Corner Cracks Research Articles

Finite element analyses of rail head cracks: Predicting direction and rate of rolling contact fatigue crack growth

A numerical framework in 3D for predicting crack growth direction and rate in a rail head is presented. An inclined semi-circular surface-breaking gauge corner crack with frictionless crack faces is incorporated into a 60E1 rail model. The investigated load scenarios are wheel–rail contact, rail bending, thermal loading, and combinations of these. The crack growth direction is predicted using an accumulative vector crack tip displacement criterion, and Paris-type equations are employed to estimate crack growth rates. Results are evaluated along the crack front for varying crack radii and crack plane inclinations. Under the combined load cases and in the presence of tractive forces, the crack is generally predicted to go deeper into the rail than under pure contact. Crack growth rates for the combined load cases are higher than (but still close to) that for pure contact. A tractive force will increase growth rates for smaller cracks, whereas a steeper (45°) inclination will decrease the growth rate under the studied conditions as compared to a shallower (25°) inclination. Results should be of use for rail maintenance planning where deeper cracks require more machining efforts.

Effects of folded fissure properties on tunnel model failure: Experiments and numerical simulations

Natural folded fissures commonly exist rather than simple straight fissures in rock masses surrounding tunnels. The presence of folded fissures significantly affects the fracture processes and failure modes of tunnel structures, thereby affecting their safety and stability. However, the research in this area is limited. Given this, this study fabricates city-gate section tunnel specimens containing folded fissures of various dip angles (α) and orientation angles (β), utilising 3D sand printing with sand and furan resin as matrix materials. Uniaxial compression fracture tests were conducted at a loading rate of 0.3 mm/min on these specimens using a digital image correlation technique to assess the impact of folded fissures on the mechanical properties and failure modes of the tunnel structures. Additionally, the improved smoothed particle hydrodynamics (SPH) method was used for damage evolution simulations during interactions between folded fissures and tunnels, and the simulation results were compared with the experimental results to verify the correctness of the method. The results show that for folded fissures, wing cracks and anti-wing cracks initiate not only from the ends but also from the bends of the fissures, whereas for straight fissures, they appear only at the fissure ends. Except for β = 0°, the interaction between folded fissures and tunnels generally results in the formation of a crack connecting the folded fissure upper end with tunnel, and the connection position varies with β. Different β values of folded fissures also influence the appearance and morphologies of top major cracks, underside major cracks, side cracks, and corner cracks around tunnels. As α increases, the overlap point of the crack and the tunnel moves from the tunnel corner to the tunnel crown. Moreover, folded fissures significantly affected the peak strength of the tunnel structures. As β increases, the peak strength first decreases, then increases, and finally decreases, reaching a minimum of 3.58 MPa at β = 90°. However, the peak strength differences are not evident under different α values. Finally, the influence of folded fissures on the cracking mechanism of the tunnel models is discussed in detail. This study provides insights into the impact of folded fissures on tunnel fracture modes and offers a reference for the application of SPH to reveal the underlying failure mechanisms of tunnel structures.

Study on the control of transverse cracks at the corners of large square billets for continuous casting of marine 945 hypo-peritectic micro-alloyed steel

Hypo-peritectic micro-alloyed steel has strong crack sensitivity, and marine 945 steel, as a kind of sub-encapsulated crystalline micro-alloyed steel, has frequent corner cracks during production, which seriously affects the production rhythm and reduces the product quality. Through the study of the morphology and composition of the matrix near the crack, the study and analysis of the high-temperature mechanical properties of the steel itself, and combined with the actual production situation, the causes of the corner cracks were clarified and the process optimisation was carried out accordingly. The results of the study show that the generation of corner cracks is due to the improper deoxidation process, uneven heat transfer inside the crystallizer and inappropriate cooling water parameters in the steelmaking process. By changing the deoxygenation method, crystallizer protective slag performance, crystallizer cooling water volume, water volume in the second cooling zone and other measures, the occurrence of billet corner cracks was effectively reduced, and the frequency of billet corner cracks was reduced from 6.3 to 2/m.

An Investigation of the Energy-Absorption Characteristics of Thin-Walled Polymer Composite C-Channels: Experiment and Stacked Shell Simulation.

Polymer composite materials are increasingly used in civil aircraft structures. The failure mode and energy-absorption characteristics of polymer composite structures have garnered significant attention from academia and industry. For thin-walled polymer composite C-channels with layups of [0/90]3s, [45/-45]3s, and [45/90/-45/0]3, low-speed axial compression tests were performed to investigate the failure modes, failure mechanisms, and energy-absorbing characteristics. After parametric studies using [0] and [90] single-element models, stacked shell models of thin-walled composite C-channels were established using the Lavadèze single-layer damage constitutive model, Puck 2000, and Yamada Sun failure criteria. The results show that these thin-walled composite C-channels exhibit a stable progressive crushing process with a local buckling failure mode, encompassing local buckling, fiber break-age, matrix cracks, delamination, and corner cracking. The stacked shell model demonstrates reasonable agreement with the progressive crushing process of thin-walled composites, accurately capturing interlayer matrix failure and interface delamination cracking behavior. A comparison of the specific energy absorption (SEA) and mean crushing force (Fmean) between the simulation and test results yields a difference of less than 6%, indicating a strong correlation between the simulation results and the experimental energy-absorbing characteristics. It also shows that a deep understanding of the parameters is helpful for accurate numerical modeling.

Formation Mechanism and Control Strategy of Transverse Corner Cracks in Continuous Casting of 55SiCr Spring Steel

An experimental study is conducted to reveal the formation mechanism of transverse corner cracks by analyzing the metallurgical structure, crack morphology, and hot ductility. The results reveal that the corner temperature of the billet tends to be lower during mold solidification, leading to the formation of grain boundary low‐plasticity ferrite. Additionally, during the straightening process, the corner temperature of the billet falls into the serious low‐temperature brittle zone with temperature lower than 850 °C. These two factors contribute to the formation and propagation of cracks along the grain boundary ferrite under straightening forces. Therefore, after the verification of mold simulation test and industrial trial, the temperature at billet (section size 150 × 150 mm2) corner is improved by increasing the mold chamfer radius from 6 to 12 mm. This adjustment ensures that the straightening temperature at billet corner elevated from 830 to 880 °C, remaining outside the serious low‐temperature brittle zone, and the maximum depth of corner cracks decreases from 741.49 to 344.89 μm.

The Crack Control Strategy Is Influenced by the Continuous Casting Process of Cr12MoV Steel

This study identifies optimal cooling rates and critical strain thresholds to prevent cracking in Cr12MoV billets during continuous casting through thermoplastic experiments, high‐temperature tensile tests, and a thermal–mechanical coupled simulation model. The result shows that the critical strain (dimensionless) for corner crack propagation at temperatures of 750, 800, and 850 °C are determined to be 0.125, 0.132, and 0.147, respectively. The critical strain for internal crack formation is found to be 0.03. The simulation results indicate that as the billet enters the first straightening roll, the corner strain is 0.035 and the central strain is 0.025. Upon exiting the fourth straightening roll, the corner strain increases to 0.038, while the central strain decreases to 0.021. Through simulation and high‐temperature confocal in situ observation, it can be concluded that a surface cooling rate of ≥5 °C s−1 effectively prevents cracks during the bending and straightening stage of continuous casting.

A physics knowledge-based neural network method for three-dimensional fracture mechanics of attachment lugs

The lug type joint serves as a critical connecting structure in aerospace vehicles, allowing for convenient assembly and disassembly, and transfer the load during service. However, cracks often initiate at the stress concentration location near the lugs hole, posing a significant threat to the safety of the aircraft structure. Therefore, evaluating the stress intensity factors for complex lug structures is of great significance. In this study, a physics knowledge-based neural network method of calculating the stress intensity factors of attachment lugs is proposed. Three types of cracked lug structures are analyzed: through-thickness crack in straight lug, quarter elliptical corner crack in straight lug and quarter elliptical corner crack in tapered lug. Various complex influencing factors in actual situations are also considered, such as the impact of the interference fit between the pin and the lug, as well as different loading directions and crack positions of tapered lugs. The stress intensity factor dataset generated by 3D finite element method, and then the neural network with its powerful learning ability and prior physical knowledge are used to achieve accurate fitting of the data. The results demonstrate that incorporating physical knowledge features significantly improves the model performance of the neural network. This method can be extended to analyze crack in structures with arbitrary geometry and complex loading conditions.

Model for rapid evaluation of corner crack widths in reinforced concrete dapped-end conections

Dapped-end connections are widely used in concrete infrastructure, and in particular in Gerber-type bridges and precast concrete buildings. The main vulnerability of such connections is the early opening of inclined cracks at the re-entrant corner of the dapped end. Such cracks are difficult to control as they propagate at low loads and open significantly before the occurrence of secondary cracks. The goal of this paper is to propose a rational model for rapid evaluation of the width of the corner cracks, including the effects of secondary cracking and, indirectly, the effect of restrained shrinkage at low loads. The model assumptions stem from detailed test observations of specimens with light and heavy reinforcement, arranged in orthogonal or diagonal layouts. The model is developed in two steps: 1) predicting the crack width at yielding of the connection, and 2) capturing the shape of the response from zero load up to yielding. A database of 42 tests with a broad range of variables is used to validate the model. It is shown that the main properties influencing the width of the corner crack are the amount and detailing of the reinforcement, and their effect is well captured by the model. It is also illustrated how the model can be used to design the reinforcement of dapped ends to meet crack width limits at service conditions.

Equilibrium-based analysis of diagonal tension failure from a re-entrant corner of an RC half-joint

For ascertaining the structural longevity of commonly used half-joint girder-to-girder connections in ageing concrete bridges, accurate strength assessments are crucially important. Existing strut-and-tie models (STM) as well as code provisions underestimate resistance to diagonal tension failure from a re-entrant corner crack, perhaps leading to unnecessary maintenance expense. We develop a more accurate equilibrium-based strength analysis of diagonal tension failure based on a faithful representation of the failure surface. We formulate equilibrium equations to represent the internal force system; strength derivation is then optimised for these equations and yield criteria. The accuracy of the new analytical method was validated through comparison with experimentally obtained strengths from 30 specimens. The analytical predictions show good agreement with the test results, with an average error of less than 2%, which is superior to STM-based prediction. Not only strength capacities but also internal force-resisting components are quantified using this analytical method. Results indicate a dependency among all internal force-resisting components and a non-dimensional parameter of a1/hd.

Structural integrity analysis with crack propagation for reactor pressure vessel nozzles based on XFEM

Structural integrity analysis for reactor pressure vessel (RPV) nozzles has been a vital issue due to the irradiation embrittlement and especially the discontinuity of geometry in RPV nozzles, which may cause a higher stress concentration and influence the structural integrity of RPV nozzles. For the linear elastic fracture mechanics (LEFM) analysis of the RPV nozzle, a postulated crack with a specific crack depth is presumed to evaluate the stress intensity factors (SIFs) whose crack depth does not grow during LEFM analysis. Thus, this paper would discuss the crack propagation for the postulated crack with a specific crack depth under the different reference temperature of nil-ductility transition (RTNDT), and the SIFs on the specific crack depth are also calculated. In recent research, the extended finite element method (XFEM) was developed, which is based on the enrichment solution space to simplify the computational fracture mechanics with the crack propagation of various discontinuities, and can be used to evaluate the crack propagation of RPV nozzle with a specific crack depth under RTNDT of −23.89, −10.89, 0.89, 2.69, and 2.89 °C. To investigate the crack propagation and structural integrity of RPV nozzles, the finite element analysis of Taiwan domestic boiling water reactor (BWR) nozzles containing the postulated circular corner crack with depths of one-fourth and one-tenth thickness along the 45° path from the tip of the nozzle corner is engaged. The present result shows that the crack was growing when the RTNDT was 0.89 °C and. In addition, the XFEM calculation of the SIFs for the specific crack depth is similar to the LEFM results.

Study on instability mechanism of soft rock roadway and pressure-relief bolt-grouting support technology

Aiming at the engineering problem of roadway deformation and instability of swelling soft rock widely existed in Kailuan mining area, the mineral composition and microstructure of such soft rock were obtained by conducting scanning electron microscopy, X-ray diffraction experiments, uniaxial and conventional triaxial tests, and the law of softening and expanding of such soft rock and the failure mechanism of surrounding rock were identified. The combined support scheme of multi-level anchor bolt, bottom corner pressure relief and fractional grouting is proposed. The roadway supporting parameters are adjusted and optimized by FLAC3D numerical simulation, and three supporting methods of multi-layer anchor bolt, bottom corner pressure relief and fractional grouting are determined and their parameters are optimized. The study results show that: the total amount of clay minerals is 53–75%, pores, fissures, nanoscale and micron layer gaps are developed, providing a penetrating channel for water infiltration to soften the surrounding rock; the three-level anchor pressure-relief and grouting support technology can control the sinking amount of the roof within 170 mm, the bottom drum amount within 210 mm, the bolts of each level is evenly distributed in tension, and the maximum stress and bottom drum displacement in the pressure relief area are significantly reduced; the pressure-relief groove promotes the development of bottom corner cracks, accelerates the secondary distribution of peripheral stress, and weakens the effect of high stress on the shallow area. Using time or displacement as the index, optimizing the grouting time, filling the primary and excavation cracks, blocking the expansion and softening effect of water on the rock mass, realizing the dynamic unity of structural yielding pressure and surrounding rock modification, has guiding significance for the support control of soft rock roadway.

A Drop-in High-Temperature Pb-Free Solder Paste that Outperforms High-Pb Pastes in Power Discrete Applications

Sn-based high-temperature lead-free (HTLF) solder pastes have been developed as a drop-in solution to replace the high-Pb solder pastes in power discrete applications. The pastes were designed to combine the merits of two constituent powders. A SnSbCuAg powder, with the melting temperature above 320°C, was designed to maintain a high-temperature performance. A SnAgCuSb powder, with a melting temperature around 228°C, was added to the paste to enhance wetting and improve joint ductility. In the design, the final joint will have the low-melting phase (the melting temperature >228oC) in a controllable quantity embedded into the high-melting SnSbCuAg matrix. HTLF-1, one of the designs, maintained the bond shear strength up to 15MPa, even around 290°C. Another design, HTLF-2, has a similar bond shear strength as Pb92.5/Sn5/Ag2.5 around 290°C, but exceeds substantially below 250°C. The power discrete components had been built with both HTLF solder pastes for both die-attach and clip-bond through the traditional high-Pb process, which demonstrated the drop-in processing compatibility. The components survived three additional SMT reflows (peak temperature of 260°C) and passed moisture sensitivity level 1. This confirmed that the maintained joint strength (comparable to or stronger than high-lead), helped to keep the joint integrity within the encapsulated components, even with melting phases in a controllable quantity existing above 228°C. Both HTLF solder pastes outperformed Pb92.5/Sn5/Ag2.5 in RDS(on) even after 1000cycles of TCT (-55/175oC), which is attributed to the intrinsic lower electrical resistivity of Sn of both HTLF pastes. Microstructural observation had shown no corner cracks for both die-attach and clip-bond joints after TCT.

A Comparative Investigation of Random Forest Regression and Artificial Neural Networks for Predicting Crack Growth Life of a Fighter Aircraft Wing Joint Under Spectrum Loading

Estimating fatigue life is challenging due to the input parameters’ statistical natures, such as the manufacturing process, scatter of service loads, microstructure, etc. Regarding fatigue life calculation in the aerospace industry, the importance of an accurate estimation becomes more critical due to strict safety, certification, service costs, and competitiveness regulations. The ability of soft computing methods to reveal complex relationships between multiple parameters and their computational speed could help predict fatigue life, especially in the service. This study compares random forest regression and artificial neural network methods to estimate the crack growth life of a fighter jet aircraft wing joint in terms of their computational time and accuracy. In addition, permutation feature importance and hyper parameter optimisation studies are conducted to extract essential features, investigate their effects on estimation performance, and fine-tune model parameters. The analysed joint is made of 7050-T7451 aluminium, widely used as a structural element in the aerospace industry. Since a hole is one of the major sources of stress concentration, and there may be many holes involved in any engineering structure, it is reasonable to assume that fatigue cracks may initiate at some of these holes during the service life of engineering structures. The crack type considered is a thru-crack around a hole, which is more severe than a corner crack. Load spectra are derived using the Fighter Aircraft Loading Standard for Fatigue (FALSTAFF) to calculate crack growth life. Considering particular service load conditions, ninety different spectra are developed, and the crack growth life of the joint is calculated based on linear elastic fracture mechanics correspondingly. Also, to simulate the worst-case scenario, friction between members and the retardation effect of load spectra are not considered when calculating crack growth life. Python’s Tensor Flow and Scikit-learn libraries are utilised to build machine learning models......

Geometry correction factor polynomials for corner crack test pieces accounting for constrained boundary conditions

Geometry correction factor polynomials for corner crack test pieces accounting for constrained boundary conditions

A Combined Numerical–Analytical Study for Notched Fatigue Crack Initiation Assessment in TRIP Steel: A Local Strain and a Fracture Mechanics Approach

In the fatigue design of metallic components using the safe-life approach, fatigue crack initiation as a development of slip systems at the nanoscale, followed by microstructurally short crack growth, is critical for the onset of structural failure. The development of reliable analytical tools for the prediction of crack initiation, although very complex due to the inherent multiscale fatigue damage processes involved, is important for promoting a more sophisticated design but, more importantly, enhancing the safety in regard to fatigue. The assessment of fatigue crack initiation life at the root of a V-shaped notch is performed by implementing a local strain and a fracture mechanics concept. In the low cycle fatigue analysis, the finite element method is used to determine the local stress–strain response at the notch root, which takes into account elastoplastic material behavior. Fatigue crack initiation is treated as the onset of a short corner crack by incremental damage accumulation and failure of a material element volume at the notch root. The finite element results are compared against established methodologies such as the Neuber and strain energy density methods. In the fracture mechanics approach, fatigue crack initiation is treated as the onset and propagation of a corner crack to a finite short crack. Fatigue experiments in two different transformation-induced plasticity (TRIP) steels were conducted to evaluate the analytical predictions and to determine the physical parameters for the definition of crack initiation. The analytical results show that the finite element method may be successfully implemented with existing fatigue models for a more accurate determination of the local stress–strain behavior at the notch tip in order to improve the assessment of fatigue crack initiation life compared to the established analytical methodologies.

Mitigation of Corner Cracking in Continuously Cast Steel Slabs Through Strain Induced Crack Opening Test

Mitigation of Corner Cracking in Continuously Cast Steel Slabs Through Strain Induced Crack Opening Test

Influence of interaction among multiple cracks on crack propagation in nozzle corner of reactor pressure vessel

This paper analyzes the interaction effects and the propagation (growth) behavior among three semi-elliptic corner cracks. Those interacting cracks are assumed to be in the stress concentration area of the inlet of the AP1000 reactor pressure vessel (RPV). The results suggest that the dominant crack will stop first under the interaction. One of the subsidiary cracks will become a long crack while the other will become a short crack. In the simulations, the interaction factor (γ) of the dominant crack and the stressintensityfactors (SIFs) of the subsidiary cracks are calculated respectively by changing the horizontal and vertical distances. The results show that the value of γ decreases with the increase of the propagation steps. With larger horizontal distances among the cracks, the maximum value of γ will rise. In addition, the maximum value of γ will rise if the subsidiary cracks are moved up vertically. Otherwise, the maximum value of γ will decrease. When the cracks get closer together, the increasing rate of the SIF of the long crack will increase while the increasing rate of the short crack will decrease, whether the subsidiary cracks are moved up or down.

Formation Mechanism and Control Technology of Transverse Corner Cracks During Slab Continuous Casting of Microalloyed Steels

Microalloyed steel with Nb, V, Ti, and other alloying elements is one of the important steel grades in the steel industry. During continuous casting process, high occurrence rate of transverse corner cracks in slabs restricts the quality and efficiency of the production of microalloyed steel grades, and it has not been effectively solved for a long time. Herein, the research progress on the formation mechanism, influential factors, and typical control technologies of transverse corner cracks during slab continuous casting are reviewed. Considering high‐temperature solidification process of microalloyed steel, the control of growth of microstructures and the precipitation behavior of carbonitrides in slab corners are supposed to be the key to avoid transverse corner cracks during slab continuous casting.

Effects of angles and shapes of a corner crack on the driving force at a set-in nozzle-cylinder in a PWR pressure vessel

Corner cracks are one of the common forms of flaws in the reactor pressure vessels (RPVs) of nuclear power plants (NPPs). Accurate evaluation of the driving force of such corner cracks is essential and critical for the efficient design and integrity assessment of the RPVs. In this study, the combined effect of the depth, length and angle of a given corner crack under the service loading was analyzed using elastic–plastic finite element analysis (EPFEA). Based on the consideration of the distribution profiles of stress intensity factor (SIF) and J integral as the crack growth along corner crack front, the crack growth stability with various crack shapes and angles was analyzed. Finite element analysis (FEA) results showed that the crack growth driving force is affected by the shape and angle of the initial crack. It is also found that the sliding mode (mode II) crack or tearing mode (mode III) crack should be taken into account in the structural integrity analysis of RPVs.

Experimental Behavior of L-Shaped and T-Shaped Cross-Laminated Timber to Evaluate Shear Walls with Openings

There is increasing interest in using cross-laminated timber (CLT) in buildings because of its high strength and stiffness. In Japan, structural design guidelines for CLT buildings were established in 2016 and construction of mid-rise buildings is increasing. Wide-panel walls can exceed widths of 10 m and integrate cut-outs for window and door openings. However, under lateral loads, corner cracks at the openings have been the most prevalent failure mechanism. To investigate the initiation and propagation of corner cracks, a series of bendings are undertaken on L- and T-shape specimens extracted from the CLT panels. In addition, three-point bending and shear tests are also carried out on beam sections extracted from the CLT panels. Three types of brittle failure were observed: bending failure of the beam or column, and rolling shear failure.