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A study was undertaken to determine the effects of several key geometry influencing factors on the impact response and energy absorption behavior of the glass fibre reinforced epoxy composites at low and intermediate energies. The energy-balance model was employed for characterising the energy absorption behavior and it depends strongly on the plate diameter and thickness. In addition, the damage vs. energy and force maps is effective in monitoring damage growth within the composite panel. The response of the composite laminate configurations characterized by different stacking sequences subjected to low velocity impacts with different impact energies have also been studied to estimate the damage initiation of composites.

The application of plant fiber-reinforced composite (PFRC) is limited due to its relatively low mechanical properties. The hybridization of a thin metal layer with plant fiber into a fiber metal laminate can largely improve the mechanical performance and the brittle fracture behavior of PFRC. However, both plant fiber and metal have difficulty bonding with the polymer matrix. In this paper, several different surface treatment methods were applied on Al alloy sheets, and the influence of surface treatments on the surface morphology and nano-mechanical properties of Al alloy were studied using an atomic force microscope (AFM). After the preparation of flax fiber&ndash, metal laminates (FFMLs) with a vacuum-assisted resin transfer molding (VARTM) technique, the nanomechanical properties of different modified FFMLs were also evaluated with an AFM. It was found that the surface treatment combination of the sulfuric acid-ferric sulfate-based treatment (P2 etching) and the silane coupling agent provided the best adhesion force and modulus for Al alloy sheets at nanoscale resolution, which contributed to the surface energy increasing and strong covalent bonds between metal and polymer matrix. The resulting manufactured FFMLs also exhibited the highest nano-mechanical properties due to the great improvement of interfacial properties between metal and matrix, which was caused by mechanical interlocking mechanism and covalent bonds between metal/fiber and resin. Macromechanical performance, including tensile and flexural properties of these modified FFMLs, was also investigated. Comparison of the modulus at the nanoscale and macroscale showed reasonable agreement, and it revealed the tough interlaminar mechanisms of these types of FFMLs.

It is well-known that the presence of the delamination in a plant fiber-reinforced composite is difficult to detect. However, the delamination introduces a local flexibility, which changes the dynamic characteristics of the composite structure. This paper presents a new methodology for composite laminate delamination detection, which is based on dynamic mechanical analysis. A noticeable delamination-induced storage modulus reduction and loss factor enhancement have been observed when the delaminated laminate was subjected to a forced oscillation compared to the intact composite laminate. For delamination area of 12.8% of the whole area of the composite laminate, loss factor of approximately 12% increase was observed. For near-to-surface delamination position, loss factor of approximately an 18% increment was observed. The results indicate that the delamination can be reliably detected with this method, and delamination position shows greater influence on the loss factor than that of the delamination size. Further investigations on different frequencies and amplitudes configurations show that the variation of loss factor is more apparently with low frequency as well as the low amplitude.

As a result of the increasing use of glass fiber reinforced plastic (GFRP) composites in engineering fields, the investigation of scale effect on impact performance for this kind of composite is essential for large scale structure design. The effects of scaling on the impact response of simply supported unidirectional GFRP were investigated through drop weight impact (DWI) tests in this study. Impact tests were undertaken over a wide range of energies to generate damages between barely visible and initiated penetration on four scale size GFRP laminates. The main impact responses including impact force, contact duration, displacement, energy absorption and damage area of scaled specimens were normalized to compare with the full-size specimen. It was found that the impact response of large sample with elastic deformation and small area of delamination can be predicted accurately according to a geometrical similar scaling law. Scale effect was found in the damage threshold force and absorbed energy of the laminates when significant internal damage occurs due to the microstructural effect becoming important in resisting impact force and absorbing impact energy. Moreover, the energy partition and effective stiffness were calculated according to the energy balance model to reveal the contribution of different modes of deformations on energy absorption for the GFRP laminates.

Effects of linear density and yarn structure on both static and dynamic mechanical properties of ramie fiber yarn reinforced composites (RYRCs) were investigated. The failure mechanisms of RYRCs were analyzed with the aid of ultrasonic C-scan and Scanning electronic microscopy (SEM). The results showed that the tensile strength of RYRCs increased gradually with increase of the linear density of the single yarns. The maximum tensile strength was obtained when the linear density reached 67.3 tex. However, a downtrend of the tensile strength was observed with further increase of the linear density of ramie single and plied yarns. The interlaminar fracture toughness was relatively high for RYRCs made from yarns with lower linear density due to the extensive fiber bridging observed during the double cantilever beam test. Meanwhile, the linear density and structure of ramie yarn had remarkable influence on the failure mode of RYRCs during the drop weight impact test.

Plant fibers (ramie, jute, sisal, kenaf, etc.) reinforced composites (PFRCs) have raised great attentions during the past decade and have been increasingly used in automotive, infrastructure, sports, etc., and even caused great interests in aerospace industry. Tremendous research work has been carried out towards improving the mechanical and realizing the functional properties of PFRCs. In recent years, the unique hierarchical microstructures of plant fibers are given special attentions and are realized to be quite essential for the properties of PFRCs, including mechanical properties, functional characteristics, and even the manufacturing qualities. The performances of PFRCs can be further improved by taking advantage of the unique microstructures of plant fibers. Therefore, in this review, the recent development of the properties, manufacturing, and applications of PFRCs with the consideration of the hierarchical microstructure of plant fibers are extensively discussed. The microstructure of plant fibers and the so caused multi-scaled interfaces of PFRCs are elaborated and the resultant methods, such as nanotechnology and hybridization techniques to improve the mechanical properties of PFRCs are discussed. Functional properties, including flammability, acoustic, damping, etc., are also extensively analyzed. Additive manufacturing and liquid molding process to make PFRCs with the emphasis on the defect formation mechanisms caused by the hierarchical microstructure of the composites are reviewed. The recent state-of-the-art researches in other high-performance cellulose-based materials by using the microfibrils-the smallest structure of the plant fibers are introduced. The review is expected to give a new insight to the researches on hierarchical materials and structures and hierarchical mechanics.

Sisal fiber reinforced composites with different amounts of resin inside the fiber lumens were prepared. The effects of resin penetration into fiber lumens on the mechanical and water-absorption properties of the composites were studied. Acoustic emission (AE) was used to characterize the fracture mechanisms of the composites by analyzing the different AE signals released during the failure process of the composites. Tensile, flexural and impact strength were all improved with the increasing of the amount of resin inside the fiber lumens due to the changes of the fracture modes. No significant effects on the tensile and flexural modulus were observed. The water absorption resistance of the composites could be improved when fiber lumens were filled with resin. The micro-failure morphologies of the composites possessing different amounts of resin inside the lumens were revealed with the aid of scanning electronic microscopy (SEM). It was found that the resin inside the lumens of the fibers strengthened the bonding between the micro fibrils within a single sisal fiber. Meanwhile, crack bridging effect of the penetrated resin could hinder the crack propagation to ensure fibers reaching their failure strain as much as possible.

The aim of this study is to characterize the interfacial fracture toughness of a micro-lattice core based sandwich structure from quasi-static to dynamic rates of loading. The modified three-point bend (MTPB) sandwich beam was used to characterize the interfacial properties of these sandwich structures. Dynamic tests were undertaken of up to 3 m/s using a purpose-built instrumented drop-weight impact tower. Data reduction was accomplished through the use of a compliance calibration procedure similar to that used for characterizing the delamination resistance of composites. The flexural properties of sandwich beams were investigated through three-point bend tests at a cross-head displacement rate of up to 3 m/s. A detailed examination of the impact region highlighted the failure processes in these systems and this was related to the data from the quasi-static flexural tests. The globalized deformation and energy absorption during progressive were also discussed.

This paper outlines the findings of a study on a range of stainless steel and titanium alloy lattice structures manufactured using the selective laser melting technique. The effect of varying key manufacturing parameters on the properties of lattice strands was studied through a series of single-filament tensile tests. The resulting failure mechanisms were investigated using a scanning electron microscope. The resulting observations have shown that the properties of these lattice strands are determined by the laser energy during the manufacturing process, which in turn is controlled by the laser power and laser exposure time. The quasi-static and low-velocity penetration behaviour of lattice core-based sandwich panels has been examined, and an aluminium foam and an aluminium honeycomb were chosen to benchmark their performance. The impact resistance of the lattice core-based sandwich structures were shown to be dependent on both the manufacturing parameters and lattice unit-cell geometry of the lattice structure. The impact resistances were improved by increasing manufacturing laser energy and lattice core density. A series of drop-weight tests at velocities up to 6 m/s have shown that the penetration behaviour of the titanium alloy lattice cores and the aluminium honeycomb cores is similar.

This paper outlines the findings of an on-going research study investigating the properties of a range of steel and titanium-based micro-lattice structures manufactured using the selective laser melting (SLM) technique. Initially, tension tests have been conducted on strands manufactured at different build angles. Micro-lattice block structures, with struts oriented at +/-45o were then tested in compression at quasi-static rates of loading. The failure mechanisms have been investigated using both optical and scanning electron microscopy. These tests have highlighted the attractive properties offered by these complex architectures.

Temperature and moisture can cause degradation to the impact properties of plant fiber-based composites owing to their complex chemical composition and multi-layer microstructure. This study focused on experimental characterization of the effect of important influencing factors, including manufacturing process temperature, exposure temperature, and water absorption, on the impact damage threshold and damage mechanisms of flax fiber reinforced composites. Firstly, serious reduction on the impact damage threshold and damage resistance was observed, this indicated excessive temperature can cause chemical decomposition and structural damage to flax fiber. It was also shown that a moderate high temperature resulted in lower impact damage threshold. Moreover, a small amount of water absorption could slightly improve the damage threshold load and the damage resistance. However, more water uptake caused severe degradation on the composite interface and structural damage of flax fiber, which reduced the impact performance of flax fiber reinforced composites.