Brian Nyvang Legarth

Low-velocity impact damage in aircraft sandwich structures can remain barelyvisible while potentially leading to critical internal degradation. This studycompares impact damage mechanisms in two honeycomb sandwich configurationsdesigned for the application as a rudder of a large passenger aircraft.A thermoplastic honeycomb design concept with a semi-open folded honeycombcore featuring wavy cell walls, and an conventional thermoset-basedreference with epoxy-based face-sheets and hexagonal Nomex® honeycombcore. Drop-weight impact tests were conducted in the transition range betweenbarely visible impact damage and visible impact damage, supportedby non-destructive failure investigation methods. The thermoplastic configurationshowed shallow residual indentation below 7J impact energy, followedby local face-sheet wrinkling along the core ribbon direction. This instability changed the damage classification and was associated with potentiallocal face-to-core disbonding rather than pronounced through-thicknesscore crushing. In contrast, the thermoset reference exhibited a conventionalindentation-dominated response, with localized honeycomb core crushing beneaththe impact location. Surface topology for the thermoplastic configurationquantified the damage growth after wrinkling onset, while X-raycomputed tomography revealed distinct internal morphologies. Exploratoryfinite-element simulations supported the interpretation that global bending,local indentation, and interface degradation governed the thermoplastic damagetransition. The findings indicate that thermoplastic sandwich conceptsmay require material- and manufacturing-specific damage-tolerance assessment,validated inspection procedures, and improved interface characterizationbefore certification-relevant implementation.

Predicting fatigue life of polymer-matrix composites with physically interpretable parameters and quantified uncertainty remains difficult on small datasets, where neither pure-physics nor pure-data-driven models suffice. This work presents a hybrid neural-physics framework that embeds a thermo-mechanical damage ordinary differential equation (ODE) in the forward path of a neural model. The ODE supplies physically meaningful parameters while the neural components capture residual variability, so that life predictions, thermo-mechanical trajectories, and material parameters are recovered jointly. A sliding-window long short-term memory (LSTM) encoder summarises cycle-resolved measurements, and a conditional variational latent represents the trajectory. A multi-channel decoder reconstructs the cyclic mechanical and energy signals, while a life-prediction head consumes the ODE end-state and is calibrated by split-conformal prediction. Material-intrinsic ODE parameters are shared across loading configurations and thermal-coupling parameters are configuration-specific. The framework is evaluated on 32 [±45]2s E-glass/epoxy specimens tested under load control at R = 0.1 in three configurations spanning two frequencies and two cooling conditions. For this system, the model predicts log10 Nf with a root-mean-square error of 0.49, and the 90 % split-conformal interval spans a factor of 4.8 in cycles with an empirical in-regime coverage of 0.84. Shared material parameters converge across 25 seed-and-fold runs with coefficients of variation at or below 1.4%. The framework matches the Basquin baseline on the headline life-prediction metric while additionally recovering interpretable material parameters and the temperature, modulus, and dissipated-energy trajectories. Coverage degrades outside the training regime and requires recalibration on new configurations.

We propose a method for quantifying the competition between tunnel and delamination cracking in laminates. Based on the analytical relation between the compliance derivative and the energy release rate, which we refer to as the compliance method, the competition between the two damage mechanisms is investigated based on compliance maps that may be precomputed, thus providing an efficient tool for predicting damage development in laminates without ad hoc fracture simulations. Tunnel and delamination cracking in an off-axis layer, much thinner than the load-carrying layers of a laminate, are used as an example to illustrate the proposed methodology. However, the compliance method is generalizable to any parameterizable cracking mechanism. The cracks are simulated in a specialized two-dimensional off-axis finite element framework, which fully captures the three-dimensional solution. The method is validated against the energy balance method for tunneling cracks and the virtual crack closure technique for delamination cracks. The method has great potential in the context of nondestructive testing, where future damage states can be predicted from precomputed or experimentally determined compliance maps, thus reducing computational and experimental costs.

The NACRA 17 is a small foiling catamaran that is lifted out of the water by two asymmetric z-foils and two rudder elevators. This paper investigates how foil deflection affects not only foil performance but overall boat behaviour using a numerical Fluid Structure Interaction (FSI) model. The deformations are solved with a solid model based on the Finite Element Method (FEM) and the flow is solved with a Reynolds Average Navier-Stokes (RANS) based Finite Volume Model (FVM). The models are strongly coupled to allow dynamic FSI simulations. The numerical model is validated by comparing it to an experimental campaign conducted at the RISE SSPA Maritime Center in Sweden.Validation shows reasonable agreement, but the model can only be considered validated for some rake angles. The large deformation of the foils is found to have a profound effect on the performance of the foils and therefore of the overall catamaran. Turbulence transition and boat speed are found to affect foil forces and, in turn, deformation. Dynamic response of the foils during boat motion as exposed to waves is investigated and finally the full boat hydrodynamic is simulated by including both foils and the rudders in various scenarios.

This work presents a constrained version of the Shear-Torsion-Bending (STB) test fixture, specially developed for testing of the mode-III face/core fracture toughness of sandwich specimens. Compared to the traditional STB test fixture, the free end of the specimen is now constrained between two support rollers, restraining the twisting deformation of the specimen resulting from the mode-III shear forces. The constrained STB test fixture was applied for mode-III fracture characterization of sandwich specimens with longitudinal slits and consisting of E-glass/epoxy face sheets and Divinycell PVC foam cores. Three density classes of the foam material were considered and tested, that is, H45, H100, and H160. Furthermore, two core thicknesses were tested for the H160 cored specimens. The experimental results from the constrained STB test method show that stable crack growth is obtained without kinking into the core. However, post-mortem investigations showed that longitudinal 45° tension cracks in the core just below the face/core interface occurred, before interface crack opening. Consequently, an apparent mode-III fracture toughness was computed using a three-dimensional Finite Element (FE) model in conjunction with the Crack Surface Displacement Extrapolation (CSDE) mode-mixity method and an optical Digital Image Correlation (DIC) based crack propagation method. Finally, based on the experimental observations presented in this paper, a general discussion of the existence of pure mode-III face/core interface propagation in foam-cored sandwich composites in relation to core and face/core bond strength is included.