Fracture Properties Of Materials Research Articles

An Improved Ordinary State-Based Peridynamic Model for Granular Fractures in Cubic Crystals and the Effects of Crystal Orientations on Crack Propagation.

Material anisotropy caused by crystal orientation is an essential factor affecting the mechanical and fracture properties of crystal materials. This paper proposes an improved ordinary state-based peridynamic (OSB-PD) model to study the effect of arbitrary crystal orientation on the granular fracture in cubic crystals. Based on the periodicity of the equivalent elastic modulus of a cubic crystal, a periodic function regarding the crystal orientation is introduced into peridynamic material parameters, and a complete derivation process and expressions of correction factors are given. In addition, the derived parameters do not require additional coordinate transformation, simplifying the simulation process. Through convergence analysis, a regulating strategy to obtain the converged and accurate results of crack propagation paths is proposed. The effects of crystal orientations on crack initiation and propagation paths of single-crystal materials with different notch shapes (square, equilateral triangle, semi-circle) and sizes were studied. The results show that variations in crystal orientation can change the bifurcation, the number, and the propagation path direction of cracks. Under biaxial tensile loading, single crystals with semi-circular notches have the slowest crack initiation time and average propagation speed in most cases and are more resistant to fracture. Finally, the effects of grain anisotropy on dynamic fractures in polycrystalline materials under different grain boundary coefficients were studied. The decrease in grain anisotropy degree can reduce the microcracks in intergranular fracture and the crack propagation speed in transgranular fracture, respectively.

A size‐dependent energy‐based strain burst criterion

AbstractThis paper presents a size‐dependent energy‐based strain burst criterion linking strength, elasticity, fracture energies and specimen size effect with stress state due to changes in boundary conditions. It proposes the concept of a ‘Burst Envelope’, a surface in three‐dimensional principal stress space derived based on energy storing and dissipation characteristics of a rock sample, taking into account the size of the specimen and potential localised failure pattern. A scalar burst index is also proposed to quantify the bursting scale. To illustrate and verify its functioning, a numerical modelling framework based on the distinct element method and employing a new cohesive‐frictional contact model is used to perform virtual strain burst experiments under different polyaxial loading‐unloading scenarios, mimicking various underground excavation scenarios. The obtained results are in good agreement with the theoretical prediction of burst occurrence. On that basis, the variation of burst possibility and magnitude are investigated with key factors, including confinement level and the material's elastic, strength and fracture properties. The effect of the specimen's aspect ratio and size on the rock burst potential is elaborated and verified using virtual strain burst experiments, facilitating the linking of the proposed theoretical framework with the evaluation of in‐situ strain bursts in rock masses around underground openings.

Insights into the fracture properties of recycled ceramic and rubber composite cement-based materials: Fracture mechanics, acoustic emission, and digital image correlation

Recycling plays a pivotal role in construction waste management, offering a path toward sustainability for the industry. Although waste ceramics can compensate for the mechanical properties of rubberized cement-based materials, the smooth glaze surface of waste ceramics increases the risk of brittle failure. This study investigated the effect of ceramic waste aggregate (CWA) on the fracture properties of rubberized mortar through the three-point bending experiment. The fracture parameters were calculated using the Double-K fracture criterion. Simultaneously, acoustic emission (AE) and digital image correlation (DIC) technologies were used to explore the fracture damage mechanism and the crack evolution process. The results indicate that the integration of 15 % and 45 % CWA significantly enhances both initiation (KICini) and unstable (KICun) fracture toughness, whereas incorporating 90 % CWA leads to a 16.8 % and 8.2 % decrease in (KICini)and (KICun), respectively. The fracture energy (GF) of rubberized mortar exhibits a continuous decline as a function of increasing CWA substitution rates. Furthermore, GMM-based RA-AF analysis elucidates a positive correlation between CWA content and tensile crack proportion in specimens. When the CWA content reaches 90 %, the proportion of tensile cracks is quantified at 29.83 %, surpassing that in standard mortar. Crack tip opening displacement (CTOD) at 0.5post-Pmax for specimens with 15 %, 45 %, and 90 % CWA substitution reveal improvements of 49.1 % and 17.2 %, and a decrease of 23.3 %, respectively, compared to standard mortar, indicating that 15 % and 45 % CWA effectively preserve the ductility of rubberized mortar. Elevated CWA substitution leads to an amplified strain localization phenomenon around the crack tip, heightening the propensity for brittle failure.

Dynamic Mechanical Response and Fracture Properties of Rock-Like Materials with Parallel and Intersecting Flaws

Dynamic Mechanical Response and Fracture Properties of Rock-Like Materials with Parallel and Intersecting Flaws

Statistical analysis of effective crack properties by microstructure reconstruction and phase-field modeling

AbstractUnderstanding the relation between the microstructure and the material’s effective behavior is an important aspect in inverse computational materials engineering. Especially in the context of localized, inelastic phenomena like plasticity and crack growth, the microstructure morphology plays a crucial role. Due to the stochastic nature of heterogeneous media, a statistical analysis over multiple simulations is necessary, since even with the same material, the simulated crack paths and effective crack lengths are highly dependent on the specific locations of microstructural features. A relevant factor that limits this type of investigation is the high cost of real microstructure data. This work presents a digital workflow for exploring the fracture properties of materials. Therein, the required statistical analyses are facilitated by an algorithm that reconstructs multiple realization of a material structure given a single example. The reconstructed structures are discretized with a regular non-conforming mesh with a diffuse interface and crack representation. Crack phase-field simulations are conducted in order to analyze the effective response. An in-depth introduction to the required methods is given together with a statistical evaluation of the conducted numerical experiments. It is concluded that the statistical variation of the effective material behavior overshadows morphological trends in the presented case. This confirms the relevance and utility of complementing simulation-based workflows with microstructure reconstruction and statistical analyses.

Effects of edge-crack orientation and depth on non-linear dynamics of laminated nanocomposite single-variable-edge plates

This paper presents an investigation into the free vibration and nonlinear dynamic analyses of new complex edge contour plate structures, known as single-variable-edge (SVE) plates. Cracks emerge at the center of the plate’s surface, and the effects of edge crack orientation and depth on the dynamical characteristics of the laminated nanocomposite SVE plates have been considered in this study. The functionally graded graphene nanoplatelet reinforced composite (FG-GPLRC) material was utilized for the edge-cracked plate that rests on a Winker-Pasternak elastic foundation. The Halpin-Tsai micromechanical model is used to predict the material characteristics, which are presumptively temperature-dependent. The rule of mixtures is employed to determine Poisson's ratio, density mass, and thermal expansion. The governing equations are established based on the theory of elasticity, and Von Karman-Donnell geometric nonlinearity assumption. To confirm the reliability and accuracy of this approach, the acquired findings are validated with those produced using Finite Element Analysis (FEA). Furthermore, the geometrical and material parameters, fracture properties, and environmental (temperature and elastic foundation) factors are also investigated in the study. The research findings hold significant implications for applications in aerospace and aeronautical, civil, and mechanical engineering.

Plasticity and ductile fracture of high-strength bolt materials under monotonic tension

The plasticity and fracture properties of high-strength bolts are crucial to the mechanical behaviours of bolted connections subjected to extreme loadings. However, the material properties of high-strength bolts have not been comprehensively studied. This paper presents a collaborative experimental and numerical investigation to characterise the material plasticity and fracture properties of normal high-strength bolts (NBs) and weathering high-strength bolts (WBs) under monotonic tension. Eighteen cylindrical tensile coupons, cut from M20, M24 and M30 grade 10.9 NBs and WBs, are tested to capture the full-range engineering stress-strain relationships. The combined Swift-Voce weighting model is modified to describe and calibrate the material true stress-strain relationship of NBs and WBs. Then, a void-growth model (VGM) incorporated with the calibrated true stress-strain relationship is used to simulate the ductile fracture of tensile coupons. The calibration process collaborating experimental results and finite element (FE) simulations to obtain material parameters of high-strength bolts is demonstrated in detail. This collaborating method is further employed to simulate the plasticity and fracture properties of normal bolts with various grades under tension, as well as grade 10.9 high-strength bolts subjected to monotonic tension at elevated temperatures in literature. FE simulation results show a high accuracy with experimental results across all specimens, proving the rationality of the proposed method for material plasticity and fracture properties of high-strength bolts.

Application of a Closed-Form Model in Analyzing the Fracture of Quasi-Brittle Materials.

Fracture failure in quasi-brittle materials poses a persistent challenge in materials science and engineering. This study presents a thorough investigation of the Boundary Effect Model (BEM), offering a nuanced understanding of the size effect on fracture properties. The conceptual framework, evolutionary process, and applicability scope of BEM are elucidated, highlighting its accuracy and reliability in calculating fracture properties across various quasi-brittle materials. Through the integration of BEM with diverse fracture tests-such as three-point bending, four-point bending, and wedge-splitting-a linear correlation between maximum failure loads and material fracture properties is established. Notably, the study demonstrates that fracture properties, determined by BEM, can be regarded as consistent material constants across specimens of varying sizes, initial notch lengths, geometries, and microstructures. Validation of the BEM's reliability encompasses the analysis of 140 fracture test results involving concrete, hard rocks, and bamboo scrimber. The synergy of non-linear and linear BEM analyses emerges as a robust approach for accurately predicting the fracture behavior of quasi-brittle materials. This comprehensive exploration sheds light on the potential of the Boundary Effect Model as a valuable tool for predicting and understanding fracture mechanics in diverse materials and scenarios. This research serves as an effective approach to accurately evaluating the fracture properties of quasi-brittle materials, which is of great practical significance for material design, engineering construction, and various industrial applications.

Structural Reliability of pressure tube of a PHWR with volumetric flaws

The volumetric flaws can get generated in the pressure tubes (PT) of PHWR (made of Zr-2.5Nb material) during the operation of the reactor. The PT material has hydrogen present during the fabrication. Additionally, the zirconium material has a tendency to pick up hydrogen from the surrounding environment and can cause initiation of a crack through delayed hydride cracking (DHC) mechanism. This crack can grow due to the DHC and has the potential to cause leakage or break of the PT. The crack formation occurs only when a set of conditions are met. The model for the crack initiation ahead of the tip of a volumetric flaws is prescribed by the CSA Standard N285.8-15. The formation of the crack is dependent of the geometry of PT, geometry of the flaw (length, depth and root radius), applied loading (due to internal pressure, residual stresses), hydrogen concentration in the material and the material tensile and fracture properties. Most of these parameters are well defined. However, some of them like flaw geometry and material properties are not known with certainty. The structural reliability based methods quantify the uncertainty using probability distributions and present results in form of probability estimates. In this work, the probability of initiation of DHC ahead of a volumetric flaw and its subsequent growth is reported. Monte Carlo based simulation method along with variance reduction techniques is used to predict the probability. This study is useful to establish the Leak-Before-Break behaviour of the PT.

Dynamic fracture toughness measurement of graphite material considering inertial effect

Graphite materials are widely used as moderators, reflectors, and core structural materials in high-temperature gas-cooled reactors. During operation, graphite materials in nuclear reactors may be subjected to dynamic loads caused by earthquakes or other factors. Therefore, the dynamic fracture properties of graphite materials need to be investigated. In this study, dynamic three-point bending drop hammer impact experiments were conducted on IG-11 graphite specimens with precracks to measure their dynamic fracture toughness under medium- and low-speed impacts using a digital image correlation method. Traditional methods for measuring fracture toughness based on ASTM standards do not consider the influence of the inertial effect, leading to unreasonably large measurement results. To address this disadvantage, the dynamic fracture toughness of IG-11 graphite was evaluated using a spring–mass model that considers the inertial effects of materials. Results indicated that the measured fracture toughness was significantly lower than that obtained based on the ASTM standard and increased almost linearly with increasing impact velocity. The toughening mechanism was analysed by observing the fracture morphology of the samples using a scanning electron microscope. The results showed that as the impact velocity increased, the fracture mode of the graphite samples gradually changed from intergranular to transgranular fracture, and the samples absorbed sufficient energy, resulting in a noticeable toughening phenomenon.

Asymmetric fracture behavior in ferroelectric materials induced by flexoelectric effect

Ferroelectric materials are widely used in actuators, exciters, and memory devices due to their excellent electromechanical properties. However, the instinctive brittleness of ferroelectric materials makes them easy to fracture under external load. Since giant strain gradient can be easily generated near the crack tip, the flexoelectric effect is indispensable in the research of fracture properties of ferroelectric materials. With the combination of time-dependent Ginzburg–Landau theory and phase-field model, the electromechanical behavior of PbTiO3 in the vicinity of the crack tip is determined in this work. The simulation results demonstrate that the domain structure near the crack tip becomes asymmetric with the flexoelectric effect. The polarization switching-induced toughening, which is characterized by the J-integral, depends on the direction of the crack relative to the original polarization orientation. Furthermore, the longitude flexoelectric coefficient f11 has more significant impact on the fracture toughness than that of the transverse flexoelectric coefficient f12 and the shear flexoelectric coefficient f44. The results of the present work suggest that the flexoelectric effect must be considered in the reliable design of ferroelectric devices.

Fatigue reliability assessment of load‐carrying cruciform welded joints with undercuts and misalignments

AbstractThe additional local stress concentration caused by undercut and misaligned defects could lead to lower fatigue strength of welded joints. The paper aims to investigate the effects of undercuts and misalignments on fatigue performance and reliability of Load‐carrying Cruciform Welded Joint (LCWJ) quantitatively by probabilistic statistics theory and fracture mechanics theory. Firstly, the geometrical characteristics of undercut and misalignment are summarized from the literature and our experiments. Subsequently, the probabilistic distributions of material fracture properties, the configurations of geometries, and flaws were evaluated under specific loading conditions. The fatigue strengths are further estimated by the probabilistic reliability theory by combining the actual fatigue data. The results demonstrate a clear disparity of fatigue crack growth threshold and resistances for the tested data of base metal and weldment. The level of defects determines the interval of fatigue life, and the larger the dispersion accompanying earlier fatigue failure occurs.

Fracture process zone in crystalline rock: effect of specimen size and shape

Ability to predict fracture properties of granular materials based on their size and loading conditions needs rigorous experimental evidence. Charcoal granite specimens of various sizes and geometries are tested under three- and four-point bending to investigate the effect on the fracture toughness of rock and size of the fracture process zone calculated using Digital Image Correlation (DIC), with validation provided by Acoustic Emission (AE). Two different methods to evaluate the length of the fracture process zone using DIC are adopted: the first one assumes that a traction free crack does not propagate in the pre-peak regime, while the second one hypothesizes that a cohesionless crack may exist at peak load before unstable crack growth. The effects of specimen size, geometry, and loading conditions are found to be more prominent for the specimen sizes used in the laboratory but are predicted to become less significant and eventually negligible as the specimen size increases. DIC results are in general agreement with localization of AE events, but the former technique is suggested due to greater resolution and ability of continuous measurements of the fracture process zone dimensions.

Dissipation mechanisms of crack-parallel stress effects on fracture process zone in concrete

The effect of crack-parallel stresses on the fracture properties of quasi-brittle materials has recently received significant attention in the fracture mechanics community. A new experiment, the so-called gap test, was developed to reveal this effect. While the finite element crack band model (CBM) with the physically realistic Microplane damage model M7 was quite successful in capturing the damage and fracture during the gap test, some questions remain, particularly the near doubling of the fracture energy at moderate crack parallel compression, which was underestimated by about 30%. Presented here is an in-depth meso-mechanical investigation of energy dissipation mechanisms in the Fracture Process Zone (FPZ) during the gap test of concrete, an archetypal quasi-brittle material. The Lattice Discrete Particle Model (LDPM) is here used to simulate the quasi-brittle material at the mesoscale, which is the length scale of major heterogeneities. The LDPM can capture accurately the frictional sliding, mixed-mode fracture, and FPZ development. The model parameters characterizing the given mix design are first calibrated by standard laboratory tests, namely the hydrostatic, unconfined compression, and four-point bending (4PB) tests. The experimental data used characteristic of the given mix design are calibrated by the Brazilian split-cylinder tests and by gap tests of different sizes with and without crack-parallel stresses. The results show that crack-parallel stresses affect not only the length but also the width of the FPZ. It is found that the energy dissipation portion under crack-parallel compression is significantly larger than it is under tension, which is caused by micro-scale frictional shear slips, as intuitively suggested in previous work. For large compressive stresses, the failure mode changes to inclined compression-shear bands consisting of axial splitting microcracks. Several complications experienced in the numerical modeling of gap tests are also discussed, and the solutions provided.

Selective laser sintered Pipe Ring Notched Tension specimens for examination of fracture properties of pipeline materials

In order to develop a non-standard method for determining the resistance to fracture and damage of pipeline materials, a new geometry of ring-shaped specimens with sharp notches or cracks has been defined. The need to develop a new method for testing of the specimens cut from the pipes arises due to the difficulties in determination of fracture mechanics parameters on thin-walled pressure pipelines, especially those with a smaller cross-section, by application of the standard procedures/specimens. Previous studies dealing with the topic of pipeline testing by non-standard methods are presented in the introductory part of the paper. In the experimental part of the research, tests on PRNT (Pipe Ring Notched Tension) and SENT (Single Edge Notched Tension) specimens are performed. Samples of these specimens were produced by SLS (Selective Laser Sintering) technique of additive production from PA12 (polyamide PA 2200) material. For the purpose of this study, a tool that is protected at the national level (in Serbia) as intellectual property is used for testing of the ring-shaped PRNT specimens. Tensile testing of both types of specimens is monitored by Aramis GOM 2M system; its operation is based on the method of digital image correlation, DIC. Additionally, finite element analyses are conducted on the PRNT and SENT geometries, enabling the calculation of the fracture mechanics parameter – Stress intensity factor K. It is concluded that the presented procedure based on the PRNT specimen has a good potential for use as a non-standard method for fracture resistance examination of pipeline materials. It can be performed on the specimens cut directly from the pipes (new or from exploitation). The dependence of the fracture resistance on the stress concentrator size is not pronounced, which means that the results depend dominantly on the material properties, rather than on geometry.

From macro fracture energy to micro bond breaking mechanisms – Shorter is tougher

We show a novel fracture behavior and properties of brittle materials not previously explored. These were made possible when merging macro to micro in fracture. Our findings are based on macro-scale fracture cleavage experiments of dynamic cracks propagating in brittle single-crystal silicon specimens, focusing on the crack’s energy-speed relationships, and the fine details of the crack front. From the micro-scale, we developed an atomistic model for bond-breaking mechanisms. These mechanisms are in the form of low-energy kink advance (migration) and high-energy kink formation (nucleation) along the crack front. The energy release rate (ERR) at crack initiation, G0, and its derivative, dG0/da≡Θ, in particular, play a major role in the novel behavior and properties. G0 and Θ are influenced by the precrack length, a0. The macro-scale experiments, the micro-scale atomistic model, and the gradient of the ERR, Θ, enabled the conclusions portrayed in this work.We identified that the cleavage energy is not a constant but bounded by the Griffith Barrier as the lower bound while the upper bound (apparently the lattice-trapping barrier) may reach 3 times the lower bound. We further suggest that a brittle material can be envisaged as comprising a pseudo-R-Curve behavior mechanism typical of metallic materials. A new and essential fracture mechanism was identified, which we term ‘quasi-propagation’, a transitional mechanism between initiation and propagation. During this mechanism, the sequence of the bond-breaking mechanisms is varying, causing an increase in the macroscale cleavage energy.The evaluated cleavage energy shows another novel behavior, that of ‘shorter is tougher’, namely, shorter cracks require higher energy to propagate, and, therefore, specimens with shorter cracks are stronger than that predicted by Griffith’s theory.

Characterization of fracture development in geometrical similar ultra-high-performance concrete (UHPC) beams using acoustic emission (AE) technique

The acoustic emission (AE) technique is a common method for evaluating the fracture and damage properties of materials. This study utilized the AE technique to analyze the fracture behavior of ultra-high-performance concrete (UHPC) beams. Experiments were conducted on three differently sized beam specimens that were geometrically similar. The center point bending tests were carried out under crack mouth opening displacement (CMOD) control to study the fracture properties of the UHPC beams. Based on the experimental study, the fracture properties such as crack propagation, parametric-based properties, and fracture process zone (FPZ) length and width were predicted in the present work. The AE waves generated during the cracking can readily differentiate between the concrete matrix failure and fiber bridging following the post-peak response in the UHPC beam. In this work, the AE parameters were used to detect the source location, fracture mode, the position of the crack, and strain localization near the crack tip. The presence of steel fibers in concrete provides complexity in identifying the source localization, and wave dispersion results in attenuation and distortion of the AEs. The occurrence of AE events was observed to increase in the post-peak part compared to pre-peak possibly due to fiber bridging, demonstrating the energy absorption capacity. Based on AE results, the length and width of the fracture process zone were found to increase with the specimen size.

High pressure and long-term gas diffusion coupled damage of composites through a multi-physical phase field framework

In extreme service environments, composites are often exposed to high pressure and prolonged chemical penetration, leading to irreversible material damage and structural failure. To address this multi-field coupling problem that has long been a concern, we develop a theoretical framework for fully coupled stress distribution, gas diffusion, heat conduction, and damage evolution of composites to reproduce complex failure modes and disclose the damage mechanisms of composites in complicated environments. To verify the theoretical feasibility, the susceptibility of material fracture properties to hydrogen content is well revealed, and a typical problem, i.e., damage of multi-layered composite hydrogen storage vessels in distinct working stages, is investigated and discussed insightfully. We highlight that (i) the fracture phase field theory is rederived under the thermodynamic consistent framework; (ii) the proposed computational model can accurately capture the heat conduction response under the coupled effects of chemical diffusion and crack propagation; (iii) through a detailed analysis of the energy evolution, the model allows precise yield of the dominant mechanisms in the diverse damage stages of the materials. This study can reproduce the different damage patterns of composite structures under the action of multi-physical fields in extreme environments, thus providing a predictive computational approach to prevent disastrous failures.

Effect of confining pressure on the compression-shear fracture properties of sandstone

The fracture characteristics of the deep rocks are susceptible to the influence of the confining pressure. Quantities of fracture tests were carried out on cracked Brazilian disc specimens to investigate the compression-shear fracture properties of sandstone materials exposed to confining pressure using the self-developed triaxial experimental system. Furthermore, the sensitivities of failure load, fracture toughness and crack propagation to the confining pressure and geometric parameters of the specimen were evaluated. The results indicate that the confining pressure environment contributes to the increase in the strength of the sandstone. The fracture toughness components (KIf and KIIf) of sandstone specimens satisfy linear correlation with the confining pressure at confining pressures from 0 to 6 MPa. Similarly, the effective fracture toughness (Keff), adequately reflecting the strengthening effect of confining pressure, satisfies a linear growth relationship with the confining pressure if the confining pressure is large enough (i.e., the confining pressure coefficientt > 1). Moreover, the crack propagation characteristics may still exhibit a tensile-shear mode under the compression-shear loading conditions supported by high values of confining pressure.

Evaluation of the Fracture Toughness KIc for Selected Magnetron Sputtering Coatings by Using the Laugier Model.

Nanoindentation is one of the methods that allows for determining the fracture properties of brittle materials. In this article, the authors present the possibility of the fracture toughness coefficient calculation of ceramic-based coatings doped by metal (W, Cr) by using the nanoindentation method with the Berkovich diamond indenter. The mechanical properties of selected coatings, such as hardness and Young's modulus, were investigated from nanohardness experiments. We analyzed the brittle fracture, which includes changes in hardness (H), Young's modulus (E), plasticity index H/E and resistance to plastic deformation H3/E2, enabled the concentration of tungsten and chromium. Due to the size of the indentation and the size of the initial cracks, it is necessary to use Scanning Electron Microscopy (SEM) to observe and measure the indentations made and the generated cracks. For evaluation of the fracture toughness in mode I, the Laugier model was chosen experimentally. The fracture toughness analysis showed that doping with concentrations of 10% W and 10% Cr causes an increase in the fracture toughness for KIc = 4.98 for TiBW (10%) and KIc = 6.23 for TiBCr (10%).