Load-crack Mouth Opening Displacement Research Articles

Effects of Mix Components on Fracture Properties of Seawater Volcanic Scoria Aggregate Concrete.

The fracture mechanism and macro-properties of SVSAC were studied using a novel test system combined with numerical simulations, which included three-point bending beam tests, the digital image correlation (DIC) technique, scanning electron microscopy (SEM), and ABAQUS analyses. In total, 9 groups and 36 specimens were fabricated by considering two critical parameters: initial notch-to-depth ratios (a0/h) and concrete mix components (seawater and volcanic scoria coarse aggregate (VSCA)). Changes in fracture parameters, such as the load-crack mouth opening displacement curve (P-CMOD), load-crack tip opening displacement curve (P-CTOD), and fracture energy (Gf), were obtained. The typical double-K fracture parameters (i.e., initial fracture toughness (KICini) and unstable fracture toughness (KICun)) and tension-softening (σ-CTOD) curve were analyzed. The test results showed that the initial cracking load (Pini), Gf, and characteristic length (Lch) of the SVSAC increased with decreasing a0/h. Compared with the ordinary concrete (OC) specimen, the P-CMOD and P-CTOD curves of the specimen changed after using seawater and VSCA. The evolution of the crack propagation length was obtained through the DIC technique, indicating cracks appeared earlier and the fracture properties of specimen decreased after using VSCA. Generally, the KICun and KICini of SVSAC were 36.17% and 8.55% lower than those of the OC specimen, respectively, whereas the effects of a0/h were negligible. The reductions in Pini, Gf, and Lch of the specimen using VSCA were 10.94%, 32.66%, and 60.39%, respectively; however, seawater efficiently decreased the negative effect of VSCA on the fracture before the cracking width approached 0.1 mm. Furthermore, the effects of specimen characteristics on the fracture mechanism were also studied through numerical simulations, indicating the size of the beam changed the fracture toughness. Finally, theoretical models of the double-K fracture toughness and the σ-CTOD relations were proposed, which could prompt their application in marine structures.

Damage and fracture investigation of self-compacting concrete in the filling layer of CRTS-III using three-point bending tests

Literature about filling layer self-compacting concrete (FLSCC) has indicated that FLSCC plays a crucial role in its application to Chinese railway track system III (CRTS-III). It is apparent that the damage characteristics of FLSCC have not been clearly investigated, as the actual working conditions are not considered enough. There exists a remaining gap about the mechanical properties and fracture behaviors of FLSCC under bending loads. In order to fill this gap, in this study, three-point bending (TPB) tests on the FLSCC beams with precast notches were performed based on the real bending situation of FLSCC in CRTS-III. Acoustic emission (AE), strain gauges and a digital camera were utilized to monitor the fresh evolutionary laws of micro- and macro-cracks as well as the corresponding damage morphology. The study presents that the monitored load-crack mouth opening displacement (P-CMPD) curves, load-strain (P-ε) curves and AE activity curves all exhibited similar distributions, including four stages with increasing bending loads. In term of the AE activities, the first apparent crack in the interior and warning point of failure can be detected by analyzing the captured AE activities of the FLSCC beams. In addition, the dominant failure mode of the specimens was determined as tensile cracks according to the rising angle-average frequency (RA-AF) criterion for potential FLSCC application in terms of the limiting ratios. This failure mode indicates that under pure bending loads, the mid-span section is subjected to the maximum bending moment, and the specimen experiences tensile damage. Therefore, for future practical application, the structural health state of FLSCC in CRTS-III could be nondestructively real-time monitored by the AE technique. Meanwhile, the calculated fracture parameters of the FLSCC specimen can reflect the cracks stability of the specimen. This study can also present experimental and theoretical references for further studies on FLSCC in CRTS-Ⅲ under other types of loads.

Bending damage and fractal characteristics of steel fiber-reinforced concrete under three-point bending test

Steel fiber-reinforced concrete (SFRC) has outstanding mechanical properties and is widely used in the construction industry. To investigate the damage characteristics of SFRC beams with different steel fiber (SF) contents (0, 15, 30, 45, and 60 kg/m3) accurately, this study analyzed the bending damage (including the load-crack mouth opening displacement curve, flexural–tensile strength, fracture toughness, and microstructure and macrostructure) and fractal characteristics of SFRC beams under three-point bending tests. The crack propagation process in the SFRC beams was monitored by the digital image correlation technique. Information on microcrack evolution and macrocrack development during the loading process was collected using acoustic emission techniques. Furthermore, the correlation dimension reflecting the development state of the cracks was obtained based on the Grassberger–Procaccia algorithm. Test results showed that the performance of concrete beams in terms of deformation and cracking significantly improved by adding SFs. The beam with an SF content of 45 kg/m3 had the highest flexural–tensile strength of 6.979 MPa, which was 1.668 times higher than that of a plain concrete beam. The beam with a 60 kg/m3 SF content had the highest fracture toughness of 1.691 MPam1/2, which was 53.87% higher than that of the plain concrete beam. The fractal analysis results showed that when the correlation dimension increased, numerous small-sized, low-evolution microcracks accumulated in the fracture process zone. When the correlation dimension decreased, the microcracks converged and formed macrocracks. The peak value of the correlation dimension can be used as precursor information for SFRC beam fracture prediction.

Synergistic effects of asphalt emulsion and fiber reinforcement on fracture properties and energy absorption of self-compacting concrete

The stability and durability of concrete structures are determined by various factors, including fracture properties. In this study, self-compacting concrete (SCC) was prepared by incorporating asphalt emulsion (AE) and fiber to improve its fracture properties and energy absorption capacity. Basalt fiber (BF) and polypropylene fiber (PPF) were used at 0.2% volume fraction, and the content of AE was 10%. Fracture parameters, including fracture energy, ductility index, and fracture toughness were evaluated by analyzing the load-crack mouth opening displacement (P-CMOD) curves obtained through a three-point bending test. In addition, quasi-static compression and gas permeability tests were conducted to evaluate the compressive energy absorption and durability, respectively. Results show that fracture energy, ductility index and fracture toughness were significantly improved after adding AE and fiber, and the specimens with PPF generally exhibited higher fracture parameters. Although the compressive strength was slightly decreased, the compressive toughness ratio was greatly improved. The stress–strain curves of specimens with AE and fiber show an obvious plastic behavior and exhibited a more gentle descending section, indicating a ductile failure mode. Moreover, gas permeability was reduced due to the filling effect of asphalt and the bridging action of fiber. Overall, incorporating AE and fiber into SCC could improve the matrix structure by creating a network with excellent toughening and bridging effects. This, in turn, optimizes crack resistance, energy absorption capacity, and durability of SCC.

Experimental and numerical investigations of crack growth of hot-rolled steel Q420C using cohesive zone model

In this paper, the application problem of cyclic cohesive zone models at different load ratios is thoroughly studied based on test data of hot-rolled steel Q420C. Firstly, the fracture toughness of Q420C steel was measured and load-crack mouth opening displacement (F-CMOD) curves were recorded. Secondly, with the help of F-CMOD curves, a comparative study of monotonic cohesive zone models was performed to calibrate model parameters. Finally, cyclic cohesive zone models with different unloading–reloading paths were used to simulate the high-cycle fatigue crack growth behaviour of Q420C steel, and their performances were compared.Research results show that plane stress assumption is moresensible when the finite element model is simplified from 3D to 2D. Rather than the conditional fracture toughness KQ, the elastic–plastic fracture toughness CTOD should be used to calculate the fracture energy of the monotonic cohesive zone model. Both cyclic cohesive zone models show good robustness towards the mesh size. When the linear scaling method is used to reduce simulation time, the cyclic cohesive zone model with an unloading–reloading path passing through the origin of coordinates is the better choice. To improve the simulation accuracyof cyclic cohesive zone models at different load ratios, the load ratio must be incorporated in the damage evolution law, and a linear relationship between accumulated cohesive length δ∑ and load ratio R was proposed forengineering applications.

Three-dimensional numerical study on mode-I local crack propagation of concrete considering boundary effect

A three-dimensional numerical method considering the boundary effect on fracture energy and initial fracture toughness was proposed to simulate the whole propagation process of mode-I local crack in concrete. The initial fracture toughness-based criterion was employed to determine the propagation of local crack surface. The numerical analyses were carried out to obtain the initial cracking load, peak load and load-crack mouth opening displacement curves for a series of concrete beams with a local crack of different sizes. Comparing with the experimental results performed in this study, the numerical results showed good agreements and applicability of the proposed method was verified. After that, the numerical analyses were conducted on the whole fracture process of local crack in concrete. The results indicated the local crack of three-point bending beam always propagated along the thickness direction firstly on the bottom surface of the specimen. The crack front was basically parallel to the upper boundary of specimen after the load decreased to 40%Pmax. For beams with a local crack of different sizes, the variations of the ratio of crack propagation area to ligament area with the ratio of load to peak load were the same so that a function expression was given to predict the increment of crack surface area under different loads. In addition, the fracture process zone evolution during the local crack propagation was discussed.

Prediction of Crack Propagation of Nano-Crystalline Coating Material Prepared from (SAM2X5): Experimentally and Numerically

The fracture and crack growth of materials can be practically and conveniently predicted through numerical analysis and linear elastics fracture mechanics. On this basis, the current study aims to present experimental work supported by a numerical technique for mimicking the crack propagation by Version 5.6 of COMSOL Multiphysics (version 5.6), used for the simulation of the coating made from Fe-based amorphous material with a thickness of 300 µm. The paper shows the effects of mixed-mode loading on cohesive zone parameters attained from load-crack mouth opening displacement (CMOD) curves. The microstructure dominates the fracture, which in mode I is altered from all-transgranular cleavage to nearly all-intergranular structure in mode II. Two common criteria for failure are linked to the mixed-mode results: Maximum energy release rate criterion (Maximum G) and maximum tensile stress criterion (Maximum S). However, distinguishing between the two criteria is made impossible by the large scatter in the data. The stress intensity factor is the basis for the. The stress intensity factor is the leading parameter facilitated by the singular element and should be estimated with accuracy. With the aim of comparing each criterion and illustrating the numerical schemes’ robustness, a number of examples are presented. It can be concluded that the Maximum G and Maximum S were successful and accurate in predicting the propagation of the Fe-based amorphous material prepared on mild steel.

Micromechanics-based model of single crack propagation in Engineered cementitious composites (ECC)

Engineered Cementitious Composites (ECC) are composites exhibiting strain-hardening behavior accompanied by the formation of multiple cracks. A special feature of ECC is the ability to tune the material composition to meet the different needs of various structural components, which benefits from the special micromechanics-based design theory of ECC. However, these existing micromechanics-based design theories are specific to the case of direct tension and has not been fully discussed and developed for the case where the ECC is subjected to bending load. In this study, a micromechanics-based crack propagation model for ECC under bending load is developed to guide the design of flexural performance of ECC. The entire crack propagation process is simulated by considering the microstructure of ECC and the crack propagation criterion. For ECCs with different types of fiber, the simulated load-crack mouth opening displacement curves agree well with experimental results and the dispersion of the ECC specimens can be captured by imparting randomness to the model. Based on this model, parametric studies of several representative parameters are performed to illustrate the applicability of the model. The results indicate that the material has a higher ultimate flexural bearing capacity near a fiber aspect ratio of 448 for the given material property parameters; and the fiber/matrix bonding should not be excessively strong. Insights from this model can facilitate the design of ECC members with excellent bending properties through fiber tailoring and surface treatment.

Tensile Performance Test Research of Hybrid Steel Fiber-Reinforced Self-Compacting Concrete.

Notched beam specimens were loaded by the three-point bending test device, and the effects of different volume contents and combinations of steel fibers on the tensile properties of hybrid steel fiber-reinforced self-compacting concrete (HSFRSCC) were studied. The failure law and strain field distribution of the specimens were studied by digital image correlation (DIC) technology. Moreover, the curves between the load and crack mouth opening displacement (CMOD) of 18 groups of hybrid steel fiber-reinforced concrete specimens were obtained, and the stress-strain curves of 18 groups of specimens were derived from the load-CMOD curves. The results show that both single and hybrid steel fibers can improve the crack deformation resistance and tensile properties of concrete, but hybrid steel fibers have a more significant improvement effect. Only when the content of steel fiber is more than 0.6% can it have a more obvious postpeak descending section, and hybrid steel fiber has higher postpeak deformation capacity and flexural toughness. The fundamental reason why concrete with hybrid steel fibers has better tensile properties is that micro and macro steel fibers cooperate with each other to resist cracks, improving the toughness of concrete after cracking. Finally, the mechanism of different size and volume content of steel fiber was analyzed from the micro level, which can be used as a reference for the engineering design of HSFRSCC in the future.

Discrete modeling of concrete failure and size-effect

Size-effect in concrete and other quasi-brittle materials defines the relation between the nominal strength and structural size when material fractures. The main cause of size-effect is the so-called energetic size-effect which results from the release of the stored energy in the structure into the fracture front. In quasi-brittle materials and in contrast to brittle materials, the size of the fracture process zone is non-negligible compared to the structural size. As a consequence, the resulting size-effect law is non-linear and deviates from the response predicted by linear elastic fracture mechanics. In order to simulate the size-effect, one needs to rely on numerical modeling to describe the formation, development and propagation of the fracture process zone. Although a number of models have been proposed over the years, it transpires that a correct description of the fracture and size-effect which accounts for boundary effects and varying structural geometry remains challenging. In this study, the Lattice Discrete Particle Model (LDPM) is proposed to investigate the effects of structural dimension and geometry on the nominal strength and fracturing process in concrete. LDPM simulates concrete at the aggregate level and has shown superior capabilities in simulating complex cracking mechanisms thanks to the inherent discrete nature of the model. In order to evaluate concrete size-effect and provide a solid validation of LDPM, one of the most complete experimental data set available in the literature was considered and includes three-point bending tests on notched and unnotched beams. The model parameters were first calibrated on a single size notched beam under three-point bending and on the mechanical response under unconfined compression. LDPM was then used to perform blind predictions on the load-crack mouth opening displacement curves of different beam sizes and notch lengths. Splitting test results on cylinders were also predicted. The results show a very good agreement with the experimental data. The quality of the predictions was quantitatively assessed. In addition, a discussion on the fracturing process and dissipated energy is provided. Last but not least, the Universal Size-Effect Law proposed by Bažant and coworkers was used to estimate concrete fracture parameters based on experimental and numerical data.

Effect of calcium leaching on the fracture properties of concrete

In this work, acceleration leaching and three-point bending (TPB) tests were performed on single-notched TPB beam specimens to explore the evolution of the fracture properties of concrete subjected to calcium leaching. The load-crack mouth opening displacement (P-CMOD) curves obtained for concrete at various leaching times were adopted to analytically calculate parameters that can reflect the fracture properties of concrete, such as the crack extension resistance (KR), fracture toughness (KIcini and KIcun), elastic modulus (E), tensile strength (ft), and fracture energy (Gf). Furthermore, a phase-field fracture model for concrete that considers calcium leaching effects is established by incorporating the time-dependent damage variables E, ft, and Gf. The results reveal that the KR curves of the leached concrete increase with crack propagation but lower and flatten with more leaching time. The fracture parametersKIcini, KIcunE, ft, and Gf rapidly decrease with leaching time in the first 60 days. However, with more leaching time, the impact of calcium leaching on the concrete fracture properties gradually diminishes. The variation in fracture parameters of leached concrete can be described by a series of exponential functions. The residual value fracture parameters of the fully leached concrete are estimated to be approximately 60–80 % of those of sound concrete. The established fracture model reproduces the P-CMOD curves of concrete at each leaching time, which verifies the rationality of the model. The numerical simulation reveals that crack propagate faster in leached concrete than in sound concrete, and leached concrete is more prone to cracking.

Fracture Mechanical Properties of Steel Fiber Reinforced Self-Compacting Concrete under Dry–Wet Cycle Sulfate Attack

Sulfate attack is the most common form of the durability damage of hydraulic concrete, and the performance degradation of cracked structural components is more significant at the position of water level change. Fly ash, a widely utilized supplementary cementitious material, can effectively improve the durability of concrete. In this paper, fly ash was used to partially replace Portland cement at 0 w%, 40 w%, 50 w%, 60 w%, and 70 w%, respectively. Through the three-point bending beam test with notch and the dry–wet cycle of sulfate attack, the change law of the fly ash content on the fracture mechanical properties of steel fiber reinforced self-compacting concrete (SFSCC) and its degradation mechanism under sulfate attack was studied. The results show that the load–crack mouth opening displacement curve of SFSCC changed from a steamed bread peak to a sharp peak under 30 dry–wet cycles of sulfate attack. The fracture toughness, peak load, and fracture energy of SFSCC with a high-volume fly ash increased with the increase in the fly ash content, while they reversed after sulfate attack. When the percentage of fly ash was 70 w%, the retention ratio of the fracture parameters was lower than that of SFSCC without incorporating fly ash, and when the percentage of fly ash was 50 w%, SFSCC had good bearing capacity, fracture mechanical properties, and corrosion resistance. The corrosion product of the reference SFSCC with 30 dry–wet cycles of sulfate attack was ettringite, whereas the SFSCC with a high-volume fly ash had no obvious corrosion products and the microstructure became looser.

Theoretical and computational investigation of the fracturing behavior of anisotropic geomaterials

The fracturing process in geomaterials is studied to characterize a potential host rock for radioactive waste, such as the kaolinite-rich Opalinus Clay formation. Because of its sedimentary genesis, this rock can be considered as a transversely isotropic geomaterial. A semi-circular bending test is here modeled based on the eXtended Finite Element Method (XFEM), to check for the formation and propagation of cracks in the rock, with a particular focus on the effect of notch dimensions and scale effects on the fracturing response of the specimen in terms of peak load. Starting with the XFEM-based results, a novel analytical formulation is also proposed to approximate the response of the material in terms of load-crack mouth opening displacement. The proposed formulation is also capable to provide a reliable estimate of the peak value and time history response, compared to some experimental predictions from literature, starting from a predefined value of initial notch depth, which could represent a useful theoretical tool for design purposes.

Phase-field modeling of fracture in high performance concrete during low-cycle fatigue: Numerical calibration and experimental validation

The development of an elasto-plastic phase-field model is presented which is able to predict nonlinear behavior of high performance concrete (HPC) during low-cycle fatigue. An elasto-plastic damage model which follows the Drucker–Prager yield criterion is formulated. For the modeling of unsymmetric tension–compression behavior of HPC two different continuous stepwise linearly approximated degradation functions for HPC in tension and in compression are used. The experimental three-point bending tests at low-cycle using notched beams of pure HPC are performed. Load–CMOD (crack mouth opening displacement) curves are plotted using the measured experimental data and residual stiffness–CMOD curves are calculated using these experimental Load–CMOD curves. The data used for the interpolation of degradation functions are calibrated using the experimental results for three-point bending tests. For that purpose the uniaxial tension and uniaxial compression tests and three-point bending beam test for HPC are simulated numerically. The accuracy of the proposed numerical model using the presented integration algorithm is verified by comparing the degradation of stiffness in numerical and experimental results.

Experimental investigation on the elastic modulus and fracture properties of basalt fiber–reinforced fly ash geopolymer concrete

A comprehensive experimental study was conducted to explore the fracture characteristics of basalt fiber–reinforced fly ash geopolymer concretes (FAGCs) with different fiber contents (0.025%, 0.05%, 0.1%, 0.15%). The fracture behaviors of the basalt fiber–reinforced FAGCs, such as load-crack mouth opening displacement curve, fracture toughness, fracture energy, crack propagation length, were investigated by three-point bending tests, and their strengthening mechanisms were explored by scanning electron microscopy. In addition, the Mori–Tanaka homogenization (MTH) method was introduced to analyze the elastic properties of the basalt fiber–reinforced FAGCs. It was found that the addition of basalt fibers improved the peak load, fracture toughness, and fracture energy of the basalt fiber–reinforced FAGCs, and the most significant improvements were noticed in the concrete with the basalt fiber content of 0.05%. Moreover, a decrease in the crack length of the basalt fiber–reinforced FAGCs was observed before the load level of the post-80% peak load. The elastic properties of the basalt fiber–reinforced FAGCs were analyzed based on the MTH theory, and numerical results were found to be in good agreement with experimental findings.

Crack extension resistance of steam-cured concrete under different curing temperature conditions

The influences of curing temperatures on the fracture characteristics for steam-cured concrete were investigated by using three-point bending (TPB) method. The measured load-crack mouth opening displacement (P-CMOD) curves were used to determine the crack extension resistance curves (KR curves) and fracture toughness. The results showed at each curing temperature, the KR curves first increased with crack growth and then approximately remained constant after the fracture process zone (FPZ) was completely developed. The KR curves for the specimens cured at 45 °C were slightly higher than those at 20 °C. However, the KR curves became lower and flatter significantly as increasing the curing temperature ranged from 45 to 95 °C. The fracture toughness also demonstrated an increase–decrease tendency with the increase of curing temperature, which was consistent to the variation of KR curves. The optimum curing temperature was found to be about 45 °C in terms of the fracture properties for steam-cured concrete. In addition, the KR curves based on different FPZ models and the stability during crack growth were also analyzed.

Assessment of Asphalt Mixture Disk-Shaped Compact Tension Test Indexes for Reflective Cracking Performance

Abstract Disk-shaped compact tension (DCT) testing is a commonly used low-temperature fracture test to determine the cracking resistance of asphalt mixtures. The current testing specification only considers the fracture energy (Gf) from load-crack mouth opening displacement test data. However, Gf does not directly take into consideration the behavior of the post peak region of the curve, which may indicate the mixture’s ability to resist crack propagation and provide insight into fracture processes (e.g., crack growth velocity). It is possible to have two DCT specimens with similar Gf values but dramatically different load-displacement responses. The main focus of this paper is to make a comparative evaluation of various performance indexes developed for DCT fracture testing with respect to field reflective cracking performance of 10 full-scale asphalt concrete overlay test sections. This study evaluates Gf in addition to three other indexes: fracture strain tolerance (FST), rate-dependent cracking index (RDCI) and a proposed DCTIndex from Minnesota Department of Transportation that considers the post peak load-displacement behavior. An equivalent performance index approach is adopted to make comparisons of test sections with varying overlay structures in terms of thickness and material properties. Results from this study showed there was relatively good agreement between all equivalent laboratory performance indexes in identifying the best and worst performing overlay sections, according to normalized field performance indexes after approximately three years of service. In general, the equivalent FST and RDCI laboratory indexes rank test sections similarly, while equivalent Gf and DCTIndex have comparable ranking.

Study on Fracture Assessment for BWR Reactor Pressure Vessel Steel by GTN model

In the JSME fitness-for service code, fracture evaluation methodology based on linear elastic fracture mechanics is provided for reactor pressure vessel (RPV) components by considering brittle fracture due to neutron irradiation in the RPV beltline region. However, in order to make a rational fracture evaluation for the RPV component such as bottom head component of boiling water reactor (BWR), where irradiation embrittlement is not concerned, it is necessary to consider the applicability of the ductile fracture evaluation methodology consistent with material fracture behavior. In this study, the applicability of the damage mechanics analysis method using Gurson-Tvergaard-Needleman (GTN) model, which is a numerical analysis method for ductile failure, was investigated for RPV bottom head component. Mechanical properties tests and fracture tests with plate specimens were performed using RPV law alloy steel (LAS) material. GTN model parameters of RPV LAS material that can reproduce stress-strain curve and load-displacement curve were determined. Fracture analyses of the plate specimens were performed using determined parameters as well. The load-Crack Mouth Opening Displacement (CMOD) curves and fracture behavior of the fracture tests were compared with the analyses results. The analyses predicted the maximum loads of the tests within an error of 4% and the fracture behavior between the tests and analyses was consistent. The results in this study clearly supported the applicability of the GTN model to RPV bottom head ring component.

Effect of notch depth ratio on mode I and mixed mode I-II fracture properties of engineered cementitious composites (ECC)

The effect of initial notch depth ratios (a/h) on the mode I and mixed mode I-II fracture properties of ECC was studied by using three-point bending test and four-point shear test, respectively. For mode I fracture, the initial fracture toughness KICini and unstable fracture toughness KICun- and KICun+ instead of KICun based on double-K fracture criterion are calculated. The mode I fracture energy GI is calculated by load-crack mouth opening displacement curve (P-CMOD). For mixed mode I-II fracture, the initial effective stress intensity factor (Keffini) are calculated by using nonlinear finite element software and the mixed mode fracture energy GI–II is analyzed. Also, crack propagation mechanism of ECC under mixed mode I-II fracture is analyzed from P-CMOD curve and load-crack mouth sliding displacement (P-CMSD) curve, and mixed mode I-II crack propagation of ECC is divided into six stage. It is found that when 0.3 ≤ a/h ≤ 0.8, the KICini does not change. While, KICun increases when a/h increases from 0.1 to 0.3, decreases when a/h increases from 0.3 to 0.7 and GI decreases with the a/h increasing from 0.3 to 0.8. GI–II is much larger than GI due to an obvious inclined fracture process zone with many micro-cracks dissipating energy in the mixed mode I-II fracture. The GI–II increases with the a/h increasing from 0.1 to 0.6 and decreases with the increase of the a/h from 0.6 to 0.8. While, the crack initiation angle increases with the increase of a/h in the mode I-II fracture.

Advanced Evaluation of the Freeze-Thaw Damage of Concrete Based on the Fracture Tests.

This paper presents the results of an experimental program aimed at the assessment of the freeze–thaw (F–T) resistance of concrete based on the evaluation of fracture tests accompanied by acoustic emission measurements. Two concretes of similar mechanical characteristics were manufactured for the experiment. The main difference between the C1 and C2 concrete was in the total number of air voids and in the A300 parameter, where both parameters were higher for C1 by about 35% and 52%, respectively. The evaluation of the fracture characteristics was performed on the basis of experimentally recorded load–deflection and load–crack mouth opening displacement diagrams using two different approaches: linear fracture mechanics completed with the effective crack model and the double-K model. The results show that both approaches gave similar results, especially if the nonlinear behavior before the peak load was considered. According to the results, it can be stated that continuous AE measurement is beneficial for the assessment of the extent of concrete deterioration, and it suitably supplements the fracture test evaluation. A comparison of the results of fracture tests with the resonance method and splitting tensile strength test shows that all testing methods led to the same conclusion, i.e., the C1 concrete was more F–T-resistant than C2. However, the fracture test evaluation provided more detailed information about the internal structure deterioration due to the F–T exposure.