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1
Freim, John, J. McKittrick, W. J. Nellis e J. D. Katz. "Development of novel microstructures in zirconia-toughened alumina using rapid solidification and shock compaction". Journal of Materials Research 11, n. 1 (gennaio 1996): 110–19. http://dx.doi.org/10.1557/jmr.1996.0014.
Testo completoAbstract (sommario):
A rapidly solidified alumina-zirconia eutectic material containing nanocrystalline t-ZrO2 has been synthesized. When heated, the microstructure contained a mixture of t-ZrO2 and m-ZrO2, each of which can facilitate toughening of the composite. Dynamic shock compaction was used to accelerate densification of the material, producing crack-free specimens with high green densities. After sintering to densities measuring ∼95% of theoretical, the shock-compacted specimens fabricated with unstabilized alumina-zirconia were extensively microcracked due to an overabundance of the m-ZrO2 phase. Experiments employing Y2O3 as a chemical stabilizer have shown that the extent of the phase transformation can be controlled, and the microstructure that developed in the stabilized material contained an acceptable level of the microcrack generating m-ZrO2 phase.
2
Sevostianov, I., L. Gorbatikh e M. Kachanov. "Recovery of information on the microstructure of porous/microcracked materials from the effective elastic/conductive properties". Materials Science and Engineering: A 318, n. 1-2 (novembre 2001): 1–14. http://dx.doi.org/10.1016/s0921-5093(01)01694-x.
Testo completo
3
Zeng, Qiu Lian, Zhong Guang Wang e J. K. Shang. "Microstructural Effects on Low Cycle Fatigue of Sn-3.8Ag-0.7Cu Pb-Free Solder". Key Engineering Materials 345-346 (agosto 2007): 239–42. http://dx.doi.org/10.4028/www.scientific.net/kem.345-346.239.
Testo completoAbstract (sommario):
Low cycle fatigue behavior of Sn-3.8Ag-0.7Cu solder was investigated under fully reversed cyclic loading, with particular emphasis on microstructural effects. The LCF behavior of the solder with equiaxed microstructure was found to differ greatly from that of the solder with a dendrite microstructure. At a given total strain amplitude, the dendrite microstructure exhibited a much longer fatigue life than the equiaxed microstructure. Such a strong microstructural effect on fatigue life arose from the difference in cyclic deformation and fracture mechanisms between the two microstructures. A large number of microcracks along grain boundaries of the equiaxed structure solder developed with increasing cycling, while for the dendrite structure solder, cyclic deformation took place along the direction of the maximal shear stress during fatigue tests and microcracks initiated and propagated along shear deformation bands. Besides, the fatigue behavior of the dendritic microstructure was very sensitive to cyclic frequency whereas the fatigue behavior of the equiaxed microstructure showed less sensitivity to cyclic frequency.
4
Kilicli, Volkan. "Development of an eutectic-based self-healing in Al–Si cast alloy". Materials Testing 64, n. 3 (1 marzo 2022): 371–77. http://dx.doi.org/10.1515/mt-2021-2045.
Testo completoAbstract (sommario):
Abstract In this study, a self-healing mechanism was developed by means of melting the eutectic structure for microcrack repair in a hypoeutectic Al–Si cast alloy. The alloy was heated just above the eutectic temperature to provide melting of the eutectic in this mechanism. The melted eutectic Si particles repair the microcrack under appropriate conditions. The microcrack formation was provided by tensile loading in Al–Si alloy tensile bars and then eutectic-based self-healing treatment was performed to ensure microcrack healing. Microcrack healing was monitored by X-ray radiography and microstructural examinations were carried out by scanning electron microscopy. The mechanical properties were investigated by tensile testing before and after the healing treatment. Eutectic-based self-healing treatment provides the healing of some microcracks in the microstructure of hypoeutectic Al–Si cast alloy. Also, 44% of yield strength, 59% of ultimate tensile strength, and 86% of total elongation have been recovered by the eutectic-based self-healing process in Al–Si alloy.
5
Balart, MarÍa J., Xinjiang Hao e Claire L. Davis. "Automated SEM/EDS Analysis for Assessment of Trace Cross-Contamination in 316L Stainless Steel Powders". Metallurgical and Materials Transactions A 53, n. 2 (1 dicembre 2021): 345–58. http://dx.doi.org/10.1007/s11661-021-06474-4.
Testo completoAbstract (sommario):
AbstractFollowing observations of microcracking in two, out of three, Additive manufactured (AM) 316L steel samples, an investigation was undertaken to ascertain the root cause. Welding diagrams, taking into account composition and process parameters, could not generally account for the experimental observations of non-cracked versus cracked AM 316L samples. EBSD phase maps in all three AM samples exhibited a fully austenitic microstructure not only in the bulk sample but also near-surface. Analysis of microcracked regions in the AM samples showed the presence of local enrichment of Ni, Cu and P. Automated SEM/EDS analysis on feedstock powder samples prepared for cross-section examination revealed a fine, foreign particulate contaminant, expected to arise from NiCrCuP alloy cross-contamination during atomization, to be completely embedded in a 316L powder particle. This type of contamination would not have been revealed on examination of powder mounted onto a SEM stub, a common approach to assess powder quality. Based on this analysis, it is recommended to consider including automated SEM/EDS analysis on powder cross-sections in any standardization protocol for quality control of powders, to increase the chances of detection and identification of fine cross-contaminants. It is also recommended that atomization of NiCrCuP alloy should no longer precede atomization of 316L alloy.
6
Myer, L. R., J. M. Kemeny, Z. Zheng, R. Suarez, R. T. Ewy e N. G. W. Cook. "Extensile Cracking in Porous Rock Under Differential Compressive Stress". Applied Mechanics Reviews 45, n. 8 (1 agosto 1992): 263–80. http://dx.doi.org/10.1115/1.3119758.
Testo completoAbstract (sommario):
Under differential compressive stress rocks exhibit nonlinear deformation that includes initial compaction, near-linear elastic behavior, and strain-hardening followed by strain-softening and dilation (or compaction in clastic rocks) and localization. This behavior derives largely from changes in the microstructure of the rocks. Much of it has been attributed to the growth of extensile microcracks. The stress-induced microstructural changes brought about by successively more complicated states of stress produced by uniaxial and triaxial compression of circular cylinders, axisymmetric stresses in hollow cylinders, and indentation by hemispheres in Indiana limestone and Berea sandstone have been preserved using Wood’s metal porosimetry. In this technique molten Wood’s metal at about 100°C is used as a pore fluid at a pressure of about 10 MPa, and the experiments are conducted using the concepts of effective stress. At the deformation state of interest, the temperature is lowered to solidify the metal, thereby preserving the microstructure as it exists under load and facilitating subsequent preparation of the specimen for microscopic study. Mode I microcrack growth is observed to occur by a variety of mechanisms such as bending, point loading and sliding cracks. The effects of this are analyzed using an elastic continuum within which Mode II displacement across microcracks and Mode I microcrack growth results from heterogeneous stress concentrations that produce local tensile stresses. While the continuum model replicates many of the observations, it fails to account for localization by en echelon arrays of extensile microcracks that precede macroscopic shear faulting. Using a “zero order” continuum approximation, the spatially stochastic distribution of grains in clastic rocks is shown to be important in the formation of the en echelon arrays of microcracks that form shear bands.
7
Ju, J. W., e Tsung-Muh Chen. "Effective Elastic Moduli of Two-Dimensional Brittle Solids With Interacting Microcracks, Part I: Basic Formulations". Journal of Applied Mechanics 61, n. 2 (1 giugno 1994): 349–57. http://dx.doi.org/10.1115/1.2901451.
Testo completoAbstract (sommario):
Statistical micromechanical formulations are presented to investigate effective elastic moduli of two-dimensional brittle solids with interacting slit microcracks. The macroscopic stress-strain relations of elastic solids with interacting microcracks are micromechanically derived by taking the ensemble average over all possible realizations which feature the same material microstructural geometry, characteristics, and loading conditions. Approximate analytical solutions of a two-microcrack interaction problem are introduced to account for microcrack interaction among many randomly oriented and located microcracks. The overall elastic-damage compliances of microcrack-weakened brittle solids under uniaxial and biaxial loads are also derived. Therefore, stationary statistical micromechanical formulation is completed. Moreover, some special cases are investigated by using the proposed framework. At variance with existing phenomenological continuum damage models, the proposed framework does not employ any fitted “material parameters. ” “Cleavage 1” microcrack growth and “evolutionary damage models” within the proposed context will be presented in Part II of this series. It is emphasized that microstructural statistical informations are already embedded in the proposed ensemble-averaged equations and, therefore, no Monte Carlo simulations are needed.
8
Li, Xu-Dong. "K Variations and Anisotropy: Microstructure Effect and Numerical Predictions". Journal of Engineering Materials and Technology 125, n. 1 (31 dicembre 2002): 65–74. http://dx.doi.org/10.1115/1.1525252.
Testo completoAbstract (sommario):
Computer experiments were performed on simulated polycrystalline material samples that possess locally anisotropic microstructures to investigate stress intensity factor (K) variations and anisotropy along fronts of microcracks of different sizes. The anisotropic K, arising from inhomogeneous stresses in broken grains, was determined for planar microcracks by using a weight function-based numerical technique. It has been found that the grain-orientation-geometry-induced local anisotropy produces large variations in K along front of microcracks, when the crack size is of the order of few grain diameters. Synergetic effect of grain orientation and geometry of broken grains control K variations and evolution along the microcrack front. The K variations may diminish at large crack sizes, signifying a shift of K calculation to bulk stress dependence from local stress dependence. Local grain geometry and texture may lead to K anisotropy, producing unusually higher/lower K at a segment of the crack front. Either K variation or anisotropy cannot be ignored when assessing a microcrack.
9
Wang, Heng, Zhanli Liu, Dandan Xu, Qinglei Zeng e Zhuo Zhuang. "Extended finite element method analysis for shielding and amplification effect of a main crack interacted with a group of nearby parallel microcracks". International Journal of Damage Mechanics 25, n. 1 (29 dicembre 2014): 4–25. http://dx.doi.org/10.1177/1056789514565933.
Testo completoAbstract (sommario):
The shielding and amplification effects of transverse array of microcracks on a main crack are investigated using extended finite element method. The interaction between macrocracks and microcracks is quantitatively characterized in terms of the stress intensity factor which is calculated by the interaction integral method and the complete stress field in the entire domain could be given without remeshing. Various distributions of microcracks with different number, location, and density are considered. For a microcrack collinear to the main crack, the numerical results agree quite well with the analytical solution. Interestingly, the shielding and amplification effects display periodicity when the main crack is placed inside the microcrack rows. In particular, the minimum stress intensity factor of the main crack which refers to the maximum shielding effect is primarily determined by the nearest microcracks. However, the maximum stress intensity factor is largely affected by the distribution and density of microcracks and even could be turned from enhancement to shielding. The results are consistent with the microcrack-toughening phenomenon observed in the experiments and are meaningful for the design of new microstructure-toughening materials.
10
Lu, Houdi, Hongtao Wang, Haitao Wang, Lie Jin, Xinxin Wu e Yu Zhou. "FM-DBEM Simulation of 3D Microvoid and Microcrack Graphite Models". Science and Technology of Nuclear Installations 2017 (2017): 1–14. http://dx.doi.org/10.1155/2017/1071709.
Testo completoAbstract (sommario):
The graphite is porous medium, and the geometry and size distribution of its structural deficiencies such as microcracks and microvoids at different oxidation degrees have a great influence on the overall performance. In this paper, we adopt the FM-DBEM to study 3D models which contain spheroidal microvoids and circular microcracks. The accuracy of this method is tested by a comparison to the theoretical solution to the problem of 2D microcrack and microvoid interaction problem. Two simulations are conducted: the simulation of graphite model containing a large number of randomly distributed microcracks and microvoids and the simulation of graphite model containing microcracks and growing microvoids. The simulations investigate the effective moduli versus the two microstructures’ density and the effect of microvoid’s growth on the SIF of microcrack.
11
Aretusi, Giuliano, Christian Cardillo, Larry Murcia Terranova e Ewa Bednarczyk. "A dissipation model for concrete based on an enhanced Timoshenko beam". Networks and Heterogeneous Media 19, n. 2 (2024): 700–723. http://dx.doi.org/10.3934/nhm.2024031.
Testo completoAbstract (sommario):
<p>A novel Timoshenko beam model enriched to account for dissipation in cement-based materials was presented in this paper. The model introduced a new variable representing the relative sliding inside microcracks within the material. In the paper, the microcrack density was not supposed to increase, assuming a small deformation regime that implied no damage growth. The model utilized an expanded version of the principle of virtual work whose contributions came from external forces, internal elastic forces, and dissipation due to the microcrack's microstructure. The elastic energy included terms related to microcrack sliding and micro-macro interactions, accounting for nonlinearity in the material behavior. Numerical simulations, conducted using the finite element method, evaluated the mechanical properties of cement-based materials under three-point flexural tests and compression tests. These tests enabled the assessment of the material dissipative behavior under cyclic loading. Results showed dissipated energy cycles and mechanical responses influenced by the microcrack mechanics. Additionally, a parametric study, varying the friction force amplitude, revealed its impact on dissipated energy. The study highlighted a non-monotonic relationship between friction force amplitude and dissipated energy, with an optimal value maximizing dissipation. Overall, the model provided insights into the mechanics of cement-based materials, particularly regarding dissipation, which was essential for understanding their behavior in structural applications.</p>
12
Eterashvili, Tamaz, T. Dzigrashvili e M. Vardosanidze. "Dislocation Clusters and Microcracks in Thin Films of LCF-Tested Austenitic Steel". Key Engineering Materials 577-578 (settembre 2013): 237–40. http://dx.doi.org/10.4028/www.scientific.net/kem.577-578.237.
Testo completoAbstract (sommario):
Austenitic chromium-nickel stainless steel CrNiNb 18-10 was studied using TEM technique. Characterizations of thin films prepared from bulk cylindrical samples after low-cycle fatigue (LCF) tests were conducted. Focus was made on the dislocation clusters, slip bands, defects and microstructure changes taking place in the steel during LCF. It is shown that microcracks occur in slip bands. Stereographic and trace analyses revealed the microcrack propagation directions. Two types of microcracks were observed: wedge-shaped and with parallel sides. The obtained results on possible reasons and mechanisms of microcrack formation in the above places are discussed in line with the theoretical assumptions and the existing literature.
13
Eterashvili, Tamaz, Temur Dzigrashvili e Manana Vardosanidze. "Trajectory and Crystallography of Crack Growth in Austenitic Steel after LCF Tests". Key Engineering Materials 592-593 (novembre 2013): 793–96. http://dx.doi.org/10.4028/www.scientific.net/kem.592-593.793.
Testo completoAbstract (sommario):
Analysis of microcrack and mesocrack formation in austenitic steel thin filmsprepared after low-cycle fatigue (LCF) testsfrom bulk samples is presented using TEM techniques. Location, orientation and interaction of microcracks with microstructure components of the steel were determined. Plastic zone ahead of mesocrack tip and the structure changes in it were analyzed. Crystallography of slip bands and deformation twins and their relation with the microcrack propagation direction were also determined. The impact of grain anisotropy and inhomogeneous distribution of stress relaxation ahead of mesocrack tip in plastic zone were considered. Influence of sizes of mesocracks [ and microcracks and their relation with the trajectory and crystallography of propagation are also discussed.
14
Bian, Gui Xue, Yue Liang Chen, Jian Jun Hu e Yong Zhang. "Fatigue Microcrack Initiation and Propagation of Aluminum Alloy under Different Stress Level and Stress Ratio". Advanced Materials Research 239-242 (maggio 2011): 1495–500. http://dx.doi.org/10.4028/www.scientific.net/amr.239-242.1495.
Testo completoAbstract (sommario):
The microstructure of fatigue fracture of aluminum alloys under various stresses and stress ratios were studied by optical microscope and scanning electron microscope, and the influences of microstructure features on microcrack initiation and propagation were investigated. The results show that the fatigue microcrack originated from surface or subsurface of specimens. And with the increase of stress ratio, fatigue crack originated from deeper subsurface at the same stress level. With the increase of stress level, fatigue crack originated from shallower subsurface or surface at same stress ratio. There is an increase in crack propagation region as the stress level decreases at the same stress ratio. Increasing of stress ratio, increases crack propagation region under same stress level. Microcrack generally originated from secondary (S phase particles) and larger particles at low stress level and high stress ratio. Microcrack generally originated from larger constituent particles at high stress level and low stress ratio. Microcracks propagation is evidently impeded by grain boundaries at low stress level and high stress ratio.
15
Eterashvili, Tamaz, T. Dzigrashvili e M. Vardosanidze. "Deviations of Microcrack during Propagation in Thin Films of Austenitic Steel and Accompanying Accommodative Processes". Key Engineering Materials 627 (settembre 2014): 297–300. http://dx.doi.org/10.4028/www.scientific.net/kem.627.297.
Testo completoAbstract (sommario):
The work deals with the transmission electron microscopy (TEM) study of thin films of chromium-nickel Х18Н10 steel. The films were prepared from bulk samples after low cycle fatigue (LCF) tests. Focus was made on the processes accompanying propagation of small microcracks. Particularly, the microstructure changes near the crack tip were analyzed in terms of accommodation processes taking place during crack propagation, such as formation of slip bands, twins etc. The authors conducted crystallographic analysis of the defects formed during crack propagation in correlation with the reasons of their initiation and homogenous length of the slip bands. Thus, the reasons of microcrack deviation from the initial direction were determined. The research has shown that the most convenient microstructure variables in the austenitic crystals of polycrystalline sample, affecting the microcrack deviation, are microstructure, crystallography and the homogenous length of slip bands.
16
Ijbara, Manhal, Kanae Wada, Makoto J. Tabata, Junichiro Wada, Go Inoue e Michiyo Miyashin. "Enamel Microcracks Induced by Simulated Occlusal Wear in Mature, Immature, and Deciduous Teeth". BioMed Research International 2018 (2018): 1–9. http://dx.doi.org/10.1155/2018/5658393.
Testo completoAbstract (sommario):
Enamel wear, which is inevitable due to the process of mastication, is a process in which the microcracking of enamel occurs due to the surface contacting very small hard particles. When these particles slide on enamel, a combined process of microcutting and microcracking in the surface and subsurface of the enamel takes place. The aim of this study was to detect microscopic differences in the microcrack behavior by subjecting enamel specimens derived from different age groups (immature open-apex premolars, mature closed-apex premolars, and deciduous molars) to cycles of simulated impact and sliding wear testing under controlled conditions. Our findings indicated that the characteristics of the microcracks, including the length, depth, count, orientation, and relation to microstructures differed among the study groups. The differences between the surface and subsurface microcrack characteristics were most notable in the enamel of deciduous molars followed by immature premolars and mature premolars whereby deciduous enamel suffered numerous, extensive, and branched microcracks. Within the limitations of this study, it was concluded that enamel surface and subsurface microcracks characteristics are dependent on the posteruptive age with deciduous enamel being the least resistant to wear based on the microcrack behavior as compared to permanent enamel.
17
Lagoeiro, Leonardo, Paola Ferreira e Cristiane Castro. "Crystallographic Control on the Development of Texture in Precipitated Quartz Grains". Materials Science Forum 495-497 (settembre 2005): 57–62. http://dx.doi.org/10.4028/www.scientific.net/msf.495-497.57.
Testo completoAbstract (sommario):
In this study we analysed microstructures and determined [c]-axis textures of quartz crystals in veins formed parallel to composition banding in naturally deformed iron oxide-quartz rocks. Only veins of few millimeters thick were sampled. These veins were formed in a regime of non-coaxial deformation under temperature of ~300°C. We made thin sections from rock slabs cut perpendicular to shear plane and parallel to shear direction. In thin sections veins are composed of large single quartz crystals of lens or rhomb-shaped blocks similar to s-porphyroclast systems. Lattice distortion (i.e. undulose extinction, gradual lattice banding and subgrain boundaries) occurs in single crystals as revealed by optical microscopy. Distortion was caused by slip of dislocations preferentially on basal planes. These are also planes along which microcracks developed. Distinct types of microcracks are individualized based on size, orientation and distribution of voids. Microcracrack voids are filled by polycrystalline quartz aggregates. In contrast to single crystals, these aggregates do not have any optical microstructure that might be related to crystal plastic process. Moreover grain size distribution are quite different from those related to dynamic recrystallized aggregates. Despite of that, polycrystalline quartz aggregates have strong [c]-axis preferred orientations. These orientations are similar to those of single crystals close to the microfracture walls. In large spaced voids c-axes orientation of quartz in polycrystalline aggregate have significant misorientation angles with respect to the single crystal [c]-axis orientation, reaching values up to 45° to the foliation plane (XY section of the finite strain). Based on microstructural and textural data we propose a mode for quartz [c]-axis texture development in both single crystals and polycrystalline aggregates that fill microcrack voids.
18
Chen, Yiwei, e Pingchuan Dong. "Modeling of Characteristics of Complex Microstructure and Heterogeneity at the Core Scale". Applied Sciences 14, n. 23 (6 dicembre 2024): 11385. https://doi.org/10.3390/app142311385.
Testo completoAbstract (sommario):
Complex pore structures and strong matrix heterogeneity distinguish carbonate rocks, but there is a lack of comprehensive methods to describe these characteristics. In this study, a integrated approach is proposed to improve the accuracy and adaptability of velocity prediction methods, using a modified squirt flow model based on microcrack structures to characterize complicated pore structures, and a mixed random medium model to represent significant heterogeneity. In addition, the microcrack structure is obtained by inversion, but different from the D-Z method, each group of microcracks corresponds to a different equivalent medium model, so as to improve the accuracy of the inversion results. And the modified squirt flow model takes into account the attenuation caused by local flow between microcracks. The random medium model simulates the inhomogeneous body in the core by adjusting the autocorrelation length a and b, the rounding coefficient n, and the angle θ. A comparative study of the measured data of five limestone and dolomite samples reveals that the P-wave prediction error of the new model is less than 5%, whereas the Biot model is less than 10%, implying that the prediction accuracy of the new model is better.
19
Rusnaldy, Rusnaldy, Pratama Eka Putra Sijabat, Paryanto Paryanto e Toni Prahasto. "Effect of Using Coolant on the Formation of Microcracks, Burr and Delamination in Bone Drilling Process". Journal of Biomedical Science and Bioengineering 1, n. 1 (8 aprile 2021): 17–26. http://dx.doi.org/10.14710/jbiomes.2021.v1i1.17-26.
Testo completoAbstract (sommario):
Direct approach for bone fracture treatment usually involves restoring the fractured parts to their initial position and immobilizing them with plates, screws and wires. This approach needs a bone surgery drilling to produce hole for screw insertion. But this drilling process causes mechanical damages, i.e microcracks, burr formation and delamination, that can reduce the stability of the fixation. One of the ways to minimize it is by using coolant. Moreover, it is noted that bone has anisotropic microstucture. The object of this study is to understand the effect of coolant on mechanical damages that occur in bone drilling and to understand the effect of microstructure difference on microcracks that occur in the drilled walls holes. Adult bovine bones and adult goat bones were used in this study as the specimens to represent differences in cortical bone microstructure. Five consecutive holes from the distal to the proximal in each specimen were generated using manual hand-drill (spindle speed (n) = 1000 rpm; drill bit (d) = 4 mm diameter) with the use of coolant as variation. The drilling holes then stained and observed using a microscope. As the result, it was found that the use of coolant can significantly reduce the drilling temperature. Microcracks, burr formation and delamination were found to be quite large in the drilling holes without coolant. However, there is no microcrack found in the drilling holes with coolant, there is only a small number of burr formation was found. In addition, it was found that the differences in bone microstructure affect the number and length of microcracks that occur in the wall of the hole. It can be concluded from this study that the application of coolant is very effective to reduce the drilling temperature and enhancing the quality of the hole generated by bone drilling and the higher the density of osteon in cortical bone, the easier the microcrack to initiate and propagate.
20
Wang, S. S., E. S. M. Chim e H. Suemasu. "Mechanics of Fatigue Damage and Degradation in Random Short-Fiber Composites, Part I—Damage Evolution and Accumulation". Journal of Applied Mechanics 53, n. 2 (1 giugno 1986): 339–46. http://dx.doi.org/10.1115/1.3171762.
Testo completoAbstract (sommario):
Cyclic fatigue damage in random short-fiber composites is studied experimentally and analytically. In the experimental phase of the study, the fatigue damage is observed to involve various forms of microcracking, originated from microscopic stress concentrators in the highly heterogeneous microstructure. In the analytical portion of the study, a probabilistic treatment of the microcracks is conducted to evaluate the statistical nature of the microscopic fatigue damage. The density and the cumulative distribution of microcrack lengths are found to follow the well known Weibull-form function, and the microcrack orientation density and cumulative distribution have expressions of a fourth-order power form of the cosθ function. Fatigue damage evolution and accumulation in the random short-fiber composite are analyzed in detail through the development of probabilistic microcrack density and distribution functions during the cyclic loading history.
21
Sakaida, Yoshihisa, Hajime Yoshida e Shotaro Mori. "Influences of Crack Face Bridging Stress and Microstructure on Fracture Toughness of Toughened Alumina Ceramics". Key Engineering Materials 462-463 (gennaio 2011): 972–78. http://dx.doi.org/10.4028/www.scientific.net/kem.462-463.972.
Testo completoAbstract (sommario):
Three types of polycrystalline alumina, one pressureless and two hot press sintered Al2O3, were used to examine the effects of the characteristics of microstructure and crack face bridging on fracture toughness. The crack opening displacements and microstructures along the pop-in crack of single edge precracked beam (SEPB) specimens were observed in situ at a constant applied stress intensity factor by scanning electron microscopy (SEM). The bridging stress distribution could be determined from the measured crack opening displacement by three-dimensional finite element analysis, and then the stress intensity factor and stress shielding effect at the crack tip could also be determined. Intergranular microcracks of toughened Al2O3 were deflected by a complicated microstructure, and crack closure due to bridging grains was observed near the crack tip. Bridging stress of Al2O3 was compressive perpendicular to the crack face and was distributed behind the crack tip. The maximum bridging stress of two hot press sintered Al2O3 was about twice as large as that of pressureless sintered Al2O3. The fracture toughness of hot press sintered Al2O3 was, therefore, higher than that of pressureless sintered Al2O3, because the total amount of bridging stress and stress shielding effect increased with increasing magnitude of microcrack deflection and the number of interlocking grains.
22
Nie, Baohua, Shuai Liu, Xianyi Huang, Haiying Qi, Binqing Shi, Zihua Zhao e Dongchu Chen. "Low Cycle Fatigue Crack Damage Behavior of TC21 Titanium Alloy with Basketweave Microstructure". Crystals 12, n. 9 (28 agosto 2022): 1211. http://dx.doi.org/10.3390/cryst12091211.
Testo completoAbstract (sommario):
Low cycle fatigue (LCF) crack initiation, propagation and damage behaviors of TC21 alloy with basketweave microstructure were investigated. The process of LCF damage was observed by a long-focus optical microscopic imaging system, and fatigue crack propagation was analyzed through in-situ SEM fatigue. The results indicated that LCF crack damage displayed different sensitivity to cyclic stress. LCF microcracks initiated from slip bands and propagated through the microcrack coalescences at high stress, while LCF cracks tended to initiate at the αL/β interface and connect with these interface microcracks. Furthermore, the LCF damage model was established on the basis of Lemaitre damage theory. When the maximum stress exceeded yield stress, LCF damage increased sharply and fatigue life decreased significantly, which agreed with experiment data.
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Xing, Wen Jing. "Microstructure of Copper Alloy Effects on Cavitation Damage". Advanced Materials Research 239-242 (maggio 2011): 575–79. http://dx.doi.org/10.4028/www.scientific.net/amr.239-242.575.
Testo completoAbstract (sommario):
The cavitation erosion behavior of ZQAl9-4-4-2 nickel-alum inium bronze in 2.4%NaCl solution was investigated by using a magnetostrictive – induced cavitation facility. The micrographs of damaged surface were observed by scanning electron microscope (SEM) and transmission electron microscopy(TEM). The results showed that the cavitation microcrack in the a phase adjacent to the k phase. They propagated and connected with each other in the a phases, resulted in the removal of a phases and detachment of the kphase from the matrix in the following test period followed. The microcracks tended to propagate parallelly to the eroded surfaces.
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Pan, Suping, Mingzhu Fu, Huiqun Liu, Yuqiang Chen e Danqing Yi. "In Situ Observation of the Tensile Deformation and Fracture Behavior of Ti–5Al–5Mo–5V–1Cr–1Fe Alloy with Different Microstructures". Materials 14, n. 19 (3 ottobre 2021): 5794. http://dx.doi.org/10.3390/ma14195794.
Testo completoAbstract (sommario):
The plastic deformation processes and fracture behavior of a Ti–5Al–5Mo–5V–1Cr–1Fe alloy with bimodal and lamellar microstructures were studied by room-temperature tensile tests with in situ scanning electron microscopy (SEM) observations. The results indicate that a bimodal microstructure has a lower strength but higher ductility than a lamellar microstructure. For the bimodal microstructure, parallel, deep slip bands (SBs) are first noticed in the primary α (αp) phase lying at an angle of about 45° to the direction of the applied tension, while they are first observed in the coarse lath α (αL) phase or its interface at grain boundaries (GBs) for the lamellar microstructure. The β matrix undergoes larger plastic deformation than the αL phase in the bimodal microstructure before fracture. Microcracks are prone to nucleate at the αp/β interface and interconnect, finally causing the fracture of the bimodal microstructure. The plastic deformation is mainly restricted to within the coarse αL phase at GBs, which promotes the formation of microcracks and the intergranular fracture of the lamellar microstructure.
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Tomić, Zoran, Tomislav Jarak, Tomislav Lesičar, Nenad Gubeljak e Zdenko Tonković. "Modelling of Fatigue Microfracture in Porous Sintered Steel Using a Phase-Field Method". Materials 16, n. 11 (3 giugno 2023): 4174. http://dx.doi.org/10.3390/ma16114174.
Testo completoAbstract (sommario):
Porosity in sintered materials negatively affects its fatigue properties. In investigating its influence, the application of numerical simulations reduces experimental testing, but they are computationally very expensive. In this work, the application of a relatively simple numerical phase-field (PF) model for fatigue fracture is proposed for estimation of the fatigue life of sintered steels by analysis of microcrack evolution. A model for brittle fracture and a new cycle skipping algorithm are used to reduce computational costs. A multiphase sintered steel, consisting of bainite and ferrite, is examined. Detailed finite element models of the microstructure are generated from high-resolution metallography images. Microstructural elastic material parameters are obtained using instrumented indentation, while fracture model parameters are estimated from experimental S–N curves. Numerical results obtained for monotonous and fatigue fracture are compared with data from experimental measurements. The proposed methodology is able to capture some important fracture phenomena in the considered material, such as the initiation of the first damage in the microstructure, the forming of larger cracks at the macroscopic level, and the total life in a high cycle fatigue regime. However, due to the adopted simplifications, the model is not suitable for predicting accurate and realistic crack patterns of microcracks.
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Shah, Kausar Sultan, Mohd Hazizan Bin Mohd Hashim, Hafeezur Rehman e Kamar Shah Ariffin. "Effect of Wet-Dry Cycling on the Microstructure of Various Weathering Grade Sandstone". Applied Mechanics and Materials 920 (5 marzo 2024): 183–87. http://dx.doi.org/10.4028/p-103kzt.
Testo completoAbstract (sommario):
This study investigated the effect of rock-water interaction on microstructural properties of various weathering grade sandstone. Sandstone samples were acquired from the Sor-Range coal mine area, Pakistan, and were investigated using a tabletop scanning electron microscope (SEM). The characteristics of microstructures from micrographs were obtained using ImageJ software. According to the findings an increase in wet and dry cycles significantly affects the microstructures (pore spaces and microcracks). The porosity and microcracks density of sandstone increases with the number of wet and dry cycles. Furthermore, the length of microcracks increases as the weathering grade increases. As variation in rock macroscopic mechanical characteristics is directly associated with the deterioration of microstructures. Therefore, analyzing the effect of water-rock interaction in various weathering grades of rock can offer a more accurate reference index for assessing the stability of geotechnical structures.
27
Han, Yongming, Xinyuan Cao, Yonghao Lu e Tetsuo Shoji. "Deformation Properties of Thermally Aged E308L Stainless Steel During Tensile Test with Carbide Effects". Materials 17, n. 24 (12 dicembre 2024): 6070. https://doi.org/10.3390/ma17246070.
Testo completoAbstract (sommario):
Microstructure and deformation properties of both unaged and aged cladding material were studied at 400 °C for 10,000 h. The results indicated that carbide formation occurred in the cladding material, while thermal aging treatment resulted in spinodal decomposition and G-phase formation in the aged ferrite phase. Furthermore, intensive straight slip bands formed in both unaged and aged austenite phases. Continual straight slip bands formed in the unaged ferrite phase, while curvilinear slip bands formed in the aged ferrite phase during the plastic deformation process. Microcracks preferred to nucleate at the points of interaction between phase boundaries and carbides, while the aged ferrite phase experienced lowered microcrack formation along the carbide/ferrite phase boundary. Microcracks propagated along the straight slip bands in the unaged ferrite phases and curvilinear slip bands in the aged ferrite phases.
28
Hu, Liang, Bo Gao, Ning Xu, Yue Sun, Ying Zhang e Pengfei Xing. "Effect of Cerium and Magnesium on Surface Microcracks of Al–20Si Alloys Induced by High-Current Pulsed Electron Beam". Coatings 12, n. 1 (5 gennaio 2022): 61. http://dx.doi.org/10.3390/coatings12010061.
Testo completoAbstract (sommario):
The effect of Ce and Mg on surface microcracks of Al–20Si alloys induced via high-current pulsed electron beam (HCPEB) was studied. Mg was revealed to refine the primary Si phase in the pristine microstructure by forming a Mg2Si phase, leading to the suppression of microcrack propagation within the brittle phase after HCPEB irradiation. The incorporation of Ce into the Al–Si–Mg alloys further refined the primary Si phase and reduced the local stress concentration in the brittle phase induced by HCPEB irradiation. Ultimately, the surface microcracks were observed to be eliminated by the synergistic effects between the two elements. For Al–20Si–5Mg–0.7Ce alloys, Ce demonstrated a homogeneous distribution in the Al matrix on the HCPEB-irradiated alloy surface, while the Mg and Si exhibited a certain degree of aggregation in the Mg2Si phase. Metastable structures were formed on the HCPEB-irradiated alloy surface, including the nano-primary silicon phase, nano-cellular aluminium structure, and nano-Mg2Si phase. Compared with alloy specimens containing Mg, the Al–20Si–5Mg–0.7Ce alloy specimens exhibited an excellent anticorrosion property after HCPEB irradiation mainly due to the combined effects of the grain refinement and microcrack elimination.
29
Kim, Chan-Joong, Ki-Baik Kim, In-Soon Chang, Dong-Yeon Won, Hong-Chul Moon e Dong-Soo Suhr. "The effect of Y2Ba1Cu1O5 addition on microstructure and formation of microcracks in the partially melted Y–Ba–Cu–O oxides". Journal of Materials Research 8, n. 4 (aprile 1993): 699–704. http://dx.doi.org/10.1557/jmr.1993.0699.
Testo completoAbstract (sommario):
In order to investigate microstructural variation by 2-1-1 addition in partially melted Y–Ba–Cu–O, a specimen resulting from 2-1-1 added to 1-2-3 was heat-treated through the peritectic temperature. Microstructure was observed on the directionally solidified region near the interface of the two samples. The 2-1-1 addition results in a homogeneous microstructure where no remnant liquid phase is present. It reduced the 1-2-3 plate thickness, as well as suppressed the formation of microcracks due to the tetragonal-to-orthorhombic phase transition or the thermal contraction during cooling from the peritectic temperature. The formation of microcracks induced by the phase transition seems to be closely related to the process of oxygen diffusion into a sample. We discuss the formation of microcracks in terms of the oxygen diffusion along the plate boundaries and of the thickness of 1-2-3 plates. The decrease in the plate thickness and the fine dispersion of 2-1-1 particles contribute suppression of the formation of microcracks and their propagation.
30
Karthik, Chinnathambi, Joshua Kane, Darryl P. Butt, William E. Windes e Rick Ubic. "Microstructural Characterization of Next Generation Nuclear Graphites". Microscopy and Microanalysis 18, n. 2 (23 gennaio 2012): 272–78. http://dx.doi.org/10.1017/s1431927611012360.
Testo completoAbstract (sommario):
AbstractThis article reports the microstructural characteristics of various petroleum and pitch based nuclear graphites (IG-110, NBG-18, and PCEA) that are of interest to the next generation nuclear plant program. Bright-field transmission electron microscopy imaging was used to identify and understand the different features constituting the microstructure of nuclear graphite such as the filler particles, microcracks, binder phase, rosette-shaped quinoline insoluble (QI) particles, chaotic structures, and turbostratic graphite phase. The dimensions of microcracks were found to vary from a few nanometers to tens of microns. Furthermore, the microcracks were found to be filled with amorphous carbon of unknown origin. The pitch coke based graphite (NBG-18) was found to contain higher concentration of binder phase constituting QI particles as well as chaotic structures. The turbostratic graphite, present in all of the grades, was identified through their elliptical diffraction patterns. The difference in the microstructure has been analyzed in view of their processing conditions.
31
Wang, Zheng, Juanping Xu, Yu Yan e Jinxu Li. "The Influence of Microstructure on the Mechanical Properties and Fracture Behavior of Medium Mn Steels at Different Strain Rates". Materials 12, n. 24 (17 dicembre 2019): 4228. http://dx.doi.org/10.3390/ma12244228.
Testo completoAbstract (sommario):
The primary task of automotive industry materials is to guarantee passengers’ safety during a car crash. To simulate a car crash, the influence of strain rates on mechanical properties and fracture behavior of medium Mn steels with different Si content (0Si without δ-ferrite and 0.6Si with about 20% δ-ferrite) was conducted using the uniaxial tensile test. The results show that ultimate tensile strength is higher, whereas total elongation is lower in 0Si than in 0.6Si. As the strain rate increases, ultimate tensile strength and total elongation decrease in both 0Si and 0.6Si; nonetheless, total elongation of 0.6Si decreases faster. Meanwhile, the area reduction of 0.6Si increases as the strain rate increases. The microcrack′s number on a rolling direction (RD)-transverse direction (TD) surface is considerably increased; nonetheless, the microcrack′s size is restrained in 0.6Si compared with 0Si. Microcracks start at γ(α′)/α-ferrite interfaces in both 0Si and 0.6Si, whereas little nucleation sites have also been found at (γ(α′)+α-ferrite)/δ-ferrite boundaries in 0.6Si. Meanwhile, δ-ferrite reveals a higher capacity for microcrack arrest. As the strain rate decreases, increased lower crack growth results in fine and even dimples on fractographs with abundant second cracks on fractographs; meanwhile, the small microcrack′s number increases, while the large microcrack′s number decreases on an RD-TD surface.
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Tuz, Lechosław, Aneta Ziewiec e Krzysztof Pańcikiewicz. "Influence of the Thermal Cutting Process on Cracking of Pearlitic Steels". Materials 14, n. 5 (8 marzo 2021): 1284. http://dx.doi.org/10.3390/ma14051284.
Testo completoAbstract (sommario):
The paper presents research results of the influence of heat input into high carbon rail steel during cutting processes on microstructure transformation and cracking. The massive block of steel prepared for rail rolling processes was cut and examined by nondestructive magnetic testing and destructive testing by microscopic examination and hardness measurements. The results show unfavorable microstructure changes where pearlite and transformed ledeburite were obtained. The effects of the presence of such microstructures are high hardness near to cutting surfaces (above 800 HV) and microcracks which grow into low hardness block cores during rolling and rail shaping.
33
Gomez, Quriaky, Oana Ciobanu e Ioan R. Ionescu. "Numerical modeling of wave propagation in a cracked solid". Mathematics and Mechanics of Solids 24, n. 9 (23 gennaio 2019): 2895–913. http://dx.doi.org/10.1177/1081286518821407.
Testo completoAbstract (sommario):
In this study, we investigate the dynamics of (damaged) materials with a nonlinear microstructure (microcracks in frictional contact) using a discontinuous Galerkin method. Although the propagation of a plane wave is associated with a complex phenomenon and the stress field loses its homogeneity, the loading pulse has an overall front wave at each moment. Thus, a macroscopic behavior can be extracted and compared with reference solutions based on analytical formulas deduced from the effective (static) elasticity models of a cracked solid. The influences of the mesh and of the microcrack pattern have been tested to choose an optimal numerical setting. We analyzed the sensitivity of the damaged pulse with respect to microcrack density, wavelength, and microcrack orientation. For small values of crack density parameter, the theoretical formula and the computed speed of the damaged pulse are very close, but for larger values there is an important gap between them. For large ratios of wavelength over crack length, the wave speed depends only on the crack density parameter. However, if this ratio is of the order of unity, the wave speed, pulse duration, and pulse amplitude are very sensitive to the wavelength. For more complex phenomena, namely blast propagation in a cracked material, we discuss how the microcrack orientation affects wave propagation and scattering.
34
Xue, Guanming, Yingchao Sun, Ling Xiang, Zhiguo Wang, Suying Hu e Zhiwen Xie. "Effect of Vacuum Annealing on Microstructure and Hot-Salt Corrosion Behavior of CoNiCrAlY/YSZ/LaMgAl11O19 Double-Ceramic Coating". Coatings 11, n. 8 (9 agosto 2021): 951. http://dx.doi.org/10.3390/coatings11080951.
Testo completoAbstract (sommario):
This study investigated the potential effect of vacuum annealing on the microstructure and hot salt corrosion behavior of CoNiCrAlY/YSZ/LaMgAl11O19 (LMA) double-ceramic coatings. A hot-salt corrosion test revealed that sprayed coatings exhibited an unsatisfactory anti-corrosion performance, and the LMA layer underwent severe fracture and corrosion degradation. Vacuum annealing induced a prominent recrystallization of the amorphous phase in LMA layer, triggering severe volume shrinkage and microcrack initiation. The recrystallization and volume shrinkage of the LMA layer were aggravated by an increase in the annealing temperature. The annealed coating with a higher fraction of the LaMA phase showed superior resistance to hot-salt corrosion. However, the salt mixture diffused simultaneously along the microcracks and eventually eroded into the YSZ layer. These results confirmed that vacuum annealing significantly enhanced the hot-salt corrosion resistance of the LMA layer. However, it deteriorated the barrier effect of the salt mixture through microcrack formation.
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Nordmark, Heidi, Alexander G. Ulyashin, John Charles Walmsley e Randi Holmestad. "A Comparative Analysis of Structural Defect Formation in Si+ Implanted and then Plasma Hydrogenated and in H+ Implanted Crystalline Silicon". Solid State Phenomena 131-133 (ottobre 2007): 309–14. http://dx.doi.org/10.4028/www.scientific.net/ssp.131-133.309.
Testo completoAbstract (sommario):
Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to compare hydrogen defects formed in p doped [001] oriented Cz silicon samples which are H+ plasma treated , H+ implanted or Si+ implanted + H+ plasma treated. Samples were studied as processed and after annealing at 250°C, 450°C and 600°C. It is found that 1 hour H+ plasma treatment at 250°C produces a low density of large defects (~100 nm) in prefered {111} plans close to the surface. H+ implantation at a dose of 3x1016 cm-2 produces high density of small (~ 20 nm) mostly {100} platelets that after 1 hour annealing at 450°C result in microcrack formation. Lower H+ implantation doses form very few microcracks at this temperature. Silicon implantation with a dose of 1015 cm2 followed by 1 hour H+ plasma treatment at 250°C and 1 hour annealing at 450°C produces similar microstructure and microcracks as the 3x1016 cm2 H+ implantation dose.
36
Li, Yongqiang, Yaoming Luo, Hangyu Du, Wei Liu, Luping Tang e Feng Xing. "Evolution of Microstructural Characteristics of Carbonated Cement Pastes Subjected to High Temperatures Evaluated by MIP and SEM". Materials 15, n. 17 (1 settembre 2022): 6037. http://dx.doi.org/10.3390/ma15176037.
Testo completoAbstract (sommario):
The microstructural evolutions of both uncarbonated and carbonated cement pastes subjected to various high temperatures (30 °C, 200 °C, 400 °C, 500 °C, 600 °C, 720 °C, and 950 °C) are presented in this study by the means of mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). It was found that the thermal stabilities of uncarbonated cement pastes were significantly changed from 400 to 500 °C due to the decomposition of portlandite at this temperature range. More large pores and microcracks were generated from 600 to 720 °C, with the depolymerization of C-S-H. After carbonation, the microstructures of carbonated cement pastes remained unchanged below 500 °C and started to degrade at 600 °C, due to the decompositions of calcium carbonates and calcium modified silica gel. At 950 °C, both uncarbonated and carbonated cement pastes showed a loosely honeycombed microstructure, composed mainly of β-C2S and lime. It can be concluded that carbonation improves the high-temperature resistance of cement pastes up to 500 °C, but this advantage is lost at temperatures over 600 °C.
37
Zhu, Tian Ping, Zhan W. Chen e Wei Gao. "Effects of Microstructure and Partial Melting on Tensile Properties of AZ91 Magnesium Cast Alloy". Materials Science Forum 546-549 (maggio 2007): 65–68. http://dx.doi.org/10.4028/www.scientific.net/msf.546-549.65.
Testo completoAbstract (sommario):
Mechanical properties of AZ91 cast alloy depend strongly on the morphology (size and distribution) of the second (β-Mg17Al12) phase. It was observed that low ductility of AZ91 alloy was attributed to the brittle nature of the β phase particles at which microcracks initiated. These microcracks then coalesced contributing to the fracture of alloy. Quantitative study on microcracking progress revealed that cast samples with coarse microstructures fractured at low strain due to the non-uniform distribution of bulk blocky β particles at interdendrite region. These fracture surfaces exhibited clear cleavage mode. Fine cast microstructure presented quasicleavage fracture mode with clear dimple and tear ridges. The partial melting (and resolidification) heat treatment improved tensile properties, which was in disagreement with the available data from literature.
38
Kraišnik, Milija, Robert Čep, Karel Kouřil, Sebastian Baloš, Aco Antić e Mladomir Milutinović. "Characterization of Microstructural Damage and Failure Mechanisms in C45E Structural Steel under Compressive Load". Crystals 12, n. 3 (19 marzo 2022): 426. http://dx.doi.org/10.3390/cryst12030426.
Testo completoAbstract (sommario):
In this paper, the microstructural damage evolution of a steel with a ferrite–pearlite microstructure (C45E) was investigated during the process of cold upsetting. The development and the accumulation of microstructural damage were analyzed in different areas of samples that were deformed at different strain levels. The scanning electron microscopy (SEM) results showed that various mechanisms of nucleation of microcavities occurred during the upsetting process. In quantitative terms, microcavities were predominantly generated in pearlite colonies due to the fracture of cementite lamellae. In addition, the mechanism of decohesion had a significant influence on the development of a macroscopic crack, since a high level of microcracks, especially at higher degrees of deformation, was observed at the ferrite/pearlite or ferrite/ferrite interfaces. It was found that the distribution of microcavities along the equatorial plane of the sample was not uniform, as the density of microcavities increased with increasing strain level. The influence of stress state, i.e., stress triaxiality, on the nucleation and distribution of microcracks, was also analyzed.
39
Zhao, Lun, Yunlong Pan, Sen Wang, Liang Zhang e Md Shafiqul Islam. "A Hybrid Crack Detection Approach for Scanning Electron Microscope Image Using Deep Learning Method". Scanning 2021 (9 agosto 2021): 1–13. http://dx.doi.org/10.1155/2021/5558668.
Testo completoAbstract (sommario):
The scanning electron microscope (SEM) is widely used in the analysis and research of materials, including fracture analysis, microstructure morphology, and nanomaterial analysis. With the rapid development of materials science and computer vision technology, the level of detection technology is constantly improving. In this paper, the deep learning method is used to intelligently identify microcracks in the microscopic morphology of SEM image. A deep learning model based on image level is selected to reduce the interference of other complex microscopic topography, and a detection method with dense continuous bounding boxes suitable for SEM images is proposed. The dense and continuous bounding boxes were used to obtain the local features of the cracks and rotating the bounding boxes to reduce the feature differences between the bounding boxes. Finally, the bounding boxes with filled regression were used to highlight the microcrack detection effect. The results show that the detection accuracy of our approach reached 71.12%, and the highest mIOU reached 64.13%. Also, microcracks in different magnifications and in different backgrounds were detected successfully.
40
Fu, Mingzhu, Suping Pan, Huiqun Liu e Yuqiang Chen. "Initial Microstructure Effects on Hot Tensile Deformation and Fracture Mechanisms of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy Using In Situ Observation". Crystals 12, n. 7 (1 luglio 2022): 934. http://dx.doi.org/10.3390/cryst12070934.
Testo completoAbstract (sommario):
The hot tensile deformation and fracture mechanisms of a Ti-5Al-5Mo-5V-1Cr-1Fe alloy with bimodal and lamellar microstructures were investigated by in situ tensile tests under scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that the main slip deformation modes are prismatic slip ({11¯00}<112¯0>) and pyramidal slip ({11¯01}<112¯0>) under tension at 350 °C. In the bimodal microstructure, several parallel slip bands (SBs) first form within the primary α (αP) phase. As the strain increases, the number of SBs in the αP phase increases significantly and multislip systems are activated to help further coordinate the increasing deformation. Consequently, the microcracks nucleate and generally propagate along the SBs in the αP phase. The direction of propagation of the cracks deflects significantly when it crosses the αP/β interface, resulting in a tortuous crack path. In the lamellar microstructure, many dislocations pile up at the coarse-lath α (αL) phase near the grain boundaries (GBs) due to the strong fencing effect thereof. As a result, SBs develop first; then, microcracks nucleate at the αL phase boundary. During propagation, the cracks tend to propagate along the GB and thus lead to the intergranular fracture of the lamellar microstructure.
41
Lv, Haiyang, Rongfeng Zhou, Lu Li, Haitao Ni, Jiang Zhu e Tong Feng. "Effect of Electric Current Pulse on Microstructure and Corrosion Resistance of Hypereutectic High Chromium Cast Iron". Materials 11, n. 11 (8 novembre 2018): 2220. http://dx.doi.org/10.3390/ma11112220.
Testo completoAbstract (sommario):
The effect of electric current pulse on the microstructure and corrosion resistance of hypereutectic high chromium cast iron was explored. The morphology of carbides in solidification microstructure was observed by an optical microscope and a scanning electron microscope and the composition was determined by an electron probe micro-analyzer. The microhardness of primary carbides and corrosion resistance of samples were also compared. Under the active of electric current pulse, the microstructure of hypereutectic high chromium cast iron was homogenized and its performance improved accordingly. On treatment by electric current, the morphology of primary carbides changed from thick long rods to hexagonal blocks or granular structures. The interlayer spacing of eutectic carbide decreased from ~26.3 μm to ~17.8 μm. Size statistics showed that the average diameter of primary carbide decreased from ~220 μm to ~60 μm. As a result, microhardness increased from 1412 HV to 1511 HV. No obvious microcrack propagation was found at the microindentation sites. The average length of microcracks decreased from ~20.7 μm to ~5.7 μm. Furthermore, corrosion resistance was remarkably enhanced. The average corrosion rate decreased from 2.65 mg/cm2·h to 1.74 mg/cm2·h after pulse current treatment.
42
Dönges, Benjamin, Claus Peter Fritzen e Hans Jürgen Christ. "Experimental Investigation and Simulation of the Fatigue Mechanisms of a Duplex Stainless Steel under HCF and VHCF Loading Conditions". Key Engineering Materials 664 (settembre 2015): 267–74. http://dx.doi.org/10.4028/www.scientific.net/kem.664.267.
Testo completoAbstract (sommario):
High frequency push-pull fatigue experiments on the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318LN) revealed that crack nucleation and crack propagation through the first grain determine significantly the lifetime of the material. Only in very few cases it was observed that fatigue samples which endured one billion load cycles without failure (run-out samples) contain microcracks which reached or overcame the first microstructural barrier (phase or grain boundary). This leads to the conclusion that in most cases the highest macroscopic stress or strain amplitude which does not lead to fatigue crack propagation through the entire first grain can be considered as the fatigue limit of the material. The present study documents that the experimentally identified fatigue mechanisms can be represented in mesoscopic finite element simulations by taking into account the effects of anisotropic elasticity, crystal plasticity, macro and micro residual stresses, plastic strain concentration in form of slip bands, crack nucleation and short crack propagation through the first grain. The current investigation shows that such simulations enable the determination of the fatigue limit of both real and synthetic microstructures. By means of real microstructures, containing slip traces and microcracks, the calculations can be verified and the required microstructural parameters can be determined.
43
Pikos, Izabela, Tomasz Rzychoń e Andrzej Kiełbus. "Microstructural Phenomenon Occurring in Elektron 21 Magnesium Alloy During Creep". Materials Science Forum 782 (aprile 2014): 339–43. http://dx.doi.org/10.4028/www.scientific.net/msf.782.339.
Testo completoAbstract (sommario):
The creep resistance of Elektron 21 magnesium alloy containing Zn, Nd, Gd and Zr has been investigated. Test has been conducted at 200°C, 225°C and 250°C with constant load amounts to 90, 120 and 150 MPa up to 100 hours. Some specimens cracked during the test. Metallographic and fractographic research has been performed in order to identify the microstructural changes occurring during the creep resistance test. Microstructure has been observed with light microscopy and scanning electron microscopy. Chemical composition of microstructural components has been investigated with energy dispersion spectroscopy. Research revealed presence of voids, microcracks and inclusions which can significantly influence creep resistance of material.
44
Yang, Hucheng, Shengrui Su, Peng Li e Jianxun Chen. "Investigation of the Microstructure Characteristics and Deformation Mechanisms of the Carbonaceous Slate under Hydromechanical Coupling". Geofluids 2023 (21 febbraio 2023): 1–16. http://dx.doi.org/10.1155/2023/5490136.
Testo completoAbstract (sommario):
Carbonaceous slate inevitably possesses microscopic pores, microcracks, cleavages, and bedding planes in complex geoenvironment during the diagenesis processes. The microstructure of carbonaceous slate changes apparently under the effects of underground water infiltration, tectonic stress, and engineering disturbance, which induces the large deformation of rock mass and influences the stability of geotechnical engineering projects. To investigate the microstructure characteristics and reveal deformation mechanisms of carbonaceous slate under the influence of water pressure and stress, the variations of the pore size distribution(PSD), connectivity of pores, and porosity of samples during water injection and triaxial compression were studied using multiple methods. The results indicated that voids include plate-like micropores and microcracks, which are discontinuous without external stress. The micropores with a size of less than 1 μm dominate in number. The flaky particles were extruded and bent at a confining pressure, which caused the intermediate pores to form and altered the PSD of the samples. Hence, the connectivity dramatically improved and resulted in increased permeability. Water with dissolved clay minerals and small particles could move between micropores and microcracks during the water injection process. The elastic deformation of the particles recovered, and the intermediate pores disappeared when the imbalanced forces on two sides of the particles were narrowed. Pore water pressure affected the effective stress state and decreased the cohesion and stiffness of the rock. In the lower stress state, the porosity had a certain range of decrease (about 0.2%) mainly due to micropore compression, while the microcrack sprouted and expanded with the increase of compressive stress, resulting in the extension of porosity. The interaction of the stress and water seepage on the slate reduced the rock strength and favored the deformation, leading to a large macrodeformation of the soft rock in the long run.
45
van der Burg, M. W. D., e E. van der Giessen. "Simulation of Microcrack Propagation in Creeping Polycrystals Due to Diffusive Grain Boundary Cavitation". Applied Mechanics Reviews 47, n. 1S (1 gennaio 1994): S122—S131. http://dx.doi.org/10.1115/1.3122807.
Testo completoAbstract (sommario):
Creep rupture in random polycrystalline aggregates is investigated numerically in terms of multi-grain cell studies using a Delaunay network modelling technique. The model involves a representation of the crystalline aggregate by means of special purpose elements attributed to each grain facet. These Delaunay elements account for elastic and creep deformations of the grains, free grain boundary sliding, as well as for the nucleation and diffusive growth of grain boundary cavities until coalescence leads to a facet microcrack. Damage accumulation is simulated numerically, until an excessive number of microcracks cause des-integration of the polycrystal. Primary attention is on the influence of randomness in the microstructure on creep rupture, either in terms of random variations of the size and shape of hexagonal grains, or in terms of random variations in the nucleation properties of grain boundaries. It is found that randomness always tends to decrease the life time. In particular, it is found that the life time depends sensitively on random variations of the geometry of the microstructure.
46
Wang, Jiang, Zhen Wang, Qingxuan Sui, Shurong Xu, Quan Yuan, Dong Zhang e Jun Liu. "A Comparison of the Microstructure, Mechanical Properties, and Corrosion Resistance of the K213 Superalloy after Conventional Casting and Selective Laser Melting". Materials 16, n. 4 (4 febbraio 2023): 1331. http://dx.doi.org/10.3390/ma16041331.
Testo completoAbstract (sommario):
K213 superalloy was fabricated by conventional casting and selective laser melting (SLM). The microstructures of the two samples were examined, and the mechanical properties and corrosion resistance of these two kinds of K213 alloy were comparatively studied. The results show that segregation of Ti occurs at the grain boundaries of the as-cast alloy, resulting in the formation of MC carbide. Many microcracks were formed in the SLM sample. Premature fracture of the as-cast alloy is caused by the precipitation of the harmful phase (Ti, Mo, Nb)C (MC). The MC carbides and microcracks in the as-cast and SLM alloys, respectively, induce tensile fracture. In comparison, the strength of the SLM sample is greater, while the elongation of the as-cast sample is greater. The oxidation resistance of the SLM sample is better at a high temperature of 800 °C. This is due to the relatively uniform composition and microstructure of the SLM alloy. However, the corrosion rate of the SLM alloy is accelerated during the electrochemical immersion corrosion process due to the existence of microcracks.
47
Stone, B. M., I. J. Jordaan, J. Xiao e S. J. Jones. "Experiments on the damage process in ice under compressive states of stress". Journal of Glaciology 43, n. 143 (1997): 11–25. http://dx.doi.org/10.3189/s002214300000277x.
Testo completoAbstract (sommario):
AbstractDuring ice-structure interaction, ice will fail in a brittle manner dominated by two processes. The first corresponds to the formation of macrocracks and the consequent spalling-off of large ice pieces. The second includes an intense shear-damage process in zones, termed critical zones, where high pressures are transmitted to the structure. The shear-damage process results in microstructural changes including microcrack formation and recrystallization. A range of tests on laboratory-prepared granular ice have been conducted to determine the fundamental behaviour of ice under various stress states and stress history, particularly as it relates to changes in microstructure. The test series was designed to study three aspects: the intrinsic creep properties of intact, undamaged ice; the enhancement of creep and changes in microstructure due to damage; and the effects of different stress paths. Tests on intact ice with triaxial confining pressures and low deviatoric stresses, aimed at defining the intrinsic creep response in the absence of microcracking, showed that an accelerated creep rate occurred at relatively low deviatoric stresses. Hence, a minimum Creep rate occurred under these conditions. Recrystallization to a smaller grain-size and void formation were observed. Ice damaged uniaxially and triaxially prior to testing showed enhancement of creep under both uniaxial and triaxial loading conditions Creep rates in triaxially damaged ice were found to be non-linear with high deviatoric stresses, corresponding to a power-law dependence of creep rate. Uniaxially damaged specimens contained microcracks parallel to the stressed direction which tended to close under triaxial confinement. Damage under triaxial conditions at low confining pressures produced small recrystallized grains near zones of microcracking. At high confining pressures, a fine-grained recrystallized structure with no apparent cracking was observed uniformly across the specimen. The recrystallization process contributes significantly to the enhanced creep rates found in damaged specimens.
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Stone, B. M., I. J. Jordaan, J. Xiao e S. J. Jones. "Experiments on the damage process in ice under compressive states of stress". Journal of Glaciology 43, n. 143 (1997): 11–25. http://dx.doi.org/10.1017/s002214300000277x.
Testo completoAbstract (sommario):
AbstractDuring ice-structure interaction, ice will fail in a brittle manner dominated by two processes. The first corresponds to the formation of macrocracks and the consequent spalling-off of large ice pieces. The second includes an intense shear-damage process in zones, termed critical zones, where high pressures are transmitted to the structure. The shear-damage process results in microstructural changes including microcrack formation and recrystallization. A range of tests on laboratory-prepared granular ice have been conducted to determine the fundamental behaviour of ice under various stress states and stress history, particularly as it relates to changes in microstructure. The test series was designed to study three aspects: the intrinsic creep properties of intact, undamaged ice; the enhancement of creep and changes in microstructure due to damage; and the effects of different stress paths. Tests on intact ice with triaxial confining pressures and low deviatoric stresses, aimed at defining the intrinsic creep response in the absence of microcracking, showed that an accelerated creep rate occurred at relatively low deviatoric stresses. Hence, a minimum Creep rate occurred under these conditions. Recrystallization to a smaller grain-size and void formation were observed. Ice damaged uniaxially and triaxially prior to testing showed enhancement of creep under both uniaxial and triaxial loading conditions Creep rates in triaxially damaged ice were found to be non-linear with high deviatoric stresses, corresponding to a power-law dependence of creep rate. Uniaxially damaged specimens contained microcracks parallel to the stressed direction which tended to close under triaxial confinement. Damage under triaxial conditions at low confining pressures produced small recrystallized grains near zones of microcracking. At high confining pressures, a fine-grained recrystallized structure with no apparent cracking was observed uniformly across the specimen. The recrystallization process contributes significantly to the enhanced creep rates found in damaged specimens.
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Huang, Xin, Aijuan Li, Zhen Huang, Yi Sun, Yumin Song e Ning Xu. "Research on Forward Problem of Rail Detection Based on Magnetoacoustic Coupling". Sensors 22, n. 15 (25 luglio 2022): 5539. http://dx.doi.org/10.3390/s22155539.
Testo completoAbstract (sommario):
According to the characteristics of rail defects, a rail microcrack detection method based on magnetoacoustic coupling effect is proposed in this paper. Firstly, the basic principle of a rail microcrack detection method based on magnetoacoustic coupling effect is described, and then the model is analyzed theoretically. Through simulation calculation, the current density distribution and Lorentz force distribution generated by electromagnetic excitation, the motion characteristics of particles under Lorentz force and the sound field distribution characteristics of magnetoacoustic signals generated by Lorentz force are obtained. Finally, an experimental platform was set up and the steel ring model was preliminarily tested. The magnetic and acoustic signals of the two steel ring boundaries excited by an electromagnetic field were collected. These signals correspond to the position distribution of the steel ring. The state change of rail microstructure will cause a change in the conductivity characteristics of rail materials, and will affect the characteristics and distribution of sound pressure in the detection. Therefore, the detection method based on the magnetoacoustic coupling effect can detect the surface microcracks of high-speed rail. This method has great feasibility and development potential in the field of rail flaw detection.
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Dong, Yijia, Chao Su, Pizhong Qiao e LZ Sun. "Microstructural damage evolution and its effect on fracture behavior of concrete subjected to freeze-thaw cycles". International Journal of Damage Mechanics 27, n. 8 (12 luglio 2018): 1272–88. http://dx.doi.org/10.1177/1056789518787025.
Testo completoAbstract (sommario):
Concrete structures in cold regions are exposed to cyclic freezing and thawing environment, leading to degraded mechanical and fracture properties of concrete due to microstructural damage. While the X-ray micro-/nano-computed tomography technology has been implemented to directly observe concrete microstructure and characterize local damage in recent years, the freeze-thawed damage evolution processes and its effect on overall mechanical performance are not well understood. In this paper, the X-ray nano-computed tomography technology and micro-scale cohesive zone model are combined to quantitatively investigate microstructural damage evolution and its effect on fracture behavior of freeze-thawed concrete samples in three-point bending tests. A two-level micro-to-macro scale finite element model is developed based on computed tomography microstructural images with microcracks due to freeze-thaw cycles. The macroscopic load–deflection curves and fracture energies are simulated and compared favorably with experimental results. Simulation results demonstrate that microcracks caused by freeze-thaw actions are the primary reason for degradation of concrete mechanical properties. Fracture behaviors of frost-damaged concrete with different mortar and interfacial transition zone strength and fracture constants are also simulated and discussed. The combined X-ray nano-computed tomography technology and cohesive zone model proposed is effective in characterizing fracture behavior of concrete and capturing freeze-thaw cycle-induced microstructural damage evolution and its effect on fracture process of concrete.