Now showing 1 - 10 of 21
  • Publication
    A risk-informed decision support tool for holistic management of fatigue design, inspection and maintenance
    (Royal Institution of Naval Architects, 2018-01-24) ; ;
    Fatigue cracks threaten integrity of marine and offshore assets and need to be managed properly during the life cycles. However, the decision making process for fatigue design and maintenance are often disconnected and probably not be optimal with respect to life cycle total costs. This paper proposes a holistic decision support tool for jointly optimizing fatigue design, inspection and maintenance decision based on risk quantification and life cycle cost analysis, taking into account the uncertainties associated with fatigue deterioration, inspection performance and repair effect. The tool can be used to support risk-informed fatigue design; inspection and maintenance decision making, so that fracture risk associated with design and operation of marine assets are controlled with the minimum life cycle total costs.
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  • Publication
    Development of probabilistic fracture mechanics method for fatigue life prediction based on EIFS concept
    A problem with fracture mechanics (FM) based fatigue analysis is that reliable information on initial crack/flaw size is often unavailable. Also, FM method cannot be applied directly to welded joints with relatively small initial flaws and long crack initiation life. This paper proposes a novel probabilistic FM method based on the equivalent initial flaw size (EIFS) concept. The initial crack size is substituted with EIFS to take both the crack initiation and propagation life into account. Three methods are tested to obtain mean value of EIFS: calibrating to S-N curve, Kitagawa-Takahashi (KT) diagram and fitting to test data. The obtained EIFSs are evaluated by comparing the predicted fatigue lives and crack evolutions with S-N curve and test crack evolution data. The suggested procedure is to derive the mean value of EIFS from S-N curve and the coefficient of variation from KT diagram.
      138
  • Publication
    TRUSS, a European innovative training network dealing with the challenges of an aging infrastructure network
    Inspections and maintenance of infrastructure are expensive. In some cases, overdue or insufficient maintenance/monitoring can lead to an unacceptable risk of collapse and to a tragic failure as the Morandi bridge in Genoa, Italy, on 14th August 2018. An accurate assessment of the safety of a structure is a difficult task due to uncertainties associated with the aging and response of the structure, with the operational and environmental loads, and with their interaction. During the period from 2015 to 2019, the project TRUSS (Training in Reducing Uncertainty in Structural Safety) ITN (Innovative Training Network), funded by the EU H2020 Marie Curie-Skłodowska Action (MSCA) programme, has worked towards improving the structural assessment of buildings, energy, marine, and transport infrastructure. Fourteen Early Stage Researchers (ESRs) have been recruited to carry out related research on new materials, testing methods, improved and more efficient modelling methods and management strategies, and sensor and algorithm development for Structural Health Monitoring (SHM) purposes. This research has been enhanced by an advanced program of scientific and professional training delivered via a collaboration between 6 Universities, 1 research institute and 11 companies from 5 European countries. The high proportion of companies participating in TRUSS ITN has ensured significant industry expertise and has introduced a diverse range of perspectives to the consortium on the activities necessary to do business in the structural safety sector.
      148
  • Publication
    Probabilistic maintenance optimization with respect to inspection quality
    Maintenance scheduling and optimization against fatigue failures is of great interest for marine and offshore engineering in terms of safety assurance, integrity management and cost control. The main challenge is to make risk-informed and optimal maintenance decisions taking into account uncertainties associated with material properties, fatigue loads, modelling, inspection and maintenance methods. While optimization of inspection times has been the objectives of many studies, the influence and optimization of inspection qualities is not very clear. This paper has applied probabilistic fracture mechanics and reliability/risk methods to optimization of inspection quality as well as inspection time and revealed the effect of inspection quality on lifetime fatigue reliability. It is found that there is a reliability-based optimum inspection quality for maintenance scheduling, which is different from the cost-based optimum one. Better inspection quality than the optimum one can lead to excessive maintenance, which occurs when the effect of maintenance is not good, and the inspection quality applied is very good. Excessive maintenance can lead to increase of both expected failure costs and maintenance costs, and thus should be avoided.
      85
  • Publication
    A probabilistic approach for joint optimization of fatigue design, inspection and maintenance
    (International Society of Offshore and Polar Engineers, 2018-06-10) ; ; ;
    This paper addresses challenges in fatigue management of marine structural assets with a holistically approach, by jointly considering fatigue design, inspection and maintenance decisions, whilst taking into account sources of uncertainties affecting life cycle performance. A risk-informed and holistic approach is proposed for jointly optimizing fatigue design, inspection and maintenance based on the same fatigue deterioration model. The optimization parameters are fatigue design factor (FDF) and inspection intervals, while the objective is to minimize expected life cycle costs (LCC). The framework is to guide design process as well as to formulate optimal maintenance strategies. The proposed approach is exemplified for the marine industry through a fatigue-prone detail in a ship structure to obtain the life cycle optimal management solution that achieves a best compromise between structural safety and life cycle costs.
      106
  • Publication
    Value of inspection in fatigue management of steel structures
    Fatigue cracking is a common problem that needs to be managed in the life cycles of steel structures. Operational inspections and repairs are important means of fatigue crack management. Driven by high relevance in safety control and budget saving, inspection and maintenance planning has been widely studied. However, the value of inspection and repairs has typically not been fully appreciated and quantified rationally before they are implemented. The basic idea of this paper is to address the planning problem with focus on repair other than on inspection. A maintenance strategy without inspection is studied and serves as comparison of a maintenance strategy with inspection. Then the value of repair and the value of inspection relative to repair can be evaluated respectively. An illustrative example is performed on a typical fatigue-prone detail in steel structures.
      77
  • Publication
    Probabilistic maintenance optimization for fatigue-critical components with constraint in repair access and logistics
    There is a need to consider repair delay and incurred failure risk in maintenance optimization for some fatigue-critical structural details in marine and offshore structures. For example, in some cases, immediate repair may not be feasible due to weather, geographical location and/or technical restrictions. Also, immediate repair may be much more expensive than well-organized delayed repair. Moreover, detected cracks may sometimes be left unattended until more cracks are found and repaired together. This paper investigates a probabilistic maintenance optimization method allowing for repair delay and the incurred failure risk. The maintenance strategy considering repair delay is optimized based on uncertainty modeling, reliability and life-cycle cost analysis. Special features of the maintenance strategy and its impacts on fatigue reliability and life-cycle costs are discussed on an illustrative example. A method to quantify the risk incurred by repair delay is proposed. It is found that repair delay can result in a significant decrease in fatigue reliability if inspection is scheduled in the late stage of service life. The benefits of the maintenance strategy to fatigue reliability and life-cycle costs are very sensitive to the inspection method. The failure risk incurred by repair delay would be the predominant risk in the life cycle.
      87
  • Publication
    Uncertainty quantification and calibration of a modified fracture mechanics model for reliability-based inspection planning
    Efficient inspection and maintenance are important means to enhance fatigue reliability of engineering structures, but they can only be achieved efficiently with the aid of accurate pre-diction of fatigue crack initiation and growth until fracture. The influence of crack initiation on fatigue life has received a significant amount of attention in the literature, although its im-pact on the inspection plan is not generally addressed. Current practice in the prediction of fatigue life is the use of S-N models at the design stage and Fracture Mechanics (FM) models in service. On the one hand, S-N models are relatively easy to apply given that they directly relate fatigue stress amplitude to number of cycles of failure, however, they are difficult to extrapolate outside the test conditions employed to define the S-N curves. On the other hand, FM models like the Paris propagation law give measurable fatigue damage accumulation in terms of crack growth and have some ability to extrapolate results outside the test conditions, but they can only be a total fatigue life model if the initial crack size was known given that they do not address the crack initiation period. Furthermore, FM models generally introduce large uncertainties in parameters that are often difficult to measure such as initial crack size, crack growth rate, threshold value for stress intensity factor range, etc. This paper proposes a modified FM model that predicts the time to failure allowing for crack initiation period. The main novelty of the modified FM model is the calibration using S-N data (i.e., inclusive of crack initiation period) for an established criterion in fatigue life and reliability level. Sources of uncertainty associated to the model are quantified in probabilistic terms. The modified FM model can then be applied to reliability-based inspection planning. An illustrative example is performed on a typical detail of ship structure, where the optimum inspection plan derived from the proposed model is compared to recommendations by existing FM models. Results demonstrate to what extent is the optimum inspection plan influenced by the crack initiation period. The modified model is shown to be a reliable tool for both fatigue design and fatigue management of inspection and maintenance intervals. 
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  • Publication
    Methodologies for Crack Initiation in Welded Joints Applied to Inspection Planning
    (World Academy of Science, Engineering and Technology, 2016-11-08) ; ;
    Over the past decades, crack propagation has been extensively studied by researchers around the word. The approach based on crack propagation models have been widely used in inspection planning. This approach has the advantage that it gives measurable fatigue damage accumulation in terms of crack propagation with time and thus crack propagation models can be updated with inspection results. However, a prerequisite for using crack propagation models in inspection planning is that parameters such as initial crack size, crack growth rate, geometry function, etc. are known.  Among those parameters, initial crack size, depending on welding quality, material and the environment, is associated with the most uncertainties because of sampling and measuring problems. Another prerequisite for using crack propagation models in inspection planning is that crack initiation period can be assumed to be negligible. Both prerequisites are challenged nowadays as manufacturing and welding techniques have been improved. Some high-quality welded joins have been proven free from detectable size of flaws and the crack initiation period can account for a large part of the whole fatigue life. This gives rise to big difficulty for inspection planning of high-quality welded joins, as there is no generally acceptable approach for modelling the whole fatigue process that includes crack initiation period. Compared to as-welded joints, reliable inspection planning is more crucial for high-quality welded joins, as they are generally designed to withstand a larger stress range. In addition, they may have shorter time for inspection as crack initiation time account for a large part of fatigue life, with a shorter crack propagation period to failure due to higher stress range. To address this problem for high-quality welded joints, a robust model accounting for the whole fatigue process needs to be developed. The core issue is how the crack initiation period can be modelled and added to the crack propagation time. To help identify this issue, this paper reviews treatment methods for crack initiation period and initial crack size in crack propagation models applied to inspection planning. Generally there are four approaches, by: 1) Neglecting the crack initiation period and fitting a probabilistic distribution for initial crack size based on statistical data, e.g. Weibull distribution or lognormal distribution; 2) Extrapolating the crack propagation stage to a very small fictitious initial crack size, so that the whole fatigue process can be modelled by crack propagation models; 3) Assuming a fixed detectable initial crack size and fitting a probabilistic distribution for crack initiation time based on specimen tests; 4) Modelling the crack initiation and propagation stage separately using small crack growth theories and Paris law or similar models. Conclusion is that in view of trade-off between accuracy and computation efforts, calibration of a small fictitious initial crack size to S-N curves is the most efficient approach.
      485
  • Publication
    Methodologies for Crack Initiation in Welded Joints Applied to Inspection Planning
    (World Academy of Science, Engineering and Technology, 2016-11) ; ;
    Crack initiation and propagation threatens structural integrity of welded joints and normally inspections are assigned based on crack propagation models. However, the approach based on crack propagation models may not be applicable for some high-quality welded joints, because the initial flaws in them may be so small that it may take long time for the flaws to develop into a detectable size. This raises a concern regarding the inspection planning of high-quality welded joins, as there is no generally acceptable approach for modeling the whole fatigue process that includes the crack initiation period. In order to address the issue, this paper reviews treatment methods for crack initiation period and initial crack size in crack propagation models applied to inspection planning. Generally, there are four approaches, by: 1) Neglecting the crack initiation period and fitting a probabilistic distribution for initial crack size based on statistical data; 2) Extrapolating the crack propagation stage to a very small fictitious initial crack size, so that the whole fatigue process can be modeled by crack propagation models; 3) Assuming a fixed detectable initial crack size and fitting a probabilistic distribution for crack initiation time based on specimen tests; and, 4) Modeling the crack initiation and propagation stage separately using small crack growth theories and Paris law or similar models. The conclusion is that in view of trade-off between accuracy and computation efforts, calibration of a small fictitious initial crack size to S-N curves is the most efficient approach.
      317