Stable Crack Research Articles

Mechanical behaviour of double lining with outer segmental lining and inner fibre reinforced concrete lining under internal water pressure

Double lining with outer segmental lining and inner reinforced concrete lining is extensively used for water conveyance tunnel with internal water pressure. In order to improve the performance of the double lining, the steel rebar and fibre reinforced concrete (R/FRC) is designed for inner lining to enhance the tensile strength of reinforced concrete (RC). Full-scale tests were conducted to compare the mechanical behaviour of double lining with inner RC and R/FRC linings. The test results showed that both types of lining failed in four stages: elastic stage, inner lining cracking stage, crack stabilization and segmental joint damage stage, and failure stage. The cracks firstly occurred in the inner lining near the segmental joints and then propagated to the whole inner lining ring. Finally, the crack width and segmental joint opening successively exceeded the thresholds, and the double lining failed until the breaking of segmental joints. Compared with inner RC lining, R/FRC lining had better performance in terms of the initial cracking load, cracking-control ability, post-cracking stiffness, service limit capacity of inner lining and waterproof capacity of segmental lining. The improvement ratios were 36.3%, 50.3%, 46.3%, 37.2% and 31.1%, respectively. The influences of fibre volume fraction, fibre aspect ratio, and reinforcement ratio of steel rebar on the mechanical performance of R/FRC lining were also investigated based on a previously proposed analytical model. The parametric analysis demonstrated that higher fibre volume fraction and fibre aspect ratio can result in more significant increase of mechanical performance of R/FRC lining. The steel fibres were found to have more significant effects on the post-cracking behaviour of the lining than same amounts of reinforcing bars. Finally, an optimized function was proposed to fulfil a balance of lining performance and economy. The experimental and analytical results indicated that R/FRC lining can substitute RC lining and perform higher mechanical performance in water conveyance tunnels.

Crack instability of concrete in fire: A new small-scale screening test for spalling

The thermal energy accumulated by the concrete cover exposed to fire is several orders of magnitude larger than the elastic energy entailed by thermal stress. Water vaporization can quickly convert this huge energy source into the mechanical work required to drive the fracturing and acceleration of concrete shards, namely the explosive spalling phenomenon.In order to fully understand the role of moisture content in this mechanism, a novel small-scale screening test setup was developed, where a planar crack is triggered in a heated concrete disk by the high thermal dilation of an embedded polymer ring. The concurrent pore saturation level is changed by the heating rate, the addition of polypropylene fibre, and the preliminary drying of samples, with direct effects on crack instability.The ability of this setup to categorize the inherent spalling sensitivity of different concrete mixes was validated against standard fire tests on prestressed slabs.

The experimental and numerical simulation of interface shear behavior between ECC and coal gangue concrete

Composite concrete structures are designed to integrate various concrete types at different levels, thereby improving their shear properties. In this paper, the effects of the interfacial keyway, reinforcement ratio, and pouring sequence on the shear strength of concrete-coal gangue concrete interface are studied by experiments and analyses. A total of 10 specimens were made, and the push-off test was carried out. Nonlinear finite element simulation was carried out using ABAQUS, and the influence of keyway angle on crack development was studied based on the extended finite element method (XFEM). It is found that the remarkable tensile and crack stability of engineered cementitious composites (ECC) has a positive effect on the shear strength of the ECC-coal gangue concrete interface. The ultimate shear strength of the interface and the ductility of the interface under shear load are significantly improved by interfacial shear reinforcement. The introduction of an interfacial keyway improves the shear strength of the concrete interface, and the number of keyways has the most significant effect on the interfacial shear strength. The angle between the crack propagation direction and interface will increase with the increase of keyway angle, the simulation results are in good agreement with the experiments.

Flexural mechanical properties of H-shaped steel-bamboo scrimber composite beams

To improve the torsional buckling resistance and enhance the load-bearing capacity and ductility of H-shaped steel beams with carbon reduction materials, the concept of combining H-shaped steel with bamboo scrimber has been proposed, in which the bamboo scrimber, an eco-friendly, lightweight, and high-strength material, promises significant structural benefits. Five full-scale specimens were designed and tested, including one H-shaped steel, two bamboo scrimber beams, and two composite beams. Additionally, further simulation tests were conducted using the verified finite element model (FEM). The experimental results revealed a remarkable increase in the ultimate bearing capacity of the composite beams by 96.5 % compared to bamboo scrimber-only beams and 72.7 % compared to steel-only beams. Meanwhile, distinct from the brittle fracture characteristics of the bamboo scrimber beams, which include instantaneous crack propagation and interlaminar cracking, the composite beams exhibited localized vertical cracks within the beam's tensile zone, effectively avoiding the torsional buckling issue of the H-shaped steel beam. The results further demonstrated that increasing the cross-sectional size of the composite beams can significantly enhance their flexural stiffness (up to 669 %), and reduce the deflection at the ultimate flexural moment. When the area of the bamboo scrimber in the cross-section was reduced with an unchanged steel profile, the initial stiffness dropped to 79 % without incurring any structural instability. Furthermore, by integrating the experimental and simulation results, a fiber element model for this composite beam structure is proposed to predict the maximum flexural moment value in the ultimate state. A corresponding calculation program was developed, and it is found that the calculated predicted ultimate flexural moment values exhibited negligible deviation from both experimental and FEM results, with AV of 1.0001, AAE of 2.21 %, and COV of 3.28 %.

Experimental investigations on the behaviour of structural-sized wood-CFRP composite beams in local fire

AbstractThe study involved combustion of 24 structural-sized beams under three-point bending subjected to substantial loading prior to ignition, reaching 90% of characteristic load-carrying capacity. A localised fire exposure zone was established proximal to the region experiencing the highest bending moment. The specimens were categorised into two groups: the first consisted of tituted by glued-laminated timber (B), and the second comprised wood-CFRP (carbon fibre reinforced polymer) composite (BW). Initial measurements encompassed pre-ignition static deflection and load. Subsequently, the specimens underwent controlled combustion, during which parameters including burning duration and deflection up to failure, were documented. Following cooling with sand, two cross-sectional slices were extracted from each fractured beam, enabling to find vector-based contours of the remaining cross-section. The charring rate and the approximate heat flux density for each test were determined, enabling a direct comparison of the results. A statistically significant number of specimens was examined, facilitating a comparative analysis between reinforced and unreinforced beams concerning failure time and form. Incorporating CFRP tapes among wooden constituents was found to increase the fire resistance of the structure, however, the thickness of the wooden material enveloping the CFRP composite emerges as a pivotal determinant. This issue needs thorough testing under standard fire in the future. Nevertheless, the fact is that adding CFRP tapes engenders a distinct form of beam collapse, transitioning from instantaneous cracking in B-beams to ductile failure in BW-beams.

Consideration of the effects of frost heave force and train loads on the cracks and propagation pattern of ballastless track slabs in cold regions

The construction of ballastless railway tracks in cold regions is susceptible to frost heave forces, which may particularly impact the stability of track slab cracks and lead to crack propagation. In this paper, the frost heave tests are conducted on the concrete specimens with prefabricated cracks to observe the evolution of the frost heave force within the cracks. The frost heave force is subsequently applied to the finite element model of the concrete specimens to calculate the stress intensity factors (SIFs). In addition, the CRTS III ballastless track model is established to investigate the crack propagation patterns under frost heave and train loads. The results show that the frost heave force in concrete cracks during freezing and thawing is divided into five stages, which produces two peaks of frost heave force. Train loading caused the SIFs of the crack tips to open and close alternately and to rise significantly under the coupling effect of frost heave load. The SIF increases approximately linearly with crack width and is higher for transverse cracks than for longitudinal cracks. Small size cracks do not possess sufficient SIF values to exceed fracture toughness, implying no crack propagation. However, as the crack width is constantly increasing, the SIF will eventually exceed the fracture toughness. Therefore, strict control over track slab's crack width is necessary for CRTS III ballastless tracks.

Investigation on the failure mechanism of hard rock cavern subjected to adjacent structural plane based on physical model experiments

Structural planes present significant safety challenges in the construction of deep hard rock caverns. However, the influence mechanism of a single dominant adjacent structural plane remains inadequately explored. This study conducted a series of physical model experiments with non-penetrating structural planes positioned adjacent to the cavern under uniaxial step compression, utilizing acoustic emission (AE) and digital image correlation (DIC) to monitor the failure process simultaneously. The results indicate that adjacent structural plane can mitigate stress-induced failure, reducing peak strength and elastic modulus by 10–42% and 13–31%, respectively. In the absence of a structural plane or a non-intersecting plane, stress-induced failure intensifies. Conversely, an intersecting plane contributes to greater crack stability. Precursor AE signals emerge earlier in the combination system. The intact cavern system shows an escalating tensile fracture ratio with loading, whereas the combination systems experience a diminishing tensile fracture ratio. Finally, particle flow code (PFC) simulation revealed that the presence of a single structural plane alters the cavern’s force chains, influencing stress concentration position. This research reveals that the structural plane has the potential to alter the cavern failure mechanism, providing insights into the complex interaction between structural planes and caverns in deep engineering.

Age and crack length effects on fracture parameters of dam gallery concrete: Experimental investigation and modeling

Cracks of varying lengths may occur in galleries throughout dam construction, operation, and management processes. This study investigates the influence of age and crack length on the fracture parameters of gallery concrete, which is crucial for understanding the mechanisms and risks associated with gallery cracking and for developing appropriate treatment measures. Wedge-splitting tests were performed on gallery concrete specimens with varying crack lengths and at different ages. It was found that the fracture parameters measured at different ages all decrease with the increase of crack length. Based on the test results, size-independent material constants such as fracture toughness and the effective length of the fracture process zone were derived and modeled using an exponential function to reflect their time-varying characteristics. A fracture model was proposed and validated, which can accurately predict the fracture behavior of gallery concrete across varying ages and crack lengths, and is applicable in practical dam gallery crack analysis for assessing crack stability and evaluating the effectiveness of treatment measures.

Effects and recommendations of crack tip modelling on compliance solutions

Elastic unloading compliance is a common technique to determine instantaneous crack size in fracture mechanics properties such as resistance curves (R-curves) and fatigue crack growth. This technique uses a polynomial, developed using finite elements models, to correlate compliance with crack depth. In these models, crack tip is usually modeled with a blunt mesh (with a crack tip radius ρ) or a sharp mesh (with element size in the width direction l), and neither are standardized in the literature. This paper shows that both affect compliance, therefore potentially impacting results of fracture mechanics properties acquired from simulations. This paper focuses on how those parameters affects compliance in SE(B) models. Finally, a parameter φ that normalizes ρ or l with the width was developed and recommended values based on compliance convergence are suggested. Controlling φ may help to ensure reliable results in fracture mechanics models where compliance needs to be determined.

Prediction of fracture failure and fracture parameters for dam gallery concrete specimens of arbitrary size and age via boundary effect model and fracture extreme theory

Cracking-associated problems may occur in the construction and operation periods of dam galleries, necessitating attention to the fracture characteristics of gallery concrete. The fracture parameters of concrete exhibit size effects and also change with age. A correct evaluation of the cracking risk or crack stability of gallery structures at any given time requires a comprehensive understanding of both the size effect and time-variant characteristics of the gallery concrete fracture parameters. In the present study, 75 wedge-splitting specimens, with effective heights ranging from 200 mm to 600 mm, were cast using gallery concrete produced by the concrete mixing system at the Baihetan Dam construction site. Subsequently, fracture tests were conducted on these specimens at curing ages spanning from 7 days to 90 days. According to the test results, the fracture parameters of concrete with a specific size exhibited an increasing trend with age within 90 days, while the fracture parameters of concrete at specific ages show a significant size effect, tending to stabilize as the effective depth of specimens gradually increases. The boundary effect model was utilized for analyzing the fracture test results of concrete specimens at different ages, yielding the size-independent double-K fracture parameters and scale parameters for concrete at each age, along with their time-varying patterns. Subsequently, equations for predicting the failure loads of concrete specimens of any age and size were established. Based on the prediction results of failure loads, the double-K fracture parameters of concrete specimens of any age and size were solved by the fracture extreme theory. The predicted failure loads and fracture parameters were in good agreement with the experimental results. The findings of this study can be applied to the full lifecycle crack risk analysis or crack stability assessment of Baihetan Dam gallery structures.

Regularities of Crack Development in Substructures of Cu–Al Alloys Near the Cracking Stability Threshold

Regularities of Crack Development in Substructures of Cu–Al Alloys Near the Cracking Stability Threshold

A coupled hydro-thermo-mechanical model based on TLF-SPH for simulating crack propagation in fractured rock mass

In this paper, a hydro-thermo-mechanical coupling model based on the smoothed particle hydrodynamics with total Lagrangian formula (HTM-TLF-SPH) is proposed to simulate the crack propagation and instability process of fractured rock mass. TLF-SPH uses the Lagrangian kernel approximation, that is, the kernel function and its gradient need only be calculated once in the initial configuration, which is much more efficient than the smoothed particle hydrodynamics (SPH) based on the Euler kernel approximation. In TLF-SPH, particles interact with each other through virtual link, and the crack propagation path of rock mass is tracked dynamically by capturing the fracture of virtual link. Firstly, the accuracy and robustness of the HTM-TLF-SPH coupling model are verified by a reference example of drilling cold shock, and the simulation results agree well with the analytical solutions. Then, the crack propagation law of surrounding rock and the evolution characteristics of physical fields (displacement, seepage and temperature fields) after excavation and unloading of deep roadway under the coupling condition of hydro-thermo-mechanical are investigated. In addition, the seepage and heat transfer processes of the surrounding rock of Daqiang coal mine under different coupling conditions are successfully simulated. Meanwhile, the effect of the boundary water pressure difference on the temperature and seepage fields under the hydro-thermal coupling condition is quantitatively analyzed.

Cause, Stability Analysis, and Monitoring of Cracks in the Gate Storehouse of a Flood Diversion Sluice

The cracks in a gate storehouse for a flood-diversion sluice of the Wangfuzhou Water Conservancy Project were studied to diagnose the structural safety. The study was based on an integrated method that involved the analysis of the cause of cracks, an assessment of the crack stability, online safety monitoring and diagnosis, and real-time monitoring and control. The results demonstrated that two dominant factors leading to cracks were the non-uniform settlement of the foundation of this gate storehouse structure combined with temperature drops in winter. By comparing crack-tip stress intensity factors at different cracking depths with concrete fracture toughness, the results showed that cracks at the gate storehouse would not continue to propagate, although some time-dependent components separated from the measuring point did not converge but tended to be closed. Therefore, the diagnosis suggests that the cracks at the gate storehouse are currently in a stable state. Additionally, the crack opening index estimated using the small probability method can be used for future real-time monitoring of crack behavior.

Stress Intensity Range Dependent Slowing Down of Fatigue Crack Growth under Strain‐Induced Martensitic Transformation of Film‐Like Retained Austenite

A clear understanding of strain‐induced martensitic transformation of filmy retained austenite (RA) near stable cracks is required. Literature shows RA transformation during fatigue crack growth (FCG) in steels containing blocky but not film‐like RA, as the latter is known to resist strain‐induced phase transformation. This work investigates the transformation in 0.2% C steel processed by direct quenching and partitioning (DQP) containing filmy and a small vol% of blocky RA and compares it with RA‐free direct quenched (DQ) steel. While the DQ steel is lath‐martensitic, DQP steel has 8.5 vol% film‐like RA evenly distributed between martensite laths. The experimental FCG rates are comparable for both steels in the low regime but increasingly differing in the Paris regime, the Paris law exponent being lower for DQP (n = 2.1) compared to DQ (n = 2.5), confirming that resistance to FCG is dependent on the stress intensity range (). Moreover, RA content near crack surface decreases during crack growth from 7.5 vol% at of 25 MPa√m to 2.3 vol% at of 54 MPa√m. Results show that higher the stress intensity range in steels with film‐like RA, the higher the degree of RA transformation and more suppression of FCG rate.

Investigation of the Effect of Nanometakaolin on the Compression Behavior of Concrete by Acoustic Emission.

To investigate the influence of nanometakaolin (NMK) on the compression behavior of concrete, acoustic emission (AE) is applied to monitor the development of cracks on ordinary concrete as a control sample and concrete with the addition of 1% or 3% NMK during the whole compressive process. The AE parameters (event, count, RA-AF, etc.) and Gaussian Mixture Model (GMM) were analyzed. The results show that the addition of NMK results in a decrease of crack number and crack width. Compared to the control concrete, the concrete with NMK inhibits the initial crack, increases the crack initiation stress and crack damage stress, and improves the energy of crack propagation. Correspondingly, we proposed a critical point where the ringing counts and amplitude fit parameters are less than a certain threshold. This index can be used as a failure precursor for crack instability. According to the criterion and the GMM, what is found from the relation between RA and AF is that the resistance to tensile and shear is improved in the case of the concrete with NMK.

Natural vibrations of circular nanoarches of piecewise constant thickness

The free vibrations of elastic circular arches made of a nano-material are considered. A method of determination of eigenfrequencies of nanoarches weakened with stable cracks is developed making use of the concept of the massless spring and Eringen's nonlocal theory of elasticity. The aim of the paper is to evaluate the sensitivity of eigenfrequencies on the geometrical and physical parameters of the nanoarch.

Fast atomic crack kinking and branching in WS2

Abstract Dynamic nanocrack propagation in 1T- and 2H-WS2 strips is studied by molecular dynamics, and the T-stress and circumferential stress in linear elastic fracture mechanics are considered. As the crack propagates, the crack-tip speed (v) experiences a rapid acceleration, and then oscillates at ∼55% (1T) and ∼65% (2H) of the Rayleigh-wave speed followed by crack kinking/branching. The critical energy release rates of crack instability are estimated to be ∼1.5 J/m2 (1T) and ∼4.0 J/m2 (2H). The crack path in 1T-WS2 exhibits higher sensitivity of strain rates for atomic asymmetry around the crack tip. Examination of the dynamic crack-tip field shows that the T-stress obtained by the over-deterministic method always fluctuates in negative, and the theoretical circumferential stress curve does not accurately capture the v-dependent atomic stress distribution. Consequently, both T-stress and circumferential stress are limited in predicting the crack kinking/branching directions, which can be attributed to the discrete crystal lattice and local anisotropy of WS2, where a preferred crack path along the zigzag surface is observed. The fracture properties of WS2 might provide useful physics for its applications in nano-devices.

Study on the influence of crack depth on the safety of tunnel lining structure

In order to study the mechanism of the influence of crack depth on the bearing capacity and stability of tunnel lining structure, the calculation model of internal force of lining with cracks is established based on the theory of fracture mechanics, and the variation of stability coefficient of lining structure with Ⅰ-Ⅱ composite cracks is analyzed. With the increase of the crack depth of the vault, the bending moment of the vault gradually decreases, the stress intensity factor increases, and the stability factor f gradually decreases. The critical depth of tensile and compressive crack instability propagation is 54.38% and 74.01% of the lining thickness, respectively. The relationship between load and displacement, internal force and contact pressure, crack propagation depth and time, and strain variation of the secondary lining vault during the loading process of cracks at different depths are analyzed. It is found that cracks have a significant impact on the bearing capacity of the secondary lining, and the deeper the cracks, the lower the bearing capacity and stability. When the crack depth is large, the tensile stress peaks in a short time, and applying smaller loads can also cause the crack to expand. When the crack depth is 0.9d (d is the thickness of the secondary lining), the bearing capacity of the structure decreases by 72.57%. The research results can provide certain theoretical guidance for the prediction and prevention of lining crack propagation during tunnel operation.

Investigation of crack recognition and spatio-temporal evolution pattern in coal samples damage

Understanding the evolution mechanism of cracks helps to evaluate the behavior and performance of rock masses and provides a theoretical basis for the mechanism of crack propagation and instability. For this purpose, a rock mechanics testing system and an acoustic emission monitoring system were used to conduct acoustic emission positioning experiments on coal samples under uniaxial compression. According to clustering theory, the distribution pattern of microcracks and the dynamic evolution process of multiple cracks were studied. Subsequently, the reasons for the change in the spatio-temporal entropy (H) and fractal dimension (D) of a single crack were revealed. The research results show that microcracks present a statistical equilibrium distribution, the Gaussian distribution model is applicable to cluster crack distribution patterns, and a machine learning method can effectively identify cracks. The fractal dimension reflects the spatial characteristics of three-dimensional elliptical cracks, and low-dimensional cluster cracks are more likely to develop into macroscopic cracks. The change of H is related to the formation process of cracks, and an abnormal H (sudden increase and sudden decrease) could provide precursor information for the instability of coal samples. This research provides a new method to study crack distributions and formations and shows the competitiveness of the method in evaluating the damage state of coal.

Effect of notch depth ratio and specimen width on mode II fracture properties of ECC

Engineered Cementitious Composites (ECCs) can be used as repair and reinforcement materials, which can improve the shear performance of the structure. Hence, doubled-notched and unnotched half-symmetric loading tests are used to study the mode II fracture properties of ECC and concrete, and the fracture failure morphology and fracture parameters of ECC and concrete are compared. The effects of the notch depth ratios of 0, 0.1, 0.3, and 0.5 and the specimen widths of 200 mm, 250 mm, and 300 mm on the mode II fracture properties of ECC and concrete are investigated. The mode II crack development of ECC can be divided into four stages: microcrack development stage, multiple cracking stage, main crack formation stage, and crack instability development stage. It can be found that ECC not only has high ultimate tensile strain and energy dissipation capacities, but also has obviously better ability to resist the unstable propagation of shear cracks than concrete. ECC can meet the high shear resistance requirements of some parts of the structure.