Showing papers on "Crack closure published in 2016"
TL;DR: In this paper, a cubic degradation function was proposed to provide a stress-strain response prior to crack initiation, which more closely approximates linear elastic behavior, and a derivation of the governing equations in terms of a general energy potential from balance laws that describe the kinematics of both the body and phase-field.
383 citations
TL;DR: In this paper, the characteristics of the acoustic emission (AE) parameters and the evolution of cracking sequences are analyzed for every cracking level, based on micro-and macro-crack damage, a classification of cracking levels is introduced.
Abstract: Determination of the cracking levels during the crack propagation is one of the key challenges in the field of fracture mechanics of rocks. Acoustic emission (AE) is a technique that has been used to detect cracks as they occur across the specimen. Parametric analysis of AE signals and correlating these parameters (e.g., hits and energy) to stress–strain plots of rocks let us detect cracking levels properly. The number of AE hits is related to the number of cracks, and the AE energy is related to magnitude of the cracking event. For a full understanding of the fracture process in brittle rocks, prismatic specimens of granite containing pre-existing flaws have been tested in uniaxial compression tests, and their cracking process was monitored with both AE and high-speed video imaging. In this paper, the characteristics of the AE parameters and the evolution of cracking sequences are analyzed for every cracking level. Based on micro- and macro-crack damage, a classification of cracking levels is introduced. This classification contains eight stages (1) crack closure, (2) linear elastic deformation, (3) micro-crack initiation (white patch initiation), (4) micro-crack growth (stable crack growth), (5) micro-crack coalescence (macro-crack initiation), (6) macro-crack growth (unstable crack growth), (7) macro-crack coalescence and (8) failure.
230 citations
TL;DR: In this article, the formation mechanism of FGA is proposed: Numerous Cyclic Pressing (NCP) between originated crack surfaces, which causes grain refinement at the originated crack wake and therefore the formation of fine-granular area (FGA).
179 citations
TL;DR: In this paper, Fatigue crack propagation tests of Ti•6Al•4V fabricated by the Wire+Arc Additive Manufacturing (WAAM) process are analyzed. But the study focused on the microstructure and residual stress effect.
151 citations
TL;DR: In this paper, the authors used previously described NO 3 − reducing bacteria, and two different porous protective carriers, to develop microbial self-healing concrete, and found that the highest crack width healed by the bacteria was 370 −±−20μm in 28 days and 480 −±-16mm in 56 days.
Abstract: Microbial self-healing of concrete has been widely investigated, yet the suggested microbial pathways are limited to ureolysis and the aerobic oxidation of carbon sources. Each of these pathways has certain environment and process related drawbacks which arise a need for an alternative pathway to proceed further. This study presents the NO 3 − reduction as an alternative microbial self-healing strategy. In the tests, we used previously described NO 3 − reducing bacteria, and two different porous protective carriers. The highest crack width healed by the bacteria was 370 ± 20 μm in 28 days and 480 ± 16 μm in 56 days. Water tightness regain up to 85% was achieved at the end of 56 days for 465 ± 21 μm crack width. Precipitates were identified as forms of CaCO 3 and were abundant in microbial specimens particularly on the inner crack surface. The findings evidence the potential of the NO 3 − reduction pathway for development of microbial self-healing concrete.
126 citations
TL;DR: In this article, a finite element analysis of stress about a blunt crack tip, emphasizing finite strain and phenomenological and mechanism-based strain gradient plasticity (SGP) formulations, is integrated with electrochemical assessment of occluded-crack tip hydrogen (H) solubility and two H-decohesion models to predict hydrogen environment assisted crack growth properties.
122 citations
TL;DR: In this paper, a crack distribution function is defined to compare and assess the effect of the microstructural parameters for the two fatigue regimes on a Rene 88DT polycrystalline nickel-base superalloy.
118 citations
TL;DR: In this paper, the effects of crack openings on crack propagation and coalescence behavior in rock-like materials under uniaxial compression were investigated, and the real-time crack coalescence processes in the specimens with different crack openings were analyzed.
Abstract: This paper experimentally investigates the cracking behavior of rock-like specimens containing artificial open flaws under uniaxial compressive loads. The present experiments mainly focus on the effects of crack openings on crack propagation and coalescence behavior in rock-like materials under uniaxial compression. The real-time crack coalescence processes in the specimens with different crack openings are analyzed. The experimental results show that the crack openings significantly affect the crack initiation stresses and the crack initiation modes. The initiation stresses of wing cracks and coplanar secondary cracks decrease with increasing crack openings. However, the initiation stress of anti-wing cracks increases with increasing crack openings. Moreover, five types of crack coalescence in the specimens containing three pre-existing open flaws under uniaxial compression are observed. The effects of crack openings on the mechanical properties of rock-like materials, which include the complete axial stress–strain curves, peak stresses, peak strains and initiation stresses, are investigated in detail.
117 citations
TL;DR: In this paper, the authors studied the effect of water intrusion on crack propagation and calculated the mechanical properties of coal such as the elastic modulus and post-peak modulus, and quantified the percentage of the stress thresholds for crack closure, crack initiation and crack damage that constitutes the peak stress.
Abstract: Studying the mechanical properties of and crack propagation in coal after water intrusion is necessary to tackle a number of geological engineering problems such as those associated with underground water storage in collieries and support for underground roadways in coal mines. To study the mechanical properties and crack development, 12 coal samples with moisture contents of 0, 2.37, 3.78 and 5.29 % were prepared for acoustic emission tests under uniaxial compression. Over about 6 days, the coal samples absorbed moisture from a humidifier in three different phases. In this period, uniaxial tests show that the peak stress, elastic modulus, strain softening modulus and post-peak modulus decreased with rising moisture content in the samples while the peak strain increased. It was further found that, by analysing the relationship between the stiffness and stress and the accumulated acoustic emission counts, all the phases of crack development can be evaluated. This is useful for studying the effect of water intrusion on crack propagation and for calculating the mechanical properties of the coal such as the elastic modulus. This investigation also quantifies the percentage of the stress thresholds for crack closure, crack initiation, and crack damage that constitutes the peak stress. These stress thresholds do not change with moisture content. Our results are of great significance for water storage in coal mines, for determination of pillar dimensions in coal mines, and for expanding the knowledge base of the mechanical properties of coal and the characteristics of crack propagation.
112 citations
TL;DR: In this article, the authors present a careful quantification of the geometrical parameters of actual concrete cracks, including density, orientation, tortuosity and width, and explore their correlation with the chloride diffusion properties of concrete.
100 citations
TL;DR: In this article, the growth of long fatigue cracks in Inconel 718 superalloy produced by selective laser melting (SLM) was investigated and the fatigue crack growth curve and the threshold value of the stress intensity factor were experimentally determined on compact-tension specimens fabricated using a RENISHAW A250 system and the recommended processing parameters.
TL;DR: In this article, a semi-elliptical surface crack was inserted into a predicted macroscopic hot spot to carry out the thermal fatigue cracking analysis under consecutive emergency braking, and the predicted peak temperature and calculated crack geometry were well in agreement with the experimental results.
TL;DR: In this article, a coupled model of lithium diffusion, mechanical stress and crack growth using a phase field method is applied to investigate how the formation of cracks depends on the size of the particle and the presence or absence of an initial crack, as well as the applied flux at the boundary.
Abstract: Fracture of storage particles is considered to be one of the major reasons for capacity fade and increasing power loss in many commercial lithium ion batteries. The appearance of fracture and cracks in the particles is commonly ascribed to mechanical stress, which evolves from inhomogeneous swelling and shrinkage of the material when lithium is inserted or extracted. Here, a coupled model of lithium diffusion, mechanical stress and crack growth using a phase field method is applied to investigate how the formation of cracks depends on the size of the particle and the presence or absence of an initial crack, as well as the applied flux at the boundary. The model shows great versatility in that it is free of constraints with respect to particle geometry, dimension or crack path and allows simultaneous observation of the evolution of lithium diffusion and crack growth. In this work, we focus on the insertion process. In particular, we demonstrate the presence of intricate fracture phenomena, such as, crack branching or complete breakage of storage particles within just a single half cycle of lithium insertion, a phenomenon that was only speculated about before.
TL;DR: In this article, a robust microstructure based fatigue framework is developed, which takes into account i) the statistical variability of the material's micro-structure, ii) the continuum scale complex heterogeneous 3D stress and strain states within the micro structure, and iii) the atomistic mechanisms such as slip-grain boundary (GB) interactions, extrusion formations, and shearing of the matrix and precipitates due to slip.
TL;DR: In this paper, a peridynamic model for fatigue cracking in homogeneous and composite materials is studied using a per-dynamic model and a set of critical damage factors are introduced to improve the stability and efficiency of fatigue cracking.
TL;DR: In this article, the authors analyzed the influence of the load direction and the phenomenon of wear on rolling contact fatigue crack growth on a UIC60 rail with accurate geometry using FEM.
TL;DR: In this article, the authors presented the results of an experimental work on the flexural behavior of three types of concrete: normal strength concrete, high strength concrete and high strength fiber concrete, using the Digital Image Correlation (DIC) technique.
TL;DR: In this article, the authors investigated the mode-I fracture strength and crack growth trajectory of polymethylmethacrylate (PMMA) samples and found that the observed differences in the crack trajectory and the fracture strength of different specimens were related to the magnitude and sign of the T-stress.
TL;DR: In this article, a general method was proposed to separate the energy dissipated during loading before crack propagation from that which is dissipated at the onset of crack propagation, which is consistent with the constant value of the crack propagation velocity measured experimentally.
TL;DR: In this article, the authors established novel criteria for determining the hydrogen compatibility of austenitic stainless steels, as well as to elucidate the mechanisms for hydrogen-assisted surface crack growth (HASCG), slow strain rate tensile (SSRT), elasto-plastic fracture toughness (JIC), fatigue crack growth and fatigue life tests were performed on Types 304, 316 and 316L steels in high pressure hydrogen gas.
TL;DR: In this paper, the authors used yield criteria to estimate the area and shape of the crack tip plastic zone using both von Mises and Tresca yield criteria, and stress maps were subsequently found by applying these yield criteria.
Abstract: This paper presents a novel methodology for the experimental quantification of the crack tip plastic zone during fatigue crack growth. It is based on the application of yielding criteria to estimate the area and shape of the crack tip plastic zone using both the von Mises and Tresca yield criteria. The technique employs strain maps calculated from displacement fields obtained by digital image correlation. Stress maps were subsequently found by applying these yield criteria. Fatigue cracks were grown in compact tension specimens made from commercially pure titanium at R ratios of 0.1 and 0.6, and the ability was explored of three different analytical elastic crack tip displacement models [Westergaard, Williams and Christopher–James–Patterson (CJP)], to predict shape and size of the crack tip plastic zone. This analysis indicated that the CJP model provided the most accurate prediction of the experimentally obtained plastic zone size and shape.
TL;DR: In this paper, industrial 2524 aluminium alloy plates with various grain sizes (0.8-298 µm) were prepared by cold rolling and heat treatment and the fatigue-crack growth rate was studied as a function of grain size through fatigue tests and microstructural observations.
TL;DR: In this article, an iterative method to simulate 3D fatigue crack propagation in crystalline materials is proposed and demonstrated on simple test cases, which relies on the computation of a damage indicator based on plastic activity around the crack tip.
TL;DR: In this article, the fatigue behavior of the weld root in orthotropic steel decks stiffened with U-ribs in relation to the loading conditions and welding details was investigated and evaluated by considering different stress ratios and weld penetration rates.
TL;DR: In this paper, the different types of plastic zones found at a crack tip under cyclic loading of a commercially available aluminium alloy are studied based on 3D finite element simulations and mechanical testing.
15 Dec 2016-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, on lifetime and failure mode.
Abstract: Fatigue crack initiation in the direct aged version of the nickel-based superalloy Inconel 718 has been investigated at room temperature in the low stress/very high cycle regime via ultrasonic fatigue testing. Three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, and Σ3 twin boundary density on lifetime and failure mode. A slight refinement in grain structure and a higher Σ3 twin boundary density is associated with substantial reductions in lifetime. Decreasing Σ3 twin boundary density for fine grain microstructures results in a change in crack initiation mechanism from strain localization within grains at the high end of the grain size distribution to cracking of NMIs. To study the early stages of crack initiation and growth, specimens with pre-cracked NMIs were also tested in order to examine the role of the surrounding grain structure. Pre-cracked NMIs mainly result in macroscopic failure initiation at NMIs independent of the wrought microstructure. However, pre-loading specimens within the plastic domain highlighted the competition in crack initiation mode between cracked-NMIs and favorably oriented twin boundaries. Crack arrest from most of the pre-cracked NMIs demonstrates that surrounding grain structure (grain orientation, local plasticity and roughness in the vicinity of crack tip due to pre-straining) play a key role in the fatigue life of components stressed in the nominal elastic regime.
TL;DR: In this paper, a model based on Poisson defect distribution is proposed to describe the effects of stress ratio on faceted crack initiation, which is in agreement with the experimental results.
TL;DR: In this article, the effects of corrosion distribution, specifically non-uniform and localized corrosion, on cracks propagation in concrete were investigated, where different corrosion distributions along rebar length were simulated using a sodium chloride pond with various sizes set on the concrete cover.
TL;DR: In this paper, the authors evaluated the effects of oriented pearlite on hydrogen assisted fatigue crack growth in a steel pipe and found that cracks propagated either parallel or perpendicular to the banded pearlite microstructure.
TL;DR: In this article, the effects of thermal damage and confining pressure on the mechanical properties of marbles including post-peak behaviors and failure modes, strength and deformation parameters, characteristic stresses in the progressive failure process had been investigated.
Abstract: Heating treatment generally causes thermal damage inside rocks, and the influence of thermal damage on mechanical properties of rocks is an important topic in rock mechanics. The coarse marble specimens drilled out from a rock block were first heated to a specific temperature level of 200, 400 and 600 °C except the control group left at 20 °C. A series of triaxial compression tests subjected to the confining pressure of 0, 5, 10, 15, 20, 25, 30, 35 and 40 MPa were conducted. Coupling effects of thermal damage and confining pressure on the mechanical properties of marbles including post-peak behaviors and failure modes, strength and deformation parameters, characteristic stresses in the progressive failure process had been investigated. Meanwhile, accompanied tests of physical properties were carried out to study the effect of thermal damage on microstructure, porosity and P-wave velocity. Finally, the degradation parameter was defined and a strength-degradation model to describe the peak strength was proposed. Physical investigations show that porosity increases slowly and P-wave velocity reduces dramatically, which could be re-demonstrated by the microscopy results. As for the post-peak behaviors and the failure modes, there is a brittle to ductile transition trend with increasing confining pressure and thermal effect reinforces the ductility to some degree. The comparative study on strength and deformation parameters concludes that heating causes damage and confining pressure inhibits the damage to develop. Furthermore, crack damage stress and crack initiation stress increase, while the ratios of crack damage stress to peak strength and crack initiation stress to peak strength show a decreasing trend with the increase of confining pressure; the magnitude of crack damage stress or crack initiation stress shows a tendency of decrease with the increasing heating temperature and the tendency vanishes subjected to high confinement.