Equivalent Initial Flaw Size Concept Research Articles

A dynamic condition assessment model of aging subsea pipelines subject to corrosion-fatigue degradation

Extension of operating lifetime of aging subsea pipelines is of great interest to oil and gas sectors. Corrosion and fatigue are the main causations of the condition degradation of subsea pipelines. A dynamic probabilistic condition assessment model based on continuous dynamic Bayesian network (DBN) is developed to support the life extension decision-making of aging subsea pipeline subjected to the corrosion-fatigue degradation. The methodology is built based on equivalent initial flaw size (EIFS) concept and a time-dependent prediction model implemented using DBN. The complex corrosion-fatigue degradation process is simplified by EIFS concept, and the crack propagation due to corrosion-fatigue is modelled using fracture mechanics model and DBN. A limit state function (LSF) is used to express the failure condition of subsea pipeline due to crack propagation. The dynamic reliability of subsea pipeline is estimated using Monte Carlo (MC) method with the probability distributions of the predicted crack sizes at different time slices. The estimated reliability is compared with the acceptable threshold to decide whether any measures are required to extend the life of subsea pipeline. The methodology is tested by a case study, and it is observed that it can be a useful tool to support life extension decision-making of aging subsea pipelines.

Crack growth analysis for rice husk reinforced polypropylene composite using equivalent initial flaw size concept

Crack growth that takes place in natural fibre polymer composite formations is dependent on several factors, whereby primary crack size is a key aspect that influences uncertainty of the crack growth. The nucleation stage is strongly affected by the fracture collapse of structures, unavoidably affecting the accuracy of the estimation of total fatigue life. In this research, fatigue crack was examined using rice husk/polypropylene composite specimens across stress loads ranging from 80 to 90% for ultimate tensile strength at the stress ratios R=0.1, 0.3, and 0.5. Consequently, the propagation rate of the crack was dependent on the stress ratio. Crack resistance showed a drop in the propagation rate of the crack rate with an increase in the R value. This effect produced more fibres/matrix fracture at high stress ratio, in comparison to the low stress ratio, which was verified further through scanning electron microscopy. Moreover, the S-N curve method was proposed, as it facilitates the deterministic total fatigue life discovery in a highly favorable manner via equivalent crack size approach. A strong consensus was observed between the model of prediction and the outcomes of the experiment.

A safe fracture fatigue life prediction based on equivalent initial flaw size

The equivalent initial flaw size (EIFS) concept was developed to provide an initial damage state for fracture mechanics-based life prediction. This study focuses on the safe fracture fatigue life prediction based on EIFS concept. A novel assessment method (M-K-T), the maximum likelihood estimation (MLE) method updated by modified Kitagawa-Takahashi (K-T) diagram method, is proposed to obtain general EIFS distribution and further predict the total fatigue life. The accuracy of the prediction method is proved by the fatigue crack propagation test of Ti2AlNb-based alloy under five different temperature (R.T., 200 °C, 400 °C, 500 °C, and 600 °C). Based on total fatigue life and uncertain damage mapping, the probability function of safe fracture fatigue life is derived considering the errors of experimental measurement and theoretical derivation.

Probabilistic Life Prediction for Reinforced Concrete Structures Subjected to Seasonal Corrosion-Fatigue Damage

The coupled effect of corrosion and fatigue deteriorates the performance of reinforced concrete (RC) structures. This paper proposes a novel framework for the probabilistic life prediction of aging RC structures under seasonal corrosion-fatigue damage based on the fracture mechanics and equivalent initial flaw size concept. A series of fatigue crack growth tests of steel bars in air and solution environments are conducted to simulate the fatigue crack growth behavior of rebar in different seasons. The framework includes three critical deterioration stages: corrosion initiation–pure fatigue crack growth, competition between corrosion pit growth and fatigue crack propagation, and structural failure. The chloride ingress, cyclic load, corrosion pit growth, concrete cracking, seasonal environmental variation, and corrosion pit-induced stress concentration are considered. Following that, an uncertainty model is established to incorporate various uncertainties associated with the load and environment. The characteristics of different stages are discussed. A parametric analysis is also performed to illustrate the influence of stress concentration, concrete severe cracking, and seasonal environmental variation on the life prediction results. Several conclusions are drawn and future work is discussed based on the proposed study.

Towards a methodology for the practical applications of the EIFS (Equivalent Initial Flaw Size) concept

This work intends to introduce a probabilistic approach for the crack propagation analysis. It presents a methodology that is based on the distribution of the initial flaw size a0. Departing from fatigue test results for specific structural components, an estimation of the initial flaw distribution is obtained, and from this distribution it becomes possible to establish the appropriate initial flaw size for crack propagation analysis for the chosen statistical distribution, such as Weibull or Log-normal. The methodology allows to apply different values of a0 for different components or structural details, reflecting the quality of the manufacturing process for the item evaluated. Two examples of practical applications of the EIFS concept are presented, corresponding to lugs and shear joints usually used for aerospace applications.