Mechanical Strength Of Composites Research Articles

Investigation of thermodynamically induced nanoparticle localization in Poly(vinylidene fluoride)/Poly(lactic acid) blends with surface-grafted carbon nanotubes

Physically anchoring multi-walled carbon nanotubes (MWCNT) onto the polymer blend interface has been extensively investigated, however, the tunable localization of polymer grafted MWCNT in immiscible polymer blends is seldom reported. Herein, poly(methyl methacrylate) (PMMA) grafted MWCNT (PMMA-g-MWCNT) were nondestructively prepared via the surface-initiated atom transfer radical polymerization. The compatibilization effects of PMMA-g-MWCNT on morphology and mechanical properties of blend composites are investigated. The results demonstrate that the fillers are distributed at the blend interface due to the high affinity between PMMA and PVDF. The interaction between PMMA and PLA changes the crystallinity of blends. Moreover, the surface modification of MWCNT enhances the interaction of filler and matrix, and the elongation at break is as high as 52 % for composites with 10 % PMMA-g-MWCNT. The fracture surface of specimen after tensile tests exhibits an obvious ductile transition with filler contents. This work systematically discussed the relationship between the PMMA-g-MWCNT distribution and the mechanical strength of composites, which provided a solid experimental foundation on the toughening mechanism of PMMA-g-MWCNT used as a compatibilizer for PVDF/PLA blends.

Mechanical and thermal properties of glass fiber reinforced polymer-clay nanocomposites

Abstract The effect of Montmorillonite (MMT) nanoclay on static mechanical and thermal characteristcs of glass fiber-reinforced polymer (GFRP) composites is studied. The static tensile properties of epoxy/MMT nanoclay, which are important properties of GFRP composites, are investigated. The predicted stress-strain curves of epoxy with 0 and 1 wt.% of nanoclay follow experiments. The predicted elastic modulus and tensile strength of epoxy with 3 wt.% of nanoclay are 74 and 230% higher than neat epoxy. For GFRP composites with 0, 1, 3, and 5 wt.% of MMT nanoclays, the measured tensile strengths are 587, 600, 628, and 724 MPa, and flexural strengths are 626, 726, 906, and 542 MPa. For composites with former nanoclays, interlaminar shear strengths are 23, 25, 31, and 28 MPa, and Izod impact strengths are 83, 94, 207, and 145 kJ/m2. Thus, mechanical strengths of composites with 0–5 wt.% of nanoclays improve considerably than pristine sample. The predicted static tensile stress-strain curves of GFRP composites with above nanoclays follow experimental behavior. For GFRP composites with 0 and 1 wt.% of MMT nanoclays, glass transition temperatures are 91 and 101°C, and storage moduli at ambient temperature are 35 and 41 GPa. The thermal conductivities of GFRP composites with 0, 1, and 3 wt.% of MMT nanoclays at ambient temperature are 0.32, 0.36, and 0.42 W/(m·K). Thus, thermal properties of composites are affected significantly on addition of nanoclays.

Oxygen permeation mass transfer modeling and onion-like micromechanical modeling to predict the effect of multilayered interface on oxidation resistance and mechanical properties of composites

To predict oxidative damage of interfaces and fibers under high temperature aerobic environment and mechanical strength of composites before and after oxidation, oxygen permeation mass transfer modeling and onion-like micromechanical modeling of multilayered interface were proposed. The oxygen mass transfer theory and oxidation damage theory of PyC interface were used to establish the two models. The oxidative damage of PyC interface was extended to (PyC-SiC)n multilayered interface, and transformation of SiC phase and microcracks caused by thermal mismatch were also considered. The law of oxygen permeation mass transfer and the evolution of fiber oxidative damage in the “Carbon fiber/(PyC-SiC)n multilayered interface” region were revealed. The mechanical strength of composites under different interface layers was researched, resembling an onion-like wrapping in layers from the core outwards. Furthermore, Cf@(PyC-SiC)n/SiC(n = 1/2/3/4) composites were prepared by chemical vapor infiltration (CVI) method. The mechanical properties of the composites were tested before and after oxidation. A comparison was made between the experimental and predicted tensile strengths before and after oxidation. The predicted results of tensile strength were found to be in good agreement with the experimental ones. In addition, the results show that PyC-SiC multilayered interface has a positive effect on the tensile strength of the composites, mainly as the cracks are deflected and branched by the multilayered interface to promote the multistage pull-out of the fibers. With the increase of interface number, the oxygen diffusion path was lengthened and zigzagged, reducing the oxidative damage of fiber and interface.

Characterisation and energy storage performance of 3D printed-photocurable resin/microencapsulated phase change material composite

The 3D fabrication of microencapsulated phase change material (MEPCM) doped resin polymer composites enables the creation of complex shapes and customized designs, opening doors for many applications in fields. This investigation fabricated a range of resin/MEPCM (20 %, 30 %, and 40 % by volume) composites using a mechanical mixing technique. This study investigates how the addition of MEPCM impacts resin matrix composite's mechanical strength, latent heat storage characteristics, and ability to regulate temperature effectively. With a 40 % MEPCM additive ratio, a pure resin porosity value of approximately 0.4 % increased to around 17 %. Thanks to the production of homogeneously dispersed MEPCM added resins with production with stereolithography (SLA), 40 % MEPCM additive enabled characteristic FTIR peaks of both MEPCM and resin to appear and, melting and solidification enthalpy values reached 87.15 j/g and 86.25 j/g, respectively. MEPCM addition enhanced the thermoregulatory properties of resin by absorbing or releasing heat during temperature fluctuations. On hotter days, 8 mm-thick composites create temperature differences exceeding 11 °C, while this difference exceeds 6 °C in the room center case. The produced 3D printed MEPCM/resin composite can be a potential material to effectively regulate the temperature of electronic devices, food packets, building materials, and electronic devices and automotive components.

Biodegradable compatibilizer modified corn stover/poly(butylene adipate‐co‐terephthalate) composites

AbstractGreat environmental and economic benefits can be achieved by utilizing the agricultural residue corn stover (CS) to prepare the biodegradable poly(butylene adipate‐co‐terephthalate) (PBAT) composites. However, CS contains polarized hydroxyl groups from its main component cellulose, thus leading to its poor wetting by hydrophobic PBAT. It is of great significance to promote the compatibility between CS and PBAT for the improvement of interfacial interactions and composite performance, but the compatibilizer modified CS/PBAT biocomposites have been rarely mentioned until now. In this study, polyethylene glycol (PEG), glycidyl methacylate (GMA), and ethylene vinyl alcohol copolymer (EVOH) were chosen to investigate the compatibilizer effect on the morphology, microstructure, thermal properties, mechanical strength, and water absorption capacity of the biodegradable CS/PBAT composites. The results reveal that the incorporated compatibilizers obviously improve the interface adhesion between CS and PBAT by reducing the hydrophilicity of CS surface. The enhanced interfacial bonding interactions effectively promote the crystallinity and mechanical strength of the CS/PBAT composites, but the initial degradation temperature and hydrophilicity of compatibilizers themselves also remarkably influence the composites’ thermostability and water absorption.Highlights The agricultural residue corn stover (CS) was used to reinforce PBAT. Effects of three different compatibilizers on the composites were studied. Compatibilizer improves the interface adhesion between CS and PBAT. Compatibilizer promotes crystallinity and mechanical strength of composites. The stability and hydrophilicity of compatibilizer affect those of composite.

Biodegradable Polyethylene-Based Composites Filled with Cellulose Micro- and Nanoparticles

Composite materials filled with cellulose particles (microcrystalline cellulose and nanocellulose) have good prospects for use in various fields. Microcrystalline cellulose (MCC) and nanocellulose (NC) were isolated by chemical and physical methods and investigated. Composite materials based on polyethylene (PE) were obtained using MCC and NC as fillers (5–20 wt.%) and maleic anhydride grafted low molecular weight polyethylene (MA-g-LMPE) as a compatibilizer. The structure and morphology of the composites and fillers were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, thermal analysis (TA), transmission electron microscopy (TEM), atomic force microscopy (AFM), and the strength properties were determined by tensile testing. An increase in the crystallinity index and mechanical strength of composites at low filler contents (up to 5 wt.%) was revealed. The size of the cellulose particles significantly affects the structure and properties of composites. Although the general picture of the effect of fillers on the crystalline structure and mechanical properties is similar, the addition of NC had a greater effect than МСС. The results of this study showed the possibility of using MCC and NC as reinforcement materials in composites, and they have biodegradable properties.

A novel one-step ultraviolet curing fabrication of myristic acid-resin shape-stabilized composite phase change material for low temperature thermal energy storage

In this work we present for the first time a simply, fast and efficient process for the fabrication of form-stable composite phase change material (PCM) by ultraviolet (UV)-curing technique. Simply by UV-curing of the PCM with a desired UV curable carrier for several minutes, a neoteric UV-cured composite that suitable for low temperature thermal energy storage (TES) can be fabricated. The feasibility of such a novel technique is demonstrated by investigating a case of myristic acid (MA) dispersed into a UV curable resin containing polyurethane acrylate (PUA). The results show that a crosslinked structure is formed in the composite after UV-curing due to the occurrence of photochemical reaction, which offer sufficient paths to accommodate the PCM, endowing the composite with the great ability to maintain the structure stabilization and prevent the liquid leakage. A good physical and chemical compatibility has been achieved in the composite and 40 wt% of UV-curing resin grants the composite optimum formula at which a melting temperature of 52 °C and an energy storage density over 210.8 kJ/kg at temperature ranged 25–100 °C are obtained. The results also reveal that the material mechanical strength can be linked to the UV-curing duration. For a given V-curing time of 5–30 mins, the composite mechanical strength is measured over 25–34 MPa, almost over twice as high as that of high-density polyethylene (HDPE) and styrene ethylene butylene styrene (SEBS) based composites. Owing to the high shape strength and excellent thermal cycling performance, such UV-curable composites can be utilized for modular design of TES devices, and also be used to precisely and directionally produce TES devices through additive manufacturing. This work exploits a new perspective for the energy-saving and time-effective fabrication of shape stabilized composite PCMs for low temperature thermal energy storage.

Lightweight and tough multilayered composite based on poly(aryl ether nitrile)/carbon fiber cloth for electromagnetic interference shielding

Electromagnetic interference (EMI) shielding materials are of vital importance in solving increasingly serious electromagnetic pollution. Currently, metal-based EMI shielding materials (ESMs) are ubiquitously used, however, their drawbacks of high density, low corrosion resistance, and poor flexibility restrict their application scenarios. By contrast, polymer-based composite materials have superior properties such as light weight, great shock absorption performance, and high corrosion resistance, which makes them promising electromagnetic interference shielding materials. In this work, an asymmetric network of the impedance matching layer and conductive shielding layer is constructed by coupling carbon fiber cloth (CFC) with magnetic porous poly(aryl ether nitrile) (PEN) polymer composite. Scanning electron microscopy confirms the alternating layered structure of the composites and the porous structure of the polymer layer. The unique designed composites achieve the highest EMI total shielding effectiveness (SET) of 37.76 dB and absorption shielding effectiveness (SEA) of 35.33 dB, which represents the successful preparation of a type of EMI material following an absorption-dominated mechanism. The CFC-3/PEN-Fe3O4 @PEI-25 demonstrates a satisfying EMI-SE/density/thickness (SSE/t) value of 543 dB·cm2·g−1, higher than most porous polymer-based EMI composites currently. In addition, the mechanical strength of composites is enhanced due to the unique alternating multilayered structure, with the highest tensile strength of 39.03 MPa, which is much higher than the obtained porous PEN membranes via the non-solvent induced phase separation (NIPs) method. This work proposes a novel approach to the construction of lightweight and resistant to secondary contamination ESMs with high performance, exerting enormous potential for wide application.

An experimental and numerical investigation on mechanical properties of 3D printed continuous glass tow preg-reinforced composites

PurposeIn this paper, continuous glass tow preg-reinforced acrylonitrile butadiene styrene (ABS) composites were fabricated by using a 3D printing method, and the purpose of this study is the investigation of the fiber preimpregnation effect on the mechanical behavior of these composites. In addition, a simple theoretical approach (mixture law), which considers the elastic behavior of reinforced composites and a numerical simulation method based on finite element method (FEM), was used to predict the tensile stress–strain behavior of ABS/glass tow preg composites in the elastic region.Design/methodology/approachDifferent groups of preimpregnated glass tows with various ABS amounts (named 2%, 10%, 20% and 30%) were prepared by the solution impregnation method. Then, preimpregnated glass tows (prepregs or tow-pregs) were fed into the printer head along with the polymeric ABS filament to print the composites. The tensile, flexural and short beam tests were conducted to evaluate the mechanical properties of the printed composites.FindingsThe first result of using tow-pregs instead of dry tows in continuous fiber 3D printing is much easier printing, printability improvement and the possibility of printing layers with low thickness, which can further increase the mechanical properties. The mechanical test results showed all of the glass prepregs improve strength and modulus in the tensile, three-point bending and short beam tests compared with neat ABS specimens, but statistical analysis showed that ABS weight percentage in the prepregs had no significant effect on the mechanical strength of composites except for the tensile modulus. Samples containing 2%-prepreg (minimum ABS amount in the tow-pregs) showed a significant improvement in tensile modulus. In the simulation section, good agreement is obtained between the model predictions and experimental tensile results. The results show that an acceptable deviation (14%) exists between the experimental and predicted value of elastic modulus by the numerical model.Originality/valueTo the best of the authors’ knowledge, this is the first study showing the effects of ABS weight percentage in prepregs on the mechanical properties of 3D printed continuous fiber-reinforced composites and predicting the mechanical behavior of 3D printed composites by numerical simulation method.

Damage Characterization of Ultra High Molecular Weight Polyethylene/ Flax/Jute Fiber Reinforced Melamine Formaldehyde Hybrid Composites using Cone Beam Computed Tomography

ABSTRACT Hybridisation is the process of reinforcing multiple fibres in a single matrix. Hybridisation has proved to enhance the mechanical strength of composites based on natural fibres and has opened avenues for using these composites in applications such as structural members for office partitions, automotive interior panels and sound proof panels. Present study deals with the hybridisation of natural fibres such as flax and jute with a synthetic fibre ultra-high-molecular-weight polyethylene (UHMWPE), using melamine formaldehyde resin. Hand lay-up and compression moulding process was used to fabricate eight layered composite panels with six different fibre orientation. In this study, the effect of stack sequence on flexural, impact and inter laminar shear strength was investigated. Maximum flexural strength of (36.07 MPa), inter laminar shear strength of (3.81 MPa) and impact strength of (63.83 kJ/m2) was noted for Stack-2 hybrid composites. Stack-4 composites with flexural strength of (18.96 MPa), inter laminar shear strength of (2.19 MPa) and impact strength of (24.61 kJ/m2) was the lowest. Analysis of the composite failure mechanisms was carried out using Cone Beam Computed Tomography (CBCT). Failure mechanisms such as angular bending, deformation, peripheral shearing, debonding and delamination were prominent.

Study on modification of ZnNb2O6/PTFE microwave composites with LCP fiber

For polymer matrix microwave composites, the agglomeration of ceramic particles at a high filling ratio will cause the microwave dielectric properties, mechanical properties and thermal conductivity of the materials to deteriorate sharply. This paper examines the reinforcement effect of LCP A950 fibers on PTFE and ZnNb 2 O 6 /PTFE composites to prevent this phenomenon. SEM, DMA, and other analysis methods showed that the incorporation of a small amount of LCP can promote the crystallization of PTFE and the mechanical strength of composites. When the volume fraction of the ZnNb 2 O 6 /PTFE/LCP three-phase is 50/45/5, the composites have a medium dielectric constant ( ε r = 6.46), extremely low loss ( t a n δ = 7.54 × 10 −4 ) and a near-zero temperature coefficient of the dielectric constant ( τ ε = +2.25 ppm/°C). Under the same conditions, the linear expansion coefficient of the material is 45.4 ppm/°C, which is 30.5% lower than that before doping with LCP, and the flexural strength is 37.8 MPa, which is an increase of 17% com、pared to the previous value. The LCP-reinforced composites have excellent overall properties, but when the LCP content is too high, its reinforcement effect on the composites is gradually weakened.

Experimental Investigation on Aquatic Waste Water Hyacinth (Eichhorniacrassipes) Plant into Natural Fibre Polymer Composite – Biological Waste into Commercial Product

Present work dealt with evaluating the aquatic wastewater hyacinth plant long fibre reinforced withepoxy polymer composite mechanical strength, absorption, characterization, thermal degradation and stability, surface morphology studies. This research work water hyacinth long fibre is used as a reinforcement material and epoxy polymer matrix material is used as a matrix phase material. By utilizing the compression moulding hot press machine the different weight percentages (20, 25, 30, and 35%) of the hyacinth composite samples areproduced.Converting the biological waste into zero waste and useful product concept is achieved in this research. In this work hyacinth, long fibre is extracted with a new novel mechanical way (fabricated machine) of the extraction process. Hyacinth long fibre composite tensile strength (mechanical strength) is varied from 36.42 to 44.62MPa, flexural strength varied from 47.86 to 59.684MPa, and impact strength varied from 0.5 to 3.5J. After the 8th hour of monitoring the composite samples are attained constant values on both water and chemical absorption studies. By utilizing thermogravimetric analysis, x-ray diffraction method, Fourier transform infrared spectroscopy method the essential functional groups present in the hyacinth composites are identified. Based on the final experiment results the hyacinth fibre composite is highly recommended to the usage of profit oriented products.

Vibrational analysis of glass/ramie fiber reinforced hybrid polymer composite

AbstractNatural fiber reinforced hybrid polymer composites have gained extensive applications in recent decades owing to their damping, low density, biodegradability, and low‐cost benefits. This research explores the impact of the surface treatment, and the number of ramie fiber layers on the vibrational and mechanical characteristics of a glass/ramie fiber reinforced hybrid polymer composite. The composite is made of outer layer (glass fiber) and inner layer (ramie fiber) at different concentrations (varying between 2, 3, and 4). The ramie was pre‐treated with sodium hydroxide (NaOH) at different weight percentages (1%, 2%, and 5%). The composites have been fabricated using the vacuum bag technique. The composites have been characterized for chemical, mechanical, morphological, and free vibrational analysis. Fourier transform infrared spectroscopy and scanning electron microscope have been used to study the chemical characteristics and morphological study of the composites respectively. The mechanical properties of the composite were evaluated through tensile and flexural testing. The hybrid composite reinforced with two layers of ramie fiber as the core with 2% pre‐treated shows the maximum tensile strength of 120 MPa and flexural modulus of 12,147 MPa. Overall, the hybridization and NaOH pre‐treatment to certain ratio helps in increase in the hybrid composite's mechanical strength compared to the natural fiber composite.

Effects of Kraft lignin and corn cob agro-residue on the properties of injected-moulded biocomposites

Lignocellulosic by-products are frequently disposed by means of combustion. This study investigates an alternative route for corn cob and Kraft lignin resources in order to support circular economy. The respective plant-based fibres and filler were compounded for the first time together with a poly(lactic acid) (PLA) matrix. Consecutively, seven different biocomposites were processed by injection-moulding and further characterized. The biocomposite containing a mixture of Kraft lignin and corn cob (12 wt% in total) exhibited the highest flexural strength (84 MPa). A proper wetting of PLA onto the corn cob particles demonstrated a good compatibility at matrix/fibre interface. PLA molecular structure changed in presence of 20 wt% lignin filler, with effect on the glass transition temperature and on the composite mechanical strength. The fibres moderately influenced composites surface tension, while Kraft lignin contributed to a slight increase of surface hydrophobicity. Surface energy (σsTotal) of composites have been estimated at 27.6, 28.7 and 27.8 mN/m for PLA/KL-20, PLA/CC-10 and PLA/KL-15/CC-5 respectively. While the polar component (σsPolar) have been estimated at 17.8, 20.0 and 18.7 mN/m for PLA/KL-20, PLA/CC-10 and PLA/KL-15/CC-5 respectively. Unlike the PLA/corn cob composite, those containing Kraft lignin were entirely biodegraded within 2 months in industrial composting conditions study. The materials could be utilized for end-use products thanks to their good mechanical and thermal properties. By adding wood-lignin and corn by-products, materials cost and carbon footprint shall decrease in comparison to pure PLA, while being a biodegradable and sustainable replacement of polyolefins.

Fabrication and investigation of mechanical properties of copper matrix nanocomposite reinforced by steel particle

Copper matrix nanocomposite reinforced by steel particles was prepared by the casting method. The disc mill and ball mill instruments were used to produce the steel particles from machining chips. The effects of the reinforcement particles' content on the microstructure, mechanical properties, fracture mechanisms, and electrical conductivity of the produced nanocomposite material were investigated. An analytical model is developed to predict the mechanical strength of the nanocomposites. The strengthening mechanisms of the reinforcement particles on the matrix and the adverse effects of the agglomeration of the particles on the composite mechanical strength are studied using the developed model. Results of the mechanical show that adding 2.5 wt% steal particles increases the yield strength, the tensile strength, the Brinell Hardness, and the Sharpy impact energy of the pure copper about 48%, 21%, 23.5%, and 300%, respectively. Elongation and ductility almost continuously increase by increasing the content of the particles. The results demonstrate the capacity of steel particles as cost-effective reinforcement material to enhance the copper matrix strength and toughness simultaneously. Furthermore, the addition of steel particles shows little adverse effect on the electrical conductivity.

Mechanical Behavior of Hydroxyapatite-Chitosan Composite: Effect of Processing Parameters

Three-dimensional hydroxyapatite-chitosan (HA-CS) composites were formulated via solid-liquid technic and freeze-drying. The prepared composites had an apatitic nature, which was demonstrated by X-ray diffraction and Infrared spectroscopy analyses. The impact of the solid/liquid (S/L) ratio and the content and the molecular weight of the polymer on the composite mechanical strength was investigated. An increase in the S/L ratio from 0.5 to 1 resulted in an increase in the compressive strength for HA-CSL (CS low molecular weight: CSL) from 0.08 ± 0.02 to 1.95 ± 0.39 MPa and from 0.3 ± 0.06 to 2.40 ± 0.51 MPa for the HA-CSM (CS medium molecular weight: CSM). Moreover, the increase in the amount (1 to 5 wt%) and the molecular weight of the polymer increased the mechanical strength of the composite. The highest compressive strength value (up to 2.40 ± 0.51 MPa) was obtained for HA-CSM (5 wt% of CS) formulated at an S/L of 1. The dissolution tests of the HA-CS composites confirmed their cohesion and mechanical stability in an aqueous solution. Both polymer and apatite are assumed to work together, giving the synergism needed to make effective cylindrical composites, and could serve as a promising candidate for bone repair in the orthopedic field.

Micro-structure and mechanical properties of microcrystalline cellulose-sisal fiber reinforced cementitious composites developed using cetyltrimethylammonium bromide as the dispersing agent

This paper reports new hierarchical cementitious composites developed using microcrystalline cellulose (MCC), sisal fibers and cetyltrimethylammonium bromide (CTAB) as the dispersing agent. MCC was dispersed in water without and with CTAB at different concentrations using ultrasonication and the optimum CTAB concentration for achieving homogeneous and stable MCC suspensions was found to be 40%. Hierarchical composites were fabricated using MCC (0.1–1.5 wt% of cement), sisal fibers (20 mm, 0.25% and 0.50 wt% of cement), 40% CTAB and tri-butyl phosphate as the defoaming agent. Mechanical strengths of composites improved significantly at 0.1 wt% MCC, which along with 0.5% sisal fibers improved compressive and flexural strengths by ~ 24% and ~ 18%, respectively. The hybrid reinforcement exhibited a synergistic effect on the fracture behavior of composites improving the fracture energy up to 40%. Hierarchical composites also showed improved fiber-matrix bonding, lower porosity and water absorption, superior hydration, carbonation resistance and durability up to 90 ageing cycles.

Effects of microvoids on strength of unidirectional fiber-reinforced composite materials

Microvoids are common defects generated during manufacturing or prolonged services of composites. Depending on the local physical environment, microvoids may form as two types, namely, matrix voids and interfiber voids. These voids are known to have detrimental effects on the load bearing capacity of composites. However, it is relatively unknown the quantitative influence of these voids on the mechanical strength of composites. Without a direct mapping between defects and ply-level constitutive behavior, high-fidelity progressive failure predictions of composite structures is challenging. To bridge the gap, a micromechanics-based finite element framework is developed to investigate composites failure in the presence of three interfiber voids and the matrix void under transverse compression, tension, shearing, and longitudinal shearing. Mechanical strengths under these four loading modes are correlated with microstructural parameters including the void volume fraction, fiber-to-void number ratio, and void orientation. In general, at the same void volume fraction, the pentagonal interfiber void lowers strength the most, followed by the square interfiber void, the triangular interfiber void, and the circular matrix void. The weakening effect is strongly associated with stress concentration in the vicinity of voids, making the fiber-to-void number ratio an important parameter as it determines the local geometry. While void orientation shows limited influence, void size is found to play an important role. Results are expected to guide design and manufacturing of composite materials to achieve less critical flaws and higher strengths. Quantitative understanding of the criticality of voids of various configurations will also assist in-situ evaluation of composite materials.

Influence of preheating on mechanical and surface properties of nanofilled resin composites.

BackgroundResin composite preheating is an innovative method that could be clinically beneficial by improving the handling properties, marginal adaptation, and surface properties of uncured nanofilled resin composite materials. There is conflict and unclear information regarding the effect of preheating on the microhardness, fracture toughness and surface roughness of nanofilled resin composites. Thus, it is important to assess whether dental clinicians can adopt preheating procedures without compromising composite mechanical strength. Objective: The purpose of this study was to evaluate the effect of preheating on microhardness, fracture toughness and surface roughness of nanofilled resin composite. Material and MethodsIn this study, one commercial nanofilled resin composite Filtek Z350 XT was used. A total of 28 disc-shaped specimens were fabricated in a Teflon mold (10 mm diameter x 2 mm thick) for Vickers microhardness indentation test and surface roughness test. The samples were divided into two groups of 14 samples each, one group of samples was light-cured at room temperature (24ºC) without preheating (non-heated group), and the other group was light-cured after preheating (preheated group). Vickers hardness measurements of 14 specimens (n=7) either preheated or non-heated of the top and bottom surfaces was measured by means of microhardness tester by applying 100 g load for 10 s. Surface Roughness measurements (Ra) were obtained from 14 specimens (n=7) either preheated or non-heated with the atomic force microscope. Fourteen single-edge-notched-beam specimens were prepared for fracture toughness test (n=7) either preheated or non-heated with measurements (2.5 x 5 x 25 mm3) and a crack 2.12 mm in length. The specimens were tested via three-point bending mode, using a universal testing machine at crosshead speed of 1.0 mm/min until failure occurred. ResultsIndependent sample t- tests revealed no significant difference between non-heated and preheated groups for all tests (p>0.05). However, for Vickers hardness test, there were significant differences between top and bottom surfaces for non-heated and preheated groups (p<0.05). Moreover, surface roughness average Ra (nm) mean values of preheated group was higher than non-heated group but no significant difference between them was found (p>0.05). ConclusionsPreheating procedure did not negatively affect microhardness, fracture toughness and surface roughness of nanofilled resin composites so preheating is recommended for the other potential clinical advantages. Key words:Preheating, nanofilled composites, microhardness, fracture toughness, surface roughness.

SiO2 aerogel-embedded carbon foam composite with Co-Enhanced thermal insulation and mechanical properties

Herein, we report the design and preparation of a carbon foam composite with the addition of SiO2 aerogel, aimed to obtain a novel thermal insulation material. SiO2 aerogel was formed via a sol-gel process and then mixed with the mesophase pitch, followed by further foaming and carbonization process to synthesize the SiO2 aerogel/carbon foam composite. The effects of SiO2 aerogel content on the morphology and performance of composite product were studied to understand the relationship between the microstructure and the thermal insulation performance and mechanical strength of the composite. The results showed that the addition of SiO2 aerogel had a significantly effect on the microstructure of the pore cells. The composite with SiO2 aerogel addition showed better thermal insulation performance as compared with the pristine carbon foam; and the composite with 11 wt % SiO2 aerogel content possessed a minimum thermal conductivity of 0.254 W/m·K. Meanwhile, the mechanical strength of the resultant composite was found co-enhanced with the addition of SiO2 aerogel. Our study paves a new avenue to realize the simultaneous enhancement of thermal insulation performance and mechanical strength of carbon foam.