Fatigue Crack Path Behavior Under Polymodal Fatigue

Abstract

The study is devoted to the prediction of crack path direction in polymodal fatigue. First experimental and theoretical results published in the literature are reviewed. It is shown that proportional and nonproportional loading conditions should be distinguished. Then, the experimental results obtained in cyclic Mode III and biaxial nonproportional Modes I + II loading are presented. These tests were carried out on two alloy steels and an aluminum alloy. The biaxial tests were performed on cruciform type specimens. In Mode III loading, it is shown that, at low applied torque, the crack surface adopts a “factory-roof” appearance with facets characteristic of local Mode I extension. It is suggested that the flat surfaces corresponding to apparent macroscopic Mode III extension observed in this study as in other ones might be an artefact. In Modes I + II loading, it is shown that the two investigated materials exhibit very different crack bifurcation angles. The results are discussed in terms of possible extended forms of the maximum tangential stress criterion, ΔkImax*, KImax* and (da/dN)max. It is shown that, as a general rule, the latter criterion gives results in better agreement with the experiments. The difficulties associated with crack closure effect are also shortly discussed.

Influence of the loading path on fatigue crack growth under mixed-mode loading

“…These two cases, which can yield very different predictions, as shown below, will be denoted by SWTa and SWTb. These two potential growth directions were considered by Hourlier et al (1985) and Dahlin and Olsson (2003). The former concluded that the direction of maximum σ n was more suitable for alloys showing a limited influence of the R ratio on their mode I kinetics, which is the case of the maraging steel investigated here (da/dN is at maximum 2.5 times higher for R = 0.7 than for R = 0, for a given K I ).…”

“…Does the loading path have an intrinsic influence, or can all this influence be captured through appropriate corrections for closure and friction effects on stress intensity factors, that is, by the use of K effective I , K effective II , K effective III ? This problem is complex and no clear answer emerges from the literature on non-proportional mixedmode I and II (Gao et al 1983;Hourlier et al 1985;Wong et al 1996Wong et al , 2000Planck and Kuhn 1999;Yu and Abel 2000;Doquet and Pommier 2004) or mode I and III (Feng et al 2006). The latter concludes: "with identical loading magnitudes in the axial and torsional directions, the crack growth and crack profiles are strongly dependent on the loading path.…”

Modelado de grieta y estimación de vida en Fretting Fatiga mediante el Método de los Elementos Finitos Extendido X-FEM

“…Además, en fretting fatiga la variación de las cargas aplicadas es no proporcional y el ratio entre los FITs no permanece constante durante los ciclos de carga (Hourlier et al, 1985;Ribeaucourt et al, 2007;Dubourg y Lamacq, 2002). En estas condiciones, las grietas generadas tras la fase de nucleación experimentan procesos de cierre de grieta con contacto entre sus caras, donde pueden encontrarse en estado de adhesión o deslizamiento con fricción (Hills y Nowell, 1994).…”

“…MTS modificado para propagación bajo carga cíclica (Hourlier) Este criterio es una extensión del criterio MTS para propagación bajo carga cíclica, y fue definido por Hourlier et al (1985). Sus autores propusieron tres variaciones del criterio MTS, asumiendo que la grieta se propaga en la dirección que corresponde al valor máximo, durante todo el ciclo, de k una hipotética grieta corta que emane de la existente, utilizando los FITs K I , K II en el extremo de grieta antes de su extensión.…”

“…Sus autores propusieron tres variaciones del criterio MTS, asumiendo que la grieta se propaga en la dirección que corresponde al valor máximo, durante todo el ciclo, de k una hipotética grieta corta que emane de la existente, utilizando los FITs K I , K II en el extremo de grieta antes de su extensión. Hourlier et al (1985) propusieron otras expresiones para calcular elángulo, vienen dadas por las Ecs. (3.48).…”