Thermal power plant steel creep deformation measurement using digital image correlation
Date
2020-12
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Following the rapid expansion of populations and industries with a concomitant rise in the demand for electricity in South Africa, increased focus has been directed to the structural health monitoring of critical components of aging thermal power plants. Long-term operation at high temperatures and loads encountered by plant components results in the degradation of material properties through creep exposure. Assessing creep damage using conventional creep testing poses several challenges in terms of large sampled material requirements for manufacturing standard specimen geometries. This is often not possible for service-retrieved material due to the limited availability for destructive sample removal. This dissertation documents a novel experimental technique that employs digital image correlation (DIC) to characterise the creep damage of service-exposed power plant steel through creep deformation measurement across nonuniform temperature and stress fields. Through the full-field property of DIC, multiple properties are measured from single specimens ofX20CrMoV12-1 (X20) piping steel supplied by Eskom in virgin and various stages of service exposure.
Initial development of the technique involves a stereo-DIC setup adapted for high temperature deformation measurement.A temperature profile ranging from 550–600°C is applied using resistive heating from a Gleeble thermo mechanical simulator and measured using infrared imaging.Near-uniform stress profiles result from appropriate specimen geometry and region of interest selection. Several temperature-dependent elastic moduli and Poisson’s ratios are measured from single specimens of virgin X20. Creating an extension to accelerated creep tests(shorter-term tests conducted at higher stresses and temperatures than encountered in service)used on virgin and various ex-service X20 states is explored.Creep curves at several temperatures are successfully realised from single specimens and are useful for identifying differences in creep resistance between ex-service levels in line with traditional damage classification through surface cavity density replication. Micro structural quantification using electron microscopy shows subgrain and M23C6precipitate growth as well as Laves phase precipitation in higher damage states of X20. For middle damage classes, it is highlighted that several complementary damage assessment methods are necessary to identify differences in material deterioration. Zener-Hollomon parameters calculated from DIC-measured strains over the temperature profiles serve as suitable damage indicators.
Single camera DIC measurement of creep curves across spatially varying stress profiles is achieved with a waisted specimen design in a traditional creep testing setup for medium-term tests. Virgin X20 creep curves at 140–150MPaare used to calibrate baseline parameters for a continuum damage mechanics model. Subsequently, the model is optimised for subgrain and precipitate parameters using the corresponding ex-service X20 creep responses. These micro structural-based parameters serve as comparative damage sensors that agree with traditional cavity and hardness-based methods. This dissertation reveals that a wealth of creep data can be extracted from fewer X20 specimens, which is highly beneficial in characterising material integrity in a supplementary manner to existing methods.Expansion to small samples through small punch creep testing is also initiated. Although beyond the scope of this work, future aspirations are that these approaches will be integrated into life management philosophies to better guide inspection and maintenance strategies.
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
Description
Thesis (PhD)--Stellenbosch University, 2020.
Keywords
Digital image correlation, Materials -- Creep, Power plant steel, Microstructure -- Characterisation, UCTD