Research

Ductile fracture and failure of marine steels

The lab's longest-running line. We measure how shipbuilding and offshore steels deform, neck and fracture under large strain, calibrate fracture models against that data, and implement them as user material subroutines so the models can be used in the explicit finite element analyses that structural assessment actually relies on.

Current work includes

• Calibration of ductile fracture models (Hosford-Coulomb, GTN, BWH instability criteria) for shipbuilding and high-tensile steels
• User material subroutines (VUMAT) for shell and solid elements, including the mesh-size regularisation that shell fracture prediction depends on
• Localised necking as a separate failure mechanism from fracture, and the strain rate and thermal softening effects that govern it
• Material testing across stress states and temperatures, down to sub-zero service conditions

Fatigue, crack growth and engineering critical assessment

This is a separate question from how a plate tears in an accident. Here the structure already contains a defect, the loading is cyclic rather than extreme, and the decision is whether the structure is fit to remain in service. We assess flaws against fracture and fatigue criteria, and work on the wave climate assumptions that fatigue life estimates rest on.

Current work includes

• Engineering critical assessment (ECA) and fitness-for-service evaluation of flawed welded structures
• Fatigue crack growth in welded joints, and the influence of weld geometry and residual stress
• Hull girder fatigue under revised wave scatter diagrams, and the sensitivity of damage ratios to that choice
• Defect assessment for alternative marine fuel containment, including low-temperature toughness of cryogenic steels

Collision, grounding and crashworthiness of ship structures

What a hull does in the seconds after an accident decides whether it floods. We simulate collision, grounding and dropped object impact up to and past the point where plating tears, using fracture criteria calibrated on our own material tests rather than a fixed failure strain, and carry the result through to accidental limit state assessment.

Current work includes

• Ship collision and grounding analysis with fracture-based failure criteria, up to accidental limit state assessment
• Plate tearing and perforation, including the aluminium structures used in high-speed craft
• Dropped object impact, and slamming loads on bow and bottom structures
• Coupled analysis of collision between floating bodies, where hydrodynamic reaction changes the energy available to damage the structure

Blast, underwater explosion and naval survivability

Naval structures are designed against loads that are rarely measured and never repeated. We predict the damage extent and the whipping response of a hull subjected to underwater explosion and to in-compartment blast, and connect that damage to whether the ship and its systems remain able to fight.

Current work includes

• Underwater explosion loading, hull girder whipping, and the effect of hull inelasticity on the whipping response
• Damage extent prediction for in-compartment explosions in warships
• Shock acceleration at equipment and armament mounts, and its sensitivity to the fracture model used
• Air blast response of marine grade aluminium plated structures

Ultimate strength and progressive collapse

The ultimate limit state of a hull girder or a stiffened panel is still computed today with methods whose numerical form has barely changed since the 1980s. We work on the collapse behaviour itself - buckling, load shedding, and interaction between failure modes - and on reformulating classical strength methods in modern differentiable computing frameworks, so that gradient-based design optimisation and sensitivity analysis become natural rather than bolted on.

Current work includes

• Progressive collapse of hull girders under combined and unsymmetrical loading, intact and damaged
• Ultimate strength of stiffened panels including welding residual stress and deformation effects
• Buckling and collapse of ring-stiffened cylindrical shells, including pressure hulls
• Differentiable reimplementation of incremental collapse methods in JAX, enabling automatic sensitivities of ultimate strength with respect to scantlings and material parameters

Where this comes from. This thrust continues the incremental-iterative tradition of simplified progressive collapse analysis developed at Newcastle University, whose methods for the elasto-plastic collapse of plates and stiffened panels were incorporated into UK naval structural design standards and the submarine design manual, and were later adopted by classification societies for merchant ships. Professor Seo held a post in marine structures at Newcastle University from 2014 to 2017.

Methods and tools. Nonlinear finite element analysis in Abaqus; Smith-type progressive collapse methods; differentiable numerical implementation in Python and JAX. See the software portfolio.

Moorings and dynamic power cables for floating offshore wind

Mooring lines and subsea dynamic power cables are among the least monitored and, statistically, the least reliable subsystems of a floating wind turbine. We work on the mechanics that sets their fatigue life: the cross-section behaviour that fixes a cable's bending stiffness, the global dynamics that sets its curvature, and the long-term damage accumulation that follows from both.

Current work includes

• Cross-section mechanics of layered power cable constructions, linking manufacturer-level cable design to the stiffness properties used in global dynamic analysis
• Shape and curvature of lazy-wave dynamic cables, where fatigue damage concentrates at the sag and hog bends and the touchdown zone
• Inter-wire slip and hysteresis in helical armour, measured in moment-curvature tests and resolved component by component
• Long-term fatigue assessment of moorings and cables under site-specific metocean conditions, connecting design load case simulation to damage accumulation

Methods and tools. OpenFAST and OrcaFlex for coupled and line dynamics; our in-house packages CableDyn and CableXsec; Bayesian filtering and surrogate modelling in Python. See the software portfolio.

Virtual sensing, monitoring and structural digital twins

Direct load measurement on a mooring line or a cable is costly, failure-prone, and rarely available along the full length of the component. Our central question is how far the load and fatigue state of a structure can be reconstructed from the measurements we can realistically obtain - platform motions, line-end responses, a handful of strain gauges - using estimators built on mechanical models rather than on correlation alone.

Current work includes

• Physics-based virtual sensing of mooring line tension from platform motion, using Kalman filtering and related state estimators built on lumped-mass line dynamics
• Inverse finite element method for reconstructing displacement and damage fields from sparse strain measurement
• System identification of cable and mooring response, including uncertainty-aware neural models used as estimators rather than as predictors
• Structural digital twins for floating wind, from OpenFAST linearisation and modal identification through to state-space export

Methods and tools. Kalman and particle filtering; inverse finite element method; NARX and neural system identification; our in-house packages VANE and FOWCON. See the software portfolio.

Floating solar and marine renewable energy structures

Offshore floating solar puts a large, shallow, highly connected array into a wave field that its freshwater predecessors never saw. We work on the hydrodynamic response of these arrays and on the structural consequences of the connector choices that hold them together.

Current work includes

• Wave-structure interaction of offshore floating solar arrays, including uniform and hybrid connector arrangements
• Seakeeping of catamaran and barge float configurations for floating solar
• Design challenges and standardisation gaps for floating solar systems deployed at sea

Peridynamic modelling of crack initiation and growth in welded steel

Classical finite element fracture mechanics needs the crack path to be anticipated by the mesh. Peridynamics is a nonlocal formulation in which cracks form and branch as a consequence of the material model rather than of the discretisation, which makes it suited to welded joints, where the crack starts at a defect nobody placed and runs through a heat-affected zone whose properties differ from the parent plate.

Current work includes

• Crack initiation and growth in welded steel structures, including the heat-affected zone and weld toe geometry
• Hydrogen effects on crack growth in structural steels, and the coupling between hydrogen transport and fracture

Emerging direction. This is a new research direction rather than an established programme. It has no publication record in the lab yet; the first manuscripts are under review.