Crack Propagation Research Articles

USAF Characteristic K Approach: A Robust Tool for Predicting Fatigue Crack Growth under Various Underload Spectra.

This paper forms part of an ongoing investigation into the tools required in linear elastic fracture mechanics (LEFM) for evaluating the durability of components designed for limited life replacement. In this study, we demonstrate that the USAF 'Characteristic K' method, when combined with the Hartman-Schijve adaptation of the NASGRO crack growth formula, can predict the impact of underloads on the propagation of small cracks in aluminum alloy AA7050-T7451 with reasonable accuracy. The published da/dN versus ΔK small crack growth curves associated with five specific underload spectra are examined. It is found that, in each case, there is reasonably good agreement between the predicted and the measured curves. To the best of the author's knowledge, this paper is the first to highlight the ability of the USAF Characteristic K approach, when coupled with the Hartman-Schijve equation, to reasonably accurately predict the growth of small cracks subjected to a range of underload spectra.

Experimental study on the mechanical characteristics of weakly cemented mudstone under different loading rates

With the gradual shift of coal mining to the western coal mining region of China, floor heave in weakly cemented mudstone roadways has become an issue affecting the safety and efficiency of coal mine production. Additionally, different mining rates can lead to fluctuating support stresses on the roof and floor of weakly cemented mudstone roadways. Therefore, obtaining a comprehensive understanding of the mechanical properties of weakly cemented mudstone at different loading rates is conducive to improving the issue of floor heave in such roadways and provides a theoretical basis for further study. In this context, a series of uniaxial mechanical tests with concurrent acoustic emission monitoring were conducted on specimens of weakly cemented mudstone under various loading rates (0.005, 0.01, 0.05, and 0.1 mm/s). The stress‒strain and acoustic emission response curves were obtained to effectively characterize the strength, deformation, damage, macroscale instability, and crack propagation characteristics of the mudstone under the influence of loading rate effects. The research results support the following findings: (1) With increasing loading rate, the peak strength and elastic modulus of weakly cemented mudstone significantly increase, while the peak axial strain and peak radial deformation significantly decrease. (2) With increasing loading rate, the stress required to trigger the expansion of weakly cemented mudstone gradually increases, and a significant power-law relationship arises between the strain of the mudstone at the start of expansion and the loading rate. (3) With increasing loading rate, the acoustic emission ringing count of weakly cemented mudstone increases: The failure of weakly cemented mudstone changes from small-range progressive failure to sudden failure, and the failure mode transitions from shear failure to tensile‒shear composite failure. (4) The studied mudstone damage variables increase with increasing loading rate, following an approximate exponential function. The conclusions obtained in this work can provide a theoretical basis for the evolution mechanism and control of floor heave in deep roadway mining.

Structural performance of fibre reinforced recycled aggregate concrete road kerb sections under monotonic and cyclic loading

In recent years, there has been a notable emphasis on waste reduction and the adoption of recycled materials within the construction industry to reduce the industry’s overall carbon footprint. This study investigates the structural performances of concrete kerb sections prepared with five different concrete mixes containing recycled concrete aggregate, recycled tyre-derived aggregates and recycled polypropylene fibres. Kerb sections were cast at a road site in a suburb of Adelaide, Australia. After the concrete hardened, sections were cut and brought to the laboratory. A large number of monotonic and cyclic load tests were conducted on the kerb sections. The load-carrying capacity, bending moment capacity, cyclic fatigue capacity, durability properties along with deformation tolerance were evaluated. Kerb sections made with concrete containing recycled aggregate and polypropylene fibre could sustain nearly 2000 cycles of loading. Kerb sections prepared with natural aggregate concrete performed comparatively better. The addition of polypropylene fibre significantly improved the post-cracking behaviour of kerb sections and can delay crack propagation and other distress when subjected to cyclic loadings such as excessive soil movement, e.g., in areas with expansive soils or prone to tree root migration. Long-term observation may be required to confirm the mechanical and durability performance improvement in real field conditions.

Towards accurate prediction of the dynamic behavior and failure mechanisms of three-dimensional braided composites: A multiscale analysis scheme

In this paper, a dynamic multiscale model is proposed for three-dimensional four-directional (3D4D) braided composites, considering the effects of strain rate and shear enhanced failure mechanism. The strain rate-dependent constitutive models are established for both resin and yarns. The accuracy of the yarn model is validated by comparing the simulated stress-strain curves with the experimental results obtained from the Split Hopkinson Pressure Bar (SHPB) tests with various strain rates. The shear enhancement coefficient of the yarn is determined from the failure envelopes with a discussion of crack propagation angles. The results show that, based on the calculated microscale properties, the mesoscale model accurately predicts both the longitudinal and transverse compressive strain-stress behavior of 3D4D braided composites at different strain rates. A comparative analysis reveals that the shear-enhanced failure model achieves a better prediction compared with other counterparts, such as Hashin-Rotem and Tsai-Wu Models. The proposed dynamic multiscale scheme for 3D braided composites is an efficient and accurate tool to characterize their multiscale features and strain rate-dependent behavior.

A hybrid finite-discrete element method for modelling cracking processes in sandy mudstone containing a single edge-flaw under cyclic dynamic loading

Rock mass deformation and failure are macroscopic manifestations of crack initiation, propagation, and coalescence. However, simulating the transition of rocks from continuous to discontinuous media under cyclic dynamic loading remains challenging. This study proposes a hybrid finite-discrete element method (HFDEM) to model crack propagation, incorporating a frequency-dependent cohesive-zone model. The mechanical properties of standard sandy mudstone under quasi-static and cyclic dynamic loading were simulated using HFDEM, and the method's reliability was verified through experimental comparison. The comparative analysis demonstrates that HFDEM successfully captures crack interaction mechanisms and accurately simulates the overall failure behavior of specimens. Additionally, the effects of pre-existing flaw inclination angle and dynamic loading frequency on rock failure mechanisms were investigated. The numerical results reveal that rock samples exhibit significantly higher compressive strength under dynamic loading compared to quasi-static loading, with compressive strength increasing with higher cyclic dynamic load frequencies. Furthermore, by analyzing the strength characteristics, crack propagation, and failure modes of the samples, insights into the failure mechanisms of rocks under different frequency loads were obtained. This study provides valuable insights into crack development and failure of rocks under seismic loads, offering guidance for engineering practices.

On crack simulation by mixed‐dimensional coupling in GFEM with global‐local enrichments

AbstractA strategy that combines the global‐local version of the generalized finite element method (GFEM) with a mixed‐dimensional coupling iterative method is proposed to simulate two‐dimensional crack propagation in structures globally represented by Timoshenko‐frame models. The region of interest called the local problem, where the crack propagates, is represented by a 2D elasticity model, where a fine mesh of plane stress/strain elements and special enrichment functions are used to describe this phenomenon accurately. A model of Timoshenko‐frame elements simulates the overall behavior of the structure. A coarse mesh of plane stress/strain elements provides a bridge between these two representation scales. The mixed‐dimensional coupling method imposes displacement compatibility and stress equilibrium at the interface between the two different element types by an iterative procedure based on the principle of virtual work. After establishing the constraint equations for the interface, the 2D elasticity model is related to the small‐scale model by the global‐local enrichment strategy of GFEM. In such a strategy, the numerical solutions of the local problem subjected to boundary conditions derived from the global‐scale problem enrich the approximation of this same global problem in an iterative procedure. Each step of the crack propagation requires a new sequence of this iterative global‐local procedure. On the other hand, the constraint equation for the interface is defined only once. The crack representation in a confined region by the global‐local strategy avoids a remeshing that would require new constraint equations. Two numerical problems illustrate the proposed strategy and assess the influence of the analysis parameters.

Origin of the enhanced creep properties of high Nb containing TiAl alloys with the addition of C and Y2O3

In this paper, Ti–43Al–6Nb–1Mo–1Cr, Ti–43Al–6Nb–1Mo–1Cr-0.5C and Ti–43Al–6Nb–1Mo–1Cr-0.5C-0.05Y2O3 alloys were prepared by induction skull melting under argon atmosphere. Creep tests were carried out on three alloys to reveal the effects of C and Y2O3 on the creep properties of alloys and to clarify the precipitation behavior of Ti2AlC and B2 phases. The results show that adding C and Y2O3 can significantly reduce the steady-state creep rate and improve the creep properties of the alloy. After adding C and Y2O3, the parallel decomposition of α2 lamellae occurs, and the fine γ lamellae precipitates from coarse α2 lamellae. The lamellar degradation occurs obviously during creep, and the reaction is α2→γ+B2. The B2 phase preferentially grows into α2 lamellae after nucleation at the lamellar interface. Furthermore, the precipitated B2 phase can induce the formation of stacking faults in α2 lamellae. The strengthening mechanism of C is mainly solid solution strengthening and the precipitation hardening. The dynamically precipitated Ti2AlC during creep can hinder the movement of dislocations and improve the bonding strength of the lamellar interface. Y2O3 mainly plays a role of second phase strengthening, which can share the stress of the matrix and prevent the crack propagation. Finally, the voids nucleation and crack propagation process of the alloy are analyzed and summarized.

Investigation into the mechanisms of Corrosion-Induced rolling contact fatigue crack initiation and propagation in pearlitic rails

Corrosion stands as a pivotal causative agent for the initiation and propagation of rolling contact fatigue (RCF) cracks on rail surfaces. This study aims to scientifically delineate the corrosion fatigue behavior of U75V rails. The study identifies the presence of oxidative corrosion and graphitization on the rail surface. Carbides near the rail surface cause stress concentration, reducing the rail’s RCF residence. Oxide corrosion engenders internal oxide inclusions within cracks, expediting RCF crack propagation. Furthermore, grain refinement and discordant deformation between soft and hard grains near the rail surface give rise to sub-surface microporosity and microcracking. These findings advance our comprehension of rail RCF and offer insights for guiding rail maintenance and informing new rail design strategies.

Influence of Hydrogen on the Failure Mechanism of Standard Duplex Stainless Steel X2CrNiMoN22-5-3 Exposed to Corrosion Fatigue

IIn a geothermal environment, cathodic protection is employed to improve resistance against corrosion fatigue. However, during the cathodic reactions under applied potential, hydrogen is generated and assimilated, leading to a reduced lifetime expectancy of high-alloyed steels. The corrosion fatigue mechanism of a standard duplex stainless steel X2CrNiMoN22-5-3 (1.4462) specimen loaded with hydrogen was studied in a corrosion chamber specifically designed for the purpose, surrounded by the electrolyte of the Northern German Basin at 369 K. The microstructural reactions resulting in hydrogen incorporation significantly decrease the number of cycles to failure of the specimen. This reduction is attributed to hydrogen enhancing crack propagation and causing early failure, primarily due to the deterioration of the mechanical properties of the ferritic phase rather than corrosion reactions or corrosive degradation.

Short Jute Fiber Reinforced Cement Mortar for Out-of-Plane Strengthening of Masonry Prisms

The retrofitting process contributes to the sustainability of the construction sector, since adopting measures to increase the lifespan of buildings reduces the need for new constructions. However, many of the materials used in this process come from nonrenewable sources and require significant water and energy consumption for production. The aim of this study is to assess the viability of using a more environmentally friendly mortar coating reinforced with short jute fibers (SJFRM) to reinforce ceramic brick masonry walls. Both coated and uncoated prisms were subjected to compression and flexural tests under two-point (line) out-of-plane loading. The reinforcement layer comprised mortar without fibers and mortars reinforced with jute fibers at levels of 2% and 4%, with lengths of 20 mm and 40 mm. Physical and mechanical tests were conducted to evaluate the properties of SJFRM in both fresh and hardened states. Results indicate that the compressive and flexural strengths were enhanced with SJFRM reinforcement due to alterations in the failure mode of the prisms. The fibers impede crack propagation in the reinforcement layer, enabling better redistribution of internal stresses in the prisms. This results in an increase of 6 to 9 times in stiffness under direct compression and up to 42 times in toughness under flexion in the prisms reinforced with SJFRM when compared to uncoated prisms.

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.

Resistance Analysis of Crack Propagation in Concrete Subjected to Hydraulic Pressure.

The KR resistance curve for hydraulic crack propagation in a concrete beam was determined and discussed. A semi-analytical method was introduced to calculate the hydraulic crack propagation in concrete. A series of concrete beams with various hydraulic pressures and initial crack depths were tested, and the hydraulic crack propagation in these beams was calculated. The calculated P-CMOD curves were first verified, and then the calculated KR resistance curve for hydraulic crack propagation was determined. Based on the test results and calculation results, the following conclusions can be drawn: The proposed analysis method can accurately predict the hydraulic crack propagation process in concrete. The KR resistance to hydraulic crack propagation in concrete decreases with the increase in hydraulic pressure but is less influenced by the initial crack depth of the test beams. In addition, the concrete beams collapse immediately under hydraulic fracturing once the KIw curve reaches the KR resistance curve. This indicates that the failure of concrete structures under hydraulic fracturing occurs immediately once the driving force of crack propagation, dominated by the hydraulic pressure in the crack, becomes significant.

Using a Negative Poisson's Ratio to Mitigate Stress Concentrations in Perforated Composite Plates

Stress concentrations pose a significant challenge in designing and optimizing composite components, often resulting in structural alterations and crack formation. This study delves into a detailed numerical and analytical analysis of stress concentration factors in composite materials, with a specific focus on the influence of negative Poisson's ratio and out‐of‐plane modulus. By exploring the interaction of these material properties, the objective is to devise effective strategies for alleviating stress concentrations in composite structures. Through analytical and numerical simulations, a robust correlation between the customized Poisson's ratio and modulus and the mitigation of stress concentration factors, particularly in epoxy/graphite‐reinforced composites are established. This tailored approach has the potential to enhance energy absorption capabilities and consequently reduce the risk of crack propagation in perforated laminated composite plates. This research findings offer valuable insights into composite material design, presenting innovative solutions for enhancing structural integrity and reducing susceptibility to stress‐related issues.

An investigation into crack initiation and extension mechanism of laminar plasma quenched-tempered rail material with different quenched zones

Laminar Plasma Quenching Tempering (LPQT) is an innovative surface treatment technique for rails, which effectively enhances both their fatigue resistance and wear resistance. This study examines the crack initiation and propagation mechanisms in LPQT-treated rail materials with distinct quenched zones using double-disc testing and metallographic analysis. The findings indicate that the crack initiation and propagation process in LPQT rails with different quenched zones is characterized by four stages. Moreover, the grain size within the spot-quenched zone of LPQT rails is notably smaller, which results in a reduced resistance to RCF crack propagation. Consequently, the crack propagation in point-quenched LPQT rail material is more intricate, with a more severe damage scenario.

Surface-Initiated Rolling Contact Fatigue on a Dent: Microstructural Evolutions through Failure Mechanism

The indentation of bearing raceways frequently contributes to bearing failures, as in gearboxes. The dents act as surface stress raisers through the Rolling Contact Fatigue (RCF) cycles, leading to crack initiation and spalling. The microstructural evolution mechanism occurring is not completely understood. Hence, to enhance bearing lifetime, it is crucial to identify microstructural parameters that influence crack initiation and propagation. Therefore, this study presents a multi-scale characterization of martensitic 100Cr6 bearings before and after RCF. It includes SEM, SEM/FIB, EBSD, TKD and ACOM-ASTAR observations. It showed that, before RCF, the bearing raceway features a 0.5 µm thick surface layer consisting in refined martensite and spread primary carbides, resulting from finishing operations. After RCF, no further grain refinement was observed along the raceway, except beneath the dent shoulder where a material plastic flow, nano-grains and ultra-fine grains of martensite, and sheared and spread primary carbides were detected up to 1.4 µm deep. Below and up to 3 µm deep, a fine grain layer is observed. The martensite morphology, size and disorientation suggest that it undergoes refinement through continuous Dynamic Recrystallisation and twinning. Crack initiation was preferentially observed in the nano-grains layer, at primary carbides/matrix interfaces. Above the crack, another refined region is observed resulting from high deformation and stresses through crack propagation. Finally, even in severely deformed areas, nanosized retained austenite islands were detected, indicating that it is not completely transformed.

Heterogeneity induced fracture characterization of concrete under fatigue loading using digital image correlation and acoustic emission techniques

Understanding the fatigue failure mechanisms in concrete is complex and remains an area of ongoing investigation. Material heterogeneity, along with the size effect, plays a significant role in specifying the crack propagation behaviour and fatigue life in case of repetitive loading conditions. In this study, the influence of material heterogeneity on the fatigue behaviour of concrete has been examined. Three different-size beam specimens with varying heterogeneity (aggregate sizes) have been considered to investigate various fracture characteristics under repetitive load conditions. Acoustic emission and digital image correlation techniques have been employed to analyze crack growth behaviour. The results revealed that stiffness degradation and crack propagation rates are faster for the specimens with smaller aggregate sizes compared to the larger aggregate size specimens of the same depth. Moreover, the fatigue life and effective crack length at failure are also higher in the case of larger aggregate sizes compared to smaller ones. Therefore, a heterogeneity-adjusted, analytical model of fatigue crack growth has been proposed. Further, the AE parameters have been utilized to characterize the tensile and shear crack dominance in different stages of fatigue crack propagation. The outcome of this study can be utilized to anticipate the crack propagation behaviour and residual fatigue life for any combination of specimen size and aggregate size.

Interfacial microstructure characteristics and failure mechanism of the laser welding-brazing steel-Al joints with various welding parameters

The microstructure details of the interface, mechanical properties and failure mechanism of laser welding-brazing (LWB) steel-Al joints with various welding parameters were comparatively investigated. The emergence of ternary Fe–Al–Si intermetallic compounds (IMCs) was observed in the interfacial area of the joints prepared utilizing ER4047 filler. In contrast, only the binary Fe–Al IMCs were formed in the interfacial area of the joints prepared employing ER5356 filler. With the same laser tilt angle, the IMCs layer of the joints prepared with ER4047 was thinner than that of the joints prepared with ER5356, implying the rapid growth of the IMCs welded via the ER5356 filler. The tensile strength exhibited by the former surpassed that of the latter. Employing the same filler metal, the average thickness of the IMCs layer of the joints with laser tilt angle of 10° was smaller than that of the joints with laser tilt angle of 20°. The strength of the joints with smaller tilt angle was better than that of the ones with bigger tilt angle. The tensile test outcomes revealed that there existed failure competition among the Al alloy base metal (Al-BM), weld metal (WM) and interface structure. If the interface structure was Fe–Al IMC with high brittleness, the cracks propagated along the interface, and finally fractured at the interface. If the interface structure was Fe–Al–Si IMC with good toughness, the crack propagation in the interface region could be hampered. In this case, the joint was prone to occur via WM failure or even Al-BM failure modes.

Fracture behaviour of reaction-bonded silicon carbide-boron carbide using digital image correlation

The current research uses the Digital Image Correlation (DIC) approach to study the fracture behaviour of Reaction-Bonded Silicon Carbide-Boron Carbide (SiC–B4C) ceramic (RBSBC). A Photron camera with 137,500 fps was used to capture the crack propagation at an interframe time frame of 2.6 ms before final fracture. The purpose was to capture the rapid brittle behaviour of the ceramic during the onset of crack propagation. The RBSBC failed in a transgranular manner like most monolithic SiC-based materials as Hexoloy-SA. The presence of randomly dispersed coarse B4C grains helps in deflecting the crack path which increases the overall crack length. The material’s resistance to crack propagation (R-curve) was measured using the crack tip opening displacement, the stress intensity at the crack tip KI, and the J-integral against crack extension. Two methods were used to determine the KI: experimentally based on the displacement values at the crack tip as obtained from the DIC images, and quasistatic based on the fracture load and the geometrical dimension of the crack. The experimentally-determined KI and J resulted in higher values compared to the quasistatic ones. However, they reflect the actual conditions at the crack tip, while the quasistatic conditions underestimate the resistance-plot. The RBSBC has higher R-curve compared to commercially available SiC-only materials as Hexoloy-SA and ABC-SiC. To improve the crack-resistance of RBSBC, the Si amount should decrease to a nm level, while the percentage of B4C grains should increase and be evenly dispersed across the material.

Assessment of resistance to crack propagation of asphalt mixtures using two parts digital image correlation

The use of recycled aggregates in the manufacturing of asphalt concrete is becoming increasingly widespread. This requires special attention to characterize their performance in terms of durability to reduce major maintenance and rehabilitation expenditures. To achieve this, it is necessary to use effective and easy-to-implement test procedures to observe and estimate the various damage and cracking processes that often lead to the structural failure of materials. Many studies have shown that the estimation of cracking energy depends on the extent of damage and cracking processes. In this context, the method of estimating the crack propagation path and the accompanying damage strongly influence the calculated value of the fracture energy. This can lead to an overestimation or underestimation of the material’s performance. Based on these considerations, our aim is to introduce a new approach for improving the estimation of cracking processes to assess resistance to crack propagation. A three-point bending test was conducted on recycled bituminous concrete samples with a single notch at two distinct temperatures, −5 °C and 10 °C. The extent of the cracking process and damage was estimated using 2PDIC, a two-part digital image correlation method. The parametric analysis of the measurements obtained by 2PDIC reveals that defining the pseudo-strain threshold enables the observation of different amplitudes and levels of strain associated with the various processes that accompany the cracking test. The results obtained for the two temperatures considered demonstrate a significant change in cracking energy, which is often overestimated and depends on the crack propagation path and the granular arrangement of the material through which the crack passes. The measurements obtained using 2PDIC also demonstrate the greater tortuosity of the crack propagation path at higher temperatures, which results in a reduction of the estimated cracking energy derived from more simplistic approaches.

Simulation of cracking processes of the rectangular cavern surrounding rock based on a continuum–discontinuum method

In a self-developed continuum–discontinuum method that considers the internal cracking of quadrilateral elements, the computational domain is divided into quadrilateral elements with high accuracy. After cracking, the cracks propagate along the boundaries of the elements and the diagonal lines of the quadrilateral elements. Deformation, cracking, and contact of rocks can be simulated using this method. For Brazilian disc rock specimens, the rapid increase of tensile crack segments corresponds to a rapid decrease of the load. Compared with the original method, crack propagation is smoother in the improved method. This is because the propagation paths of cracks are fewer in the original method, and the propagation of cracks is hindered For uniaxial compressive rock specimens, when two shear cracks intersect, one crack may dominate, forming a main crack, and its direction is consistent with the theoretical direction obtained using the Mohr − Coulomb criterion. For the cavern surrounding rock models, some long and narrow shear cracks appear in the surrounding rock, and their shapes are like logarithmic spirals. An interval exists between the shear cracks, and the more to the interior of the surrounding rock, the larger the interval between cracks. The higher the hydrostatic pressure, the faster the shear crack propagates and the easier it is to form a shear slip line. With an increase of cavern size, the depth and range of the shear cracks increase. With a decrease of the internal friction angle, the interval between the cracks increases, and the propagation direction of the cracks become more internal.