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.

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.

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.

Interleaving composite laminates with nanofibrous mat is one of the most reliable methods for increasing interlaminar fracture toughness. The present study seeks to find out how the damage mechanisms of carbon fiber reinforced polymers (CFRPs), subjected to the mode-I and mode-II fracture tests, are affected while those are modified by interleaved Polyamide 66 (PA66) electrospun layers. For this goal, acoustic emission (AE) and scanning electron microscope (SEM) techniques were used for assessing the damage mechanisms. The mode-I test results showed that adding nanofibers could decrease matrix cracking, fiber breakage, and fiber/matrix debonding by 92%, 27%, and 87%, respectively. The AE demonstrated that no fiber breakage occurred during mode-II loading in both non-modified and nanomodified specimens which was validated by SEM images. On the other hand, the two other damage modes, i.e. matrix cracking and fiber/matrix debonding, decreased about 97% in the nanomodified laminates.

The present study evaluates the toughening capability of electrospun PA66 nanofibers for carbon/epoxy composite laminates subjected to mode II loading conditions at elevated temperatures. The Dynamic Mechanical Analysis (DMA) test showed that the glass transition temperature of the produced nanofibers is in a range of ∼60–80 °C. Accordingly, End-Notched Flexure (ENF) carbon/epoxy specimens interleaved by a 50 μm-layer of electrospun PA66 nanofibers were subjected to the quasi-static mode II loading at room temperature (∼25 °C), 100 °C, 125 °C, and 160 °C. At room temperature, the mode II interlaminar fracture toughness (G_IIC) of the nano-modified specimen was ∼4 times higher than the virgin specimen (non-modified) (3.12 kJ/m² vs 0.81 kJ/m²). The results showed that G_IIC of the virgin specimen was independent of temperature. However, in the case of the nano-modified specimen, although the G_IIC did not change from room temperature to 100 °C (3.12 kJ/m² vs 3.09 kJ/m²), by further increasing temperature to 125 °C and 160 °C, G_IIC dropped by 34% and 43% respectively (2.05 kJ/m² and 1.77 kJ/m² respectively). 3D surface scans and Scanning Electron Microscopy (SEM) images of the fracture surface revealed three reasons for decreasing the toughening capability of the PA66 nanofibers at high temperatures: a) the crack crosses the nano-layer less at high temperatures, b) the dominant damage mechanism at low temperature is “cohesive failure”, the damage propagation within the nanolayer, while at higher temperatures “adhesive failure”, the debonding of the nanolayer from carbon fibers, plays a critical role in the fracture, and c) severe plastic deformation of nanofibers at high temperatures.

The present study is focused on the characterization of the fatigue damage features in carbon/epoxy laminates under mode-II loading conditions. To this aim, a sinusoidal cyclic load was applied to the End-Notched Flexural (ENF) specimens and the fatigue behavior of specimens was investigated. Scanning Electron Microscope (SEM) was used to identify the damage features on the fracture surface, i.e. fiber imprints, cusps, roller cusps, and striations. It was found that the fatigue damage features, such as cusps and striations, completely depended on the fatigue crack growth rate, da/dN. In addition, a linear relationship between the fatigue striation space and the strain energy release rate range (ΔGs) and the hysteresis loop area was established. The Acoustic Emission (AE) method was also employed to characterize the damage features. The obtained results showed that higher AE energy indicates larger and rougher cusps and striation features.

This paper presents a concise state-of-the-art review on the use of Fiber Reinforced Polymers (FRPs) in bridge engineering. The paper is organized into commonly used FRP bridge components, and different materials/manufacturing techniques used for repairing and construction of FRP bridges. Efforts have been made to give a clear and concise view of FRP bridges using the most relevant literature. FRPs have certain desired properties like high strength to weight ratio, and high corrosion and fatigue resistance that make them a sustainable solution for bridges. However, as FRPs are brittle and susceptible to damage, when safety is concerned, critical parts of the bridges are made as hybrids of FRP and conventional materials. Despite significant studies, it has been found that a comprehensive effort is still required on better understanding the long term performance and end-of-life recycling, developing cost-effective and flexible manufacturing processes such as 3D printing, and developing green composites to take full advantages of FRPs.

This paper investigates the effect of temperature and hybridization on the impact damage evolution and post-impact residual strength of hemp/epoxy, basalt/epoxy and their hybrid laminates, using mechanical and acoustic emission (AE) based analysis. To start with, the specimens were impacted by a drop weight impact tower machine at two temperatures of 30 °C and 65 °C and then they were subjected to a three-point bending test for the assessment of their residual strength, while online AE signals were recorded during the test. The mechanical behavior of the laminates was evaluated through measurement of the impact force and absorbed energy. AE response of the slope of cumulative rise angle (RA) was used for identification of the severity of the impact-induced damage in the laminates. In addition, the sentry function was computed on the basis of the correlation between the mechanical strain energy stored in the materials and the acoustic energy propagates by fracture events, enabled evaluation of the amount of impact-induced damage. These results showed the hybridized laminates having a better resistance to impact damage at the elevated temperature (65 °C) compared with the non-hybridized laminates, whereas, in the case of the ambient temperature (30 °C), basalt/epoxy laminates had a higher impact damage resistance than other configurations. This study reveals the capability of the proposed AE-based methods to investigate the effect of temperature and hybridization of composite laminates.

This study aims to use the passive and active acoustic-based health monitoring methods for impact damage assessment of composite structures. To this aim, a Carbon Fiber Reinforced Polymer (CFRP) composite plate was fabricated and subjected to a simulated low-velocity impact by performing repeated quasi-static indentation tests where a loading-unloading-reloading test profile with 5 repetitions was adopted. Two Acoustic Emission (AE) broadband sensors and a network of eight piezoelectric (PZT) sensors were attached on the composite plate surface. AE (passive method) was employed during the loading and reloading phases of the indentation tests to online monitor the critical damage occurrence and also specify the damage type while scanning of the plate with Lamb waves (active method) was done to localize the damage when the structure was unloaded. Felicity Ratio (FR) index which was calculated based on the AE data could accurately detect that critical damage occurred during the 5th loading-unloading-reloading stage when the structural integrity dropped to 60% of its initial stage. Furthermore, Lamb wave signals of central frequency 150 kHz localized the impact damage with error of 0.89 cm (3.6% error respect to the shortest dimension of the scanned area).

The increasing use of Adhesively-bonded joints in industrial applications resulted in more attention to damage assessment in these joints. The aim of the present study is to characterize the damage in bi-material double-lap adhesively-bonded joints by Acoustic Emission (A E). Two different structural adhesives, representing a ductile (Methacrylate-based) and brittle (epoxy-based) types, were used to bond C F R P skins to a steel core. The fabricated joints were loaded in tension while damage evolution was monitored by A E . Due to the difference in the fracture nature of the adhesives "brittle vs. ductile", different damage mechanisms occurred in the specimens; including adhesive layer failure, steel deformation, adhesive/adherends interfacial debonding and delamination in the C F R P skin. In order to distinguish and classify these damage types by A E, the AE features of each damage mechanism were first obtained by conducting some standard tests on the individual constituent materials. Then, these AE reference patterns were used to train an ensemble decision tree classifier. Finally, the trained model classified the AE signals of the double-lap tests and the images captured by camera were utilized to verify AE results. This study demonstrates the potential of AE technique for damage characterization of the adhesively-bonded bi-material joints.