Parts manufactured with Laser Powder Bed Fusion (LPBF) are drawing interest in the adhesive joints research because of their high surface roughness, which is usually associated with good adhesion. This work aims to assess the adhesion strength of the inherent surface morphology of LPBF manufactured titanium.

Double Cantilever Beam (DCB) tests were carried out to determine the mode I fracture toughness of joints comprising as-printed titanium (Ti6Al4V) adherends, namely titanium-titanium secondary bonded and titanium-Carbon Fibre Reinforced Polymer (CFRP) co-bonded joints. The effect of high-temperature oxidation on the fracture toughness was also evaluated by testing a batch of joints in which the titanium underwent a post-printing thermal treatment. The as-printed specimens were compared to the same type of joints but with sandblasted titanium adherends to evaluate the effect of this surface pre-treatment on the value of fracture toughness.

The results indicate that non-oxidised titanium joints with untreated adherends had an average of 11% higher fracture toughness than their sandblasted counterparts. On the other hand, sandblasting proved beneficial for oxidised joints, increasing the fracture toughness by 64% on average over the untreated samples.@en

Here we investigate the adhesive properties of Selective Laser Melted (SLM) titanium surfaces in metal-composite cobonded joints without any prior surface treatment, to explore the inherent surface roughness of SLM parts to potentially create strong adhesive bonds. Double Cantilever Beam (DCB) tests were carried out to assess the joints mode I fracture toughness involving untreated SLM titanium and woven Carbon Fibre Reinforced Polymer (CFRP) adherends, co-bonded with an epoxy film adhesive. The same type of DCB joints, but now with sandblasted SLM titanium adherends, were tested to compare the adhesion strength of the untreated SLM surface versus the sandblasted surface. The findings reveal that joints with untreated SLM titanium adherends exhibit similar toughness to those with sandblasted SLM titanium adherends, indicating that the surface morphology of as-printed SLM titanium is suitable for manufacturing robust adhesive joints. The elimination of the surface treatment in the manufacturing of adhesive joints with SLM adherends could increase the interest of many industrial fields towards the use of adhesive bonding over other joining techniques.@en

This work aims to improve the damage tolerance of secondary adhesively bonded joints under quasistatic mode I loading conditions by architecting the carbon fibre-reinforced polymer substrates’ stacking sequences [1]. Double Cantilever Beam tests show that architecting the stacking sequence of the laminates composite substrates in combination with the adhesive layer’s fracture toughness affects the crack onset and triggers different crack paths throughout the joints’ thickness. In specimens bonded with a low-toughness bi-component adhesive, the tailored design, including a co-cured toughening layer, could increase the effective fracture toughness of the composite bonded joints up to 200%. From this study, it was possible to recognise the complexity and benefits of moving from the traditional cohesive failure to outbreaking multiple crack path propagation. @en

Aiming to increase damage tolerance of adhesively bonded joints, this work explores the influence of CFRP layup of the adherends on the crack onset and crack propagation of composite bonded joints under mode I loading. Quasi-static Double Cantilever Beam tests were performed using four different CFRP layups bonded with two adhesives. Parallel to the experimental program, finite element analyses were performed to aid in understanding and identifying the various damage mechanisms in each specimen type. The results show that the CFRP layup and adhesive fracture toughness significantly influence the joint fracture phenomena at crack onset and further crack propagation. An enhancement of the joint's mode I fracture toughness values at crack onset was observed in the specimens where a crack competition between the propagation within the bondline and the composite's layers was triggered. During crack propagation, the fracture toughness of the joint increases at crack deflections between the different plies of the CFRP layup until reaching the 0° ply, where sudden delamination occurs. It has been shown that CFRP layup tailoring is a promising toughening method that, when carefully designed, has the potential to increase the maximum effective fracture toughness up to 100% when compared to pure cohesive failure.

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Using fiber-reinforced composite patches for repairing damaged structures made of metal or/and concrete is an interesting and widely available solution on the market using synthetic materials. These repairing patches are bonded on the structures’ surfaces to increase their strength against internal stresses, as well as protect them from external physico-chemical attacks, thereby limiting crack propagation. Natural fibers offer a potential alternative to replacing glass or carbon fibers commonly used for bonded repair patches. Similarly, bio-based polymers represent an important sustainable alternative for partially or entirely replacing the petroleum-based polymers. In this study, an epoxy matrix reinforced with flax fiber is proposed as the material for the patches, and bonded to a steel plate using four different types of adhesive materials, including a castor-oil derived polyurethane resin. Floating roller peel tests were performed to assess the adhesion and viability of these new patches. The resulting peeling loads and fracture surface analysis are presented. Polyurethane demonstrates promising performance for epoxy-to-steel joints, but major improvements of the bio-based polyurethane application process and curing conditions may be necessary for its successful industrial implementation.@en

Understanding the relationship between the sensors’ outputs and the damage evolution within the joints is becoming increasingly crucial to improving structural health monitoring systems and collecting data to improve the joint’s design. Therefore, a study of the acoustic emission method associated with visual fracture evaluation was proposed to give insights into the toughening of composite bonded joints and better understand the relationship between the acoustic emission features and the damage mechanism involved. Thus, two different layups were proposed for the substrates: [0]8 and [0/902/0]S. In addition, a toughened epoxy adhesive with an embedded carrier (AF163-2k) was used to bond the substrates. Five specimens of each stacking sequence were tested under quasi-static mode I loading conditions. A travelling microscope and a regular digital camera were used on the lateral sides of the specimens to track the crack propagation paths. One piezoelectric sensor linked to the AMSY-6 Vallen system was used to assess the acoustic emission features produced within the joints during the tests. Unsupervised machine learning algorithms based on artificial neural networks and the Morlet continuous wavelet transformation were used to pattern recognition of the acoustic emission data. Self-organising maps, together with k-means algorithms, were used for data clustering. Following that, the acoustic emission features of each cluster were associated with the insights obtained from the crack propagation images. Finally, it was observed that the different layups triggered simultaneous toughening mechanisms. The combination of the acoustic emission and the visual evaluation was crucial for a deeper understanding of the underlying phenomena.@en

Delamination growth in fibre reinforced polymer composites is generally evaluated with experiments that have been standardized for quasi-static load conditions. These tests characterize unidirectional delamination growth in mode I (DCB), mode II (ELS or ENF) of mixed mode conditions (MMB). However, little attention is paid in literature to the applicability of these tests to in-service delamination problems that are generally characterized by planar delamination growth. In this study, the relation between planar delamination growth, induced by transverse quasi-static indentation loading, and these unidirectional delamination tests was investigated. To that aim, prior planar delamination growth tests reported in literature, performed at EPFL, were analysed to identify up to what extent this planar growth could be correlated to the concepts of strain energy release and strain energy density. Once this appeared to successful, an experimental setup was designed to measure the delamination boundary during the transverse indentation loading of planar delamination specimens made of nontransparent carbon fibre reinforced polymer composites. With that set-up, quasi-static and fatigue planar delamination growth experiments were performed, and delamination contours could be successfully captured. While the quasi-static tests revealed limited growth, evaluation with numerical simulations revealed that the indentation force required to extend the delamination quasi-statically would cause damage to the specimen. This is attributed to the increasing length of the delamination contour when delaminations expand, which is not the case with standard unidirectional specimen. With the fatigue tests, however, delamination growth was achieved, but interestingly enough two phases were observed; first the delamination propagated in a planar fashion, while at some point in time work did not exceed an apparent threshold. Instead of no growth, however, the delamination still increased but then in a transverse manner. What makes this study of particular interest, is that the strain energy density as criterion could capture the strain energy offered (work) along the entire delamination contour, while the strain energy release rate described the resistance to delamination growth. This latter observation is in agreement with the original concept employed by Griffith when he formulated the basis of linear elastic fracture mechanics. This presentation present the experiments performed, the analysis of results, and will conclude with a proposal how to relate standard unidirectional tests to planar growth, considering that these standard tests contain little to no information on transverse phenomena with respect to strain energy density (work) and strain energy release (dissipation). @en