Publications
176 results found
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
Nowell D, 2017, CHALLENGES IN ENGINEERING EDUCATION IN THE UK, 7th International Conference on Mechanics and Materials in Design (M2D), Publisher: INEGI-FEUP, Pages: 13-14
Nowell D, 2017, DESIGN AGAINST FATIGUE FAILURE: CHALLENGES AND SOLUTIONS, 7th International Conference on Mechanics and Materials in Design (M2D), Publisher: INEGI-FEUP, Pages: 3-4
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
Nowell D, O Connor SJ, Dragnevski KI, 2015, Measurement and analysis of fatigue crack deformation on the macro- and micro-scale, FRATTURA ED INTEGRITA STRUTTURALE, Pages: 1-7, ISSN: 1971-8993
Hills DA, Nowell D, 2014, Mechanics of fretting fatigue-Oxford’s contribution, TRIBOLOGY INTERNATIONAL, Vol: 76, Pages: 1-5, ISSN: 0301-679X
This article presents an historical review of studies of the mechanics of fretting fatigue, concentrating on those carried out at the University of Oxford. It includes, particularly, the seminal initial studies conducted under the direction of Prof J.J.O’Connor in the early 1970s, and brings the picture up to date. It concludes with some consideration of what the next stage of challenges facing fundamental studies are. (C) 2013 Elsevier Ltd. All rights reserved.
Fleury RMN, Paynter RJH, Nowell D, 2014, The influence of contacting Ni-based single-crystal superalloys on fretting fatigue of Ni-based polycrystalline superalloys at high temperature, TRIBOLOGY INTERNATIONAL, Vol: 76, Pages: 63-72, ISSN: 0301-679X
Fretting fatigue tests at high temperature were carried out with single-crystal and polycrystalline alloys. The influence of the crystallographic orientation of contacting single-crystal pads on the lives of polycrystalline specimens is investigated. An analytical formulation of the contact problem between two anisotropic cylinders is used to study the changes in peak stress and contact width due to the orientation of the single-crystal. Finite element analysis is also used to study the variation of stress near the contact edge due to crystallographic orientation. An area averaging method is suggested to correlate fretting fatigue and plain fatigue curves. Using finite element analysis combined with the area averaging method, a better correlation was found between fretting fatigue results and plain fatigue. (C) 2014 Elsevier Ltd. All rights reserved.
Achintha M, Nowell D, Fufari D, et al., 2014, Fatigue behaviour of geometric features subjected to laser shock peening: Experiments and modelling, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 62, Pages: 171-179, ISSN: 0142-1123
Finite element models, using the eigenstrain approach, are described that predict the residual stress fields associated with laser shock peening (LSP) applied to aerospace grade aluminium alloys. The model was used to explain the results of laboratory fatigue experiments, containing different LSP patch geometries, supplementary stress raising features and different specimen thickness. It is shown that interactions between the LSP process and geometric features are the key to understanding the subsequent fatigue strength. Particularly relevant for engineering application, is the fact that not all instances of LSP application provided an improvement in fatigue performance. Although relatively deep surface compressive residual stresses are generated which can resist fatigue crack initiation in these regions, a balancing tensile stress will always exist and its location must be carefully considered. (C) 2013 Elsevier Ltd. All rights reserved.
Mulvihill DM, Brunskill H, Kartal ME, et al., 2013, A Comparison of Contact Stiffness Measurements Obtained by the Digital Image Correlation and Ultrasound Techniques, EXPERIMENTAL MECHANICS, Vol: 53, Pages: 1245-1263, ISSN: 0014-4851
The digital image correlation (DIC) and ultrasound techniques have both previously been employed to measure the contact stiffness of real engineering interfaces, but a comprehensive comparison of these techniques has not previously been carried out. Such a comparison is addressed in the present paper. The principal novelty in this work is that DIC and ultrasound are used to simultaneously measure contact stiffness in the same tests and on the same contact interface. The results show that ultrasound measures somewhat higher contact stiffness magnitudes than DIC: at an average normal contact pressure of 70 MPa, ultrasound was around three times stiffer. Given that the techniques are vastly different in their measurement approach (DIC measures micron-scale relative displacements from external side-on images of the interface, while ultrasound uses the reflection of an ngstrom scale ultrasonic perturbation from the interior of the interface itself), this level of agreement is thought to be encouraging. The difference in results can partly be explained by consideration of inherent physical differences between the techniques which have previously received little attention. Ultrasound measurement will always give the local elastic ‘unloading stiffness’ (even at a plastically deforming contact); whereas, a load-deflection technique like DIC, will give the ‘loading stiffness’. The reason for this difference is discussed in the paper and tests carried out under increasing tangential load in the pre-sliding regime illustrate this difference experimentally. Under normal loading, the increase in real contact area obscures the effect to some extent as both DIC and ultrasound stiffnesses increase with normal load. The results suggest that rough interfaces may be satisfactorily modelled as a variable stiffness spring whose stiffness increases with contact pressure as the smooth contact case is approached.
Achintha M, Nowell D, Shapiro K, et al., 2013, Eigenstrain modelling of residual stress generated by arrays of laser shock peening shots and determination of the complete stress field using limited strain measurements, SURFACE & COATINGS TECHNOLOGY, Vol: 216, Pages: 68-77, ISSN: 0257-8972
This paper presents a hybrid explicit finite element (FE)/eigenstrain model for predicting the residual stress generated by arrays of adjacent/overlapping laser shock peening (LSP) shots where the use of a completely explicit FE analysis may be impractical. It shows that for a given material, the underlying eigenstrain distribution (in contrast to the resulting stress field) representing a laser shock peen is primarily dependent on the parameters of the laser pulse and the number of overlays rather than the precise component geometry. Consequently the residual stress introduced by complex laser peening treatments can be built up by using static FE models and superposition of individual eigenstrain distributions without recourse to further computationally demanding explicit FE analyses. It is found that beneath a small LSP array the magnitude of the compressive residual stress is higher than for a wider array of LSP shots and that with increasing numbers of layers the compressive stress increases as does the depth of the compressive zone. The model predictions for the eigenstrain distributions are compared well with experimental measurements of plastic strain (full-width-at-half-maximum) obtained by neutron diffraction. The eigenstrain method is also extended to construct the full residual stress field using measured residual elastic strains at a finite number of measurement locations in a component. (C) 2012 Elsevier B.V. All rights reserved.
Nowell D, Kartal ME, De Matos PFP, 2013, Digital image correlation measurement of near-tip fatigue crack displacement fields: constant amplitude loading and load history effects, FATIGUE & FRACTURE OF ENGINEERING MATERIALS & STRUCTURES, Vol: 36, Pages: 3-13, ISSN: 8756-758X
Recent work by de Matos and colleagues employed digital image correlation to measure near tip displacement fields for fatigue cracks in 6082 T6 aluminium alloy. The main focus of this work was to directly measure fatigue crack closure, but the measurements can also be used to examine conditions at and ahead of the crack tip. In this paper, the results are re-analysed and compared to two crack-tip deformation models. The first assumes simple elastic deformation (according the Westergaard solution). This allows the history of crack-tip stress intensity to be examined. Reasonable agreement with the elastic model is obtained, although there is a residual stress intensity caused by the plastic wake, which gives rise to crack closure. The second model examined is a simple elasticplastic assumption, proposed by Pommier and colleagues. This can be applied to constant amplitude loading, although the results obtained here are very similar to the elastic case. A slightly more complex load case (a single overload in an otherwise constant amplitude variation of load) gives a much more complicated crack-tip history. Here, the importance of crack-tip plastic displacement, represented by the second term in Pommier’s model becomes much clearer. Load history effects are captured by the residual value of this term and its associated displacement fields as well as by stress intensity factor. The implications for further modelling and experimental work are discussed.
James MN, Yates JR, Susmel L, et al., 2013, Characterisation of Crack Tip Stress Fields, Proceedings of the 1st Joint International Journal of Fatigue/Fatigue & Fracture of Engineering Materials & Structures conference, held in Forni di Sopra, UD, ITaly, 7-9 March, 2011, INTERNATIONAL JOURNAL OF FATIGUE, Vol: 46, Pages: 1-1, ISSN: 0142-1123
James MN, Yates JR, Susmel L, et al., 2013, Guest Editorial: Special Issue on Characterisation of Crack Tip Stress Fields, FATIGUE & FRACTURE OF ENGINEERING MATERIALS & STRUCTURES, Vol: 36, Pages: 1-2, ISSN: 8756-758X
Medina S, Nowell D, Dini D, 2013, Analytical and Numerical Models for Tangential Stiffness of Rough Elastic Contacts, TRIBOLOGY LETTERS, Vol: 49, Pages: 103-115, ISSN: 1023-8883
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- Citations: 68
Mintzas A, Nowell D, 2012, Validation of an H-cr-based fracture initiation criterion for adhesively bonded joints, ENGINEERING FRACTURE MECHANICS, Vol: 80, Pages: 13-27, ISSN: 0013-7944
The singular stress fields that develop around wedges found in adhesively bonded scarf and double lap joint configurations are fully determined by employing Williams’ eigenfunction expansion method in combination with a path independent contour integral method. The intensities of the near-tip stress fields (H) are then used as fracture initiation parameters in order to predict the strengths of these joints. The predicted strengths are compared to experimental results found in literature and the conditions for the validity of the proposed H-cr-based fracture initiation criterion are examined. (C) 2011 Elsevier Ltd. All rights reserved.
Mulvihill DM, Kartal ME, Nowell D, et al., 2011, An elastic-plastic asperity interaction model for sliding friction, TRIBOLOGY INTERNATIONAL, Vol: 44, Pages: 1679-1694, ISSN: 0301-679X
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- Citations: 76
Cartwright RP, Paynter RJH, Hills DA, et al., 2011, Long term wear of complete contacts subject to fretting, WEAR, Vol: 271, Pages: 2821-2825, ISSN: 0043-1648
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- Citations: 2
Kartal ME, Mulvihill DM, Nowell D, et al., 2011, Measurements of pressure and area dependent tangential contact stiffness between rough surfaces using digital image correlation, TRIBOLOGY INTERNATIONAL, Vol: 44, Pages: 1188-1198, ISSN: 0301-679X
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- Citations: 58
Mulvihill DM, Kartal ME, Olver AV, et al., 2011, Investigation of non-Coulomb friction behaviour in reciprocating sliding, WEAR, Vol: 271, Pages: 802-816, ISSN: 0043-1648
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- Citations: 42
Achintha M, Nowell D, 2011, Eigenstrain modelling of residual stresses generated by laser shock peening, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 211, Pages: 1091-1101, ISSN: 0924-0136
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- Citations: 73
Kartal ME, Mulvihill DM, Nowell D, et al., 2011, Determination of the Frictional Properties of Titanium and Nickel Alloys Using the Digital Image Correlation Method, EXPERIMENTAL MECHANICS, Vol: 51, Pages: 359-371, ISSN: 0014-4851
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- Citations: 43
Kartal ME, Barber JR, Hills DA, et al., 2011, Partial slip problem for two semi-infinite strips in contact, INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE, Vol: 49, Pages: 203-211, ISSN: 0020-7225
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- Citations: 11
Kartal ME, Mulvihill DM, Nowell D, et al., 2011, Measurement of tangential contact stiffness in frictional contacts: the effect of normal pressure, 8th International Conference on Advances in Experimental Mechanics: Integrating Simulation and Experimentation for Validation, Publisher: TRANS TECH PUBLICATIONS LTD, Pages: 321-326, ISSN: 1660-9336
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- Citations: 3
Achintha M, Nowell D, 2011, Eigenstrain modelling of residual stresses generated by arrays of LSP shots, 11th International Conference on the Mechanical Behavior of Materials (ICM), Publisher: ELSEVIER SCIENCE BV, Pages: 1327-1332, ISSN: 1877-7058
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- Citations: 7
Mulvihill DM, Kartal ME, Nowell D, et al., 2011, Investigation of the friction variation with sliding which is commonly observed in individual fretting test cycles, 8th International Conference on Advances in Experimental Mechanics: Integrating Simulation and Experimentation for Validation, Publisher: TRANS TECH PUBLICATIONS LTD, Pages: 213-218, ISSN: 1660-9336
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- Citations: 2
Nowell D, Paynter RJH, De Matos PFP, 2010, Optical methods for measurement of fatigue crack closure: moire interferometry and digital image correlation, FATIGUE & FRACTURE OF ENGINEERING MATERIALS & STRUCTURES, Vol: 33, Pages: 778-790, ISSN: 8756-758X
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- Citations: 29
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