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Bonetti, Rossella (2025) Development of in-service monitoring based models for remaining life management of thermally loaded ageing structures. PhD thesis, University of Nottingham.

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Abstract

The primary aim of this thesis is to propose and develop a novel and practical life assessment methodology for power plant applications that uses more proactively both periodic and routine inspection data in conjunction with online operational data. This methodology facilitates the computation of the remaining life of high temperature components and essentially links the plant data routinely collected during outage inspections, via online monitoring or by testing of material samples to the life prediction models. High temperature pipework will be used as an example of the proposed approach and deployable creep life prediction models are reviewed that could function with these plant datasets. Of critical importance is that the output from the life prediction is used to inform plant operations on any necessary changes in order to mitigate damage accumulation.

A holistic empirical lifing approach, which accommodates the on-site information of replica, hardness and strain, is established. The approach is based on an extensive review and analysis of a large amount of outage inspection data on ageing high temperature parent ½Cr½Mo¼V (CrMoV) material and has been used to illustrate how such routinely collected inspection data can be better utilised to provide the plant operator with predictions of residual creep life. The model differentiates between long term and persistent thermal softening behaviour revealed by change in hardness over time and short-term creep cavitation that accelerates material damage. Importantly, the models developed are designed to be used iteratively with surface replica and hardness data available from an outage inspection. The study shows that the availability of more data will enable further refinements, but more importantly, it emphasises the importance of systematically capturing these data and processing them at the time of inspection to forecast residual life and then updating and tuning the model periodically at future inspections. The capture of strain data from pipe diametral measurements is also a routine outage activity and this data is included in a case study to demonstrate the capabilities in the residual life forecast by the new methods.

An integrated life assessment procedure for structures operating under thermomechanical loading is also developed. The methodology uses a viscoplasticity based framework combined with the R5 life assessment code. The viscoplastic

constitutive model used for the stress-strain analysis is derived from the Chaboche-Lemaitre formulation from which can be directly obtained the required parameters for the R5 assessment as stress relaxation per cycle and the elastic follow-up factor. The R5 procedure is therefore significantly simplified. The proposed life assessment procedure is demonstrated on a martensitic steel (FV566) industrial gas turbine rotor under a typical start up – shut down operation. The effect of creep-fatigue interaction at different locations within the rotor structure is assessed and the remaining life at each location is calculated. A sensitivity study is performed at half load, which shows an increase in lifetime of the rotor compared to the full load condition.

The developed lifing model techniques are backed up with a physical explanation by investigating the softening behaviour of a P91 steel due to its microstructural changes under creep testing. The creep degradation behaviour of ex-service-exposed P91 steel is investigated during interrupted creep tests at 660°C and 80 MPa using a number of material characterization techniques including TEM, SEM, EBSD, EDS and optical microscopy to identify the microstructural evolution and the associated deformation mechanisms during creep at temperature. Microhardness has also been measured in order to evaluate the softening mechanism. Under the creep conditions examined, microstructural degradation is found to be governed by the disappearance of the lath sub-structure, lath widening and recrystallization, dislocation density reduction, coarsening of M23C6 and creep cavitation while MX and Laves phases are stable. Hardness evolution, extrapolated from hardness data obtained from uniaxial creep tests, is used to characterize the softening of the material. On this basis, hardness decrease is justified in term of the aforementioned microstructural changes. Implications of the findings for specific in-service life management in thermal plant piping systems are finally addressed.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Shipway, P.H. Sun, Wei
Keywords:	Power plants; Component life; Inspection data; Assessment methodology; Microstructural changes; Lifing model techniques
Subjects:	T Technology > TJ Mechanical engineering and machinery
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID:	80535
Depositing User:	Bonetti, Rossella
Date Deposited:	29 Jul 2025 04:40
Last Modified:	29 Jul 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/80535

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