The aim of this study was to characterise the microstructural organisation of staple carbon fibre-reinforced polymer composites and to investigate their mechanical properties. Conventionally, fibre-reinforced materials are manufactured using continuous fibres. However, discontinuous fibres are crucial for developing sustainable structural second-life applications. Specifically, aligning staple fibres into yarn or tape-like structures enables similar usage to continuous fibre-based products. Understanding the effects of fibre orientation, fibre length, and compaction on mechanical performance can facilitate the fibres’ use as standard engineering materials. This study employed methods ranging from microscale to macroscale, such as image analysis, X-ray computed tomography, and mechanical testing, to quantify the microstructural organisations resulting from different alignment processing methods. These results were compared with the results of mechanical tests to validate and comprehend the relationship between fibre alignment and strength. The results show a significant influence of alignment on fibre orientation distribution, fibre volume fraction, tortuosity, and mechanical properties. Furthermore, different characteristics of the staple fibre tapes were identified and attributed to kinematic effects during movement of the sliver alignment unit, resulting in varying tape thicknesses and fuzzy surfaces.

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Adhesive bonding is one of the most suitable joining technologies in terms of weight and mechanical performance for current carbon fiber reinforced polymer aircraft fuselage structures. However, traditional joint topologies such as single overlap joints induce high peel stresses, resulting in sudden failure and low joint strength when compared to metal adherends. This drawback in using carbon fiber reinforced polymer is hindering their performance and efficiency in full-scale structures where joints are essential. The goal of this paper is to review how the joint design can help to improve the lap shear strength of composite bonded joints, to recognize the challenges that still need to be understood and to give insight into new opportunities. The focus is thereby on means to increase the matrix-dominated out-of-plane strength of the adherend in order to postpone delamination failure, as it is known to be the most prone type of failure of composite bonded joints. The paper is divided in two main parts: firstly, a review of topology-related and material-related design parameters is given and secondly, future opportunities to improve out-of-plane strength of CFRP bonded joints yet to be explored are discussed.

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This study aims at assessing the sensitivity of the ultrasonic welding process for joining epoxy- to thermoplastic-based composites sensitivity to the heating time. For that, carbon fibre (CF)/epoxy adherends with a co-cured PEI coupling layer were ultrasonically welded to CF/polyetheretherketone (PEEK) adherends at different heating times. Process-induced changes in the meso and microstructure of these welds were identified and correlated to the weld strength. Subsequently, a processing interval, i.e., a range of heating times resulting in less than 10% decrease of weld strength, was defined. As, expected, the dissimilar composite welded joints were more sensitive to the heating time than the CF/PEEK to CF/PEEK welded joints. However, this effect was less pronounced than expected, since a relatively wide processing interval could be obtained provided that the coupling layer had a sufficient thickness.

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The goal of this study is to explore new topologies for adhesively bonded composite overlap joints in order to improve their strength under tensile loading. Multiple stacked overlaps, also referred as finger joints, are compared with single overlap topologies. The quasi-static tensile behaviour of single lap joints with two overlap lengths 12.7 mm and 25.4 mm are compared to finger joints with 1 and 2 stacked overlaps through thickness with constant 12.7 mm overlap length. Two composite adherend stacking sequences are tested for each topology [0/90]_4s and [90/0]_4s. A non-linear FE-analysis is performed to analyse the shear and peel stresses along the adhesive bond line. A difference in peak shear and peel stress, at the tip of the bonded region could be observed: (i) for 1 finger, the peak peel stress is higher than in the single lap joint configurations because the beneficial effect of avoiding eccentricity in the finger joint is outperformed by the detrimental effect of reducing to half the adherend stiffness at the overlap; (ii) for 2 fingers, the stress field changes significantly leads to a 23% decrease in peak shear and 33% in peak peel stress, compared to the single lap joint topologies. In addition, experimental lap shear tests are performed and monitored using acoustic emission technique, to follow the damage events. Different trends at damage initation and at maximum load are believed to result from how the damage propagates inside the joint. A topology with 2 fingers and layup [90/0]_4s, which fails entirely inside the adherend, provides the lowest peak shear and peel stress and the highest load at damage initiation. It is however outperformed in maximum load by a single lap joint topology with layup [0/90]_4s, with mostly cohesive failure. It is further found that, unlike in single overlap topologies, the most dominant stress component for damage initiation inside the finger joints is the in-plane tensile stress, at the butt joint resin pockets, rather than peel stresses at the overlap region. Lastly, if weight efficiency is the main requirement, a finger joint design can effectively replace a single overlap joint design. However, for absolute maximum joint strength, the single overlap joint is a better choice than the finger joint.

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This study investigates the early fatigue damage of cross-ply carbon/epoxy laminates. The aim is to unfold the damage accumulation process, understand the interaction between different damage mechanisms, and quantify their contribution to stiffness degradation. Tension-tension fatigue tests were performed, while edge observation and DIC technique monitored the damage evolution. It was found that different accumulation process and interactive levels between transverse matrix cracks and delamination exist for specimens with similar stiffness degradation. A linear increase of stiffness degradation was observed with the increase of matrix crack density, while the growing trend of stiffness degradation converged with the increase of delamination.

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The aim of this study is to evaluate the enhanced off-axis properties of thin plies to improve the performance of adhesively bonded carbon fiber reinforced polymers. Single lap bonded joints with three different ply thicknesses of 200 μm, 100 μm and 50 μm were tested under quasi-static tensile loading. Acoustic Emission and Digital Image Correlation were used to monitor the damage and strain evolution of the overlap area during testing. 3D post-mortem failure analysis of the fracture surfaces were performed using a 3D profiling microscope. Experimental results show an increase of 16% in the lap shear strength and an increase of 21% in the strain energy when using the 50 μm instead of 200 μm ply thicknesses. However, Acoustic Emission measurements show that the damage initiation is postponed up to a 47% higher load when using 50 μm instead of 200 μm ply thicknesses. Moreover, the total amount of acoustic energy released from initiation up to final failure was significantly less with thin plies. A non-linear finite element analysis up to damage initiation indicates that with decreasing ply thickness, the damage onset inside the composite is postponed to higher loads and moves away from the adhesive interface towards the mid-thickness of the adherend. It is found that, decreasing the single ply thickness of laminated composite adherends in a single overlap bonded joint increases the maximum load and delays damage initiation of the joint, however the damage progression till final failure is more sudden.

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Adhesive bonding is one of the most suitable joining technologies in terms of weight and mechanical performance for current carbon fiber reinforced polymer aircraft fuselage structures. However, traditional joint topologies such as single overlap joints induce high peel stresses, resulting in sudden failure and low joint strength when compared to metal adherends. This drawback in using carbon fiber reinforced polymer is hindering their performance and efficiency in full-scale structures where joints are essential. In this thesis, novel design concepts are proposed to tackle the challenge of poor out-of-plane properties of composite adherends that limit the performance of composite single lap bonded joints, by making use of the three design parameters: stacking sequence, ply thickness and overlap stacking. Design parameters of carbon fiber reinforced polymer bonded joints can be classified in three categories: Global topology relates to the global geometry of the joint, for example whether it is a single or a double overlap joint topology. Local topology refers to features that affect only a local region of the entire bond line, for example a certain spew fillet geometry or a tapered edge of the adherend. The third category describes any design parameters which are related to the specific materials of the adhesive and the adherends. The adherends themselves consist of laminated plies, which can be tailored, for example in terms of ply thickness or stacking sequence. These laminate specific design parameters are the core of this work. For all adherend laminate designs studied in the context of this thesis, the following approach is chosen: Single lap bonded joints were manufactured varying the design features (stacking sequence, ply thickness and/or overlap stacking). The experimental campaign consisted of quasi-static tensile tests using Acoustic Emission and Digital Image Correlation to monitor the damage and strain evolution of the overlap area during testing. 3D post-mortem failure analysis of the fracture surfaces was conducted using a 3D profiling microscope. Parallel to the experiments, a Finite Element Analysis is performed up to damage initiation, taking into account non-linear geometry and elasto-plastic behaviour of the adhesive. Damage initiation loads and strain fields are numerically predicted and validated with experimental data. Stacking sequence: Single overlap bonded joints with four different composite adherend stacking sequences are tested and numerically simulated, in order to evaluate the effect of the layups on the quasi-static tensile failure of the bonded joints. The results show that increasing the adherend bending stiffness postpones the damage initiation in the joint. However, this is no longer valid for final failure. The ultimate load is influenced by how the damage progresses from crack initiation up to final failure. For similar bending stiffness, a layup that leads to the crack propagating from the adhesive towards the inside layers of the composite increases the ultimate load. The failure mode is highly influenced by the orientation of the interface lamina in contact with the adhesive, such that, a 0° interface ply causes failure within the bond line, while a 90° interface ply causes failure inside the composite adherend. Ply thickness: Another way to improve the out-of-plain properties of the laminate is to decrease their ply thickness. Single lap bonded joints with three different ply thicknesses of 200m,iv 100m and 50m are tested. Experimental results show an increase of 16% in the lap shear strength and an increase of 21% in the strain energy when using the 50m instead of 200m ply thicknesses. Acoustic Emission measurements show that the damage initiation is postponed up to a 47% higher load when using 50m instead of 200m ply thicknesses. Moreover, the total amount of acoustic energy released from initiation up to final failure is significantly less with thin plies. A failure analysis of the numerical results up to damage initiation indicates that with decreasing ply thickness, the damage onset inside the composite is postponed to higher loads and moves away from the adhesive interface towards the mid-thickness of the adherend. Overlap stacking: In a third approach a change of global joint topology is achieved with multiple stacked overlaps, also referred as finger joints, by using the ply interleaving technique. The quasi-static tensile behavior of single lap joints with two overlap lengths 12.7mm and 25.4mm are compared to finger joints with 1 and 2 stacked overlaps through the thickness with a constant 12.7mm overlap length. Two composite adherend stacking sequences, [(0/90)s]4 and [(90/0)s]4, are tested for each topology. A difference in peak shear and peel stress at the tip of the bonded region can be observed: (i) the peak peel stress in the 1-finger joint is higher than in the single lap joint configurations because the beneficial effect of avoiding eccentricity in the finger joint is outperformed by the detrimental effect of reducing to half the adherend stiffness at the overlap; (ii) for 2 fingers, the stress field changes significantly with a doubled bonding area and leads to a 23% decrease in peak shear and 33% in peak peel stress, compared to the single lap joint topologies. It is concluded that a quasi-isotropic layup may not be the best choice in terms of tensile joint strength. In order to improve tensile strength up to damage initiation, the layup should be optimized for bending stiffness, while up to final failure, a stacking sequence that yields to a complex crack path inside the composite can lead to higher ultimate loads. Decreasing the single ply thickness of laminated composite adherends in a single overlap bonded joint increases the maximum load and delays damage initiation of the joint. However, the damage progression till final failure is more sudden. Comparing single overlap with finger joint topologies, different trends at damage initiation and at maximum load are believed to result from how the damage propagates inside the joint. A topology with 2 fingers and layup [(90/0)s]4, which fails entirely inside the adherend, provides the lowest peak shear and peel stress and the highest load at damage initiation. It is however outperformed in maximum load by a single lap joint topology with layup [(0/90)s]4, with mostly cohesive failure. It is found that, unlike in single overlap topologies, the most dominant stress component for damage initiation inside the finger joints is the in-plane tensile stress, at the butt joint resin pockets, rather than peel stresses at the overlap region. If weight efficiency is the main requirement, a finger joint design can effectively replace a single overlap joint design. However, for absolute maximum joint strength, the single overlap joint is a better choice than the finger joint. In total, all three approaches lead to an increase in joint strength, either till damage initiation (Chapter 3, 4, 5) or till final failure (Chapter 3).@en

Single lap bonded joints with four different composite adherend stacking sequences were tested and numerically simulated. The aim was to evaluate the effect of the layups on the quasi-static tensile failure of the bonded joints. The study shows that increasing the adherends bending stiffness postpones the damage initiation in the joint. However, this is no longer valid for final failure. The ultimate load is influenced by how the damage progresses. For similar bending stiffness, a layup that leads to the crack propagating from the adhesive towards the inside layers of the composite increases the ultimate load. The failure mode is highly influenced by the orientation of the interface lamina in contact with the adhesive, such that, a 0° interface ply causes failure within the bond line, while a 90° interface ply causes failure inside the composite adherend. Finally, it is concluded that a quasi-isotropic layup may not be the best choice in terms of tensile joint strength. In order to improve tensile strength up to damage initiation, the layup should be optimized for bending stiffness, while up to final failure, a stacking sequence that yields to a complex crack path inside the composite can lead to higher ultimate loads.

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The goal of this study is to investigate new designs of composite bonded joints in order to improve their strength under tensile loading. Multiple stacked overlaps are compared with single overlap designs. The concept of multiple stacking is well known as ply-interleaving technique for co-curing dissimilar materials. For a secondary bonding process, a similar concept is used in tongue-and-groove joints. However, it is so far limited to one stacking level due to the complexity of the design. By means of thin unidirectional layers, the tongue-and-groove design is expanded further to two stacking sequences and applied to secondary bonding of CFRP adherends. Single lap joints of 12.7 and 25.4 mm overlap length were compared to finger joints with 1 and 2 overlaps of 12.7 mm overlap length, stacked through the thickness. Specimens were tested according to ASTM D-5868-01. The initial and final failure load were recorded. The study shows that for the same overlap length in a multi-stacked configuration, there is a potential for higher average lap strength, in comparison with an increase in overlap length of a single overlap. This effect might be mainly due to the reduction of secondary bending moment and by load distribution over multiple interfaces.

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