Interlaminar Shear Strength Values Research Articles

INFLUENCE OF CURING PROCESS INTERRUPT ON ILSS OF THIN CARBON EPOXY LAMINATES

Autoclave curing process of any composite part lasts for about 5 to 6 hrs during which the composite parts will undergo physical & chemical changes due to the polymerization process. Any interrupts or breakdown during curing process due to mechanical / power / controller failures can affect the properties and in turn results in improper curing of a composite part. Hence there is a prime need to analyze the effects of process interrupt at critical stages of the curing process on part quality and strength. Efforts are made in this paper to study and analyse the effects of curing process interrupts on the properties of Hexcel 913 carbon /epoxy composite laminates of thickness 2 mm at five critical stages (75 0 C, 90 0 C, 110 0 C, 120 0 C, and 135 0 C ) of a cure cycle. The ILSS (Inter laminar Shear Strength) is analyzed as per Dutch Institute for Norms (DIN) standards and the results are compared with the master specimen. The results exhibits low strength at cure temperatures 90, 110 and 120 o C for process interrupt of 60 mins due to significant decrease in temperature during the process interrupt and eventually result in incomplete resin transition at critical Tg points from 90 to 120 deg.C. The laminates cured with process interrupt for 60 mins at 75 o C have reduced 0.5 % of the ILSS properties and between 90 to 120 o C the ILSS has reduced by 4 - 8% of the reference laminate properties. Hence the autoclave break down / power failure for more than 40 mins at 90 o C and 30 mins at 120 o C is not recommended for curing of composite parts of thickness less than 3 mm as the part strength gradually decreases .However these conditions vary for curing thicker composite laminates, since it consists of more layers and will have high thermal gradients between the layers during curing. Mathematical equations are formulated to determine the Avg. ILSS values at particular cure temperature for different process interrupt periods. This helps the designer or the manufacturer to evaluate and analyse the ILSS values due to process interrupts at critical points during curing process.

Dynamic mechanical property and interlaminar shear strength of the carbon fabric reinforced polyetherimide composite based on the polyetherimide resin films

In this study, the laminates reinforced with the concentrated nitric acid or acetone treated carbon fabrics were prepared based on polyetherimide films with different thickness including 30 μm and 50 μm, respectively. 4 Harness Satin (4HS) and 5 Harness Satin (5HS) carbon fabrics were used as the reinforcements of the composite laminates. Three impact factors, including the polyetherimide film thickness, fabric treatment method and fabric type, were considered in this study. Interlaminar shear strength (ILSS) measurement shown that the thicker polyetherimide film (50 μm), 4HS carbon fabric and the nitric acid treatment of the fabric could be used to increase the ILSS value because of the improvement of the interfacial property for the laminate. The 50 μm thickness polyetherimide films used in the laminates improved the storage modulus, and decreased the glass transition temperatures (Tgs) by DMA or DSC. It was because that the better interfacial property and the stronger mobility of the polymer chain under the greater residual internal stress in the laminate was obtained with the increase of the polyetherimide film thickness. The nitric acid treatment of the fabric increased the Tgs measured by DMA and DSC, and decreased the tan δ peak values of the laminates because of the stronger interfacial adhesion between the fiber and the resin and the decreased mobility of the polymer chain. In addition, the effects of above three impact factors on the ILSS, storage modulus, loss modulus, tan δ and the Tg of the laminate were discussed in detail by ILSS, DMA and DSC measurement, respectively. POLYM. COMPOS., 38:663–672, 2017. © 2015 Society of Plastics Engineers

Preparation of boz/glass fibers/cyanate ester resins laminated composites

A novel kind of high performance laminated composites of Bis allyl benzoxazine/Quartz fibers/Cyanate ester (Bz-ally/QFs/CE) have been developed. The effects of Bz-ally on the moisture resistance, mechanical, dielectric, and thermal properties of Bz-ally/QFs/CE laminated composites were investigated systematically. Compared with QFs/CE laminated composites, the flexural strength and the interlaminar shear strength value of the Bz-ally/QFs/CE laminated composites were improved markedly with up to 25.2% and 22.3% increasing magnitude, respectively. Meanwhile, the Bz-ally/QFs/CE laminated composites exhibit lower dielectric constant and loss than QFs/CE laminated composites over the testing frequency from 10 to 60 MHz. In addition, the thermal stability and moisture resistance of Bz-ally/QFs/CE laminated composites were also superior to that of Boz/QFs/CE laminated composites. POLYM. COMPOS., 38:523–527, 2017. © 2015 Society of Plastics Engineers

Preparation of emulsion‐type thermotolerant sizing agent for carbon fiber and the interfacial properties of carbon fiber/epoxy resin composite

ABSTRACTA modified resin was synthesized through the reaction between dodecylamine and tetraglycidyldiaminodiphenylmethane (TGDDM), which was used as the film former of sizing agent for carbon fiber (CF). The sizing agents were prepared through phase inversion emulsification method. Fourier transform infrared spectroscopy (FTIR) was utilized to analyze the modified resin. Particle sizes of the sizing agents were tested to evaluate their stabilities. Differential scanning calorimetry (DSC) results demonstrated that the glass transition temperature (Tg) of the modified TGDDM is much higher than the Tg of the cured epoxy resin E‐44. The influences of the sizing treatment on CF were investigated by abrasion resistance, fluffs, and stiffness tests. The maximum abrasion resistance increased by 172.8%, compared with the abrasion resistance of the desized CF. Interlaminar shear strength (ILSS) results of the CF/TGDDM composites indicated that the interfacial adhesion between CF and matrix resin was greatly improved after CF was sized. The maximum ILSS value could obtain a 29.16% improvement, compared with the ILSS of the desized CF composite. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41882.

Nano-enhanced composite materials under thermal shock and environmental degradation: A durability study

Carbon nanotubes (CNTs) were incorporated at 0.5 wt% in epoxy resin using sonication at two different levels of amplitude (50% and 100% of 400W nominal sonication power). The CNTs modified epoxy systems were used to manufacture carbon fibre reinforced laminates (CFRPs). All specimens were subjected to thermal shock and hygrothermal exposure. The presence of CNTs did not alter the water absorption profiles for the epoxy resin, but it resulted in a spectacular 40% reduction in the water uptake at equilibrium for the CFRPs. The interlaminar shear strength of the CFRPs was not significantly affected by the thermal shock cycles; however it was reduced by 50–60% after hygrothermal exposure. The addition of CNTs led to slightly lower interlaminar shear strength values in the as-manufactured state. However their presence did not accelerate the deterioration of the strength after the environmental exposure. Although the addition of CNTs did not significantly influence the thermomechanical properties of the resin, they were beneficial in the case of the CFRPs since (i) they enhanced the storage modulus and glass transition temperature and (ii) limited the deterioration of these properties after thermal shock and hygrothermal exposure. The amplitude level during sonication which determined the dispersion state and length of the CNTs had a clear effect on the durability of the studied systems.

The effect of argon and air plasma treatment of flax fiber on mechanical properties of reinforced polyester composite

Flax fibers were modified by argon and air atmospheric pressure plasma treatments to improve the mechanical properties of flax fiber-reinforced unsaturated polyester composites. Plasma treatments were carried out at plasma powers of 100, 200, and 300 W. Both plasma surface treatments were conducted to improve the tensile strength, tensile modulus, flexural strength, flexural modulus, interlaminar shear strength (ILSS), Mode I interlaminar fracture toughness (GIC), and Mode II interlaminar fracture toughness (GIIC). Moreover, the maximum improvement in the mechanical properties was obtained after air plasma treatment of flax fiber at a plasma power of 300 W. Tensile strength, flexural strength, ILSS, GIC, and GIIC values of flax fiber-reinforced polyester composites increased by nearly 34%, 31%, 39%, 35%, and 42%, respectively. However, for argon plasma-treated flax fiber-reinforced polyester composites, the mechanical properties of composite increased up to argon plasma power of 200 W.

Effects of temperature and loading speed on interface-dominated strength in fibre/polymer composites: An evaluation for in-situ environment

The present investigation intends to study the influence of crosshead velocity and in-situ environmental conditioning i.e. high temperature and cryogenic temperature on micromechanical performance of glass fibre/epoxy, carbon fibre/epoxy and Kevlar fibre/epoxy polymer composites. 3-point short beam shear tests were conducted on the conditioned specimens to evaluate the interfacial properties and failure modes which are related to mechanical properties of the composites. The effect of crosshead velocity (within the range 1-103mm/min) on the interlaminar shear strength (ILSS) of all the three composite systems at different temperatures was studied. The glass transition temperature (Tg) of conditioned samples were measured by differential scanning calorimetry (DSC) in the temperature range of 25°C to 150°C temperature. At 1mm/min loading rate, for both glass/epoxy and carbon/epoxy composites maximum increase in ILSS value was about 85.72% with respect to ambient, while for Kevlar/epoxy composite 31.77% reduction in ILSS was observed at -100°C temperature.

Experimental study on the mechanical behavior and microstructural assessment of Kevlar/epoxy composites at liquid nitrogen temperature

AbstractThe present investigation emphasized the understanding of the mechanical behavior and progressive failure modes of Kevlar/epoxy-reinforced laminated composites subjected to a three-point bend test at liquid nitrogen (LN2) temperature (77 K). The tests were conducted on the woven fabric laminates with different loading speeds and characterized by scanning electron microscopy (SEM). The effect of LN2 temperature on the interlaminar shear strength (ILSS) of Kevlar/epoxy composites with various loading speeds was observed. The results indicate that the values of ILSS were increased at higher crosshead speeds compared to ambient temperature. The fibrillation progression and matrix failure (riverline marking) was observed by SEM. A change in crosshead speed may result in the variation of failure modes.

An assessment of mechanical behavior and fractography study of glass/epoxy composites at different temperatures and loading speeds

Effect of loading rate on fracture and mechanical behavior of autoclave cured glass fiber/epoxy prepreg composite has been studied at various loading (striking) rates (0.01-103mm/min). The maximum load carrying capacity and strain at yield continuously increases with increasing loading speed. The interlaminar shear strength (ILSS) value is high at low loading speed and becomes low at high loading speed with the transition of loading rate at approximately 300mm/min. The formation of steps, welt interfacial failure and cleavage formation on matrix resin i.e. localized plastic deformation processes were dominating mechanisms for specimens tested at low loading rates, while brittle fracture of fiber, fiber pull-out and impregnation were dominating mechanisms for specimens tested at loading rates of 800mm/min or higher.

Preparation of POSS/Quartz fibers/cyanate ester resins laminated composites

Quartz fibers (QFs) and two polyhedral oligomeric silsesquioxane (POSS) (epoxy-POSS and hydroxy-POSS) are performed to regulate the dielectric properties, heat resistance, and mechanical properties of cyanate ester (CE) resins. With the introduction of POSS, the dielectric constant and dielectric loss tangent values of the POSS/QFs/CE laminated composites are decreased obviously. The heat resistance properties and flexural strength values of the laminated composites are increased, but the interlaminar shear strength values of the laminated composites are decreased slightly. The EP-POSS/QFs/CE laminated composites have relatively better dielectric, heat resistance, and mechanical properties than those of HO-POSS/QFs/CE laminated composites. POLYM. COMPOS., 36:2017–2021, 2015. © 2014 Society of Plastics Engineer

Combination effect of physical drying with chemical characteristic of carbon nanotubes on through-thickness properties of carbon fiber/epoxy composites

The present work studied the combination effect of physical drying with chemical modification of carbon nanotubes (CNTs) on some through-thickness properties of carbon fiber/epoxy composites. Different drying methods of heat drying and freeze drying were utilized to affect CNT organization form in carbon fiber/CNTs preforms and composites: The adoption of heat-drying method made CNTs more inclined to form aggregates accompanied with randomly scattered CNTs, while continuous CNT networks could always be assembled when freeze drying method was employed. The formation mechanism of such CNT networks was discussed, and could be described as “freeze drying within confined space.” Chemical characteristic of CNTs was controlled by choosing different solutions of non-functionalized CNTs (NOCNTs) or hydroxyl-modified CNTs (OHCNTs). As a consequence, CNT networks modified composites, especially that with OHCNTs formed networks, displayed significantly better electrical performance than composites with CNT aggregates and scattered CNTs; NOCNT networks and scattered OHCNTs made the corresponding composites possess higher interlaminar shear strength (ILSS) value, whereas OHCNT networks impaired ILSS while enhancing flexural strength and modulus of composites.

Amino‐functionalization of carbon fibers through electron‐beam irradiation technique

AbstractAmino‐functinonalized carbon fibers were achieved via electron‐beam (EB) irradiation in Diethylenetriamine (DETA) solution and triethylene tetramine (TETA) solution at 200 kGy. Different graft monomer concentrations were investigated to find the optimal concentration of each graft monomer. X‐ray photoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy were applied to investigate chemical composition and functional groups, topography and disorder degree of amino‐functionalized carbon fibers surface. Meanwhile, adsorption ability and interfacial adhesion between modified carbon fibers and epoxy resin were determined by TGA and interlaminar shear strength (ILSS). It is found that amino‐functionalized carbon fibers which had rougher and more active surface performed better adsorption ability on epoxy resin than untreated fibers. The optimal ILSS values of carbon fiber (treated with DETA and TETA) reinforced composites were 21.37 MPa and 18.28 MPa, which were much higher than that of untreated fiber reinforced composites. The comprehensive results demonstrated that in this condition, the optimal grafting concentrations of both DETA and TETA were 1.5 mol/L. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40274.

Experimental study on the mechanical behavior of carbon/epoxy composites with a carbon nanofiber-modified matrix

Material property characterization tests were performed on satin weave carbon/epoxy composites, where the epoxy resin was modified with 2 wt% carbon nanofibers prior to infusion into a continuous carbon fiber preform. Uniaxial tension tests, tension-tension fatigue tests, and fracture tests were initially performed on 0–3 wt% carbon nanofibers-reinforced epoxy specimens in order to determine the carbon nanofiber weight fraction leading to the optimal mechanical properties of the modified epoxy matrix. In general, the elastic modulus of the modified epoxy increased with increasing carbon nanofibers weight fraction. The ultimate tensile strength, fatigue life, and fracture toughness also increased significantly with increasing amounts of carbon nanofibers and reached maximal values at a carbon nanofiber weight fraction of 2 wt%. The improvement in tensile properties of carbon nanofiber–modified epoxy became more pronounced for specimens loaded at higher strain rates. Further increases in nanofiber content, however, resulted in a relative decrease in nanocomposite strength and fatigue life; this is likely due to local stress concentrations associated with poorly dispersed carbon nanofibers. Vacuum-assisted resin infusion molding was used to fabricate hybrid composite panels consisting of woven carbon fabric and epoxy resin modified with 2 wt% carbon nanofibers. Uniaxial compression, open-hole compression, and short beam shear tests were performed to assess the effect of carbon nanofibers on matrix-dominated composite properties. Hybrid composites containing 2 wt% of carbon nanofibers in the epoxy matrix resulted in compressive strength, open-hole compressive strength, and interlaminar shear strength values that were 19.8%, 27.8%, and 15.8% greater, respectively, than those for woven fabric composites prepared with neat epoxy. Quasi-static uniaxial tension tests and tension-tension fatigue tests of hybrid composite specimens also led to similar enhancements in the composite ultimate strength and fatigue life relative to composites specimens infused with neat epoxy. Scanning electron microscopy images of composite micro-fracture surfaces indicated that randomly distributed carbon nanofibers provide some crack bridging, reduce crack opening, and lead to crack turning for small cracks. Such mechanisms are likely responsible for the improvement in mechanical properties.

Preparation of continuous carbon nanotube networks in carbon fiber/epoxy composite

In order to optimize carbon nanotube (CNT) dispersion state in fiber/epoxy composite, a novel kind of CNT organization form of continuous networks was designed. The present work mainly discussed the feasibility of preparing continuous CNT networks in composite: Fiber fabric was immersed into CNT aqueous solution (containing dispersant) followed by freeze drying and pyrolysis process, prior to epoxy infusion. The morphologies of fabric with CNTs were observed by Scanning Electron Microscope. The relationship between CNT networks and flowing epoxy resin was studied. Properties of composite, including out-of-plane electrical conductivity and interlaminar shear strength (ILSS), were measured. The results demonstrated that continuous and porous CNT networks formed by entangled CNTs could be assembled in fiber fabric. Most part of them were preserved in composite due to the robustness of network structures. The preserved CNT networks significantly improved out-of-plane electrical conductivity, and also have an effect on ILSS value.

Influence of surface properties on the interfacial adhesion in carbon fiber/epoxy composites

A polyacrylonitrile‐based carbon fiber was electrochemically oxidized in an aqueous ammonium bicarbonate solution with current density of up to 2.76 A/m2 at room temperature. X‐ray photoelectron spectroscopy revealed that the oxygen content increased with increasing current density before approaching saturation. The increase can be divided into two regions, the rapid increase region (0–1.78 A/m2) and a plateau region (1.78–2.76 A/m2). The surface chemistry analysis showed that the interlaminar shear strength (ILSS) value of the carbon fiber/epoxy composite could be improved by 24.7%. The carbon structure was examined using Raman spectroscopy in terms of order/disorder in the graphite structure and the results indicated that the relative percentage of graphite carbon in the form of sp2 hybridization increased above a current density of 1.39 A/m2. The increasing non‐polar graphite carbon on the carbon fiber surface decreased the surface energy. As a result, both the surface free energy () and its polar component () decreased when current density increased above 1.78 A/m2. The ILSS value had no direct relationship with the nature and surface density of the oxygen‐containing functional groups nor with the carbon structure. It is the surface free energy (), especially the polar component (), which played a critical role in affecting the interfacial adhesion of carbon fiber/epoxy composites. The ILSS value changed with increasing current density and could be divided into three distinct regions, as chemical interaction region (I), anchor force region (II) and matrix damage region (III). Copyright © 2013 John Wiley & Sons, Ltd.

Environmental effect on the mechanical properties of commingled-yarn-based carbon fibre/polyamide 6 composites

The main objective of this experimental investigation was to evaluate the changes from accelerated ageing on selected properties of carbon fibre/polyamide 6 composites based on hybrid yarns. In this study, two types of mechanical tests were performed to measure the environmental influence on the material properties. They are three-point bending to measure the flexural strength and stiffness, and short beam three-point bending to measure the interlaminar shear strength. The 10-mm-thick quasi-isotropic carbon fibre/polyamide 6 composites with 52% volume fraction of carbon fibre to be tested were manufactured by autoclave consolidation. The test samples were dried, and subsequently exposed to 60℃ and 100% relative humidity at different lengths of time up to 2500 h, followed by drying at 23℃ and 50% relative humidity. Few samples were additionally completely dried at 70℃ in vacuum for 21 months. Tests were also performed on as manufactured and dried material at low temperature (–45℃) and high temperature (115℃). The measured mechanical properties decreased with exposure time at 60℃ and 100% relative humidity. Both the bending stiffness and the strength degrade to a level of about 65%, whereas interlaminar shear strength drops to about 87% of the property values of the unexposed (initially dried) material. The bending stiffness and strength at −45℃ are about 87% of the properties at room temperature, whereas at 115℃ the stiffness drops to 75% and the strength drops to 60% of the properties at room temperature. The interlaminar shear strength values also drop to about 75% at both −45℃ and 115℃. Extreme temperatures and long-time exposure to humidity of quasi-isotropic carbon fibre/polyamide 6 laminates can thus reduce the bending stiffness and strength by up to 35% and the interlaminar shear strength by up to 25%.

Effect of environmental degradation and mechanical properties of polyamide 6-grafted carbon fiber/poly(3-hydroxybutyrate-co- 3-hydroxyvalerate) composites

The measured mechanical properties of carbon fiber/polyamide 6 (CF/PA6) laminates decreased with exposure time at 60°C and 100% relative humidity. Both the bending stiffness and the strength degraded to a level of about 65%, whereas interlaminar shear strength (ILSS) dropped to about 87% of the property values of the unexposed material. The bending stiffness and strength at −45°C are about 87% of the properties at room temperature, whereas at 115°C, the stiffness drops to 75% and the strength drops to 60% of the properties at room temperature. The ILSS values also drop to about 75% at both −45°C and 115°C. Extreme temperatures and longtime exposure to humidity of quasi-isotropic CF/PA6 laminates can thus reduce the bending stiffness and strength by up to 35% and the ILSS by up to 25%.

Matrix modification with silane coupling agent for carbon fiber reinforced epoxy composites

To improve interfacial adhesion between carbon fiber and epoxy resin, the epoxy matrix is modified with N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (YDH602) and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (YDH792), respectively. And the effect of matrix modification on the mechanical performance of carbon/epoxy composites is investigated in terms of tensile, flexural and interlaminar properties. The flexural properties indicate that the optimum concentration of silane coupling agents YDH602 and YDH792 for the matrix modification is approximately 0.5 wt% of the epoxy resin system, and the mechanical properties of the YDH792-modified epoxy composites is better than that of the YDH602-modified epoxy composites at the same concentration. Compared to unmodified epoxy composite, the incorporation of 0.5 wt% YDH792 results in an increase of 4, 44 and 42 % in tensile, flexural and interlaminar shear strength (ILSS) values of the carbon/epoxy composite, respectively, while the corresponding enhancement of tensile and flexural modulus is 3 and 15 %. These improvements in mechanical properties can be considered to be an indication of better fiber/matrix interfacial adhesion as confirmed by SEM micrographs of the fracture surface after interlaminar shear testing. The viscosity of the modified epoxy resin system can be reduced by incorporation of silane coupling agent YDH792, which is beneficial for fiber impregnation or wetting during liquid composite molding process.

The gamma irradiation of PBO fiber on the mechanical and tribological properties of PTFE composites

The mechanical and tribological behavior of gamma irradiated poly(p-phenylene benzobisoxazole) (PBO) fiber filled polytetrafluoroethylene (PTFE) composites was investigated. The gamma irradiated PBO fiber composite had the highest inter-laminar shear strength value of all the combinations because its higher bond strength may have hindered a large fiber/matrix debonding. X-ray photoelectron spectroscopy results indicate that the contents of polar groups on the surface of gamma irradiated PBO fiber increase compared to PBO fiber. The wear tests were conducted on a ring-on-block apparatus using composite block against polished metal counterparts under dry sliding conditions. It can also be found that gamma irradiation treatment was helpful to the improvement of the anti-wear ability of the PTFE composite which was related to the abrasive wear mechanism.

Effects of different fluorination routes on aramid fiber surface structures and interlaminar shear strength of its composites

Poly-p-phenylene-benzimidazole-terephthalamide (PBIA) fiber was surface modified by direct fluorination under three different routes. The fiber was dried under vacuum to remove physisorbed water trapped on it and then fluorinated by the fluorine and oxygen gases or by the fluorine gas only. Results show that the interlaminar shear strength (ILSS) value of these two kinds of fluorinated fiber reinforced epoxy resin was 43.9MPa and 51.0MPa, which was improved about 14.0% and 32.5% compared with that of the virgin fiber (38.5MPa), respectively. In the third route, the fiber was fluorinated by the fluorine and oxygen gases without removing physisorbed water, and the ILSS value decreased for nearly 31.2%, i.e. from 38.5MPa to 26.5MPa. X-ray photoelectron spectroscopy (XPS) showed that oxygen-containing and fluorine-containing chemical groups were introduced onto the fiber surface after fluorination, providing a stronger chemical bonding to polymeric matrices. Scanning electronic microscopy (SEM) indicated that the surface morphology of the fluorinated PBIA fiber varied with the different fluorination routes. A mass of compact micro groove structures was formed by the route that the fiber was dried to remove physisorbed water and then fluorinated with fluorine gas only. And these structures would markedly improve the ILSS of the composites. But, a mass of unstable flake surface structures was formed by the route that the fiber was fluorinated with the fluorine and oxygen gases without removing physisorbed water. And these structures would be the weak interface between the fiber and matrix and decrease the ILSS, even a lot of polar chemical groups were bonded onto the fiber surface as well.