Publications
176 results found
Mackay C, Nowell D, 2023, Informed machine learning methods for application in engineering: a review, Proceedings of the Institution of Mechanical Engineers Part C: Journal of Mechanical Engineering Science, Vol: 237, Pages: 5801-5818, ISSN: 0954-4062
Machine Learning (ML) has proved to be successful at identifying and representing underlying relationships in large data sets which would be difficult to process manually. However, the large amounts of data required for unsupervised learning mean that these traditional approaches encounter problems where data is sparse. In addition, these models are often used with insufficient regard for the details of the underlying optimization process. This poses a problem in engineering where the ability to explain model predictions (explainability) is often a prerequisite. There is a particular issue where ML methods may reach a conclusion which does not agree with existing physical understanding. Further, for problems where some of the underlying physics is already known, the traditional ML approach is effectively using large data sets to “re-learn” existing physical understanding. A potential solution to these issues is the incorporation of physical domain knowledge into the model or its training process to produce Informed Machine Learning. This paper provides an overview of the current state of informed machine learning for application in engineering. Firstly, the definition of explainable machine learning is explored. A selection of methods that incorporate physical priories into the machine learning pipeline is then described, leading to a review of current applications of informed machine learning in engineering. As a result of this analysis, a taxonomy is developed which provides a potential path for method development.
Hattingh D, Vormwald M, Taylor D, et al., 2023, Foreword to the special issue, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 175, ISSN: 0142-1123
Yuan J, Salles L, Nowell D, et al., 2023, Influence of mesoscale friction interface geometry on the nonlinear dynamic response of large assembled structures, Mechanical Systems and Signal Processing, Vol: 187, ISSN: 0888-3270
Friction interfaces are unavoidable components of large engineering assemblies since they enable complex designs, ensure alignment, and enable the transfer of mechanical loads between the components. Unfortunately, they are also a major source of nonlinearities and uncertainty in the static and dynamic response of the assembly, due to the complex frictional physics occurring at the interface. One major contributor to the nonlinear dynamic behavior of the interface is the mesoscale geometry of a friction interface. Currently, the effects of the interface geometry on the nonlinear dynamic response is often ignored in the analysis due to the high computational cost of discretizing the interface to such fine levels for classical finite element analysis. In this paper, the influence of mesoscale frictional interface geometries on the nonlinear dynamic response is investigated through an efficient multi-scale modeling framework based on the boundary element method. A highly integrated refined contact analysis, static analysis, and nonlinear modal analysis approach are presented to solve a multi-scale problem where mesoscale frictional interfaces are embedded into the macroscale finite element model. The efficiency of the framework is demonstrated and validated against an existing dovetail dogbone test rig. Finally, the effects of different mesoscale interface geometries such as surface waviness and edge radius, are numerically investigated, further highlighting the influence of mesoscale interface geometries on the nonlinear dynamics of jointed structures and opening a new research direction for the design of friction interfaces in friction involved mechanical systems.
Kouanga C, Jones J, Revill I, et al., 2023, A variable amplitude fretting fatigue life estimation technique: formulation and experimental validation, Tribology International, Vol: 178, ISSN: 0301-679X
The aims of the research work summarised in this paper are twofold. The first goal is to make available a large number of new experimental results generated by testing specimens of grey cast iron under both constant and variable amplitude fretting fatigue loading. The second goal is to formulate an advanced fretting fatigue design approach based on the combined use of the Modified Wӧhler Curve Method, the Theory of Critical Distances and the Shear Stress-Maximum Variance Method. The validation exercise based on the experimental results being produced demonstrates that the proposed methodology is a powerful tool suitable for designing mechanical assemblies against fretting fatigue.
Blades L, Hills D, Nowell D, et al., 2022, The effects of external loading on low displacement wear rates of unlubricated steels, WEAR, Vol: 490-491, ISSN: 0043-1648
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- Citations: 1
Kouanga CVT, Nowell D, Joyce RSD, et al., 2022, Critical Plane and Critical Distance Approaches to Assess Damage Under Variable Amplitude Fretting Fatigue Loading, 9th International Conference on Fracture, Fatigue and Wear (FFW), Publisher: SPRINGER-VERLAG SINGAPORE PTE LTD, Pages: 161-173, ISSN: 2195-4356
Fantetti A, Mariani S, Pesaresi L, et al., 2021, Ultrasonic monitoring of friction contacts during shear vibration cycles, Mechanical Systems and Signal Processing, Vol: 161, ISSN: 0888-3270
Complex high-value jointed structures such as aero-engines are carefully designed and optimized to prevent failure and maximise their life. In the design process, physically-based numerical models are employed to predict the nonlinear dynamic response of the structure. However, the reliability of these models is limited due to the lack of accurate validation data from metallic contact interfaces subjected to high-frequency vibration cycles. In this study, ultrasonic shear waves are used to characterise metallic contact interfaces during vibration cycles, hence providing new validation data for an understanding of the state of the friction contact. Supported by numerical simulations of wave propagation within the material, a novel experimental method is developed to simultaneously acquire ultrasonic measurements and friction hysteresis loops within the same test on a high-frequency friction rig. Large variability in the ultrasound reflection/transmission is observed within each hysteresis loop and is associated with stick/slip transitions. The measurement results reveal that the ultrasound technique can be used to detect stick and slip states in contact interfaces subjected to high-frequency shear vibration. This is the first observation of this type and paves the way towards real-time monitoring of vibrating contact interfaces in jointed structures, leading to a new physical understanding of the contact states and new validation data needed for improved nonlinear dynamic analyses.
Parel K, Paynter R, Nowell D, 2020, Linear relationship of normal and tangential contact stiffness with load, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 476, ISSN: 1364-5021
Measurements with digital image correlation of normal and tangential contact stiffness for ground Ti-6Al-4V interfaces suggest a linear relationship between normal contact stiffness and normal load and a linear relationship between tangential contact stiffness and tangential load. The normal contact stiffness is observed approximately to be inversely proportional to an equivalent surface roughness parameter, defined for two surfaces in contact. The ratio of the tangential contact stiffness to the normal contact stiffness at the start of tangential loading is seen to be given approximately by the Mindlin ratio. A simple empirical model is proposed to estimate both the normal and tangential contact stiffness at different loads for a ground Ti-6Al-4V interface, on the basis of the equivalent surface roughness and the coefficient of friction.
Blades L, Hills D, Nowell D, et al., 2020, An exploration of debris types and their influence on wear rates in fretting, WEAR, Vol: 450, ISSN: 0043-1648
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- Citations: 15
Díaz FA, Vasco-Olmo JM, López-Alba E, et al., 2020, Experimental evaluation of effective stress intensity factor using thermoelastic stress analysis and digital image correlation, International Journal of Fatigue, Vol: 135, Pages: 1-10, ISSN: 0142-1123
During the last decades, the debate over the mechanisms governing fatigue crack shielding has been mainly focused on demonstrating the existence of fatigue crack closure and the difficulties on quantifying the induced stress during crack propagation. Hence, most adopted experimental methods have been based on the direct or indirect measurement of contact loads between crack surfaces as the crack starts closing. Nevertheless, these methods depend on many factors sometime difficult to control, which has contributed to question their reliability by many authors. For this reason, two modern well established, full-field, non-contact experimental techniques, namely Thermoelastic Stress Analysis (TSA) and 2D Digital Image Correlation (2D-DIC), have been analysed to evaluate the influence of crack shielding during fatigue experiments conducted on two aluminium alloys (Al2024-T3 and Al7050) tested at different stress ratios. In the particular case of TSA, the technique appears to have a great potential in the evaluation of fatigue crack shielding since crack tip events are inferred directly from the temperature changes occurring at the crack tip rather than from remote data. Experimental data from both techniques have been employed in combination with two different mathematical models based on Muskhelishvili’s complex potentials to infer the effective range of stress intensity factor. Results from both techniques agree quite well, showing a variation in the stress intensity factor range as the R-ratio changes from 0.1 to 0.5 and illustrating the potential ability of both techniques to account for the shielding effect due to crack closure.
Schwingshackl CW, Nowell D, 2020, The Measurement of Tangential Contact Stiffness for Nonlinear Dynamic Analysis, Pages: 165-167, ISSN: 2191-5644
Nonlinear dynamic models for frictional interfaces require a number of input parameters to allow a realistic representation of the contact interface. Interface geometry and static pressure distributions can be obtained reliably from numerical analysis. However, it is also necessary to measure the friction coefficient, and the tangential and normal contact stiffness. The tangential contact stiffness plays a significant role in the dynamic response, but is very challenging to measure. In this paper quasi-static and dynamic experiments developed at the University of Oxford and at Imperial College London respectively, will be compared and discussed. Of particular interest is the dependence of the stiffness on the static normal load and the overall contact area.
Yin L, Reddyhoff T, Nowell D, 2020, Detailed investigation of brake squeal - Improvement of the squeal test rig and comparison between results and predictions, Pages: 2155-2163
Brake squeal is a long-standing problem in the vibration and tribology fields. This irregular noise causes irritation for vehicle users and passers-by, who may think that the brake components are problematic, although the brake system is working as designed. This study investigates brake squeal through a combination of experiment and simulation. An improved version of the pin-on-disc test rig is developed for squeal testing. Complex eigenvalue analysis is used to extract instability from the finite element model and different friction laws are examined in order to improve prediction.
Yin L, Reddyhoff T, Nowell D, 2020, Y Detailed investigation of brake squeal - Improvement of the squeal test rig and comparison between results and predictions, International Conference on Noise and Vibration Engineering (ISMA) / International Conference on Uncertainty in Structural Dynamics (USD), Publisher: KATHOLIEKE UNIV LEUVEN, DEPT WERKTUIGKUNDE, Pages: 2155-2163
Lopez-Crespo P, Moreno B, Nowell D, 2019, Recent progress on experimental characterisation of fatigue and fracture behaviour of materials, Journal of Strain Analysis for Engineering Design, Vol: 54, Pages: 363-363, ISSN: 0309-3247
Cavalheiro JVS, Nowell D, 2019, Low-cycle fatigue behaviour of an aero-engine disk alloy under non-proportional loading, 12th International Conference on Multiaxial Fatigue and Fracture (ICMFF), Publisher: E D P SCIENCES, Pages: 1-8, ISSN: 2261-236X
The quest for higher efficiency and fuel economy has pushed aeroengines to challenging levels. In order to become more efficient, engines must run at higher bypass ratios and temperatures, resulting in extreme operating conditions for their hottest section. Nickel based superalloys have been used for this application for the past 50 years due to high fatigue strength at elevated temperatures. This paper investigates the deformation behaviour and fatigue lives of a powder metallurgy Nickel-based superalloy developed for discs of high-pressure turbines, i.e. the most demanding section of aeroengines. For that six different non-proportional load paths were carefully selected, where five of them present the same degree of load non-proportionality, to explore load path dependency and the effects of non-proportional multi-axial loading on fatigue lives. Results confirm an additional cyclic hardening caused by load non-proportionality and its detrimental effect on fatigue life. Lives for non-proportional tests were around three times shorter than fatigue lives for proportional tests at comparable stress levels.
Nowell D, Sahadi Cavalheiro JV, 2019, Approaches to fatigue life prediction under multiaxial loading, 12th International Conference on Multiaxial Fatigue and Fracture
Fantetti A, Tamatam, Volvert, et al., 2019, The impact of fretting wear on structural dynamics: experiment and simulation, Tribology International, Vol: 138, Pages: 111-124, ISSN: 0301-679X
This paper investigates the effects of fretting wear on frictional contacts. A high frequency friction rig is used to measure the evolution of hysteresis loops, friction coefficient and tangential contact stiffness over time. This evolution of the contact parameters is linked to significant changes in natural frequencies and damping of the rig. Hysteresis loops are replicated by using a Bouc-Wen modified formulation, which includes wear to simulate the evolution of contact parameters and to model the evolving dynamic behaviour of the rig. A comparison of the measured and predicted dynamic behaviour demonstrates the feasibility of the proposed approach and highlights the need to consider wear to accurately capture the dynamic response of a system with frictional joints over its lifetime.
Nowell D, Nowell SC, 2019, A comparison of recent models for fatigue crack tip deformation, Theoretical and Applied Fracture Mechanics, Vol: 103, Pages: 1-6, ISSN: 0167-8442
Fatigue crack propagation occupies much of the life of engineering components, particularly in the short crack regime. It is important to understand the mechanisms of propagation in order to carry out damage tolerance assessment and to predict component service life. The paper describes experiments carried out at macro- and micro-scale using digital image correlation to measure near-tip displacements. From these, various key parameters governing crack growth are extracted, and different models of fatigue crack deformation are validated. In particular, it is concluded that the Pommier and Hamam and the CJP models for fatigue crack displacement and stress fields have rather similar approaches to capturing the effects of crack tip plasticity.
Kristnama AR, Xu X, Nowell D, et al., 2019, Experimental investigation of high velocity oblique impact and residual tensile strength of carbon/epoxy laminates, Composites Science and Technology, Vol: 182, Pages: 107772-107772, ISSN: 0266-3538
Composite components are required to be resilient against Foreign Object Damage (FOD) induced by localised high velocity impact events. Here an experimental investigation into high velocity oblique impacts and residual tensile strength of thin quasi-isotropic carbon/epoxy laminates is reported. Oblique (45°) impacts between 100 m/s and 350 m/s were carried out using 3 mm steel cubes on the edge and the centre of the laminates, mounted as a cantilever beam. Impact induced damage was characterised using X-ray Computed Tomography (CT) and the residual strength of impacted laminates was determined through quasi-static tensile tests. The residual strength shows a strong dependence on the impact damage size, characterised in terms of fibre fracture width and delamination area. Machined notches were then investigated and compared to impacted laminates in terms of residual strength.
Nowell D, Nowell PW, 2019, A machine learning approach to the prediction of fretting fatigue life, Tribology International, Vol: 141, Pages: 1-8, ISSN: 0301-679X
The paper analyses some fretting fatigue results from the literature, reported by Nowell and bySzolwinski and Farris. The principal variables of contact size, peak pressure, remote specimentension, and tangential force ratio are identified and these are used to construct an Artificial NeuralNetwork (ANN), aimed at predicting total fretting fatigue life. The network is trained and validatedusing 90% of the data, and its success at predicting the results for the remaining 10% of unseen datais examined. The network is found to be very effective at separating the results into low life and‘run-out’ groups. It is less successful at predicting lives for the low life specimens, but this is largelydue to the difficulty of incorporating the runout and finite life tests together in the same dataset.The approach is seen to be potentially useful and identifies contact size as a key variable. However,the results highlight the need for significant numbers of experimental results if the method is to beused effectively in future. Nevertheless, the trained network comprises a useful tool for theprediction of future experimental results with this material.
Fleury R, Salvati E, Nowell D, et al., 2019, The effect of surface damage and residual stresses on the fatigue life of nickel superalloys at high temperature, International Journal of Fatigue, Vol: 119, Pages: 34-42, ISSN: 0142-1123
A methodology for evaluating the effect of surface damage in the fatigue life of nickel superalloys is presented in this paper. Dents generated due to low velocity impacts of hard objects were simulated using a finite element (FE) model. The residual stress distribution underneath the dent root obtained numerically was compared with the measurements on experimentally simulated damaged specimens using ring-core milling at the micron scale through a combined Focused-Ion Beam and Digital Image Correlation technique (FIB-DIC). The numerical and experimental results for the residual stress show good agreement in terms of residual stress trends and magnitudes. The residual stress distribution obtained via the FE model was subsequently used in a fatigue short crack growth model for an estimation of the fatigue life of dented specimens. The fatigue life predictions were then compared with experimental fatigue results for the nickel superalloy at high temperatures. The comparison shows a significant improvement in the prediction of fatigue life of parts with superficial damage due to careful consideration of the residual stresses around the damage.
Sahadi J, Nowell D, Paynter R, 2018, Fatigue life prediction for Waspaloy under biaxial loading, Theoretical and Applied Fracture Mechanics, Vol: 97, Pages: 1-14, ISSN: 0167-8442
This investigation revisits biaxial fatigue experiments carried out with nickel-based superalloy Waspaloy. Recently, yield criteria extended to multiaxial fatigue and stress-based approaches have been analysed, and their performances in correlating biaxial test data have been evaluated. It has been concluded that despite giving reasonable results, the parameters do not properly represent the physical behaviour of the material. An extension of the study is therefore executed using the strain based critical plane approaches proposed by Wang-Brown and Fatemi-Socie, and the energy-based approaches proposed by Smith-Watson-Topper, Liu and Ince-Glinka. Reasonably good fatigue life predictions are obtained with all criteria. However, for low cycle fatigue regime, best correlation is obtained with the Liu parameter.
Nowell D, Dragnevski K, O'Connor S, 2018, Investigation of Fatigue Crack Models by Micro-scale Measurement of Crack Tip Deformation, International Journal of Fatigue, Vol: 115, Pages: 20-26, ISSN: 0142-1123
The paper describes displacement measurements taken in the vicinity of a fatigue crack tip using digital image correlation. In-situ loading is employed in a scanning electron microscope to get measurements very close to the tip. The results are compared to the usual elastic model of crack tip deformation and to the Christopher, James and Patterson model, which is critically discussed. It is shown that the use of an elastic model with a residual stress intensity caused by crack tip shielding gives a good representation of the experimental results.
Kouanga C, Jones J, Revill I, et al., 2018, On the estimation of finite lifetime under fretting fatigue loading, International Journal of Fatigue, Vol: 112, Pages: 138-152, ISSN: 0142-1123
The aim of this paper is to formulate and validate an alternative design approach suitable for predicting finite lifetime of mechanical assemblies subjected to constant amplitude (CA) fretting fatigue loading. The design methodology being proposed is based on the use of the Modified Wӧhler Curve Method (MWCM) applied in conjunction with both the Theory of Critical Distance (TCD) and the Shear Stress-Maximum Variance Method (-MVM). In more detail, the TCD, applied in the form of the Point Method (PM), is used to take into account the damaging effect of the multiaxial stress gradients acting on the material in the vicinity of the contact region. The time-variable linear-elastic stress state at the critical locations is then post-processed according to the MWCM which is a bi-parametrical criterion that estimates fatigue lifetime via the stress components relative to those planes experiencing the maximum shear stress amplitude. Thanks to its specific features, the MWCM is capable of modelling not only the presence of non-zero mean stresses, but also the degree of multiaxiality and non-proportionality of the local load history being investigated. In this setting, the -MVM is used to calculate the stress quantities relative to the critical plane whose orientation is determined numerically by locating that plane containing the direction experiencing the maximum variance of the resolved shear stress. The accuracy and reliability of the proposed design methodology was checked against a number of experimental data taken from the literature and generated by testing four different metallic materials. The agreement between experiments and estimates being obtained strongly supports the idea that the proposed approach can be used to perform a rapid assessment of mechanical assemblies damaged by in-service fretting fatigue loading.
Nowell D, Salvati E, Qi P, 2018, Crack closure modification in dwell-fatigue, 13th International Conference on Advance s in Experimental Mechanics
Fleury RMN, Nowell D, 2017, Evaluating the influence of residual stresses and surface damage on fatigue life of nickel superalloys, 11th International Conference on Fatigue Damage of Structural Materials, Publisher: ELSEVIER SCI LTD, Pages: 27-33, ISSN: 0142-1123
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- Citations: 31
Nowell D, Dragnevski KI, O'Connor SJ, 2017, Measurement and analysis of fatigue crack deformation at the micro-scale, FRATTURA ED INTEGRITA STRUTTURALE, Pages: 197-202, ISSN: 1971-8993
Hills DA, Nowell D, Barber JR, 2017, KL Johnson and contact mechanics, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART C-JOURNAL OF MECHANICAL ENGINEERING SCIENCE, Vol: 231, Pages: 2451-2458, ISSN: 0954-4062
Sahadi JV, Paynter RJH, Nowell D, et al., 2017, Comparison of multiaxial fatigue parameters using biaxial tests of Waspaloy, International Journal of Fatigue, Vol: 100, Pages: 477-488, ISSN: 0142-1123
Fleury RMN, Nowell D, Sui T, et al., 2017, Characterisation of handling and service surface damage on Nickel alloys caused by low velocity impacts of blunt hard objects, Mechanics of Materials, Vol: 107, Pages: 45-55, ISSN: 0167-6636
This paper presents a characterisation of surface damage, more specifically dents, caused by low velocity impacts of blunt objects on RR1000 Nickel superalloys. These are representative of damage that may occur during handling and service of components during manufacturing or maintenance. The characterisation of dents produced in laboratory tests is carried out both in terms of their geometry and the residual stresses the damage. A finite element model is presented and the results are validated in terms of dent geometry produced for different impact velocities. The stress distribution predicted by the numerical model is also compared with experimentally measured stresses via X-ray diffraction for validation of the model. The residual stresses obtained from the finite element (FE) model and their implications to fatigue and crack propagation lives are also discussed here.
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