Ultrasonic-assisted ruthenium-catalyzed one-pot synthesis of biscoumarins

The Ru-grafted zeolite beta was found to be an excellent heterogeneous catalyst for one-pot synthesis of biscoumarins. This reaction was carried out under reflux condition and ultrasonic irradiation. On the other hand, the catalyst could be recovered for the subsequent reactions and reused without appreciable loss of activity

Mechanochromic Hybrid Composites for Structural Health Monitoring

The present paper reports an overview of mechanochromic self-reporting thin-ply hybrid composite sensors, which are designed to visually indicate overload in structures. These sensors, made from combinations of high-strain and low-strain materials, change appearance earlier than the final fracture, providing a clear visual cue of damage like delamination or strain overload. They can be used for various applications, for example, overload monitoring, and to detect barely visible impact damage in composites

Characterisation and application of bio-inspired hybrid composite sensors for detecting barely visible damage under out-of-plane loadings

Traditional inspection methods often fall short in detecting defects or damage in fibre-reinforced polymer (FRP) composite structures, which can compromise their performance and safety over time. A prime example is barely visible impact damage (BVID) caused by out-of-plane loadings such as indentation and low-velocity impact that can considerably reduce the residual strength. Therefore, developing advanced visual inspection techniques is essential for early detection of defects, enabling proactive maintenance and extending the lifespan of composite structures. This study explores the viability of using novel bio-inspired hybrid composite sensors for detecting BVID in laminated FRP composite structures. Drawing inspiration from the colour-changing mechanisms found in nature, hybrid composite sensors composed of thin-ply glass and carbon layers are designed and attached to the surface of laminated FRP composites exposed to transverse loading. A comprehensive experimental characterisation, including quasi-static indentation and low-velocity impact tests alongside non-destructive evaluations such as ultrasonic C-scan and visual inspection, is conducted to assess the sensors’ efficacy in detecting BVID. Moreover, a comparison between the two transverse loading types, static indentation and low-velocity impact, is presented. The results suggest that integrating sensors into composite structures has a minimal effect on mechanical properties such as structural stiffness and energy absorption, while substantially improving damage visibility. Additionally, the influence of fibre orientation of the sensing layer on sensor performance is evaluated, and correlations between internal and surface damage are demonstrated

A review on self-reporting mechanochromic composites: an emerging technology for structural health monitoring

Recently emerging mechanochromic systems are becoming highly attractive for structural health monitoring (SHM) purposes in various industries, such as civil, wind, and aerospace, to improve the safety and performance of structures. These are based on self-reporting polymer composites which provide a light-weight sensor with an easy-to-read visual cue for SHM purposes. The present paper reports a critical overview of mechanochromic self-reporting approaches and discusses the outlook for future development in the field. Design principles and cutting-edge applications of the main physical- and chemical-based self-reporting mechanisms, i.e., mechanochromism based on dye-filled materials, modified polymers, structural color materials, and smart hybrid composite sensors, are presented with special attention to SHM. These emerging sensors create a new generation of user-friendly, cheap, and power-free SHM systems, guaranteeing economic and technological advantages that will open up new horizons for innovative, safer, and lighter composite products with significantly lower maintenance costs

Barely visible impact damage detection in composite structures using deep learning networks with varying complexities

Visual inspection is one of the most common non-destructive testing (NDT) methods that offers a fast evaluation of surface damage in aerospace composite structures. However, it is highly dependent on human-related factors and may not detect barely visible impact damage (BVID). In this research, low velocity impact tests with different energy levels are conducted on two groups of composite panels, namely ‘reference’ and ‘sensor-integrated’ samples. Then, the results of impact tests, together with C-scan and visual inspection images, are analysed to define the BVID range and create an original image dataset. Next, four different deep learning models are trained, validated and tested to capture the BVID only from the images of the impacted and non-impacted surfaces. The results show that all four networks can learn and detect BVID quite well, and the sensor-integrated samples reduce the training time and improve the accuracy of deep learning models. ResNet outperforms other networks with the highest accuracy of 96.2% and 98.36% on the back-face of reference and sensor-integrated samples, respectively. The proposed damage recognition method can act as a fast, inexpensive and accurate structural health monitoring tool for composite structures in real-life applications

Investigating the fatigue behaviour of quasi-isotropic pseudo-ductile thin-ply carbon/glass epoxy hybrid composites

This paper investigates the fatigue behaviour of pseudo-ductile Quasi-Isotropic (QI) interlayer hybrids with un-notched and open-hole configurations. Two different types of QI pseudo-ductile hybrids were evaluated; HighC, with carbon to glass thickness ratio of 0.29, that is made of thin-ply M46JB-carbon/epoxy and thin-ply Xstrand-glass/epoxy prepregs, and LowC, with carbon to glass thickness ratio of 0.19, that is made of thin-ply T300-carbon/epoxy and standard-ply S-glass/epoxy prepregs. The hybrid configurations were loaded at 4 Hz in tension–tension fatigue without any initial damage and at different percentages of the pseudo-yield stress (σpy) at which damage initiates. It was observed that there is no stiffness reduction, after 100000 cycles, for a stress level of 80% and 50% of the σpy for the un-notched and open-hole laminates, respectively. By increasing the stress level to 90% and 70% of the σpy for the un-notched and open-hole laminates, respectively, there is a gradual stiffness reduction due to the appearance of matrix cracking and delamination in LowC, but no gradual reduction and no visible damage were observed for HighC. The final failure is more brittle and happens at a lower number of cycles for HighC compared with LowC. Different damage extents were observed for the investigated laminates before the final sudden failure due to variables such as the ply thickness, the cyclic energy release rate and the interfacial fracture toughness

Hybrid composite sensors for improved visual inspection of impact damage

Advanced visual inspection techniques are essential for ensuring the structural integrity and reliability of laminated fibre reinforced polymer (FRP) composite structures. Given the ever increasing applications of FRP composites in various industries such as aerospace, wind turbine, and automotive, the demand for accurate, efficient, and non-destructive methods to monitor their health becomes paramount. Traditional inspection methods often fall short in detecting defects or damage in composite structures, which can compromise their performance and safety over time. A prime example of this is barely visible impact damage (BVID) caused by out-of-plane loadings such as indentation and low-velocity impact that can considerably reduce the residual strength. Therefore, developing advanced visual inspection techniques is essential for early detection, localisation, and characterisation of defects, thereby enabling proactive maintenance and extending the lifespan of composite structures. The work in this thesis explores the viability of using hybrid composite sensors for detecting BVID in laminated FRP composite structures. Drawing inspiration from the colour-changing mechanisms found in nature, hybrid composite sensors composed of thin-ply glass and carbon layers are designed and attached to the surface of the laminated FRP composites exposed to out-of-pane loadings. A comprehensive experimental characterisation, including quasi-static indentation and low-velocity impact tests alongside non-destructive evaluations such as ultrasonic C-scan and visual inspection, is conducted to assess the sensor’s efficacy in detecting BVID. After this, a complementary numerical and theoretical study is conducted to optimise the sensor’s design through a parametric study, enabling the evaluation of key design parameters to tailor the sensor for specific applications. This numerical model can serve as a cost-effective and reliable tool for fast parametric design studies. The effectiveness of the sensor is further examined, particularly in automated visual inspection, when employing deep learning-assisted techniques. These deep learning models use images captured from the surfaces of both damaged and intact composites, enabling assessment of the sensors’ potential in enhancing damage pattern recognition and classification. Ultimately, the proposed sensing technology is implemented on curved FRP composite panels, serving as a real-life case study representative of composite gas cylinders. The findings of this research offer insights into the design, characterisation, and application of bio-inspired hybrid composite sensors, thereby enhancing visual inspection capabilities for detecting BVID in composite structures

Validation and Applications of the Material Point Method

The Material Point Method (MPM) is a modern finite element method that is classified as a point based method or meshless method, while it takes the advantage of two kinds of spatial discretisation that are based on an arbitrary Eulerian-Lagrangian description of motion. The referenced continuum is represented by the material points, and the motions are tracked through a computational background mesh, that is an arbitrary constant mesh which does not move the material. Hence, in the MPM mesh distortion especially in the large deformation analysis is naturally avoided. However, MPM has been employed to simulate difficult problems in the literature, many are still unsatisfactory due to the lack of rigorous validation. Therefore, this thesis firstly provides a series of simple case studies which any numerical method must pass to test the validity of the MPM, and secondly demonstrate the capability of the MPM in simulating difficult problems such as degradation of highly swellable polymers during large swelling that is currently difficult to handle by the standard finite element method. Flory’s theory is incorporated into the material point method to study large swelling of polymers, and degradation of highly swellable polymers is modelled by the MPM as a random phenomenon based on the normal distribution of the volumetric strain. These numerical developments represent adaptability of the MPM and enabling the method to be used in more complicated simulations. Furthermore, the advantages of this powerful numerical tool are studied in the modelling of an additive manufacturing technology called Selective Laser Melting (SLM). It is shown the MPM is an ideal numerical method to study SLM manufacturing technique. The focus of this thesis is to validate the MPM and exhibit the simplicity, strength, and accuracy of this numerical tool compared with standard finite element method for very complex problems which requires a complicated topological system

A unified comparative study on the classical lamination theory, hole drilling, slitting, and curvature measurement methods for assessment of residual stress in laminated polymeric composites

Residual stresses are balanced, non-uniform, undesirable, and often harmful in laminated polymer composite materials. Different methods are developed to measure these stresses in composites. In this manuscript, three experimental measurement methods, namely ‘hole drilling’, ‘slitting’, and ‘curvature’ methods, are considered, and the results are compared with those of classical lamination theory (CLT). It is demonstrated that the hole drilling technique can be used to measure residual stress in the near-surface areas of the structures, while the slitting method is often employed to assess residual stresses through the depth of specimens. Moreover, the curvature method shows great potential for estimating the overall residual stress quality in laminated composites with un-symmetrical configurations

Visual inspection of impact damage in composite materials

Visual inspection is one of the most common nondestructive evaluation techniques that is performed through vision. This chapter will summarize visual inspection principles for detecting impact damage in composite materials. Effective parameters and available standards for visual inspection of low-velocity impact damage are introduced, and the relevant challenges are discussed. Current developments in improving visual inspection, including the use of artificial intelligence algorithms, as well as smart coating layers for improved visibility of impact damage are reported. Recent case studies are included, and future research directions are discussed