Multi-scale approach of the instrumented indentation technique on the fracture toughness estimation

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 volume through a variable damage, introduced by Kachanov [4], related to the surface density of the microdefects. On the other hand, the works carried out over ductile materials by Li et al. [3] have been performed only with nanoindentation data preventing a discussion on the scale-­‐effect. (...

Multi-scale approach of the instrumented indentation technique on the fracture toughness estimation

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 volume through a variable damage, introduced by Kachanov [4], related to the surface density of the microdefects. On the other hand, the works carried out over ductile materials by Li et al. [3] have been performed only with nanoindentation data preventing a discussion on the scale-­‐effect. (...

Mechanical properties of thermally sprayed porous alumina coating by Vickers and Knoop indentation

Depending on the thermal spraying conditions, coatings obtained can present different defects, like pores, cracks and/or unmelted particles, and different surface roughnesses, that can affect the determination of the hardness and elastic modulus. The present work investigates the mechanical properties, determined by means of Knoop and Vickers indentations, of a plasma as-sprayed alumina coating, obtained with a nano-agglomerated powder sprayed using a PTF4 torch, in order to highlight how the surface defects interfere into the indentation process. As a main result, Knoop indentation compared to Vickers one gives less dispersive results (15% and 33%, respectively), that are, in addition, more representative of the coating properties. The mean values obtained are 110 ± 40 GPa for the elastic modulus and 1.75 ± 0.42 GPa for the hardness. In addition, and for the two indenter types used, multicyclic indentation has been performed because it allows a more appropriate characterization of such heterogeneous coatings due to the representation of the mechanical properties as a function of the indentation load and/or the penetration depth, leading to more reliable results according to the depth-variability of the coating microstructure

Mechanical properties by indentation of aluminium nitride nanometric thin film

Les propriétés mécaniques des films minces sont généralement déterminées par nanoindentation pour éviter l’influence du substrat. En effet, nous savons que le substrat influence cette mesure dès lors que l’indenteur pénètre à plus de 10 % de l’épaisseur du film pour la dureté, ce chiffre pouvant être ramené à 1 % pour le module d’élasticité. Pour des films extrêmement minces pour lesquels la mesure directe des propriétés ne serait pas possible, l’application de modèles pour séparer la contribution du substrat de la mesure est alors nécessaire. Dans ce travail, la dureté et le module d’élasticité d’un film de nitrure d’aluminium de 250 nm d’épaisseur déposé par Magnetron Sputtering ont été déterminées par nanoindentation. Pour réduire l’influence de l’incertitude de l’épaisseur du film sur la détermination des propriétés mécaniques, nous proposons de masquer cette épaisseur dans les termes de lissage de plusieurs modèles et d’étudier leurs convergences. Concernant le module d’élasticité, nous avons observé que les valeurs suivaient une courbe typique en S entre deux asymptotes, une qui tend vers la valeur du module du film pour les très faibles profondeurs d’indentation et l’autre vers celle du substrat pour les plus fortes charges. Dans ces conditions, le modèle d’Antunes et al. utilisant le paramètre de Gao généralement utilisé dans de telles études, car ne faisant intervenir aucun coefficient de lissage, ne peut représenter correctement l’évolution des points expérimentaux. C’est pourquoi nous proposons d’utiliser une loi du type Avrami qui permet de bien prendre en compte ces deux tendances. Finalement, nous obtenons 10 GPa pour la dureté et 150 GPa pour le module d’élasticité en accord avec les données de la littérature.The mechanical properties of thin film are usually determined by nanoindentation in order to circumvent the influence of the substrate. Indeed, it is recognized that the substrate interferes into the measurement when the indenter penetrates more than 10% of the film thickness for the hardness determination and 1% for the elastic modulus determination. For very thin films for which a direct measurement of the mechanical properties is not possible, models must be applied to separate the contribution of the substrate into the measurement. In this work, hardness and elastic modulus of aluminum nitride film of 250 nm of thickness deposited by Magnetron Sputtering have been determined by nanoindentation. In order to reduce the influence of the uncertainty of the film thickness value on the mechanical property determination, we suggest to introduce the thickness term into the fitting parameters of the different models and to study the convergence of these models. Concerning the elastic modulus, we observed that the experimental data varied following a typical S-curve between tow asymptotes, one which tends toward the value of the film modulus for very low indenter displacements and the other toward that of the substrate for the highest indentation loads. In these conditions, the model proposed by Antunes et al. using the Gao’s function in which no fitting parameters are involved cannot adequately represent the experimental data variation. That is the reason why we propose to apply similarly to the Avrami’s law which allows to take into account the tendencies at the two extremum. Finally, we obtained values close to 10 GPa and 150 GPa for the hardness and the elastic modulus, respectively, in accordance with the literature data

Influence of volcanic ash, rice husk ash, and solid residue of catalytic pyrolysis on the flame-retardant properties of polypropylene composites

AC

Obtaining biodegradable films through the management of organic waste containing starch and chitosan [Obtención de películas biodegradables mediante la gestión de residuos orgánicos que contienen almidón y quitosano]

International audienc

High Cycle Fatigue Damage Evaluation of Steel Pipelines Based on Microhardness Changes During Cyclic Loads: Part II

ACTIInternational audienceThe hardness of a material shows its ability to resist to microplastic deformation caused by indentation or penetration and is closely related to the plastic slip capacity of the material. Therefore, it could be significant to study the resistance to microplastic deformations based on microhardness changes on the surface, and the associated accumulation of fatigue damage. The present work is part of a research study being carried out with the aim of proposing a new method based on microstructural changes, represented by a fatigue damage indicator, to predict fatigue life of steel structures submitted to cyclic loads, before macroscopic cracking. Here, Berkovich indentation tests were carried out in the samples previously submitted to high cycle fatigue (HCF) tests. It was observed that the major changes in the microhardness values occurred at the surface of the material below 3 μm of indentation depth, and around 20% of the fatigue life of the material, proving that microcracking is a surface phenomenon. So, the results obtained for the surface of the specimen and at the beginning of the fatigue life of the material will be considered in the proposal of a new method to estimate the fatigue life of metal structures

Estudio del comportamiento mecanico de compuestos de mezcla polimerica con adicion de cáscara de arroz y compatibilizante

L utilisation et la mise au point de polymères renforcés sont des activités déjà anciennes et de nombreuses études leur ont déjà été consacrées. Parallèlement à l utilisation des matières plastiques la question de leur recyclage se pose avec de plus en plus d acuité. Le travail présenté cherche à comprendre le rôle d un renfort constitué d écorce de riz sur le comportement mécanique d un mélange de polyoléfines. Un plan d expérience est construit sur la base 1) de mélanges de polyoléfines recyclés 2) d ajout d écorce de riz et 3) d ajout d agent compatibilisant et 4) sur le mode d élaboration par injection ou compression. L adhérence de l écorce de riz avec la matrice polymère a été examinée en relation avec sa microstructure et sa composition. Il est mis en évidence que l addition d écorce de riz seule réduit les propriétés de traction mais que l addition d agent compatibilisant favorise la déformation du polymère. La différence est notable entre les valeurs des propriétés de traction des éprouvettes élaborées par injection ou par compression. Cependant, les résultats peuvent être comparés valablement à l aide d un paramètre capacité de charge que nous proposons comme le rapport entre la tension maximale et la déformation maximale qui renseigne sur la capacité globale de résistance et de déformation aux conditions à rupture. Du point de vue du processus physico-chimique on peut conclure que l addition de compatibilisant améliore l adhérence entre les composants polymères et de l écorce de riz, et par voie de conséquence la surface de rupture. L indice de capacité de charge que nous proposons pourrait être considéré comme un facteur de conception pertinent.The use and development of reinforced polymers are long-standing activities and many studies have already been done on the subject. With the development of the use of plasticsthe question of recycling becomes more and more acute. The work presented here reflects these concerns by seeking to understand the role of a reinforcement consisting of rice husk on the mechanical behavior of a mixture of polyolefins. The experimental design which was used was built on the base 1) of polyolefin blends 2) the addition of rice wheat and 3) the addition of compatibilizer and 4) the mode of preparation by injection or compression. The adhesion of rice husk with the polymer matrix has been examined in relation to its microstructure and composition. It was also revealed that the addition of rice wheat alone reduces the tensile properties but that the addition of compatibilizer promotes the deformation of the polymer blend. A noticeable difference was found between tensile properties of specimens prepared by injection or compression. However, the results can be validly compared if we introduce a parameter called "load capacity" that we propose as the ratio between the maximum voltage and maximum strain which provides information on the overall ability of strength and deformation conditions at fracture. From the standpoint of physical-chemical process it can be concluded that the addition of compatibilizer improves the adhesion between the polymer components and rice wheat and consequently the fracture surface. The index of "load capacity" that we propose could be considered a relevant design factor.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF