Damage evolution in coated cemented carbides under compressive contact loads

This work deals principally with two important issues and their interrelation: the evolution of damage in coated cemented carbide tools used for cold forming, and the assessment of mechanical behavior of cemented carbides under compressive contact loads. Damage evolution in ironing tools is qualitatively investigated by planar and cross-sectional microscopical observations. Several mechanisms of damage are identified by taking into account the circumferential distribution along the surface of the ironing tools: coating wear and/ fracture, plastic deformation and fracture of the substrate. In the context of damage location in the substrate, the emphasis is put on the study of damage following all processing steps in substrate and coating preparation. This analysis aims at determining whether the substrate is weakened or not by the mechanical and chemical pre-treatment prior to coating. Spherical indentation testing is used as an experimental procedure to reproduce the compression stress state during cold forming. Large indenter tips (300 microns) are employed for testing cemented carbides in an effort to minimize the effects of material structural inhomogeneities. The behavior of cemented carbides under compressive loads is governed by the reversibility of deformation induced by indentation. A unique feature revealed by analysis of experimental load-displacement indentation curves is the positive energy balance, in that a gain of energy is evidenced after a loading-unloading cycle. Two hypotheses are developed for interpreting such a behavior: the strain induced martensitic transformation of the cobalt ductile phase and the thermal residual stresses in the composite. Substrate weakening subsequent to mechanical and chemical pretreatment prior to coating is expected to have a particular role on the indentation behavior of cemented carbides. The impact of substrate damage on the indentation parameters is identified by a parametric study using finite element simulations in which the damage is considered uniform. In reality, the damage distribution over the surface is rather irregular. In this context, a statistical approach is used for assessment of the material indentation parameters. The failure of coating/substrate systems under compressive contact loads is investigated by finite element simulations. The first failure events are identified by considering that system failure occurs either by substrate plastic deformation or coating fracture. The response of coated systems to spherical indentation is synthesized in damage diagrams which can predict whether the coating or the substrate fails first. The possibility for using such diagrams for assessing the evolution of damage in coated cemented carbide tools is highlighted.

Mechanical properties of MC multimetallic carbides in steel

Lionel Michelet

Carbides constitute a class of materials used for many applications, where excellent mechanical properties, such as stiffness, hardness and fracture toughness, are required. The machining industry is a typical example where carbide are crucial materials. Whether it be in tool steels or in cermets, the most performant machining alloys are actually composed of ternary carbides, i.e. contain two different metallic elements in addition to carbon, present as a reinforcement in a metallic matrix. Ternary carbides constitute better reinforcements providing enhanced mechanical properties in comparison with binary carbides; however, our knowledge of ternary carbides per se is insufficient, and only the properties of the alloys have been characterized. The main reason explaining such a lack of data is the relative small size of these particles, in the micro-range, which complicate the measurement of their mechanical properties. This thesis aims to contribute towards filling this gap by means of micromechanical methods able to measure locally the main properties of individual microscopic carbide particles. This is now possible thanks to the recent developments of experimental techniques such as nanoindentation and FIB. The challenges of this thesis are then to be able to 1) produce carbides particles embedded in iron with tailored compositions, 2) test mechanically individual particles and 3) explain the evolution of the properties with changes in composition, in order to find an optimal composition characterized by enhanced mechanical properties. The main challenges come from the small dimension of the particles (< 40 µm) and the fact that such particles are embedded in a steel matrix, more compliant than the particles, which can then bias the measurement of elastic properties. Standard nanoindentation techniques cannot measure accurately the mechanical properties of such matrix/particle combinations and new methods have been developed in this thesis in order to overcome this problem. Carbide particles have been grown in situ by arc-melting cast iron with transition metal high purity chips, in order to properly tune the composition. Three mechanical properties have been measured: elastic modulus, hardness and fracture toughness, by indenting a polished surface, or by bending up to fracture notched micro-cantilever beams. This trio of properties is essential in order to characterize properly a material in view of an application in machining industry. By using binary carbide compositions as references, we have characterized five ternary carbide systems, namely (Ti,W)C, (Ti,V)C, (Ti,Ta)C, (Ti,Nb)C and (Ta,V)C, as well as the quaternary (Ti,Ta,V)C system. The ratio of the metallic elements has been varied in order to cover the entire range of compositions. For the ternary systems, we found compositions that exhibited both a higher hardness and modulus than the two corresponding binary carbides, enhancing the properties by ~15-20%. By investigating the filling of atomic bonds involving d-orbitals, we can provide a partial explanation to the elastic modulus and hardness evolution with composition, the maxima in hardness corresponding to a valence electron concentration between 8.4 and 8.6. The quaternary system aims to further improve mechanical response of the ternary carbides: it allows to highlight a very promising quaternary composition, characterized by the combination of both a very high modulus and hardness of respectively 636 and 41 GPa.

EPFL2018

Comportement mécanique des milieux granulaires sous sollicitations cycliques

Frédéric Mayoraz

The following work deals with the characterisation of the behaviour of road bases granular material, made of tunnel excavation material and loaded by passing vehicle axles. This granular media, don't correspond generally to the standards demanded for the construction of road pavements. The thesis is a part of the project COST 337 ("Unbound granular materials for road pavements") whose aim is to develop a guideline for the design of road pavements in Europe (material production, laboratory tests, modelling of mechanical behaviour). At the beginning, several loadings of road pavements are described (mechanical, thermal, hydric), concentrating on mechanical loading. Full-scale tests and finite element simulations led to the knowledge of the order of magnitude of stresses and deformations as well as to the understanding of the rotation of the principal stresses, due to the passing of a vehicle axle. This theoretical part is followed by laboratory cyclic tests with sand to estimate the influence of the principal stress rotation on the deformability of granular media and the importance to take this phenomenon into account for the modelling. Further laboratory tests were carried out on high diameter samples (160 and 250 mm) made of tunnel excavation material. Those tests were made by performing a large number of cycles (80'000 to 250'000 cycles) and various stress paths. The aim of the tests was to measure the evolution of elastic (stiffening of material) and plastic (accumulation of permanent deformations) material characteristics during the cycles. Tools have been developed in the framework of image analysis to assess the eventual change of morphology of this non-standard granular media. Finally the modelling of the accumulation of plastic deformations is discussed in accordance to the theory of elasto-plasticity and visco-plasticity. To remove the limits of elasto-plasticity to simulate a large number of cycles, a visco-plastic approach is developed. The elegant approach, replacing the time by the number of cycles, allows to approximate the permanent deformations of a road pavement in dependence on the stress path. It provides a tool to estimate the rutting in road pavements.

EPFL2002

Probing the buckling of pressurized spherical shells

Arefeh Abbasi, Pedro Miguel Nunes Pereira de Almeida Reis, Dong Yan

The prediction of the critical buckling conditions of shell structures is plagued by imperfection sensitivity. Non-destructive testing through point-load probing has been recently proposed to map the stability landscape of cylindrical shells. However, the counterpart procedure for spherical shells is still debatable. Here, we focus on the mechanical response of pressurized spherical shells containing a single dimple–like defect to a point probe. Combining experiments, finite element modeling, and existing results from classic shell theory, we characterize the nonlinear force-indentation response of imperfect shells at different pressurization levels. From these curves, we seek to identify the critical buckling pressure of the shell. In particular, the indentation angle is varied systematically to examine its effect on the probing efficacy. We find that the critical buckling point can be inferred non-destructively by tracking the maxima of the indentation force–displacement curves, if the probe is implemented sufficiently close to the defect. When probing further away from the defect, the test fails in predicting the onset of buckling since the deformation due to indentation remains localized in the vicinity of the probe. Using FEM simulations and shallow shell theory, we quantify the characteristic length associated with this localized deformation, both in the linear and nonlinear regimes. Our results demonstrate the limiting conditions of applicability for the usage of probing as a non-destructive technique to assess the stability of spherical shells.