Search

Background The use of the Taylor-Quinney coefficient for introducing a thermal dissipation term into material models relies on understanding its dependencies. These are usually determined through extensive experimentation, wherein temperature variations are monitored in a test piece during mechanical loading. Objective This study aims to reduce the cost and time necessary to determining the dependencies of the Taylor-Quinney coefficient by proposing a novel small specimen inverse testing method and demonstrating its use on aluminium alloy 7175. Methods The method proposed is based on mechanical testing of a novel small ring specimen in parallel with FEA simulations. In the experiments, small rings of 7175-T7351 aluminium alloy, 20 mm in outer diameter, were loaded between two pins for different pin displacement rates (namely 1, 1.5 and 2 mm/s) at room temperature and the local specimen temperature field was monitored using an infra-red thermal camera. Fully coupled thermal-mechanical simulations of the tests were performed using a range of Taylor-Quinney coefficients, and the resulting temperature evolutions compared to the experimental results in order to determine appropriate coefficient values for the material. Results The method presented shows good repeatability and allows for clear observation of thermal dissipation. Taylor-Quinney values ranging 0.51-0.59 are reported for the 7175 alloy, in line with values reported in the literature for similar alloys. Density, specific heat capacity and thermal conductivity, fundamental thermal material properties necessary for the simulations, are also reported for the alloy. Conclusions The method detailed shows promise for determining Taylor-Quinney coefficients in a wide range of experimental conditions and is proposed as a cheap and fast alternative to full-scale specimen testing of Taylor-Quinney coefficients. Taylor-Quinney values obtained for 7175 aluminium are shown to be much lower than the value of 0.9 often proposed for materials.

Small specimen mechanical testing is an exciting and rapidly developing field in which fundamental deformation behaviours can be observed from experiments performed on comparatively small amounts of material. These methods are particularly useful when there is limited source material to facilitate a sufficient number of standard specimen tests, if any at all. Such situations include the development of new materials or when performing routine maintenance/inspection studies of in-service components, requiring that material conditions are updated with service exposure. The potentially more challenging loading conditions and complex stress states experienced by small specimens, in comparison with standard specimen geometries, has led to a tendency for these methods to be used in ranking studies rather than for fundamental material parameter determination. Classifying a specimen as ‘small’ can be subjective, and in the present work the focus is to review testing methods that utilise specimens with characteristic dimensions of less than 50 mm. By doing this, observations made here will be relevant to industrial service monitoring problems, wherein small samples of material are extracted and tested from operational components in such a way that structural integrity is not compromised. Whilst recently the majority of small specimen test techniques development have focused on the determination of creep behaviour/properties as well as sub-size tensile testing, attention is given here to small specimen testing methods for determining specific tensile, fatigue, fracture and crack growth properties. These areas are currently underrepresented in published reviews. The suitability of specimens and methods is discussed here, along with associated advantages and disadvantages.

This paper explores how the requirement for energy storage capacity will grow as the penetration of renewables increases. The UK’s electric grid is used as a case study. The paper aims to provide insight on what is the most economical solution to decarbonize the electric supply. A two-dimensional study varying the penetrations of wind and solar PV is carried out to identify the most appropriate generation mix for the country. The study is based on 9 years of demand and generation data with a 1hr resolution. It discusses the risk of underestimating the storage capacity needed, by failing to capture the inter-annual variability of renewables and analyzes the economic trade-off between over-generation (curtailment) and storage capacity. It also aims to determine the percentage of over-generation that minimizes the total cost of electricity. Results suggest that the UK could need a storage capacity of approximately 43 TWh to decarbonize its electricity supply. This figure considers a generation mix of 84% wind +16% solar PV, a roundtrip storage efficiency of 70%, and 15% of curtailment. Based on current costs of bulk energy storage technologies, this storage capacity translates into an investment of ∼£165.3 billion or approximately 7% of the country’s GDP.

The wide scale use of small specimens in routine testing programs could significantly reduce material resource requirements (factors of 10 are easily achievable). This is a major benefit to situations where there is not enough material to manufacture conventional, full-size specimens, such as first-stage gas turbine blade roots. However, limitations exist due to concerns over size effects, manufacturing difficulties, uncertainties related to the application of representative loading conditions and complex interpretation procedures of non-standard data. Due to these limitations, small specimen testing techniques have been mostly applied in ranking exercises and to determine approximate or simple material parameters such as Young’s modulus, minimum creep strain rate and fracture toughness. The small ring method is a novel, high sensitivity small specimen technique for creep testing that has been extended in the present work to the determination of tensile material properties. The main advantages of the small ring specimen are that it is self-aligning and has a large equivalent gauge length in comparison to other small specimens, resulting in much higher testing sensitivity. In the present work, this specimen type mimics conventional, full-size, monotonic testing, allowing for observations of elastic and plastic material response to be made. Wrought aluminium alloy 7175-T7153 small rings were tested at room temperature at 5 different loading (displacement) rates and the results compared to conventional, full-size, monotonic specimen equivalents. Finite element analysis was conducted in order to evaluate the equivalent gauge section and equivalent gauge length in the small ring specimen (which varied between circa 0.35–1.4 mm2 and 25–45 mm, respectively) to facilitate these comparisons. An analytical solution has also been derived in order to validate the finite element analysis.

The first reported in-situ synchrotron X-ray diffraction experiments for the inertia friction welding process are presented. The evolution of the microstructure around the weld interface has been quantified throughout the process., The widespread use and development of inertia friction welding is currently restricted by an incomplete understanding of the deformation mechanisms and microstructure evolution during the process. Understanding phase transformations and lattice strains during inertia friction welding is essential for the development of robust numerical models capable of determining optimized process parameters and reducing the requirement for costly experimental trials. A unique compact rig has been designed and used in-situ with a high-speed synchrotron X-ray diffraction instrument to investigate the microstructure evolution during inertia friction welding of a high-carbon steel (BS1407). At the contact interface, the transformation from ferrite to austenite was captured in great detail, allowing for analysis of the phase fractions during the process. Measurement of the thermal response of the weld reveals that the transformation to austenite occurs 230 °C below the equilibrium start tem­per­ature of 725 °C. It is concluded that the localization of large strains around the contact interface produced as the specimens deform assists this non-equilibrium phase transformation.

Curved laminates in aero-structures, such as the L-angle sections where webs and flanges meet, are prone to delamination due to high interlaminar stresses in these regions. Some efforts to investigate delamination in these structures can be found in the literature but commonly structures are limited to unidirectional layups or modelling approaches are constrained to the cohesive element based methods. In this work, multi-directional L-angle laminates were manufactured using unidirectional prepregs and tested under four-point bending load conditions to examine the interface damage. Acoustic emission technique was used to assist the capture of damage initiation and propagation. Three interface modelling strategies for predicting delamination, namely cohesive element, cohesive surface and perfectly bonded interface were used in the numerical study. The interface damage behaviour was successfully predicted by the simulation methods and differences among the strategies were compared.

For critical component application, such as aerospace turbine rotors, it is imperative to be able to make accurate in-service material behaviour and component life predictions for both design and monitoring of component life. The development of such predictive capability is dependent on the quality of the experimental data from which the material parameters are derived. This paper shows the effect that scatter which may be present within experimental data, manifesting itself within the constitutive parameters derived from this data, has on the resulting fatigue crack initiation life of the nickel-based superalloy RR1000. Industrial relevance was added to this investigation by the use of flight representative thermomechanical fatigue loading cycles and state of the art material behaviour and fatigue crack initiation models used within the finite element simulations conducted. The effect of the scatter in to the modelling approach on the outcoming predictions is made via a Monte-Carlo analysis. This analysis consisted of running the same simulation several times, but with the experimentally determined and validated baseline constitutive parameters varied via correction factors built into the model, for each run via a singular value decomposition procedure. It was found that small scatter in has only a very localised scatter out effect on the crack initiation predictions under the flight representative loading. Funding Agencies|Clean Sky 2 Joint Under-taking under the European Unions Horizon 2020 research and innovation programme [686600]

The present work introduces a small volume, high throughput testing method that may be used to quickly, and with high spatial fidelity, investigate local material properties in additively manufactured materials. A case study application is presented here, considering components made from the alloy Ti-6Al-4V by the powder bed fusion process. Novel tensile small ring samples are produced at multiple locations over the print volume and are tested in order to estimate local tensile material responses. An inverse method is implemented in order to estimate representative material properties at each testing location by optimising initial values against sequential finite element model results. Initial estimates themselves are found through a geometrically non-linear semi-analytical model that approximates elastic material response. A good level of repeatability is noted between small ring tests conducted at specific build locations, suggesting a limited effect of build height on constitutive response and robust testing/analysis methodology. Small ring tests indicate a mean Young's modulus of 82.83 GPa (with a standard deviation of 4.57 GPa) and a mean yield stress of 655.58 MPa (with a standard deviation of 115.73 MPa). Full sized “conventional” tests, published in the author's previous work, indicate a Young's modulus of 114.45 GPa and a yield stress of 771.266 MPa. Limited fractography has indicated that there is a wide variation in porosity across the build volume, suggesting that the deviations in local material properties are due to the use of a reduced print volume in specimen manufacture.

The current paper describes TMF crack growth behaviour in an advanced nickel-based superalloy. Changes in behaviour are examined which occur as a function of the phase angle between applied stress and temperature. The fractography of the failed specimens reveals changes from transgranular to intergranular growth between high and low phase angle tests as a result of the onset of high temperature damage mechanisms. More targeted testing has also been undertaken to isolate the contributions of these mechanisms, with specific transitions in behaviour becoming clear in 90° diamond cycles, where dynamic crack growth and oxidation strongly interact.

The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also provides guidance for pre-cracking and use of different specimen geometries as well as Digital Image Correlation imaging for crack monitoring. The majority of the tests have been carried out on a coarse grain polycrystalline nickel-base superalloy using two phase angles, Out-of-Phase and In-Phase cycles with a triangular waveform and a temperature range of 400-750 degrees C. Funding Agencies|European UnionEuropean Union (EU) [686600]

Packed bed thermoclines have attracted considerable interest as an economical method for storing large amounts of thermal energy. They are a constituent part of a range of proposed thermo-mechanical energy storage systems, such as Adiabatic Compressed Air Energy Storage (ACAES) and Pumped Thermal Energy Storage (PTES). The low cost of the thermal storage media (crushed rock or gravel) means that even with the cost of the required compression and expansion equipment, these systems potentially have a lower Levelised Cost of Storage than batteries, especially for grid scale storage. Packed bed thermoclines rely on a stratified temperature gradient (thermal front) between heated material at the top and cooler material at the bottom. The stability of this thermal front can affect the exergetic efficiency of the store. We present a simple criterion for the stability of a thermal front and show that during discharge of a hot store, a small cold perturbation in the thermal front can develop into a cold tunnel that propagates ahead of the main thermal front. By contrast, the presence of a small hot perturbation at the thermal front prior to charging with hot gas is shown to be quickly dissipated. We also calculate a theoretical critical perturbation size required for a cold tunnel to develop ahead of the thermal front. Below this size transverse thermal diffusion is able to dissipate perturbations before they can develop. Three dimensional Computational Fluid Dynamics simulations are used to accurately visualise thermal front instabilities and also to quantify their effect on the exergetic efficiency of a cycling thermal store. Adding a small high void fraction region near the bottom of the thermal store caused a significant disruption of the thermal front on each discharge cycle and resulted in a 4.5% increase in the exergy loss rate. Low void fraction adjacent to the walls of the thermal store, which typically occurs during packing, caused a more significant 63% increase in the exergy loss rate relative to a uniformly packed thermal store.

The optimization of a packed bed for utility-scale applications is presented herein. The paper discusses comprehensively the effects that particle size, aspect ratio and storage mass have on the exergy losses of the store throughout a complete working-cycle and seeks to provide a clear reference of what is an adequate range of aspect ratios to consider for the design of a grid-scale packed bed. A one-dimensional model that accounts for temperature-dependent properties of both, storage medium and heat transfer fluid, and self-discharge losses is used for performing the analyses. The working cycle considered for the modelling work is a 24 h long sinusoidal profile (12 h charge/12 h discharge) with a 10 MW peak power and a total energy storage requirement of 79.4 MWhth. The results show that a substantial improvement in performance can be achieved by adopting a configuration based on an aspect ratio between 0.5 and 0.8 and fine-tuning the particle size for the specific shape of the container. Furthermore, the study reveals that increasing the thermal storage mass leads to a considerable increase in efficiency. It is determined that a design with 50% additional storage mass, an aspect ratio of 0.6 and a particle size of 3.7 mm, is the optimum configuration from a techno-economic perspective, having a roundtrip exergy efficiency of 98.24%. Additionally, the paper presents a brief study on the effect of the frequency of the work-cycle, which serves as an introduction to the concept of optimizing a thermal storage system by splitting the load into multiple frequency components.

Flywheels are an attractive energy storage solution for many reasons; high turnaround efficiencies, long cycling lives and high “ramp-up” power rates have all been noted in the literature. Novel flywheel based hybrid energy storage systems have also been suggested by several authors which, due to the inherent partitioning of power sources in the system architecture, provide capacity for flywheels to deliver/receive energy over a comparatively large range of time scales and loading frequencies. Accommodating grid power fluctuations at the millisecond to second time scale is an ever growing problem that almost all grids undergoing de-carbonisation are facing. Synchronous flywheel energy storage systems have the attractive capability of being able to replace “real” (passively controlled) inertia with “real” inertia in a cheap and very robust manner. Flywheel design at the grid scale warrants careful consideration, as for static energy storage applications (i.e. those not used in transportation) the main driving factor is the reduction of manufacturing and material costs. It is paramount that material is used effectively, i.e. it is sufficiently stressed such that the flywheel is not oversized (and therefore expensive) while simultaneously guarding against the likelihood of catastrophic failure during service. Fatigue has the potential to be a serious life limiting mechanism due to fluctuating rotational speeds, however in depth analysis is lacking in the literature. The present work looks to quantify the severity of fatigue in flywheels which re-establish grid inertia by applying fatigue design methods (such as the rainflow cycle counting method and the generalised strain amplitude methods of Ince and Glinka for fatigue lifing) to loading scenarios that represent grid frequency fluctuations. Importantly flywheels are sized based on different limit stress criteria, thereby enabling differing levels of structural capacity usage between designs. For the realistic design cycles considered in the present work (representative of a large scale grid undergoing normal frequency fluctuations) all projected lives are extremely large, suggesting that fatigue is not a limiting factor and that any of the tested design methodologies is viable. Significant improvements in energy density and cost per unit of energy stored may however be achieved if elastic–perfectly-plastic (Tresca based) design criteria are implemented over simple strictly elastic variants. Neglecting containment costs for simplicity, improvements in energy density of ≈ 74 % and cost per unit of energy stored of ≈ 290 % are demonstrated to be achievable.

To date, the complex behaviour of small punch creep test (SPCT) specimens has not been completely understood, making the test hard to numerically model and the data difficult to interpret. This paper presents a novel numerical model able to generate results that match the experimental findings. For the first time, pre-strained uniaxial creep test data of a P91 steel at 600 ∘C have been implemented in a conveniently modified Liu and Murakami creep damage model in order to simulate the effects of the initial localised plasticity on the subsequent creep response of a small punch creep test specimen. Finite element (FE) results, in terms of creep displacement rate and time to failure, obtained by the modified Liu and Murakami model are in good agreement with experimental small punch creep test data. The rupture times obtained by the FE calculations which make use of the non-modified creep damage model are one order of magnitude shorter than those obtained by using the modified constitutive model. Although further investigation is needed, this novel approach has confirmed that the effects of initial localised plasticity, taking place in the early stages of small punch creep test, cannot be neglected. The new results, obtained by using the modified constitutive model, show a significant improvement with respect to those obtained by a ’state of the art’ creep damage constitutive model (the Liu and Murakami constitutive model) both in terms of minimum load-line displacement rate and time to rupture. The new modelling method will potentially lead to improved capability for SPCT data interpretation.

This paper explores how the energy storage capacity required by an electric grid increases with the penetration of renewable generation. The paper uses the UK as a case study and aims to quantify the amount and duration of energy storage that the country will need to fully decarbonize its electric supply. The paper also studies the effect that the mix of renewables (wind + solar) has on the storage capacity needed and highlights that a greater mismatch between the generation and demand profiles will require a larger energy storage capacity. Therefore the right generation mix for the region should be used. Results show that the UK will need a storage capacity of approximately 7.63 TWh (~4 days) to achieve an overall renewable penetration of 100%. Two important considerations are made: i) the mix between wind and solar is 79–21% and ii) a 5% of over-generation (and curtailment) is allowed. Assuming that the storage capacity is provided by compressed air systems (CAES) and considering the current costs of renewable generation, this scenario attains a levelized cost of electricity of ~61 £/MWh. This scenario achieves the lowest possible LCOE despite the fact that 5% of the generated electricity is wasted and still paid-for. If a strict rule of zero-net curtailment was in place, the storage capacity required would almost double (14.93 TWh) and the final cost of electricity would be ~2.3% higher.

Inertia plays an important role in the function of electrical grid stability. When there is a difference between supply and demand grid frequency will change. Inertia resists this change and limits the rate of change of frequency (RoCoF) giving time for power stations to change their supply to match demand. Traditional thermal power stations have significant amounts of inertia in their generators and steam turbines. Renewable energy sources such as wind and solar tend to have little or no inertia and as a result grid inertia has reduced and will continue to do so in the future, causing issues of grid stability. There are various ways to address this problem including: replacing grid inertia, increasing the maximum level of RoCoF allowed, or minimising the largest generator or load on the grid. Inertia hasn't traditionally been a traded commodity as it has been a by-product of large thermal power generation. This is likely to change in the future with the increase in non-synchronous generation. The costs and methods of creating grid inertia are somewhat novel fields. This paper surveys some of the options of creating grid inertia. It also presents SHyKESS which is a flywheel based method of creating grid inertia and primary frequency control.

Compressed air is an attractive energy storage solution that can address many of the problems associated with operating large electricity grids with high levels of renewable penetration. The mature nature of the technology makes compressed air a robust and cheap alternative to batteries that is particularly applicable to offshore generation. Storage tanks (pressure vessels) must be utilised if geological alternatives, such as solution mined salt caverns, are not available or cannot be excavated in a particular deployment. Tanks are typically expensive, however it is possible to realise significant improvements (over 50%) in cost per unit exergy stored if “real gas effects” of air are exploited. Economic benefits resulting from realistic air property dependencies rely on storing air at low temperatures, circa −40°C. In this temperature range concerns are raised over the integrity of common pressure vessel materials; a transition from ductile to brittle failure modes is observed in many BCC (body centred cubic) steels that limits the size of “safe” (non-propagating) flaws in the vessel and increases the potential for fast fracture/catastrophic failure. Autofrettage is a manufacturing process in which a beneficial compressive stress state at the internal wall of a pressure vessel is induced by over pressurising the cylinder during manufacture. Autofretteage allows larger flaws or defects (such as cracks) to be safely accommodated in a design, compared to an identical vessel that has not undergone autofrettage. In this work autofrettage is investigated as a method which can allow cold compressed air energy storage to be realised. Safe operating pressures and temperatures are determined for vessels that have undergone autofrettage. These are then compared to similar calculations for more “simple” vessel designs (i.e. without autofrettage) and economic arguments are developed for the adoption of cold compressed air storage. Costings (cost per unit exergy stored) are not significantly sensitive to the additional effort required to autofrettage a vessel (due to the pressure levels involved).

Societal pressures are mounting on electricity operators to operate traditional fossil-fuel power plants in an efficient and flexible manner in conjunction with renewable power plants. This requires the uses of high frequency start up – shut down load profiles in order to better match market demands. As such, high temperature/pressure components such as steam pipe sections and headers experience fluctuating mechanical and thermal loads. There is therefore an industrial need for the accurate prediction of fatigue and creep damage in order to estimate remnant component life. In the present work, a continuum damage model has been coupled with a Chaboche unified viscoplastic constitutive model in order to predict stress-strain behaviour of a P91 martensitic steel (a material used for power plant steam pipes) due to cyclic plasticity and damage accumulation. The experimental data used here are from the previous work [1]. Cyclic fully reversed strain controlled experiments (±0.4%, ±0.25% and ±0.2% strain ranges) and cyclic test with a dwell period (±0.5% strain ranges) for a P91 martensitic steel under isothermal conditions (600°C) are utilised. The physically relevant material parameters are determined and optimised using experimental results. Although many material parameter identification procedures can be found in the literature [1-6], there are uncertainties in determining the limits for the parameters used in the optimisation procedure. This could result in unrealistic parameters while optimising using experimental data. The issue is addressed here by using additional dwell test to identify the limits for stress relaxation parameters before using Cottrell’s stress partition method to identify the limits for strain hardening parameters. Accumulated stored energy for damage initiation criterion and damage evolution parameters are also extracted from the experimental results. The estimated failure lifetimes for ±0.4%, ±0.25% and ±0.2% cases are 1600, 4250 and 9500 cycles, respectively, as opposed to 1424, 3522 and 10512 cycles as given by experiments.

Non-destructive testing techniques are widely applied in industry for the evaluation of quantities of interest without inflicting additional damage accumulation. Crack detection and monitoring is a prime example of where non-destructive testing is valuable. Among the variety of non-destructive testing techniques, the direct current and alternating current potential difference methods, which are based on the principle that an electrical potential field around a conductive specimen is disturbed by the presence of geometric irregularities (or “features”), have received a great deal of attention in the literature. This is mainly due to the high levels of accuracy associated with these techniques and good estimations of crack initiation and propagation having been achieved. A critical review of the evolution and applications of potential difference methods is presented in this paper. Potential difference methods are capable of providing accurate and continuous measurements with simple installation and exclude the requirement of visual access under harsh service conditions. Alternating current potential difference methods require lower current input than direct current equivalents and hence provide higher sensitivity and offer better noise rejection but are vulnerable to capacitance effects and are more expensive. Calibration curves can be determined analytically, numerically, or by direct or analogue experimental techniques with each method offering strengths and limitations. Application of these should be determined in accordance with the specific scenario. The performance of electric probes (of voltage measurements and current injection) on top- and side-face of C(T) and SEN(B) specimens are reviewed in detail as case examples. Specific guidance in normalising measurements and eliminating errors from thermoelectric effects can be implemented in order to improve the accuracy of PD methods. Abundant results have been obtained by applying PD methods in monitoring cracks geometries under aggressive conditions such as corrosion, high temperature, creep and cycled loading.

The wide scale market penetration of numerous renewable energy technologies is dependent, at least in part, on developing reliable energy storage methods that can alleviate concerns over potentially interrupted and uncertain supplies. Many challenges need to be overcome, not least among them is allowing capacity for the wide range of time scales required to ensure grid stability. In thermal power plant, high frequency/short duration demand fluctuations, acting at the milliseconds to several seconds time scale, are addressed passively by the inertia of the grid. Here, grid inertia can be thought of as the mechanical inertia of spinning steel in steam and gas turbines. This allows time for active control measures to take effect at the tens of second to hours time scale and for the system to recover without a supply frequency deviation that is noticeable to the customer. It is of paramount importance that, as thermal plant is retired, renewable energy generation and storage systems account for the loss of this inertia. In the literature, strategies to address the loss of “real” inertia have often relied on emulation rather than actual replacement. The present work focuses on the preliminary development of a novel energy storage system that makes use of real inertia to address short term supply/demand imbalances while simultaneously allowing for extended depths of discharge. The concept looks to combine flywheel and compressed fluid energy stores in order to power a synchronous generator. By combining these energy storage technologies through a differential drive unit, DDU, it is anticipated that the benefits of high system inertia can be exploited in the short term while allowing energy to be continually extracted from the flywheel in the long term during storage discharge. The use of a DDU makes the present design particularly novel and distinct from other hybrid systems. In essence, this inclusion allows energy to be extracted entirely from the flywheel, inducing “real” inertia, or entirely from the secondary store, inducing “synthetic” inertia, or some combination of the two. Fundamental sizing calculations for a 50 MW system with 20 MWh of storage capacity are presented and used to design a suitable control system that allows for the operation of both primary flywheel and secondary compressed fluid energy stores. The transient behaviour of the system is simulated for several charge/discharge time profiles to demonstrate response stability for the system. Comments on system turnaround efficiency, which is dependent upon loading history but for the intended applications can be considered to be greater than 90% are also made here, along with a case study application to an isolated Californian solar powered grid.

The trend in power generation to operate plant with a greater frequency of on/partial/off load conditions creates several concerns for the long term structural integrity of many high temperature components. The Green's function method has been used for many years to estimate the thermal stresses in components such as steam headers by attempting to solve the un-coupled thermal stress problem for a unit temperature step. Once a Green's function for a unit temperature step has been determined, realistic or actual component temperature profiles can be discretised and the time dependent stress profile reconstructed using Duhamel's theorem. Stress fluctuations can therefore be estimated and damage due to fatigue mechanisms can be quantified. A potential difficulty with this method is that Green's function approximations are determined for a single analysis point in a structure. This is because Green's functions are approximated by fitting a trial function to the results of finite element (FE) simulations. While a user can make some judgement on which point in a structure will give the “worst case” (or life limiting) conditions, it is foreseeable that points of interest will be dependent on the specific analysis conditions, such as the stub penetration geometry and the loading condition considered. The neural network approach described in this paper provides a means where transient thermal stress models of complex components (here taken to be steam headers) can be generated relatively quickly and used pro-actively to assess and modify plant operation. A range of header geometries have been considered to make the network applicable over an industry relevant envelope. Coefficients of determination ( R 2 ) are typically above 0.92 when reconstructed (from neural network results) unit temperature step stress profiles are compared against “true” FEA results. Mean errors in the stress profiles are, for the majority of cases, less than 10 % . Suggestions are also made on possible future improvements to the method through the use of additional constraints on the reconstructed stress profiles.

Rapid thermal ageing has been observed in the literature for certain aerospace aluminium alloys under conditions which are foreseeable in their intended component applications. While several experimental programs have explored this phenomenon in the laboratory, efforts to incorporate these effects in unified constitutive models have, to date, been lacking. In the present work, a modified elastic-viscoplastic material model has been fitted to 7175-T7351 aluminium alloy data under fully-reversed uniaxial (strain-controlled) isothermal loading conditions at 160 °C and 200 °C. These temperatures were chosen in order to represent nominal and extreme (but still operationally relevant) conditions experienced by aero-engine gearbox components. The modified elastic-viscoplastic model is able to accurately predict the hysteresis loops of the strain-controlled fatigue data of each sample at 160 °C and 200 °C. Additionally, it was found that the isotropic hardening can be effectively de-coupled into material ageing and mechanical softening components.

Societal and environmental pressures are forcing thermal power plant operators to deviate greatly from the generation strategies of the past. The application of high frequency start up/shut down/partial load cycles to components that may well be outside their design life makes research into the long-term integrity of at risk assets paramount. Decoupled thermal/mechanical analyses have been used in the literature to estimate anisothermal fatigue in header components, with convective boundary conditions typically assumed for internal surfaces in order to determine heat fluxes and hence a temperature field. In reality, convective coefficients are heavily dependent upon local velocity profiles. In the present work, computational fluid dynamics is used in order to better approximate the steam flow in a real power plant header, leading to a convection coefficient field that is used to solve the thermal problem. Anisothermal fatigue analysis is finally conducted using a Chaboche type model. The results of computational fluid dynamics have illustrated that heat transfer coefficient values can vary (spatially) by a factor of 5.49 over the internal header wall, with noticeable hot spots in the wake of the stub penetrations. Peak differences of 6.47 % in accumulated plastic strain levels have been observed between simulations conducted with constant (simplified) and variable (computational fluid dynamics derived) thermal boundary conditions.

Crack size determination using electrical potentials both in service and in the laboratory has been undertaken for many years. In the laboratory this has mainly concentrated on the measurement of crack depth, with either alternating current (AC) or direct current (DC) supplies. Some work to determine the varying depth along the width of cracks as an inspection tool of in service parts using mapping methods has been done. This has used both AC and DC utilising various models to understand the data recorded, in Alternating Current Potential Drop (ACPD) a range of frequencies have been used to give various skin depths. The resulting analyses have been grouped into two groups 'thin skin' and 'thick skin', in the thin skin case the skin depth is significantly smaller than the depth of the crack 1/10th of the crack depth whereas in the thick skin cases are for cases where skin depth is over this limit. Some work has been carried out to try and unify these two approaches. The work presented here looks to develop a method using variable frequency ACPD to resolve further information about cracks growing in laboratory specimens. A system has been developed to rapidly sweep a wide frequency band and record voltage drop across a crack or feature. A selection of steel samples with known geometries and features have been used to trial and benchmark the technique. These samples have a range of cross sections as well as machined features or a range of shapes and sizes to simulate a range of crack geometries. This work has been approximated using a 2D computational model. This has been done using a reduced thickness approach.

A fatigue crack initiation model based on damage accumulation via a fatigue memory surface in conjunction with a plastic strain energy parameter was evaluated for thermomechanical fatigue loading in a gas turbine disc alloy. The accumulated damage in each hysteresis loop was summed up, and it was assumed that the damage at the stable state is repeated until failure occurs. Crack initiation occurs when enough fatigue damage has been obtained, and the number of cycles can thus be directly determined. The fatigue damage is highly coupled to the constitutive behaviour of the material, where the constitutive behaviour was modelled using a non-linear hardening description. Based on this, a stable state was achieved and the obtained damage could be extracted. A user-defined material subroutine was implemented, incorporating both the constitutive description and the fatigue damage accumulation. The framework was adopted in a finite element context to evaluate the thermomechanical fatigue crack initiation life of the disc alloy RR1000. From the evaluation it could be seen that a good prediction of the thermomechanical fatigue life was achieved compared to performed experiments.

This paper explores the use of steel wire wound pressure vessels for reducing the cost of aboveground air storage for small-scale CAES systems. CAES technology is often only considered for large scale applications because underground salt caverns are only cost-effective for capacities in the order of hundreds of MWh. Besides the size restriction, CAES systems are geographically limited to places with suitable salt deposits. The cheapest pressure vessels available on the market have a nominal pressure of 250 bar (suitable for CAES applications) and can be found for about ∼80$/kWh; which is approximately 12 times the overall cost per unit exergy storage of an underground cavern. Such high costs would impede a CAES system to be competitive. The wire-winding approach proposed allows reducing the volume of steel required to fabricate a vessel up to a 35%, depending on the specific configuration of the vessel. A conventional container made from solid plates and rated for 250 bar, requires between 0.08 and 0.11 m3 of steel per m3 of storage volume, which translates into material costs in the range of $16.5 to $21.7 per kWh of exergy storage capacity. In contrast, a wire-wound vessel (rated for the same pressure) requires between 0.068 and 0.07 m3 of steel wire per m3 of capacity (considering wires with a yield strength of 1700 MPa). This represents material costs between $8.5 and $12.4 per kWh of exergy storage capacity. Considering that wire-winding is much simpler than the fabrication of a conventional tank made of solid steel plates, it is estimated that wire-wound vessels can be produced for less than 35 $/kWh.

In many situations where the characterisation of the mechanical behaviour of a specific material is required, source material for manufacture of conventional test specimens may be at a premium. Examples include the validation of new alloys for use in the power industry, the description of the heat affected zone (HAZ) of weldments or performing a remnant life study on an in service component (such as steam pipe work used extensively in the power generation industry). The potential for a limit in sample material has necessitated the development of small specimen designs and associated test methods, particularly for the determination of the creep behaviour of a sample material. The small punch creep test (SPCT) has the potential to characterise the full uniaxial creep curve (as the specimen is taken to fracture). It is for this reason that the small punch creep test has attracted much interest from the research community. Owing to the complex deformation mechanism interactions experienced in the smal...

Small specimen testing techniques are potentially highly useful procedures which could be used to determine many fundamental material properties from relatively small amounts of sample material. Small specimens can be manufactured from scoop samples taken from the surface of a component, allowing for continued operation after sampling. In the past, it has been assumed that, provided the wall thickness dimension around the specimen sample site is greater than the minimum design requirement, the action of scoop sampling will not greatly compromise the future operation of the component. Little study has been completed, however, to verify this assumption or to evaluate the likely reduction in remnant life due to scoop sampling. In the present study, a finite element investigation is made into the effects of scoop sampling on the stress states and failure lives of power plant straight pipes under steady-state creep conditions. Loading conditions considered include an internal pressure load only (assuming closed-end conditions), as well as system loading (in the form of additional axial and in-plane bending moment loading). Typically, the application of system loading increases the likelihood of failure being controlled by the stress states in the vicinity of a scoop excavation. This leads to potentially significant reductions (up to approximately 90%) in creep life of straight piping components. Parametric equations (valid for a range of pipe geometries and scoop cut depths) have been proposed, which can estimate the stress riser effect in the vicinity of an excavation sample under closed-end and additional axial loading conditions.

The accurate prediction of creep life is of great importance for the structural integrity of high temperature components, such as those used in power generation plant, if safe, efficient and economically responsible operation is to be achieved. Continuum damage mechanics can be used in conjunction with finite element analysis to provide a fundamental step in modelling creep failure. Material constants used in these models are often derived from accelerated creep rupture tests, performed using higher stresses or temperatures than would normally be experienced by real components. In this paper, a comparative assessment of extrapolated failure times for several creep damage models has been under taken for uniaxial, notched bar, closed end straight pipe section and idealised pipe bend geometries (note the pipe geometry was typical of that used in power generation plant) using finite element software. Material constants for each model were determined using creep and creep rupture tests performed on P91 materials under a uniaxial condition and using notched bar specimens to obtain the material properties related to the multiaxial stress state. In all cases, a hyperbolic sine function creep law was found to consistently give reduced failure times, compared to power law based models (e.g. Liu–Murakami and Kachanov-Robotnov), when stresses below those used in the creep tests were considered.

Pipe bends represent geometric discontinuities in the steam pipe systems of power plants, therefore understanding the behaviour of these potential locations of weakness is of great industrial importance for component inspection, design and analysis. Due to the high operating temperatures encountered, the failure mechanism of creep is a justified concern. Furthermore, while the geometry of pipe bends appears to be simplistic, the manufacturing process employed results in variations to the critical dimensions of the pipe bends. It is these variations in geometry that can cause potentially significant differences in peak steady-state rupture stress magnitude (approximately 48% in some of the cases considered in the present work). Through analysis of industrial data, several novel non-dimensional parameters have been established, allowing for (with suitable constraint equations depending on the type of bends analysed) the approximation of the complexity of pipe bend geometry in only a few dimension factors. Using these factors, systematic finite element analysis (FEA) studies may be completed with these non-dimensional parameters taking account of a range of geometry variation. Using this philosophy, the stress states and failure lives of pipe bends of the same type (i.e. Hot Reheat or Main Steam) with similar, but not identical, dimensions may be estimated and compared using approximations of the peak rupture stress function. By way of example, this procedure is applied to Main Steam and Hot Reheat type 90° pipe bend geometries. The accuracy of interpolation for the stress function is also analysed, along with comments on failure locations and possible future improvements.

The use of small specimens in routine testing would reduce resource requirements, however, limitations exist due to concerns over size effects, manufacturing difficulties, uncertainties related to the application of representative loading conditions, and complex interpretation procedures of non-standard data. Due to these limitations, small specimen testing techniques have been mostly applied for ranking exercises and to determine approximate or simple material parameters such as Young’s modulus, creep minimum strain rate and fracture toughness. The small ring method is a novel, high sensitivity small specimen technique for creep testing and has been extended in the present work for the determination of tensile material properties. Wrought aluminium alloy 7175-T7153 was tested at room temperature at 5 different loading rates. Finite element analysis was completed to evaluate the equivalent gauge section and equivalent gauge length in order to compare uniaxial tensile testing results and small ring specimen tensile testing results. An analytical solution has also been derived in order to validate the finite element analysis. It was discovered that the finite element analysis model was suitable, validated by both experimental results and analytical solution as well as that small ring specimens can be used to acquire same stress/strain data as uniaxial specimens.

Thermo-mechanical coupling is a critical component in the thermodynamics of irreversible processes and is related to the dissipation of thermal energy during plastic straining. The Taylor-Quinney coefficient may be thought of as a ratio between thermally dissipated energy and plastic work, thereby giving insight into the thermo-mechanical coupling term. The inclusion of this parameter in a meaningful way is complicated by the various dependencies that the Taylor-Quinney coefficient may be subject to (e.g. loading rate and temperature). Determination of these dependencies is usually achieved through extensive experimentation, wherein temperature variations are monitored (with reference to an unloaded control sample) in a test piece during mechanical loading. There are practical limitations in full size testing methods however, not least relating to the location of full sized control and loaded samples in an environment chamber/furnace while simultaneously maintaining (control sample) temperature uniformity and high resolution temperature measurement (in the loaded sample). The present work details a method based on a novel small specimen testing technique that is currently under development at the University of Nottingham. A small ring of 7175-T7351 aluminum alloy (approximately 10mm in diameter and 2mm in thickness) is loaded between two pins at room temperature, with the local specimen temperature field monitored during monotonic deformation using an infra-red thermal camera. Experimental results are compared for different pin loading rates (namely 0.1mm/s, 1mm/s, and 10mm/s), with particular emphasis placed on localised temperature variations in areas of expected high plasticity. Differences of approximately 0.6°C were observed between 0.1mm/s and 1mm/s tests, with higher temperatures recorded in the latter. Higher temperatures were also noted at small specimen locations associated with localised plasticity. Fundamental thermal material properties are reported for the 7175 alloy in order to facilitate future analysis and heat equation solution efforts (working towards Taylor-Quinney coefficient determination).

Journal article A service-aged P91 steel was used to perform an experimental program of cyclic mechanical testing in the temperature range of 400 °C–600 °C, under isothermal conditions, using both saw-tooth and dwell (inclusion of a constant strain dwell period at the maximum (tensile) strain within the cycle) waveforms. The results of this testing were used to identify the material constants for a modified Chaboche, unified viscoplasticity model, which can deal with rate-dependant cyclic effects, such as combined isotropic and kinematic hardening, and time-dependent effects, such as creep, associated with viscoplasticity. The model has been modified in order that the two-stage (nonlinear primary and linear secondary) softening which occurs within the cyclic response of the service-aged P91 material is accounted for and accurately predicted. The characterization of the cyclic viscoplasticity behavior of the service-aged P91 material at 500 °C is presented and compared to experimental stress–strain loops, cyclic softening and creep relaxation, obtained from the cyclic isothermal tests. Science Foundation Ireland [Grant No. SFI/2010/IN.1/I3015] peer-reviewed

Pipe bends are regions of geometric discontinuities in the pipe systems used in power plants and most industry recorded failures have been located around similar regions. Understanding these potential locations of weakness is therefore of great interest for the safe and economic operation of piping components. Increased predictive accuracy would assist in component design, condition monitoring, and retirement strategy decisions. Modeling of piping components for finite element analysis (FEA) is complicated by the variation of the cross section dimensions (changes in wall thicknesses or cross section ovality) around the pipe bend due to the manufacturing procedure implemented. Quantities such as peak rupture stress and creep rupture life can be greatly affected by these geometric variation (Rouse, J. P., Leom, M. Z., Sun, W., Hyde, T. H., Morris, A., “Steady-state Creep Peak Rupture Stresses in 90 Pipe Bends with Manufacture Induced Cross Section Dimension Variations”International Journal of Pressure Vessels and Piping, Volumes 105–106, May–June 2013, pp. 1–11). Three dimensional (3D) models can be used to approximate to the realistic level of detail found in pipe bends. These simulations may however be computationally expensive and could take a considerable amount of time to complete. Two dimensional (2D) axisymmetric models are relatively straight forward to produce and quick to run, but of course cannot represent the full geometric complexity around the pipe bend. A method is proposed that utilises multiple 2D axisymmetric pipe bend models to approximate the result of a 3D analysis through interpolation, thus exploiting the greatly reduced computing time observed for the 2D models. The prediction of peak rupture stress (both magnitude and location) is assessed using a simple power law material model. Comments are made on the applicability of the proposed procedure to a range of bends angles (90 deg, 60 deg, and 30 deg), as well as the effect of the stress exponent (n) and tri-axial (α) material constants. Provided that peak stresses do not occur at the bend/straight interface, the magnitude and location of the peak rupture stress can be predicted by the 2D axisymmetric interpolation method with a typical percentage difference of less than 1%.

A procedure to estimate material constants for the unified Chaboche viscoplasticity model from experimental data has been published elsewhere; however several critical assumptions are made to enable this and potential numerical problems can limit the effectiveness of the optimisation. Pragmatic optimisation procedures are therefore required to determine the material properties accurately and efficiently. This is made more complex by the presence of several deformation mechanisms and their interactions. Automation is critical due to the large amounts of data generated in testing. Complications that inhibit this process can arise due to factors such as experimental scatter. In this paper, a general optimisation framework is discussed and investigated using data from isothermal tests on a P91 steel at 600°C. Potential obstacles in the procedure are addressed and solutions (such as pre-optimisation experimental data ‘cleaning’) are suggested. Methods to maximise the amount of confidence a user has in a particular optimised constant set are also discussed.

Determining representative material constant sets for models that can accurately predict the complex plasticity and creep behaviour of components undergoing cyclic loading is of great interest to many industries. The Chaboche unified visco-plasticity model is an example of a model that, with the correct modifications, shows much promise for this particular application. Methods to approximate material constant values in the Chaboche model have been well established; however, the need for optimisation of these parameters is vital due to assumptions made in the initial estimation process. Optimisation of a material constant set is conducted by fitting the predicted response to the experimental results of cyclic tests. It is expected that any experimental data set (found using the same values of test parameters such as temperature; the dependency of which is not accounted for in the original Chaboche model) should yield a single set of optimised material parameters for a given material. In practice, this may not be the case. Experimental test programs usually include multiple loading waveforms; therefore, it is often possible to optimise for separate, different sets of material constants for the same material operating under comparable conditions. Several optimisation strategies that utilise multiple sets of experimental data to form the objective functions in optimisation programs have been applied and critiqued. A procedure that evaluates objective functions derived from the multiple experimental data types simultaneously (i.e. in the same optimisation iteration) was found to give the most consistently high-quality fitting. In the present work, this is demonstrated using cyclic experimental data for a P91 steel at 600 °C. Similar strategies may be applied to many constitutive laws that require some form of optimisation to determine material constant values.

It may be inevitable in the design and analysis of most high temperature components (such as power industry pipe work) that variations in load and/or temperature will occur in normal operation. This presents complications in the prediction of the response of such components due to potential hardening or softening effects caused by the accumulation of plastic strain. Furthermore, interactions between hardening (or softening) behaviour and creep may be observed, particularly in high temperature applications. In this paper, the Chaboche model is described as it has the potential to represent this type of behaviour. An optimisation procedure for fine tuning material constants is developed and presented. This is a key step as the determination of initial estimates requires several assumptions to be made. Several potential pitfalls in optimisation procedures are described and addressed, mainly through the application of experimental data cleaning as a pre-processing procedure. This removes unavoidable experimental scatter that inhibits optimisation. Investigations into the effects of variations in the initial conditions on optimised material constant values and the number of data points selected on computational times are made to aid in the application of similar optimisation procedures. The superior fitting given by the implementation of an optimisation procedure is verified by applying it to the results of strain controlled cyclic tests of a P91 steel at 600°C.

The threat of thermal fatigue is an increasing concern for thermal power plant operators due to the increasing tendency to adopt “two-shifting” operating procedures. Thermal plants are likely to remain part of the energy portfolio for the foreseeable future and are under societal pressures to generate in a highly flexible and efficient manner. The Green’s function method offers a flexible approach to determine reference elastic solutions for transient thermal stress problems. In order to simplify integration, it is often assumed that Green’s functions (derived from finite element unit temperature step solutions) are temperature independent (this is not the case due to the temperature dependency of material parameters). The present work offers a simple method to approximate a material’s temperature dependency using multiple reference unit solutions and an interpolation procedure. Thermal stress histories are predicted and compared for realistic temperature cycles using distinct techniques. The proposed interpolation method generally performs as well as (if not better) than the optimum single Green’s function or the previously-suggested weighting function technique (particularly for large temperature increments). Coefficients of determination are typically above 0 . 96 , and peak stress differences between true and predicted datasets are always less than 10 MPa.

© 2020 Elsevier Ltd An understanding of rate dependency over a wide range of time scales is vitally important in approximating the transient response of critical components operating in extreme environments. Many examples of viscoplastic model formulations can be found in the literature, wherein all rate dependency is assumed to occur after yielding. Such models neglect any viscous effects during elastic deformation. In the present work, a unified viscoelastic – viscoplastic material model is developed for the Nickel superalloy RR1000. Particular emphasis is placed on model parameter determination, which is accomplished using standard cyclic plasticity and stress relaxation experimental data.