Crack Initiation Load Research Articles

Higher Toughness of Metal-nanoparticle-implanted Sodalime Silicate Glass with Increased Ductility

In this report, we propose a novel framework for toughening brittle oxide glass originated from enhanced ductility by implanting a secondary material comprising different mechanical properties. To do so, copper-metal nanoparticles are implanted into the subsurface layer of commercial soda-lime silica glass by using the electrofloat method. The crack initiation load of the implanted glass is found to be comparable to the glass chemically strengthened in ordinary tempering conditions. By observing crack propagation and stress distribution from cross-section, it is found that the crack propagation stops within the metal nanoparticle implanted layer, due to the stress dissipation or relaxation. The copper-implanted glass shows improved toughness with decreased hardness. The toughening mechanism of the composite glass is theoretically studied using molecular dynamics calculations on an amorphous silica model with copper nanoparticles embedded, and Peridynamics fracture simulations for indentation on a glass sheet model whose surface was implicitly modeled as the copper-implanted oxide glass. The experimentally observed phenomena of intrinsic toughening were well explained by the series of the conducted simulations.

STUDY ON THE INFLUENCE OF COMPRESSIVE STRESS ON THE COMPRESSION SHEAR CRACK PROPAGATION

In this paper, the problem of crack initiation and propagation under compression shear load is investigated experimentally. The influence of far-field compression stress perpendicular to the crack surface on the initiation and propagation of the crack in brittle material is emphasizeded. With the help of the two-axis loading test machine, the two shaft displacements and forces can be adjusted freely during loading. Thus a kind of single-edge symmetric double-crack compression-shear specimen is designed. During the loading, the compression stress stoped as it reaches a series of preset values, while the shear stress increases monotonically till to the crack initiation and propagation. Then the crack initiation and propagation law of compression shear crack under different preset compression stresses can be obtained. It can be find from the test that as the compression stresses relatively smaller, along with the preset compression stress increases, the crack initiation angle increases and the deviation of crack propagation path from the initial crack direction changes severely. As the compression stress reached a definated value, the initiation angle is not changed. It can be find from the test sample that the two crack surfaces are contacted after this definated value of compression stress. The results show that the effect of compression stress on the crack propagation can be divided into two stages according to whether the crack surface contacted or not. Before the crack closed, the influence of compression stress on crack initiation load, crack initiation angle and propagation path increases,when the preset compression stress increases the crack initiation load and the crack initiation angle increase, and the propagation path deviates from the initial crack direction severely. After the crack closed, although the compression stress increases, the crack initiation angle and the critical compression shear stress ratio are keep unchanged. In addition, there existes a certain correspondence between the crack initiation angle and the compression-shear stress ratio at the time of crack initiation. When the ratio of compression-shear stress at the crack initiation increases, the crack initiation angle increases, and vice versa.

Effect of vanadium oxide addition on thermomechanical behaviors of borosilicate glasses: Toward development of high crack resistant glasses for nuclear waste disposal

The effect of of V2O5 additions (up to 8 mol%) on the physical properties (e.g., density, glass transition temperature and refractive indices) and mechanical behaviors (e.g., hardness, fracture toughness, elastic modulus) of a soda-lime borosilicate glass with a composition of 60.2SiO2-16.0B2O3-12.6Na2O-3.8Al2O3-5.7CaO-1.7ZrO2 (mol%) have been investigated systematically. It was found that introducing V2O5 decreases the glass transition temperature and hardness while increases the refractive index and crack initiation load. Composition-property relationships, especially on mechanical properties, were examined in order to provide insights for the development of new glass materials with outstanding mechanical properties. The results show that V2O5 can be a glass component for engineering and insuring high crack resistant glasses and glasses with long-term stability. Additionally, elastic properties as a function of temperature were studied using Brillouin light scattering and temperature-independent elastic moduli behaviors were observed.

Fracture sequences during elastic–plastic indentation of brittle materials

Abstract

The effects of initial crack length on fracture characterization of rubbers using the J-Integral approach

The effects of initial crack length on the mode I fracture properties of crosslinked elastomers were investigated. A J-integral approach was implemented using single-specimen test procedures. Quasi-static fracture tests were carried out on filled and unfilled acrylonitrile-butadiene rubbers with various pre-cut (initial) crack lengths, a0, at a fixed rate of extension, using single edge notch rectangular bar specimens. Data from these experiments were analyzed and crack resistance curves (R curves) were obtained. A detailed analysis of the single-specimen method was carried out to establish correlation with the fracture properties derived from the more involved multi-specimen method. The weak dependence of geometry factor on rubber composition and displacement (in the neighborhood of crack initiation displacement) was verified. The crack initiation resistance and tearing modulus of the materials were calculated based on the R curve for each initial crack length. Fracture properties of lightly cracked specimens (a0/W<0.4, where W is the specimen width), including crack initiation resistance, strain at crack initiation, tearing modulus and maximum load and extension before fracture, were found to show significant variations with a0. These variations are characterized and discussed for the filled and unfilled rubber materials.

Determination of fracture energy and fracture toughness of two types of concrete and their interface

In this study, three-point bending tests are performed on high-strength concrete, low-strength concrete, cementation of high and low-strength concrete with interface between them and interface reinforced by steel fibre, respectively. The corresponding crack initiation load, peak load, external works at several loading points, fracture energy, pre-failure damage characteristics and double-K fracture toughness parameters are analysed based on test data utilising the theory of fracture mechanics. The quantitative comparisons of these values are made for different types of specimens. It reveals that the interface causes a great reduction on crack initiation load, peak load, the fracture energy, initiation fracture toughness and unstable fracture toughness. The pre-failure process of specimens with interface develops faster than other intact specimens. In addition, the steel fibre can reinforce the interface, especially, in increasing the sustainability at the post failure stage.

Differences in surface failure modes of soda lime silica glass under normal indentation versus tangential shear: A comparative study on Na+/K+‐ion exchange effects

AbstractThe effects of exchanging Na+ with K+ on the mechanical and mechanochemical properties of a soda lime silica (SLS) glass were investigated. It is known that replacing smaller modifier ions with bigger ions in the silicate glass network, at temperatures below the glass transition (Tg), produces a compressive stress in the subsurface region that enhances resistance to mechanical damages. This study found that when Na+ ions in SLS are exchanged with K+ ions at 400°C, the hardness, indentation fracture toughness, and crack initiation load of the surface are increased, which is consistent with the chemical strengthening effect. However, the resistance to mechanochemical wear in a near‐saturation humidity condition (relative humidity RH = 90%) is deteriorated. When K+ ions are exchanged back with Na+ ions at 350°C, the wear resistance in high humidity conditions is recovered. These results indicate that the improvement of mechanical properties under indentation normal to the surface is irrelevant with the resistance to mechanochemical wear under tangential shear at the surface. Based on the analysis of the surface chemical composition, silicate network structure, and hydrogen‐bonding interactions of hydrous species in the subsurface region, it is proposed that the leachable Na+ associated with non‐bridging oxygen and subsurface hydrous species in the silicate network play more important roles in the mechanochemical wear of SLS at high RH.

Nanoindentation and nanoscratch behavior of ZnO:Pr thin films deposited by DC sputtering

Abstract

The effects of thermal treatments on the subcritical crack growth of Pingdingshan sandstone at elevated high temperatures

Increasing the deeper understanding of the thermal damages and failure mechanisms of sandstone undergoing thermal treatments at different temperatures is a key concern for deep-mining and underground coal gasification processes. In this research study, a scanning electron microscope (SEM) apparatus, JSM-5410LV, which was equipped with a built-in digital electro-hydraulic servo loading system, was applied to carry out a series of three-point bending tests on Pingdingshan sandstone following heat treatments at elevated and high temperatures ranging from 25 to 600 °C. The subcritical crack initiation load, peak load, and elastic modulus were found to increase with the increases in the thermal treatment temperatures until a maximum was achieved at 125 °C, after which decreases were observed. However, it should be noted that there were sudden drops observed for the specimens after the thermal treatment temperature reached 150 °C due to the thermal diffusivity of the cement. The subcritical crack growth length was theoretically calculated, and a digital speckle correlation method (DSCM) was applied to verify the initial load and subcritical crack growth length. It was found that the fracture toughness fluctuated significantly when the thermal treatment temperature ranged from 25 to 125 °C, and reached a peak of 47.45 MPa mm0.5. It was also observed that, as the temperature was raised from 175 to 600 °C, the fracture toughness gradually decreased. The subcritical crack growth mode was determined to be intra-granular cracking following the thermal treatments below 125 °C, while a mixture of intra-granular and trans-granular cracking occurred in the specimens which experienced thermal treatments of 175 °C. The relationship between heat treatment temperature and subcritical crack growth was derived, which could be used to develop geothermal energy extraction from the critical temperature resources.

Simple mechanics model and Hertzian ring crack initiation strength characteristics of silicon nitride ceramic ball subjected to thermal shock

This study investigated the ring crack initiation strength characteristics of silicon nitride ceramic balls subjected to thermal shock. Sphere indentation tests were conducted on ceramic balls following water quenching and furnace cooling operations performed in air and vacuum. The ring crack radius was significantly smaller because of the very thin silicon oxide (SiO2) layer formed owing to heat treatment in air at high temperatures. In addition, the ring crack initiation load and Weibull shape parameter of water-quenched ceramic balls decreased with increasing temperature difference and remained relatively unaffected by the high-temperature oxidation. This phenomenon was mainly due to microscopic damage, which occurs near the surface of ceramic balls, caused by transient thermal stress developed during water quenching. A simple mechanics model based on the constant energy release rate criterion has been proposed for comparing experimental results obtained in this study against theoretical predictions, and results of the said comparison adequately verify strength characteristics of the aforementioned ceramic balls subjected to thermal shock.

New insights into nanoindentation crack initiation in ion‐exchanged sodium aluminosilicate glass

AbstractThe effect of ion‐exchange on the fracture behavior and the threshold load is investigated for radial crack initiation resulting from cube‐corner indentation. Both tin and air sides of the sodium aluminosilicate float glass are considered. The threshold load and mechanical properties are experimentally measured by nanoindentation. A qualitative explanation of crack initiation is developed by analyzing the stresses at the indentation site. The ion‐exchanged glasses show a lower threshold load for radial crack initiation with a cube‐corner indenter than the raw glass, and this is due to a higher crack driving stress for ion‐exchanged glasses. However, the compressive stress on the surface of the ion‐exchanged glasses can inhibit the expanding of the radial cracks. The air side always shows higher values for the threshold load than the tin side before and after ion‐exchange, which is in accordance with the calculated crack driving stress results.

Multi-parameter average strain energy density factor criterion applied on the bi-material notch problem

The stress field near a bi-material notch tip has the singular character. The power of singularity is lower in comparison to the case of a crack. The stress field can be described by the asymptotic stress series, in which each term contains generalized stress intensity factor and stress term exponent. The terms can be either singular or non-singular depending on the value of stress term exponent. The exponents are determined as solution of eigenvalue problem and depend on material properties and local geometry. The factors are calculated by analytical-numerical approaches and depend on global geometry and applied loading. The singular terms become unbounded when approaching the bi-material notch tip while the non-singular terms vanish. The non-singular terms increase precision in stress description on larger distances from the notch tip. The crack initiation conditions from the notch tip can be assessed by the average strain energy density factor (SEDF) criterion. In the case of bi-material notch problem, the crack can initiate in the direction of global minimum, the local minimum or the interface. The actual crack initiation load is calculated for each of abovementioned directions considering the individual fracture toughnesses. Contrary to the case of a crack, the direction of minimum of SEDF changes in dependence on the distance from the tip, therefore the average value over specific distance is used. The multi-parameter criterion considers singular and non-singular stress terms. Therefore in cases, when the specific distance is large, the non-singular terms provide means to improve precision of prediction of crack initiation parameters.

Ductile fracture initiation with consideration of strain concentration and stress triaxiality near crack fronts in compact tension specimens of hydrided irradiated Zr-2.5Nb materials with split circumferential hydrides

Ductile fracture initiation with consideration of strain concentration and stress triaxiality near crack fronts in compact tension specimens of hydrided irradiated Zr-2.5Nb materials with split circumferential hydrides is investigated by three-dimensional finite element analyses. The results indicate that plastic strain concentration is observed in the middle of the ligaments ahead of the crack front. A strain-based failure criterion with consideration of stress triaxiality is developed from the Gurson yield model. With the failure criterion, the necessary fraction of the load for crack initiation is about 0.60–0.70 to fracture the ligaments when compared to that for a CT specimen without split circumferential hydrides.

Numerical simulation of rock failure under static and dynamic loading by splitting test of circular ring

The fracture propagation and failure process in rock are subjects of great interest in rock mechanics. In the present study, an approach combining the finite element method (FEM) and the discrete element method (DEM), named ELFEN, is adopted to simulate and investigate the failure process of a typical hard rock (Carrara marble) under static and dynamic splitting ring tests. The FDEM software is firstly validated by simulating the test of a Brazilian disk under static loading. Then, the failure of circular ring specimens is numerically studied under static and dynamic loading, with the ratio of internal to external diameter (represented by λ) of the specimen groups varying from 0.1 to 0.6, with increments of 0.1. Under static loading, with the increase in internal diameter, the failure mode is transformed from diametrical splitting to four-fan-shaped failure (when λ≥0.4). Under dynamic loading, four-fan-shaped failure occurs from geometric axial symmetry to axial asymmetry, which means that, with increasing internal diameter, the tensile cracks along the horizontal diametrical direction gradually deviate toward the top loading platen. The peak load of the circular ring has a descending trend with the increase in internal diameter under both loading conditions. A rational load value (peak load or crack initiation load) is suggested to calculate the tensile strength of circular rock ring specimens under the splitting ring test. Finally, the numerical results are compared with previous experimental results. The good agreement between the two sets of results, in terms of the failure modes and variation trend of the indirect tensile strength with increasing internal diameters, verify the accuracy and applicability of the FDEM approach.

Chemical tempering of soda lime silicate glasses by ion exchange process for the improvement of surface and bulk mechanical strength

The aim of this work is improving mechanical properties of soda-lime glass by imparting compression stress on the surface through ion exchange and investigate the effect of process parameters on chemical strengthening. KNO3 salt bath was used for ionic exchange at varying temperatures and durations. Treated samples were investigated for their optical transmittance, surface potassium ionic concentrations and concentration profiles from surface to bulk using EDS and line scan EDS techniques. Effect of process temperature and time on the amount of mechanical strengthening was determined through the bending strengths (four-point), scratch resistances and indentation behaviors as micro hardness, crack formation load limit and cracking probability. Results were evaluated regarding ionic concentration variations. Transmittance decreased by 2% after the treatment. Surface potassium concentration increased with increasing temperature and was increased to become saturated in a short period. Potassium concentrations decreased from surface to bulk, and diffusion reached to deeper layers with increasing time and temperature. Ionic exchange improved micro hardness by 8–17%, threshold loads for indentation crack initiation 2–10 times, cracking probability down to 50%, strength up to 3.5–4 times and scratch resistance 2–4 times. Crack types changed between radial, median and lateral; depending on the degree of chemical tempering.

Mechanical properties of macrosynthetic fibre-reinforced concrete exposed to sulfate attack

The performance and durability of concrete is always affected by atmospheric conditions, chemical ions and mechanical loads, which reduce its bearing capacity, leading to the occurrence of accidents. Macrosynthetic fibre-reinforced concrete, as an alternative material, is used to improve the corrosion resistance of underground structures. In this study, accelerated corrosion tests were designed to investigate the change in compressive strength, elastic modulus and fracture toughness of concrete soaked in sulfate environment. Microscopic monitoring techniques, including scanning electron microscopy and energy-dispersive X-ray spectroscopy, were adopted to find changes in the structure. Compared to macrosynthetic fibre-reinforced concrete and ordinary concrete soaked in sulfate corrosion environment, the results indicated that macrosynthetic fibre improved the structure compactness of concrete; the losses of elastic moduli and compressive strength of the specimens soaked in sulfate were reduced. The maximum crack initiation load of macrosynthetic fibre-reinforced concrete was enhanced. Through microstructure monitoring, it is found that macrosynthetic steel fibre contributes to a remarkable improvement in the fracture resistance of concrete.

Influence of steel fibers extracted from waste tires on shear behavior of reinforced concrete beams

The accumulation of worn‐out tires creates health and fire hazards, and researchers are busy to incorporate waste tire rubber in concrete instead of burning or dumping. After shredding of waste tires, steel beads were separated by magnetic separator. The potential utilization of wasted steel fibers in concrete was investigated in this experimental work; fresh and hardened properties of fiber‐reinforced concrete (RC) were evaluated and compared with the conventional concrete. Shear behavior was investigated by conducting a four‐point bending test on RC beams. Five types of RC beams were cast; two control specimens were cast, with and without stirrups, by using conventional concrete. Three types of beams were cast by using 0.40, 0.65, and 1.00% fibers by volume. Workability of fiber RC was lower than the conventional concrete. Linear increase in failure load was observed with an increase in the dosage of fibers, and a 64.3% increase in failure load was observed after 1% addition as compared to control specimen. Mid‐span deflection, crack initiation load, and ultimate shear load were also increased with the addition of fibers. Crack spacing was reduced by 64.2% of specimen, with 1% dosage of fibers as compared to a control specimen without any stirrups, and a reduction in crack width was also observed with the addition of fibers.

Numerical Representation of Multiple Premature Failures in Steel-Plated RC Beams

Realizing the importance of widely used technique of plating for flexural retrofitting of reinforced concrete (RC) beams and its drawbacks due to premature failure(s), present work concentrates in developing a finite element tool model capable of successfully capturing multiple premature failure modes and their corresponding behaviors. The model is simple but focused; the capability and accuracy of the results have been validated through test literature, particularly focusing on the load capacities of beams at progressive stages of failure modes; which is from crack initiation through to complete failure, such as the load of crack initiation, first crack and complete failure. Acceptable accuracy is shown in terms of crack type(s), crack patterns, sequence, location and direction of propagation through the innovative use of cohesive zone model (CZM). The model clearly explains that debonding and peeling, although originating from a same location for most cases, are extensions of different types of cracks.

Interaction between Deformation and Crack Initiation under Vickers Indentation in Na2O–TiO2–SiO2 Glasses

An observed minimum in the critical crack initiation load at Poisson's ratio (ν) of 0.21-0.22 in Na2O-TiO2-SiO2 glasses was investigated. Vickers indentation was used to examine hardness and average cracking length, fracture toughness was measured through the single-edge pre-cracked beam method (SEPB), and volumes of densification and shear flow around indents were measured using atomic force microscopy (AFM). Relations between the critical crack initiation load and hardness, average crack length, fracture toughness, and the volume fractions of densification and shear flow were studied. No correlations were observed between hardness, average crack length, or fracture toughness with the critical crack initiation load. A link between the minimum in crack initiation load and a change in deformation mechanisms (densification vs. shear flow) was observed.

Mode I fracture behaviour of sandstone after heat treatment

The stability of rock in engineering applications, such as exploiting geothermal resources and underground coal gasification, is seriously affected by its fracture behaviour during or after high-temperature exposure. Three-point bending (TPB) tests on sandstone samples with different pre-existing crack lengths after heat treatment were conducted to investigate the influence of temperature and crack length on the mode I fracture properties. As the temperature increased, the fracture toughness first increased and then decreased gradually, reaching a peak value at 200°C. While the a/h (the ratio of the crack length to the height of the sample) was between 0·29 and 0·37, the crack length had no detectable effect on fracture toughness. The crack initiation load, critical tensile strain and the propagation process of mode I cracks were obtained based on the uniqueness of the crack propagation path under TPB and maximum strain theory.