Fatigue Crack Paths 2003

DamageAccumulation and CrackPaths under Variable

Loading Modes

F. Morel1 and L. Flacelière1

1 E.N.S.M.A.,

Laboratoire de Mécanique et de Physique des Matériaux (LMPM),U M R6617,

Site du Futuroscope, BP 109, 86960 Futuroscope Cedex, France.

Tel. 33 (0)5 49 49 82 39

Fax. 33 (0)5 49 49 82 38

e-mail : morel@lmpm.ensma.fr

ABSTRACTW.henstructures and components are submitted to multiaxial and variable

amplitude loading, the damage accumulation process and the crack path under such

complex loading are not easy to predict. An original high cycle multiaxial fatigue life

prediction method was proposed by Morel to account for some microplasticity

mechanisms usually encountered in metallic materials at the scale of the grains. The

aim of this work is to study the conditions of damage accumulation and the orientation

change of the crack path when a variable amplitude sequence is composed of different

constant amplitude loading blocks. More exactly, two different loading modes, torsion

and tension, will be applied in the same sequence. The predictions of the critical plane

fatigue damage model are compared to fatigue tests under sequence of tension and

torsion carried out on a mild steel. Other data from the literature relative to sequence

of non similar load conditions (including multiaxial loading) are also analyzed.

I N T R O D U C T I O N

In the industry, most of the components that resist high cycle fatigue are subjected to a

multiaxial state of stress [1]. Sometimes, the applied sequences are composed of

different loading modes which makes the damage accumulation process and the crack

path difficult to predict. To tackle this problem, a relevant fatigue damage model has to

be used. This model must be able to deal with a variable amplitude loading composed

for instance of tension and torsion loading modes.

The aim of this paper is to understand how to carry out the damage accumulation

with a critical plane damage model for a successive application of loading blocks

presenting different stress states. An original high cycle multiaxial fatigue life

prediction method was proposed by Morel [2-4] to account for some microplasticity

mechanisms usually encountered in metallic materials at the scale of the grains. This

damage model is based on the mesoscopic approach introduced by Dang Van [5] and

developed by Papadopoulos [6,7]. It has been shown when dealing with constant