Specific Fracture Energy Research Articles

Fracture parameters of alkali-activated aluminosilicate composites with ceramic precursor: durability aspects

Four sets of alkali-activated aluminosilicate (AAAS) composites based on ceramic precursors were studied in terms of their characterization by mechanical fracture parameters as a basis for considerations of durability. AAAS composites made of brick powder as a precursor and alkaline activator with various silicate moduli (Ms = 0.8, 1.0, 1.2, 1.4, and 1.6) were investigated. The sets of AAAS composites differed in terms of the used filler: quartz sand or brick rubble. Two different precursor particle size ranges of 0–1 mm and 0–0.3 mm were used for both types of filler. The test specimens had nominal dimensions of 40 × 40 × 160 mm and were provided with a notch at midspan after 28 days of hardening. The notches were cut up to 1/3 of the height of the specimens. The specimens were subjected to three-point bending fracture tests during which force vs. deflection (F–d) and force vs. crack mouth opening displacement (F–CMOD) diagrams were recorded. Tensile strength ft,ID and specific fracture energy GF,ID values were identified using the inverse method based on a neural network ensemble. The obtained F–CMOD diagrams were subsequently evaluated using the double-K fracture model supported by the ft,ID and GF,ID values. The double-K model allows the quantification of two different levels of crack propagation: initiation, which corresponds to the beginning of stable crack growth, and unstable crack propagation.

Определение трещиностойкости образцов с шевронным надрезом с использованием трехточечного изгиба

A method is proposed for the experimental determination of the specific fracture energy of a material under loading by a 3-point bending of a rectangular specimen with a chevron notch. It is shown that at the initial stages of crack propagation in the chevron notch zone, a linear dependence of the specimen compliance on the crack length is observed. This makes it possible to calculate the specific fracture energy of the material without using the phenomenological equations used under standard test conditions. The calculations were carried out for technical titanium VT1-0.

Experimental Research on the Bending and Fracture Characteristics of Cotton Stalk

HighlightsA two-factor randomized block design was used to study the influence of experimental factors on indicators.Specific fracture energy can indicate the relationship between mass and power.A cotton stalk model was established using the discrete element method (DEM).Abstract. Effectively chopping of the mixture of mulch film and cotton stalk recycled by machine is the only way to achieve subsequent separation of the materials. Cotton stalk is one of the main components of the mixture. According to the working principle of a chopping device, the bending and fracture characteristics of cotton stalk samples were measured. A two-factor random block design was used to study the effects of moisture content and sample location on the plant on the mechanical characteristics of the stalk samples. According to the results, the specific fracture energy of the stalk samples was calculated. The results showed that the relationship between the moisture content and bending performance of the samples was an inverse proportional function in general. However, when the moisture content was 20% to 30%, the fracture energy in the double-support bending tests was low, which was therefore the most suitable condition for chopping. In addition, a cotton stalk model was established using the discrete element method (DEM), and the optimal parameter combination was determined. Compared with the actual test results, the model error of the peak bending force was 1.20%. This study can support the analysis of chopping device simulation and material preparation in experimental research. Keywords: Bending fracture characteristics, Cotton stalk, Discrete element method, Three-point bending test.

Fracture bi-linear model parameters and brittleness of high-performance concretes for paving: why fatigue tests matters and cannot be spared?

ABSTRACT: Fracture tests on two high-performance concrete for pavements were performed using the wedge-split test. The results allowed perceiving the same specific fracture energy for both concrete. However, the crack opening during the tests as well as the characteristic length of the materials resulted in roundly distinct behaviors in terms of brittleness. Previous fatigue studies for both concretes are then fairly readdressed pointing out the unavoidable needs for studying the concrete brittleness as a parametric way for selecting suitable concrete proportions and materials as well as to reach its microstructural behavior in order to acquire an appropriate judgment about its fatigue performance.

Influence of different treatment and amount of Spanish broom and hemp fibres on the mechanical properties of reinforced cement mortars

This paper presents experimental investigations into the behaviour of cement mortar micro-reinforced with natural hemp and Spanish broom fibres. The fibre treatment process was carried out with: alkali (2.5, 5, 6, 8, 10 and 15% NaOH solution, and 2.5 or 5% NaOH and 2% Na2SO3 mixed solution), seawater, and a combination of alkali (5% NaOH) and seawater. The changes in fibre structure caused by the chemical treatment were monitored by FTIR, TG/DTG and XRD analysis. Fibres were added to mortar specimens in amounts of 0.34, 0.5, 0.68 and 1 vol%. The flexural and compressive strengths were tested on mortar specimens after 56 days. The σ–δ diagrams are presented, and the specific fracture energy is determined based on the curve from the plot of load (N) vs. displacement (mm). The tensile strength of the fibres was determined on specimens treated in the same way. Treatment in different solutions showed that increased pH values of the treatment medium have a great influence on the increase in crystallinity, for both Spanish broom and hemp fibres. The results presented here showed that natural fibres do not have a significant influence on the mechanical properties of the mortar (compressive and flexural strengths), but do significantly increase its ductility. In addition, it has been shown that specimens with a lower proportion of natural fibres give better mechanical properties and higher ductility of the material. It was observed that both the mortar preparation and the method of fibre installation significantly affect the mortar properties. Seawater as a means of treating fibres was proven to be the most ecologically and economically acceptable treatment method. Spanish broom fibres showed large potential for reinforcing cement composites.

Comprehensive fracture tests of concrete for the determination of mechanical fracture parameters

AbstractAdvanced experimental evaluation of the mechanical fracture properties of quasi‐brittle materials is an important part of the design, assessment, and computational modeling of structures made of these materials. The aim is to determine parameters independent of the test configuration or the size of the test specimens. This paper presents the results of an extensive experimental program which is part of the development of a comprehensive multilevel approach for experimental–computational determination of the mechanical fracture parameters of concrete. Three‐point bending and wedge‐splitting tests were performed simultaneously using three different geometrically similar specimen sizes and two well‐separated depths of initial notches. Before the start of the main testing program, the pilot tests that were performed were aimed at the design of the most appropriate concrete mixture. Special attention was paid to the selection of a maximum aggregate grain size. In order to study the process of concrete aging and the development of its mechanical properties, the accompanying destructive and non‐destructive tests were carried out at different ages of hardening. Fracture parameters were evaluated using the effective crack‐length formulation and work‐of‐fracture method. Finally, the size‐independent specific fracture energy was back‐calculated from the measured specific fracture energy.

3D Finite‐Element Simulation of Wedge Splitting Test of Non‐Standard Ceramic Double Cantilever Beam with Chevron Notch

Small‐scaled non‐standard specimens with chevron notch (CN) from Zirconia ceramics are probed both numerically and experimentally by wedging. The original construction in the form of double cantilever beam (DCB) with CN is considered for the fracture toughness determination of ZrO2 specimens. The fracture toughness is determined in experiment on wedging of DCB with CN, with application of analytical formula derived by one of the authors and maximum force, bending the cantilever's arm. The 3D finite‐element simulations of wedge splitting test are performed using two approaches: cohesive zone modeling (CZM) approach and element elimination technique (EET). The specific fracture energy defined in experiments is taken as the input parameter for CZM simulations of crack propagation. The effect of parameters, namely, cohesive strength, and friction coefficient, between ceramic wedge and ceramic cantilever, is considered. Different traction–separation laws are implemented, and the resulting force–displacement curves are compared with experimentally obtained ones. In the CZM approach, considering the fracture energy and the predefined crack path, matching of experimental and numerical results is not achieved. In EET, in which the restrictions on the crack paths and the requirements for inputting the fracture energy are not imposed, a good match with the experimental results is achieved.

Role of particle shape in determining tensile strength and energy release in diametrical compression of natural silica grains

Extensive particle fracture has been noticed in projectile penetration tests. However current models for predicting projectile penetration depths do not consider particle fracture. Therefore, in order to understand the role of particle shape on single grain strength and the subsequent comminution process, single grain crushing tests on sand grains of different shapes and sizes were performed. High-resolution, three dimensional images of grain surface, were created using confocal microscope and fracture of the grains were captured with high-speed imaging. Acoustic emissions during single grain crushing was used to estimate the energy released during grain fracture. It was found that local stress fields influence particle strength and in natural granular material surface flaws maybe the critical flaw causing fracture. The estimated critical flaw size causing fracture were submicron in size. The mass specific fracture energy increased with increasing failure stress and decreasing particle size and is significantly greater than that computed from pure diametrical breaking. Except for large angular grains, mass specific energies for different shapes were similar. Angular sands required multiple events to break. The outcome of this work can be utilized in geomechanics to understand the comminution process and in power industry to estimate the energy required for comminution.

Determination of the Specific Fracture Energy of the Material under Shear Deformation

We present a procedure for the evaluation of the strain and fracture energy of the material under the conditions of shear deformation based on the application of the method of digital image correlation. The established characteristics are used to determine the stress-strain state and service life of drill columns and shafts of hydraulic turbine in bending and torsion.

Fracture Parameters of Alkali-Activated Aluminosilicate Composites with Ceramic Precursor

This paper deals with selected alkali-activated aluminosilicate composites with a ceramic precursor in terms of their characterization using mechanical fracture parameters. Three composites were studied. They were manufactured using brick powder as a precursor and an alkaline activator with a dimensionless silicate modulus of Ms = 1.0, 1.2 and 1.4. The test specimens were nominally 40 × 40 × 160 mm in size and had a central edge notch with a depth of 1/3 of the specimen’s height. At least 6 specimens made of each composite were tested at the age of 28 days. The specimens were subjected to three-point bending tests, during which diagrams showing force vs. deflection at midspan (F–d diagrams) and force vs. crack mouth opening displacement (F–CMOD diagrams) were recorded. After the processing of these diagrams, values were determined for the static modulus of elasticity, effective fracture toughness (including its initiation component from the analysis of the first part of the F–CMOD diagrams), effective toughness and specific fracture energy using the effective crack model, Work-of-Fracture method, and Double-K fracture model. After the fracture experiments had been performed, compressive strength values were determined for informational purposes from one part of each specimen that remained after testing. In order to obtain visual information about the internal structure of the composites before and after the mechanical testing, the selected specimen was examined via X-ray microtomography. Tomographic measurements and image processing were performed for the qualitative and quantitative evaluation of internal structural changes with an emphasis on the calculation of porosimetry parameters as well as the visualization of the fracture process zone. The fractal dimension of the fracture surface and fracture process zone was determined. The porosity and microstructure images of selected samples taken from specimens were assessed.

Lotus Effect and Friction: Does Nonsticky Mean Slippery?

Lotus-effect-based superhydrophobicity is one of the most celebrated applications of biomimetics in materials science. Due to a combination of controlled surface roughness (surface patterns) and low-surface energy coatings, superhydrophobic surfaces repel water and, to some extent, other liquids. However, many applications require surfaces which are water-repellent but provide high friction. An example would be highway or runway pavements, which should support high wheel–pavement traction. Despite a common perception that making a surface non-wet also makes it slippery, the correlation between non-wetting and low friction is not always direct. This is because friction and wetting involve many mechanisms and because adhesion cannot be characterized by a single factor. We review relevant adhesion mechanisms and parameters (the interfacial energy, contact angle, contact angle hysteresis, and specific fracture energy) and discuss the complex interrelation between friction and wetting, which is crucial for the design of biomimetic functional surfaces.

Fracture behavior and thermal shock resistance of alumina-spinel castables-Effect of added fused zirconia-alumina

This study aims to clarify the influence of fused zirconia-alumina (FZA) on the fracture behavior and thermal shock resistance (TSR) of alumina-spinel castables, and discover the relationship between toughness parameters and TSR. Alumina-spinel castables with different FZA contents (0, 2, 4, 6, 8, and 10 wt.%) were prepared using tabular alumina, fused spinel, calcium aluminate cement, ultrafine alumina powder, and FZA as raw materials. The fracture behavior and TSR of alumina-spinel castables were determined by a wedge splitting test (WST) and a water quenching test, respectively. The results show that the flexibility of alumina-spinel castables can be significantly enhanced by introducing FZA. With increasing FZA content, the load-displacement curves change from typical brittle fracture behavior to non-linear fracture behavior. The flexibility parameter (ratio of GF/σNT) of the castables increases from 8.3 μm to 13.6 μm when FZA content increases from 0 wt.% to 10 wt.%. The flexibility improvement of castables containing FZA can be attributed to the formation of microcracks caused by the transformation of ZrO2. The TSR of alumina-spinel castables can also be significantly improved by adding FZA. The ratio of residual cold modulus of rupture (CMOR) of the castables increases from 12.3% to 34.2% when FZA content increases from 0 wt.% to 10 wt.%. The improvement in TSR can be attributed to a decrease in the stored elastic energy and a subsequent increase in the specific fracture energy of the castables. Generally, alumina-spinel castables with non-linear fracture behavior have good thermal shock resistance. There is a significant correlation between the flexibility parameter (GF/σNT ratio) and the ratio of residual CMOR; the correlation coefficient is 0.96.

Sustainable fiber reinforced cementitious panels containing PCM: Mechanical and thermal performance

An experimental study was planned and executed for the application of Phase Change Materials (PCM) containing fiber-reinforced cementitious panels on buildings. The objective of the research was to enhance the thermal performance of the panels. Panels with the dimensions of 60x120x2.5 cm were produced and experimental investigations about the thermal and the mechanical performance of the composites were carried out. PCM containing composites showed higher latent heat capacity and lower thermal conductivity. Reinforcement with chopped fibers compensated the strength loss due to PCM in cementitious panels. Specific fracture energy of the panels increased with increase of PCM ratio. PCM containing fiber reinforced cementitious panels showed great potential for energy efficient buildings with enhanced thermal and mechanical properties.

Effect of Curing Temperature in the Alkali-Activated Brick Waste and Glass Powder mortar and Their Influence of Mechanical resistances

In this study, compressive strength values were measured at different curing times(7,14 and 28 days).The alkali-activation of the brick and glass powder body with potassium water glass having a silicate modulus of 3. Compressive strengths, flexural strength and specific fracture energy of the specimens stored at 40° C and 60° C are evaluated at 28-days. The study demonstrates that the storage temperature of specimens and the content of the alkaline solution have a significant influence on all mechanical properties of the studied materials.
 Keywords: brick waste, glass powder, curing temperature, alkali-activated.

Evaluation of the Energy Losses for Quasibrittle Fracture Based on the X-Ray Investigations of Newly Formed Surfaces

The thickness of plastically deformed layer formed under the conditions of quasibrittle fracture as a result of propagation of a preliminarily formed fatigue crack is found as a result of X-ray investigations. We estimate the residual stresses of the second kind. By using these stresses, we compute the specific fracture energy. It is shown that the thickness of plastically deformed layer coincides with the height of asperities on the fracture surface.

Fracture properties of jointed rock infilled with mortar under uniaxial compression

Grouting is commonly used to fill rock joints to provide reinforcement in geotechnical engineering. Understanding the fracture behavior of grouted rock joints depends on the characterization of infilled rock joints. In this work, mortar-filled rocks with grooved granite joints an inclination of 45° and infilling mortar layer between granite joints were simulated and a pre-existing cracks were assumed in mortar layers. Uniaxial compressive tests were performed on 20 specimens with different initial crack lengths a. Mix mode I-II fracture occurred in joints due to compression-shear condition. Peak load Pmax and fracture energy G0 were determined based on experimental load-deformation curves. The obtained results showed that both peak load Pmax and specific volume fracture energy Gf-V were observed to decrease with increasing initial crack length a. A simple bilinear distribution of local volume fracture energy gf-V was assumed to evaluate the effect of front boundary on specific volume fracture energy Gf-V to obtain the size-independent real fracture parameter of mortar-filled rock joints GF-V. Experimental results were also obtained by the established front boundary effect. It was observed that volume fracture energy and front boundary zone length were GF-V = 1.7 N/mm2a* = 17.5 mm, respectively, which were consistent with experimental observations.

Effect of alkali-silica reaction on the fracture properties of confined concrete

Alkali-silica reaction (ASR), which was recently discovered in nuclear power plant structures commonly without shear reinforcement, has previously been shown to induce anisotropic expansion in confined concrete. The fracture properties (strength, stiffness, and specific fracture energy) of ASR-induced anisotropically-damaged concrete specimens were quantified by varying both the damage level and relative direction of the ASR-induced cracking orientation against the loading direction corresponding to the fracture propagation. The effect of different orientations (0, 45, and 90° relative to the notch of the specimen) of expected ASR-induced cracks on the fracture properties was investigated using a wedge-splitting test (WST). Specimens without ASR expansion generally showed the highest fracture properties; however, the specific fracture energy was highest for ASR-affected specimens in which the expected orientation of ASR-induced cracks was perpendicular to the WST specimen notch. Specimens in which the ASR-induced cracks were parallel to the notch exhibited the lowest strength and fracture energy.

Fracture behavior of magnesia refractory materials under combined cyclic thermal shock and mechanical loading conditions

AbstractTo investigate the effect of cyclic thermal shock and mechanical loading on the fracture behavior of magnesia refractories showing different brittleness, the as‐received and cyclic thermally shocked specimens are subjected to monotonic and cyclic wedge splitting test. The whole duration of test is monitored by digital image correlation and acoustic emission. Both thermal and mechanical fatigue resistance increase with the reduction of brittleness. Repetitive thermal shock results in pronounced reduction of strength. However, the specific fracture energy and nonlinearity increase after exposure to thermal shock due to the expanded micro‐crack network inducing the development of a significant fracture process zone. Periodic loading mainly leads to the decrease of strain bearing capacity, as the fatigue loads favor the extension of crack tip instead of fracture process zone expansion. The combined application of periodic thermal shock and mechanical loads gives a new insight into the progressive damage behavior of refractory under critical conditions.

Dynamic fracture investigations of ultra-high performance concrete by spalling tests

Ultra-High Performance Concrete (UHPC) is an innovative cement based material with superior strength for engineering structures. The mechanical properties of the cementitious composite depend on its exact composition and need to be determined experimentally. Whereas several standard tests for the static loading regime exist, dynamic data such as tensile strength and failure load are difficult to measure.This contribution presents an experimental method to identify the material properties under impact loading. By means of a modified Hopkinson pressure bar, cylindrical UHPC specimens of different compositions without fiber are examined in strain rates of approximately 30s-1. Assuming uniaxial wave propagation, the resulting stress states are analyzed and the specimens’ dynamic elastic modulus, tensile strength and specific fracture energy are determined. Numerical fracture simulations in the sense of an inverse analysis prove the experimental approach valid. The agreement between both, experimental observations and numerical results, show that the obtained parameter are reliable and suitable for predictive simulations.

Evaluation of fracture properties of cancellous bone tissues using digital image correlation/wedge splitting test method

The fracture mechanics (FM) parameters of cancellous bone tissues are very important from a clinical point of view especially for the bone cement augmentation. From the literature review, one can observe that the experimental determination of fracture mechanic parameters of cancellous bone are still lacking. This can be due to the conditions associated with the unstable crack propagation in the cancellous bone and lack of tools to extract and measure the parameters (like crack opening displacement (COD) and crack length) in the course of fracture tests, which are necessary to evaluate the fracture properties. To address above mentioned, a platform was developed integrating an optical measurement technique like digital image correlation (DIC) with classical wedge splitting test (WST) method to extract precise and real crack tip positions, crack opening displacement (COD) at each load step. These indeed used for the evaluation of the fracture mechanic properties (fracture toughness, specific fracture energy (Gf)) of the cancellous bone. Two approaches were used to evaluate the fracture mechanic properties of the bone. The first method is based on the global approach, which was widely used in the literature and the second method is based on the local approach. In this local approach, the local fracture energy (Gi) during the course of the test was evaluated, which give access to local fracture mechanics. The results evaluated by both the methods were in good accordance and compared with available literature. In addition, an attempt made to retrieve the real crack tip position at each load step during the test.