Deducing the Fatigue Crack Growth Rates of Natural Flaws in Silicon Nitride Ceramics: Role of R‐Curves

Fatigue failure is a concern when high‐strength, high‐toughness silicon nitride ceramics are used in mechanical components and the growth of natural flaws will determine the usable upper bound strength. In this study a fracture resistance curve (R‐curve) model is incorporated into an established method for deducing natural flaw growth rates from a combination of strength and fatigue life data for smooth specimens. Experimental data for a commercial silicon nitride, SL200, were examined. When compared with results deduced using a constant fracture toughness model, the new method gives more physically realistic growth rate results. Specifically, by incorporating the R‐curve the deduced fatigue threshold is equal to the reported intrinsic toughness for crack propagation of 2.2 MPa√m, whereas the constant fracture toughness model gives a physically unrealistic threshold value. Furthermore, much better agreement is achieved with the growth rates measured using macroscopic compact‐tension specimens. Overall, it is concluded that the R‐curve effect should not be ignored when deducing the fatigue crack growth rates of natural flaws in high‐toughness silicon nitride ceramics.     

Direct adjustment procedure for weight functions of graded materials

The weight function procedure simplifies the determination of stress intensity factors. If the weight function is known for a crack in a component of homogeneous material, the stress intensity factor can be obtained by multiplying this function by the stress distribution and integrating it over the crack length. In the case of graded materials, weight functions are seldom available in the literature. The main point of this paper is to demonstrate whether or not the approximate direct adjustment procedure for the derivation of weight functions is also valid in the case of graded materials. In this study it has been found that the procedure is applicable and no changes are necessary compared with the procedure for homogeneous materials.     

An analysis of the creep of hot pressed silicon nitride in bending

The creep behaviour of hot pressed silicon nitride is investigated in four-point-bend tests at temperatures of about 1200° C. By use of appropriate creep laws for the primary creep range as well as for the secondary range the experimental results can be well described analytically.     

The crack opening displacement field of semi-elliptical surface cracks in tension for weight functions applications

Application of the weight function method for calculating energy release rates or averaged weighted stress intensity factors requires that both the stress intensity factors and the crack opening displacements are known for a reference load case. This report gives an approximative solution for the crack opening displacement field of a semi-elliptic surface crack under pure tension loading. As a practical example the energy release rates are calculated for bending and compared with results available in the literature. Also a test procedure is described for checking the quality of approximative stress intensity factors.

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Résumé Pour appliquer la méthode des fonctions pondérées au calcul des vitesses de relaxation de l'énergie ou des facteurs d'intensité de contraintes moyens pondérés, il faut que soient connus les facteurs d'intensité de contraintes et les déplacements d'ouverture de la fissure, dans un cas de sollicitation de référence. La rapport fournit une solution approximative pour le champ de COD correspondant à une fissure superficielle semi-elliptique soumise à une charge de traction pure. Comme exemple pratique, on calcule les vitesses de relaxation de l'énergie correspondant à la flexion, et on se compare avec les résultats publiés dans la littérature. On décrit également une procédure d'essai en vue de vérifier la qualité des facteurs d'intensité de contraintes approchés.

     

A fracture-mechanical theory of subcritical crack growth in ceramics

An interpretation is proposed of the power law describing the relation between subcritical crack growth rate and stress intensity factor. It is based on the idea that thermal transients both break and re-establish bonds. The effects, which occur during these processes are mathematically described using a Morse potential. Already the rough model employed provides enough information on bond breaking to understand the principle of subcritical crack growth.     

Determination of v-K-curves of hot-pressed silicon nitride at elevated temperatures

The problem of the peeling of an elastic layer from the rigid wall of a cylindrical cavity is considered. A closed form analytical solution is obtained and numerical results are presented revealing characteristic features of the phenomenon.

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Résumé On considère le problème du pelage d'une couche élastique à partir de la paroi réputée d'une cavité cylindrique. On obtient une solution analytique de forme fermée; les résultats numériques qui sont présentés révèlent les composantes caractéristiques du phénomène.

     

Determination of approximative weight functions for straight through cracks

The method of determining approximative weight functions, well known for straight through surface cracks (edge cracks), is extended to internal straight through cracks. The procedure is outlined in detail for central cracks in endless strips of infinite and finite widths. For the finite width strip the coefficients of the weight function are given in a closed form.

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Résumé On étend aux fissure internes traversantes et directes la méthode de détermination des fonctions pondérales approximatives, dont l'application aux fissures droites débouchant en surface (fissures de bord) est bien connue.On décrit en détail la procédure utiisée dans le cas de fissures centrales dans des bandes sans fin de largeur infinie be finie.Pour les bandes de largeur finie, les coefficients de la fonction pondérale sont fournis sous une forme fermée.