Non-isothermal Loading Conditions Research Articles

A thermo-mechanical coupled constitutive model for clay based on extended granular solid hydrodynamics

The thermo-mechanical (T-M) coupling behavior of clay can be difficult to predict due to its complicated coupling mechanism, irreversibility and OCR dependency. To date, several constitutive models were proposed based on the phenomenological theory of elasticity and plasticity, which are able to simulate some of the T-M coupling behavior but lacks the fundamental description of coupling dissipation mechanism. The theory of Granular Solid Hydrodynamics (GSH) originally developed for single phase and isothermal granular solid has provided an alternative framework for the constitutive modeling of granular geo-materials. This paper first introduces the T-M coupled fundamental behavior of clay and its existing constitutive models. Then a constitutive model of a single phase and isothermal granular solid based on GSH is briefly outlined, followed by an extension of this GSH-based model made to account for a cohesive saturated granular solid, e.g. clay subjected to non-isothermal loading conditions. Based on the extension of GSH, a T-M coupled constitutive model is established, which allows for a better understanding of the physical mechanism underlying the observed T-M coupled behavior of clay. Verification of the model is conducted by predicting the thermally induced volumetric deformation and the undrained thermal failure of saturated Kaolin clay and Boom clay.

Strip yield model application for thermal cyclic loading

An extension of the strip yield model towards the calculation of crack opening and closure under non-isothermal loading conditions is the main prerequisite for its applicability in thermal fatigue analyses. The aim of this contribution is to implement a new proposal to this feature. Here, the yield stress is allowed to be location and time dependent. With this extension, the cyclic crack tip opening displacements, ΔCTOD, can be calculated. They are assumed to be related uniquely to the crack growth rate. The corresponding empirical relationship is derived from conventional isothermal fatigue crack growth behaviour of steels. Its integration leads to fatigue crack growth curves and lives.

Fatigue assessment of thermal cyclic loading conditions based on a short crack approach

Thermal loading conditions of nuclear power plant components cause local stress-strain hystereses. For the fatigue life prediction of nuclear power plant components under thermal cyclic and structural loading a new method based on the local strain approach is to be presented. This method involves finite-element simulations as well as the experience gathered from lifetime assessment methods based on short crack models. The local stresses and strains are obtained from coupled-field FE-analyses. The calculation of the hysteresis-loops relies on appropriate material models and experimentally verified temperature-dependent material parameters in order to describe the elasto-plastic behavior of the material as realistically as necessary. Due to the temperature dependence of the material parameters the resulting hysteresis loops are of non-conventional shapes and similar to those observed under multiaxial non-proportional structural loading. Hence, fatigue methodologies developed for non-proportional loading conditions during the past years bear good prospects for successful application under non-isothermal loading conditions

Oxidation assisted fatigue crack growth under complex non-isothermal loading conditions in a nickel base superalloy

This paper deals with the prediction of fatigue crack growth at high temperatures in the N18 nickel base superalloy, which is employed by Snecma for turbine disc applications. This material and other nickel base superalloys were widely studied in the past under isothermal conditions and constant amplitude fatigue. Dwell time effects are observed which are attributed, in this material, to grain boundary oxidation. The main objective of this research is to use this knowledge to model the fatigue crack growth rate in the N18 nickel base superalloy when complex “missions” are encountered. This implies variable amplitude and non-isothermal loading conditions (450–650 °C). For this purpose, an incremental fatigue crack growth model which was originally developed for isothermal variable amplitude loading conditions was extended so as to be applicable to non-isothermal loading conditions. In addition, the incremental form of the fatigue crack growth law in this model is very useful to account for the coupling effect between fatigue and time-dependent phenomena such as creep or oxidation. In the present case, the effect of the environment was modelled as a competition between two phenomena: a detrimental effect of grain boundary oxidation ahead of the crack tip and a beneficial effect of the growth of a passivation layer of oxides on the freshly created crack surfaces. The model was used to simulate fatigue crack growth under complex cycles at high temperature and the comparisons with experimental results are satisfactory.

Features of low-cycle endurance evaluation under isothermal and nonisothermal loading conditions

Features of low-cycle endurance evaluation under isothermal and nonisothermal loading conditions