Initiation Fracture Toughness Research Articles

Analysis of crack propagation in onshore wind turbine foundations using the double-K fracture model

The load-bearing mechanism of the embedded steel ring to foundation connection used for onshore wind turbines causes high stress concentrations in the concrete around the base flange. Cracks initiate and propagate through the foundation from the base flange. The crack growth under normal operating and extreme loading conditions reduces the stiffness and load-bearing capacity of the tower to foundation connection. This in turn affects power production and could lead to economic losses if failure of the foundation occurs. This paper investigated the stability of a horizontal crack propagation in a typical onshore wind turbine foundation using the double-K fracture propagation criterion. The study involved experiments on two sub-part models and numerical simulation of a large-diameter onshore wind turbine foundation. Using the numerical results, the stress intensity factors at the tip of a horizontal crack emanating from the outer edge of the base flange were calculated at different stages of loading. The stress intensity factors were compared with the initial and unstable fracture toughness of concrete to assess the stability of the crack propagation. It was found that for vertical displacements of the embedded steel ring exceeding 9 mm, the crack propagation became unstable. Based on the crack propagation behaviour, a method was proposed to assess the safety of onshore wind turbine foundations using the vertical displacements of the embedded steel ring.

The effect of cell shape anisotropy on fracture toughness of low-density brittle foams

Low-density brittle foams, although not expected to carry mechanical loads, have to retain structural integrity during service in order to deliver the expected functional characteristics. The fracture toughness of low-density foams exhibits anisotropy when foam cells acquire anisometric shape due to, e.g., the presence of a preferential direction for free-rise foams, which have cells elongated in the foam rise direction. In the current study, the effect of foam morphology on anisotropy in mode I and mode II initiation fracture toughness is evaluated numerically. A lattice model of foams with elongated and generalised Kelvin cells is employed that provides a more realistic representation of foam morphology than rectangular grids studied previously. The results obtained are compared with an analytical scaling model based on parallelepiped unit cell. The predicted toughness anisotropy is found to agree with the experimental results for low-density rigid polyurethane foams with moderate cell shape anisotropy.

Orientation-dependent micromechanical behavior of nacre: In situ TEM experiments and finite element simulations

Nacre's superior mechanical properties and failure behavior are strongly orientation-dependent due to its brick-and-mortar microstructure. In this work, the anisotropic microscopic deformation and the resulting macroscopic mechanical properties are evaluated under different loading conditions. Our in situ transmission electron microscopy deformation experiments and finite element simulations reveal that nacre possesses enhanced indentation resistance along the direction normal to the tablets through delocalization of indentation-induced deformation by taking advantage of its layered structure. In addition, nacre's ability to recover from large deformations is observed. We study the strong loading direction dependence of nacre's macroscopic mechanical properties and elucidate the underlying microscopic deformation patterns in the tablets and the soft matrix. Particularly, its performance along the transverse direction is optimized to withstand the loading conditions in nature. We show the importance of the vertical matrix for the initial stiffness and fracture toughness of the composite. These findings provide guidelines for designing nacre-inspired artificial composites with enhanced mechanical properties. Statement of significanceNacre is widely recognized as an excellent structural model for designing bio-inspired tough and strong artificial composites. Due to its brick-and-mortar microstructure, it exhibits loading direction-dependent mechanical behavior. In this contribution, we investigate the macroscopic mechanical properties and microscopic deformation behavior of nacre under different loading conditions by means of in situ TEM deformation tests and FE simulations. It is found that effective elastic moduli and microscopic deformation strongly depend on the loading direction. The organic matrix is highly deformable. The indentation resistance along the direction normal to tablets is enhanced via deformation delocalization. Our quantitative and qualitative results provide guidelines on optimizing the mechanical properties of nacre-inspired novel composites.

Experimental Research on Fracture Characteristics of Reactive Powder Concrete in Different Volume Content of Steel Fiber

The fracture characteristics of reactive powder concrete in different volume content of steel fiber are researched in this paper, combining with the photoelastic coating method and the mathematical analytical software MATLAB. More precisely, the wholly integrated initiation, stable propagation, and final failure stages of precast cracks in three-point bending specimens of reactive powder concrete in different volume content of steel fiber are directly recorded and systematically analyzed. Besides, the photoelastic fringe distribution graphs from the vicinity of the precast cracks of specimens are obtained. Based on the experimental results and fracture mechanics theory, the mechanical property of reactive powder concrete, fracture energy, ductility index, initial fracture toughness, and unstable fracture toughness are quantitatively and qualitatively analyzed. The research achievements indicate that there is a stable crack propagation process before the instability failure of the three-point bending beam specimen of reactive powder concrete. The fracture process of the structure includes three stages, namely, the crack initiation, stable propagation, and instability failure. The order of the photoelastic fringe, critical effective crack length, crack initiation load, and maximum load of test parameters in the three-point bending beam specimen of reactive powder concrete increases with the increase of the fiber volume ratio. Based on the numerical treatment for the whole curves of each specimen group, the softening curve and the double fracture parameters of reactive powder concrete are obtained. Besides, the calculated critical effective joint length, initiation toughness, toughness increment, and unstable fracture toughness increase with the increase of the steel fiber volume ratio, by analyzing the measured initiation load, maximum load, and critical effective joint length. The research results can be treated as an important basis and reference for the engineering design and safety assessment of reactive powder concrete.

Effect of Moisture Absorption on Mode II Fracture Behavior of Fumed Silica Reinforced Hybrid Fiber Composite

ABSTRACT This work aims to investigate the influence of water ingression on Mode II interlaminar fracture behavior of a fumed silica-reinforced jute-kevlar hybrid nanocomposite. Shear mode of failure is characterized by using End Notched Flexure (ENF) specimens. Thirteen different composites are prepared based on four layering sequences (i.e., Jute-Jute-Jute-Jute [JJJJ], Jute-Kevlar-Kevlar-Jute [JKKJ], Kevlar-Jute-Jute-Kevlar [KJJK], and Kevlar-Kevlar-Kevlar-Kevlar [KKKK]) and four nanofiller weight proportions (0, 1.5, 3, and 4.5%), with the aim of studying the impact of the layering sequence and nanofiller proportion. Initiation fracture toughness (GIIC init.) is reported for dry and wet conditions of the composites using the Simple Beam Theory (SBT) data reduction method. Among the reported stacking sequences, the stacking sequence JKKJ possesses the highest fracture-resistant material in both dry and wet conditions because of the active involvement of the fiber and matrices at the interlaminar layer resulting from interlaminar crack propagation. The water-aged composite JKKJ with a 3% nanofiller weight proportion shows the highest initiation fracture toughness (GIIC init.) value, 1811.4 J/m2. The composite with the stacking sequence JKKJ and 3% nanofiller weight proportion is recommended, which is proved to be the best Mode II fracture-resistant material under wet conditions among the reported composites.

Crack resistance properties of carbon nanotube-modified concrete

Incorporating carbon nanotubes (CNTs) into concrete can improve its fracture toughness. The crack resistance properties of reinforced concrete with CNTs were tested by ring test and three-point bending beam fracture test. The fracture parameters of reinforced concrete with CNTs were analysed by using double-K fracture parameters and the results showed that CNTs can improve the crack resistance properties. When the CNT content was 0.10 wt%, the compressive strength at 28 days and the splitting tensile strength of concrete were increased by 1.7 and 11.7%, respectively. When the CNT content was 0.15 wt%, the cracking time of concrete under semi-closed and closed conditions was extended by 42.1 and 41.2%, respectively. The initiation fracture toughness, the unstable fracture toughness and the fracture energy of concrete were increased by 31.0, 32.4 and 24.6%, respectively. It was found that CNTs delay the propagation of micro-cracks by bridging across the micro-cracks in concrete and strengthening the interfacial transition zone.

Evolution on fracture properties of concrete during steam curing

This paper investigated the fracture properties of concrete after steam curing ranging from 4 to 48 h at a typical constant temperature of 60 °C. The evolution pattern of fracture properties for concrete during steam curing was revealed, and the equivalent maturity was adopted to estimate the development of fracture parameters. The results indicated similar to the mechanical parameters, the initial fracture toughness KIcini exhibited an increase-decrease tendency as increasing steam curing hold (SCH) time. However, the unstable fracture toughness KIcun and fracture energy GF were found to increase monotonously with SCH time, leveling off approximately between 16 h and 48 h. The equivalent maturity approach was proved to be available to evaluate the fracture parameters of steam-cured concrete with a relative error of less than 10%. In addition, the crack extension resistance during whole fracture process was also analyzed, and it was found the SCH time should be kept within 16 h at 60 °C for the development rate of fracture properties. The test results can provide a basis for the control of cracks of concrete in steam curing process.

The fracture toughness analysis on the basalt fiber reinforced asphalt concrete with prenotched three-point bending beam test

In this study, the fracture resistance of the basalt fiber reinforced asphalt concrete (BFRAC) constructed in The Republic of Mozambique was investigated with prenotched three-point bending beam tests. Fracture energy and the double-parameter fracture model, i.e., initial fracture toughness (Ki), and ultimate fracture toughness (Ku), were utilized to evaluate the fracture behaviors of BFRAC. The test results indicate that the double-parameter fracture model proposes more rational and convincing explanations to the BFRAC over the fracture energy. According to the double-parameter fracture model, a remarkable enhancement on fracture resistance of BFRACs with 4.5% and 5.0% ratios achieved as the fiber content increased while a gradual increase exhibited for the fracture resistance of BFRAC with 4.0% b/a ratio and 0.2% fiber. It indicates that the fracture resistance of BFRAC will be enhanced with the increase of the basalt fiber dosages as long as the asphalt volume is enough for wrapping aggregates and basalt fiber, causing more cohesive and adhesive capacity.

Research on the effect of thermal treatment on the crack resistance curve of marble using notched semi-circular bend specimen

The crack resistance curve is widely used to describe the complete fracture process of quasi-brittle materials. In the paper, the crack resistance curves of thermally treated marble were tested through the notched semi-circular bend specimens. The acoustic emission (AE) and digital image correlation (DIC) techniques were used to monitor the fracture behaviors of the specimens during the test. The AE results indicated that the crack will initiate from the notch when the axial load is less than the peak load. And, when the axial load reaches the peak load, the DIC results demonstrated that there exists a large fracture process zone (FPZ) in front of the crack notch. The effect crack length was calculated by the compliance method, which is coincided with the DIC results. The amplitude of the crack resistance curves decreases with the increase of treatment temperature, which suggested that the fracture resistance of the specimen declines due to thermal treatment. Based on the K-resistance curves, the initial fracture toughness, unstable fracture toughness, and fracture toughness when FPZ is fully formed have been determined for the thermally treated specimens. The results showed that these parameters both decrease in the increase of treatment temperature. Moreover, The FPZ length first increases and then decreases with the increase of normalized crack length and the critical and maximum non-dimensional FPZ length tends to increase with the increase in treatment temperature.

Fracture properties of concrete exposed to different sulfate solutions under drying-wetting cycles

The fracture properties of concrete subjected to drying-wetting cycles in different types of sulfate solutions were investigated. Five exposure conditions, i.e. 5% and 10% sodium sulfate solutions, 5% and 10% magnesium sulfate solutions (by mass) and water were prepared. Three-point bending tests were carried out at the corrosion periods of 1, 2, 3, 5, 9 and 15 months with the digital image correlation (DIC) technique. The initial and unstable fracture toughness were calculated by the proposed method, and the evolution of the fracture process zone (FPZ) was studied. Results indicate that (1) with the increase of corrosion time, an initial increase stage and a followed decrease stage for the fracture toughness were noticed. The values of unstable fracture toughness obtained by the analysis method agreed well with those obtained by the experimental method. (2) The FPZ length increased linearly with the crack propagation until a full FPZ development, and then decreased gradually. The maximum FPZ length decreased slightly with the corrosion time. (3) Using DIC technique, the fracture path can be easily identified from the horizontal strain field, and the evolution of the FPZ can be determined by the horizontal displacement with high accuracy.

Damage Model Prediction of Crack Initiation and Propagation in Five Fracture Geometries for X80 Pipeline Steel

Abstract Test specimens used to calibrate damage-mechanics models are designed to produce a range of stress triaxialities and Lode angles to accurately capture the fracture envelope of the metal. Many of these specimens have lower constraint than deeply notched fracture specimens that undergo stable tearing and have high constraint at the crack tip. Often just one or two fracture geometries are used to calibrate the model. In this work, the ability of a damage model to capture variability associated with constraint at a crack tip, particularly for crack initiation, is assessed. A recent round-robin (Wilkowski et al., 2019, “1st Round-Robin for Exploring the Effects of Constraint on Fracture Initiation Toughness for Surface-Cracked Pipe/Fittings,” Panel Session held in Conjunction With ASME PVP2019, San Antonio, TX) study on the initiation toughness of X80 provided data for five fracture specimens in which crack tip constraint varied. As a damage model for X80 was not available, the well-calibrated modified Mohr–Coulomb (MMC) damage models from literature for X65 and X70 were used as a starting point for the model. Experimental data from the single compact tension C(T) specimen were used to slightly modify the X65 and X70 models to capture the X80 fracture response. The MMC damage model was applied in finite element analysis (FEA) to simulate both the crack initiation and propagation responses of single-edge-notched-tension (SENT) and surface-cracked pipe specimens. Except for a low J-integral at initiation predicted for the C(T) specimen, the remaining predicted responses for force, pressure, and initiation were in good agreement with the experimental data provided in the round-robin.

Microstructural Controls on Mixed Mode Dynamic Fracture Propagation in Crystalline and Porous Granular Rocks

AbstractBrittle fracture propagation in rocks is a complex process due to significant grain‐scale heterogeneity and evolving stress states under dynamic loading conditions. In this work, we use digital image correlation and linear elastic fracture mechanics to make instantaneous measurements of the opening (mode I) and in plane shear (mode II) components of the stress intensity field during dynamic mixed mode crack initiation and propagation in crystalline and granular rocks. Both rock types display some similar fracture behaviors as observed in engineered materials, including rate dependent fracture initiation toughness and a direct relationship between propagation toughness and crack velocity; however, measured propagation toughness is higher than quasi‐static values at crack velocities well below the branching velocity in both rocks. Additionally, due to grain scale controls on the fracture process, mixed mode crack propagation is fundamentally different between these two rock types. Mixed mode propagation is energetically more favorable than pure opening mode propagation in sandstone, while the opposite is true in granite. Furthermore, following initiation, propagation in granite occurs so as to minimize the mode II contribution, irrespective of the initiation conditions, while fractures in sandstone maintain a non‐negligible mode II contribution during propagation across the sample.

Utilizing bamboo biochar in cement mortar as a bio-modifier to improve the compressive strength and crack-resistance fracture ability

Concrete is an obvious source of global carbon dioxide emissions. In recent years, many kinds of pulverized biochar materials have been used in cement composites to improve the carbon capture ability as well as to enhance the mechanical properties. In this study, bamboo biochar is used to replace some cement to make biochar mortar, in order to improve the compressive strength and crack-resistance ability. By the measurement of the main oxidation chemicals of bamboo biochar, it is found that bamboo biochar meets the requirements of class N – pozzolan and has pozzolanic reaction ability. Thus, it can be used as a useful admixture to mortar. The compressive strength of biochar mortar is higher than that of the control mortar without bamboo biochar, due to the filling and self-curing effects of bamboo biochar. The logarithmic formula and negative exponential formula are used to describe the age strength relationship of biochar mortar at early period. Then the initial and final compressive strength are introduced to illustrate the influence of biochar dosage on the compressive strength of mortar during different age. On another hand, biochar mortar has larger initial cracking fracture toughness and unstable fracture toughness than the control mortar, indicating that bamboo biochar is beneficial for mortar to resist crack initiation and propagation, although it decreases the critical crack length. Considering the compressive strength and fracture toughness, 1–3 wt% replacement of bamboo biochar to cement is suggested as the optimal dosage for mortar in this study.

Experimental study on fracture mechanics of cementitious materials reinforced by graphene oxide–silica nanocomposites

This study aims to improve the fracture properties of cementitious materials by adding graphene oxide (GO) and graphene oxide–silica nanocomposites (GO-nanoSiO2). The pristine GO was coated with nano-silica to form GO-nanoSiO2 composites. Based on the three-point bending test, the fracture toughness and fracture energy of cementitious materials were calculated to provide comprehensive fracture mechanics characterization. The fracture surface morphology and the crack development in mortar were detected by ultra-depth microscopy and scanning electron microscopy. The results demonstrate that the addition of GO-nanoSiO2 is more beneficial to improve the fracture properties of cementitious materials than pristine GO and the amount of nano-silica grafted on GO surface affects its enhancement effect. The initial fracture toughness, unstable fracture toughness and fracture energy of mortar reinforced by GO-nanoSiO2 (0.05 wt% GO and the mass ratio of nano-SiO2/GO is 2.8) are increased by 92.2%, 64.7% and 77.5%, respectively. GO-nanoSiO2 improves the roughness of mortar fracture surface, changes the propagation mode of cracks, and makes the crack propagate along a more tortuous path which contributes to energy dissipation.

A Numerical Method for Applying Cohesive Stress on Fracture Process Zone in Concrete Using Nonlinear Spring Element.

Aiming at the numerical simulation of the entire crack propagation process in concrete, a numerical method is proposed, in which cohesive stress on the fracture process zone (FPZ) is simulated and applied by a nonlinear spring element. Using displacement control, the cohesive stress values on the FPZ are obtained from solving a system of nonlinear equations through an iterative process. According to a crack propagation criterion based on initial fracture toughness, the approach adds the spring elements to finite element analysis when simulating mode I crack propagation in standard three-point bending notched concrete beams with different strengths, initial crack ratios , and depths (D). The simulated load versus displacement (P-Delta) curves are performed to recalculate the fracture energy and verify the accuracy of cohesion in the proposed method. The simulated load versus crack mouth opening displacement (P-CMOD) curves are consistent with the previous experimental results. Subsequently, the variations of the FPZ length and the crack extension resistance ( curves are studied according to the proposed iterative approach. Compared with the existing methods using a noniterative process, the iterative approach generates a larger maximum FPZ length and curve where the FPZ length is mainly determined by the fracture energy, tensile strength, and geometry shape of the beam, and the curve is primarily determined by the fracture energy and FPZ length. The significant differences in numerical results indicate that the applying cohesion is essential in numerical simulation. It is reasonable to conclude that the proposed nonlinear spring element is more applicable and practical in the numerical simulation of the concrete mode I crack propagation process by improving the accuracy of the cohesion applied on the FPZ.

Fracture performance improvement and fracture process simulation of concrete for shotcrete use in hot and dry environments

AbstractIn order to investigate the fracture performance of concrete in hot and dry tunnel environments, the fracture parameters are tested through three‐point bending tests on notched beams. The effects of basalt and steel fibers on the fracture properties in hot and dry environments are explored, and the effects of beam height and notch length on fracture parameters of concrete are analyzed by using numerical calculations. The results show that the fracture toughness of concrete in hot and dry environments is obviously lower than that in the standard curing condition. The addition of basalt and steel fibers has little effect on the initial fracture toughness but can significantly improve the unstable fracture toughness and fracture energy. The peak load and maximum fracture process zone length decrease with the increase of the notch length and increase with the increase of the beam height, while the unstable fracture toughness is basically not affected by the size changes of concrete beams.

Unravelling the lamellar size-dependent fracture behavior of fully lamellar intermetallic [formula omitted]-TiAl

Strengthening of metals by incorporating nano-scale coherent twin boundaries is one of the important breakthroughs of recent years in overcoming the strength-ductility trade-off. To this effect, also twin boundaries in nano-lamellar lightweight Ti-Al alloys promise a great potential, but their contribution to the deformation and fracture behavior needs to be better understood for designing optimal microstructures. To this end, we carry out linear elastic fracture mechanics informed large-scale atomistic simulations of fully lamellar microstructures consisting of the so-called ”true twin” boundaries in γ-TiAl. We find that nano-scale lamellae are not only effective in improving the fracture toughness and crack growth resistance, but also that the lamellar size controls the crack tip mechanisms. We identify a critical lamella thickness in the region between 1.64 and 3.04 nm, above which the crack tip events are primarily dislocation-based plasticity and the critical fracture initiation toughness exhibits an increasing trend with decreasing lamella size. Below the critical thickness, a decline in fracture toughness is observed and the crack tip propagation mechanisms are quasi-brittle in nature, i.e. the cleavage of atomic bonds at the crack tip is accompanied by plasticity events, such as twin-boundary migration and dislocation nucleation. A layer-wise analysis of the unstable stacking fault energy, the energy barrier for dislocation nucleation, that the critical thickness is of a similar value as the distance from the twin boundary at which bulk properties are restored.

Fracture behaviors of HVFA-SCC mixed with seawater and sea-sand under three-point bending

This paper aims to study the fracture behaviors of high-volume fly ash-self-compacting concrete (HVFA-SCC) mixed with seawater and sea-sand (SWSS) or freshwater and river sand (FWRS). Three-point bending test were performed on 24 notched beams fabricated with varying in replacement ratio of fly ash (0%, 30%, 50%, and 70%) and the type of water and sand (SWSS and FWRS). The initial and unstable fracture toughness of these test specimens are determined by the double- K fracture model. The effect of fly ash replacement ratio and type of water and sand on the fracture parameters is analyzed and discussed. In addition, the cohesive fracture toughness of all the test specimens is calculated by using Gauss–Chebyshev integral method and the weight function method based on the bilinear tensile softening curve given in CEP-FIP Model Code. A comparison of fracture toughness parameters of determined from the experimental approach and analytical approaches is presented in these SCC specimens. Results show that SCC mixed with SWSS replacing FWRS can improve the unstable fracture toughness and fracture energy, and decrease its brittleness behavior. The cohesive fracture toughness of SWSS-SCC specimens is underestimated by these analytical methods based on the tensile softening curve given in CEP-FIP Model Code.

Experimental and Numerical Investigation of Fracture Behaviors of Steel Fiber–Reinforced Rubber Self-Compacting Concrete

This paper presents the experimental and numerical studies of flexural-fracture behaviors of steel fiber–reinforced rubber self-compacting concrete (SRSCC) materials. The scrap-tires rubber aggregate was used to partially (10%, 15%, and 25%) replace the fine aggregate based on its volume for SRSCC samples, and the microsteel fiber was introduced with an addition ratio of 0.2% based on the entire mixture volume. The plain self-compacting concrete (SCC) and the rubberized SCC specimens were produced for comparison. The three-point bending test on the single-edge notched beam showed the increased flexural strength and total fracture energy of SRSCC samples by adding steel fiber and rubber. The critical fracture parameters including initial fracture energy (Gf) and fracture toughness (KІc) were determined based on the Load-CMOD curves and two parameters fracture model. With these properties, the bilinear tension-softening model (aggregate interlock effect) and trilinear tension-softening model (aggregate interlock and fiber-bridging effects) were calibrated for normal SCC and SRSCC, respectively. The tension-softening functions were utilized in the FEM model to predict the flexural-fracture behaviors of corresponding specimens, and the numerical simulation results showed reasonable agreement with experiments. Overall, the experimental testing data and numerical simulation model can reveal the detailed fracture behavior of SRSCC for future improved material design.

Influence of Graphite Powder on the Mechanical and Acoustic Emission Characteristics of Concrete

In this paper, uniaxial compressive strength (UCS) test and three-point bending (TPB) test, together with an acoustic emission (AE) system, were performed to investigate the mechanical properties and AE characteristic changes of concrete with different graphite powder (GP) content. The results show that: (1) Poor adhesion and low interlocking of graphite with cement stone increase the initial defects of concrete, reducing its elastic modulus and the cyclo-hoop effect, and thus weakening the compressive strength. (2) For concrete with a low graphite content, the second sharp rise in ringing counts or energy released during the compressive process can be regarded as a failure alarm. However, as GP content increases, the second sharp rise fades away, while the first sharp rise becomes more visible. At high GP content, the first sharp rise is better for predicting failure. (3) The initial defects caused by GP significantly lower the initial fracture toughness, but its bridging effect greatly increases the critical crack mouth opening displacement and thus significantly enhances the unstable fracture toughness of concrete, by up to 9.9% at 9% GP content. (4) In contrast to compressive process, the sharp increase in AE signals preceding failure during the fracture process cannot be used to predict failure because it occurs too close to the ultimate load. However, as GP can significantly increase the AE signals and damage value in the stable period, such failure precursor information can provide a safety warning for damage development.