Crack Propagation Research Articles

A simplified method for predicting overflow-induced crack propagation in gravity dams using genetic algorithm and material-based model

Cracks are always a serious concern in the stability analysis of gravity dams. One of the main reasons for the initiation of cracks is overflows. In evaluating dam safety under flood conditions, it is crucial to consider the balance between the strength of the dam body and the propagation of cracks. In this context, simplified equations serve as valuable tools in dam design as they offer a quick and efficient way to estimate the behavior of the dam structure. In this study, firstly using a polygonal material-based model (P-MBM), which is improved in considering the softening behavior of joints, the micro-crack propagation in masonry material is investigated. Then, based on performing 81 numerical simulations in a parametric study on the geometrical and geotechnical properties of the dam subjected to overflow, a great database of the behavior of different dams is investigated. Then, using the genetic algorithm, a set of equations is proposed, and their accuracy is validated through UDEC simulations and some theoretical methods from existing literature. For all models, the crack tends to be horizontal at the initial phase. Then, due to increasing the compressive stresses on the downstream side, the fractures tend to the dam toe. The results also indicate that the proposed equations can reasonably determine the behavior of gravity dams and the development of cracks in the dam body. The outcomes highlighted the considerable effects of geometry and geotechnical properties on dictating the trajectory of crack growth.

Study on the mesoscopic mechanical behavior and damage constitutive model of micro-steel fiber reinforced recycled aggregate concrete

This study employs in-situ 4D CT experimental techniques to investigate the mesostructured changes and mechanical behavior of micro steel fiber-reinforced recycled aggregate concrete (MSF-RAC) under uniaxial compression and cyclic loading conditions. The research aims to address the scientific problem of limited understanding of how micro steel fibers affect the mesoscopic mechanical behavior and structural damage characteristics of recycled aggregate concrete. High-resolution CT scanning technology was utilized for real-time observation of crack initiation and propagation, revealing the micro-mechanisms by which micro steel fibers enhance RAC. The study found that incorporating micro-steel fibers significantly improves the density of the interfacial transition zone (ITZ), inhibits crack development, and enhances the mechanical properties of the concrete. Based on these observations, an improved stiffness degradation damage constitutive model was proposed, which comprehensively considers the distribution orientation of micro steel fibers and ITZ characteristics. The model effectively predicts the stress-strain relationship of MSF-RAC under various conditions, demonstrating high accuracy and practicality. These findings provide important theoretical support and experimental evidence for the optimal design of RAC, showcasing significant engineering application value by promoting the sustainable use of recycled materials in construction.

Linear fractal evolution characteristics of rock crack distributions during loading process

To investigate the fractal characteristics of rock crack distributions during the loading process, discrete element method was used to make rock samples with joints and record the crack propagation. The Box-counting method was used to quantitatively analyze the fractal dimension of the crack distribution at each moment, and the relationship between the crack fractal dimension and strain ratio was established based on fractal theory. The results indicated that the relationship between the fractal dimension of the crack distribution and strain ratio showed a strong linear characteristic. By transforming this linear relationship into a linear function, the slope of the function was found to be linked to the failure patterns of the sample, and a refinement coefficient (damage-fracture reduction factor) was identified from the slope as an effective basis for determining the degree of sample damage and fracture. The damage-fracture reduction factor can be categorized: 0.25–0.5 (spilt and fracture), 0.5–0.9 (synergy between fracture and damage), 0.9–1 (microcrack asymptotic damage). Owing to the linear fractal characteristics, an expression for the damage variables influenced by failure patterns can be established from the geometric aspect. In addition, the linear fractal characteristics of the cracks were verified in other acoustic emission and crack extension experiments.

Research on mechanical properties of steel-polypropylene fiber concrete and application of beam structure

Concrete faces the difficulties of low tensile strength and poor crack resistance in building structures. In order to remedy this deficiency. In this paper, steel-polypropylene hybrid fiber reinforced concrete (SPFRC) was prepared by adding steel fiber (SF) and three kinds of polypropylene fiber (PF) to C50-grade concrete. The mechanical properties and microstructure of SPFRC were studied with different fiber combinations and content, obtaining the best hybrid combination. Based on this, the bending resistance and cracking of SPFRC beam members were investigated. The results demonstrate that the addition of fiber improves the compressive strength of ordinary concrete by 0.16% ∼ 17.69%, the splitting tensile strength by 15.18% ∼ 47.45%, and the bending strength by 3.54% ∼ 26.77%. Compared with single-fiber concrete, the hybrid fiber can achieve better internal microstructure, which further enhances the mechanical properties of the material. Hybrid fibers overlap within concrete beams, effectively redistributing stress and inhibiting the formation and propagation of cracks. For the three types of SPFRC beams, the cracking load is increased by 14.29% ∼ 28.57% compared with PC beam, the ultimate bearing capacity is increased by 9.68% ∼ 19.35%. The optimal dosage is determined as 1.0% SF, 0.6% Embossed polypropylene fiber (PBF). It provides reference for the application of SPFRC in flexural members.

Enhancing thixotropic properties of epoxy resin and mechanical properties of epoxy resin thermosets by polyethyleneimine functionalized carbon nanotubes

Epoxy resin (ER) thermosets are widely applied in various fields due to their high mechanical strength, thermal stability and chemical corrosion resistance. However, the brittleness of ER thermosets and low thixotropic property of ER matrix largely limits their further development. Here, this article develops branched polyethyleneimine (PEI) functionalized carbon nanotube (PEI@CNT) as high-performance nanofiller for simultaneously strengthening and toughening ER thermosets. Benefiting from good dispersity and rich amine groups of PEI@CNT, effective interfacial bonding between PEI@CNT and ER is achieved. The ER thermoset with 0.5 wt% PEI@CNT shows increase in tensile strength by 24.8 % and fracture elongation by 30.0 % compared with pristine ER thermoset. The enhanced mechanical properties of PEI@CNT modified ER thermosets can be ascribed to the chemical interaction between PEI@CNT and ER matrix as well as the formed crack propagation, which can dissipate a lot of fracture energy, further improving the strength and toughness of the ER composites. More importantly, the thixotropic property of ER matrix is largely improved after adding the PEI@CNT filler. This work reports a high-performance nanofiller for enhancing mechanical properties of ER thermosets and processability of ER matrix.

An Investigation of Tensile, Fatigue, and Fracture Behavior of 3D-Printed Polymers

Abstract The progression of manufacturing technology has significantly benefited from the adoption of 3D printing techniques, which enable the production of parts with intricate geometries. However, it is important to acknowledge that components fabricated through this additive manufacturing method frequently manifest defects and are prone to failure under severe conditions. Therefore, a thorough examination of the mechanical properties of these parts is essential to effectively reduce the failure. This study aimed to explore the mechanical properties of two prevalently used 3D-printed polymers, specifically Onyx and acrylonitrile butadiene styrene (ABS), by integrating computational and experimental analyses. The experimental study utilized a material testing system and digital image correlation (DIC) technology, while the computational analysis covered the finite element (FE) modeling of the 3D-printed samples. The research focused on evaluating the tensile strength and fatigue resistance of the specimens printed in various orientations, alongside a detailed investigation of their fracture behavior. The crack propagation analysis was carried out using the DIC system and the separating morphing and adaptive re-meshing technology (SMART) scheme in ansys. It was observed that upright build orientation produced the weakest samples for axial loading and specimens with notches failed earlier than those without. Moreover, Onyx was found to have a higher resistance to fracture or failure compared to ABS. The FE modeling results demonstrated strong agreement with the experimental results, validating their accuracy and reliability in characterizing the critical mechanical response of 3D-printed parts rapidly and cost effectively.

Analysis of Fracturing Expansion Law of Shale Reservoir by Supercritical CO2 Fracturing and Mechanism Revealing

The rapid expansion of reservoir fractures and the enlargement of the area affected by working fluids can be accomplished solely through fracturing operations of oilfield working fluids in geological reservoirs. Supercritical CO2 is regarded as an ideal medium for shale reservoir fracturing owing to the inherent advantages of environmental friendliness, excellent capacity, and high stability. However, CO2 gas channeling and complex propagation of fractures in shale reservoirs hindered the commercialization of Supercritical CO2 fracturing technology. Herein, a simulation method for Supercritical CO2 fracturing based on cohesive force units is proposed to investigate the crack propagation behavior of CO2 fracturing technology under different construction parameters. Furthermore, the shale fracture propagation mechanism of Supercritical CO2 fracturing fluid is elucidated. The results indicated that the propagation ability of reservoir fractures and Mises stress are influenced by the fracturing fluid viscosity, fracturing azimuth angle, and reservoir conditions (temperature and pressure). An azimuth angle of 30° can achieve a maximum Mises stress of 3.213 × 107 Pa and a crack width of 1.669 × 10−2 m. However, an apparent viscosity of 14 × 10−6 Pa·s results in a crack width of only 2.227 × 10−2 m and a maximum Mises stress of 4.459 × 107 Pa. Additionally, a weaker fracture propagation ability and reduced Mises stress are exhibited at the fracturing fluid injection rate. As a straightforward model to synergistically investigate the fracture propagation behavior of shale reservoirs, this work provides new insights and strategies for the efficient extraction of shale reservoirs.

Fatigue Crack Propagation Across the Multiple Length Scales of Technically Relevant Metallic Materials

The fundamentals of our understanding of fatigue crack propagation were formed more than 60 years ago by Paul C. Paris. Since then, the run toward new metallic materials and alloys with ever finer-grained microstructures has had a large impact on research. Along with enormous variation of the microstructural length scales (i.e., grain size), the essential parameters for the description of fatigue crack growth, such as the crack propagation rate and plastic zone size, also exhibit an immense change from the subnanometer to the micrometer regime. These enormous variations in the fatigue crack growth behavior's controlling parameters motivate this contribution. This article presents an overview of the effect of grain size, from the millimeter to the nanometer grain-size regime, on fatigue crack propagation of mainly ductile metals and alloys with an attempt to summarize the most important findings and underlying physical phenomena, including with respect to selected materials such as pure iron, nickel, and austenitic and pearlitic steel.

A computational approach for phase-field model of quasi-brittle fracture under dynamic loading

AbstractA computational model is formulated for studying dynamic crack propagation in quasi-brittle materials exposed to time-dependent loading conditions. Under such conditions, inertial effects of structural components play an important role in modelling crack propagation problems. The computational model is proposed within the theory of regularised cracks which uses a damage-like internal variable. Here, fracture considers phase-field damage which gives rise to a material degradation in a narrow material strip defining the regularised crack. Based on the energy formulation using the Lagrangian of the system, the proposed computational approach introduces a staggered scheme adopted to solve the coupled system and providing it in a variational form within the time stepping procedure. The numerical data are obtained by quadratic programming algorithms implemented together with a finite element code.

Application of time-temperature superposition principle for cracking characterization of asphalt mixtures using semi-circular bend test

In recent decades, numerous laboratory tests have been developed to characterize the cracking performance of asphalt mixtures. Among these, the semi-circular bend (SCB)-based Illinois flexibility index test (I-FIT) (AASHTO T 393) has emerged as a favored method due to its straightforward specimen preparation and better sensitivity to mixture variables. However, the cracking susceptibility of asphalt mixtures is significantly influenced by temperature and loading rate, leading to a discrepancy in performance between standard test conditions and real-world scenarios. To address this challenge, this study explores the applicability of the time-temperature superposition principle (TTSP) for the SCB test. The study evaluates four mixtures incorporating reclaimed asphalt pavement (RAP) and/or reclaimed asphalt shingles (RAS) across various loading rates and temperature combinations. Findings suggest that TTSP can be applied to the SCB test without significantly impacting cracking performance indices such as flexibility index and post-peak slope. Furthermore, the study also demonstrates that TTSP can be applied in the crack propagation phase during the post-peak domain of the mixtures. The findings of the study will allow laboratories to develop more flexible testing plans without compromising the accuracy and reliability of performance parameters by adjusting the loading rate corresponding to the test temperature and vice versa. Consequently, this approach can potentially make a balanced mix design (BMD) practice more implementable.

Restoration of Strength in Polyamide Woven Glass Fiber Organosheets by Hot Pressing: Case Study of Impact and Compression after Impact

Thermoplastic composite organosheets (OSs) are increasingly recognized as a viable solution for automotive and aerospace structures, offering a range of benefits including cost-effectiveness through high-rate production, lightweight design, impact resistance, formability, and recyclability. This study examines the impact response, post-impact strength evaluation, and hot-pressing repair effectiveness of woven glass fiber nylon composite OSs across varying impact energy levels. Experimental investigations involved subjecting composite specimens to impact at varying energy levels using a drop-tower test rig, followed by compression-after-impact (CAI) tests. The results underscore the exceptional damage tolerance and improved residual compressive strength of the OSs compared to traditional thermoset composites. This enhancement was primarily attributed to the matrix’s ductility, which mitigated transverse crack propagation and significantly increased the amount of absorbed energy. To mitigate impact-induced damage, a localized hot-pressing repair approach was developed. This allowed to restore the post-impact strength of the OSs to pristine levels for impact energies below 40 J and by 83.6% for higher impact energies, when OS perforation was observed. The measured levels of post-repair strength demonstrate a successful restoration of OS strength over a wide range of impact energies, and despite limitations in achieving complete strength recovery above 40 J, hot-pressing repair emerges as a promising strategy for ensuring the longevity of thermoplastic composites through repairability.

Monitoring damage evolution of SiCf/PyC/SiC mini-composites using electrical resistivity: Crack opening distance and interphase retention rate based electromechanical modeling

Electrical resistance is closely related to the damage of ceramic matrix composites (CMC) such as matrix crack, crack opening distance (COD), and interphase retention rate, giving it the potential to become a new non-destructive testing (NDT) technique. An electro-mechanical experiment method was designed for the tensile test of ceramic matrix mini-composites (CMMC). An optical in-situ tensile test was performed to obtain the pattern of matrix crack propagation. The result confirms that matrix crack saturation may not occur before the material fractures. An electromechanical model considering the COD was established to identify the damage situations. A new method for preparing ceramic matrix micro-composites composed of a single fiber and a single-layer pyrolytic carbon (PyC) interphase was realized. The accurate in-situ resistivity of the PyC was measured based on the micro-composites.

Entropy‐based method considering nonlinear hardening effect to predict the crack propagation life of superalloy GH4169 at elevated temperature

AbstractThis paper aims to determine the relationship between thermodynamic entropy generation and fatigue crack propagation life of superalloy GH4169 at 300–650°C. The entire specimen was considered as the thermodynamic system. The plastic energy dissipation in the crack tip was obtained by finite element simulation utilizing the Chaboche nonlinear hardening model. Then the cyclic entropy generation rate (CEGR) and the accumulated entropy generation are calculated by combining simulation and experimental methods. Results show that the CEGR is a power function of the stress intensity factor range, and it is almost a constant at fatigue failure. The fatigue fracture entropy (FFE) increases as fatigue cycles at failure increase at constant temperature, but it first decreases and then increases when temperature increases from 300 to 650°C. A fatigue life prediction model based on the thermodynamic damage parameter is established and verified by comparison with experimental results and available data in the literature.

Study of Prefabricated Crack Propagation on Monocrystalline Silicon Surfaces for Grinding Damage Analysis

Crack generation and propagation are critical aspects of grinding processes for hard and brittle materials. Despite extensive research, the impact of residual cracks from coarse grinding on the cracks generated during fine grinding remains unexplored. This study aims to bridge this gap by examining the propagation law of existing cracks under indentation using the extended finite element method. The results reveal that prefabricated cracks with depths less than the crack depth produced on an undamaged surface tend to extend further without surpassing the latter. Conversely, deeper prefabricated cracks do not exhibit significant expansion. A novel method combining indentation and prefabricated cracks with fracture strength tests is proposed to determine crack propagation. Silicon wafers with varying damaged surfaces are analyzed, and changes in fracture strength, measured by the ball-on-ring method, are utilized to determine crack propagation. The experimental results confirm the proposed crack evolution law, validated by damage assessments across different grinding processes, which is suitable for crack damage. The findings demonstrate that residual cracks from coarse grinding are negligible in predicting the maximum crack depth during fine grinding. This research provides a crucial foundation for optimizing the wafer thinning process in 3D stacked chip manufacturing, establishing that changes in fracture strength are a reliable indicator of crack propagation feasibility.

A length-free phase field model for epoxy adhesive materials based on modified Drucker–Prager criterion

Characterizing the cracking behavior of epoxy adhesive material is of great importance for composite structures. Recently, the phase field model has emerged as the preferred method for predicting the crack propagation. This paper presents a length-free phase field model considering the characteristics of epoxy adhesive materials. The proposed model is formulated within a framework that decomposes the phase field fracture driving force, ensuring that it adequately considers the tension/compression asymmetry of the epoxy adhesive materials. The effects of hydrostatic pressure, deviational stress, and friction are incorporated into the model by employing a modified Drucker-Prager failure criterion. The failure criterion is included in the phase field model using the effective stress invariants. Furthermore, the internal length scale, previously considered to lack real physical meaning, is reformulated based on material strength. This model addresses the challenge of measuring the internal length of epoxy materials. It serves as a valuable reference for applying phase field models to epoxy materials. The accuracy of the model is validated through an L-shaped plate simulation, and an attempt using the model to predict the crack development in thick epoxy adhesive joints is undertaken. Several representative samples are simulated, including three double cantilever beams with different configurations. The numerical results demonstrate a good correlation with experimental data, highlighting the potential of the model in simulating the crack propagation in epoxy adhesive joints.

Effect of subgrain microstructure on the mechanical properties of Invar 36 specimens prepared by laser powder bed fusion

Invar 36 samples were fabricated via laser powder bed fusion (LPBF) at various energy densities. In this work, a parameter range for the preparation of full-density Invar 36 samples was determined. The microstructural evolution and tensile properties of the materials were studied in the as-LPBFed state, and the role of subgrain structures on the fracture mechanism was investigated. According to the results of this analysis, as the energy density increased, the density first increased and then decreased. The maximum density (99.94 %) reached 65 J/mm3, and the simple density reached full density at 40–90 J/mm3. Due to constitutional supercooling, the microstructure of the alloy is composed of cellular subgrains and columnar subgrains. Columnar subgrains grow perpendicular to the boundary of the molten pool to the center of the molten pool, while cellular subgrains primarily exist in the remelting area of the molten pool. In the full density range, the ultimate tensile strength of the samples reaches 437–484 MPa, and differences in the morphology, distribution, and micronicomechanical properties of the subgrains are responsible for the differences in macroscopic properties. At 40, 65, and 90 J/mm3, the elastic moduli of the columnar subgrains are approximately 155.1 GPa, 161 GPa, and 162.4 GPa, respectively; the elastic moduli of the cellular subgrains are approximately 136.2 GPa, 140.2 GPa, and 139.9 GPa, respectively, which means that in the elastic deformation stage, the columnar subgrains are more resistant to elastic deformation; in the plastic deformation stage, different subgrains have different effects on crack propagation; the columnar subgrains grow perpendicularly to the melt pool boundary, which has a hindering effect on the extension of microcracks; and the cellular subgrains have a homogeneous grain size so that the microcracks can be easily extended along the cellular subgrain boundaries.

Investigation on anisotropic toughness and cracking behavior of woven lay-up Fiber Metal Laminates with two-level customized interfaces

Interlaminar toughening for metal-polymer interfaces in woven lay-up Fiber Metal Laminates (w-FMLs) is a critical concern, demanding isotropic mechanics and higher energy dissipation. Customizing interfaces with interlocking structures like dimples or grooves has proven effective in meeting these requirements, while the resulting two-level features introduce greater complexity in toughening effects, and interfacial mechanics are influenced by factors such as fiber orientation and resin distribution. To understand the anisotropy effect and toughening mechanics induced by different two-level features, three types of two-level customized interfaces of w-FMLs are examined. Edge shear tests are utilized to evaluate interfacial mechanics and toughness, with a focus on tracing crack propagation behaviors. Through numerical simulation and characterization, the toughening effect and its mechanisms are unveiled, revealing that differences in toughness pinning effect and local plastic deformation of the customized interfaces are the primary determinants of interlaminar toughness. Additionally, a coupling effect between the woven core and interfaces is observed, showing that the superposition of the tougher orientation of the woven core on the strong toughness pinning orientation of the interfaces is a crucial factor in determining excellent overall interlaminar toughness.

Influence of confining pressure on rock fracture propagation under particle impact

Revealing the influence of confining pressure on the propagation and formation mechanism of rock cracks under particle impact is significant to deep rock excavation. In this study, the three-dimensional fracture reconstruction of the rock after particle impact was carried out by CT scanning, and the stress and crack field evolution of the rock under particle impact were analyzed by PFC2D discrete element numerical simulation. The results demonstrate that after particles impact, a fracture zone and intergranular main crack propagation zone are formed in the rock. The shear stress and tensile stress caused by compressive stress are the main reasons for the formation of the fracture zone, while the formation of the intergranular main crack propagation zone is mainly due to tangential derived tensile stress. The confining pressure induces prestress between rock particles such that the derived tensile stress needs to overcome the initial compressive stress between the particles to form tensile fractures. And the increase in the confining pressure leads to increases in the proportion of shear cracks and friction effects between rock particles, resulting in an increase in energy consumption for the same number of cracks. From a macroscopic perspective, the confining pressure can effectively inhibit the generation of cracks.

X-IGA Used for Orthotropic Material Crack Growth

In this paper, we propose a new approach for numerically simulating the growth of cracks in unidirectional composite materials, termed extended isogeometric analysis, evaluating the maximum stress intensity factor and T-stress. To validate our approach, we used a small anisotropic plate with two edge cracks, beginning with formulating the governing equations based on the energy integral method, Stroh’s Formula, and the Elastic Law describing the behaviour of anisotropic materials, while considering boundary conditions and initial states. A MATLAB code was developed to solve these equations numerically and to post-process the tensile stress and the stress intensity factor (SIF) in the first mode. The results for the SIF closely match those obtained using the extended finite element method (X-FEM), with a discrepancy of only 0.0021 Pa·m0.5. This finding underscores the credibility of our approach. The extended finite element method has demonstrated robustness in predicting crack propagation in composite materials in recent years, leading to its adoption by several widely used software packages in various industries.

On the anisotropic fracture behaviour of AA2050-T84 thick plate

Anisotropy in the fracture behaviour of AA2050-T84 was investigated using 2D digital image correlation (DIC)-assisted 3-point bending fracture toughness test on specimens with crack propagation direction oriented along the normal direction (ND) and rolling direction (RD). The tensile properties along the RD indicated slightly higher yield strength (482 ± 8 MPa) than ND sample (469 ± 9 MPa), while the fracture toughness in terms of energy release rate was higher when the notch was oriented along the ND (35.3 ± 1 kJ/m2) compared to the RD (18.5 ± 0.9 kJ/m2). Fractography of the fractured surfaces and detailed electron backscatter diffraction (EBSD) analysis of the microstructure along crack paths unveiled higher crack tortuosity with extensive secondary cracking in the TD-ND specimen than in the TD-RD. DIC measurements clearly showed a larger deformation zone for the TD-ND specimen also as which corroborated the plastic zone size determined using linear elastic fracture mechanics (LEFM). Additionally, the crystallographic model of tilt and twist angle differences between the neighbouring grains was employed to explain the increase in crack path tortuosity and higher dissipation of plastic work contributing to superior fracture toughness for the TD-ND sample.