Since creep-fatigue is mainly studied in uniaxial tension, it is shown here how to proceed to perform both experiments and calculations under multiaxial loading and when the temperature varies both in time and space. The constitutive equations used are those of elasto-visco-plasticity coupled or not, to damage, with isotropic and kinematic hardening. It is shown that the unified damage law first proposed for ductile failure and then for fatigue may also be applied to multiaxial creep-fatigue interactions with a new expression for the damage threshold. The procedure for the identification of material parameters is described. Finally, it is shown that the uncoupled calculation procedure, where damage is calculated as a postprocessing of an elasto-visco-plastic computation, gives satisfactory results in comparison to the fully coupled analysis ; the latter being more acurate but very expensive in computer time.

Multiaxial Creep Fatigue under Anisothermal Conditions

Because creep–fatigue is mainly studied in uniaxial tension, it is shown here how to proceed to perform both experiments and calculations under multiaxial loading and when the temperature varies both in time and space. The constitutive equations used are those of elasto-visco-plasticity coupled or not, to damage, with isotropic and kinematic hardening. It is shown that the unified damage law first proposed for ductile failure and then for fatigue may also be applied to multiaxial creep–fatigue interactions with a new expression for the damage threshold. The procedure for the identification of material parameters is described in detail. Finally, it is shown that the uncoupled calculation procedure, where damage is calculated as a post-processing of an elasto-visco-plastic computation, gives satisfactory results in comparison to the fully coupled analysis; the latter being more accurate but very expensive in computer time.

Multiaxial creep–fatigue under anisothermal conditions

Higher order asymptotic analysis of crack tip fields under mode II creeping conditions

During service at high temperature, stabilized stainless steels are well known to present a serious form of intergranular cracking in the Heat Affected Zones (HAZ) referred to as the reheat cracking phenomenon which is very deleterious for the structural integrity of welds made in this steel. The purpose of the present study is to present quantitative results on mechanical behaviour and the damage at high temperature of simulated HAZ which show different sensitivities to intergranular cracking. Creep laws are determined and an intergranular damage model is identified using the local approach to fracture based on mechanical tests performed at 600 °C and metallurgical observations on smooth specimens and two types of notched specimens.

Modelling creep damage in heat affected zone in 321 stainless steel. Part I: Quantitative study of intergranular damage

A unified method to solve higher order asymptotic crack-tip fields of mode I, mode II and mixed mode I/II crack in power-law creeping solids

This study deals with the understanding of creep crack initiation and creep crack growth in 316 LN austenitic stainless steel. To evaluate the respective role of the creep strain and that of the main principal stress on intergranular creep damage, original creep experiments were performed on sharp notched thin wall tubular specimens in 316 LN steel tested in torsion at 600°C. The specimens contained a sharply notch perpendicular to their axis. This resulted in overall Mode II loading. In the first part of the paper, the results of four experiments are reported, then, in the second part, a damage model identified previously and based on quantitative observations of grain boundary damage is used to simulate creep crack initiation and creep crack growth. Good agreement between the experimental results and the numerical simulations is found.

Mode II creep crack initiation in 316 LN stainless steel: experiments and modelling

This study deals with the evaluation of local approach to assess the mechanical integrity of cracked components submitted to cyclic and steady load at elevated temperature. In this approach, a fracture criterion based on calculated intergranular damage ahead of the crack tip is introduce to simulate both crack initiation and crack propagation in 316 L type austenitic stainless steel. This numerical method, based on finite element computations is firstly described. Then numerical results are compared with sets of experimental data for two specimen geometries during load controlled tests. A damage criterion in good agreement with previous studies leads to promising load-displacement response, crack initiation and crack growth curves.

/pdf/local-approach-numerical-simulations-of-creep-and-creep-yxb18dh5np.pdf

Local Approach : Numerical Simulations of Creep and Creep-Fatigue Crack Initiation and Crack Growth in 316L SPH Austenitic Stainless Steel

This study concerns the application of the local approach to creep-fatigue crack initiation and creep crack growth in components submitted to combined mechanical and thermal loadings leading to intergranular damage. The purpose is to assess the remaining life of a structure with a crack detected during in service inspection or with a geometrical singularity. A new numerical method based on finite element computations and on a damage model relying on quantitative observations of grain boundary damage is proposed. The numerical results obtained are in good agreement with experimental data. They also fit them better than the classical global approach predictions.

Local approach applied to creep-fatigue crack initiation and crack growth in circumferentially notched 316L tubes under combined tension and cyclic thermal shocks

D. Poquillon

Papers

Mode II creep crack initiation in 316 LN stainless steel: experiments and modelling

Local Approach : Numerical Simulations of Creep and Creep-Fatigue Crack Initiation and Crack Growth in 316L SPH Austenitic Stainless Steel

Local approach applied to creep-fatigue crack initiation and crack growth in circumferentially notched 316L tubes under combined tension and cyclic thermal shocks