Maximizing Interlaminar Fracture Toughness in Bidirectional GFRP through Controlled CNT Heterogeneous Toughening

"Interleaving" is widely used for interlaminar toughening of fiber-reinforced composites, and the structure of interleaving is one of the important factors affecting the toughening efficiency of laminates. Several experiments have demonstrated that compared to continuous and dense structures, toughening layers with structural heterogeneity can trigger multiple toughening mechanisms and have better toughening effects. On this basis, this work further investigates the application of heterogeneous toughening phases in interlaminar toughening of bidirectional GFRP. CNT was selected to construct toughening phases, which was introduced into the interlaminar of composites through efficient spraying methods. By controlling the amount of CNT, various structures of CNT toughening layers were obtained. The fracture toughness of modified laminates was tested, and their toughening mechanism was analyzed based on fracture surface observation. The results indicate that the optimal CNT usage (0.5 gsm) can increase the initial and extended values of interlayer fracture toughness by 136.0% and 82.0%, respectively. The solvent acetone sprayed with CNT can dissolve and re-precipitate a portion of the sizing agent on the surface of the fibers, which improves the bonding of the fibers to the resin. More importantly, larger discrete particles are formed between the layers, guiding the cracks to deflect in the orientation of the toughened layer. This generates additional energy dissipation and ultimately presents an optimal toughening effect.

Enhancing the Longitudinal Compressive Strength of Freeform 3D-Printed Continuous Carbon Fiber-Reinforced Polymer Composite Laminate Using Magnetic Compaction Force and Nanofiber Z-Threads

Low fiber-direction compressive strength is a well-recognized weakness of carbon fiber-reinforced polymer (CFRP) composites. When a CFRP is produced using 3D printing, the compressive strength is further degraded. To solve this issue, in this paper, a novel magnetic compaction force-assisted additive manufacturing (MCFA-AM) method is used to print CFRP laminates reinforced with carbon nanofiber (CNF) z-threads (i.e., ZT-CFRP). MCFA-AM utilizes a magnetic force to simultaneously levitate, deposit, and compact fast-curing CFRP prepregs in free space and quickly solidifies the CFRP laminate part without any mold nor supporting substrate plate; it effectively reduces the voids. The longitudinal compressive test was performed on five different sample types. ZT-CFRP/MCFA-AM samples were printed under two different magnetic compaction rolling pressures, i.e., 0.5 bar and 0.78 bar. Compared with the longitudinal compressive strength of a typical CFRP manufactured by the traditional out-of-autoclave-vacuum-bag-only (OOA-VBO) molding process at the steady-state pressure of 0.82 bar, the ZT-CFRP/MCFA-AM samples showed either comparable results (by -1.00% difference) or enhanced results (+7.42% improvement) by using 0.5 bar or 0.78 bar magnetic rolling pressures, respectively.

Synthesis, Characterization and Application of Biobased Unsaturated Polyester Resin Reinforced with Unmodified/Modified Biosilica Nanoparticles

This paper presents sustainable technology for environmentally friendly composite production. Biobased unsaturated polyester resin (b-UPR), synthesized from waste polyethylene terephthalate (PET) glycosylate and renewable origin maleic anhydride (MAnh) and propylene glycol (PG), was reinforced with unmodified and vinyl-modified biosilica nanoparticles obtained from rice husk. The structural and morphological properties of the obtained particles, b-UPR, as well as composites, were characterized by Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The study of the influence of biosilica modification on the mechanical properties of composites was supported by hardness modeling. Improvement of the tensile strength of the b-UPR-based composite at 2.5 wt.% addition of biosilica modified with vinyl silane, named "b-UPR/SiO2-V" composite, has been achieved with 88% increase. The thermal aging process applied to the b-UPR/SiO2-V composite, which simulates use over the product's lifetime, leads to the deterioration of composites that were used as fillers in commercial unsaturated polyester resin (c-UPR). The grinded artificially aged b-UPR composites were used as filler in c-UPR for the production of a table top layer with outstanding mechanical properties, i.e., impact resistance and microhardness, as well as fire resistance rated in the V-0 category according to the UL-94 test. Developing sustainable composites that are chemically synthesized from renewable sources is important from the aspect of preserving the environment and existing resources as well as the extending their life cycle.

Recent Advances in Limiting Fatigue Damage Accumulation Induced by Self-Heating in Polymer-Matrix Composites

The self-heating effect can be considered as a catastrophic phenomenon that occurs in polymers and polymer-matrix composites (PMCs) subjected to fatigue loading or vibrations. This phenomenon appears in the form of temperature growth in such structures due to their relatively low thermal conductivities. The appearance of thermal stress resulting from temperature growth and the coefficient of thermal expansion (CTE) mismatch between fibers and neighboring polymer matrix initiates and/or accelerates structural degradation and consequently provokes sudden fatigue failure in the structures. Therefore, it is of primary significance for a number of practical applications to first characterize the degradation mechanism at the nano-, micro- and macroscales caused by the self-heating phenomenon and then minimize it through the implementation of numerous approaches. One viable solution is to cool the surfaces of considered structures using various cooling scenarios, such as environmental and operational factors, linked with convection, contributing to enhancing heat removal through convection. Furthermore, if materials are appropriately selected regarding their thermomechanical properties involving thermal conductivity, structural degradation may be prevented or at least minimized. This article presents a benchmarking survey of the conducted research studies associated with the fatigue performance of cyclically loaded PMC structures and an analysis of possible solutions to avoid structural degradation caused by the self-heating effect.

Identification of Representative Equivalent Volumes on the Microstructure of 3D-Printed Fiber-Reinforced Thermoplastics Based on Statistical Characterization

The present work describes a methodology to compute equivalent volumes representing the microstructure of 3D-printed continuous fiber-reinforced thermoplastics, based on a statistical characterization of the fiber distribution. In contrast to recent work, the methodology herein presented determines the statistically equivalent fiber distribution directly from cross-section micrographs, instead of generating random fiber arrangements. For this purpose, several regions, with different sizes and from different locations, are cropped from main cross-section micrographs and different spatial descriptor functions are adopted to characterize the microstructures in terms of agglomeration and periodicity of the fibers. Detailed information about the adopted spatial descriptors and the algorithm implemented to identify the fiber distribution, as well as to define the location of cropped regions, are given. From the obtained statistical characterization results, the minimum size of the equivalent volume required to be representative of the fiber distribution, which is found in the cross-section micrographs of 3D-printed composite materials, is presented. To support the findings, as well as to demonstrate the effectiveness of the proposed methodology, the homogenized properties are also computed using representative equivalent volumes obtained in the statistical characterization and the results are compared to those experimentally measured, which are available in the literature.

Efficient Use of Carbon Fibers as Heating Elements for Curing of Epoxy Matrix Composites

The aim of this study is to achieve a fully cured thermoset matrix that is heated by a direct electric current passing through the reinforcement fibers i.e., the Joule heating effect. Two types of fibers were used as heating elements for curing the epoxy resins. Kanthal resistance fibers were used as reference heating elements and subsequently, they were replaced by a Torayca Carbon Tow of the same radius. The specimens were cured by the heat produced by a direct electric current passing through the fibers and achieving temperatures of 50 °C and 70 °C. Specimens cured in a conventional oven were also manufactured, to compare the resistance heating method to the conventional one. Next, all specimens were mechanically characterized in a quasi-static three-point bending mode of loading and experimental results were compared to derive useful conclusions concerning the applicability of the technique to polymer/composite materials mass production. Finally, a preliminary economical study concerning power consumption needed for the application of both the traditional oven curing and the carbon fibers heating elements use for the manufacturing of the same amounts of materials is presented, showing a maximum financial benefit that can be achieved, on the order of 68%.

Effect of Atmospheric-Pressure Plasma Treatments on Fracture Toughness of Carbon Fibers-Reinforced Composites

In this study, nano-scale fillers are added to epoxy matrix-based carbon fibers-reinforced composites (CFRPs) to improve the mechanical properties of multi-scale composites. Single-walled carbon nanotubes (SWCNTs) used as nano-scale fillers are treated with atmospheric-pressure plasma to introduce oxygen functional groups on the fillers’ surface to increase the surface free energy and polar component, which relates to the mechanical properties of multi-scale composites. In addition, the effect of dispersibility was analyzed through the fracture surfaces of multi-scale composites containing atmospheric-pressure plasma-treated SWCNTs (P-SWCNTs) under high load conditions. The fillers content has an optimum weight percent load at 0.5 wt.% and the fracture toughness (K_IC) method is used to demonstrate an improvement in mechanical properties. Here, K_IC was calculated by three equations based on different models and we analyzed the correlation between mechanical properties and surface treatment. Compared to the composites of untreated SWCNTs, the K_IC value is improved by 23.7%, suggesting improved mechanical properties by introducing selective functional groups through surface control technology to improve interfacial interactions within multi-scale composites.

Value-Added Use of Invasive Plant-Derived Fibers as PHBV Fillers for Biocomposite Development

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a promising biobased, biodegradable thermoplastic with limited industrial applications due to its brittleness and high cost. To improve these properties, lignocellulosic fibers from two invasive plants (Phalaris arundinacea and Lonicera japonica) were used as PHBV reinforcing agents. Alkali treatment of the fibers improved the PHBV-fiber interfacial bond by up to 300%. The morphological, mechanical, and thermal properties of the treated fibers were characterized, as well as their size, loading, and type, to understand their impact on performance of the biocomposites. The new biocomposites had improved thermal stability, restricted crystallization, reduced rigidity, and reduced cost compared with PHBV. Additionally, these novel biocomposites performed similarly to conventional plastics such as polypropylene, suggesting their potential as bio-alternatives for industrial applications such as semirigid packaging and lightweight auto body panels.

X-ray Computed Tomography for the Development of Ballistic Composite

This paper presents the results of research on ballistic panels made of polymer–matrix composites (PMCs). The analysis covers two types of composites produced by the authors based on high-density polyethylene (PEHD) and polypropylene (PP) reinforced with aramid fabric. Ballistic tests were carried out with the use of two types of projectile: 0.38 Special, and 9 × 19 Parabellum, which are characterized by different velocity and projectile energy. The study presents the X-ray computed tomography (XCT) analysis for structure assessment of ballistic panels and its impact behavior, further compared to the results of computer simulations conducted using the numerical analysis. The quality of the manufactured panels and their damage caused by a ballistic impact was assessed using a multi-scale geometry reconstruction. The mesoscale XCT allowed the internal composite geometry to be analyzed, as well as a unit cell of the representative volume element (RVE) model to be built. The RVE model was applied for homogenization and finite element (FEA) simulation of projectile penetration through the ballistic panel. The macroscale XCT investigation allowed for the quantitative description of the projectile’s impact on the degree of delamination and deformation of the panels’ geometry.

Investigation on Ultrasonic Welding Attributes of Novel Carbon/Elium® Composites

Joining large and complex polymer–matrix composite structures is becoming increasingly important in industries such as automobiles, aerospace, sports, wind turbines, and others. Ultrasonic welding is an ultra-fast joining process and also provides excellent joint quality as a cost-effective alternative to other joining processes. This research aims at investigating the welding characteristics of novel methyl methacrylate Elium^®, a liquid thermoplastic resin. Elium^® is the first of its kind of thermoplastic resin, which is curable at room temperature and is suitable for mass production processes. The welding characteristics of Elium^® composites were investigated by optimizing the welding parameters with specially designed integrated energy directors (ED) and manufactured using the Resin transfer molding process. The results showed a 23% higher lap shear strength for ultrasonically welded composite joints when compared to the adhesively bonded joints. The optimized welding time for the ultrasonic welded joint was found to be 1.5 s whereas it was 10 min for the adhesively bonded joint. Fractographic analysis showed the significant plastic deformation and shear cusps formation on the fractured surface, which are typical characteristics for strong interfacial bonding.

Electrical Resistance Prediction for Functionalized Multi-Walled Carbon Nanotubes/Epoxy Resin Composite Gasket under Thermal Creep Conditions

Carbon nanotube-based conductive polymer composites (CPC) showed great potentials for self-sensing and in situ structural health monitoring systems. Prediction of the long-term performance for such materials would be a meaningful topic for engineering design. In this work, the changing behavior of the long-term resistance of a multi-walled carbon nanotubes/epoxy resin composite gasket was studied under different temperature and loading conditions. Glass transition strongly influenced the resistance behavior of the composite during the thermal creep process. Similar to classical Kelvin-Voigt creep model, a model considering both the destruction and recovery processes of the conductive network inside the CPC was established. The long-term resistance variation can be predicted based on the model, and the results provided here may serve as a useful guide for further design of smart engineering structural health monitoring systems.

Mechanical, Electrical and Rheological Behavior of Ethylene-Vinyl Acetate/Multi-Walled Carbon Nanotube Composites

This paper investigates the rheological, mechanical and electrical properties of a Ethylene-Vinyl Acetate (EVA) polymer filled with 1, 3 and 5 wt.% multi-walled carbon nanotubes (MWCNTs). The melt flow and pressure-volume-Temperature (pvT) behaviors of the EVA/MWCNT composites were investigated using a high-pressure capillary rheometer, while the electro-mechanical response was investigated on injection-molded samples. Rheological experiments showed that the melt shear viscosity of the EVA/MWCNT composite is dependent on nanotube loading and, at high shear rates, the viscosity showed temperature-dependent shear thinning behavior with a flow index n < 0.35. The specific volume of the EVA/MWCNT composite decreased with increasing pressure and MWCNT wt.%. The transition temperature, corresponding to the pvT crystallization, increased linearly with increasing pressure, i.e., about 20 to 30 °C when cooling under pressure. The elastic modulus, tensile strength and stress at break increased with increasing MWCNT wt.%, whereas the strain at break decreased, suggesting the formation of MWCNT secondary agglomerates. The electrical conductivity of the EVA/MWCNT composite increased with increasing MWCNT wt.% and melt temperature, reaching ~10⁻² S/m for the composite containing 5 wt.% MWCNTs. Using the statistical percolation theory, the percolation threshold was estimated at 0.9 wt.% and the critical exponent at 4.95.