Effect Of Void Content Research Articles

Effect of Void Content on the Mechanical Properties of GFRP for Ship Design

Defects such as voids in composite materials often degrade the mechanical properties of laminates. Even if these materials are manufactured based on the design requirements, there is a possibility of instability occurring in these composite structures. In this study, several prototypes were developed based on changes in composite ship design conditions (glass fiber weight fraction and fabric combination type) using a hand lay-up approach. The fabrication quality was quantitatively defined using the burn-off test, and statistical analysis was performed. A combination of chopped strand mat and woven roving material laminates possessed relatively less void content in the entire glass content (Gc) region (30–70 wt%) compared to a chopped strand mat single-material laminate. The effect was more pronounced in the high-Gc region (50–70 wt%) than that in the normal-Gc region (30–50 wt%). The composite hull plate can be designed seamlessly according to changes in fabrication quality. To ensure safety, the thickness of the laminate must be greater than that specified in the ISO standards, regardless of the combination type in the normal-Gc region. As a result of the void content considered, the flexural strength in the single laminate decreased by 15.02%. Furthermore, 3.33% of the flexural strength calculation decreased in the combined laminate compared to that in the ISO rules. Thus, a single CSM material can be designed to be thicker than a combined-material laminate with the same Gc, while considering the void content on the mechanical properties.

Experimental investigation and micromechanics-based damage modeling of tensile failure of polymer concrete reinforced with recycled PET bottles

Epoxy-based polymer concretes (PCs) behave in a quasi-brittle manner and suffer from low tensile strength. Adding different kinds of fillers has been widely used to improve the ductility of these materials and change the failure mechanisms. In this study, we used polyethylene terephthalate (PET) fillers from recycled bottles to enhance the tensile strength and ductility of PCs and prevent sudden brittle failure. Two different sizes of fillers (i.e., fine and coarse) are considered and the Brazilian disk configuration is used to measure the indirect tensile strength of reinforced PCs. Experimental results showed that the addition of coarse PET fillers to the PC has more significant effects on tensile strength and failure mechanism than fine PET fillers. Then, micromechanical damage analyses are conducted via finite element simulations of two-dimensional representative volume elements. The effects of void content and recycled PET fillers on the tensile strength and ductility of reinforced PCs were studied in the micromechanical simulations. Since the Brazilian disk examines the indirect tensile behavior, a relationship was proposed between the direct micromechanical tensile strength and the indirect macro mechanical tensile strength. The new procedure is experimentally validated for the prediction of the tensile strength of both reinforced and unreinforced PCs.

Effects of void content on the moisture uptake and mechanical strength of a glass/epoxy composite

The effect of voids on the moisture uptake and transverse modulus and strength of a unidirectional glass/epoxy composite has been examined. To create voids, a foaming agent was mixed with the liquid epoxy resin prior to infusion. Micrographs of polished cross-sections indicate that voids are close to spherical in shape appearing in the matrix and at the fiber/matrix interface. The fiber, matrix, and void contents were analyzed by micrograph, burn-off, and density (Archimedes) methods. The void contents in the void-free and void-containing composites were respectively close to zero and 21%. Specimens were immersed in distilled water at 40°C until saturation. The weight gain was monitored periodically. For the void-free specimens, saturation moisture contents of about 0.60%. The void-containing specimens absorbed much more water, about 5%. The Fickian diffusion model provided good fits to experimental data although the transport mechanism for the void-containing composite appeared non-Fickian. Young’s modulus of the void-free composite was slightly affected by water absorption, although, the transverse tensile strength dropped significantly. Voids were found to substantially reduce transverse modulus and strength about 70%, for both dry and water saturated specimens. Moisture content analysis was in good agreement with experiments for the void-free composite but over-predicted the moisture content for the voidcontaining composite. Micromechanics predictions of the transverse modulus were within 21% of the experimental results for void-free and void containing composites at dry and water saturated conditions.

A DMA-Based Approach to Quality Evaluation of Digitally Manufactured Continuous Fiber-Reinforced Composites from Thermoplastic Commingled Tow

Direct digital manufacturing of continuous fiber-reinforced thermoplastics exhibits the potential to relieve many of the constraints placed on the current design and manufacture of composite structures. At present, the additive manufacturing of continuous fiber-reinforced thermoplastics is demonstrated to varying extents; however, a comprehensive investigation of manufacturing defects and the quality of additively manufactured high fiber volume fraction continuous fiber-reinforced thermoplastic composites is limited. Considering the preliminary nature of the additive manufacturing of continuous fiber-reinforced thermoplastics, composites processed in this manner are typically subject to various manufacturing defects, including excessive void content in the thermoplastic matrix. Generally, quality evaluation of processed composites in the literature is limited to test methods that are largely influenced by the properties of the continuous fiber reinforcement, and as such, defects in the thermoplastic matrix are usually less impactful on the results and are often overlooked. Hardware to facilitate the direct digital manufacturing of continuous fiber-reinforced thermoplastic matrix composites was developed, and specimens were successfully processed with intentionally varied void content. The quality of the additively manufactured specimens was then evaluated in terms of the measured maximum storage modulus, maximum loss modulus, damping factor and the glass transition temperature by means of dynamic mechanical analysis (DMA). DMA allows for thermomechanical (i.e., highly matrix sensitive) evaluation of the composite specimens, specifically in terms of the measured elastic storage modulus, viscous loss modulus, damping factor and the glass transition temperature. Within the tested range of void contents from roughly 4–10%, evaluation by DMA resulted in a distinct reduction in the maximum measured storage modulus, maximum loss modulus and glass transition temperature with increasing void content, while the damping factor increased. Thus, the results of this work, which focused on the effect of void content on DMA measured properties, have demonstrated that DMA exhibits multi-faceted sensitivity to the presence of voids in the additively manufactured continuous fiber-reinforced thermoplastic specimens.

Effects of void content on flexural properties of additively manufactured polymer composites

Mechanical characterizations are inevitable to the research and development in additive manufacturing (AM). Most mechanical tests for composite materials are relatively expensive, time-consuming, and challenging. Therefore, three-point flexural testing is widely adopted as it offers an easy and fast method to assess AM composite's performance. The correlations between the flexural and other mechanical properties in AM composites are, however, not understood. In this work, multiscale modeling is utilized to connect micro-scale composite properties to macro-scale flexural simulations and experiments. Two AM composite samples of continuous carbon fiber-reinforced polyetheretherketone (PEEK) are considered. The void volume fractions in these samples are ~10 and 5%, typical of the composite's AM process. First, elastic constants for the two samples, considering their porosity, are predicted via micro-mechanical finite element analysis (FEA). A test model is then constructed to carry out detailed macro-scale FEA stress analysis in flexural tests. Specifically, flexural strength of AM composites is correlated to their tensile and compressive strengths. The findings of this study can pave the way for rapid characterization, process optimization, and new materials development in AM composites.

The effect of molding process on thermomechanical properties of feather nonwoven reinforced polyester composites

Composites reinforced with feather fibers have attracted the attention of scientists due to their low cost, availability, renewable character and low density. This experimental study emphasizes the effect of molding techniques on the mechanical, thermal and biodegradability properties of feathers nonwoven reinforced polyester composite (FP). The composites were prepared by three methods: resin transfer molding (RTM), infusion and vacuum molding techniques. The morphological analysis revealed excellent compatibility of feather fiber in the matrix. The FP composites made by the three methods showed good insulation performance with thermal conductivity values ranging from 37 mW/(m.K) to 39 mW/(m.K) at 10 °C and the lowest value was observed for the sample developed by the RTM technique. Moisture absorption and the effect of void content on the moisture absorption depend on a number of factors: type of reinforcement, matrix material, molding process and fiber content .The FP composites developed by the vacuum molding technique have shown higher water absorption due to the presence of a high void content. The tensile strength and young's modulus of the FP composites were also examined in this research; the mechanical results showed that the molding techniques clearly affected the performance of the composites. The results obtained in this study have potential applications where high environmental resistance, low cost and durable materials are required.

Experimental Study of Hollow-core Slab Containing Waste PET Bottles

This study investigated the utilization of plastic-waste concrete as an effort to reduce urban waste problems. The waste plastic bottles were utilized to form the hollows of the hollow-core slabs (HCSs). The bottles were made of polyethylene terephthalate (PET). As a part of green research to reuse waste material, shredded PET was also added to the concrete mixture to improve the HCS strength. The cast-in-site HCS could be constructed without any difficulties. Three parameters were investigated: the effects of void content, shredded PET content, and steel-fiber (SF) content on the HCS ultimate bending capacity (Mu). Fifteen specimens were tested under static loads until failure, and the results were compared with those of the solid slab. Two different void contents 19% and 24% were studied. The other parameters were the shredded PET content (0.5% and 0.7%) and the SF content (0.19% and 0.32%). The Mu values of the HCS specimens were 12% to 16% less than that of the solid slab. However, the strengths were still within the theoretical capacity of the slab. The addition of the shredded PET could improve the HCS bending capacity by 18% to 38% compared with that of the solid slab. Similar results were also found for the specimens with SFs, whose Mu values were 11% to 46% greater than that of the solid slab.

Microscale finite element analysis for predicting effects of air voids on mechanical properties of single fiber bundle in composites

Air voids produced in the manufacturing process have significant influence on the mechanical performances of single fiber bundle in carbon fiber-reinforced composites. The microscale finite element model of fiber bundle with voids is developed to predict mechanical properties and failure mechanisms. The failure responses of fiber and matrix in bundle are initiated by the maximum stress and Stassi failure criteria. The sudden stiffness degradation law is adopted to capture brittle behaviors. Both available theoretical models for voids and non-voids are used to validate the numerical model without voids and with different percentages of voids including 0.15, 0.5, 1, 2, 3, 4 and 5%. Effects of void contents on stress–strain responses of bundle are studied. The matrix damage development is studied regarding transverse compression and out-of-plane shear loading. The micro-stress analysis of matrix and voids in three-RUC model under different loadings is performed. The simulated elasticity and strength properties correlate well with theoretical results for voids and non-voids. The strengths and moduli gradually decrease with increasing void contents except longitudinal properties. The stress concentrations of bundle are mainly influenced by the loading direction.

An experimental investigation on the effect of particle size on the thermal properties and void content of Solid Glass Microsphere filled epoxy Composites

This paper investigates about the thermal characterization of Solid glass micro-sphere (SGM) filled epoxy composites. SGMs of different sizes are embedded in epoxy resin to fabricate composites by hand-layup technique. The composites for various SGM content ranging from 0 to about 35 vol % are thus fabricated and the effective thermal conductivities (keff) of the composites are estimated. The theoretical values are then compared with keff values obtained from the experiment. This study shows that the incorporation of SGm results in an improvement in thermal insulation capability of the polymer. Further, the influence of size and content of SGMs in the extent of reduction of keff was studied. Also, the effect of void content on improving insulation capability of the composites was analysed.

Influence of voids on the compressive failure behaviour of fibre-reinforced composites

Abstract The subject of this work is the micromechanical investigation of the influence of voids on the compressive failure behaviour of fibre-reinforced polymers (FRP). Although, the effect of void content on the mechanical properties was investigated in previous literature, a local examination of the failure mechanisms is essential to understand how the presence of voids reduces the compressive strength in FRP. Therefore, the complex three dimensional structure of a composite, including several voids, were analysed and a reduced mechanical model composite (focusing on one unique void) was derived. The experimental investigation of the model composite included the description of the stress- and strain behaviour of the matrix using photoelasticity. Additionally, a numerical examination of a parameterised model composite was conducted. From the experimental studies the failure initiation and propagation could be observed. Supported by the findings from numerical simulation, foundation and alignment of fibres as well as fibre-matrix bonding has the most impact on the failure initiation.

A simplified fabrication process for biofiber-reinforced polymer composites for automotive interior trim applications

A simplified process was developed for fabricating natural bio-based fiber-reinforced polymer composites for applications such as automotive interior trim substrates. Biofiber (Kraft pulp fiber) and four types of thermoplastic polymers (PP, two PP/PE polymer blends, and PLA) were first wet-formed into fiber/polymer mats and the mats were made into composites using a match-mold thermoforming process. The effects of void content on the composite tensile and flexural properties were investigated. Impact resistance and heat deflection temperature were tested and acoustic absorption coefficients of the composites were evaluated as well. Two types of prototype panels (2-D and 3-D) containing biofiber/polymer composite substrates with a bonded synthetic leather decorative surface were successfully made using this process. Biofiber/PP composites had comparable performance to the commercially available non-wood natural fiber/PP composite counterparts for the properties investigated in this study at the same density level (0.8 g/cm3).

The Residual Bending Properties after Impact of CFRP Laminates with Voids

The effects of voids on the residual bending strength after impact of CFRP laminates were discussed in this paper. Impact tests on the CFRP laminates with two porosity levels were conducted at five impact energy levels of 3J, 6J, 9J, 12J and 15J at the room temperature. The bending strength and the bending modulus of non-impacted specimens decreased 0.67% and 0.086% with the porosity increasing from 0.33% to 0.71%. Compared to non-impact specimens, the residual bending strength after impact (BAI) and bending modulus after impact significantly decrease with increasing impact energy. The void contents had the negative effects on the BAI after impact of the specimens at the same impact energy, while no obvious effects of void contents on the bending modulus are observed.

EFFECTS OF POROSITY ON ULTRASONIC CHARACTERISTIC PARAMETERS AND MECHANICAL PROPERTIES OF GLASS FIBER REINFORCED COMPOSITES

Voids are inevitable in the fabrication of fiber reinforced composites and have a detrimental impact on mechanical properties of composites. Different void contents were acquired by applying different vacuum bag pressures. Ultrasonic inspection and ablation density method were adopted to measure the ultrasonic characteristic parameters and average porosity, the characterization of voids' distribution, shape and size were carried out through metallographic analysis. Effects of void content on the tensile, flexural and interlaminar shear properties and the ultrasonic characteristic parameters were discussed. The results showed that, as vacuum bag pressure went from -50kPa to -98kPa, the voids content decreased from 4.36 to 0.34, the ultrasonic attenuation coefficient decreased, but the mechanical strengths all increased.

The effects of void contents on the long-term hygrothermal behaviors of glass/epoxy and GLARE laminates

In this paper the effect of voids formed in both the glass/epoxy and GLARE laminates on their long-term hygrothermal behaviors are evaluated. By adopting different autoclave pressures during a cure cycle, the specimens with different void contents ranging from 0.5% to 2.0% were obtained for both materials. Two types of hygro and thermal aging tests were then carried out according to water absorption in distilled water and thermal cyclic fatigue up to 1500 cycles, respectively. The resulting behaviors of degradation for both materials due to two types of aging tests were evaluated by the test for interlaminar shear strength which is highly affected by those types of aging. Experimental results indicate that the autoclave pressure enhances a certain extent of the GLARE’s interfacial bonding with the low void contents. It therefore leads to an increase in the resistance to the hygro and thermal agings.

Effect of Void Content on the Moisture Absorption in Polymeric Composites

This study assesses the effect of porosity in the moisture absorption of polymeric composites. The preparation of composites samples with porosity, control, and measurement of void content, and moisture absorption behavior of carbon/epoxy laminates are discussed. Rheological analyses are used to characterize the viscosity behavior of prepregs either in the presence or absence of moisture. Polymeric composites with different levels of voids were produced and submitted to hygrothermal conditioning to study the moisture absorption behavior. It was found that the rate of water uptake and the maximum level of moisture absorption in carbon/epoxy composites depends on the void content and specimen geometry. The dependence on the void content may be quite significant. The nature of the matrix system was also verified to affect the moisture absorption properties of the composite.

Formation of Microscopic Voids in Resin Transfer Molded Composites

Performance of composite materials usually suffers from process-induced defects such as dry spots and microscopic voids. While effects of void content in molded composites have been studied extensively, knowledge of void morphology and spatial distribution of voids in composites manufactured by resin transfer molding (RTM) remains limited. In this study, through-the-thickness void distribution for a disk-shaped, E-glass/epoxy composite part manufactured by resin transfer molding is investigated. Microscopic image analysis is conducted through-the-thickness of a radial sample obtained from the molded composite disk. Voids are found to concentrate primarily within or adjacent to the fiber preforms. More than 93% of the voids are observed within the preform or in a so-called transition zone, next to a fibrous region. In addition, void content was found to fluctuate through-the-thickness of the composite. Variation up to 17% of the average void content of 2.15% is observed through-the-thicknesses of the eight layers studied. Microscopic analysis revealed that average size of voids near the mold surfaces is slightly larger than those located at the interior of the composite. In addition, average size of voids that are located within the fiber preform is observed to be smaller than those located in other regions of the composite. Finally, proximity to the surface is found to have no apparent effect on shape of voids within the composite.

Effect of void content on the behavior of advanced fiber reinforced composite materials

Effect of void content on the behavior of advanced fiber reinforced composite materials