement (CMOD), and high-precision displacement sensors and strain gauges tracking deformation and strains at various positions. The study yielded significant experimental results, and a finite element (FE) model of the CWP was developed using the nonlinear finite element method (FEM) to validate the experimental data against simulation results. The FE model was then used to investigate the influence of material properties on crack propagation resistance and driving force curves. A method for determining the ultimate tensile strain capacity (UTSC) of pipelines based on actual material fracture toughness was proposed. This research contributes valuable experimental data for further exploration of fracture behaviour in large-scale pipeline girth welds and offers insights for safety evaluations of such welds.

Thermal Buckling and Postbuckling Analysis of Cracked FG-GPL RC Plates Using a Phase-Field Crack Model

A phase-field crack model is developed for numerical analysis of thermal buckling and postbuckling behavior of a functionally graded (FG) graphene platelet-reinforced composite (FG-GPLRC) plate with a central crack. The inclined central crack is represented according to a stable, effective phase-field formulation (PFF) by introducing a virtual crack rotation. The problem is formulated using first-order shear deformation theory (SDT) incorporated with von Kármán geometric nonlinearity. And it is approximated by combining regular Laplace interpolation functions and crack-tip singular functions in the framework of the 2D extended natural element method (XNEM). Troublesome shear locking is suppressed by applying the concept of the MITC (mixed-interpolated tensorial components)3+ shell element to the present numerical method. The results demonstrate the effectiveness of this method in accurately predicting the critical buckling temperature rise (CBTR) and the thermal postbuckling path. In addition, the parametric results reveal that the CBTR and postbuckling path of the FG-GPLRC plate are distinct from those of the FG carbon nanotube-reinforced composite (FG-CNTRC) plate and remarkably affected by the parameters associated with the crack and graphene platelet (GPL).

Numerical calibration for fracture parameters of three-point bending semi-circular specimens

This study aims to refine the fracture characterization of three-point bending semi-circular specimens used in rock fracture toughness assessments. The primary objective is to improve the accuracy of such evaluations by developing numerical simulations of specimens with pre-engineered cracks of varying geometries. Numerical simulations were conducted using the finite element method. The interaction integral method was employed to quantify the stress intensity factors (SIFs) and T-stress at crack tips. Initially, the model’s accuracy was validated by replicating stress singularities at crack tips in a benchmark circular disk with a central straight crack. Following validation, dimensionless fracture parameters specific to the three-point bending semi-circular specimens were calibrated. The numerical results demonstrate that the dimensionless stress intensity factor (YI) increases with both the relative crack length (a/R) and the spacing between support points. Notably, for relative crack lengths a/R ≤ 0.5, the dimensionless T-stress assumes negative values, initially decreasing and then increasing as the relative crack length increases. The findings of this study provide valuable insights into the fracture behavior of three-point bending semi-circular specimens with pre-engineered cracks. Based on the observed trends in the dimensionless fracture parameters, it is recommended that relative crack lengths within the range of 0.2–0.6 be used to maintain the accuracy of rock fracture toughness tests. The finite element method used in this study serves as a robust tool for calibrating fracture parameters, thereby laying a strong foundation for the application of these specimens in rock fracture toughness evaluations.

Mixed finite element implementation of plane crack problems within strain gradient elasticity

AbstractAn approach is proposed and applied to solve boundary value problems within the strain gradient elasticity theory. A mixed variation formulation of the finite element method is used, where stresses, strains, and displacements are considered as independent variables. This significantly simplifies the pre‐requirement for approximating functions and allows one to use the simplest triangular finite elements with a linear approximation of displacements. Global unknowns in a discrete problem are nodal displacements, while the strains and stresses in nodes are treated as local unknowns. This discrete problem is solved by a modified iteration procedure, where the global stiffness matrix for classical elasticity problems is treated as a preconditioning matrix with fictitious elastic moduli. The modeling plane crack problem (tension of a square plate with a central crack) is solved. The obtained solution agrees with the results available in the literature. Good convergence is achieved by refining the mesh for all scale parameters.

Prediction model of fatigue crack propagation life of corroded steel plate considering pit characteristics

Pitting corrosion notably increases stress concentration, thus accelerating fatigue crack initiation and propagation at the pit base. This research quantifies the relationship between pit dimensions and fatigue crack development by introducing crack extension ratios (γa​ and γc)​. Employing a series of finite element models, the influence of pit characteristics on the stress intensity factor (K) at the crack tip is assessed. Based on these assessments, a formula for computing the K-value is proposed. Furthermore, a two-stage fatigue life prediction model for both partial and complete penetration is developed using the Paris equation. Results indicate that pits substantially affect the K-value during partial penetration. Specifically, as γa approaches zero, the K-value approaches zero, and as γa approaches one, it aligns with the K-value of a semi-elliptical surface crack. Conversely, in the complete penetration phase, the influence of the pit on the K-value is negligible, and the K-value can be calculated according to the plate with a central penetrating crack. Experimental validation confirms that the model generally maintains a prediction error within 10%.

Image-driven prediction of fatigue crack growth in metal materials via spatiotemporal neural network

This study proposes an image-driven model based on the SimVP spatiotemporal neural network (STNN) to predict the fatigue crack growth (FCG) in aluminum alloys. This methodology represents a novel usage of STNNs for FCG analysis. It does not require repetitive modeling, extensive computations, or conventional mechanical assumptions. The datasets used during this study were gathered from fatigue experiments with a variety of crack positions, angles, and load levels; they contained a total of 17,925 image frames obtained from DIC measurements. Subsequently, the displacement fields were interpolated onto uniform grids and then augmented, so they could be fitted into an STNN. The proposed method was validated using specimens with edge and central cracks subjected to loads equal to 15.0 % and 20.0 % of the ultimate load. The generalization capability of the proposed method was studied by predicting the FCG under load levels and crack angles outside the training set. In addition, its predictive capability was investigated for both short and long step sizes by employing datasets in which the image data were collected at varying intervals. The overall structural similarity index measurement values were greater than 0.968, and the root mean square errors were held within 0.025 mm. The predicted displacement fields, crack lengths, and crack growth rates agreed well with experimental measurements.

Transient thermal fracture analysis of a honeycomb layer with a central crack

Auxetic honeycomb materials with negative Poisson’s ratio received rapidly growing attention in recent years owing to their exceptional thermomechanical properties in constructing sandwich composites. The objective of this article is to investigate the transient thermal behavior and fracture risk of a honeycomb layer with a central crack subject to a sudden thermal shock by theoretical modeling. Two different material configurations along both orthogonal directions, as well as the conventional and auxetic hexagonal alumina honeycomb cells, are examined to illustrate their effects on the thermal stress intensity factors. To solve the thermoelastic governing equations subject to complex boundary conditions, the methodology of integral transform with singular integral equation is employed. The numerical results demonstrate that the auxetic honeycombs can reduce the thermal stress intensity factors significantly compared to their conventional counterparts. The ratio of stress intensity factors in auxetic to non-auxetic honeycombs under the same absolute value of the internal cell angle θ decreases monotonically with increasing |θ|. In addition, the effects of the relative density, crack length, and crack position are investigated. Our findings would provide a more comprehensive understanding of the honeycomb’s fracture behaviors and contribute to the material design of the sandwich composites with honeycomb cores.

Experimental Research on the Low-Cycle Fatigue Crack Growth Rate for a Stiffened Plate of EH36 Steel for Use in Ship Structures

This paper presents a straightforward approach for determining the low-cycle fatigue (LCF) crack propagation rate in stiffened plate structures containing cracks. The method relies on both the crack tip opening displacement (CTOD) and the accumulative plastic strain, offering valuable insights for ship structure design and assessing LCF strength. Meanwhile, the LCF crack growth tests for the EH36 steel were conducted on stiffened plates with single-side cracks and central cracks under different loading conditions. The effects of stress amplitude, stress ratio, and stiffener position on the crack growth behavior were examined. Fitting and verifying analyses of the test data were employed to investigate the relationship between CTOD and the crack growth rate of EH36 steel under LCF conditions. The results showed that the proposed CTOD-based prediction method can accurately characterize the LCF crack growth behavior for stiffened plate of EH36 steel for use in ship structures.

Atomistic and continuum Ascertainment of The crack tip stress fields in anisotropic elastic cubic media

The article provides a comparative analysis of stresses associated with the crack tip in an anisotropic elastic medium with cubic crystalline symmetry of elastic properties under mixed loading conditions obtained by two fundamentally different approaches: atomistic simulations and continuum fracture mechanics methods. The atomistic modelling approach is premised on the application of the molecular dynamics (MD) method implemented in the open source code program Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The continuum mechanics approach is based on the classical elasticity theory of anisotropic media, in which the mechanical fields associated with the crack tip are represented by the crack tip stress and displacement series expansions generalizing the well-known classical asymptotical presentation of M. Williams to the case of anisotropic media. Using the MD method, a large series of computations of the loading of monocrystalline copper and aluminum plates with a face-centered cubic crystal lattice weakened by a central crack applying the embedded atom potential (EAM) was implemented. MD modeling is aimed at determining the atomic stresses and strains near the crack tip. The computed atomic stresses were compared with the stress field determined by the continuum elasticity theory of anisotropic media for crystal lattices with cubic symmetry of elastic properties. A comparative analysis was carried out for the angular dependencies of the stress and strain tensor components at various selected distances from the crack tip for the entire range of mixed deformation forms starting from pure tensile loading and ending with forms close to pure transverse shear. It is ascertained that the crack tip fields determined on the basis of two fundamentally different approaches, precisely, discrete and continuum, are completely consistent with each other. It is established that mathematical methods of continuum fracture mechanics can be applied to describe stress, strain and displacement fields at the atomic level.

Study on fracture toughness of ultra-thin stainless steel foils

Abstract Ultra-thin stainless steel foils have garnered significant attention due to their exceptional mechanical properties, including light weight, high strength, toughness, corrosion resistance, oxidation resistance, high temperature resistance, and superior shielding properties. In this study, we conducted experimental tests to evaluate the mechanical properties of ultra-thin stainless steel foils, focusing on the determination of Young’s modulus and yield stress for two distinct thicknesses of steel foils. Furthermore, employing both experimental and numerical simulation methodologies, we investigated the impact of wrinkles on the fracture toughness of steel foils containing central cracks subjected to tensile loading. The findings presented in this paper not only contribute to a comprehensive understanding of the mechanical behavior of ultra-thin stainless steel foils but also serve as a fundamental basis for subsequent numerical simulations and fracture toughness estimations. This research lays the groundwork for advancing the development and utilization of ultra-thin stainless steel foils across various industrial applications.

Assessing Treatments to Mitigate End-Face Cracking in Air-Dried Acacia dealbata Logs

Acacia dealbata Link, known as Mimosa in Portugal, is an invasive hardwood species with potential for construction use, but research is limited. The available stock of small-diameter juvenile wood logs can help reduce this gap, but tangential cracking at log ends challenges fastener connections. This study evaluated different treatments to control and reduce end-face cracking in small wood logs during air drying, an economical and environmentally friendly procedure. The extreme two-thirds of sixteen Mimosa logs were subjected to two treatments: one with longitudinal kerfs 15 mm deep along the length (two and three kerfs) and the other with a hollow in the center up to half the length (16 mm and 30 mm diameters). Over 219 days of air drying and compared with the central part, kerfing treatments significantly reduced outer-wood tangential cracking (p < 0.001), with the three kerfs also reducing crack numbers (p < 0.05) but increasing significantly cracks near the pith (p < 0.01). The 30 mm central hollow significantly reduced central perforation cracking (p < 0.05). Prospectively, the results suggest that a combined treatment approach involving cross-cuts could help reduce and/or control end cracking, thereby improving the suitability of wild Mimosa logs for construction use.

A Novel Analysis of the Size Effect on Stress Intensity Factor for Center and Edge Cracks in Tension by the Net-Section Mechanics

A Novel Analysis of the Size Effect on Stress Intensity Factor for Center and Edge Cracks in Tension by the Net-Section Mechanics

Atomic simulations of crack propagation in Ni-Al binary single crystal superalloy with a central crack

Nickel (Ni)-based single-crystal superalloys are of great importance in the aircraft industry due to their excellent mechanical properties, and cracks as unavoidable defects may affect the mechanical performances of materials dramatically. In this paper, large scale molecular dynamics (MD) simulations are carried out to understand the deformation mechanisms of Ni-based single crystal with a central crack under tension. Here, the effects of matrixes (γ, γ′ and γ/γ′), strain rates (1 × 109 s−1 ∼ 3 × 109 s−1) and temperatures (300 K∼900 K) on the role of crack propagation are considered. It is observed that dislocations and slip systems in the γ′ model are concentrated near the crack, resulting in the rapid expansion of dislocation, which leads to the fastest crack growth speed and early fracture. While the crack propagation rate of γ and γ/γ′ models are relatively slow, due to the combined action of the Lomer-Cottrell lock and stacking fault tetrahedron structure and Stair-rod dislocation, which hinders crack propagation. In addition, deformation at increased strain rates and/or reduced temperatures, lead to superior yield stress and Young′s modulus for models with a central crack at γ/γ′ interface. On the other hand, high temperature and high strain rate will promote crack propagation in the γ phase, and the higher the strain rate and/or temperature, the faster the crack propagation speed will be. These results will enrich our understanding on the crack propagation and evolution mechanisms in Ni-based single crystal superalloy.

Spalling characteristics of sandstone containing rectangle hole with gradient cavity height: An experimental and DEM numerical study

Spalling failure commonly exists during the progressive excavation of underground engineering, and the stepped excavation of underground mining stope is rarely simulated in previous studies. Therefore, the biaxial compression test is finished by sandstone samples containing rectangular hole (with different cavity heights), and the corresponding numerical simulation is conducted by the discrete element method. Based on the mechanical, optical and acoustic data captured during the test, the main results and conclusions are as follows: 1) There are two paths to activate the typical spalling failure, the first way is caused by the rapid energy release (manifested as sudden AE burst), and the second way is that the released energy accumulates to a threshold over a long period and finally causes large-scale damage. 2) The relatively larger cavity height delayed the initiation of central tensile failure in the roof & bottom surrounding rocks and the spalling failure in sidewalls. 3) The relatively larger confining pressure contributes to the early construction of the spalling failure and limits the extension of the central tensile crack bands. 4) The construction of spalling failure is very fast, the ejected rock plates are caused by several rounds of spalling failure, and the final V-shaped groove is built by the spalling failure and internal crushing failure.

On how pseudo-ductility modifies the translaminar fracture toughness of composites and the nominal strength of centre-cracked specimens

Among the efforts to revert the traditionally brittle characteristic of laminated composites, pseudo-ductility relies on utilising hybridisation to stimulate sub-critical damage mechanisms. However, how such pseudo-ductility would translate into an increase in material toughness or an improvement in the strength of the sub-components remains unclear. To elucidate this, we perform a numerical study departing from a parameterised pseudo-ductile model implemented in a finite element model. We use non-dimensional analysis to investigate the effect of the two most relevant parameters: pseudo-ductile strain (ɛd) and the ratio of ultimate strength to pseudo-ductile yield strength (σf/σy). We infer material toughness from the simulation of Compact Tension specimens, and it is shown to increase linearly with ɛd, and non-linearly with σf/σy but tends to a plateau. Then, the simulation of Centre Cracked scaled specimens reveal that the nominal strength increases on the elastic limit extreme (large specimens) but decreases below a given size.

Experimental investigation on multiple-site fatigue failure of the hollow compressor impeller

This study conducts a systematic experimental investigation on multiple-site damage (MSD) in aero-engine hollow compressor impellers, focusing on MSD fatigue fracture growth and coalescence behaviors and mechanisms. Comparative analyses of literature research, standard specimen tests, and full-scale disk test experiences conclude that elevated cyclic radial stresses, especially in thin-walled disk webs, facilitate the occurrence of MSD fractures. Extending from previous numerical studies, a full-scale hollow impeller fatigue test on a high-speed spin tester revealed an over 153 mm circumferential through-wall MSD failure in the outlet web, accounting for 33 % of its circumference. Challenges in traditional nondestructive inspections for such complex structures emphasize the effectiveness of synergetic fractography analysis in investigating the MSD failure. Thousands of scanning electron microscope (SEM) images, examined at macro and micro levels with the assistance of metallography and stereomicroscopy, unfolded the MSD fracture features of crack initiation, propagation, coalescence, and interaction. Besides the well-known macroscopic rachet marks, this study is the first to identify and analyze various microscopic MSD merging features, such as terraces, plastic ridges, shear sliding surfaces, cliffs, plateaus, and lowlands. Differences in fracture merging and growth features enable us to distinguish MSD component cracks. Quantitative fractography analysis revealed that the crack initiation occurred 1126 test cycles earlier, and that the overall crack propagation life slightly surpassed 1646 cycles as new cracks merged and joined. Diverse crack growth durations and rates across 11 fracture paths indicate asynchronous initiations and confirm interactions among component cracks. Cracks initiate flatter near the central crack colony but appear bumpier and more pronounced plastic deformation at the periphery. The overall MSD fracture evolution process was backtracked through synergetic fracture assessments, detailing the merging process of varied component crack colonies, enhancing our understanding of the spatial–temporal evolvement of cracking scenarios in actual aero-engine impellers.

Study on overburden failure characteristics and displacement rule under the influence of deep faults

Deep faults significantly impact the structural stabilities and deformation behaviors of their overburden rocks, which are key factors in underground engineering and geological hazard research. Considering the problem of deep-fault-overburdened breaking during mining of super-thick coal seams and taking the Yaoqiao Coal Mine as the research object, the mining fracture evolution characteristics and overburden displacement law of the non-structured and fault-bearing overburden corresponding to fully mechanized caving mining are compared and analyzed using numerical simulations and physical similarity simulations. The results of this study show the following: 1) The fracture height of the overlying rock presents a specific change law with advancement of the working face; the initial rapid increase to a maximum height of 74 m is achieved when the working face advances to 90 m; with the development of the plastic zone indicating past yield, the fracture height decreases to 54 m and becomes stable, and the final caving angle of the fracture stabilizes at 70°. 2) In coal mining under normal fault conditions, when the working face advances from the upper to lower walls, the roof forms a masonry beam structure that slows down fault activation and crack development. When moving away from the fault, the overburden movements and water-conducting cracks are fewer, and the crack height is lower than that without faults. When approaching the fault, the influence of the faults in the fracture zone increases, and the height of fracture development reaches the maximum value after crossing the fault, highlighting the significant influence of the fault on fracture development. 3) Through a similarity simulation test, it is shown that the overburden caving zone is further compacted by the overburden rock and that the roof collapses in a large range, resulting in rapid upward development of the overburden rock cracks and separation of the central overburden rock cracks that are gradually compacted and closed. These findings are expected to have important theoretical and practical significance for deep underground engineering design, geological disaster prevention, and fault activity monitoring.