21 research outputs found
Macro- and micro-mechanical perspectives on creep-fatigue interaction in Type 316L stainless steel
No full textCreep-fatigue of Type 316L stainless steel under asymmetric waveforms (specifically slow tension-fast compression, S-F, and fast tension-slow compression, F-S) has been understudied, despite its significant implications as demonstrated in this work. This study bridges macro- and micro-mechanical perspectives through a combined approach, involving high-temperature testing, scanning electron microscopy, X-ray computed tomography, neutron diffraction, and crystal plasticity modelling. Macro-mechanical tests revealed distinct deformation behaviours under S-F and F-S waveforms with and without a 1-hour tensile dwell at 550 °C, with S-F reducing lifespan in both fatigue and creep-fatigue conditions. Post-mortem analyses revealed distinct fracture morphologies induced by tensile dwell, with creep-fatigue S-F specimen exhibiting more pronounced intergranular-dominant fracture and higher internal defect volume. It also exhibited the highest number fraction of medium-sized (10–40 μm) microcracks, which correlates with its shortest fatigue life and more creep damage accumulation. Higher grain-level deformation incompatibility was observed during tensile dwell in the S-F load waveform. Crystal plasticity modelling revealed that the higher tensile stress amplitudes during S-F loading stem from increased dislocation density, with average densities at peak tensile strain during the saturation cycle reaching 186 μm⁻² for S-F and 147 μm⁻² for F-S waveforms. These findings establish a strong link between macroscopic and microscopic behaviours under asymmetric loading, emphasising the potential of S-F waveforms for cost-effective creep-fatigue experiment design. Furthermore, for the asymmetric waveforms studied, creep-fatigue life assessment using the ductility exhaustion method demonstrates greater accuracy than those based on the time fraction method
Characterising and modelling fracture in functional pet foods
No full textThe underlying basis of this research is that food oral breakdown heavily depends on the mechanical properties of the material being masticated. So far, researchers have been measuring mechanical properties to establish sensory panels which are useful towards designing food. Although this methodology has proven industrially popular, it displays weakness in addressing complex product development and optimisation tasks. Specifically, mastication involves varying parameters across consumers, such as speed, teeth geometry, bite force, jaw motion, as well as friction. Therefore, this study considers more advanced techniques to help predict how the food will perform during oral processing. A Finite Element (FE) model was developed, where the first bite on a starch based food was simulated, based on digital pet teeth geometry. The food material model was constructed from uniaxial compression and tension data as well as fracture toughness values through the essential work of fracture (EWF) and cutting tests. The main focus is on the development of suitable material constitutive laws including damage, in order to predict deformations and crack patterns in the food item, as experienced during chewing. Emphasis is also given to determining the true material fracture toughness by assessing the validity of the EWF and cutting methods on highly dissipative materials, which leads to the development of a new, more accurate and convenient test method. The composition effect on the mechanical response was also studied by comparing four starch based recipes. The FE jaw force versus displacement results match the experimental data obtained by a physical replicate of the bite model, lending weight to the approach as a powerful tool in understanding food breakdown and in establishing relations between the mechanical properties and sensory attributes. The study also reflects industry needs for time and cost efficient techniques towards product development and optimisation.Open Acces
Ratchetting and creep failure in twin-wall turbine blades experiencing severe thermal and centrifugal loading
No full textTwin-wall structures can be cooled both externally and internally, raising great potential for use in high-temperature applications. However, their increased geometric complexity imposes a range of potential failure mechanisms for consideration in design. The primary aim of this study is to identify the nature of such mechanisms by constructing Bree type interaction diagrams for idealized double-wall systems under cyclic thermomechanical loading that shows the combination of loading conditions for which cyclic plasticity (leading to fatigue failure)-creep ratchetting occur. Through an extension of the classical Bree analysis, we determine analytical boundaries between different regimes of behavior. We also quantify the effects of wall thickness ratio, temperature field, and yield and creep material properties. Local cyclic plasticity is shown to dominate over structural/global ratchetting when the yield strength reduces with temperature and/or when the temperature gradient through the hot wall thickness dominates over the temperature difference between the walls. Thus, we conclude that global ratchetting is unlikely to occur in the practical loading range of Nickel-based twin-wall turbine blades, but instead these systems suffer from local fatigue at cooling holes and excessive creep deformation. This is verified by 3D cyclic finite element (FE) simulations, demonstrating that the analytical approach provides a powerful, cost-effective strategy for providing physical insight into possible deformation mechanisms in a range of thin-walled components; highlighting the key trade-offs to be considered in design; and directing the use of computer methods toward more detailed calculations
Transient thermo-mechanical response and failure in selected turbine cooling geometries
No full textDecarbonising the aviation industry is imperative, as CO2 accumulates in our atmosphere. Reducing the specific fuel consumption of gas turbine engines is achievable by increasing the turbine entry temperature (TET). A reduction in TET promises reduced emissions and lower costs to operators and consumers. Increasing engine temperatures requires more efficient cooling systems to ensure adequate safety and component life. A double-walled effusion-cooled system promises improved performance. It harmonises several existing technologies to produce favourable aerothermal performance. A growing wealth of literature is focused on the mechanical performance of such a system. Prior analytical and computational modelling of the thermal and stress fields requires confirmation and calibration of experimental results. This work aims to establish a new standardised experimental methodology to assess the aerothermal and mechanical performance of these new systems.
An engine-representative flat-plate test section, or coupon, was designed and modelled alongside a test rig, the Effusion Life and Stress Addition (ELSA). The coupon was designed to promote failure at the film hole wedge, the region created near the film holes’ intersection with the hot-walls’ exterior. This was achieved by removing pedestals from the non-test region and reducing kinematic constraints and the local stress field. Further stress mitigation was achieved by scaling the outermost pedestals and associated fillets, which was shown to reduce local stress by 41%. ELSA employed a combination of vertical-cavity surface-emitting lasers and a compound parabolic concentrator to supply engine-representative heat fluxes of up to 7.4 W/mm2 , and laboratory-temperature cooling air of up to 200 standard litres per minute to achieve engine-representative thermal gradients. Coupons obtained a maximum measured temperature of 1050 ◦C with thermal differences of up to 365 ◦C. The surrounding rig remained below 75 ◦C at all measured locations. The control settings did not exceed 50% of the available laser power, indicating that higher temperatures and thermal ramp rates are possible.
Engine-representative metal temperatures and distributions were evaluated for Inconel 625 and CMSX-4 coupons. The predicted crack initiation location for circular holes was confirmed at film hole wedges for most components. The results showed a negligible dependence on the thermal gradient, normal to the coupon, in determining crack initiation. Instead, a dependence on the peak temperature was displayed. Experiments with varying normal thermal differences for the same maximum temperature show little to no effect on the number of cycles to crack initiation. An increased maximum temperature with an identical thermal gradient led to rapid buckling failure in less than 30 cycles. Furthermore, elliptical holes outperformed circular holes for identical internal geometries. This was with respect to both aerothermal and thermo-mechanical performance. In the ELSA test facility, elliptical holes with identical cross-sectional areas required an increase in laser power and a decrease in coolant to achieve identical thermal distributions. Elliptical holes endured three times more cycles than circular holes before developing cracks. Finally, the single-crystal CMSX-4 coupon was shown to survive eight times longer than a 3D-printed Inconel 625 component for an identical geometry and thermal field.
A numerical model was developed to extract the specific elastic stress state from a transient simulation of the thermal cycling profiles. It estimated an elastic nominal stress state, membrane stresses far from features, as a boundary-value problem. The boundaries were approximated on the basis of known thermal data of the quasi-steady hold region of the testing. The results of the finite elements showed reasonable agreement with the experimental values. A sub-domain approach was used to model the stresses at a critical point: the film-hole exterior wedge. Elasto-plastic models were used to estimate the total strain range. The results were compared to an established empirical Manson-Coffin relation to estimate the number of cycles to crack initiation. All models underestimated the fatigue life of CMSX-4 relative to the observed experimental values. An alternate estimate for the fatigue life, based on crystal plasticity, was then applied. The new estimated fatigue lives straddled the experimental results. A Molski-Glinka model offered a conservative estimate, while a constant-strain approach overestimated fatigue life
2D and 3D thermoelastic phenomena in double wall transpiration cooling systems for gas turbine blades and hypersonic flight
No full textDouble wall transpiration cooling (DWTC) systems allow the operating temperature of gas turbines to be increased above current levels using conventional cooling technologies, promising further enhancements in engine efficiency and a reduction in fuel consumption and emissions. Alongside the outstanding cooling performance of DWTC systems, there are structural integrity implications which have not been fully evaluated. Theoretical and Finite Element (FE) analyses presented here evaluate thermal stresses arising in a range of double wall configurations in two-dimensional (2D) and three-dimensional (3D) space. Geometric effects of wall spacing, wall thickness ratio, connecting pedestal spacing between the walls and pedestal thickness, are found to have a similar effect on the stress state in both the 2D and 3D models, and are highly sensitive to the underlying structural constraints. Although an inner cool wall may be essential for applying internal impingement cooling in the system to enhance its aerothermal performance, at the same time the inner wall increases compression in the hot outer wall, due to the kinematic constraint that the two walls must extend equally. As a result, the critical compressive stresses in the hot wall can be relieved by using a thinner inner wall. Our study determines the essential nominal stress fields that drive stress concentrations and associated local creep-plasticity-fatigue effects at critical features in DWTC systems, such as effusion holes and pedestals
Analytical shakedown, ratchetting and creep solutions for idealized twin-wall blade components subjected to cyclic thermal and centrifugal loading
No full textDouble-wall transpiration cooling systems offer the potential for performance improvements over conventional single wall systems in aerospace applications. Here we idealise the geometry in terms of a constrained 2 bar system which allows the development of analytical expressions for the entire range of possible mechanical responses under out-of-phase thermomechanical loading. The application of Koiter's shakedown theorem along with equilibrium is a powerful strategy for identifying mechanisms by which the structure can incrementally collapse (ratchetting). We show that twin wall systems with zero mechanical loading can ratchet in the compressive direction when severe thermal mismatch occurs, and that ratchetting is replaced by reverse plasticity as the bar thickness difference increases. Mechanisms exist where plastic strains do not occur at the extremes of the loading cycle. The degradation of yield strength of Ni alloys with temperature modifies drastically the response, while additional creep induced ratchetting and creep failure processes are shown to occur at extreme temperatures within a cycle. Our solutions aim to provide physical insight into the response of double-wall transpiration cooled Ni-based turbine blades
Crystal plasticity analysis of fatigue-creep behavior at cooling holes in single crystal Nickel based gas turbine blade components
No full textWe build a crystal plasticity finite element framework to investigate slip localisation and fatigue-creep behaviour at the cooling holes of single crystal Nickel (Ni) based components under cyclic thermomechanical loading. The total slip rate is decomposed into a thermally activated dislocation glide rate which dominates at moderate/low temperatures (T) and/or high stresses, and a climb rate which dominates at high temperatures and increases as inelastic strain accumulates. This formulation captures the monotonic and long-term creep response of Ni alloys in the wide range 20 <T < 1100 °C and indicates that room temperature plasticity during unloading increases the high temperature creep rate during loading (creep dwell), eventually increasing the total slip accumulation per cycle; the effect depends on the way the inelastic strain accumulates upon successive slip reversals. Elastic material anisotropy is shown to modify drastically the stress concentration around holes such that slip tends to localise at locations where the max principal stress, tangent to the hole surface, aligns with stiff crystallographic directions. This highlights the importance of plastic and creep anisotropy and creates new avenues for optimising hole shape to minimise slip activity. Our study brings to light key material-component relationships that concern the wider material science, high temperature and fatigue communities.</p
Multiscale analysis of thermomechanical stresses in double wall transpiration cooling systems for gas turbine blades
No full textDouble wall transpiration cooling (DWTC) is a new technology that allows the gas turbine inlet temperatures to be increased beyond current levels to promote higher engine efficiency. DWTC systems consist of outer hot and inner cooler walls, connected by pedestals, which contain film cooling and impingement holes, respectively. In order to employ these new systems, an evaluation of the stresses that drive fatigue and ratchetting at critical stress raisers is essential. We present a modelling framework which combines Computational Fluid Dynamics (CFD)-heat transfer solutions for the temperature field in DWTC systems, with theoretical and Finite Element (FE) elastic solutions for the thermal (T) stress and centrifugal (CF) stress fields. We demonstrate that uniaxial tensile CF loading causes much higher stress concentration factors (SCF) at cooling holes and wall-connecting pedestals than the thermally induced biaxial stresses. A theoretical framework is developed, supported by FE studies, that captures the dependence of the SCF on important geometric parameters, such as wall thicknesses, pedestal height and hole size, spacing and inclination angle, which provides important information for the optimisation of these systems. A key observation of relevance to both conventional and non-conventional turbine blade designs, is that the superposition of tensile CF stresses to compressive T stresses is beneficial for the performance at the critical film hole features; for double wall blades, however, the superposition degrades the performance at impingement holes and pedestals, as in these locations the T stresses are also tensile. These stresses can be balanced by using an optimal wall thickness ratio. Our elastic solutions can be readily used in analyses for predicting structural ratchet boundaries based on shakedown theory and the local cyclic strain range that drives thermomechanical fatigue in DWTC systems
Creep-plasticity-fatigue calculations in the design of porous double layers for new transpiration cooling systems
No full textNew porous double layer (PDL) transpiration cooling technologies can allow gas turbine entry temperatures to be increased beyond current limits towards higher engine efficiency. However, PDL systems require inclined film holes with stress concentration factors in excess of 3.8. Combination of thermoelastic Finite Element (FE) analysis with Neuber type local strain approaches gives similar cyclic strain range predictions with cyclic plasticity-creep FE analysis. Fatigue crack initiation at film holes occurs with a low number of cycles due to excessive plasticity. Our study establishes links between elastic-inelastic analyses and between material phenomena-PDL geometry and indicates pathways of improving life
Designing against severe stresses at compound cooling holes of double wall transpiration cooled engine components
No full textAdvanced thermal protection technology is key for allowing hotter gas turbine cycle temperatures towards minimising fuel consumption and emissions. While effusion holes are essential for reducing heat flux into the structure through the formation of an air cool layer between the hot gas flow and the solid, they can shorten component life due to local stress raising effects. Through Finite Element (FE) analysis, we evaluate the severity of these effects in double wall transpiration cooling (DWTC) systems under thermal loading and identify how mechanical performance can be improved by modifying global and local geometric features. Increasing effusion hole inclination to 60° to the surface normal leads to extreme stress concentration factors (SCFs), which can exceed 5. Eliminating ellipticity in the effusion hole surface is shown to offer enormous performance benefits, by decreasing the SCF by 50%. A narrow spacing between the wall-connecting pedestals implies shorter hole-hole and hole-pedestal distances, which also leads to a reduction of SCFs. Our elastic solutions can be readily used in fatigue life calculations based on Neuber type local strain approaches. They also establish the basis for understanding the response of the new systems under combined thermal-mechanical loading
