The present study proposes a comprehensive integrity assessment approach for a full-scale adhesively-bonded bi-material joint for maritime applications. The joint represents a cross-section of the bond-line connection of a ship with a steel hull and a sandwich composite superstructure. The full-scale joint consists of a sandwich composite core adhesively bonded to two U-shaped steel brackets. The joint was subjected to a quasi-static loading profile including 6 load cycles up to the final failure. Each load cycle was followed by a dwell segment holding the joint at the maximum displacement for 30 s and then unloading to 50% of the maximum displacement. Three Structural Health Monitoring (SHM) techniques including Acoustic Emission (AE), Fiber Optic Sensor (FOS), and Digital Image Correlation (DIC) were employed during the test to assess the damage state of the joint. Moreover, a Finite Element Model (FEM) was developed to simulate the evolution behavior of different damage mechanisms in the joint and the FE results were compared against the experimental findings. The obtained results showed that the integration of all the employed techniques could successfully detect the damage initiation, assess the severity of the damage, localize the critical regions of the joint, and distinguish the different damage mechanisms.

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Three-point bending analysis, experimental and numerical, is conducted for three different sizes, 30*30 mm, 40*40 mm and 60*60 mm, of one-sided square cross-ply TEXIPREG HS 160 RM carbon-fiber-reinforced polymer (CFRP) patches as well as intact and notched specimens. The tests are carried out with the aim to study the influence of the patch’s size on the flexural properties of the specimen. Hashin-2D damage initiation criterion is adopted to investigate the failure in each ply, and the damage evolution is represented by the stiffness degradation. The intact and notched laminates flexural properties are investigated as well as notch sensitivity. The bonded patch repairs efficiency is investigated in the light of ultimate load, flexural modulus, peel and shear stresses along the bond-line as well as the damage initiation coefficient at critical zones. A significant advancement in the flexural modulus and ultimate load for the repaired laminates is noticed with increasing the patch’s size. A Parametric numerical study is conducted to investigate the influence of the patch size on the damage initiation coefficient at the critical zones. The recommended patch’s size is 44*44 mm in order to avoid failure initiation at the patch corner overlap on the parent laminate.

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Thicker bondlines along with manufacturing-tolerant and fracture-resistant adhesives are trends visible across different industries, especially maritime. In this work, two contrasting adhesives: an elastic-brittle epoxy-based, and a nonlinear-ductile methyl methacrylate (MMA) are characterized and compared via tensile, compact tension (CT) and double cantilever beam (DCB) testing, for steel-to-steel adherends. Significant differences are captured between the two bonding materials in terms of the energy required for crack growth: in the MMA “ductile” adhesive is ∼4 and 10 times more than for the epoxy “relatively brittle” adhesive for CT and DCB testing, respectively. While epoxy bondlines fail due to a symmetric in-plane crack, the MMA bondlines experience multiple cracking originating from high deformations and Poisson's effects. Moreover, in the case of MMA adhesive, the DCB testing led to plastic deformation of steel adherends. The existing evaluation protocols are adopted for data reduction and the effect of plastic dissipation is theoretically addressed. Despite adherend plasticity, it is concluded that the crack growth is driven by the elastic energy release, and thus, after small correction taking into account initial adherend plasticity, the existing simple models can still be used. This study highlights the potential use of ductile adhesive instead of the commonly used brittle ones to significantly improve the adhesively bonded joints in maritime applications in which thick bondlines, manufacturing-tolerant and fracture-resistance characteristics are required.

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The aim of this research was to investigate the self-healing potential of damaged Al joints when bonded using novel eco-epoxide adhesives derived from tannic acid (TA). Two eco-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were produced. The effect of the eco-epoxy components on the self-healing ability was assessed in terms of the energy dissipation recovery after partial failure in a double cantilever beam (DCB) test, which was compared to the reference epoxy (R). The self-healing process required 2 h and 2 bars in an autoclave at 180 °C. Techniques such as DSC, FTIR and DMA showed residual activity and potential self-healing capability of the used adhesives. A combination of two monitoring techniques, Digital Image Correlation (DIC) and Acoustic Emission (AE), was used to monitor the strain distribution and damage propagation in the DCB specimens. The healing index for adhesives R, B and A was found to be 8.9%, 3.0%, and 82.5% respectively. The findings of this work highlighted the potential of using bio-based epoxy adhesives in structural adhesive bonding, as well as the prospect of utilizing their self-healing ability to restore the strength of such bonded parts.

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The effect of the weaving architecture and the z-binding yarns, for 2D and 3D woven composites on the low-velocity impact resistance of carbon fibre reinforced composites, is investigated and benchmarked against noncrimp fabric (NCF). Four architectures, namely: NCF, 2D plain weave (2D-PW), 3D orthogonal: plain (ORT-PW) and twill (ORT-TW), were subjected to 15 J impact using a 16 mm-diameter, 6.7 kg hemispherical impactor. Nondestructive techniques, including ultrasonic C-scanning, Digital Image Correlation (DIC) and X-ray computed tomography (CT) were used to map and quantify the size of the induced barely visible impact damage (BVID). The energy absorption of each architecture was correlated to the damage size: both in-plane and in-depth directions. The 3D architectures, regardless of their unit-cell size, demonstrated the highest impact resistance as opposed to 2D-PW and the NCF. X-ray CT segmentation showed the effect of the higher frequency of the z-binding yarns, in the ORT-PW case, in delamination and crack arresting even when compared to the other 3D architecture (ORT-TW). Among all the architectures, ORT-PW exhibited the highest damage resistance with the least damage size. This suggests that accurate design of the z-binding yarns’ path and more importantly its frequency in 3D woven architectures is essential for impact-resistant composite structures.@en

The development of new adhesives has allowed to expand the application of bonding into the most diverse industrial fields. This review article presents the commonly used experimental methods for the investigation of mechanical performance of adhesively bonded joints in the aerospace, wind energy, automotive and civil engineering sectors. In these sectors, due to their excellent intrinsic properties, composite materials are often used along with conventional materials such as steel, concrete and aluminium. In this context, and due to the limitations that the traditional joining techniques present, adhesive joints are an excellent alternative. However, standardized experimental procedures are not always applicable for testing representative adhesive joints in these industries. Lack of relevant regulations across the different fields is often overcome by the academia and companies’ own regulations and standards. Additional costs are thus mitigated to the industrial sectors in relation with the certification process which effectively can deprive even the biggest companies from promoting adhesive bonding. To ensure continuous growth of the adhesive bonding field the new international standards, focusing on actual adhesive joints’ performance rather than on specific application of adhesive joints are necessary.

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This study is devoted to the use of acoustic emission technique for a comprehensive damage assessment, that is, damage detection, localization, and classification, of an aeronautical metal-to-composite bonded panel. The structure comprised a titanium panel adhesively bonded to carbon fiber–reinforced plastic omega stringers. The panel contained a small initial artificial debonding between the titanium panel and one of the carbon fiber–reinforced plastic stringers. The panel was subjected to a cyclic increasing in-plane compression load, including loading, unloading, and then reloading to a higher load level, until the final fracture. The generated acoustic emission signals were captured by the acoustic emission sensors, and digital image correlation was also used to obtain the strain field on the surface of the panel during the test. The results showed that acoustic emission can accurately detect the damage onset, localize it, and also trace its evolution. The acoustic emission results not only were consistent with the digital image correlation results, but also managed to detect the damage initiation earlier than digital image correlation. Finally, the acoustic emission signals were clustered using particle swarm optimization method to identify the different damage mechanisms. The results of this study demonstrate the capability of acoustic emission for the comprehensive damage characterization of aeronautical bi-material adhesively bonded structures.@en

The aim of this paper is to study the self-healing capability of fractured Al joints bonded with novel eco-epoxide adhesives synthesized from a bio-renewable raw material (tannic acid – TA). Two synthesized eco-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were used as a replacement for the toxic epoxy component based on Bisphenol A. The effect of the eco-epoxy components on the self-healing capability was measured as a recovery of shear strength in a single lap joint (SLJ) test after complete failure, which was compared to the reference epoxy (R). The self-healing procedure was performed in an autoclave at 180 °C for 2 h and 2 bars. A combination of two monitoring techniques, Digital Image Correlation (DIC) and Acoustic Emission (AE), was used to monitor the strain distribution and damage propagation in the SLJ. The measured shear stress of A and B adhesives in the SLJ had values in the range of 2.3–5.1 MPa. A fracture analysis showed complete adhesive failure for all the tested adhesives, which was not affected by the self-healing process. Out of all adhesives, only the A adhesive demonstrated the capability to heal. The recovery of the shear strength for adhesive A was higher than 50% of the virgin case. In addition, the AE analysis managed to capture a clear distinction between the signals for the virgin and the self-healed tests for adhesive A. Results obtained in this study highlighted the promising potential of using bio-based epoxy adhesives in structural adhesive bonding with the possibility of using self-healing in the recovery of the strength of such bonded joints.@en

Two synthesized eco-epoxy components based on TA: (A) glycidyl ether and (B) glycidyl phosphate ester, are used, as a replacement for the Bisphenol A (BPA) based epoxy component, for bonding aluminum (Al) and carbon fiber reinforced polymer (CFRP). Their effect on the mode I fracture toughness (GI) is evaluated by Double Cantilever Beam (DCB) testing while using Digital Image Correlation (DIC) for in-situ crack tip monitoring. Compared to the reference adhesive, an improvement of (GI) of Al (43%) and CFRP (100%) is obtained when using adhesive B. Moreover, regardless of the adherend material, a stick-slip pattern of crack growth is observed. Weak adhesion of the reference adhesive leads to an adhesive failure vs. a cohesive-adhesive failure in the case of adhesive B. On the contrary, the modification of adhesive A has an adverse effect on the GI of Al (-33%) and CFRP (-78%) as opposed to their reference counterparts.@en

This work investigates the effect of long-term environmental exposure on the performance of composite-to-metal bonded joints. Specimens are manufactured using a carbon-fiber reinforced polymer (CFRP) co-bonded to a steel member with epoxy adhesive and aged in a salt spray chamber. The mixed-mode fracture behavior of the non-aged and aged specimens is assessed using the mixed-mode bending (MMB) test apparatus. The fracture energy is calculated using the finite elements method and an analytical approach, the strain-based method (SBM). The SBM showed to be a simple and accurate method to obtain the total fracture energy and the fracture mode ratio of the bi-material specimen. Ageing increased the fracture toughness at crack initiation by 27% for specimens loaded at 20% mode II and 7% for specimens loaded at 15% mode II. This can be related to the shear behavior and plasticization of the adhesive material. During crack propagation, the fracture toughness remained similar for specimens loaded at 20% mode II and decreased by 15% in specimens loaded at 15% mode II. Fractography analysis together with chemical characterization showed that the penetration of moisture at the edges of the fracture surface produced adhesive failure in these regions affected by moisture. Moreover, the failure mode at the unaffected regions of the fracture surface shifted from cohesive to a combination of thin-layer cohesive and adhesive failure after ageing. The results contributed to describe the effect of ageing on the fracture behavior of bonded materials.@en

The goal of this study was to investigate the effect of the structure of Mn-Al layered double hydroxide (LDH) on the adhesion behavior of composite adhesives on two Al alloys (L3005 and L8079). The composite adhesives were made out of the UV-curing Bisphenol A glycidylmethacrylate/triethylene glycol dimethacrylate (BT) as polymer matrix and the addition of 1, 3, and 5 wt. % of Mn-Al LDH as adhesion enhancers. Adhesion was evaluated by using the micro Vickers hardness testing procedure. The wetting angle of composite adhesives to the Al substrates was measured and compared to the adhesion parameter b obtained from the microhardness tests. The highest increase in adhesion was observed for BT with 5 wt. % of Mn-Al LDH on L3005 substrate, which was more than 15 times higher than the adhesion for the neat BT. The morphological segregation of composite adhesives after the contact with Al substrates was examined by optical microscopy and a higher compatibility of Mn-Al LDH particles with L3005 substrate was found. The methods used for the adhesion properties assessment suggested that the Mn-Al LDH was the best adhesion enhancer of the BT matrix for L3005 substrate containing a higher content of Mn and surface hydroxyl groups.@en

To improve their adhesion strength, polymeric surfaces are usually modified through different treatments. This study investigates the effect of mechanical, chemical, and energetic treatments on the bonding strength of ethylene propylene diene methylene (EPDM), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS) materials. Three adhesives based on different chemical compositions, namely silicone, polyurethane, and modified-silane (MS) polymer, were considered. Results show that the effect of the applied treatments on the adhesion strength of EPDM surfaces is insignificant. Only a slight improvement is obtained in the case of polyurethane-based adhesive, while the failure modes remained adhesive. As for PVC, most treatments were effective in the case of the silicone-based adhesive, especially grit blasting, primer, and UV/ozone treatments. Only UV/ozone treatment improved the adhesion strength and altered the failure mechanisms of this material when polyurethane and MS-based adhesives are used. The adhesion of ABS increased and the failure modes changed from adhesive to cohesive for most treatments. Particularly, a significant improvement is obtained when primer coating and UV/ozone radiation are applied. This comparative study paves the way for the design of polymeric joints with highly enhanced adhesion performance.@en

This paper presents a new process for obtaining eco-epoxide adhesives synthesized from bio-renewable raw material (tannic acid-TA) and used for bonding lightweight materials (aluminum (Al) and carbon fiber reinforced polymer (CFRP). Two synthesized bio-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were used as a replacement for the toxic epoxy component based on Bisphenol A. The effect of eco-epoxy components on the interface adhesion was measured by the determination of adhesion parameter b, which was compared to the reference epoxy (REF). The increase of adhesion parameter b was 77.5% for A and 151.5% for B. The adhesion of both eco-adhesives was tested using the bell peel test (BPT) with the Al and CFRP adherends. When compared to REF, the average peel load for B was 17.6% (39.3%) and 58.3% (176.9%) higher for the Al and CFRP adherends, respectively. Complete adhesion failure of REF reflected the weak adhesion to both Al and CFRP, which was improved by the addition of eco-epoxy components A and B showing the presence of cohesive failure. The microhardness testing method of interface adhesion was proven to be a fast and reliable testing method, providing a qualitative indication in adhesive selection.

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This study is devoted to the damage characterization of Non-Crimp Fabric (NCF), 2D plain-woven (2D-PW) and 3D orthogonal plain-woven (ORT-PW) carbon/epoxy laminates, subjected to compression after multiple-impact loading, using Acoustic Emission (AE). The ultrasonic C-scan images showed that the interlaminar damage area induced by the single and 3-impact in ORT-PW architecture is 3 and 2 times smaller than NCF and 2D-PW architectures respectively. The impacted specimens were then subjected to the in-plane compression load. Two indices, one based on the mechanical response and another one based on the AE behavior of the laminates, were proposed to compare the performance of different architectures. These indices showed that the ORT-PW had the best performance among all the architectures. Finally, AE was used to distinguish the different damage mechanisms including: matrix cracking, intra and inter-yarn debonding, defected-fiber breakage, intact-fiber breakage and z-binder fiber breakage in the CAI tests of the architectures.@en

Mechanics of double-lap Steel-to-CFRP adhesively-bonded joints loaded in tension are investigated experimentally using Digital Image Correlation (DIC) and Acoustic Emission (AE), analytically using a one-dimensional closed-form solution and numerically with Finite Element analysis. The double-lap bi-material joints are fabricated of a steel core adhesively bonded to two CFRP skins with adhesive thickness of ~ 8 mm, using an Epoxy-based and MMA-based adhesives. In order to capture the in-plane deformation of the joint, full field strain/displacement maps are obtained using DIC. This data is used to validate the shear-lag model predictions of the adhesive shear stress/strain distribution as well as the linear-elastic Finite Element Model (FEM) results. In addition, they are used to capture the susceptible damage locations and their effect on the displacement contour maps, strain distribution and load transfer between the joint's different constituents. A correlation between the DIC displacement and the AE signals is obtained for damage detection in both joints. Moreover, a good agreement amongst the analytical, FE and DIC strain/stress distributions along the bond-line is observed. This study introduces the analytical shear-lag model as an alternative to predict the stress state in thick-adhesive double-lap joints, with an acceptable level of accuracy and robustness.@en

The increasing interest for the application of adhesive joints in naval superstructures motivates researchers to gain an in-depth understanding of the mechanical behaviour and failure mechanisms of these joints. This work reports on an experimental study of the deformation behaviour and damage evolution of a full-scale multi-material joint using different instrumentation techniques. Adhesively bonded joints of steel to sandwich panel components have been subjected to quasi-static tensile tests during which the global deformation of the joint and local strain distributions were monitored using digital image correlation (DIC). During one particular tensile test, fibre optic Bragg sensors (FBG) were also applied to the specimen’s surface at different locations in order to quantify the evolution of local strains. Additionally, acoustic emission (AE) sensors were installed in order to monitor damage initiation and evolution with increasing levels of imposed deformation. This test showcased adhesive failure at the interface of the steel adherend and the adhesive, while cohesive failure was observed within the adhesive and skin failure at the interface between adhesive and the composite skin of the sandwich panel. The post-mortem observed failures modes were compared to the acoustic events that originated during the test due to damage initiation and propagation within the joint. The evolution of the different sensor signals, i.e. the damage expressed as cumulative AE energy and local strains measured with Bragg sensors and DIC, are mutually compared and acceptable correlation is found.@en

Tailored fibre placement (TFP) is an embroidery-based technology that allows the fibre tows to be placed exactly where they are most needed for structural performance and stitched into position on a compatible textile or polymer substrate. In this study commingled carbon-nylon fibre tows were utilised to produce thermoplastic cross-ply net-shaped preforms using TFP. Four TFP composite plaques were manufactured; baseline (blank), machined-hole, tailored-hole-1 and tailored-hole-2. Steering the tows was used to create the hole in tailored-hole-1 and tailored-hole-2. In comparison to the design of tailored-hole-1, a different fibre trajectory, with a circular reinforcement around the hole, was suggested for the tailored-hole-2. Fibre volume fraction, optical microscopy, X-ray-CT scans, tensile and open-hole tests were carried out. With the exception of the baseline sample, the modified design of tailored-hole-2 composite exhibited the highest axial strength and modulus compared to the machined-hole and tailored-hole-1 composites. Only the tailored-hole-2 specimens exhibited less than 10% reduction of the notched strength compared to the un-notched strength. This study highlights the importance of the stress/load-paths and associated fibre-orientations. While TFP can be an extremely valuable design tool for composite preforms and resulting structural components, a deep understanding of stress distributions is inevitable to achieve optimal TFP-design.@en

The aim of the present study is to characterize the damage in bi-material steel-to-composite double-lap adhesively-bonded joints using Acoustic Emission (AE). Two different structural adhesives, a ductile (Methacrylate-based) and brittle (Epoxy-based), were used to bond CFRP skins to a steel core. The fabricated joints were loaded in tension while damage evolution was monitored by AE. Due to the difference in the fracture nature of the adhesives “ductile vs. brittle”, different damage mechanisms were observed; including cohesive failure within the adhesive layer, steel deformation, failure at the adhesive/adherends interface (adhesive failure) and delamination in the CFRP skin. To classify these damages by AE, the AE features of each damage mechanism were first obtained by conducting standard tests on the individual constituents. Then, these AE reference patterns were used to train an ensemble decision tree classifier. The best parameters of the ensemble model were obtained by Bayesian optimization, and the confusion matrix showed that the model was sufficiently trained with the accuracy of 99.5% and 99.8% for Methacrylate-based and Epoxy-based specimens respectively. Afterwards, the trained model was used to classify the AE signals of the double-lap specimens. The AE demonstrated that the dominant damage mechanisms in the case of the Methacrylate-based were cohesive and adhesive failures while in the case of the Epoxy-based they were CFRP skin failure and adhesive failure. These results were consistent with the Digital Image Correlation, Fiber Optic Sensor and camera results. This study demonstrates the potential of AE technique for damage characterization of adhesively-bonded bi-material joints.@en

Thermoplastic composites are in high demand and continually growing in use due to their inherent properties. Commingled fibre is one of the recent solutions developed for thermoplastic composites, but has not yet ready for weaving. An alternative to commingling approach, co-weaving of reinforcing and thermoplastic fibres is investigated in this study. In this work, the carbon and nylon fibres were woven separately through the warp and weft directions, respectively. A 5-multilayer 3D weave architecture was designed to produce the co-woven fabrics. By varying the weft filling or pick density, a set of woven fabrics of different thermoplastic content was obtained. The hot press was used to consolidate the composite plaques. Samples of the thermoplastic composites were physically characterised through density, fibre volume fraction and void content and then optically investigated. The composite samples were also mechanically tested to determine the interlaminar shear strength via the short beam bending test. The result proves that co-weaving method for thermoplastic composites is a feasible approach as the composite shows a low void content of approximately 1.14 percent. Comparing the four co-woven composites tested in this study it is found that the maximum achievable strength (ILSS) is ~ 41.36MPa in the case of lowest matrix (PA66) and the highest fibre (CF) contents, i.e. 13PA/7CF.@en

This paper investigates the effect of the fabric architecture and the z-binding yarns on the compression after multiple impacts behavior of composites. Four fiber architectures are investigated: non-crimp fabric (NCF), 2D plain weave (2D-PW), 3D orthogonal plain (ORT-PW) and twill (ORT-TW) weave. The specimens were subjected to single and multiple low-velocity impacts at different locations with the same energy level (15 J). Non-destructive techniques including ultrasonic C-scanning, X-ray CT and Digital Image Correlation (DIC) are employed to quantitatively analyze and capture the Barely Visible Impact Damage (BVID) induced in the specimens. Although the absorbed energy was approximately the same, damage was the least in 3D woven architectures. In the case of compression after impact, 3D woven composites demonstrated a progressive damage behavior with the highest residual strength (∼92%) while 2D plain weave and NCF specimens showed suddenly catastrophic damage and the residual strength of ∼65% and ∼55% respectively.@en