Experimental investigation of thickness effects in the manufacture of thick-section composite structures using liquid thermoplastic resin

Tidal turbine blades require thick-sections (10 - 100 mm) to withstand challenging operating conditions and high loads. The growing demand for sustainable materials is driving interest in recyclable liquid thermoplastic resins. These room-temperature processable resins may, in time, rival thermosets as the material of choice for blade manufacturing. Like other liquid resins, their use in thick-laminate production proves challenging as heat generated during polymerisation must be controlled to avoid boiling and defects. Although innovative low-exotherm grades are now available, their behaviour in thick-laminate manufacturing remains poorly understood. This study provides novel insights into the exothermic polymerisation profiles of two low-exotherm grades (Elium ® 188 XO and Elium ® 191 XO/SA). Industry-relevant findings on process times are also reported to guide their implementation. Five thermocouples embedded in each 10-ply laminate tracked temperature evolution over 12 hours. Elium ® 188 XO reached a higher exothermic peak temperature (86.5°C) than Elium ® 191 XO/SA (63.2°C). Nonetheless, both maximum temperatures remained below their respective boiling points, indicating minimised risk of exotherm-induced defects. A delayed exothermic reaction in Elium ® 191 XO/SA extended its process time, implying a slower manufacturing rate. The enhanced understanding of the polymerisation behaviour of liquid thermoplastic resins in thick-section laminates will allow researchers and practitioners to establish their suitability for tidal turbine blades.

7 - Fatigue behavior of thick composite laminates

This paper summarizes results of a theoretical and experimental study on the nonlinear deformation behavior of thermoplastic matrix composites. The experimental work focuses on the processing and characterization of laminates of AS4 graphite/poly phenylene sulfide (PPS), E glass/PPS, and graphite/PPS-glass/PPS hybrids. The theoreti cal study develops a generic material constitutive model applicable for all thermoplastic matrix composites that exhibit nonlinear stress-strain response. On the lamina level, a stress-based nonlinear stress-strain model for a unidirectionally reinforced ply is transformed into a mixed stress/strain-based model. Then, a model for the nonlinear material response of a multi-directional laminate is developed. The laminate model pre dicts the nonlinear compliances based on the current effective laminate stresses and strains, and does so without need for iteration. In these models, the full three-dimensional stress and strain states have been retained so the models are applicable for both thin and thick section composites. While the three-dimensional nature of the models is discussed in detail elsewhere [1], the emphasis of the present paper is on the nonlinear deformation response. There is good agreement between the theoretical predictions and experimental results for laminates of graphite/PPS, glass/PPS, and their hybrids. The paper also pre sents scanning electron micrographs to portray microscopic failure modes that may cause additional, damage-induced, nonlinear response in these laminates.

Elium (novel methyl methacrylate (MMA)) resin is a liquid thermoplastic resin curable at room temperature and a possible replacement for epoxies. The main objective of this work is to evaluate the mechanical characteristics of novel Kevlar fabric reinforced Elium composites with different thicknesses. The plain-woven structure Kevlar/Elium laminates were manufactured with 1.5 mm and 2.5 mm thicknesses through vacuum-assisted resin infusion moulding, where 8 and 12 layers of woven fabrics were used, respectively. The effect of laminate thickness was measured in terms of mechanical (tensile, flexural, shear, and dynamic mechanical analysis (DMA)) and physical (density and fibre volume fraction (FVF)) characteristics. The density of the laminates was found in the range of 1.18–1.31 g cm−3. FVF was 50.69 and 52.27% for 1.5 and 2.5 mm thick laminates, respectively. The composite with 1.5 mm thickness exhibited the highest tensile strength (667.9 MPa) and flexural strength of 330.7 MPa. Conversely, the highest interlaminar shear strength measured for 2.5 mm thick laminate is 16.5 MPa. The DMA analysis recorded the highest storage and loss modulus for 2.5 mm thickness laminates. The fractography analysis confirmed the quantified experimental observation of excessive interface debonding and delamination. Elium composites may be suitable for high-end structural applications, including marine and aircraft structures.

Static behaviour and damage of thick composite laminates

Mechanical testing of glass-fibre-reinforced composite (GFRP) plates made of twill fabric and a thermoplastic recyclable infusion resin is presented. The considered thermoplastic resin, ELIUM®, is made of poly-methylmethacrylate and can be infused with properly tuned vacuum techniques, in the same manner as all liquid resin. Tensile, flexural, and drop-dart impact tests were carried out to assess the mechanical properties of the composites considering different fabrication conditions, such as the different degassing pressure before infusion and three different infusion vacuum pressures. The work reports a methodology to infuse ELIUM resin at a relatively high vacuum pressure of 0.8 bar. X-ray microtomography analysis showed that the produced laminates are free of defects, differently from what was reported in the literature, where void problems related to a vacuum infusion pressure higher than 0.3-0.5 bar were pointed out. Vacuum pressure values influence the mechanical characteristics of the laminate: when higher vacuum pressures are adopted, the mechanical properties of the GFRP laminates are enhanced and higher values of elastic modulus and strength are obtained. On the other hand, degassing the resin before infusion does not influence the mechanical properties of the laminates. A maximum bending and tensile strength of 420 and 305 MPa were reached by using the vacuum infusion of 0.8 bar with an elastic modulus of 18.5 and 20.6 GPa, respectively. The density of the produced laminates increases at higher vacuum infusion pressure up to a maximum value of 1.81 g/cm3 with the fibre volume fraction of each laminate.

Fiber reinforced composites, due to their higher specific strength and specific stiffness, are replacing many metallic structures. Of these, thick composite laminates are of high interest in various, millitary, transportation and marine applications for their use in ballistic and shock protection. One such application is in Composite Armored Vehicle (CAV) integral armor comprising of thick section composite that serves as the primary load-bearing component. The current solution of the structural backing laminate utilizes an S2-glass/epoxy system processed using automated fiber placement method. While proven structurally suitable, this method is time consuming as well as expensive. This paper presents several alternative cost-effective manufacturing solutions for fabricating composite laminates of 20 mm (0.8 in.) nominal thickness (made of 45 layer, 2 × 2 twill weave S2-glass with 933 sizing/vinyl ester C-50 resin), consisted with them CAV application in focus. They include Vacuum Assisted Resin Transfer Molding (VARTM) and Vacuum Assisted Resin Infusion Modeling (VARIM) and their variations. The effectiveness of different affordable processing approaches adopted in fabricating the structural laminate is compared in terms of static and dynamic compression response of the laminations. Static studies have been conducted on thick composites using specimen based on Army Material Technology Laboratory’s (AMTL) recommendation for thick section composites, while dynamic response is studied on cubic specimen samples using a Split Hopkinson Pressure Bar (SHPB).

Carbon Fiber Reinforced Composite (CFRP) is widely used in deep-sea pressure-resistant structures. With the increase in submergence depth demand leading to the increase in the thickness of the CFRP shell plate, there is a significant thickness effect on its compression performance. In order to study the mechanism of the decrease in compression performance of the laminate, uniaxial compression tests, interlaminar shear tests, out-of-plane tensile tests, damage characterization, and FEM analysis were carried out on three thicknesses of laminates. The results showed that the compressive strength, interlaminar shear strength, out-of-plane tensile strength of laminates and FEM compression model decreased by 10.3%, 12.7%, 23.6%, and 13.6% when the thickness of the laminate was increased from 2 mm to 12 mm. Concurrently, the compression failure mechanism is transformed from the overall strength failure to the instability–crush failure mode caused by the initial delamination. The effects of out-of-plane tensile strength and interlaminar shear strength on compressive properties were also considered. It provides support for the regulation of compression performance of large-thickness laminates and the safety of deep-sea pressure-resistant structures in service.

Punch shear tests have been proven to simulate most of the damage mechanisms observed under ballistic impact. A phenomenological model has been developed to quantify the elastic and absorbed energies as a function of displacement during punch. This model is used to quantify the damage mechanisms of thick-section composites as a function of displacement, and number of pre-defined delamination planes. It has been identified that different damage mechanisms as a function of displacement can be correlated with the load-displacement curve of punch shear tests. Three significant damage mechanisms are identified. Energy absorbed by these damage mechanisms are partitioned and quantified for a thick-section composite made from plain-weave S-2 glass and toughened API epoxy SC15 resin. The effect of pre-defined delamination planes on energy absorption and damage mechanisms are also quantified.

Effect of thickness and reinforcement configuration on flexural and impact behaviour of GFRP laminates after exposure to elevated temperatures

In consequence of an increased interest in using endless carbon fibre reinforced thermoplastic composites (TPC), automated and highly productive processing technologies for cutting and trimming steps of consolidated materials are sought. In this paper, the influence on the thermal effect caused by laser cutting with respect to static strength properties of TPC based on a polyphenylene sulfide (PPS) matrix is studied. For the cutting experiments, consolidated TPC laminates at varying thicknesses up to s = 3.1 mm and a disc laser emitting at a wavelength of ! = 1030 nm at a maximum output power of PL = 2 kW are used. For the first time, the resulting magnitude of the heat affected zone (HAZ) at the cutting edge of the composite material is correlated with interlaminar shear strength tests. The results are compared to specimens prepared by milling and abrasive water jet cutting. Depending on the laminate thickness, the laser treated TPC samples show compara- ble properties to those of conventionally processed specimens. A reduced load bearing area, as a consequence of damaged fibre-matrix-adhesion due to laser impact, is identified as main factor for the reduction of interlaminar shear strengths for higher laminate thicknesses.

A multi-field coupled model was developed to simulate the flow–compaction behavior of thick composite laminates manufactured by the autoclave process based on Darcy’s law and the effective compaction stress theory. The model was verified by comparing the predictions with the experiment results of a thick unidirectional laminate. The results show that the resin flow and compaction of fiber bed start from the top surface and gradually spread into the interior region, and the non-uniform resin flow along the thickness direction causes a gradient distribution of fiber volume fraction in the thick composite part. A cross-plied composite laminate model with a thin interlaminar layer was constructed, and the effect of the interlaminar transverse permeability on the flow–compaction behavior of the thick cross-plied laminate was numerically analyzed. The results indicate that the thick cross-plied composite laminate with high interlaminar transverse permeability has the similar flow–compaction process with that of the thick unidirectional laminate. An interlaminar layer with low transverse permeability impedes the resin flowing out from the interior of the thick cross-plied composite laminate and causes a lower fiber volume fraction compared with that in a unidirectional laminate.

This paper outlines the current design trends in food packaging, its main environmentally friendly material alternatives, and industrial processing technologies. In this respect, this important product has undergone several evolutions throughout history. Initially acting as a containment device, it has later evolved into a source of information and even a marketing platform for food companies, always with a view to extending shelf life. However, these functionalities are highly dependent on the materials used and their properties. In this respect, plastics have conquered the food packaging market due to their affordability and flexibility. Nevertheless, environmental concerns have arisen due to their impact on the environment, in addition to the introduction of stricter industry regulations and increased consumer environmental awareness. Therefore, this work found that the current design trends in food packaging are toward sustainability, reducing packaging complexity, with easier recycling, and material selection that combines both sustainability and functionality. In the case of bioplastics as a sustainable alternative, there is still room for improvement in their production, with careful consideration of their raw materials. In addition, their technical performance is generally lower, with challenges in barrier properties and processability, which could be addressed with the adoption of Industry 4.0.

A novel hybrid damage monitoring approach to understand the correlation between size effect and failure behavior of twill CFRP laminates

Effect of laminate thickness on low-velocity impact of GFRP/epoxy composites