Novel adherend laminate designs for composite bonded joints

Research output: ThesisDissertation (TU Delft)

3 Downloads (Pure)

Abstract

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).
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Benedictus, R., Supervisor
  • Teixeira De Freitas, S., Advisor
Award date13 Nov 2020
DOIs
Publication statusPublished - 2020

Keywords

  • CFRP
  • Adhesive bonding
  • Lap joining

Fingerprint Dive into the research topics of 'Novel adherend laminate designs for composite bonded joints'. Together they form a unique fingerprint.

Cite this