Research presentation abstracts
13 July 2021
Mr Danie Louw
Fracture prediction of support structure mimicking notched tensile samples
Localized heating and cooling cycles that a component experiences while being produced by Laser Powder Bed Fusion causes stress and strain which may cause the component or it’s support structures to fracture. The stress and strain in a component being produced with Laser Powder Bed Fusion can be effectively modelled with finite element software by the inherent strain method, but to be able to predict and therefore avoid in-situ fracture the modelled stress and strain must be used with a relevant fracture criterium. The work presented here tests the accuracy of various criteria for predicting fracture of a support structure like geometry.
15 June 2021
Mr Llewellyn Cupido
Mass-space equivalence: a reduction approach to a fundamental understanding of matter
Mass can [informally] be defined by what is (i.e., the matter, which means the density, 𝜌) that is confined within an 𝑛th −dimensional space (ℝ𝑛). If (𝑛=3), it is known as a volume (𝑉), i.e., ℝ3≡𝑉 (⇒𝑚=𝜌𝑉). Thus an intrinsic property of an object. A photon, a type of elementary particle and the most fundamental entity of energy, is massless. However, it transfers energy to other subatomic particles (with mass), conserving energy, but not mass. The current understanding is that photons have momentum (𝑚𝑣≠𝑝=ℎ𝑓/𝑐) which is a function of frequency (𝑓, [ 𝐬−𝟏].) and speed (𝑐, [ 𝐦.𝐬−𝟏].) ) as it propagates through space. The proportionality constant (ℎ – Planck’s constant, [ 𝐉.𝐬−𝟏].) is necessary to convert to units of energy [ 𝐉 ]. The problem is that base SI units of [ 𝐉 ] is [ 𝐤𝐠.𝐦𝟐.𝐬−𝟐 ], i.e. mass is being enforced “subtlety” upon a particle through its interaction with a proton. So it begs the question, what is mass? The hypothesis is that mass and space (or volume) is equivalent and that through an abstract reduction, an actual proof can be proposed, which in itself does not violate the conservation laws and is tangible to common sense.
25 May 2021
Mr Devan Atkinson
Selecting the optimal subset size for Digital Image Correlation
Digital Image Correlation (DIC), capable of determining the displacements and deformations experienced by a specimen from images captured of it, has seen widespread use in experimental solid mechanics as an optical metrology technique. However, the DIC method is complicated, being composed of several intricate elements, and as such the displacements results are highly sensitive to many factors. In particular, assuming that an image set of satisfactory quality has been obtained, the choice of processing parameters, particularly subset size and shape function order, during the correlation process greatly influence the certainty of the displacement results computed. The goal of this project is to develop a framework to determine the optical subset size and shape function parameters such that the obtained displacement results are reliable and accurate, obtained in a timely manner (avoiding the need to find the best parameters through trial-and-error) and independent of the user. To achieve this the first phase of the project involved creating a highly modular DIC framework for 2D and stereo DIC. In addition to these frameworks being simple, and well detailed in order to serve as an educational tool to newcomers to the field of DIC who wish to obtain a working understanding of the process, these frameworks are modular in that they allow the subset size, subset shape and shape function order to be assigned on a per subset basis allowing easy integration with adaptive strategies (which attempt to assign the most appropriate processing parameters, such as the subset size, for the DIC analysis under consideration). The second phase of this project aims to develop an adaptive strategy to select the most appropriate subset size for each subset based on the light intensity pattern.
25 May 2021
Ms Marli Heynemann
Functionally Stiff Titanium Lattice Structure for Bone Reconstruction
Bone reconstruction is a complicated orthopaedic procedure which describes the surgical reconstruction of limbs that show signs of severe skeletal trauma. Current reconstructive solutions include autograft, allograft and endoprosthesis. This study focuses on endoprosthesis fabricated from Ti-6Al-4V used for reconstruction of segmental bone loss in the lower limb, specifically the femur bone. A common cause of implant failure is bone resorption around the implant due to stress shielding. This is caused by the significantly higher stiffness of the metallic implant compared to stiffness of the bone in which it is fixated. Moreover, the incompatibility of the directional stiffness of the implant and the bone also causes stress shielding. The aim is to prevent implant failure due to stress shielding by mimicking the anisotropic stiffness of bone. This is explored by using an analytical model of a body centred cuboid unit cell to design a Ti-6Al-4V lattice structure with specific stiffness values in the longitudinal and transverse directions.
11 May 2021
Mr Gerrit Ter Haar
PhD defense ‘mock’ presentation
Laser powder bed fusion produced Ti-6Al-4V: microstructural transformations and changes in deformation behaviour through thermal treatments
The research investigates three temperature regions for thermal treatments post-fabrication and one in-situ approach. Thermal treatments between 750 – 960 °C are used to develop insight into the microstructure’s large-scale morphological transformation. Thermal treatments at 960 °C achieve fragmentation and grain globularisation. This is followed by quenching to attain a superior bi-modal microstructure. Thermal treatments at temperatures below 650 °C are used to develop insight into the initial stages of martensite decomposition phase transformation and material stress relief. Microhardness and tensile properties revealed material embrittlement. Fine precipitates in the microstructure were identified using high-resolution transmission electron microscopy. Based on these findings, two theories for the cause of material embrittlement based on two possible transformation routes of the initial stages of martensite decomposition are proposed. Mechanical properties of laser powder bed fusion produced Ti-6Al-4V depend on build orientation. Although the unique columnar-shaped and textured prior-β microstructure is identified as a probable cause of deformation anisotropy, limited insight into the cause of anisotropic deformation exists. Thermal treatments above 975 °C are used to globularise the columnar prior-β grain morphology. Microstructural anisotropy between two orientations is quantified using electron backscatter diffraction maps, and the influence of microstructure on deformation and crack initiation is studied. Insight into the deformation behaviour and the identified relation between prior-β crystallographic texture and α-lath morphological texture is used to formulate a theory of the probable cause of material deformation anisotropy. The study lastly investigates a novel approach to thermal treatments by using high-energy process parameters to achieve in-situ heating. An iterative approach for part optimisation using non-default process parameters is undertaken. While findings indicate that an improved microstructural and residual stress state can be achieved, detrimental effects of part oxidation and part-edge bulging are also observed.
6 July 2020
Dr Melody Van Rooyen
Creep damage assessment of ex-service 12 % Cr power plant steel using digital image correlation
Deterioration assessment of materials is essential to the continued effective operation of critical components in thermal power plants, especially given the current power generation challenges faced by South African plant operators. One of the main tasks of thermal plant maintenance engineers is establishing the degree of damage experienced by in-service components, such as steam piping, that operate at high temperatures (> 550 °C) and high loads (steam pressures > 15 MPa). Long term service exposure results in a deteriorative material phenomenon known as creep. Within progressive inspection philosophies, traditional laboratory-based creep testing is often difficult to conduct on ex-service steel due to the limited material availability from which to machine standard specimen geometries [1]. This work investigates the measurement of short-term creep strain curves at several stresses and at 600 °C from a single test through the use of a non-traditional specimen geometry together with full-field strain measurement using digital image correlation (DIC) – imaging technology used for strain measurement. Of particular interest is ex-service X20CrMoV12-1 (X20) which is currently widely in use in older South African power plants. These curves serve as input data to an Oruganti continuum damage mechanics model whereby microstructural-specific damage parameters can be extracted [2].
It is shown that the developed DIC technique allows multiple damage parameter extraction through continuum damage mechanics whilst preserving material economy of service-retrieved X20. These damage parameters give a comparative indication of the microstructural degradation with structural evidence of this obtained directly through scanning electron microscopy observations of micro-grains and precipitates. As corroborated by previous studies [1], subgrain growth is particularly a good indicator of damage in comparison to more traditional methods such as surface replication and void counting currently employed by local power utilities.
Such an approach is ideal for life monitoring of critical power station components when used in parallel with traditional techniques. Assessing the degree of creep exhaustion of power engineering alloys guides maintenance strategies for repair and replacement schedules to ensure reliable plant operation without the need for unplanned shut-downs due to breakdowns.
REFERENCES
[1] van Rooyen, M., Becker, T.H., Westraadt, J.E., & Marx, G. (2019). Creep damage assessment of
ex-service 12 % Cr power plant steel using digital image correlation and quantitative
microstructural evaluation, Materials, 12(19). Available: https://www.mdpi.com/1996-
1944/12/19/3106 [2020, March 11].
[2] Oruganti, R., Karaadge, M., & Swaminathan, S. Damage mechanics-based creep model for 9-
10%Cr ferritic steels, Acta Materialia, 59(5), 2145-2155.
6 July 2020
Mr Gerrit Ter Haar
The influence of microstructural texture and prior beta grain recrystallisation on the deformation behaviour of LPBF produced Ti-6Al-4V
Mechanical properties of laser powder bed fusion produced Ti-6Al-4V components have been shown to be anisotropic with respect to the build orientation. The control and predictability of mechanical properties during manufacturing is important in achieving parts that consistently achieve adequate tensile properties. While studies have identified the columnar prior-beta grain structure as a key cause in mechanical anisotropy, little is understood of the underlying microstructural differences with respect to the build direction and how this controls anisotropic deformation behaviour on a grain-level. Little-to-no studies have demonstrated effective post-process approaches to improve anisotropy. This study investigates microstructural texture differences in two build orientations before and after post-process heat treatments and in what way the microstructure controls deformation-and-failure behaviour. The study applies uni-axial tensile tests to samples built “vertically” and “horizontally” with reference to the build plate. Microscopy techniques of scanning electron microscopy imaging and back scatter diffraction are used for microstructure characterisation and deformation mode identification. Results identify key crystallographic and morphological textural differences in the two build-orientations. Heat treatments above the β-transus successfully globularise prior beta grains thereby improving microstructural and mechanical anisotropy. The use of electron backscatter diffraction demonstrates key morphological textural features that control slip, microcrack initiation and final fracture
25 June 2020
Mr Devon Atkinson
A 117 Line 2D Digital Image Correlation Code Written in MATLAB
Digital Image Correlation (DIC) has become a popular tool in the field of experimental solid mechanics to measure the displacement and deformation experienced by specimens as they are loaded. Although there are several publications which explain DIC in its entirety while still catering to newcomers to the concept, these publications neglect to discuss how the theory presented is implemented in practice. This gap in literature, which this paper aims to address, makes it difficult to gain a working knowledge of DIC which is necessary in order to contribute towards its development. The paper attempts to address this by presenting the theory of a 2D, subset based DIC framework that is predominantly consistent with state-of-the-art techniques, and discussing its implementation as a modular MATLAB code. The correlation aspect of this code is validated showing that it performs on par with well-established DIC algorithms and thus is sufficiently reliable for use in the field of experimental solid mechanics. This paper therefore serves as an educational resource to bridge the gap between the theory of DIC and its practical implementation. Furthermore; although the code is designed as an educational resource, its validation combined with its modularity makes it attractive as a starting point to develop the capabilities of DIC
25 June 2020
Prof Thorsten Becker
Near-threshold fatigue crack growth rates of Laser Powder Bed Fusion produced Ti-6Al-4V
When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependent on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependent crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependent on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V
21 October 2019
Mr Nic Macallister
Small scale testing for SLM Ti6Al4V (ELI) component certification
With the SLM methodology having gained traction in the manufacturing sector, printing titanium alloy components for use in high performance biomedical and aerospace applications is now feasible. Certification of SLM components is a necessary step for safe and further implementation, however, can be inefficient and costly. Using sub-standard test specimens and non-contact testing techniques such as DIC to determine tensile, fracture and fatigue performance characteristics is a potential solution. This research aims to address each of: tensile, fracture toughness and fatigue property determination using small scale testing, with further attention given to the influence of the as-built surface finish and number of prior-β grains in specimen cross section. Furthermore, incorporation of predictive FEM (Finite element modelling) is presented as a possible value adding extension to the research planned. Discussion on current standpoint and future planning for each of the property acquisitions will be presented.
21 October 2019
Mr Ben Parker
Blending of Powders for
Ti6Al4V Metal Additive Manufacturing
Powder bed fusion (PBF) is a metal additive manufacturing (MAM) technique in which three dimensional components are produced through selectively laser melting in a layer-by-layer fashion. MAM techniques allow for the production of high-quality components with the benefits of design complexity and minimal material wastage, as opposed to traditional subtractive manufacturing methods. PBF typically utilises pre-alloyed (PA) Ti-6Al-4V powder, which is predominantly only suitable for the aerospace and biomedical industries given its high cost point. In-situ alloying of elemental powders in PBF would reduce the powder production costs. This will: enable the rapid development of new alloys with the addition of various alloying elements in variable ratios; allow for the production of larger scale components; and enable MAM techniques to break into more commonplace industries. Thorough powder characterisation is required to ensure the suitability of new elemental powder blends for MAM and is crucial to produce components of high quality.
30 September 2019
Mr Llewellyn Cupido
Spinodal decomposition and nucleation in thermally exposed Duplex Stainless Steel
Solid-solid transformation are a diffusional driven process and is basically subdivided into the non-classical spinodal decomposition diffusion, and the classical nucleation and growth. Spinodal decomposition will start spontaneously when the ideal thermodynamic and kinetic conditions are achieved and compositionally evolve through sinusoidal wave. This process causes a phase separation in the matrix that forms a solute-rich and solute-poor phase but maintaining the initial crystal structure. Nucleation, also known as precipitation during thermal aging, forms a completely new phase. During a recent study, it was observed that there seems to be a dependence of nucleation on spinodal decomposition, but literature it is often considered as if not related. This is due to limitations in the approach of existing models in predicting both phenomena independently. This leads to inadequate predictions that overestimate mechanical properties and fail to describe the actual phenomena.
05 September 2019
Mr Raymond Botete
Simulation of Microstructure for Titanium Alloys Formed In-situ Using Selective Laser Melting
Selective laser melting (SLM) is a powder-bed based Additive Manufacturing (AM) technique that uses a laser power to melt powder materials in a layer-wise fashion to build functional parts directly based on computer-aided design (CAD) data. This technique has become increasingly popular over the past decades for the manufacture of metallic parts. The widespread interest is largely stimulated by a wide range of advantages offered in comparison to other traditional manufacturing methods. These advantages lie in its ability to achieve highly complex and customisable parts at a high material use efficiency. However, this process is limited by the unavailability of suitable pre-alloyed powders as starting material. This has prompted the need to discover new powder feedstock compositions more suitable for SLM applications. The best approach to achieve this is by in-situ alloying within SLM using powder feedstock made from commercially available heterogeneous powders. However, the microstructural inhomogeneity becomes a huge concern with defects such as porosity and segregation. Therefore, this study aims to develop a numerical model that can simulate and predict the microstructure of titanium alloys formed in-situ using SLM. The main objectives include: (i) investigation powder mixing procedures that will produce powder feedstock suitable for SLM processing, (ii) develop a numerical model to predict microstructure by simulation melting, solidification as well as evaporation during SL. The microstructural features of interest are limited to phase fraction, grain size and structure as well as porosity. Ti-6Al-4V alloy would be used as the baseline for the investigations.
12 August 2019
Mr Georgino Tshikwand
Deformation Mechanisms of Laser Powder Bed Fusion Ti6Al4V Diamond and Octet Truss Lattice Structures
Lattice structures with customized stiffness, strength, and specific strain energy absorption allow for the design of light-weight, loadbearing structures, suitable for functional engineering applications. In this work, we studied the deformation mechanisms of two well-known lattice structure designs produced by laser powder bed fusion (L-PBF) of Ti6Al4V. Two different finite element analysis (FEA) approaches were used to simulate deformation under compression: one using 1D beam elements, the other using tetrahedral 3D solid elements. The results were compared to physical compression tests of L-PBF lattice structures. The octet-truss structure was found to deform by a combination of 45° and 135° shear bands and compressive buckling of struts whereas the diamond lattice structure was found to deform by 45° high shear bands.
25 July 2019
Mr Danie Louw
Cracking of parts during Laser Powder Bed Fusion
During Laser Powder Bed Fusion, a component is formed by repeatedly consolidating thin layer of powder on top of one another to represent a cross section of the part being formed. The shrinking of each layer imposes stress on the underlying part. In the absence of restraint, the part may warp, but even if there is restraint the part may crack and de-laminate. The conditions leading to cracking is not well understood and the current project aims to define criteria that may be used to accurately predict cracks
01 July 2019
Mr Devan Atkinson
Digital Image Correlation: Practical implementation of a 2D Digital Image Correlation framework as a MATLAB code
Digital image correlation (DIC) has become a widely adopted technique in experimental solid mechanics for measuring full-field displacements and strains experienced by a specimen. Consequently there are many publications which give a comprehensive breakdown of the mathematical theory of the DIC method; however, these publications do not discuss in detail how the DIC method, that is presented, is implemented in practice as a code. This is detrimental for newcomers to the concept of DIC because the steep learning curve of DIC makes it difficult for these newcomers to obtain a working knowledge of DIC from purely theoretical resources. This paper aims to bridge this gap between the mathematical theory of DIC and its practical implementation by presenting the mathematical theory of a 2D, local DIC framework, that is predominantly consistent with the current stateof-the-art practices, and explaining how this DIC framework is implemented as a practical MATLAB code. This DIC code is then validated by using the SEM 2D DIC Challenge image sets in order to test its capabilities. The results show that the proposed DIC framework, and subsequent code, determines displacements and strains with sufficient accuracy and precision to be considered reliable enough for use in experimental solid mechanics applications. This paper is to be published in the hope that providing a resource specifically aimed at newcomers to the concept of DIC will encourage these newcomers to get involved in the DIC community by using this code as a starting point to develop their own code.
10 June 2019
Mr. Jacques Piek
Comparison of slurry formulations for gel-casting of titanium
There is very little experimental data published on gel-casting for metals using low-toxic systems. Two binder systems for titanium gel-casting are investigated as alternatives to the HEMA/MBAM system in order to improve the gel-casting process for titanium. Methacrylamide (MAM) is chosen as the main monomer of the first system and the second binder system uses Isobam® as main constituent. The performance of the two binder systems are studied by evaluating the slurry formulation, mixing process, quality and ease of the gel-casting process and the microstructure and properties of the final gel-cast part. The best slurry formulation for each binder system is determined by a rheology study.
10 June 2019
Mr Preyin Govender
Sinter-infiltration of Ti6Al4V
This research investigates the processing and characterization of sintered Ti-6Al4V powder compacts, infiltrated with aluminium. Two powder blends were mixed; namely, commercially pure titanium mixed with a 60Al-40V master alloy, and an elemental blend of titanium, aluminium (6 wt%) and vanadium (4 wt%). These powders were die compacted at 400 MPa and sintered under high vacuum for 2 hours at 1100 °C and 1200 °C, respectively. Infiltration of the sintered samples was attempted by placing aluminium powder compacts on top of the samples while heating them to 700°C under a nitrogen atmosphere. The results of these experiments shall be presented along with any obstacles and future work that needs to be completed
29 May 2019
Mr Nur Dhansay
The fatigue crack growth rate threshold of Selective Laser Melting Ti6Al4V
Fatigue loading is the most common cause of structural failure in a part. Selective laser melting (SLM) has become a popular research topic over past decades, due to its manufacturing abilities, with specific interest gained in the aerospace and biomedical industries in which parts used in these industries typically undergo fatigue loading and failure. A drawback of the SLM process is the large residual stresses, martensitic microstructure and porosities produced, which has shown to negatively affect the fatigue behaviour of the material. These mechanisms vary in magnitude, depending on the printing process parameters, producing a range of fatigue behaviour which is found in literature. Thus, demonstrating that a better understanding of these mechanisms is required on fatigue behaviour for the confidence and qualification of SLM parts in industry. In literature a large focus on the fatigue life approach has been given and similarly, but to a lesser extent, the fatigue crack growth rate (Paris regime). However, there is a lack of research concerning the fatigue crack growth rate threshold. This research focuses on the fatigue crack growth rate threshold of SLM Ti-6Al-4V, in three build orientations, using the load-shedding technique, to better understand the effects of residual stress and microstructure on crack growth. The effect of residual stress and microstructure on the fatigue crack growth is shown and insight of these mechanisms on fatigue crack growth rate threshold are discussed.
29 May 2019
Mr Gerrit Ter Haar
Selective laser melting produced
Ti6Al4V: Influence of low temperature thermal treatment on stress relief and martensite decomposition phase transformation
There is no doubt that metal additive manufacturing (MAM) has revolutionized modern manufacturing in the 21st century. Through bringing innovative capabilities to structural design and the democratization of manufacturing, MAM processes such as selective laser melting (SLM) have captured the attention of both the aerospace and biomedical industries. Concern over achievable material quality, however hampers the acceptance of this technology by industry. For example, inherent to the SLM process is high residuals stress and an unfavorable brittle microstructure. This study complements the on-going research efforts into SLM-produced Ti6Al4V to established post-process heat treatments to relieve residual stress and improve part ductility. Current research understanding of SLM-produced TI6Al4V material change below 600 °C is limited. Interest into low temperature annealing is stimulated by its lower energy consumption and the recent implementation of in-situ base-plate pre-heating. This work aims to develop a deeper understanding of the mechanisms of material change at low temperatures and apply this knowledge to improving part quality. Experimental heat treatments of SLMproduced Ti6Al4V samples were carried out varying temperatures and hold times below 600 °C. Analysis of material change was performed through hardness measurements, electron microscopy and x-ray diffraction. Tensile tests were used to determine mechanical properties. Key results of this study are: the development of methodologies to monitor the progress of the initial stages of martensite decomposition. A significant increase in rate of residual stress relief from ~480 °C to ~580 °C. The identification of a non-classical precipitation hardening through the nucleation of fine β particles and the subsequent strengthening observed. The work demonstrates the rate of residual stress relief at low temperatures, the initially stages of martensite decomposition and its influence on material properties.
22 May 2019
Prof Thorsten Becker
The structural integrity of Additively Manufactured Ti6Al4V: Materials, process and build attributes linked to part life predictions
In Additive Manufacturing (AM) of metallic parts, where applications are often directed toward biomedical and aerospace applications, part integrity has become a major research focus. Inherent to the AM process are high residual stresses, porosity, and a unique microstructure. These have been shown to depend on the choice of built parameters and the material’s underlying properties. Of concern is the influence of the aforementioned on the part’s structural integrity. This work explores the current understanding of the material, process and build attributes of AM manufactured Ti6Al4V parts. Particularly, the work focuses on fatigue life predictions by exploring the initiation, propagation and fast fracture of AM produced Ti6Al4V test samples and how these correlates to the material, process and build attributes. An attempt is made to parameterize the influence of residual stress, porosity, and microstructure on fatigue life predictions. The work, for the first time, links process and part attributes to form an understanding of the AM process and its implication on fatigue life in AM produced Ti6Al4V parts.
08 April 2019
Ms Melody van Rooyen
Creep Damage Assessment Of Ex-Service 12 %Cr Power Plant Steel Using Digital Image Correlation and Quantitative Microstructural Evaluation
With the very recent rotational load shedding implemented by Eskom that all South Africans experienced, the nightmare of slowed economic growth and productivity impediments for communities, universities and industries have shown their teeth in a similar manner to events in 2015. Four years later and the current aging fleet of coal-fired power stations are still experiencing plant shutdowns due to the ongoing failure of critical components, including boiler tubes and main steam piping. This has revitalised research pertaining to the life monitoring of such components which are subjected to high temperatures and loads for an extended time period. Material damage manifests as a result of a phenomenon known as creep that can result in component failure, plant downtime and more load shedding if not properly monitored. Traditionally, such creep damage is investigated by Eskom maintenance engineers using metallographic replication and void (cavities formed during creep) counting. When additional characterisation is needed, lengthy and expensive conventional creep tests (lasting in excess of 10 000 hours) are conducted to obtain a single creep strain over time which is not suitable to the immediate repair/replacement decisions required during shutdowns as South African industries stand inoperable with no power. This paper aims to discuss alternative means of damage characterisation of a 12% Cr power plant steel known as X20CrMoV12-1 (X20 hereafter) using Digital Image Correlation (DIC hereafter) techniques as well as advanced microstructural characterisation. Multiple creep curves are obtained in an accelerated fashion using the full-field abilities of DIC on an X20 specimen subjected to a non-uniform temperature field in a thermomechanical simulator. When this type of analysis is applied to ex-service X20 of varying degrees of aging, a clear difference between the corresponding damage levels is noted, as also confirmed by several microstructural degradation indicators associated with creep.
18 March 2019
Dr Mahmoud Mostafavi
Department of Mechanical Engineering University of Bristol
Effect of plasticity on creep deformation
The life of nuclear power stations in the UK are threatened by two issues: cracking in the core nuclear graphite and failure of boilers. The Advanced Gas Cooled Reactors in the UK are the only high temperature reactors (nominally 525oC) in the world which have been operated long term in the creep regime. Since the boilers are located within the concrete pressure vessel, they cannot be changed or easily repaired. Therefore, the creep failure of the boilers end the life of the reactor. The shut down and start-ups of the reactor, which can be up to 300 cycles during the lifetime of the reactor, can induce plastic deformation in the boiler which can accelerate or impede the creep deformation. However, so far, the effects of this prior loading has been neglected in the creep life calculations. This work is aimed to underpin developing the current assessment methodologies further so the effects of prior plasticity is included in the creep life calculations. How: The creep life of high-temperature steels can be significantly affected by the prior monotonic and cyclic plastic loadings. This effect is partly due to the generation of intergranular strains from the grain-scale elastic and plastic anisotropic deformation during plastic loading. In this work the effect of these plasticity generated intergranular strains on the subsequent creep strain accumulation behaviour in stainless steel is investigated. An in- situ synchrotron diffraction experiment was conducted at 550oC, in which the sample was loaded incrementally to different level of plasticity, followed by a displacement-controlled creep dwell at each stage. The lattice strains of 4 grain families were measured during the loading and the creep dwells. It was found that the intergranular strains generated during the plastic deformation significantly affect the relative magnitude of creep strain accumulation in different grain families. A subtle but significant difference has been observed between the creep intergranular strain accumulation behaviour and the plastic intergranular strain accumulation behaviour in different grain families which can be used to validate a micromechanical model developed for creep and plastic deformation. The micro mechanical strains accumulation and relaxations measured from the experiment were compared with the prediction of the novel crystal plasticity finite element micromechanical model. A good overall match was found between the experiment and the model regarding the magnitude of stress relaxation after various level of plasticity. The validated crystal plasticity model is a powerful tool to predict creep deformation behaviour of engineering alloys after various complex prior plastic deformation.