Interlaminar Shear Strength Values Research Articles

Interlaminar Shear Strength of Medium-Density Fiberboard (MDF) Measured by Three-Point Short-Beam Shear Test and Comparison with that Obtained by Asymmetric Four-Point Bending Test

In this research, interlaminar shear strength of medium-density fiberboard (MDF) as obtained by a three-point short-beam shear test was examined. The three-point short-beam shear tests were conducted on MDF, which were used in a previous research on the asymmetric four-point bending tests. Interlaminar shear strength was measured using the specimens with rectangular and I-shaped cross sections with varying the span lengths. It is less useful to obtain the interlaminar shear strength by the three-point short-beam shear test than the asymmetric four-point bending test because the effective span length for obtaining an appropriate value of interlaminar shear strength is extremely restricted.

Nanographite-reinforced carbon/carbon composites

Carbon/carbon composites (C/Cs) with nanographite platelets (NGP) filler in a matrix derived from phenolic resin were produced. Different weight concentration (0.5, 1.5, 3, 5wt.%) NGP were introduced by spraying the NGP during the prepreg formation. The NGP-reinforced C/Cs were characterized for effect of NGP concentration on microstructure, porosity, interlaminar shear strength (ILSS), flexural, ultrasonic and vibration damping behavior. At 1.5wt.% NGP C/C, the highest values of ILSS observed was 10.5MPa (increased by 22%), flexure strength of 142.4MPa (increased by 27%), flexural modulus of 59.2GPa (increased by 68%) and porosity of 18.8% (reduced by 17.5%) in comparison to neat (without NGP) densified C/C. Ultrasonic testing revealed an average increase of 15% through the thickness Young’s modulus of NGP-C/C; (3.12GPa at 1.5wt.% NGP). A 20% average decrease in the damping ratio of the first four modes of vibration was observed in 1.5wt.% NGP densified C/C. At low concentration (⩽1.5wt.%) the NGP filled in the pores, cracks and debonded interface but at concentration higher than 1.5wt.% NGP lost their effectiveness due to agglomeration. The required cycles for desired density/properties are projected to be less compared to neat C/C due to less porosity observed in ⩽1.5wt.% NGP concentration C/C.

Effect of carbon fiber surface chemistry on the interfacial properties of carbon fibers/epoxy resin composites

Polyacrylonitrile-based carbon fibers were electrochemically oxidized under a wide variety of current densities, and then surface changes in the morphological structures and the chemical compositions were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. Finally, the effects of carbon fiber surface chemistry on the interfacial properties of carbon fiber reinforced composites were researched. The results showed that surface contaminants generated during the production process were removed and the relative contents of oxygen and nitrogen elements increased by 322% and 432%, respectively, after the electrochemical oxidation. Increases in the O/C and N/C ratios resulted in the increase in the interlaminar shear strength values of composites. However, much higher O/C and N/C ratios led to decline in the interlaminar shear strength values. Carboxyl groups were responsible for strong interactions between carbon fiber surfaces and the epoxy resin. When the current densities were more than 5 A/m2, the interfacial bonding between carbon fibers and the epoxy resin became weak and the interlaminar shear strength values also decreased.

Effect of low current density electrochemical oxidation on the properties of carbon fiber‐reinforced epoxy resin composites

Changes in surface physicochemical structures of polyacrylonitrile‐based carbon fibers resulted from low current density electrochemical oxidation were monitored by scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS). The relationship between the interlaminar shear strength (ILSS) values of carbon fiber‐reinforced polymers (CFRPs) and carbon fiber surface chemistry including elemental ratios and the relative content of oxygen‐containing functional groups were researched. SEM results revealed that the electrochemical oxidation got rid of surface contaminants generated during the production process. XPS analysis showed that the relative contents of oxygen and nitrogen increased by 446% and 202%, respectively, after the electrochemical oxidation. Carbon fiber surface chemistry was of paramount importance to the interfacial properties of CFRPs. The higher the carbon fiber surface activity, the better the interfacial bonding was, and an increase in the acidic‐group contents was responsible for a higher ILSS value. However, when the current density increased to 1.0 A/m2, the interfacial bonding between carbon fiber and the epoxy resin became weak which led to the decline in ILSS values. Copyright © 2012 John Wiley & Sons, Ltd.

Thermal shock behavior of hydrothermally conditioned e-glass fiber/epoxy composites

The glass fiber-reinforced polymer composites suffer from significant moisture absorption properties, moisture due to both the ambience and direct immersion in ordinary water during the course of their practical applications. The quantity of moisture absorbed and its effect on the composite component in any engineering application is also severely influenced by the temperature fluctuations. Hence the present work aims at evaluating the glass fiber/epoxy composite under hydrothermal exposure coupled with up and down-thermal shocks for various lengths of time. Inter-laminar shear strength (ILSS) values of shocked and non-shocked composite specimens were determined using INSTRON-1195 testing system. Low-temperature differential scanning calorimetry was used to monitor the glass transition temperature (Tg). A longer hydrothermal exposure showed an increase in the ILSS value after an initial low value. The ILSS seemed to vary with up and down-thermal shock differently for hydrothermally treated sample. Despite little variation in Tg value, the depression in the same was observed under up-thermal shock compared to down-thermal shock conditions. The fractographs of shocked and non-shocked specimen, as obtained from scanning electron microscope, divulged the chief mode of failure viz. fiber fragmentation and delamination.

Effect of ammonium-salt solutions on the surface properties of carbon fibers in electrochemical anodic oxidation

The surfaces of polyacrylonitrile-based carbon fibers were treated by an electrochemical anodic method. Three different kinds of ammonium-salt solutions namely NH4HCO3, (NH4)2CO3 and (NH4)3PO4 were respectively chosen as the electrolytes. The effect of these electrolytes on the surface structure was studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The results showed that longitudinal grooves on the fiber surface became more well-defined and much deeper after surface treatment, and the root mean square roughness (RMS) of carbon fiber surface increased from 4.6nm for untreated fibers to 13.5nm for treated fibers in (NH4)3PO4 electrolytes. The concentration of oxygen and nitrogen atomic on the fiber surface increased after surface treatment. The tensile strength of oxidized fibers had an obvious decrease, whereas the interlaminar shear strength (ILSS) value of corresponding carbon fiber reinforced polymers (CFRPs) increased in a large extent. The intensity of oxidative reaction varied with the change of ammonium-salt solutions and electrochemical oxidation in (NH4)3PO4 electrolyte was of the most violence. The corresponding mechanism was also discussed and the result showed that the higher the concentration of OH− ions in the electrolytes, the violent the oxidative reaction happened.

Mechanical anisotropy in unidirectional glass fabric reinforced oligomeric siloxane modified polyester composites

In this study, the effect of incorporation of oligomeric siloxane into unsaturated polyester on mechanical behavior of unidirectional glass fiber/polyester composites has been investigated by means of tensile, flexural and short beam shear tests. The amount of oligomeric siloxane added into unsaturated polyester was in the range 1–3 % by weight of the glass fabrics. Mechanical tests were conducted at different angles (0 °, 45 °, and 90 °) with respect to fiber direction. The higher siloxane content exhibited a tendency to have greater tensile, flexural and interlaminar shear strength values in machine direction, bias direction and cross direction. From Scanning electron microscopy images, the presence of polyester particles on the unidirectional glass fiber surface confirmed better adhesion.

Synthesis and characterization of novel epoxy‐modified waterborne polyurethanes and their use in carbon fiber sizing

AbstractNovel epoxy‐modified waterborne polyurethanes (EPUs) were synthesized from toluene 2,4‐diisocyanate, poly(ethylene glycol) (PEG), dimethylolbutyric acid, and 2,3‐epoxy‐1‐propanol (glycidol). The chemical structures of the EPU products were characterized by 1H‐NMR and Fourier transform infrared spectroscopy. The thermal properties of the EPUs were examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA showed that the EPUs had good thermal stability, with an initial decomposition temperature higher than 250°C. DSC analysis showed that the glass‐transition temperature of EPU1 was higher than that of EPU2. When the EPUs were heated above 150°C, the curing reaction took place. In this study, the EPUs were used for carbon fiber sizing. The mechanical interfacial strength between the fibers and the matrix was evaluated by scanning electron microscopy (SEM) and interlaminar shear strength (ILSS) testing. The SEM images of the fracture sections of the composites proved that the interfacial adhesion between the fibers and the matrix improved after the EPU sizing treatment. The composites exhibited the highest value of ILSS after the EPU1 sizing treatment. The excellent properties of the EPUs confer it with potential applications in carbon fiber sizing. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The modification of Kevlar fibers in coupling agents by γ-ray co-irradiation

Kevlar fibers were treated in three kinds of coupling agents' solutions by Co-60 gamma-ray co-irradiation. After the treatment, the interlaminar shear strength (ILSS) values of Kevlar fibers/epoxy composites were all improved. Surface elements of the fibers were determined by energy dispersive X-ray microanalysis (EDX). X-ray photoelectron spectroscopy (XPS) indicated that the oxygen/carbon ratio of the treated fibers was increased and Fourier transform infrared (FT-IR) spectrum confirmed the increase in the polar groups at the fiber surface. The tensile strength of the fibers was evaluated by statistical analysis using the Weibull distribution. The wettability of the fiber surface was also enhanced by the treatment. The possible mechanisms of gamma-ray co-irradiation treatment are proposed by the radical reactions. The results indicated that gamma-ray co-irradiation technique modified the physicochemical properties of Kevlar fibers and improved the interfacial adhesion of its composites.

Improvement of PBO fiber surface and PBO/PPESK composite interface properties with air DBD plasma treatment

The interface of fibrous composites is a key factor to the whole properties of the composites. In this study, the effects of air dielectric barrier discharge (DBD) plasma discharge power density on surface properties of poly(p‐phenylene benzobisoxazole) (PBO) fiber and the interfacial adhesion of PBO fiber reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composite were investigated by several characterization methods, including XPS, SEM, signal fiber tensile strength, interlaminar shear strength, and water absorption. After the air DBD plasma treatment at a power density of 41.4 W/cm3, XPS analysis showed that some polar functional groups were introduced on the PBO fiber surface, especially the emergence of a new oxygen‐containing group (−O–C = O group). SEM observations revealed that the air DBD plasma treatment had a great influence on surface morphologies of the PBO fiber, while the signal fiber tensile strength results showed only a small decline of 5.9% for the plasma‐treated fiber. Meanwhile, interlaminar shear strength value of PBO/PPESK composite was increased to 44.71 MPa by 34.5% and water absorption of the composite decreased from 0.46% for the untreated specimen to 0.27%. The results showed that the air DBD plasma treatment can effectively improve the properties of the PBO fiber surface and the PBO/PPESK composite interface. Results obtained from the above analyses also showed that both the fiber surface and the composite interface performance would be reduced when an undue plasma discharge power density was applied. Copyright © 2011 John Wiley & Sons, Ltd.

Effects of compaction and UV exposure on performance of acrylate/glass‐fiber composites cured layer by layer

AbstractWith an aim to reducing manufacturing costs, in general and specifically to provide a solution to the thick laminate curing depth issue for composite materials, UV curing technology was combined with a fiber placement process to fabricate acrylate/glass‐fiber composites. A novel layer‐by‐layer UV in situ curing method was employed in this article and interlaminar shear strength (ILSS) tests and SEM were used to evaluate the effect of processing parameters, including compaction force and UV exposure dose, on ILSS. The SEM images from short‐beam strength test samples and the results of ILSS showed that the fibers' distribution was uniform in the cured matrix resin resulting from the compaction forces and that beneficially influenced the ILSS of the composite greatly. However, the matrix resin produced large shrinkage stresses when it reached a high degree of conversion (DC) in one‐step, which resulted in poor interlaminar adhesion. In addition, the fast curing speed of UV on the composite resulted in poor wetting between fiber and resin, and accordingly resulted in lower ILSS. To overcome these problems and obtain high ILSS value composites, an optimized compaction force and UV exposure dose were determined experimentally. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Improvement of the interfacial adhesion between PBO fibers and PPESK matrices using plasma‐induced coating

AbstractThis work deals with the plasma‐induced coating process on the surface of PBO fibers to obtain a strong interfacial adhesion between the poly(p‐phenylene benzobisoxazole) (PBO) fibers and the poly(phthalazinone ether sulfone ketone) (PPESK) matrices. The process consisted of four steps: (a) plasma preactivation of PBO fibers; (b) immersion in an epoxy resin solution; (c) drying and then soaking with the PPESK solution; (d) shaped by compression molding technique. The orthogonal experiments used in this study enable the determination of the significant experimental parameters that influence efficiency of the process by comparing the values of ILSS. The order of their influences was the concentration > power > treating time > treating pressure. The results of the interlaminar shear strength (ILSS) and water absorption showed that the ILSS of the composite increased by 56.5% after coating, meanwhile the water absorption declined to 0.32%. The changes of the surface chemical composition, the surface morphology, and the surface free energy of fibers were studied by FTIR spectroscopy, atomic force microscope (AFM), and dynamic contact angle analysis (DCAA), respectively. Fracture mechanism of the composite was examined by scanning electron microscope (SEM). The results indicated that plasma‐induced coating process was an efficient method to enhance the interfacial adhesion of PBO fibers and PPESK matrices. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Oxygen plasma treatments of jute fibers in improving the mechanical properties of jute/HDPE composites

The surfaces of jute fabrics have been oxygen plasma treated using low frequency (LF) and radio frequency (RF) plasma systems at different plasma powers (30, 60, and 90 W) for 15 min to improve the mechanical properties of jute fiber/HDPE (high density polyethylene) composites. The effect of oxygen plasma treatment on the functional groups of jute fibers was examined by X-ray photoelectron spectroscopy (XPS) analysis. Effects of oxygen plasma treatments on the mechanical properties of jute fiber/HDPE composites were investigated by means of tensile, flexure, and short-beam shear tests. Surface morphology of the fractured surfaces of composites was observed by using scanning electron microscopy (SEM). When RF plasma system was used, the interlaminar shear strength (ILSS) values of the composites increased with increasing plasma power. Similarly, in LF plasma system, ILSS values showed a tendency to increase at plasma powers of 30 and 60 W. However, increasing of plasma power to 90 W decreased the ILSS value of jute/HDPE composite. Also, tensile and flexure strengths of the composites showed similar trends.

Chemical treatments of bamboo to modify its moisture absorption and adhesion to vinyl ester resin in humid environment

Moisture absorption is a major concern in the case of natural fibers used as reinforcement in structural composites. Bamboo strip as a kind of natural material has been used to reinforce vinyl ester resin, in this study. The moisture absorption behavior of the bamboo strips and their adhesion to vinyl ester resin at different levels of relative humidity (RH) during specimen fabrication are investigated. To improve moisture resistance and the adhesion to vinyl ester resin under high RH conditions, the bamboo strips are subject to different chemical treatments, that is with alkaline, pre-alkaline and acetic anhydride, pre-alkaline and potassium permanganate, and pre-alkaline and silane. The study of the equilibrium moisture content of modified bamboo strips shows that it is slightly increased by alkaline treatment, while it is reduced by the other three methods, especially by acetylation due to its well-known esterification. The interfacial shear strength (IFSS) by pull-out test is greatly improved after the modification and it decreases steadily as the fabrication RH levels increase for both untreated and treated samples. At high level RH (80% RH), IFSS is almost negligible except acetylated specimens. Results indicate that the severe negative impact on the IFSS by fabrication RH levels is not so considerable if the bamboo strips are subjected to acetylation as pretreatment. The interlaminar shear strength values based on short-beam shear test of unidirection bamboo strip mats/vinyl ester resin composites are well consistent with IFSS data.

Improvement and mechanism of interfacial adhesion in PBO fiber/bismaleimide composite by oxygen plasma treatment

The improved interfacial adhesion of PBO fiber-reinforced bismaleimide composite by oxygen plasma processing was investigated in this paper. After treatment, the maximum value of interlaminar shear strength was 57.5 MPa, with an increase of 28.9%. The oxygen concentration of the fiber surface increased, as did the surface roughness, resulting in improvement of the surface wettability. The cleavage and rearrangement of surface bonds created new functional groups O C O, N C O and N O, thereby activating the fiber surface. And long-time treatment increased the reaction degree of surface groups while destroyed the newly-created physical structures. The enhancement of adhesion relied primarily on the strengthening of chemical bonding and mechanical interlocking between the fiber and the matrix. The composite rupture planes indicated that the fracture failure shifted from the interface to the matrix or the fiber.

Kinetic and thermodynamics analysis of water absorption in unidirectional fiber reinforced composites by polyethersulphone and polyphenylence sulfide

In the present work, carbon fiber, polyethersulphone and polyphenylence sulfide were the material choice. By a solution and suspension impregnation technique and hot-pressing process, a unidirectional laminates was produced. The values of Interlaminar Shear Strength (ILSS) showed PPS widen the scope of shaping temperature without negative effect upon the interface binding force among the CF and resins. The moisture gain curves of composites by different matrix were compared with those of the neat resins in order to determine the interface combination status during moisture sorption process. The results proved PPS reduced water absorption by weakening the polarity of benzene and forming compact structure by crystallization. The difference in absorption heat of two kinds of matrix and composites suggested that the interface play an important role during the diffusion procession. Comparing to the pseudo-first-order model, the pseudo-second-order model is in excellent agreement with the experimental data of the composite consist of hybrid resins.

Improvement of Interfacial Adhesion of Glass Fiber/Epoxy Composite by Using Plasma Polymerized Glass Fibers

This study intends to produce plasma polymer thin films of γ-glycidoxypropyltrimethoxysilane (γ-GPS) on glass fibers in order to improve interfacial adhesion of glass fiber-reinforced epoxy composites. A low frequency (LF) plasma generator was used for the plasma polymerization of γ-GPS on the surface of glass fibers at different plasma powers and exposure times. X-ray photoelectron spectroscopy (XPS) and SEM analyses of plasma polymerized glass fibers were conducted to obtain some information about surface properties of glass fibers. Interlaminar shear strength (ILSS) values and interfacial shear strength (IFSS) of composites reinforced with plasma polymerized glass fiber were evaluated. The ILSS and IFSS values of non-plasma polymerized glass fiber-reinforced epoxy composite were increased 110 and 53%, respectively, after plasma polymerization of γ-GPS at a plasma power of 60 W for 30 min. The improvement of interfacial adhesion was also confirmed by SEM observations of fractured surface of the composites.

A study of the effect of oxygen plasma treatment on the interfacial properties of carbon fiber/epoxy composites

AbstractIn this work, effects of the interface modification on the carbon fiber‐reinforced epoxy composites were studied. For this purpose, the surface of carbon fibers were modified by oxygen plasma treatment. The surface characteristics of carbon fibers were studied by X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), dynamic contact angle analysis (DCAA), and dynamic mechanical thermal analysis (DMTA), respectively. The interlaminar shear strength (ILSS) was also measured. XPS and AFM analyses indicated that the oxygen plasma treatment successfully increased some oxygen‐containing functional groups concentration on the carbon fiber surfaces, the surface roughness of carbon fibers was enhanced by plasma etching and oxidative reactions. DCAA and DMTA analyses show that the surface energy of carbon fibers increased 44.9% after plasma treatment for 3 min and the interfacial bonding intensities A and α also reached minimum and maximum value respectively. The composites exhibited the highest value of ILSS after oxgen plasma treated for 3 min. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Enhancement of the mechanical properties of glass/polyester composites via matrix modification glass/polyester composite siloxane matrix modification

Enhancement of the mechanical and vibrational properties of glass/polyester composites was aimed via matrix modification technique. To achieve this, unsaturated polyester was modified by incorporation of oligomeric siloxane in the concentration range of 1–3 wt%. Modified matrix composites reinforced with woven roving glass fabric were compared with untreated glass/polyester in terms of mechanical and interlaminar properties by conducting tensile, flexure, and short-beam shear tests. It was found that after incorporation of 3 % oligomeric siloxane into the polyester matrix, the tensile, flexural, and interlaminar shear strength (ILSS) values of the resulting composite increased by 16, 15, and 75 %, respectively. The increases in ILSS as well as in tensile and flexural properties were considered to be an indication of better fiber/matrix interaction as confirmed by SEM fractography images. Furthermore, the effect of oligomeric siloxane incorporation on the vibrational properties of the composites was investigated by experimental modal testing and the natural frequencies of the composites were found to increase with increasing siloxane concentration.

Role of processing on interlaminar shear strength enhancement of epoxy/glass fiber/multi-walled carbon nanotube hybrid composites

Interlaminar shear strength (ILSS) of hybrid composites made from glass fiber and multi-walled carbon nanotube (MWCNT) modified epoxy is compared with unmodified epoxy/glass fiber composites (control). By combining the techniques of high speed mechanical stirring and ultrasonic agitation, 0.5% MWCNT by weight was dispersed in epoxy to prepare a suspension. Composites were manufactured by both injection double vacuum-assisted resin transfer molding (IDVARTM) and the flow flooding chamber (FFC) methods. Compression shear tests were conducted on the manufactured samples to determine ILSS. The effect of processing history and batch-to-batch variability of materials – glass fiber preform, resin and carbon nanotubes – on ILSS of samples made by both techniques was investigated. Statistical analysis of the measured ILSS values for hybrid composites and their comparison with the control specimens clearly show that hybrid composites made by the FFC process resulted in (i) significant ILSS enhancement relative to the control and to IDVARTM specimens and (ii) better repeatability than the IDVARTM process. A description of both IDVARTM and FFC processes and their role in dispersing the nanotubes between the fabric layers was discussed.