Publications

Abstract of: Strength evaluation and failure prediction of bolted and adhesive glass/steel joints

This paper investigates the use of bolted and brittle/ductile adhesive connections in glass structures. Two benchmark designs of shear connections are introduced and tested experimentally in quasi-static tensile tests. The designs consist oftempered glass and aluminium substrates while steel splices are used for the load application. In addition, material characterisation testing for the glass and the adhesive is performed and the outputs are used for the numerical simulation of the same joints. Pressure-sensitive, plasticity and failure models are introduced and calibrated to accurately capture the behaviour of the adhesives. Good agreement between the experimental observations and numerical predictions is achieved. The results show that both types of adhesive joints outperform bolted joints while counter-intuitively the lower strength ductile adhesive achieves consistently higher joint strength compared to the brittle adhesive. The numerical analyses highlight that while brittle adhesive joints fail once the fracture strain of the adhesive has been reached, while for ductile adhesives an extensive plastic zone develops near the areas of stress concentrations thereby delaying the damage initiation.

Author response for "Mechanical and interfacial characterisation of leading‐edge protection materials for wind turbine blade applications"

Multilayer Leading Edge Protection systems of Wind Turbine Blades: A review of material technology and damage modelling

In the immediate future, wind power will provide more electricity than any other technology based on renewable and low-emission energy sources. As a result, the size of offshore wind turbines has increased to harvest more wind energy in order to achieve the 2050 EU carbon neutral targets. The use of composites opens great prospects in the design and manufacture of the wind turbine blades due to their optimization versatility but composites perform poorly under impact and are sensitive to environmental factors. To combat this, blade manufacturers employ polymer-based surface coatings, caps or tapes to protect the composite structure. However, it is the repeated impact of rain droplets combined with the high blade tip speed, which are mostly contributing to the erosion of wind turbine blades [1]. The hindering of leading-edge erosion could be obtained through its multilayer material optimization i.e. Leading Edge Protection LEP [2]. Both the surface erosion and the intra-layer adhesion are affected by the shock wave propagation through the thickness of the LEP system produced from the collapsing water droplet after impact [3]. It is necessary to increase the interfacial fracture toughness resistance of the multy-layered system from the surface to the interface boundaries to damp the surface damage and avoid subsurface delamination [4]. Therefore, validated models considering the developed multicomplex stress states and the material degradation due to environmental loads are required for design purposes toward anti-erosion protection performance. This investigation summarizes the review of the current literature conducted in the framework of the IEA Wind TCP (International Energy Agency Wind Technology Collaboration Programme) - Task 46 Erosion of wind turbine blades [5]. It focuses on two main issues: firstly, the LEP material configuration used in industry considering the blade integration technology and, secondly, the modelling techniques and numerical procedures currently used to predict both wear surface damage and interface delamination failure. This work will allow for the identification of gaps within the research that can be explored during IEA Wind Task 46.

Damage mechanisms of adhesively bonded joints of thin tow-based discontinuous composites

• The adhesive bonding of thin Tow-Based Discontinuous Composites (TBDCs) was studied experimentally • High-toughness adhesives promoted damage within the TBDC micro-architecture, reaching fracture toughness values of 800 J/m² • The fracture toughness of TBDC joints was significantly higher than that of UD and cross-ply laminates • Thin TBDCs can be used as toughening layers in composite bonded structures due to their high fracture toughness • Acoustic Emission and fractography revealed complex co-existing mechanisms, dominated by matrix cracking and fibre/matrix debonding Tow-Based Discontinuous Composites (TBDCs) are a new class of composite materials that combine high strength and stiffness with in-plane isotropy making them of interest in high-end structural applications. Despite their potential, efficient connection methods are currently lacking and the adhesive bonding behaviour of TBDC structures remains unexplored. This work, therefore, seeks to address this gap by analysing the quasi-static performance of TBDC adhesive joints under mode I loading condition. Double Cantilever Beam (DCB) tests were performed using two adhesives with contrasting toughness levels: a moderate (∼600 J/m 2 ) and a high toughness adhesive (> 2400 J/m 2 ). When a moderate-toughness adhesive was used, a combination of cohesive failure and composite damage was observed, with only a small scatter in the experimental results. In contrast, the use of the high-toughness adhesive led to a shift in damage mechanisms towards the composite micro-architecture, resulting in fracture toughness values in the region of 800 J/m 2 , with a larger experimental scatter. Acoustic Emission analysis identified matrix cracking and fibre/matrix debonding as the dominant damage mechanisms. These findings were validated by the post-mortem fractography analysis via Scanning Electron Microscopy. This work therefore provides the first detailed analysis of the damage mechanism in adhesively bonded TBDCs, which have potential in aerospace and automotive applications.

Strength evaluation and failure prediction of bolted and adhesive glass/steel joints

This paper investigates the use of bolted and brittle/ductile adhesive connections in glass structures. Two benchmark designs of shear connections are introduced and tested experimentally in quasi-static tensile tests. The designs consist of tempered glass and aluminium substrates while steel splices are used for the load application. In addition, material characterisation testing for the glass and the adhesive is performed and the outputs are used for the numerical simulation of the same joints. Pressure-sensitive, plasticity and failure models are introduced and calibrated to accurately capture the behaviour of the adhesives. Good agreement between the experimental observations and numerical predictions is achieved. The results show that both types of adhesive joints outperform bolted joints while counter-intuitively the lower strength ductile adhesive achieves consistently higher joint strength compared to the brittle adhesive. The numerical analyses highlight that while brittle adhesive joints fail once the fracture strain of the adhesive has been reached, while for ductile adhesives an extensive plastic zone develops near the areas of stress concentrations thereby delaying the damage initiation.

A 3D voxel-based mesostructure generator for finite element modelling of tow-based discontinuous composites

Tow-based discontinuous composites manufactured with ultra-thin tapes display high stiffness, strength, and in-plane isotropy, thus competing with composite laminates. Their complex 3D microstructure affects the mechanical response, in turn demanding 3D generators that capture the tape waviness, resin pockets, and thickness and fibre content variations. The present work proposes an automated numerical framework combining a 3D voxel-based mesostructure generator with finite element models. A modified 3D random sequential absorption technique is developed with bin-guided allocation, draping, and thickness control. A statistical study is used to size the statistical volume elements and predict the elastic properties of thick, thin, and ultra-thin tow-based discontinuous composites. The results are compared with the experimental values from the literature. Despite uncertainties in physical tape properties, the resulting stiffnesses are predicted with good accuracy.

Hydrogen permeability of thin-ply composites after mechanical loading

Hydrogen is a sustainable alternative to conventional fuels, and it may be obtained with near zero carbon footprint. However, hydrogen storage remains a key challenge, and the use of composite tanks has gained significant interest over the last few years. In addition, thin-ply composites promote fibre damage by delaying matrix microcracking and free edge delamination. In this work, the H2 permeation/diffusion performance of virgin and mechanically loaded thin cross-ply laminates is studied. In addition, Scanning Electron Microscopy (SEM) is used to identify defects and micro-damage in the laminates and explain the experimental values. The study shows that the hydrogen (H2) barrier performances of thin-ply composites are lower than conventional metallic systems. Obtained permeability values, however, resulted well below the allowable limits for most combinations of temperature and pressure and remain unaffected despite the application of high tensile strains showing that permeation is not accelerated.

Mechanical and interfacial characterisation of leading‐edge protection materials for wind turbine blade applications

Abstract Modern wind turbine blades experience tip speeds that can exceed 110 m/s. At such speeds, water droplet impacts can cause erosion of the leading edge, which can have a detrimental effect on the performance of the wind turbine blade. More specifically, rain erosion is leading to both reduced efficiency and increased repair costs. The industry is using polymeric coatings—leading‐edge protection (LEP) materials—to protect the blades but those are also prone to rain erosion. In this work, LEP materials that are currently used by the industry for the protection of wind turbine blades were selected and their performance assessed. The LEP materials were characterised in terms of mechanical properties by using different experimental methods, and they were also assessed in terms of durability by performing rain erosion testing (RET). Finally, the damage and failure mechanisms observed were further investigated using CT scanning. This paper provides an insight to the properties of LEP materials, their durability, and the damage and failure mechanisms they experienced during rain erosion.

Durability of an adhesively bonded joint between steel ship hull and sandwich superstructure pre-exposed to saline environment

This paper outlines an experimental investigation into the durability of large-scale adhesively bonded joints with a thick layer of methyl methacrylate adhesive (MMA). Ageing has been performed by immersion in a 3.5 wt% NaCl solution for 10 weeks at 50°C. Two aged and one unaged specimen were subjected to tensile testing, and three aged and one unaged specimen were loaded up to ~ 3.5 million fatigue cycles followed by a residual tensile test. The ductility of the adhesive is affected by ageing and fatigue testing. Despite a decrease in ductility, the plastic zone development was adequate for the required strain redistribution without compromising the joint performance (strength and stiffness) demonstrating the fatigue tolerance of the joint. The shear, longitudinal, and peel strain values in the adhesive bulk are evaluated by digital image correlation. The shear strength values are significantly higher than the requirements following from the design. All specimens failed by sudden delamination of the composite plate. Post-mortem analysis showed no corrosion travel at the interface of steel and adhesive.

Failure prediction and optimal selection of adhesives for glass/steel adhesive joints

Prediction of moisture diffusion and failure in glass/steel adhesive joints

Abstract Glass/steel adhesive joints are being used increasingly in the construction industry as they offer significant structural advantages over conventional mechanical fastener approaches. However, adhesive joints are also known to be sensitive to moisture diffusion into the bondline, which reduces the interfacial bonding strength for hybrid glass/steel substrates. The effect of moisture on the performance degradation of glass/steel adhesive joints has been successfully predicted assuming adhesive property degradation but requires experimental determination of the affected moisture ingress zone. This study utilizes a multi-physics numerical approach implemented via the commercial finite element code Abaqus 2020, which firstly simulates moisture ingress into the adhesive/glass interface and subsequently couples the diffusion effects with a cohesive zone modelling approach for damage initiation and propagation. The numerical predictions are calibrated against experimental data on glass/steel Double Cantilever Beam (DCB) specimens, which are bonded with a ductile methacrylate adhesive (Araldite 2047–1). The modelling approach is then validated against the experimental response of large double lap shear joints of a significantly different bondline geometry. It is demonstrated that the numerical model successfully predicts the critical exposure time for partial to complete joint degradation enabling the development of engineering guidelines for life-time prediction of various joint geometries.

Development of cohesive zone models for the prediction of damage and failure of glass/steel adhesive joints

Fatigue performance and damage characterisation of ultra-thin tow-based discontinuous tape composites

Tow-cased discontinuous composites are an attractive alternative material to conventional continuous composites as they offer in-plane isotropy, enhanced manufacturability allowing to achieve complex 3D shapes with high curvatures and local reinforcement in critical areas, while also maintaining high strength and stiffness, therefore expanding the design space significantly. In addition, the use of ultra-thin tapes and optimised manufacturing methods can increase the mechanical properties even further and change the damage mechanisms. Fatigue, however, could be a limiting design factor, as the fatigue behaviour of these materials has not been fully characterised. This work presents a complete study on the fatigue response of ultra-thin tow-based discontinuous composites: fatigue S-N curves are measured, and the damage and failure mechanisms are characterised utilising optical and scanning electron microscopy. Finally, a critical interpretation of the results is also presented by comparing the performance of ultra-thin tow-based discontinuous composites against other similar fibre reinforced composites and metals. It is shown that the optimised manufacturing methods combined with low tape thickness leads to enhanced quasi-isotropic fatigue performance. In addition, the fatigue limit was raised significantly compared to other discontinuous composites, and the tow-based discontinuous composites outperformed their metal counterparts when the results were normalised with density.

Strength analysis and failure prediction of thin tow-based discontinuous composites

Tow Based Discontinuous Composites (TBDCs) are a new class of composite materials which combine in-plane isotropy, high strength and stiffness and enhanced manufacturability. However, due to their complicated micro-architecture, characterising the performance of these materials and predicting their response is challenging. This work develops a complete experimental and analytical framework which identifies all the key properties in the performance of the TBDCs, characterises them experimentally and builds an analytical predictive tool for both the stiffness response and the strength of the TBDC material. Fractography is also utilised to identify the damage mechanisms and correlate them with the analytical predictions. A parametric study is developed which shows the critical effect that the tape thickness and mode II fracture toughness have on the TBDCs. Finally, the performance of the material is compared to similarly developed TBDCs from the literature and shows the significant strength and stiffness increases recorded through the combination of the thin high-modulus tapes and the increased fibre volume fractions.

Durability of an adhesively bonded joint between steel ship hull and composite superstructure pre-exposed to marine environment.

This paper outlines an experimental investigation into the durability of large-scale adhesively bonded joints immersed in a 3.5 wt% NaCl solution for 10 weeks at 50°C. Two aged and one unaged specimen were subjected to tensile testing up to failure, and three aged and one unaged specimen were loaded up to ~3.5 million fatigue cycles followed by a residual tensile test. The shear, longitudinal, and peel strain values in adhesive bulk (evaluated by the digital image correlation (DIC) technique) are significantly higher at the gripped sides due to the asymmetrical design of the steel brackets. The shear strength and stiffness of statically and fatigue tested specimens are found to be identical. The shear strength values are significantly higher than the requirements following from the design. All specimens failed by sudden delamination of the composite plate. Post-mortem analysis showed no corrosion travel at the interface of steel and adhesive.