Wedge Splitting Test Research Articles

Effects of alkali-silica reaction on the fracture energy of mass concrete: Experimental investigations using the wedge splitting test

This work aims to study the effects of alkali-silica reaction (ASR) on the fracture energy of reactive mass concrete mixtures (maximum aggregate sizes of 38 mm and 76 mm), characteristic of an existing hydro-electric facility. More than 40 large concrete blocks were casted and stored in a controlled environmental chamber for more than 1100 days. The blocks as well as cylindrical reference cores were monitored in expansion. Wedge splitting (WS) tests with different notch to depth ratios, were performed at different ASR advancements to assess the evolution of mechanical and fracture parameters of concrete. Digital Image Correlation (DIC) technique was used to monitor the tests. Damage Rate Index (DRI) technique was also performed on cores extracted from the blocks, following the completion of the splitting tests. Important size and heterogeneity effects were observed on the assessed ASR expansion and fracture energy. By filtering these effects, it was possible to assess for the first time in literature the evolution of size-independent fracture energy with respect to intrinsic volumetric expansion of concrete. The findings suggest that fracture energy increases for moderate levels of expansion, due to crack branching within the splitting crack, interacting with pre-existing ASR-induced cracks. This phenomenon was not observed for the case of a few horizontally cast blocks where the splitting crack ran parallel to pre-existing ASR cracks, indicating an anisotropy in ASR damage effects on the fracture energy of concrete.

The optimal design of double-edge wedge-splitting tensile test for ultra high performance concrete

The double-edge wedge-splitting (DEWS) test is a new indirectly tensile test, those of stress distribution similar with the uniaxial tension state. However, there are no standard rules regarding the splitting specimen and groove dimensions suitable for the UHPC tensile properties. In this paper, DEWS test, conventional splitting test, and direct tensile test were performed to measure the tensile strength and tensile stress vs. crack opening response of UHPC. The digital image correlation (DIC) technique was utilized to detect the crack propagation paths and fracture modes. Two specimen shapes, three groove angles and three fiber types are utilized to this investigation. The experimental results of 18 DEWSs under tensile loading, including peak strength, tensile stress-COD curves, fracture modes and properties, and the crack paths, are presented. The results show that the UHPC tensile performance used with cubic specimen is less sensitive to groove angle than in the cylinder. An important finding is that DEWS test used with 45° grooved cube specimen presents the lowest variation in peak strength and stress-COD curve for UHPC tensile behaviors. After comparison, the tensile strengths using DEWS test and direct tension test are essential equal in first-cracking and peak strength. The crack paths and fracture mode are basically the same inside the DEWS and direct tension specimens. Accordingly, the DEWS test used with 45° grooved cubic specimen is suggested for measuring the UHPC tensile property.

Brittleness reduction of Al2O3–C refractories: Microstructure evolution and role of carbon fibers

Carbon fibers (Cf) were incorporated into Al2O3–C refractories to enhance their toughness in this work. The results indicated the 0.3 wt% Cf-adding improved the high-temperature bending strength to 1.93 MPa from 1.07 MPa, and led to a 55 % enhancement in thermal shock resistance. The above enhancement could attributed to the in-situ formed SiC coating which was observed on the carbon fiber's surface and strengthened the interface bonding between the carbon fiber and refractory matrix. A solid-liquid diffusion reaction was employed to describe the formation process of in-situ SiC layer on the carbon fibers. Furthermore, the wedge-splitting test with digital images was carried out to investigate the crack-growth trajectory during the splitting process. It was confirmed that Cf-adding could enhance the strength and reduce brittleness with the crack-bridging, pullout, and interlock behaviors of carbon fibers in the refractory matrix.

A computational approach to integrate three-dimensional peridynamics and two-dimensional higher-order classical elasticity theory for fracture analysis

AbstractAs a nonlocal alternative of classical continuum theory, peridynamics (PD) is mathematically compatible to discontinuities, making it particularly attractive for failure prediction. The PD theory on the other side can be computationally demanding due to its nonlocal interactions. A coupling between PD and refined higher-order finite element method (FEM) integrates their salient features. The present study proposes a computational approach to couple three-dimensional peridynamics with two-dimensional higher-order finite elements based on classical elasticity. The bond-based PD modeling is considered in a region where damage might appear while refined finite element modeling is used for the remaining region. The refined finite elements employed in this study are based on the 2D Carrera Unified Formulation (CUF), which provides 3D-like accuracy with optimized computational efficiency. The coupling between PD and FEM is achieved through the Lagrange multiplier method which permits physical consistency and compatibility at the interface domain. An adaptive convergence check algorithm is also proposed to achieve predetermined accuracy in the solution with minimum computational effort. Simulations of quasi-static tension tests, wedge splitting tests and L-plate cracking tests are carried out for verification. In-depth analysis shows that the present approach can reproduce the linear deformation, material degradation and crack propagation in an effective way.

Direct validation of 3D meso-scale fracture modelling of UHPFRC by in-situ micro X-ray CT wedge-split tests and parametric studies

3D meso-scale finite element models of ultra high performance fibre reinforced concrete (UHPFRC) based on in-situ micro X-ray Computed Tomography (CT) images are developed and validated in this study. The CT images at 16.9 μm voxel resolution from a progressive wedge-split test were converted into meso-scale 3D tetrahedron meshes. The short fibres, embedded in the mortar matrix, were modelled by truss elements with equivalent elastoplastic constitutive laws transformed from single fibre pullout load–displacement curves, so as to indirectly model the fibre–matrix interfaces. A concrete damage plasticity model was used to simulate damage and fracture in the mortar. The simulated load–displacement curves, final crack patterns, and load-crack opening curves were found in good agreement with the in-situ CT test results, and the non-vertical crack path was significantly affected by the overall orientation of fibres bridging the crack. Further simulations with all the fibres perpendicular to the tensile splitting direction showed that the peak load and fracture energy increased by 42 % and 45 % respectively from 1 % to 3 % fibre volume fraction. This indicates the need to optimize the fibre orientation for best mechanical performance according to the loading conditions.

Evaluation of tensile failure behavior of ultra high performance concrete under double-edge wedge splitting and direct tension loadings

UHPC tensile strength is often measured indirectly through splitting tests and flexural tests substituted for direct tensile tests. However, the tensile strain-hardening and multiple cracking performance of UHPC is highly sensitive to the testing techniques and the specimen shape used. In this paper, the double-edge wedge splitting (DEWS) test and direct tensile test were performed to evaluate the tensile strain-hardening properties and damage evolution of UHPC. The digital image correlation (DIC) technique was utilized to detect the cracking opening displacement (COD) and evaluate the damage evolution in the tensile stress vs. crack opening response. The results show that the tensile strength between DEWS and direct tensile tests in UHPC strain-hardening and strain-softening stages had a high correlation value of 0.98. Moreover, the difference in cracking strengths in the DEWS and direct tensile tests was less than 5 %, and those of peak strengths were fewer than 5.23 MPa. An improvement in relative COD increment was observed in DEWS and direct tensile tests before 90 % of the peak load at the post-peak branch. Results indicate that the DEWS testing is an efficient way to evaluate the UHPC tensile failure properties.

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.

Finite element modelling of refractories fracture process zone with gradient enhanced damage models

This study investigates the numerical simulation of fracture behaviour in quasi-brittle materials like magnesia spinel refractories using the Gradient-Enhanced Damage (GED) model. It focuses on the complex modelling of these materials non-linear responses and compares conventional and variant GED models through a wedge splitting test. The results demonstrate that all GED models show a good fit to experimental data. However, the conventional GED model falls short in accurately depicting the fracture process zone. In contrast, the localizing GED model more accurately represents the fracture process zone, limiting spurious damage distribution, but requires finer meshing, elevating computational demands. The stress-based variant reduces spurious damage but is less effective comparatively. The study also assesses the role of heterogeneous strength distribution in replicating realistic crack patterns as observed in experiments.

Effect of Fiber Orientation on the Fatigue Behavior of Steel Fiber-Reinforced Concrete Specimens by Performing Wedge Splitting Tests and Computed Tomography Scanning

This paper shows the relationship, in steel fiber-reinforced concrete, between fiber orientation and fatigue response through the combined use of computed tomography (CT), digital image processing (DIP) software and wedge splitting test (WST). The WST cubes were extracted from conventional 150 × 150× 600 concrete prisms and a groove and notch were carved on different faces in such a way that in half of the test specimens the fibers are oriented mostly perpendicular to the breaking surface and, in the other half, the fibers are mostly oriented parallel to the breaking surface. Fiber orientation was obtained using a CT device and DIP software from a miniprism extracted from the previously mentioned concrete prisms. The results show that there is a strong correlation between the crack-sewing fiber orientation on the one hand and fatigue life and crack opening rate per cycle on the other hand. Cubes with a higher percentage of fibers perpendicular to the crack surface (i.e., with a higher efficiency index) show a longer fatigue life and a lower crack opening rate per cycle, while cubes with a lower efficiency index show a shorter fatigue life and a higher crack opening rate per cycle.

R3-DICnet: an end-to-end recursive residual refinement DIC network for larger deformation measurement.

Digital image correlation (DIC) is an optical metrology method for measuring object deformation and has been widely used in many fields. Recently, the deep learning based DIC methods have achieved good performance, especially for small and complex deformation measurements. However, the existing deep learning based DIC methods with limited measurement range cannot satisfy the needs of real-world scenarios. To tackle this problem, a recursive iterative residual refinement DIC network (R3-DICnet) is proposed in this paper, which mimics the idea of the traditional method of two-step method, where initial value estimation is performed on deep features and then iterative refinement is performed on shallow features based on the initial value, so that both small and large deformations can be accurately measured. R3-DICnet not only has high accuracy and efficiency, but also strong generalization ability. Synthetic image experiments show that the proposed R3-DICnet is suitable for both small and large deformation measurements, and it has absolute advantages in complex deformation measurement. The accuracy and generalization ability of the R3-DICnet for practical measurement experiments were also verified by uniaxial tensile and wedge splitting tests.

Brittleness reduction of low-carbon Al2O3-C refractories with in-situ formation MgAl2O4/CNTs layer between aggregate and matrix

Crack propagation at the aggregate/matrix interfaces of refractories favors their brittleness reduction and better thermal shock resistance. Design of suitable interfaces is highly desirable in refractories. With coating of magnesium citrate and nickel nitrate hexahydrate catalyst on corundum aggregates, a thin MgAl2O4/CNTs interfacial layer of porous structure was formed between aggregate and matrix in low-carbon Al2O3-C refractories. The formation mechanisms of this interfacial layer are pyrolysis of magnesium citrate, reaction with corundum into MgAl2O4 and growth of CNTs. Comparison study of mechanical results shows that this compound interfacial layer contributes to an increase in cold crushing strength, and brittleness reduction assessed with wedge splitting tests. The 3-times water quenching tests testified that brittleness reduction brings about a mild loss of cold modulus of rupture. The results support the conclusion that aggregate/matrix interface design is an effective method to reducing the brittleness and eventually improving the thermal shock resistance of refractories.

Fracture parameter identification by Digital Image Correlation and Finite Fracture Mechanics for millimeter-scale samples

In situ wedge splitting tests are conducted on millimeter-size PMMA samples. A method to determine the material tensile strength and critical energy release rate (ERR) from digital image correlation (DIC) and a full finite element implementation of the coupled criterion (CC) is proposed. The best material parameter identification is achieved when the CC implementation based on measured displacements on a contour of a subdomain is performed, providing the crack initiation length remains small compared to the subdomain size.

Assessment of fracture behavior of mono-carbonate (C4AcH11) bonded alumina-spinel castables by wedge splitting test

In this study, nano CaCO3 was added to CAC to prepare mono-carbonate hydrate (C4AcH11). The mechanical properties, pore structure, and fracture behavior at high temperatures of mono-carbonate bonded alumina-spinel castables were assessed. The results showed that the plate-like mono-carbonate promotes early setting behaviors and higher demolding strength. The sintering effects of nano CaCO3 compensate for the strength decay at intermediate temperatures. Moreover, well-distributed CaO sources are essential to forming smaller and elongated CA6 interlocking laminar structures and contribute to the refinement of pore structures (<10 µm) after firing at 1600 °C. The fracture behavior of CAC-bonded alumina-spinel castables evaluated at 1400 °C shows that the deformation of fine grains/matrices and pores leads to typical nonbrittle fracture.

Implementation of the adaptive phase-field method with variable-node elements for cohesive fracture

In this paper, an adaptive phase-field model based on variable-node elements and error-indicator is presented to predict cohesive fracture evolution. The phase-field cohesive-zone model is characterized by the parameterized energetic degradation and crack geometric functions. A staggered iteration scheme is used to solve the coupled non-linear system of displacement field and phase field. The adaptive local refinement algorithm facilitated with error-indicator and variable-node elements is developed to improve computational efficiency. To determine the adaptive local refinement regions, we apply the phase-field threshold to characterize the crack regions and a novel history field threshold to denote crack tips. The variable-node elements are acted as transition elements to flexibly yet simply link the fine and coarse meshes. In addition, a fast method to generate the variable-node elements is given. Several representative numerical examples are studied to demonstrate the performance and accuracy of the proposed adaptive phase-field model, consisting of a mixed-model fracture of L-shape panel, the wedge-splitting test, the classic three point bending experiment and a plate with a hole and multi-cracks under tensile/shear loadings. The computer codes can be accessed at: https://github.com/hhuztc/Adaptive-PFM.git.

Investigation of the influence of impurities typical in secondary raw materials on the behavior of high alumina castables – Part 1: Design of the castables, setting properties and high temperature fracture behavior

The research was carried out using a primary high alumina raw material that was selectively spiked with typical elements found in secondary raw materials. These artificially spiked raw materials were processed into cement-free high alumina monolithics by 1:1 substitution of the corresponding high alumina matrix fraction in the formulation. Regardless of the impurity, the rheology and setting behavior of the developed model refractory castables showed a very high degree of comparability with the reference material. The same workability allowed the production of comparable test pieces for hot wedge splitting tests. The effects on high temperature fracture behavior observed in this study can therefore be attributed solely to the impurities.

Modeling and fracture behavior of mesoscale concrete considering actual aggregate shapes and placement domain shapes

The establishment of meso-concrete models under complex boundary conditions still faces great challenges at present. In this study, a numerical framework for constructing concrete meso-structures in irregularly placement domains is developed based on the dot matrix idea. In addition, a solution to quickly obtain numerous 3D aggregates with realistic morphology as well as 2D cross-section aggregates is proposed. On this basis, wedge splitting (WST) tests with different coarse aggregate contents are conducted and the numerical results agree well with experimental results. The fracture mechanism of the wedge splitting specimen was then investigated by analyzing the contact force chain and crack evolution. Finally, parametric analysis was used to investigate the impact of aggregate and mortar strength on the fracture characteristics of WST specimens. The results show that the numerical framework has good robustness. Tensile stress is the driving force for crack extension in WST specimens, the area of the pure tensile stress zone and the distance between the front end of the pure tensile stress zone and the crack tip decreases as the crack extension length increases. With the increase of coarse aggregate content, the deflection effect of the crack is intensified, the crack path becomes more tortuous, and the fracture zone range also increases. The mechanical properties of concrete and the crack extension path are significantly influenced by the strength of the mortar and the aggregate, but the aggregate does not contribute as much to the overall strength of the concrete structure as the mortar matrix does.

Integrated approach for quality control of fibre-reinforced sprayed concrete for tunnel lining

The quality control of fibre-reinforced sprayed concrete (FRSC) for tunnel linings has evolved significantly in recent years. One of the main innovations concerns the control of FRSC in the lining using small-scale tests of cores extracted from the tunnel instead of the traditional approach, based on the characterization of panels sprayed on moulds. This study combines experimental and numerical investigations to evaluate if constitutive models derived from small-scale tests, such as the double punch test (DPT) and double-edge wedge splitting (DEWS) test can be used to predict the behaviour of larger panels in the round panel test (RPT) ASTM C1550. Results confirm the link between the new and the traditional approaches. Findings demonstrate that small-scale tests can be considered as a viable method to evaluate the mechanical behaviour of FRSC both in regular quality control programs and for verifying an existing tunnel lining condition.

Evaluation of fracture behavior of lightweight magnesia‐based refractories via wedge splitting test along with acoustic emission detection

AbstractThe effect of spinel powder on the fracture behavior and mechanical properties of lightweight magnesia‐based refractories containing microporous magnesia aggregates with high apparent porosity (37.4%) were investigated by the wedge splitting test (WST) with the digital image correlation and acoustic emission. With the addition of spinel powder, lightweight magnesia spinel refractories showed a higher cold compressive strength compared with lightweight pure magnesia refractories. From the WST, the addition of spinel powder increased the specific fracture energy and characteristic length of lightweight magnesia spinel refractories, which improved the crack propagation resistance. The increased tortuosity of main crack and a higher ratio of crack propagation along the aggregates/matrix interface were main reasons for reducing the brittleness of lightweight magnesia spinel refractories. Besides, acoustic emission (AE) signal activity indicated that the propagation of pregenerated micro‐cracks by the thermal mismatch and the development of fracture progress zone were primary ways to consume energy in lightweight magnesia spinel refractories. The reduced proportion of crack propagation within aggregates was also detected by the peak frequency of AE signals in lightweight magnesia spinel refractories. For microporous magnesia aggregates with high apparent porosity (37.4%), lightweight magnesia spinel refractories also showed reduced brittleness fracture behavior than lightweight pure magnesia refractories.

A methodology for performing and evaluating miniaturized wedge splitting tests accompanied by digital image correlation

This methodology study investigated the application of a miniaturized wedge splitting test (WST) accompanied by digital image correlation (DIC) for the novel analysis of fracture mechanisms occurring during fracture of quasi-brittle refractories. Results from representative miniaturized WST were compared to standard WST results. Accuracy was improved by excluding experiments where unstable crack growth occurred or when the crack propagated perpendicular to the notch direction. Besides a sample size effect, a limitation of the fracture process zone (FPZ) size occurred. This FPZ limitation reduced the specific fracture energy G‘F from 112.4 N/m for the standard WST to 55.4 N/m for the miniaturized WST. Due to the sample size effect, the brittleness number was 0.194 and lower than for the standard WST with 0.414. The miniaturized WST enabled a more detailed evaluation of the acting mechanisms by analyzing the load–displacement curves together with the DIC. Grain bridging, microcracking, crack deflection/branching mechanisms and friction effects were evaluable. The analysis of the miniaturized WST could prove the supposed mechanisms acting during the standard WST. However, not all observed mechanisms also occurred in the standard WST due to the difference in the samples' brittleness.

Interfacial fracture performance of engineered cementitious composites and fire-damaged concrete

This study investigated the interfacial fracture properties of engineered cementitious composites (ECC) and fire-damaged concrete using bi-material wedge splitting specimens. The full load versus crack mouth opening displacement (CMOD) curves of the bi-material specimens were obtained using wedge splitting tests, and the fracture toughness and fracture energy were calculated. The effects of the maximum temperature experienced by the concrete (20–700 °C) and the interfacial roughness (0–5 mm) on the interfacial damage mode, fracture toughness, and fracture energy were studied. The results indicated that when the maximum temperature experienced by the concrete was lower than 400 ℃ or the interface was smooth, the damage mode of the specimens was primarily adhesive failure. However, when the maximum temperature experienced by the concrete was equal to or greater than 400 °C and the interface was rough, cohesive failure often occurred in the specimens. Owing to the change in the damage mode, the bi-material specimens had the highest values of the fracture toughness and fracture energy at 400 °C. When the maximum temperature experienced by the concrete exceeded 400 °C, both the fracture toughness and fracture energy decreased with the increasing temperature. In this study, the fracture toughness and fracture energy of the bi-material specimens first increased and subsequently decreased with the increaseing roughness, and the maximum fracture toughness and fracture energy values were obtained when the interface roughness was 3 mm. The study results would provide a reference for the surface treatment when ECC was used in the strenthening of fire damaged concrete.