Publications
183 results found
Zheng J-H, Davies CM, Lin J, et al., 2017, Constitutive modelling of a T74 multi-step creep ageing behaviour of AA7050 and its application to stress relaxation ageing in age formed aluminium components, International Conference on the Technology of Plasticity, ICTP 2017, Pages: 281-286
© 2017 The Authors. Published by Elsevier Ltd. This work focuses on validating a model, which is defined using load controlled creep ageing data, from strain-controlled stress relaxation ageing tests. A set of phenomenological constitutive equations were proposed. The material constants within the equations were determined from multi-step (120 °C× 6 h +177 °C ×7 h) creep-ageing test data. Temperature effects on the internal state variables (i.e. precipitate size, volume fraction and dislocation density) were also considered. This model was further used to predict the stress relaxation phenomena under the same ageing condition. Although similar stress reduction trends and features were observed, the stress reduction was significantly over-predicted. The over-prediction of the stress reduction magnitude suggests that improvements should be applied to the current modelling method for simulating spring-back after creep age forming.
Zheng J, Davies CM, Lin J, 2017, Comparison of creep deformation rates during load and strain controlled multi-step creep ageing tests on AA7050, 20th International ESAFORM Conference on Material Forming, Publisher: AMER INST PHYSICS, ISSN: 0094-243X
Narayanan A, Dubey K, Davies C, et al., 2017, The creep of alloy 617 at 700 C: Material properties, measurement of strain and comparison between Finite Element Analysis and Digital Image Correlation, International Journal of Solids and Strictures, Vol: 129, Pages: 195-203, ISSN: 0020-7683
Future generations of power plants, such as the Ultra-Super-Critical (USC) power plants, are being designed to be operated at more extreme pressures and temperatures in order to achieve higher efficiency. One candidate material for components is Inconel alloy grade 617, a nickel based superalloy, which is expected to possess better creep resistance in comparison to other types of alloys Bhadeshia and Honeycombe (2011); Evans and Wilshire (1993). At present there is little available data or information about the behaviour of this material at the temperature of interest (700° C) and hence there is a need to evaluate its properties under these conditions.This paper details experimentation on Alloy 617 to evaluate its uniaxial behaviour under tension and creep at 700° C, using the results obtained to develop a creep damage model based on power law creep in conjunction with the Cocks-Ashby void growth approach Cocks and Ashby (1982) for creep in a multiaxial stress state. Finite Element (FE) simulations are compared to experimental results obtained by Digital Image Correlation (DIC), which is used in order to validate the effectiveness of a power law creep damage model. Results made using a novel electrical strain sensor using ACPD principles supplement this work to draw comparisons between the response of the sensor and the strain field experienced by the specimen.
mehmanparast A, Davies CM, 2017, The influence of inelastic pre-straining on fracture toughness behaviour of Type 316H stainless steel, Engineering Fracture Mechanics, Vol: 188, Pages: 112-125, ISSN: 0013-7944
The effects of inelastic deformation, in the form of plastic and creep pre-strain, on the fracture toughness behaviour of Type 316H stainless steel have been investigated in this study. Material pre-conditioning effects on the strain distribution fields ahead of the crack tip have been investigated using digital image correlation technique. Fracture toughness tests have been performed on compact tension specimens made of the as-received, plastic pre-strained and creep pre-strained materials. The influences of specimen side groove and pre-cracking type on the fracture toughness behaviour of 316H stainless steel have also been examined. The test results have shown that the fracture toughness values decrease by increasing the percentage of inelastic pre-strain introduced into the material. Moreover, the generated R-curves and the subsequent fracture toughness values have been found sensitive to the specimen side groove. Finally, it has been observed that local creep damage has more severe impact on the fracture toughness of the material compared to global creep deformation.
Ali AM, Newman S, Hooper P, et al., 2017, The effect of implant position on bone strain following lateral unicompartmental knee arthroplasty. A biomechanical model using digital image correlation, Bone and Joint Research, Vol: 6, Pages: 522-529, ISSN: 2046-3758
ObjectivesUnicompartmental knee arthroplasty (UKA) is a demanding procedure, with tibial component subsidence or pain from high tibial strain being potential causes of revision. The optimal position in terms of load transfer has not been documented for lateral UKA. Our aim was to determine the effect of tibial component position on proximal tibial strain.MethodsA total of 16 composite tibias were implanted with an Oxford Domed Lateral Partial Knee implant using cutting guides to define tibial slope and resection depth. Four implant positions were assessed: standard (5° posterior slope); 10° posterior slope; 5° reverse tibial slope; and 4 mm increased tibial resection. Using an electrodynamic axial-torsional materials testing machine (Instron 5565), a compressive load of 1.5 kN was applied at 60 N/s on a meniscal bearing via a matching femoral component. Tibial strain beneath the implant was measured using a calibrated Digital Image Correlation system.ResultsA 5° increase in tibial component posterior slope resulted in a 53% increase in mean major principal strain in the posterior tibial zone adjacent to the implant (p = 0.003). The highest strains for all implant positions were recorded in the anterior cortex 2 cm to 3 cm distal to the implant. Posteriorly, strain tended to decrease with increasing distance from the implant. Lateral cortical strain showed no significant relationship with implant position.ConclusionRelatively small changes in implant position and orientation may significantly affect tibial cortical strain. Avoidance of excessive posterior tibial slope may be advisable during lateral UKA.
Ahn J, He E, Chen L, et al., 2017, The effect of Ar and He shielding gas on fibre laser weld shape and microstructure in AA 2024-T3, Journal of Manufacturing Processes, Vol: 29, Pages: 62-73, ISSN: 1526-6125
The effect of using argon and helium shielding gas on weld quality, defect formation and microstructure of laser welded aluminium alloy 2024-T3 was investigated. Full penetration autogenous welds were made at a constant laser power of 4.9 kW using a continuous-wave (CW) fibre laser at travel speeds of 3.0–5.0 m/min and focal positions of +4 to −4 mm. To investigate this effect, a comparison was made between Ar and He by examining the weld quality in terms of the face and root weld width, the weld width ratio; and the presence of welding defects including undercut, underfill, reinforcement, porosity and crack. Optical metallography, energy-dispersive X-ray spectroscopy and micro-hardness indentation testing on weld cross-sections were used to identify how the chemical and physical properties of the shielding gases and the characteristics of the fibre laser affect the overall weld geometry. Based on the results, it was believed that relatively small influence of ionisation on fibre laser induced plume enhanced the welding process stability and lowered the threshold power density for keyhole formation. Both Ar and He shielding gases could therefore, be used effectively to produce good quality welds. However, at the lowest speed and also at the maximum focal position, higher ionisation potential and thermal conductivity of helium resulted in an excessive weld width when He was used even though, the overall weld quality was better than that with Ar.
Davies CM, Cacciapuoti B, Sun W, et al., 2017, An evaluation of the capability of data conversion of impression creep test, Materials at High Temperatures, Vol: 34, Pages: 415-424, ISSN: 0960-3409
High temperature power plant components are now working far beyond their operative designed life. Therefore, establishing their in-service material properties is a matter of significant concern. Reliable secondary creep data can be produced by impression creep test (ICT), which requires a small volume of material that can be scooped from in-service critical components. This paper presents an overview of ICT to highlight the capability in determining the minimum creep strain rate (MSR) data by use of conversion relationships that relates uniaxial and impression creep tests data. Some existing ICT data are critically reviewed. Furthermore, this paper presents some new impression creep test data and their correlation with uniaxial data, obtained from a number of steels at different stresses and temperatures. The presented data, in terms of MSR against the reference uniaxial stress, are used to assess the capability and reliability of the ICT method.
Tarnowski KM, Dean DW, Nikbin KM, et al., 2017, Predicting the influence of strain on crack length measurements performed using the potential drop method, Engineering Fracture Mechanics, Vol: 182, Pages: 635-657, ISSN: 0013-7944
The potential drop (PD) crack growth measurement technique is sensitive to strain accumulation which is often erroneously interpreted as crack extension. When testing ductile materials these errors can be significant, but in many cases the optimum method of minimising or suppressing them remains unknown because it is extremely difficult to measure them experimentally in isolation from other sources of error, such non-ideal crack morphology. In this work a novel method of assessing the influence of strain on PD, using a sequentially coupled structural-electrical finite element (FE) model, has been developed. By comparing the FE predictions with experimental data it has been demonstrated that the proposed FE technique is extremely effective at predicting trends in PD due to strain. It has been used to identify optimum PD configurations for compact tension, C(T), and single edge notched tension, SEN(T), fracture mechanics specimens and it has been demonstrated that the PD configuration often recommended for C(T) specimens can be subject to large errors due to strain accumulation. In addition, the FE technique has been employed to assess the significance of strain after the initiation of stable tearing for a monotonically loaded C(T) specimen. The proposed FE technique provides a powerful tool for optimising the measurement of crack initiation and growth in applications where large strains are present, e.g. J-R curve and creep crack growth testing.
Tarnowski KM, Davies CM, Dean DW, et al., 2017, Experimental determination of C* using a hot unload and a modified reference stress method, 14th International Conference on Fracture, Pages: 466-467
© 2017 Chinese Society of Theoretical and Applied Mechanics. All Rights Reserved. During creep crack growth (CCG) testing, large errors in the calculated elastic and plastic load-line displacement (LLD) rates, associated with assumptions of ideal crack geometry and power-law hardening material behavior, significantly influence the experimentally determined value of the crack tip characterizing parameter, C*. To mitigate these errors a novel method of interpreting CCG test data is proposed which combines experimental observations at the start and end of the test, with a modified version of the reference stress method. Using finite element analysis it has been demonstrated that this method can significantly improve the accuracy of the elastic and plastic LLD rates predicted throughout a CCG test.
Kapadia P, Davies C, Pirling T, et al., 2017, Quantification of residual stresses in electron beam welded fracture mechanics specimens (vol 106, pg 106, 2017), INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 113, Pages: 255-255, ISSN: 0020-7683
Li N, Zheng J, Zheng K, et al., 2017, A fast ageing method for stamped heat-treatable alloys, WO2017021742 A1
Nasser M, Davies CM, Nikbin K, 2017, THE INFLUENCE OF AGR GAS CARBURISATION ON THE CREEP AND FRACTURE PROPERTIES OF TYPE 316H STAINLESS STEEL, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
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- Citations: 4
Davies CM, Garg P, Hooper PA, 2017, DEFORMATION AND FRACTURE BEHAVIOUR OF 316L STAINLESS STEEL MANUFACTURED THROUGH SELECTIVE LASER MELTING, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Davies CM, Thomlinson H, Hooper PA, 2017, FATIGUE CRACK INITIATION AND GROWTH BEHAVIOUR OF 316L STAINLESS STEEL MANUFACTURED THROUGH SELECTIVE LASER MELTING, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Riley M-R, Davies CM, Garwood S, 2017, DEVELOPMENT OF A NOVEL TEST PROCEDURE TO INVESTIGATE THE IMPACT OF STRAIN GRADIENTS ON THE FATIGUE ENDURANCE OF STAINLESS STEEL, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Sancho A, Hooper PA, Davies CM, 2017, DUCTILE DAMAGE ASSESSMENT USING CONTINUUM DAMAGE MECHANICS AND METHODOLOGY FOR HIGH STRAIN-RATE DAMAGE ANALYSIS, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
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- Citations: 1
Tarnowski KM, Davies CM, Nikbin KM, et al., 2017, EXPERIMENTAL DETERMINATION OF ELASTIC AND PLASTIC LLD RATES DURING CREEP CRACK GROWTH TESTING, ASME Pressure Vessels and Piping Conference, Publisher: AMER SOC MECHANICAL ENGINEERS
Kapadia P, Davies C, Pirling T, et al., 2016, Quantification of residual stresses in electron beam welded fracture mechanics specimens, International Journal of Solids and Structures, Vol: 106-107, Pages: 106-118, ISSN: 1879-2146
Residual stress measurements have been made in a range of electron beam welded samples to study how the weld induced residual stresses redistributed during fabrication of compact tension, C(T), specimens. The samples were manufactured from Type 316H stainless steel in the ex-service material condition and in material which had been preconditioned by inducing 8% plastic strain. Measurements made using neutron diffraction, slitting and the contour method were generally in good agreement and showed residual stress components of up to three times the base material's yield strength existed in the samples. When sectioning a sample to perform the contour method, large elastic deformations occurred at the cut tip due to the large residual stresses present. A correction was applied to the measured surface displacements to account for this deformation. Neutron diffraction measurements were made at various stages of the fabrication process, which showed significant stress redistribution occurred as the welded samples were machined into C(T) specimens. However the tensile stresses near the crack tip of the C(T) specimens remained large and could significantly influence subsequent crack growth tests.
Ahn J, He E, Chen L, et al., 2016, Prediction and measurement of residual stresses and distortions in fibre laser welded Ti-6Al-4V considering phase transformation, Materials & Design, Vol: 115, Pages: 441-457, ISSN: 0261-3069
Residual stresses and distortions due to time dependent and localised heating imposed during fibre laser welding a 2.0 mm thick titanium alloy Ti-6Al-4V sheet were studied. Sequentially coupled thermo-metallurgical-mechanical simulations were performed to predict welding induced residual stresses and distortion in the fibre laser weld sample, and validated using an experimental database including weld pool geometry and temperature fields. Residual stress measurements were taken using X-ray and neutron diffraction techniques and distortion measurements were recorded using a coordinate measuring machine (CMM). The influence of thermally driven non-isothermal diffusional and diffusionless solid state phase transformations on welding residual stresses and distortions were considered in the numerical model. An internal state variables approach was used to represent the transformed volume fraction of different microstructural phases as a function of cooling rate and peak temperature, and the volumetric change due to temperature variations and phase transformations were calculated. In addition, post weld heat treatment (PWHT) as a method for reducing residual stresses was examined.
Ahn J, Chen L, He E, et al., 2016, Effect of filler metal feed rate and composition on microstructure and mechanical properties of fibre laser welded AA 2024-T3, Journal of Manufacturing Processes, Vol: 25, Pages: 26-36, ISSN: 1526-6125
The influence of aluminium alloy 4043 filler wire feed rate on the weld quality and mechanical properties of high power 5 kW fibre laser welded aluminium alloy 2024-T3 was investigated. Loss of volatile alloying elements such as magnesium and other elements including copper and silicon which all contributed to the hot crack sensitivity was measured using energy dispersive X-ray spectroscopy at different filler wire feed rates. High feed rates of above 4.0 m/min produced instabilities, whereas, low feed rates below 2.0 m/min did not sufficiently modify the chemical composition of the weld pool. The optimum feed rate was found to be in the range between 2 and 3 m/min, where the corresponding dilution ratio of around 9–12% in the weld pool with less than 0.6% silicon content reduced the percentage of Mg2Si and also decreased the solidification temperature and total shrinkage during freezing. The addition of filler metal reduced the risk of welding defects and improved ductility to over 3.5% and a fairly higher tensile strength of around 380 MPa than without. Microstructural examination showed that the addition of filler wire increased the number of finer dimples within the weld, resulting in a purely ductile fracture behaviour, as well as reduced micro hot cracks and porosities.
Pan R, Davies CM, Zhang W, et al., 2016, The effectiveness of cold rolling for residual stress reduction in quenched 7050 aluminium alloy forgings, Metal Forming 2016, Pages: 521-527, ISSN: 1013-9826
© 2016 Trans Tech Publications, Switzerland. Residual stresses are often introduced into aluminum alloys through quenching processes performed to generate the required microstructure. Such residual stresses are known to be deleterious to the integrity of the component.Methods tomitigate residual stresses in quenched components are therefore of great importance. Cold rolling has been proposed as an effective technique to remove residual stresses in large components. In this work, the effectiveness of cold rolling in reducing the residual stresses in quenched blocks AA7050 has been quantified using the neutron diffraction technique. Neutron diffraction measurements have been performed on two blocks one quenched and the other quenched & cold rolled block. Comparing the residual stress distributions pre and post rolling it has been found that cold rolling almost eliminates the tensile residual stresses in the core of the block, however it generates large tensile residual stresses in a shallow region near the surface of the block.
Sancho A, Cox MJ, Cartwright T, et al., 2016, Experimental techniques for ductile damage characterisation, Procedia Structural Integrity, Vol: 2, Pages: 966-973, ISSN: 2452-3216
Ductile damage in metallic materials is caused by the nucleation, growth and coalesce of voids and micro-cracks in the metal matrix when it is subjected to plastic strain. A considerable number of models have been proposed to represent ductile failure focusing on the ultimate failure conditions; however, only some of them study in detail the whole damage accumulation process. The aim of this work is to review experimental techniques developed by various authors to measure the accumulation of ductile damage under tensile loads. The measurement methods reviewed include: stiffness degradation, indentation, microstructure analysis, ultrasonic waves propagation, X-ray tomography and electrical potential drop. Stiffness degradation and indentation techniques have been tested on stainless steel 304L hourglass-shaped samples. A special interest is placed in the Continuum Damage Mechanics approach (CDM) as its equations incorporate macroscopic parameters that can represent directly the damage accumulation measured in the experiments. The other main objective lies in identifying the strengths and weaknesses of each technique for the assessment of materials subjected to different strain-rate and temperature conditions.
Ahn J, Chen L, Davies CM, et al., 2016, Parametric optimisation and microstructural analysis on high power Yb-fibre laser welding of Ti-6Al-4V, Optics and Lasers in Engineering, Vol: 86, Pages: 156-171, ISSN: 1873-0302
In this work thin sheets of Ti–6Al–4V were full penetration welded using a 5 kW fibre laser in order to evaluate the effectiveness of high power fibre laser as a welding processing tool for welding Ti–6Al–4V with the requirements of the aircraft industry and to determine the effect of welding parameters including laser power, welding speed and beam focal position on the weld microstructure, bead profile and weld quality. It involved establishing an understanding of the influence of welding parameters on microstructural change, welding defects, and the characteristics of heat affected zone (HAZ) and weld metal (WM) of fibre laser welded joints. The optimum range of welding parameters which produced welds without cracking and porosity were identified. The influence of the welding parameters on the weld joint heterogeneity was characterised by conducting detailed microstructural analysis.
Narayanan A, Maharaj C, Kelly M, et al., 2016, Recent developments in measuring creep strain in high temperature plant components, Strain, Vol: 52, Pages: 467-477, ISSN: 1475-1305
Accurate measurements of creep strain are necessary to evaluate the condition and predict the remaining life of power plant constituent materials. Optical techniques are appropriate for this purpose as they are a non-contact method and can therefore be used to measure strain without requiring direct access to the surface. Within this class of techniques, the Auto-Reference Creep Management And Control (ARCMAC) camera system can be used to calculate the strain between two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two iterations in system design, the Conventional ARCMAC and Digital Single-Lens Reflex (DSLR) ARCMAC.Experiments are conducted to determine the absolute limit of accuracy of the systems in comparison to a strain gauge, and the relative accuracy across several orders of magnitude until specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated temperatures to evaluate the effect of temperature on the gauges and to investigate whether its accuracy diminishes in creep conditions.It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 µε or less. In accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable displacement transducer when used to measure creep strain. Strain variations (under 500 µε) were noted on a steel plate subjected only to operational temperature and no stress. This error is very reasonable compared to a critical strain value of 93,000 µε in a given high temperature-service material. Digital Image Correlation (DIC) results using the DSLR ARCMAC system show approximately 4% error in measurement for plastic strains in the specimen. The two measures of strain measurement (using ARCMAC and DIC) can serve to complement each other.
Corcoran J, Hooper P, Davies C, et al., 2016, Creep strain measurement using a potential drop technique, International Journal of Mechanical Sciences, Vol: 110, Pages: 190-200, ISSN: 0020-7403
This paper will demonstrate the use of a potential drop sensor to monitor strain. In particular, the suitability of the technique to high temperature or harsh environment applications presents an opportunity for monitoring strain in components operating under creep conditions. Monitoring creep damage in power station components is a long standing technological challenge to the non-destructive evaluation community. It is well established in the literature that strain rate serves as an excellent indicator of the progress of creep damage and can be used for remnant life calculations. To facilitate the use of such strain rate based evaluation methods, a permanently installed, strain sensitive, potential drop technique has been developed. The technique has very simple and robust hardware lending itself to use at high temperatures or in harsh environments. Strain inversions are presented and demonstrated experimentally; a room temperature, plastic deformation experiment is used for validation and additionally an accelerated creep test demonstrates operation at high temperature (600 °C+). Excellent agreement is shown between potential drop inverted strain and control measurements.
Mehmanparast A, Davies CM, Dean DW, et al., 2016, Effects of plastic pre-straining level on the creep deformation, crack initiation and growth behaviour of 316H stainless steel, International Journal of Pressure Vessels and Piping, Vol: 141, Pages: 1-10, ISSN: 1879-3541
Politis NJ, Politis DJ, Davies CM, et al., 2016, A method of determining unified viscoplastic constitutive equations for hot forging simulations, Pages: 251-261, ISSN: 1013-9826
Constitutive equations have been used extensively to accurately describe material properties over a wide range of temperatures and strain rates in numerical simulations. In this paper, an algorithmic method of determining the constants of such constitutive equations is presented. The Genetic Algorithm implementation utilising MATLAB is described, and example fits to experimental data are presented.
Alang NA, Davies CM, Nikbin KM, 2016, Low Cycle Fatigue Behaviour of Ex-Service P92 Steel at Elevated Temperature, 21st European Conference on Fracture (ECF), Publisher: ELSEVIER SCIENCE BV, Pages: 3177-3184, ISSN: 2452-3216
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- Citations: 7
Ejaz M, Davies CM, Dean DW, 2016, Interpretation of creep crack growth data for 1/2 CMV steel weldments, 21st European Conference on Fracture (ECF), Publisher: ELSEVIER SCIENCE BV, Pages: 903-910, ISSN: 2452-3216
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Mehmanparast A, Davies CM, Nikbin K, 2016, Creep-Fatigue Crack Growth Testing and Analysis of Pre-strained 316H Stainless Steel, 21st European Conference on Fracture (ECF), Publisher: ELSEVIER SCIENCE BV, Pages: 785-792, ISSN: 2452-3216
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