Horizon 2020 Marie Skłodowska-Curie Innovative Training Network

Guang Zou

BSc, MSc
Home/Guang Zou
Guang Zou 2017-09-18T11:46:17+00:00
Early Stage Researcher
Lloyd’s Register EMEA (United Kingdom)

Project 5: Integrity management of ship structures

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Research Interests:

Marine and subsea engineering; Finite element analysis; Physical model testing; Probabilistic modelling and reliability assessment; Health monitoring; Fatigue and fracture; Shell buckling; Composite mechanics; Fluid-structure interaction; Ice load and modelling; Structural design and construction

Biography:

He obtained his BSc in Naval Architecture and Marine Engineering from Wuhan University of Technology, and MSc in Mechanics of Ship and Marine Structures from China Ship Research & Development Academy. His MSc project applied toroidal shells in subsea pressure structures. He developed a simplified mechanics model for stiffened toroidals, and defended his MSc thesis with distinction.

He worked as a research engineer at China Ship Scientific Research Centre for 3.5 years. Further to his MSc project, he investigated stiffened toroidals analytically and experimentally in a NSFC-funded project, in which he was deputy project manager. He also engaged in other national projects about development of underwater vehicles and stations, where he developed his capabilities in numerical simulation, scaled-model testing, engineering design, field measurements and testing.

He joined Lloyd’s Register EMEA in Dec 2015 as research fellow for European TRUSS project, where he is developing probabilistic models for reliability of passenger ships.

Research Outputs:

  • Du Q.H., Zou G., Zhang B. W., Wan Z. Q. (2015), “Simplified theoretical solution of circular toroidal shell with ribs under uniform external pressure”, Thin-Walled Structures, 96 (11):49-55. DOI: http://dx.doi.org/10.1016/j.tws.2015.07.019
  • Zou G., Du Q.H., Zhang B.W. (2014), “Characteristic analysis of pressure toroidal structures on strength”, Ship Science and Technology, 36 (2):14-19.
  • Du Q.H., Bian R. G., Zou G. (2013), “Assembled toroidal structures”, Patent No. 102182207A.
  • Zou G., Peng X.N, Du Q.H. (2012), “Theoretical solution and essential research on stiffened toroidal shells”, Journal of Ship Mechanics, 16 (1):83-92.
Publications in TRUSS

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. -> External link to Publisher’s version

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. -> Link to full text in repository

A problem with fracture mechanics (FM) based fatigue analysis is that reliable information on initial crack/flaw size is often hard to obtain. Also, FM method can’t 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 curves, 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 curves and test crack evolution data. The suggested procedure is to derive the mean value of EIFS from S-N curves and the coefficient of variation from KT diagram.  -> Link to full text in repository

 Fatigue cracks pose threats to the integrity of welded structures and thus need to be addressed in the whole service lives of structures. In-service inspections are important means to decease the probability of failure due to uncertainties that cannot be accounted for in the design stage. To help schedule inspection actions, the decline curve of reliability index with time needs to be known. A predictive tool is normally developed based on crack propagation models neglecting the crack initiation stage, which leads to conservative predictions for fatigue life. Inspection plans built on those predictions are far from optimal, especially for welds with relatively long crack initiation life. This paper proposes to use a fracture mechanics based reliability analysis method that takes the crack initiation stage into account via the concept of Time-To-Crack-Initiation (TTCI). The optimum inspection plan for a fatigue prone ship structural component is derived by the new approach and compared to the commonly-used method that only considers crack propagation life. Two inspection planning approaches are tested to investigate the influence of incorporating crack initiation period: (i) target reliability approach and, (ii) equidistant inspection times approach. With each planning approach, two inspection methods are adopted: close visual and magnetic particle inspection. The paper concludes with recommendations on the inspection method and planning approach to adopt while considering and without considering the crack initiation stage. -> Link to full text in repository

DOI: https://doi.org/10.1201/9781315210469-340

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. -> Link to full text in repository