Numerical modelling of the quasi-brittle behavior of materials by considering microcracks effect (original) (raw)
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Dynamic damage and fracture processes in brittle materials
Dynamic Behavior of Materials, 2024
Brittle materials such as, rocks, concrete and high-performance concrete, glass, ceramics, ice..., are materials abundantly present in our everyday life and materials involved in many industrial applications or in various protective solutions. Achieving a good understanding of their strain-rate sensitivity in relation with their microstructure remains a major issue. In addition, the development of micromechanics-based models is of major importance in view of improving the predictive capabilities of analytical and numerical models and in order to better explain the role of material parameters involved in the strain-rate and pressure sensitivity of brittle materials. In the present chapter the damage and fracture processes involved in various brittle materials are discussed. Several experimental techniques offering the capability to characterize and analyse these damage modes are presented. Next, some micromechanics-based models attending to describe these mechanisms are detailed. Experimental and modelling approaches allow highlighting the main microstructural parameters driving the behaviour of brittle materials at high loading rates.
Quantitative characterization of microcracking in brittle materials by finite element modeling
Engineering Fracture Mechanics, 1995
A numerical procedure is described for directly simulating the generation of a microcrack process zone around a crack during loading. The interactions between microcrack-microcrack and microcrack-main crack were realized and certain effects of interaction predicted by the theoretical model are corroborated by the numerical analyses. However, as our model takes several interactions, e.g. interactions among microcracks, into account, some essential features of microcracking revealed by the simulation are physically different from the prime hypothesis of the continuum model and provide some insights into the toughening mechanisms of microcracking.
Computational materials …, 2011
Multiphase steels have become a favoured material for car bodies due to their high strength and good formability. Concerning the modelling of mechanical properties and failure behaviour of multiphase steels, representative volume elements (RVE) have been proved to be an applicable approach for describing heterogeneous microstructures. However, many multiphase steels exhibit inhomogeneous microstructures which result from segregation processes during continuous casting. These segregations lead to a formation of martensite bands in the microstructure causing undesirable inhomogeneities of material properties. The aim of this work is to develop an FE evaluation procedure for predicting a microcrack formation provoked by banded martensitic structures. A micromechanism based damage curve was applied as a failure criterion for the softer ferritic matrix in the microstructure in order to simulate the propagation of cracks resulting from the failure of martensitic bands. The parameters of the damage curve were determined by in situ miniature bending tests and tensile tests with notched samples. The presented approach provides the basis for an assessment criterion of the component safety risk of multiphase steels with inhomogeneous microstructures.
An approach for fracture of quasi-brittle materials
Engineering Fracture Mechanics, 1990
An R-curve approach for fracture of quasi-brittle materials is proposed in this paper. An R-curve is defined as an envelope of fracture energy release rate of specimens with different sizes but the same initial notch length. By assuming that an effective traction-free critical crack is the function of an initial crack length contained in a material, an expression of the R-curve with two parameters can be derived by solving a differential equation. The parameters of the R-curve can be uniquely determined according to Kfc and CTOD, for positive geometry specimens, and according to K$ and dK,/da = 0 for negative geometry specimens. Load-CMOD and loaddisplacement response can be predicted based on the proposed R-curve by requiring that the crack driving force and crack growth resistance are equal at every equilibrium crack length. The predicted curves show a good agreement with the wide range of experimental results. Pre-critical stable crack pro~gation of q~si-bottle materials can be well described by the present approach.
International Journal of Fracture, 2003
A computational approach to the optimization of service properties of two-phase materials (in this case, fracture resistance of tool steels) by varying their microstructure is developed. The main points of the optimization of steels are as follows: (1) numerical simulation of crack initiation and growth in real microstructures of materials with the use of the multiphase finite elements (MPFE) and the element elimination technique (EET), (2) simulation of crack growth in idealized quasi-real microstructures (net-like, band-like and random distributions of the primary carbides in the steels) and (3) the comparison of fracture resistances of different microstructures and (4) the development of recommendations to the improvement of the fracture toughness of steels. The fracture toughness and the fractal dimension of a fracture surface are determined numerically for each microstructure. It is shown that the fracture resistance of the steels with finer microstructures is sufficiently higher than that for coarse microstructures. Three main mechanisms of increasing fracture toughness of steels by varying the carbide distribution are identified: crack deflection by carbide layers perpendicular to the initial crack direction, crack growth along the network of carbides and crack branching caused by damage initiation at random sites.
Engineering Fracture Mechanics, 2006
The integrity of PWR pressure vessels is assured by keeping the crack tip stress intensity factor below the toughness of the material under monotonic isothermal loading. To study the effects of sudden cooling associated with a thermal gradient, a specially modified compact specimen has been developed. This has been used to carry out tests in the transition zone with different loading-temperature sequences liable to call the conventional criteria into question. The test is described in detail in Part I of this article [Chapuliot S, et al. Thermomechanical analysis of thermal shock fracture in the brittle/ductile transition zone. Part I: Description of the tests. Engng Fract Mech, 72, 2005, 661-73].
Proposal of new damage model for thermal shock based on dynamic fracture on the brittle materials
Journal of Non-Crystalline Solids, 2005
In the current paper, we propose that the mechanical resistance of the brittle materials under dynamic loading can be used to predict the thermal shock damage resistance via a new proposed damage criterion. The increase of the mechanical resistance based on high loading rate has been investigated by experimental methods for ÔLead Crystal GlassÕ including the static and dynamic loadings by universal ÔINSTRONÕ and Ôsplit Hopkinson pressure bars (SHPB)Õ. The relationship between the measured static and dynamic strengths of materials has been compared and developed via Tuler and ButcherÕs dynamic criterion. ÔModified Brazilian disk (MBD)Õ specimens are used to obtain indirect tensile stress concentration at notch tip. The fracture process under various loading rates has been observed by optical and scanning electronic microscope (SEM) to study the fracture phenomenon according to the high loading rate effect for Lead Crystal Glass. The dynamic stress due to high loading rates can be considered as the thermal stress during thermal shock phenomenon with high heat convection coefficient values which leads to high gradient temperature, low duration time and intrinsic damage feature like cracks and flakes for concerned material. The transient thermal stress analysis has been carried out by numerical method for MBD specimen subjected to rapid cooling condition. The numerical results and experimental dynamic resistance of selected material are utilized to develop a new semi-empirical damage criterion based on the continuum damage mechanics (CDM) including dynamic material resistance, critical thermal shock time which corresponds to maximum transient thermo-mechanical stress and heat convection coefficient evolution according to the thermal shock severity.
Brittle-ductile transition and scatter in fracture toughness of ferritic steels
2005
Here we analyze a dislocation simulation model for the brittle-to-ductile transition of Ferritic Steels, a typical multiphase material. The crack tip plastic zones are simulated as arrays of discrete dislocations emitted from crack-tip sources and equilibrated against the friction stress due to lattice and obstacles. The crack-tip gets blunted and the emitted dislocation arrays modify the elastic field of the crack. The combined stress field of the crack and the emitted dislocations describe an elasto-plastic crack field. The simulated crack system involves microcracks embedded in the plastic zone of a macrocrack. The inherent scatter in fracture toughness measurements are studied by using a size distribution for microcracks, distributed on the plane of the macrocrack. The scatter in fracture toughness measurements is found to be an effect of the size distribution of microcracks rather than their spatial distribution on the matrix ahead of the crack plane.
Inorganic Materials, 2018
It has been shown by means of EBSD techique that fracture of ferritic steel in ductile-brittle transition temperature region, along with the formation of previously discribed cleavage microcracks, results in the formation of ductile microcracks. It has also been shown that microstructure of plastic zones under brittle and ductile fracture components produced by the main crack propagation differ significantly. Better developed plastic zone under ductile fracture component protects steel from overstress. The plastic zone under brittle fracture surface, apparently, has a reduced local plasticity. Consequently, the cleavage microcracks formation precedes the fracture process. During the main crack formation such microcracks occur in steel microvolumes located both in front of its tip and in adjacent to its edges microvolumes. Further propagation of the main crack is realized in steel which already contains scattered cavities and reduces to ductile fracture of the connections between them.
A heuristic approach to microcracking and fracture for ceramics with statistical consideration
Theoretical and Applied Fracture Mechanics, 2000
Microcracking damage and toughening are examined for ceramics. These eects have been found to depend on the material microstructure and macrocrack growth. Isotropic damage, attributed to random distribution of microcrack location, length and orientation can be associated with a disordered microstructure and a non-uniform residual stress ®eld. When the applied stress is the main cause of cracking, the microcrack distribution is no longer random such as a system of quasi-parallel cracks. To highlight the eect of crack interaction, discrete models are advanced where damage is simulated by a distribution of microcracks. The dilute concentration assumption is invoked to simplify the analysis.