167 results found
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.
Blades L, Hills D, Nowell D, et al., 2021, The effects of external loading on low displacement wear rates of unlubricated steels, Wear, ISSN: 0043-1648
Whilst contact loading is known to affect wear, the general stress field is rarely considered. Steel fretting (and low amplitude reciprocating) wear contacts typically develop through a transient regime and wear by multiple mechanisms. It is expected that if these mechanisms were controlled by plasticity or fatigue, the wear rate would be altered by external stresses. Whether or not these stresses must be accounted for is an important consideration. This paper assesses the sensitivity of wear rate to external stresses, experimentally. An apparatus was designed to apply external loads to fretting wear contacts. The wear rates throughout the tests were insensitive to changes in external load, indicating that wear models need only model stresses due to contact loading, and external loads can be disregarded.
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
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.
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
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.
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
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
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
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
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.
Fleury RMN, Paynter RJH, Nowell D, 2017, Estimation of the coefficient of friction in partial slip contacts between contacting nickel superalloys, TRIBOLOGY INTERNATIONAL, Vol: 108, Pages: 156-163, ISSN: 0301-679X
Salvati E, O Connor S, Sui T, et al., 2016, A study of overload effect on fatigue crack propagation using EBSD, FIB-DIC and FEM methods, Engineering Fracture Mechanics, Vol: 167, Pages: 210-223, ISSN: 0013-7944
Abrupt increase in the maximum load during fatigue cycling modifies the deformation conditions at the crack tip, causing plastic flow that may lead to crack closure, introducing residual stress and hardening. The net consequence of these effects is notable crack growth retardation. Despite decades of research in the field, controversy persists regarding the role of each specific mechanism and their interaction. Resolving these issues with the help of experimental observation is related to the difficulty of obtaining local residual stress information at appropriate resolution. The present study examines the effect of overload on fatigue crack grown in a Compact Tension (CT) specimen of aluminium alloy AA6082 (BS HE30). Fatigue crack was grown in the sample under cyclic tension (R = 0.1). After the application of a single overload cycle, fatigue loading was recommenced under previous cycling conditions. The crack morphology was investigated using Scanning Electron Microscopy (SEM). Electron Backscattered Diffraction (EBSD) was used to map grain orientation and crystal lattice distortion (pattern quality) in the vicinity of the crack. EBSD analysis of intra-granular misorientation allowed the qualitative analysis of the region around the crack tip location at the time of the overload application. Observations are discussed with a view to identify the roles of crack closure and residual stress effects. Residual stress was evaluated at salient locations around the crack retardation site using the FIB–DIC method which combines the use of Focused Ion Beam (FIB) and Digital Image Correlation (DIC) for residual stress measurement at the (sub)micron-scale. The residual stress field due to overload occurrence was also simulated using Finite Element Method (FEM), and the results compared with experimental observations.
O'Connor SJ, Nowell D, Dragnevski KI, 2016, Measurement of fatigue crack deformation on the macro- and micro-scale: Uniform and non-uniform loading, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 89, Pages: 66-76, ISSN: 0142-1123
Baietto M-C, Burns J, Nowell D, et al., 2015, International Conference on Fatigue Damage of Structural Materials, International Journal of Fatigue, Vol: 82, Pages: 119-119, ISSN: 0142-1123
Nowell D, 2015, Strain Analysis: Past, Present, and Future, JOURNAL OF STRAIN ANALYSIS FOR ENGINEERING DESIGN, Vol: 50, Pages: 411-411, ISSN: 0309-3247
Achintha M, Nowell D, 2015, Residual stress in geometric features subjected to laser shock peening, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART C-JOURNAL OF MECHANICAL ENGINEERING SCIENCE, Vol: 229, Pages: 1923-1938, ISSN: 0954-4062
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