Lightweight structures for marine applications: from testing to design

The main objective of the thesis was to promote a broader integration of lightweight sandwich structures in the marine industry. Amongst all possible material combinations, all-aluminium sandwich structures were selected as the focus of the study. Weight reduction and materials sustainability are becoming of primary importance for marine structures, but the simultaneous application of both of these approaches is seldom considered. The literature review regarding the use of sandwich structures for marine applications, highlighted the predominance of the composite category, whose disposal and recycling are both difficult and expensive. Therefore, a solution to both lightweight requirements and environmental issues could lie in the application of all-metal sandwich structures, and in particular of aluminium sandwich structures, which combine low-density and excellent mechanical properties with good recyclability and sustainability. In view of these considerations, a deeper knowledge on aluminium sandwich structures, based on experimental and analytical analysis, is required to support a safe and reliable design and encourage their application. For this purpose, aluminium sandwich structures (AHS) with honeycomb core were chosen to perform an extensive experimental investigation. Particular attention was paid to two of the most critical loading condition, which may result from common in-service events: impact and fatigue loading. Low-velocity impact tests were performed on six different AHS configurations, both single and double-layer. The latter were introduce in order to improve the crashworthiness of sandwich structures. Double-layer panels displayed a progressive collapse sequence, depending on the core arrangement and on the cell size. Such observations suggested the possibility to obtain energy absorbing structures with a controlled deformation. A theoretical evaluation was applied to investigate the mono-layer impact response and preliminary considerations on the existence of a size effect were drawn. The fatigue analysis, which was seldom considered in previous literature on AHS, was performed applying three-point bending loading conditions. A preliminary static analysis was performed both under three and four point bending conditions. The static tests allowed the identification of the static bending strength and the identification of cell walls buckling buckling as the main phenomena involved in AHS bending response. The influence of boundary conditions on fatigue life and on collapse modes were investigated by considering different supports spans. For one condition the S-N curve was obtained and its equation was compared to literature results. Two different collapse mechanisms were observed depending on the supports span: for larger supports span a fracture of the tensioned skin was observed, whereas lower supports span produced core shear. In both cases, failure occurred suddenly and this should be taken into consideration in real applications. An analytical model was applied to predict fatigue collapse modes and limit loads. A fatigue failure map describing the relationship between supports span, collapse modes and fatigue limit loads was obtained, in order to provide a quantitative tool for aluminium honeycomb sandwich structures design. Other innovative and efficient solution to the requirements of lightness and good mechanical performance for marine structures - with particular attention to crashworthiness - could be provided by the application of biomimetic principles. The idea of taking inspiration from natural strategies and structures to cope with engineering problems, was introduced and developed in collaboration with Trinity College of Dublin. Bamboo was selected as the natural structure to analyse and mimic, especially for what concerns impact behaviour. Bamboo samples were subjected to impact both on the outside and on the inside surface. It was found that the impact strength is correlated to the thickness and to the impact side. Impact tests were also performed on specimens whose outside surface had been abraded and on whole cylindrical sections. The role of graded and hierarchical structure in impact response, suggested some guidelines for bio-inspired structures design. Four bamboo-inspired structures were designed, based on the idea of combining corrugated panels with different geometrical characteristics to resemble bamboo graded and hierarchical structure. The design choices took into considerations also the feasibility limitations for large components, which is often the case for marine structures. Some samples of the bio-inspired solutions were made using 3D printing and tested in compression. The best performance was obtained by the structure which more closely replicates bamboo’s hierarchy. In addition, buckling theory was applied to predict bio-inspired structures performance and good agreement between experimental and analytical results was observed. Finally, a comparison of the performances of aluminium honeycomb sandwich structures and glass-fibre reinforced plastics (GFRP) sandwich panels for marine applications was provided, in order to assess the feasibility and the benefits of AHS application. The comparison was first based on the identification of the bending stiffness as a mechanical parameter equivalence, to guide the replacement of existing GFRP sandwich panels with AHS. Material charts reporting bending stiffness against other design parameters showed the significant improvements in terms of weight and volume reduction achievable with aluminium sandwich structures. Starting from bending stiffness equivalence, further suggestions for the design of AHS were introduced: a graphical approach based on plots of stiffness requirements, weight reduction goal and failure modes was applied for the identification of the main design variables. In conclusion, a case study regarding the possible substitution of a GFRP-based ship balcony overhang with an equivalent aluminium honeycomb sandwich structure was outlined. A preliminary numerical investigation to support the design of full-scale tests was developed. The results showed the possibility to simultaneously significantly reduce the weight, leave the geometry almost unchanged, and improve the mechanical response.