Multiaxial Variable Amplitude Fatigue of Aluminium–Steel Welded Joints with In-Situ Full-Field Measurement

About the project

About the Project There is no funding attached to this project. The Industry and Innovation Research Institute (I2Ri) draws on talents, expertise and facilities across Sheffield Hallam University. The vision is to be the leading provider of applied research excellence delivering materials, computing, science and engineering innovations meeting the development needs of industry. PhD Research Topic The structural integrity of lightweight hybrid structures is becoming increasingly critical in sectors such as automotive, rail, offshore and renewable energy. Aluminium-to-steel welded joints offer an attractive route to combine weight reduction with mechanical performance; however, their fatigue behaviour under realistic service loading remains insufficiently understood. While existing studies have largely focused on constant amplitude loading, real components experience complex multiaxial variable amplitude fatigue, often involving non-proportional loading histories, load sequence effects and transient overloads. This project will address this gap by developing a rigorous experimental and modelling framework to characterise and predict the fatigue performance of aluminium–steel welded tubular joints subjected to multiaxial variable amplitude loading. Building on recent advances in multiaxial fatigue assessment, the project will move beyond simplified loading scenarios and provide new insight into fatigue damage mechanisms under realistic service conditions. Aims and Objectives The overarching aim is to establish physically sound and practically applicable methodologies for fatigue life prediction of aluminium-to-steel welded joints under complex loading histories. Specific objectives include: · Generate a comprehensive experimental dataset under multiaxial variable amplitude loading (combined tension–torsion, proportional and non-proportional paths). · Quantify the influence of load sequence, overloads/underloads, and mean stress evolution on fatigue damage accumulation. · Apply and critically assess advanced fatigue criteria (e.g. MWCM, critical plane approaches, and TCD-based methods) under variable amplitude conditions. · Develop improved life prediction strategies suitable for engineering design. Experimental Programme A central component of the project will be an extensive experimental campaign on coldArc®-welded AA6082-T6 aluminium to S235 steel tubular joints. Specimens will be tested using state-of-the-art servo-hydraulic multiaxial fatigue rigs capable of applying synchronised tension–torsion loading with arbitrary load histories. Loading scenarios will include: · Variable amplitude spectra representative of real service conditions · Block loading sequences to investigate interaction effects · Non-proportional loading paths with evolving phase angles · Load histories incorporating overloads and spectrum truncation A key novelty of this project is the integration of in-situ full-field deformation measurements using Digital Image Correlation (DIC). Advanced Measurement and Analysis (DIC) DIC will be employed to capture high-resolution strain fields at the weld toe and surrounding material during cyclic loading. This will enable: · Direct observation of strain localisation and crack initiation mechanisms · Quantification of cyclic plasticity and ratcheting effects under variable amplitude loading · Identification of critical locations and characteristic material lengths for fatigue assessment · Validation of local stress–strain approaches and numerical models By linking full-field strain evolution to fatigue life, the project will provide unprecedented insight into the mechanics governing damage initiation in dissimilar welded joints. Modelling and Fatigue Assessment The experimental results will be used to evaluate and extend established fatigue assessment methods, including: · Nominal stress approaches (IIW, Eurocode 9) adapted for variable amplitude loading · Local approaches such as the Theory of Critical Distances (TCD) and fictitious notch radius methods · Critical plane-based multiaxial fatigue criteria incorporating load path dependency Special attention will be given to damage accumulation models capable of accounting for sequence effects (e.g. non-linear Miner’s rule extensions or energy-based approaches). The goal is to identify robust yet practical methodologies for industrial application. Impact and Relevance This project will deliver: · New experimental evidence on fatigue behaviour under realistic loading conditions · Validated methodologies for fatigue life prediction of hybrid welded structures · Design-relevant guidance for engineers working with aluminium–steel joints The outcomes will be directly relevant to industries seeking lightweight, durable and reliable structural solutions, particularly where variable amplitude loading is unavoidable. Candidate Profile Applicants should have a strong background in mechanical engineering, materials engineering or a related discipline. Knowledge of fatigue, solid mechanics, or experimental methods is desirable. Experience with optical measurement techniques or numerical modelling would be advantageous but is not essential. Why Apply? This PhD offers the opportunity to work at the forefront of fatigue and structural integrity research, combining advanced experimentation with cutting-edge modelling. You will gain expertise in multiaxial fatigue, full-field measurement techniques, and engineering design methodologies, while contributing to challenges of clear industrial importance. The project will be conducted within a dynamic and internationally recognised research environment, with strong links to industry and opportunities for collaboration and dissemination. Eligibility All applicants should hold a strong undergraduate degree (2.1 or above) and/or a relevant masters qualification (or expectation of the same). We welcome applications from all candidates irrespective of age, pregnancy and maternity, disability, gender, gender identity, sexual orientation, race, religion or belief, or marital or civil partnership status. International candidates are required to provide an IELTS certificate with a score of at least 6.5 overall, and a minimum of 6.0 in all components. For further information on English Language requirements, please click here . For further details on entry requirements, please click here . How to apply All applications must be submitted using the online application form . In your application, be sure to include the title of the project that you are applying for. As part of your application, please upload: · A research proposal (max. 1500 words) in your own words, briefly outlining the proposed research, the current knowledge and context referencing key background literature; a proposed methodology or approach to answer the key questions, and any potential significance or impact of the research · Copy of your highest degree certificate · Non-UK applicants must submit IELTs results (or equivalent) taken in the last two years and a copy of their passport. Applicants must provide 2 references, with at least one to be academic. References must be received directly from the referees. We strongly recommend you contact the lead academic, (Prof Luca Susmel - L.Susmel@shu.ac.uk ) , to discuss your application. For information on how to apply please visit https://www.shu.ac.uk/research/degrees

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