Advances in Mechanical Behaviour, Plasticity and Damage

D. Miannay, J.C. Dupré, J.M. Georges, M. Bornert, M. Cherkaoui, R. Schirrer, T. Thomas, S. Pommier, A. Pineau, P. Costa, D. François, A.B Vannes, A. Lasalmonie, D. Jeulin, D. Marquis, F. Vaillant, H. Burlet, M. Berveiller, M. Berveiller

• Editore: Elsevier Science
• Anno: 2000
• ISBN:
  9780080552750
• Argomento: Technik & Maschinenbau
• Categoria: Maschinenbau

Since its inception in 1991, EUROMAT has been held each year on behalf of the Federation of European Materials Societies (FEMS), and alternates between general and topical prospectives.
This year's theme, Advances in Mechanical Behaviour, Plasticity and Damage, was proposed by the Societe Francaise de Metallurgie et de Materiaux (SF2M) to FEMS.

This publication contains a selection of papers presented at the EUROMAT 2000 Conference, held in Tours, France on 7-9 November 2000. The aim of this Conference was to concentrate mainly on recent advances made in the investigation of the relationship between microstructures of materials and their mechanical behaviour; including, fundamentals, modelling and applications. Encompassed in the Conference's aim is the nurturing of the synergistic effect between the theoretical and applied areas in this field. This was achieved by addressing important basic and practical aspects of the mechanical behaviour and damage of materials whilst also providing significant links between various complementary approaches.

All kinds of materials are covered and topics that were covered include the mechanics of solid polymers, microstructurs and micromechanisms, and the collective behavior of defects which looks at the interaction of multiple defects in a system.

Fatigue

Fatigue Crack Initiation and Crystallographic Crack Growth in an Austenitic Stainless Steel

C. Chauvot; M. Sester Fraunhofer Institut für Werkstoffmechanik D-79108 Freiburg, Germany

ABSTRACT

In the classical (crack growth) concepts used to predict the lifetime of specimens or components under cyclic loading, initiation and early growth are neglected. Instead, fictitious starter cracks are prescribed. Experiments with specimens made of the austenitic stainless steel with the german designation X6 CrNiNb 18-10 (approximately steel 316) showed that a significant part of the lifetime is spent for the initiation and crystallographic growth of microstructurally short cracks. In the present work the crack growth behaviour was studied with the classical fracture mechanics concept and with an effective crack length approach. Models for crack initiation and crack growth in the first grains of a polycrystalline aggregate are proposed.

KEYWORDS

Stainless steel

fatigue

crack initiation

microstructurally small cracks

crack growth

lifetime prediction

MATERIAL AND EXPERIMENTAL PROCEDURES

The material employed was an austenitic stainless steel with the chemical composition as given in Table 1. Low-cycle fatigue tests were conducted under push-pull loading at room temperature and at 280°C. Cracks on the specimen surface were monitored by means of a replication technique. This technique allows to record the development of surface defects during the course of the loading. Cracks of a few microns could be detected with this method.

Table 1

Chemical composition, % by weight

C	Si	Mn	p	S	Cr	Mo	Ni	Nb	Ti	Al	N2	Cu	Co	B	Ta
0.026	0.22	1.88	0.009	0.001	18.45	0.36	10.80	0.350	0.020	0.020	0.026	0.05	0.03	0.0015	0.006

RESULTS

The crack growth data given here concern only symmetrical push-pull tests at room temperature. Fig. 1 shows constant-amplitude fatigue crack propagation results for four strain amplitudes (R_ε=-1), plotted in terms of crack length, a, as a function of the number of cycles, N. It can be seen that one crack (main crack) became predominant and leads to rupture of the specimen. The other cracks often do not grow very much. Cracks smaller than some grain sizes follow a microstructural path. Cracks which become longer than several grain sizes grow perpendicular to the loading axis. Most cracks initiated at slip bands, or grain boundaries and twin boundaries. Crack initiation near inclusions was seldom observed. As an example, a series of replicas is given for a certain crack in Fig. 2. Fig. 3 shows the Z_D-Wohler diagrams (for tests at room temperature and tests at 280°C) obtained for our material. This elastic plastic Zo-Parameter, which can be compared with the elastic fracture mechanics parameter K, is defined in equation (5). With the help of the Z_D-Parameter normalized with the temperature dependent yield strength σ_y, the data falls in a single scatter band.

Fig 1 Crack length as function of the number of cycles (measurements with the replication technique)

Fig 2 Series of replicas, specimen LRCx08, ε_a=0.25%, R_ε=-1, crack 4

Fig 3 Z_D-Wöhler diagrams for push-pull tests at room temperature and 280°C

MODELLING

Fatigue crack initiation is described with the dislocation pileup model of Tanaka and Mura [1]. This model is intended for initiation in slip bands. The Tanaka-Mura model gives a Manson-Coffin-type relationship between the local plastic shear strain ampl...