Crack Propagation Research Articles

Optimizing Fatigue Performance in Gradient Structural Steels by Manipulating the Grain Size Gradient Rate

As a new type of high-performance material, gradient structural steel is widely used in engineering fields due to its unique microstructure and excellent mechanical properties. For the prevalent fatigue failure problem, the rate of change in the local grain size gradients along the structure (referred to as the gradient rate) is a key parameter in the design of gradient structures, which significantly affects the fatigue performance of gradient structural steel. In this study, a new method of ‘Voronoi primary + secondary modeling’ is adopted to successfully establish three typical high-strength steel models corresponding to the convex-, linear-, and concave-type gradient rates for gradient structures, focusing on the stress–strain response and crack propagation in structural steel with different gradient rates under cyclic loading. It was found that the concave gradient rate structural model is dominated by finer grains with larger volume fraction, which is conducive to hindering fatigue crack propagation and has the longest fatigue life, which is 16.16% longer than that of the linear gradient rate structure and 23.66% longer than that of the convex gradient rate structure. The simulation results in this study are consistent with the relevant experimental phenomena. Therefore, when regulating the gradient rate, priority should be given to increasing the volume fraction of fine grains and designing a gradient rate structure dominated by fine grains to improve the fatigue life of the material. This study presents a new strategy for designing engineering materials with better service performance.

FORMULATION OF A MODE I CRACK PROPAGATION MODEL BY ANALYZING THE STRESS FIELD AT THE CRACK TIP

One of the phases in the development of materials is knowledge of their possible degradation. At this stage, taking into account the influence of temperature variation remains essential for certain materials. The objective of this work is to take this aspect into account by formulating a mode I crack propagation model by studying the stress field in the vicinity of the crack tip. To do this, starting from Hookes law generalized in linear elasticity, we use the asymptotic expressions in displacements of the crack tip of Moes et Al. in order to obtain the stress field model. To validate the model, a comparison is made with data from the literature. The observations show a very high agreement of results. The advantage of this model is to take into account the role of temperature in the degradation of materials by analyzing crack propagation. It will be used in our future work.

An adaptive phase field approach to 3D internal crack growth in rocks

Modeling the 3D internal crack under compression entails complex fracture mechanics (mode I-II-III fracture), resulting in substantial computational costs and challenges in characterizing fracture morphology characterization for Phase Field Method (PFM) simulations. In the present paper, we developed a 3D adaptive crack propagation model integrated into the PFM to study the internal cracks in rocks. The proposed locally refined solution was implemented within the isogeometric analysis (IGA) framework, utilizing PHT-splines and employing the phase field variable as an error indicator to accurately capture non-smooth crack surfaces. The simulation begins with a coarser mesh, and the spatial discretization is adaptively refined in real-time as the computation progresses and the phase variable threshold is reached. By comparing our results with previous numerical tests, we validated the effectiveness of our model in simulating crack growth. Notably, a smaller length scale parameter led to a higher peak force in notched square plates under tension, and our adaptive PFM successfully reduced errors caused by length scale parameters. We further validated our model by comparing it with uniaxial compression experiments on 3D-printed resin samples with internal penny-shaped cracks, achieving good agreement with the experimental data. Using the 3D adaptive PFM, we predicted changes in crack morphology and fracturing mechanisms by varying the inclination angles of internal penny-shaped cracks. Our numerical results showed that as the inclination angle increased, wing cracks initiated closer to the end tips of the original crack, and their length decreased due to the wings wrapping effect. The corresponding fracturing mechanisms transition from mode I-III fracture (0°) to mode I-II-III fracture (30°, 45°, 60°), and eventually to mode I fracture (90°) as inclination angles increase. Comparing 2D and 3D models, we found that the growth of wing cracks in 3D was significantly influenced by the intermediate principal stress. We conclude that the extended 3D adaptive PFM-based model accurately predicted complex mechanical behavior and crack growth patterns in rock.

Bedding effect on progressive crack propagation in layered sandstone grotto roof

Roof failure significantly threatens the safety and stability of grottoes. For the layered roof of sandstone grottoes, the failure process is closely related to bedding effects of crack propagation. Taking the Yuanjue Cave as an example, the progressive failure process of the layered roof was summarized. The mechanisms of crack propagation in the roof were analyzed theoretically, and bedding effects together with influencing factors of crack propagation were explored. Results showed that the instability process of the layered roof in sandstone grottoes can be summarized into four stages: tensile fracture of the roof, vertical expansion of cracks, horizontal deflection of cracks, and instability collapse of blocks. The development and propagation of cracks constituted the intrinsic cause of the instability of layered roofs. A fracture mechanics model for crack propagation in layered roofs was established considering the bedding effect, and the criteria for vertical expansion of cracks in grotto roofs was derived. Moreover, the fracture criterion of crack competition was modified in combination with the bedding effect, revealing that an increase in interlaminar cohesion inhibited the crack deflection mode. The crack deflection mode in grotto roofs was jointly determined by the stress at the crack-tip (σfxx and σfyy), the strength of the rock layers and bedding plane (σ't, μ and c). To control the crack propagation in roofs, the engineering reinforcement of grotto roofs can be achieved by weakening bedding effects and indirectly reducing the roof span. This study is of great significance for revealing the progressive failure process and optimizing the reinforcement scheme of layered roofs in grottoes.

Investigation on the fatigue crack propagation of rock-concrete interface under fatigue loading below the initial cracking load

The fatigue crack propagation of the composite rock-concrete interface under constant-amplitude flexural fatigue loading below the initial cracking load was investigated in the present study. Fatigue pre-crack initiation and propagation tests were first conducted on three types of rock-concrete interfaces under three-point bend loading. It was shown that for the rock-concrete interface, the damage evolution throughout the fatigue loading process can be divided into deceleration and acceleration stages according to the rate curves of flexural stiffness, deformation and crack propagation. The crack propagation rate exhibited two distinct regions when plotted on a log scale, namely the small-crack growth region and the long-crack growth region. The small-crack growth region for the studied rock-concrete interfaces was not particularly pronounced. The long-crack growth region was the Paris’ law region and included both the acceleration and part of the deceleration stages. Based on the experimental results, the fatigue crack propagation rate in the Paris’ law region was then described using the empirical Paris’ law. The exponent m1 in the Paris’ law was found to be approximately constant, with a value of m1 ≈ 21 for the rock-concrete interface. The pre-factor C1 was found to be dependent on both the load ratio R and the type of the rock-concrete interface (represented by an elastic mismatch parameter α). By introducing R and α into the empirical Paris’ law, an adjusted Paris’ law was proposed. The adjusted Paris’ law can describe the fatigue crack propagation rate in a unified way for the same rock-concrete interface of different R and different interfaces in the scope of this research. Based on the adjusted Paris’ law, the fatigue cycles during the crack propagation and fatigue lifetimes of all tested rock-concrete interface specimens were reasonably predicted. For practical applications, an approach was suggested involving a combination of the adjusted Paris’ law and a material coefficient β (approximately β ≈ 0.143 in this study) to estimate the fatigue lifetimes of structures or specimens with existing pre-cracks at the rock-concrete interface.

A closed-form solution of dowel action based on beam on elastic foundation theory and fracture mechanics

This paper proposes a new mechanical model to describe the dowel action with the aim of using the model to gain a deeper understanding of the unstable dowel splitting cracking observed in shear experiments of beams without shear reinforcement. The model was developed by combining beam on elastic foundation (BEF) theory and fracture mechanics. The proposed model is able to predict the whole evolution process of dowel action until the propagation of the dowel splitting crack becomes unstable. The model theoretically proves that the development of a dowel splitting crack can become unstable under certain conditions, therefore leading to the unstable shear failure of the whole member. In addition to the derivation of the analytical model, the paper also validates the model using data from the literature. Finally, an analytical solution of the critical shear displacement that triggers the unstable dowel splitting crack is derived. It can be used to improve the failure criterion initially proposed in the Critical Shear Displacement Theory (CSDT).

Strong and Tough MXene Bridging-induced Conductive Nacre.

Nacre is a classic model, providing an inspiration for fabricating high-performance bulk nanocomposites with the two-dimensional platelets. However, the "brick" of nacre, aragonite platelet, is an ideal building block for making high-performance bulk nanocomposites. Herein, we demonstrated a strong and tough conductive nacre through reassembling aragonite platelets with bridged by MXene nanosheets and hydrogen bonding, not only providing high mechanical properties but also excellent electrical conductivity. The flexural strength and fracture toughness of the obtained conductive nacre reach ~282 MPa and ~6.3 MPa m1/2, which is 1.6 and 1.6 times higher than that of natural nacre, respectively. These properties are attributed to densification and high orientation degree of the conductive nacre, which is effectively induced by the combined interactions of hydrogen bonding and MXene nanosheets bridging. The crack propagations in conductive nacre are effectively inhibited through crack deflection with hydrogen bonding, and MXene nanosheets bridging between aragonite platelets. In addition, our conductive nacre also provides a self-monitoring function for structural damage and offers exceptional electromagnetic interference shielding performance. Our strategy of reassembling the aragonite platelets exfoliated from waste nacre into high-performance artificial nacre, provides an avenue for fabricating high-performance bulk nanocomposites through the sustainable reutilization of shell resources.

Strong and Tough MXene Bridging‐induced Conductive Nacre

AbstractNacre is a classic model, providing an inspiration for fabricating high‐performance bulk nanocomposites with the two‐dimensional platelets. However, the “brick” of nacre, aragonite platelet, is an ideal building block for making high‐performance bulk nanocomposites. Herein, we demonstrated a strong and tough conductive nacre through reassembling aragonite platelets with bridged by MXene nanosheets and hydrogen bonding, not only providing high mechanical properties but also excellent electrical conductivity. The flexural strength and fracture toughness of the obtained conductive nacre reach ~282 MPa and ~6.3 MPa m1/2, which is 1.6 and 1.6 times higher than that of natural nacre, respectively. These properties are attributed to densification and high orientation degree of the conductive nacre, which is effectively induced by the combined interactions of hydrogen bonding and MXene nanosheets bridging. The crack propagations in conductive nacre are effectively inhibited through crack deflection with hydrogen bonding, and MXene nanosheets bridging between aragonite platelets. In addition, our conductive nacre also provides a self‐monitoring function for structural damage and offers exceptional electromagnetic interference shielding performance. Our strategy of reassembling the aragonite platelets exfoliated from waste nacre into high‐performance artificial nacre, provides an avenue for fabricating high‐performance bulk nanocomposites through the sustainable reutilization of shell resources.

Wpływ ograniczenia naprężenia na stabilność masy skalnej

A huge number of factors controls rock mass failure, but it is mainly influenced by the state of stress and in particular on the bearing capacity and failure mechanism of the massif. The evaluation of rock mass strength in confined and unconfined compression, as well as its tension strength, are key issues to understand rock mass behaviour prior to failure. A connection between the laboratory analyses of the rock mass and the practical use of the obtained data is presented in the current work. The strength properties, confinement effect and failure mechanisms are successfully studied in volcanic rock specimens from an underground mine. In order to estimate the confinement effect on rock mass strength properties, different confined compression stresses on rock specimens are applied. In addition, the crack initiation and propagation in rock samples are observed and rock mass failure mechanisms are studied. The obtained data is used for stability analyses of an underground openings through determination of the safety factor. The obtained results of the safety factors underlined the influence of the confining stress on the rock mass. The tendency of increasing values of the shear safety factor and decreasing values of the tensile safety factor as confinement increases is found. This is an important observation that would allowed more accurate predictions of the stable and unstable zones of the underground openings to be carried out, and thus the stability of the rock mass to be improved.

Effect of trace element arsenic on the high cycle fatigue properties and the carbide uniformity in high carbon chromium bearing steel

AbstractIn this paper, the mechanical properties and microstructures of bearing steels with arsenic contents of 30, 40, and 120 ppm have been investigated. The results show that As‐40 steel has the highest tensile strength but lower fatigue strength than As‐120 steel. The fatigue crack propagation rate of As‐120 steel is relatively low, mainly due to the increase in ΔKth. After quenching and tempering, As‐120 steel has larger prior austenite grains and more undecomposed, elongated carbides appear, reducing the uniformity of the steel. For carbides with an aspect ratio greater than 2, they cannot grow by the Oswald mechanism of spontaneous migration but are spheroidized by self‐cracking. The undissolved elongated carbides have a lower chemical potential, which increases the diffusion tendency of arsenic in the matrix. SIMS and EPMA analyses show that there is no significant distortion of the trace element arsenic.

Experimental Investigation into the Failure Mechanisms of Fabric Membrane Materials with Defective Flat Round Cracks

The emphasis of this study is placed on the investigation into the failure mechanisms of the fabric membranes when exposed to such defective cracks. This experimental study investigates the initial crack of a flat circle and conducts a uniaxial shear test on the membrane materials. The deformation of the membrane materials is obtained through an optical non-contact scanner. Our study has been conducted to assess the crack propagation of fabric membrane materials at different angles. The relationships between crack width and stress together with stress and strain are also obtained. Based on the results, a mechanic of failure on the membrane was proposed. Moreover, new findings into the ductility and energy absorption of the fabric membrane materials have been established to inform the failure mechanisms.

Bio-Inspired Helicoidal Composite Structure Featuring Graded Variable Ply Pitch under Transverse Tensile Loading

Biostructures found in nature exhibit remarkable strength, toughness, and damage resistance, achieved over millions of years. Observing nature closely might help develop laminates that resemble natural structures more closely, potentially improving strength and mimicking natural principles. Bio-inspired Carbon Fiber-Reinforced Polymers (CFRP) investigated thus far exhibit consistent pitch angles between layers, whereas natural structures display gradual variations in pitch angle rather than consistency. Therefore, this study explores helicoidal CFRP laminates, focusing on the Non-Linear Rotation Angle (NLRA) or gradual variation to enhance composite material performance. In addition, it compares the strength and failure mechanisms of the gradual configuration with conventional helicoidal and unidirectional (UD) laminates, serving as references while conducting transverse tensile tests (out-of-plane tensile). The findings highlight the potential of conventional and gradual helicoidal structures in reinforcing CFRP laminates, increasing the failure load compared to unidirectional CFRP laminate by about 5% and 17%, respectively. In addition, utilizing bio-inspired configurations has shown promising improvements in toughness compared to traditional unidirectional laminates, as evidenced by the increased displacement at failure. The numerical and experimental analyses revealed a shift in crack path when utilizing the bio-inspired helicoidal stacking sequence. Validated by experimental data, this alteration demonstrates longer and more intricate crack propagation, ultimately leading to increased transverse strength.

In situ TEM nanomechanical study on enhanced toughness of graphene-nanoparticle nanocomposite

Although theoretical studies and experimental research have shown outstanding mechanical properties of graphene, its brittleness and low toughness restrict its widespread industrial use. Here, we conducted a study to examine the impact of encapsulated Ag nanoparticles on the mechanical properties of graphene through in situ nanomechanical testing in an aberration-corrected transmission electron microscope (TEM). The presence of Ag nanoparticles was found to decrease crack propagation speed, enabling the TEM observation of crack deflection around the nanoparticles at nanoscale, as well as real-time measurement of the concomitant increase in stress. Using our nanomechanical testing data, we were able to quantitatively estimate the stress intensity factor, which serves as a measurement of fracture toughness for graphene, and then compared our results with previously reported values. We further discuss the role of nanoparticles in the propagation of cracks and their potential to enhance the toughness of materials. These findings have practical implications for the development of high-performance graphene composite materials.

Optimization of quasi-β forging parameters to control trimodal microstructure parameters and performance of TC21 forgings

This study aims to explore the influence of distinct forging parameters on microstructure and mechanical properties of TC21 forgings. The trimodal microstructure, which includes equiaxial α (αeq), lath α (αlath), and fine α (αfine) phases, was achieved via quasi-β forging and dual heat treatment. Subsequently, mechanical properties were evaluated through tensile, impact toughness (ak), and fracture toughness (KIC) tests. Results indicate that the content of αeq is higher at lower forging heating temperature, whereas the content of αlath is diminished and the size of αfine remains relatively small. This tendency reverses at higher forging heating temperature due to the increased driving force of the phase transition. The rise in αlath content implies more pronounced interface strengthening and plastic deformation, which significantly contributes to strength compared to αfine and αeq but exerts a weaker influence on plasticity. Moreover, the combined deformation and fracture of αlath and αfine exert a significant influence on the initiation and propagation of cracks in ak and KIC specimens. The limited coordination of deformations exhibited by αlath leads to preferential stacking dislocations and shear fractures at interfaces, which are further influenced by its higher aspect ratio affecting crack propagation paths. This not only increases the required energy for initiating cracks but also raises energy consumption during their propagation. Crucially, the presence of αfine significantly enhances the dislocation accumulation and the plastic deformation zone at the crack tip, thereby playing a pivotal role in determining local deformation capacity and crack deflection frequency.

Application and prospective of sand-type 3D printing material in rock mechanics: a review

PurposeThis paper aims to investigate the application of 3D printing technology, particularly using sand-type materials, in the creation of artificial rock models for rock mechanics experimentation.Design/methodology/approachUsing a comprehensive analysis, this research explores the utilization of 3D printing technology in rock mechanics. Sand-type materials are specifically investigated for their ability to replicate natural rock characteristics. The methodology involves a review of recent achievements and experimentation in this field.FindingsThe study reveals that sand-type 3D printing materials demonstrate comparable properties to natural rocks, including brittle characteristics, surface roughness, microstructural features and crack propagation patterns.Research limitations/implicationsWhile the research establishes the viability of sand-type 3D printing materials, it acknowledges limitations such as the need for further exploration and validation. Generalizability may be constrained, warranting additional research to address these limitations.Originality/valueThis research contributes insights into the potential application of sand-type 3D printing materials in indoor rock physics experiments. The findings may guide future endeavors in fabricating rock specimens with consistent structures for practical rock mechanics applications.

Inspired by nacre: α-ZrP three-dimensional network structures for interlaminar toughening of laminates

This work study enhancing the mechanical properties of composite laminates by using a three-dimensional network structure that mimics the nacre-like structure. The optimal interlaminar structure was developed by utilizing α-ZrP with ultrasonic-assisted exfoliation for 6 minutes. The mode-I critical energy release rate (GIC) and mode-II critical energy release rate (GIIC) of the toughened composite are 104.44% and 74.20% higher than those of the unmodified sample. Observation of the region around the crack by SEM and OM reveals that the three-dimensional network structure across the fiber fabric undergoes energy dissipation behaviors, such as fracture, slip and pull-out when cracks penetrate thin walls. In addition, the effect of the thin-walled structure of the α-ZrP aerogel on interlaminar crack propagation is discussed, taking into account the inhomogeneous and multi-surface of the composite interfaces, and the toughening mechanism is investigated.

Influence of Interaction between Microcracks and Macrocracks on Crack Propagation of Asphalt Concrete.

This paper aims to reveal the interaction relationship between microcracks and macrocracks and the influence of the interaction on the crack propagation behavior. A theoretical model of asphalt concrete was established for the interaction between microcracks with different crack densities and a macrocrack. And a meso-structure model of AC-13 dense-graded asphalt concrete was established by combining the Talyor medium method and the DEM (discrete element method). Macro and micro parameters, such as the stress-strain characteristics, crack evolution parameters, and crack tip stress field, were obtained through a semi-circular bend virtual test and used to study the characteristics of crack propagation under the interaction between microcracks and the macrocrack. The results indicate that the interaction has an effect throughout the process of asphalt concrete damage, and shows shielding and acceleration effects as the microcrack density changes. When the microcrack density is low (f3 ≤ 0.8), the crack propagation process, which is affected by the interaction effect, exhibits significant differences, and the interaction effect shows the shielding effect. When the microcrack density is high (f3 > 0.8), the fracture stage is mainly affected by the interaction effect, which shows the acceleration effect. The results provide a predictive theoretical and numerical model for low-temperature cracking of asphalt pavement, and theoretical support for the design, maintenance, and upkeep of long-life pavement.

Study on rolling contact fatigue crack initiation and propagation in U75V rail treated by laminar plasma

Surface discrete hardening by laminar plasma (HLP) enhances rail wear resistance by rapidly quenching surfaces to form discrete martensitic zones. In comparative field tests on a North China freight railway, HLP-treated U75V rails supported twice the total passing weight before reaching wear limits, compared to untreated rails. However, HLP treatment increases susceptibility to rolling contact fatigue (RCF) cracks, including interface cracks from mechanical differences, impact cracks from wheel-rail contact, deformation cracks from cumulative deformation, and matrix cracks extending from interfaces. The ratchet effect and seesaw mechanism, due to plastic deformation accumulation and quenching area warping, are crucial for crack initiation and propagation. This research advances understanding of HLP-treated rails' tribological behavior, emphasizing the necessity for customized maintenance strategies.

Conductive polymer composites for resistive flexible strain sensors

The emergence of wearable devices has spurred demand for strain sensors that are flexible, stretchable, and sensitive. Traditional strain sensors often lack stretchability due to the brittle nature of their materials. Conductive polymer composites (CPCs) are well-suited for this application due to their excellent flexibility, stretchability, and sensitivity, making them the material of choice for strain sensors. Resistive strain sensors, a common type of CPCs-based sensor, are favored for their cost-effectiveness, straightforward manufacturing process, and ease of signal collection. This review explores the sensing mechanisms of various resistive strain sensors, including percolation, crack propagation, contact resistance change and tunneling effect. It examines the impact of different fabrication processes and structural designs on sensor performance parameters including sensitivity, stretchability, linearity, and repeatability. Additionally, the advantages and challenges of integrating discrete sensors into high-density strain sensor arrays are discussed. Finally, we introduce the wide-ranging applications of CPCs-based strain sensors in multiple fields.

Microstructural, texture, and crystallographic analysis of SiC particle incorporation in double-sided friction stir welding of dissimilar aluminium alloys

This paper investigates the impact of incorporating SiC nanoparticles into the welded interface joint of dissimilar aluminium alloys 2024 and 7075. Double-sided friction stir welding (DS-FSW) was performed to investigate the impact of added nanoparticles on the joint's mechanical properties and crack growth rate. Microstructural analysis of the fractured surface was conducted to assess the influence of nanoparticles on crack propagation, while field emission scanning electron microscopy (FESEM) was utilized to examine the grain size distribution across different weld zones. On analyzing the DS-FSW joint with SiC-reinforced particles using electron backscattered diffraction (EBSD), predominant shear texture components such as B/B̅ and C are observed. Additionally, there are deformed textures like Rotating cube (H) {001} <110> and F {111} <112>, along with recrystallization texture elements such as Goss {110} <001>, P {011} <112>, and Cube {001} <101>. Mechanical properties, determined through hardness and tensile tests, were compared between nanocomposite weldments and base metal to assess the improvement in mechanical properties following the inclusion of SiC particles. Crack growth tests were performed using compact test (CT) specimens at a maximum load of 5 kN and a stress ratio of R = 0.1. The addition of SiC nanoparticles to the joint area led to a significant decrease in grain size and an increase in hardness within the stir zone, compared to the base metal and samples without the powder. The tests revealed an approximately 62 % increase in tensile strength and a notable increase in microhardness of the weld zone of the aluminium alloy metal matrix composite (AAMMCs).