THE INFLUENCE OF THE HARDENING STATE ON TIME DEPENDENT DAMAGE AND ITS CONSIDERATION IN A UNIFIED DAMAGE MODEL

A novel numerical damage model has been developed to combine Fick’s second law of diffusivity with microstructure modelling of environmentally assisted creep damage. The environmental acceleration of material degradation has been modelled and compared with experimental observations. The combined multi-site damage and crack growth model for creep and environmentally assisted time-dependent material oxidation/carburisation based on a gas/solid interface diffusion and non-linear time-dependent creep mechanism is proposed. Numerical predictions are presented to develop a methodology for component lifing. The model allows for the development of a hardened layer due to surface oxidation and predicts damage and cracking during subsequent creep under an applied load. The simulated grain mesh structure used can replicate surface healing or diffuse intergranular cracking and material depletion emanating from the gas/solid surface interface by quantifying the strength ratios between grain and grain boundaries. In this article, oxidation/carburisation is estimated both analytically and numerically using Fick’s diffusion laws and carbon/steel diffusion flux properties available in the literature. It is also shown that carbon diffusion distribution can be related to grain hardening due to carburisation as well as grain/grain boundary strength ratios which could vary as much as a factor of 0.5. The model is validated by comparing with actual oxidation/carburisation data for the long-term oxidised 9-12 Cr steels operating at high temperatures. Finally, it is shown that the mode and rate of surface oxidation and hardening, depending on whether the material is homogenous or contains micro-cracks substantially affects the life time of a component under high temperature creep loading.

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A two-scale time-dependent damage model based on non-planar growth of micro-cracks

Journal of the Mechanics and Physics of Solids ◽

10.1016/j.jmps.2010.07.018 ◽

2010 ◽

Vol 58 (11) ◽

pp. 1928-1946 ◽

Cited By ~ 22

Author(s):

Bertrand François ◽

Cristian Dascalu

Keyword(s):

Damage Model ◽

Time Dependent ◽

Model Based ◽

Micro Cracks ◽

Planar Growth

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A new cumulative damage model for time-dependent reliability analysis of deteriorating structures

Proceedings of the Institution of Mechanical Engineers Part O Journal of Risk and Reliability ◽

10.1177/1748006x19886157 ◽

2019 ◽

Vol 234 (2) ◽

pp. 290-302

Author(s):

Junxiang Li ◽

Jianqiao Chen ◽

Zhiqiang Chen

Keyword(s):

Reliability Analysis ◽

Damage Model ◽

Aging Effect ◽

Cumulative Damage ◽

Time Dependent ◽

Sustained Load ◽

Aging Effects ◽

Sustained Loads ◽

Cumulative Damage Model ◽

Transient Loads

Performance and reliability of structures will deteriorate with the effects of loads, environment, and interior factors of materials. In this article, a novel cumulative damage model is developed for time-dependent reliability analysis of deteriorating structures. The deterioration is a combination of three stochastic processes: the gradual deterioration posed by aging effects, the sudden deterioration caused by transient loads, and the additional deterioration introduced by sustained loads. The aging effect is modeled as a gamma process, while the transient load is described by a Poisson process. The sustained load is modeled by a stationary binomial process and a Poisson square wave process, respectively. The load threshold for three different scenarios are all considered and applied to not only the transient loads but also the sustained loads. The time-dependent reliability of deteriorating structures is then evaluated based on this model via semi-analytical methods or numerical simulation methods. Three numerical examples and an example involving a natural gas pipeline are used to validate the effectiveness of the proposed model for computing the time-dependent reliability.

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