Influence of Microstructure on the Fracture Induced by a Hard Cutting Indenter (original) (raw)
Related papers
Interaction of two hard indenters in the cutting process of quasi-brittle materials
2002
In this paper, an attempt is made to find some general relations for the microcutting process in brittle or quasi-brittle materials, under different hypotheses of microscopic failure behaviour. Fracture patterns in homogeneous brittle solids are obtained by the Finite Element Method in the framework of Linear Elastic Fracture Mechanics (LEFM). On the other hand, microstructural heterogeneities are taken into account by Lattice Model simulations. When two indenters are acting in parallel, their mutual distance plays an important role. If the indenters are very close, they behave like a unique large indenter, whereas if the distance is relatively large, their mechanical interaction vanishes. In addition, when the distance is approximately three or four times their radius, the mechanism of chipping (with formation of secondary chip between the two parallel grooves) can take place, improving the ratio of removed volume to spent energy and then the demolition ability of the two indenters.
Numerical analysis of the cutting interaction between indenters acting on disordered materials
International Journal of Fracture, 2005
In this paper, an attempt is made to find some general relations for the microcutting process in brittle or quasi-brittle materials, under different hypotheses of microscopic failure behavior. Fracture beneath the indenters and sudden chip formation are the main dissipation mechanisms taken into consideration. Fracture patterns in more homogeneous brittle solids are obtained by the Finite Element Method in the framework of Linear Elastic Fracture Mechanics (LEFM). On the other hand, the quasi-brittle response due to microstructural heterogeneities is taken into account by Lattice Model simulations. The analysis is not limited to the more common study of a single indenter. When two indenters are acting in parallel, their mutual distance plays an important role. If the indenters are very close, they behave like a unique larger indenter, whereas if the distance is relatively large, their mechanical interaction vanishes. In addition, when the distance is approximately three to four times their dimension, the mechanism of chipping (with formation of secondary chip between the two parallel grooves) can take place, improving the ratio of removed volume to spent energy and then the demolition ability of the two indenters. Some comparisons are proposed between the presented approach and more sophisticated and computationally demanding models from the literature, as well as with experimental data. The analysis should provide useful hints for the optimal design of super-abrasive tools.
Numerical analysis of indentation fracture in quasi-brittle materials
Engineering Fracture Mechanics, 2004
The process of indentation of brittle and quasi-brittle materials has been investigated both from the experimental and the theoretical point of view. As far as we know, only a few studies have tried to explain the mechanics of cutting due to an indenter which penetrates inside the material. In this paper, an attempt is made to find some general relations for the cutting process in brittle and quasi-brittle materials, under different hypotheses for the microscopic failure behaviour. Fracture patterns in homogeneous brittle solids are obtained by the finite element method in the framework of linear elastic fracture mechanics. Microstructural heterogeneities are taken into account by the lattice model simulation. Although the reality is often much more complex than the theoretical models applied, the study provides interesting indications for improving performance of cutting tools.
Numerical modelling of intergranular fracture in polycrystalline materials and grain size effects
In this paper, the phenomenon of intergranular fracture in polycrystalline materials is investigated using a nonlinear fracture mechanics approach. The nonlocal cohesive zone model (CZM) for finite thickness interfaces recently proposed by the present authors is used to describe the phenomenon of grain boundary separation. From the modelling point of view, considering the dependency of the grain boundary thickness on the grain size observed in polycrystals, a distribution of interface thicknesses is obtained. Since the shape and the parameters of the nonlocal CZM depend on the interface thickness, a distribution of interface fracture energies is obtained as a consequence of the randomness of the material microstructure. Using these data, fracture mechanics simulations are performed and the homogenized stress-strain curves of 2D representative volume elements (RVEs) are computed. Failure is the result of a diffuse microcrack pattern leading to a main macroscopic crack after coalescence, in good agreement with the experimental observation. Finally, testing microstructures characterized by different average grain sizes, the computed peak stresses are found to be dependent on the grain size, in agreement with the trend expected according to the Hall-Petch law. SOMMARIO. In questo articolo, il fenomeno della frattura intergranulare nei material policristallini è studiato mediante un approccio di meccanica della frattura non lineare. Il modello non locale di frattura coesiva per interfacce con spessore finito recentemente proposto dai presenti autori è impiegato per descrivere il fenomeno di separazione ai bordi di grano. Da un punto di vista modellistico, considerando la dipendenza dello spessore dei bordi di grano dalla dimensione del grano stesso, si è ottenuta una distribuzione delle proprietà meccaniche delle interfacce. Essendo la forma ed i parametri del modello non locale della frattura coesiva dipendenti dallo spessore dell'interfaccia, si ottiene una distribuzione di energie di frattura come conseguenza della variabilità statistica della microstruttura del materiale. Usando tali dati si conducono simulazioni di meccanica della frattura su elementi di volumi rappresentativi (RVE) in 2D e si determinano le rispettive curve di tensione-deformazione. La frattura è il risultato di un insieme di microfessure diffuse che danno luogo alla propagazione di una fessura macroscopica principale, in ottimo accordo con quanto osservato sperimentalmente. Infine, testando microstrutture dotate di diversi diametri medi dei grani, si osserva come le tensioni di picco siano dipendenti dal diametro del grano, secondo un trend in accordo con la legge di Hall e Petch.
A fracture mechanics model to study indentation cutting
Fatigue & Fracture of Engineering Materials & Structures
Many cutting processes, such as chopping, slicing, and carving, consist in 2 different stages: an initial stage of indentation, in which the cutting tool is pushed into the material under the action of an external force, and a second stage, where the target material undergoes a progressive separation. This second stage is characterised by the formation of a fracture surface followed by the cut propagation due to the increasing external force, until eventually a steady state might occur. The purpose of this paper is to analyse the cutting process by means of some concepts of fracture mechanics and discuss the occurrence of the steady state. A simple model is used to obtain an analytic expression of the stress intensity factor at the tip of the cut and investigate the evolution of the fracture process. It is found that the cut propagation depends on the wedge sharpness. The analytic results are compared with finite element analyses, where the effect of tip blunting due to plasticity is taken into account. The influence of the cutting tool geometry is also discussed.
Tectonophysics, 2001
An account on the role of higher order strain gradients in the mechanical behavior of elastic-perfectly brittle materials, such as rocks, is given that is based on a special grade-2 elasticity theory with surface energy as this was originated by Casal and Mindlin and further elaborated by the authors. The fundamental idea behind the theory is that the effect of the granular and polycrystalline nature of geomaterials (i.e. their microstructural features) on their macroscopic response may be modeled through the concept of volume cohesion forces, as well as surface forces rather than through intractable statistical mechanics concepts of the Boltzmann type. It is shown that the important phenomena of the localization of deformation in macroscopically homogeneous rocks under uniform tractions and of dependence of rock behavior on the specimen's dimensions, commonly known as size or scale effect, can be interpreted by using this ‘non-local’, higher order theory. These effects are demonstrated for the cases of the unidirectional tension test, and for the small circular hole under uniform internal pressure commonly known as the inflation test. The latter configuration can be taken as a first order approximation of the indentation test that is frequently used for the laboratory or in situ characterization of geomaterials. In addition, it is shown that the solution of the three basic crack deformation modes leads to cusping of the crack tips that is caused by the action of ‘cohesive’ double forces behind and very close to the tips, that tend to bring the two opposite crack lips in close contact, and further, it is demonstrated that the fracture toughness depends on the size of the crack, and thus it is not a fundamental property of the material. This latter outcome agrees with experimental results which indicate that materials with smaller cracks are more resistant to fracture than those with larger cracks.
Can J Earth Sci, 1987
Conventional Vickers microhardness testing can be used to determine mode I fracture toughness, KIc, as well as hardness, H. The crack length versus indenter load formulation previously used in geology indicates that KIc for quartz increases with decreasing load. This analysis is unreliable because it neglects residual stresses, which depend on the modulus to hardness ratio (EIH). A recent analysis incorporating this effect gives results compatible with published bend-test determinations. Earlier results systematically under-or overestimate KIc depending on whether (EIH)1/2 is greater or less than 4.15. An extensive SEM study to determine three-dimensional indent-fracture morphology reveals a region of high backscattered electron contrast immediately adjacent to the indenter contact. TEM analysis shows this is an intensely deformed region with very high fracture density and evidence of partial melting. A submicrometre-scale blocky structure, perhaps resulting from cataclasis, occurs between fractures. The analysis shows that "plastic" deformation associated with indentation in qua* is controlled largely by fracturing, although the absence of voids suggests some crystal-plastic accommodation process may also operate. La mkthode conventionnelle de microduretk de Vickers peut servir i determiner non seulement la duretC (H) mais aussi le tCnacitC de fissuration mode-I (K,,). La formation de la longueur des fissures versus l'empreinte-charge appliquCe utilisCe auparavant en gCologie kvkle que KIc pour le quartz augmente avec la diminution de la charge. Cette analyse n'est pas fiable, car elle ne tient pas compte des contraintes ksiduelles, lesquelles dkpendent du rapport module d'klasticitk i duretC (EIH). Une Ctude kcente incluant cet effet foumit des ksulats qui sont en accord avec les mesures reportks pour des essais de flexion. Les ksultats obtenus antkrieurement sous-ou surestimaient syst6matiquement KIc selon que la valeur de (EIH)"2 etait sup6rieure ou infkrieure i 4,15. Une Ctude dCtaillCe au MEB de la morphologie tridimensionnelle empreinte-fissure kvele la pksence d'une kgion d'tlectrons ktrodiffusks fortement contrastante localiske immkdiatement au voisinage du contact du marqueur. L'analyse au MET kvkle qu'il s'agit d'une kgion intenskment dkformke, caractCrisCe par une forte densit6 de fissures, et qui pksente des indices d'une fusion partielle. Une structure polydrique i 1'Cchelle subrnicroscopique, probablement produite par cataclase, apparait entre les fissures. L'analyse montre que la dCformation "plastique" associ6e B l'empreinte dans le quartz est contdlte principalement par la fissuration malgk que l'absence de vides suggkre l'intemention d'un processus d'adaptation de type cristallin-plastique ma1 dCtermin6.
Multi-scale approach of the instrumented indentation technique on the fracture toughness estimation
2015
Instrumented Indentation Technique (IIT) is widely used to determine the mechanical properties of materials. The elastic modulus is usually determined by applying the methodology proposed by Oliver and Pharr [1] who supposed that its value is independent of the indentation depth. However, some authors [2, 3] have observed a decrease of the elastic modulus when the indenter displacement increases which allowed them to introduce a continuous damage theory used afterwards to estimate the fracture toughness of ductile materials. The assumption made by the authors is that a damage in the region very close to the bottom of the indent results in the formation of microvoids which leads to the variation of the elastic modulus as a function of the indenter displacement. Starting from this observation, Lee et al. [2] proposed an energy model based on the Griffith’s theory and the continuous damage mechanics (CDM) which states that the elastic modulus variation is related to the fraction void v...