Fracture Mechanics Research Articles

Non-oscillatory fracture mode partition for interface crack under mixed-mode loading

The oscillatory crack-tip field in the classical interface fracture mechanics solution often leads to complications in identifying the mode mixity in bimaterial interface fracture problems. In this paper, a non-singular series solution for the continuum problem of a crack on the cohesive-spring interface under mixed-mode loading is presented and compared to the classical oscillatory solution. A Gaussian process regression model was also developed to enable quick evaluation of the phase angle for interface delamination characterization by using mixed-mode bending fracture test.

Improvement of butt-fusion welding procedure and performance evaluation method for thick-walled bimodal polyethylene pipes

High-density polyethylene (HDPE) pipes have been continuously improved by the progress made by the resin industry and pipe manufacturers. Together with the advanced technology of welding equipment in terms of structure reliability and automations, HDPE pipes have been gradually applied to higher pressure level, bigger diameter, and thicker walls in operations which have become the preferred piping system for nuclear safety application in transporting cooling seawater. There are substantial differences in parameter settings when applying heat and force during different welding procedures, as well as the complicated evaluation methods for mechanical properties, which causes communication barriers between engineers and researchers. Thus, it is necessary to comprehensively study the welding procedures and evaluation methods to standardize the evaluation protocols in the industry. In this paper, based on the assumptions of temperature effect and molecular chain diffusion dynamics at the welding interface, the optimization of the welding procedure is proposed. Some well-established evaluation methods have been used for performance assessment and their feasibilities are discussed. The idea of creating crack along the weld interface is adopted as new method to evaluate the material ability of fracture resistance. Specimens with unilateral notch and weld interface crack are proposed and subjected for quasi-static axial tensile test. The fracture stress and fracture energy at the welding interface are used to differentiate the quality of the joint from different welding procedures. The fracture morphologies of the weld interface are analyzed. A complete picture is drawn for the mechanical fracture resistance of the weld interface. The method used in this paper can be corelated to the pipeline design conditions, therefore could be applied to the real-time engineering applications. Moreover, the evaluation method proposed in this paper is simple, reliable, and easy to implement.

Interface failure of segmental tunnel lining strengthened with steel plates based on fracture mechanics

Interface failure of segmental tunnel lining strengthened with steel plates based on fracture mechanics

Mechanism of overburden fracture induced earthquakes in coal seam mining

Mining-induced earthquakes are unnatural seismic events that frequently occur in high-position hard and thick rock strata during coal mining. Considering the frequent occurrence of strong mining-induced earthquakes in the Dongtan mining area, this study analysed the fracture migration characteristics of hard and thick rock strata and the focal mechanism of mining-induced earthquakes based on Volasov’s thick-plate and moment tensor inversion theories. The results showed that the main key strata were difficult to break under single-panel mining conditions because of the thick and high-strength rock strata and breakage of the main key strata is caused by multiple-panel mining. Volasov’s thick-plate theoretical calculation indicated an initial fracture span of the main key strata was 314 m, which is consistent with the actual mining distance of the working face. This verified that strong mining-induced earthquakes were induced by the initial fracture of the main key strata. In coal mining, the pure shear failure type of mining-induced earthquakes indicated the highest percentage, and the shear fracture of rock strata was the primary cause of strong mining-induced earthquakes. The dip angle of the focal fracture surface in mining-induced earthquakes was generally within 15°. Through an analysis of the focal mechanism of mining-induced earthquakes, it has a certain guiding role in explaining the mechanism of mining-induced earthquakes.

Tension and Shear Behaviour of Basalt Fiber Bio-Composites with Digital Image Correlation and Acoustic Emission Monitoring.

This research investigates the mechanical behavior and damage evolution in cross-ply basalt fiber composites subjected to different loading modes. A modified Arcan rig for simultaneous acoustic emission (AE) monitoring was designed and manufactured to apply quasi-isotropic shear, combined tensile and shear loading, and pure tensile loading on specimens with a central notch. Digital image correlation (DIC) was applied for high-resolution strain measurements. The measured failure strengths of the bio-composite specimens under different loading angles are presented. The different competing failure mechanisms that contribute to the local reduction in stress concentration are described. Different damage mechanisms trigger elastic waves in the composite, with distinct AE signatures that closely follow the sequence of fracture mechanisms. AE monitoring is employed to capture signals associated with structural damage initiation and progression. The characteristic parameters of AE signals are correlated with crack modes and damage mechanisms. The evolution of AE parameters during the peak load transition is presented, which enables the timely AE detection of the maximum load transition. The combination of DIC and AE monitoring improves understanding of the mechanical response and failure mechanisms in cross-ply basalt fiber composites, offering valuable insights for possible performance monitoring and structural reliability in diverse engineering applications.

Analysis of fracture mechanism and fracture energy prediction of lightweight aggregate concrete

The fracture energy of lightweight aggregate concrete (LWAC) is comparatively lower than that of normal weight concrete (NWC), primarily due to the propensity for fracture exhibited by lightweight aggregates and their characteristics that modify the damage softening mechanism at the crack tip. Few studies have been done on how to precisely predict the LWAC fracture energy. In this work, 111 sets of three-point bending test data were gathered, and the impact of aggregate types and size, concrete density, water-cement (w/c) ratio, and compressive strength on the total average fracture energy (GF) and the initial fracture energy (Gf) was thoroughly evaluated. Five fracture energy prediction methods’ applicability were compared, and a fracture energy prediction expression appropriate for LWAC was presented. Finally, the three-point bending test was used to confirm the prediction. The results show that the fracture energy of LWAC is significantly influenced by changes in aggregate and mortar characteristics. The compressive strength has substantial correlations with the fracture energy of LWAC. After adjusting the aggregate and mortar coefficients, the prediction formula can correctly predict Gf and GF.

Elastic solids under frictionless rigid contact and configurational force

A homogeneous elastic solid, bounded by a flat surface in its unstressed configuration, undergoes a finite strain when in frictionless contact against a rigid and rectilinear constraint, ending with a rounded or sharp corner, in a two-dimensional formulation. With a strong analogy to fracture mechanics, it is shown that (i.) a path-independent J–integral can be defined for frictionless contact problems, (ii.) which is equal to the energy release rate G associated with an infinitesimal growth in the size of the frictionless constraint, and thus gives the value of the configurational force component along the sliding direction. Furthermore, it is found that (iii.) such a configurational sliding force is the Newtonian force component exerted by the elastic solid on the constraint at the frictionless contact. Assuming the kinematics of an Euler–Bernoulli rod for an elastic body of rectangular shape, the results (i.)–(iii.) lead to a new interpretation from a nonlinear solid mechanics perspective of the configurational forces recently disclosed for one-dimensional structures of variable length. Finally, approximate but closed-form solutions (validated with finite element simulations) are exploited to provide further insight into the effect of configurational forces. In particular, two applications are presented which show that a transverse compression can lead to Eulerian buckling or to longitudinal dynamic motion, both realizing novel examples of soft actuation mechanisms. As an application to biology, our results may provide a mechanical explanation for the observed phenomenon of negative durotaxis, where cells migrate from stiffer to softer environments.

Numerical and Preliminary In Situ Investigation on Roadway Excavation Using Static Expansion Mechanical Fracturing

This paper introduced a new nonexplosive roadway excavation method, combining the reserved free space technology and the static expansion mechanical fracturing technology, where the former is implemented by the gasbag, while the piston splitter is for the latter. The numerical model of roadway excavation was set up via PFC3D to investigate the mechanical fracturing performance, including the single-hole fracturing and the hole network fracturing. The results show that the reasonable hole margin is about 1.0–1.5 m, and the optimal column spacing of the hole network pattern is 1.0 m, after comprehensively analyzing the fracturing performance and the splitting force evolution. Moreover, the mechanical fracturing excavation method was applied to construct a parking chamber in the Kalatongke Mine, to preliminarily verify the feasibility of the static expansion mechanical fracturing technology. The in situ investigation results indicate that the excavation footage is about 0.8 m with the piston splitter when adopting a hole margin of 1.0–1.5 m. To sum up, the preliminary field application and the numerical simulation result both support the feasibility of mechanical fracturing, and the reasonable fracturing hole margin is about 1.0 m.

Flexible Strain Sensors Based on an Interlayer Synergistic Effect of Nanomaterials for Continuous and Noninvasive Blood Pressure Monitoring.

The continuous, noninvasive monitoring of human blood pressure (BP) through the accurate detection of pulse waves has extremely stringent requirements on the sensitivity and stability of flexible strain sensors. In this study, a new ultrasensitive flexible strain sensor based on the interlayer synergistic effect was fabricated through drop-casting and drying silver nanowires and graphene films on polydimethylsiloxane substrates and was further successfully applied for continuous monitoring of BP. This strain sensor exhibited ultrahigh sensitivity with a maximum gauge factor of 34357.2 (∼700% sensitivity enhancement over other major sensors), satisfactory response time (∼85 ms), wide strange range (12%), and excellent stability. An interlayer fracture mechanism was proposed to elucidate the working principle of the strain sensor. The real-time BP values can be obtained by analyzing the relationship between the BP and the pulse transit time. To verify our strain sensor for real-time BP monitoring, our strain sensor was compared with a conventional electrocardiogram-photoplethysmograph method and a commercial cuff-based device and showed similar measurement results to BP values from both methods, with only minor differences of 0.693, 0.073, and 0.566 mmHg in the systolic BP, diastolic BP, and mean arterial pressure, respectively. Furthermore, the reliability of the strain sensors was validated by testing 20 human subjects for more than 50 min. This ultrasensitive strain sensor provides a new pathway for continuous and noninvasive BP monitoring.

Seismic resistance analysis of a plywood house structure based on ABAQUS fracture mechanics

Plywood is a more affordable material for the residential construction industry due to its lower cost and easy availability. However, earthquakes are the biggest barrier for wood structures to stand for a long time, as the materials can easily deform or collapse when subjected to seismic forces. This paper aims to explore the performance of plywood structures during an active earthquake by conducting a series of tests and simulations. In this study, plywood property was explored in the four-point bending experiment to determine its stress-strain relationship, and the results were used to identify the materials young modulus, a key parameter for designing more resilient structures. In addition, a finite element model of a plywood house structure was developed in the ABAQUS application, and the seismic movements with five different velocity values were applied to determine the stress distribution and deflections of the structure. The results provide insights into the dynamic behavior of plywood structures under different seismic movements.

Dynamic damage and fracture mechanism of curved ripper cutting reinforced concrete composites: Numerical simulation combined with engineering practice

Shield tunnels inevitably cross existing structures like reinforced concrete pile foundations and concrete walls due to the spatial interlocking of underground structures in the construction process. The objective of minimizing cutter damage during the cutting of reinforced concrete pile foundations is widely pursued in engineering. In this study, a numerical model of curved ripper cutting reinforced concrete composites was established, with the engineering background of Nanjing Metro Line 10 shield tunnel crossing the pile foundation of Youth River Bridge. The dynamic damage and fracture of reinforced concrete composites under the cutting of single, double, and multiple rippers were comprehensively investigated. The effects of the operating parameters and geometric parameters on the cutting performance were discussed. Finally, a statistical analysis was carried out to evaluate the damage of 148 rippers and the fracture of rebar in an actual engineering project. The simulation results reveal that coordinated cutting of five rippers has optimized the fracture mechanism of the rebar, weakening the compression effect between the ripper and the rebar, and highlighting the tensile fracture of the rebar. It is appropriate to set the alloy blade angle at 90° and the blade width at 10 mm for practical engineering applications. The penetration has a significantly greater influence compared to cutting speed. In practical engineering, adopting a low penetration and high rotation speed grinding mode is advantageous for reducing blade fractures. The central fishtail cutter and the rippers in the outer area of the cutterhead are more prone to damage during the pile cutting process and warrant additional reinforcement measures. The findings provide comprehensive and innovative insights into shield cutting reinforced concrete pile foundations.

Research on Fracture Mechanism of Roof Plate and Filling Technology of Working Face Over-air Tunnel

For a long time, the faults on both sides of the working face have been the technical problems faced by the coal mining face. Aiming at the situation that both sides of 7509 comprehensive workings in Wuyang coal mine are near to big faults, and through several long and short hollow alleys, the theoretical analysis, numerical simulation and other methods are used to study the fracture mechanism of the roof plate of comprehensive workings in both sides of the faults and the technology of safe and fast hollowing of the workings. Through theoretical calculation and numerical simulation, it is finally determined that the filling strength of 4 MPa is the most suitable. And through the field industrial test, it is proved that the working face can realize rapid, safe and stable mining after filling with ultra-high water material.

Understanding the fracture mechanisms of Ni–Co–Cr-type superalloys: Role of precipitate evolution and strength degradation

Microstructure stability directly affects the performance degradation of hot-end structural parts such as turbine discs. In this research, long-term (10–10000 h) thermal exposure experiments were conducted on a Ni–Co–Cr type superalloy at 800 °C. The mechanical properties were obtained through high-temperature (750 °C) tensile tests. Subsequently, the microstructure evolution process, including intragranular γ′ and μ phases, and the initiation and propagation of defects were systematically and thoroughly studied. The results showed that intragranular γ′ precipitates coarsened under thermal exposure. After 100 h of thermal exposure, μ phases were observed near the intergranular γ′ precipitates and grain boundaries. The size and volume fraction of the μ phase initially increased, and then remained relatively stable. Numerous cracks formed on the plane of the μ phases and intergranular γ′ precipitates. With an increase in tensile stress, these cracks interconnected and propagated along the intergranular γ′ precipitates, leading to the formation of pores. Moreover, because the size of the intragranular γ′ precipitates exceeded 80 nm, the critical resolved shear stress (CRSS) was primarily activated by the Orowan bypassing mechanism. Notably, the CRSS of dislocation motion, such as stacking fault, strong-coupled shearing, and Orowan bypassing, decreased with increasing intragranular γ′ precipitate size. Therefore, the intragranular strength also weakened and the intragranular γ′ precipitates coarsened under continuous thermal conditions. This research on precipitate evolution and failure mechanisms will contribute to a better understanding of failure mechanisms for Ni–Co–Cr-based superalloys.

Virtual reassembling of 3D fragments for the data-driven analysis of fracture mechanisms in multi-component materials

This paper introduces a novel method for characterizing fracture mechanisms in multi-component materials using 3D image data gained by X-ray computed tomography (CT) measurements. In mineral liberation, the understanding of these mechanisms is crucial, particularly whether fractures occur along the boundaries of mineral phases (intergranular fracture) and/or within mineral phases (transgranular fracture). Conventional techniques for analyzing fracture mechanisms are focused on globally comparing the surface exposure of mineral phases extracted from image measurements before and after fracture. Instead, we present a virtual reassembling algorithm based on image registration techniques, which is applied to 3D data of multi-component materials before and after fracture in order to determine and characterize the individual fracture surfaces. This enables us to conduct a local quantitative analysis of fracture mechanisms by voxelwise comparing adjacent regions at fracture surfaces. A quantitative analysis of fracture mechanisms is especially important in the context of recycling processes. As primary deposits are decreasing worldwide and the focus is shifting on reducing wast, the interest in secondary raw materials has increased. However, lower-concentrated, but valuable materials are often overlooked in recycling routes designed for higher-concentration materials and dissipate. Therefore, efforts are being made to enrich valuable elements, such as lithium, as engineered artificial minerals through pyrometallurgical processes. The subsequent liberation through comminution processes, such as crushing, is essential for the recovery of valuable minerals. A better understanding of crushing processes, especially fracture mechanisms in slags, is crucial for the success of recycling. The reassembling algorithm presented in this paper is evaluated through a simulation study, followed by an application to a naturally occurring ore and a slag resulting from a recycling process.

Fracture mechanisms and suppressed temperature effects of YBCO superconducting materials dominated by grain boundaries

YBa2Cu3O7-δ (YBCO) superconducting materials are typical polycrystalline materials, and the presence of intrinsic grain boundaries (GBs) is the main reason that affects superconductivity, in which high-angle GBs lead to low critical current density. There is no doubt that GBs also play an important role in the mechanical properties and fracture mechanisms of YBCO superconducting materials. In this work, the uniaxial tensile molecular simulations have been performed to investigate the fracture mechanisms of YBCO superconducting materials dominated by different types of GBs. The (100) single crystal and (110) twinning boundary models have been employed as comparative references. The results show that the low-angle GBs of YBCO consist of discontinuous dislocation cores, while the high-angle GBs are composed of a series of structural unit arrangements. Meanwhile, Young's modulus and ultimate tensile strength of YBCO can be enhanced by twin boundaries, while the other GBs have different effects on Young's modulus and ultimate tensile strength. In addition, the expansion of dislocation cores and the deformation of characteristic structural units are the main fracture mechanisms. More significantly, a novel temperature effect has been found to be significantly suppressed under the high-angle GBs. These findings not only provide a deep understanding of the GB structures, but may also provide guidance for modulating the mechanical properties of YBCO superconducting materials.

Fracture mechanics approach to minimum reinforcement design of fibre-reinforced and hybrid-reinforced concrete beams

Fracture mechanics approach to minimum reinforcement design of fibre-reinforced and hybrid-reinforced concrete beams

Direct FE[formula omitted] failure prediction of fused-filament fabricated parts

The rapid development of additive manufacturing technology and the consequential microstructural variations have brought the simulation aspects into focus. To capture an accurate mechanical response, multiscale methods are used to determine the macroscopic constitutive behaviour. Among these methods, the recently introduced Direct FE2 (DFE2) technique has shown some success in predicting material behaviour via the direct incorporation of representative microstructure volumes into the macroscopic structure. The present study highlights the predictive modelling capabilities of DFE2 linked with an element elimination technique (EET) in investigating the performance of fused-filament fabrication (FFF) samples under tension and compression. Parts were manufactured by FFF with various raster angles to vary the failure characteristics. Their mechanical responses and fracture behaviours were simulated by DFE2 and benchmarked against conventional finite element (FE) analysis, highlighting that the results of DFE2 were competitive against FE. Most importantly, the combination of EET with DFE2 resulted in predictive insights regarding changes in the failure surface for samples with varying raster angles.

Dynamic Mechanical Response and Fracture Characteristics of Multi-flawed Rocks Exposed to Hydrostatic Confinements

Dynamic Mechanical Response and Fracture Characteristics of Multi-flawed Rocks Exposed to Hydrostatic Confinements

Coupled slow strain rate and acoustic emission tests for gaseous hydrogen embrittlement assessment of API X65 pipeline steel

Hydrogen embrittlement (HE) sensitivity of API X65 pipeline steel was evaluated in this study through the simultaneous application of slow strain rate tests (SSRT) and acoustic emission (AE) monitoring on hollow dog-bone samples. Exposure to hydrogen gas decreased the elongation to fracture percentage with a significant rise in embrittlement index for the higher pressure of 10 MPa. SEM images of the fracture surfaces demonstrated that the fracture mechanism changed from ductile to ductile–brittle under 1 MPa pressure, and to predominantly brittle with quasi-cleavage characteristic when testing under 10 MPa hydrogen gas. Classification of the AE events based on waveform analysis revealed a mainly shear mode fracture for 10 MPa nitrogen sample, while a dominant tensile mode fracture was identified for the hydrogen charged samples.

Crystal plasticity modeling and analysis for the transition from intergranular to transgranular failure in nickel-based alloy Inconel 740H at elevated temperature

The precipitation-strengthened Nickel-based alloy Inconel® 740H® (IN740H) exhibits increased ductility at higher applied strain rates during quasi-static tensile tests at an elevated temperature of 760 °C. The examination of fracture surfaces in this context reveals a noteworthy transition of underlying fracture mechanisms from transgranular to intergranular fracture as the applied strain rate decreases from 1×10−3/s to 0.83×10−4/s. To thoroughly understand the mechanical response of IN740H under these conditions, this study develops a crystal plasticity finite element (CPFE) model. This model incorporates various deformation mechanisms including dislocation slips, climb, and grain boundary sliding, which are relevant to the test conditions. The model is calibrated using data from both tensile tests at different strain rates and creep tests across a broad stress range at 760 °C, enabling the accurate determination of model parameters for each mechanism. Simulation results well captured the experimental observations of different failure modes. At higher strain rates, the model shows a dominance of dislocation slip leading to heterogeneous plastic deformation and formation of transgranular shear bands causing the failure, while at lower strain rates, an increased activity of grain boundary sliding causes grain boundaries crack leading to intergranular failure.