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• Journal article
  Shao Z, Li N, 2017,

  A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions

  , Journal of Visualized Experiments, Vol: 122, ISSN: 1940-087X

  This protocol proposes a novel biaxial testing system used on a resistance heating uniaxial tensile test machine in order to determine the forming limit diagram (FLD) of sheet metals under hot stamping conditions.

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• Conference paper
  Rounthwaite N, McGilvery CM, Jiang J, Williams R, Giuliani F, Britton TBet al., 2017,

  A chemical and morphological study of diesel injector nozzle deposits - insights into their formation and growth mechanisms

  , SAE 2017 World Congress and Exhibition, Publisher: SAE International, Pages: 106-114, ISSN: 1946-3960

  Modern diesel passenger car technology continues to develop rapidly in response to demanding emissions, performance, refinement, cost and fuel efficiency requirements. This has included the implementation of high pressure common rail fuel systems employing high precision injectors with complex injection strategies, higher hydraulic efficiency injector nozzles and in some cases <100µm nozzle hole diameters. With the trend towards lower diameter diesel injector nozzle holes and reduced cleaning through cavitation with higher hydraulic efficiency nozzles, it is increasingly important to focus on understanding the mechanism of diesel injector nozzle deposit formation and growth. In this study such deposits were analysed by cross-sectioning the diesel injector along the length of the nozzle hole enabling in-depth analysis of deposit morphology and composition change from the inlet to the outlet, using state-of-the-art electron microscopy techniques. Deposits produced in the injector nozzles of the industry standard fouling test (CEC F-98-08 DW10B bench engine) were compared with those formed in a vehicle driven on a chassis dynamometer, using a drive cycle more representative of real world vehicle conditions, to explore the effects of differing drive cycles and engine technologies. Fouling in all tests was accelerated with the addition of 1ppm zinc neodecanoate, as specified in the CEC DW10B test. This in-depth characterisation revealed a complex multi-layered system of deposits inside the diesel injector nozzle. Through analysing these layers the mechanisms enabling the initial deposit formation and growth can be postulated.

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• Journal article
  Jiang J, Dunne F, Britton T, 2017,

  Toward predictive understanding of fatigue crack nucleation in Ni-based Superalloys

  , JOM, Vol: 69, Pages: 863-871, ISSN: 1047-4838

  Predicting when and where materials fail is a holy grail for structural materials engineering. Development of a predictive capability in this domain will optimize the employment of existing materials, as well as rapidly enhance the uptake of new materials, especially in high-risk, high-value applications, such as aeroengines. In this article, we review and outline recent efforts within our research groups that focus on utilizing full-field measurement and calculation of micromechanical deformation in Ni-based superalloys. In paticular, we employ high spatial resolution digital image correlation (HR-DIC) to measure surface strains and a high-angular resolution electron backscatter diffraction technique (HR-EBSD) to measure elastic distortion, and we combine these with crystal plasticity finite element (CPFE) modeling. We target our studies within a system of samples that includes single, oligo, and polycrystals where the boundary conditions, microstructure, and loading configuration are precisely controlled. Coupling of experiment and simulation in this manner enables enhanced understanding of crystal plasticity, as demonstrated with case studies in deformation compatibility; spatial distributions of slip evolution; deformation patterning around microstructural defects; and ultimately development of predictive capability that probes the location of microstructurally sensitive fatigue cracks. We believe that these studies present a careful calibration and validation of our experimental and simulation-based approaches and pave the way toward new understanding of crack formation in engineering alloys.

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• Conference paper
  Shao Z, Li N, Lin J, 2017,

  A New Damage Model for Predicting Forming Limits under Hot Stamping Conditions

  , The International Conference on Plasticity, Damage, and Fracture 2017
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• Journal article
  Shao Z, Li N, Lin J, Dean Tet al., 2017,

  Formability evaluation for sheet metals under hot stamping conditions by a novel biaxial testing system and a new materials model

  , International Journal of Mechanical Sciences, Vol: 120, Pages: 149-158, ISSN: 0020-7403

  Hot stamping and cold die quenching has been developed in forming complex shaped structural components of metals. The aim of this study is the first attempt to develop unified viscoplastic damage constitutive equations to describe the thermo-mechanical response of the metal and to predict the formability of the metal for hot stamping applications. Effects of parameters in the damage evolution equation on the predicted forming limit curves were investigated. Test facilities and methods need to be established to obtain experimental formability data of metals in order to determine and verify constitutive equations. However, conventional experimental approaches used to determine forming limit diagrams (FLDs) of sheet metals under different linear strain paths are not applicable to hot stamping conditions due to the requirements of rapid heating and cooling processes prior to forming. A novel planar biaxial testing system was proposed before and was improved and used in this work for formability tests of aluminium alloy 6082 at various temperatures, strain rates and strain paths after heating, soaking and rapid cooling processes. The key dimensions and features of cruciform specimens adopted for the determination of forming limits under various strain paths were developed, optimised and verified based on the previous designs and the determined heating and cooling method [1]. The digital image correlation (DIC) system was adopted to record strain fields of a specimen throughout the deformation history. Material constants in constitutive equations were determined from the formability test results of AA6082 for the prediction of forming limits of alloys under hot stamping conditions. This research, for the first time, enabled forming limit data of an alloy to be generated at various temperatures, strain rates and strain paths and forming limits to be predicted under hot stamping conditions.

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• Journal article
  Shao Z, Li N, Lin J, Dean TAet al., 2016,

  Development of a new biaxial testing system for generating forming limit diagrams for sheet metals under hot stamping conditions

  , Experimental Mechanics, Vol: 56, Pages: 1489-1500, ISSN: 0014-4851

  Conventional experimental approaches used to generate forming limit diagrams (FLDs) for sheet metals at different linear strain paths are not applicable to hot stamping and cold die quenching processes because cooling occurs prior to deformation and consistent values of heating rate, cooling rate, deformation temperature and strain rate are not easy to obtain. A novel biaxial testing system for use in a Gleeble testing machine has been adopted to generate forming limits of sheet metals, including aluminium alloys, magnesium alloys and boron steel, under practical hot stamping conditions in which heating and cooling occur. For example, the soaking temperature is about 900 °C and the deformation temperature range is 550–850 °C for boron steel [1] and the soaking temperature is about 535 °C and the deformation temperature range is 370–510 °C for AA6082 [2]. Resistance heating and air cooling were introduced in this pioneering system and the thermal analysis of different heating and cooling strategies was investigated based on a type of cruciform specimen. FE models with a UAMP subroutine were used to predict temperature fields on a specimen in ABAQUS 6.12. Digital image correlation (DIC) system was used to record strain fields of a specimen by capturing images throughout the deformation history and its post-processing software ARAMIS was used to determine forming limits according to ISO standards embedded in the software. Heating and cooling strategies were determined after the analysis. Preliminary results of forming limit curves at the designated temperatures are presented in order to verify the feasibility of this new method.

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• Journal article
  Li N, Shao Z, Lin J, Dean TAet al., 2016,

  Investigation of uniaxial tensile properties of AA6082 under HFQ® Conditions

  , Key Engineering Materials, Vol: 716, Pages: 337-344, ISSN: 1013-9826

  © 2016 Trans Tech Publications, Switzerland. For a metal forming process, the uniaxial tensile properties of a material are the most fundamental and important properties to investigate. Solution heat treatment, forming and in-die quenching (HFQ®) is a patented process to form complex shape panel components using aluminium alloys at high efficiency and low cost. A Gleeble materials thermo-mechanical simulator was used to conduct uniaxial tensile testing of AA6082 under HFQ® conditions. A set of grips were specially designed to reduce the heat loss of specimen during testing in a Gleeble and allow the strain measurement by using digital image correlation (DIC) system. A large dog-bone specimen with parallel length of 80mm was designed to minimise the temperature gradient along the gauge section. Temperature gradient was measured and uniaxial tensile tests were conducted at the range of deformation temperature of350-535 °C and the range of strain rate of 0.1-4/s. The uniaxial tensile properties of AA6082 at different temperatures and strain rates under HFQ® conditions were summarised and the viscoplastic response of the material was discussed.

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• Journal article
  Dunne FPE, Guan Y, Britton TB, Jiang J, Chen B, Zou Jet al., 2016,

  Crystal Plasticity Modelling and HR-DIC Measurement of Slip Activation and Strain Localisation in Single and Oligo-crystal Ni Alloys under Fatigue

  , International Journal of Plasticity, Vol: 88, Pages: 70-88, ISSN: 0749-6419

  Single crystal (CMSX4) and oligocrystal (MAR002) nickel have been studied using three-point beambending under conditions of cyclic loading. SEM images have enabled identification of slip activation,and high resolution digital image correlation has been utilized to quantify the developing strain fieldsand the strain localization in both single and oligocrystals in fatigue. The single and oligocrystalmicrostructures have been replicated within crystal plasticity finite element models and the fatigueloading analysed such that grain-by-grain comparisons of slip may be carried out. Single and multipleslip activation, slip localisation and microstructure-sensitive stress evolution have been examined.Single crystal bend fatigue gives rise to non-symmetric slip fields and localisation depending oncrystallographic orientation. Modelling correctly captures slip activation and the developing nonsymmetricslip fields. Oligocrystal slip is markedly heterogeneous, with grain misorientations drivingstrong variations, also reasonably captured by the model. Microstructure behaviour is found to varyspatially and include elastic-plastic hysteresis which is stable, and which undergoes mean stressrelaxation so that plastic shakedown occurs. Remarkable variations occur between locations eitherside of grain boundaries, providing appropriate opportunities for fatigue crack nucleation.

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• Journal article
  Shao Z, Bai Q, LI N, shi Z, lin J, Dean Tet al., 2016,

  Experimental investigation of forming limit curves and deformation features in warm forming of an aluminium alloy

  , Proceedings of the Institution of Mechanical Engineers Part B - Journal of Engineering Manufacture, Vol: 232, Pages: 465-474, ISSN: 0954-4054

  The determination of forming limit curves and deformation features of AA5754 aluminium alloy are studied in this article. The robust and repeatable experiments were conducted at a warm forming temperature range of 200 °C–300 °C and at a forming speed range of 20–300 mm/s. The forming limit curves of AA5754 at elevated temperatures with different high forming speeds have been obtained. The effects of forming speed and temperature on limiting dome height, thickness variation and fracture location are discussed. The results show that higher temperatures and lower forming speeds are beneficial to increasing forming limits of AA5754; however, lower temperatures and higher forming speeds contribute to enhancing the thickness uniformity of formed specimens. The decreasing forming speed and increasing temperature result in the locations of fracture to move away from the apexes of formed specimens. It is found that the analysis of deformation features can provide a guidance to understand warm forming process of aluminium alloys.

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• Conference paper
  Shao Z, Li N, Politis D, Bai Q, Lin Jet al., 2016,

  Analysis on Experimental Techniques for Generating FLD at ElevatedTemperatures

  , 2nd International Conference on Advanced High Strength Steel and Press Hardening (ICHSU2015), Publisher: World Scientific

  The evaluation of the formability of sheet material is of great importance for forming complex-shapecomponents in automotive applications. However, for hot and warming forming conditions, thisusually requires formability tests with specialist devices and testing procedures and is difficult toobtain accurate and comparable results due to the lack of testing standards. In this paper, the conceptand development of strain-based forming limit diagram (FLD) are introduced. The testing methodsto experimentally determine the FLD, classified into formability tests and planar tensile tests, arewidely reviewed. The applications of both types to obtain FLD at elevated temperatures for differentforming processes are introduced, and the advantages and disadvantages of the methods areanalysed.

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• Conference paper
  Shao Z, Li N, Politis D, Bai Q, Lin Jet al., 2016,

  Analysis on Experimental Techniques for Generating FLD at Elevated Temperatures

  , International Conference on Hot Stamping of UHSS (ICHSU2015)
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• Journal article
  Shao Z, Bai Q, Lin J, 2014,

  A novel experimental design to obtain forming limit diagram of aluminium alloys for solution heat treatment, forming and in-die quenching process

  , Key Engineering Materials, Vol: 622-623, Pages: 241-248, ISSN: 1013-9826

  © (2014) Trans Tech Publications, Switzerland. Solution heat treatment, forming and in-die quenching (HFQ) is a patented process to form complex shape metal components at a high efficiency and a low cost. Conventional experiment approaches to determine forming limit curves (FLCs) at different strain paths are not applicable for the HFQ forming process. A novel biaxial tensile test rig is designed to overcome the difficulties and determine the FLCs at high temperatures based on the commercial Gleeble machine. This test device employs the circle plate and connecting rod mechanism in order to achieve different strain states, such as uniaxial tension, plane strain and biaxial tension. Resistance heating and air cooling are adopted to obtain an isothermal environment and to control cooling rates in Gleeble respectively. The designs of the cruciform specimen for this test are also introduced in this paper.

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