Apparent Interfacial Shear Strength Research Articles

Fabrication of carbon nanotube epoxy prepreg towards lightweight structural composites

Despite improvements in carbon nanotube fiber tenacities, their shear strength continues to limit their performance in composite applications. Shear strength can be improved through resin penetration into the nanotube fiber hierarchical microstructure to enhance internal interfaces. In this work, a manufacturing process uses an ionic liquid and epoxy to simultaneously stretch and infiltrate nanotube roving resulting in a nanotube-epoxy-ionic liquid prepreg. The ionic liquid also serves as a latent curing agent/hardener for the prepreg. After curing, the resulting composite fiber has comparable tenacity and specific modulus to fibers made without epoxy but with improved shear properties. The dry core shear failure mode typically observed in CNT fibers was eliminated and the apparent interfacial shear strength was improved by over an order of magnitude.

Prediction of interfacial behaviour of single flax fibre bonded to various matrices by simulation of microdroplet test

The microdroplet test is commonly used to determine the apparent interfacial shear strength (IFSS) of fibre-reinforced microcomposites. A deeper analysis of the test outcome can provide meaningful information about the fibre/matrix interface behaviour if a predictive approach is adopted. In this study, this predictive approach was used to investigate the quality of interface for polymer drops bonded single flax fibre at the microscale. Microdroplets of five thermoplastics matrices were prepared with single flax fibres. Microbond test was performed to assess the force–displacement curves of the studied composite systems. In addition, a finite element (FE) modelling methodology was adopted to quantify the interfacial role by proposing an interfacial constitutive law including the debonding stage. The numerical sensitivity results reveal the leading role of the interfacial stiffness as well as the fibre–matrix separation displacement in triggering the debonding behaviour. In addition, the numerical responses show strong matching with experimental trends using the proposed interfacial model for a wide variety of fibre/matrix interactions. The identification of the mechanical behaviour of the considered composite system shows that the best performing system is flax fibre/PLA, allowing a maximum fibre–matrix separation of 156 µm and an interfacial stiffness of 47 GPa/mm. The worst performing system is flax fibre/PP, which has a limited fibre–matrix separation of 55 µm. This study concludes that the proposed numerical model is able to capture the interfacial shear behaviour of polymeric drops bonded to a single flax fibre, which allows its extension at the mesoscale for a given arrangement of flax fibres in the bio-based composites.

Effect of temperature on the fiber-matrix adhesion in glass fiber reinforced thermoplastics

The apparent interfacial shear strength (IFSS) between sized glass fiber and neat high-density polyethylene (HDPE), HDPE with a maleic anhydride coupling agent, polyamide 6 (PA6), and polybutylene terephthalate (PBT) has been measured from 20 °C to 80 °C. A modification of a custom set-up involving the single fiber pullout test has been used to perform the characterizations. The calculation of radial stresses on the fiber due to polymer shrinkage during sample cooling has been conducted using data from dynamic mechanical analysis and thermomechanical analysis. A strong inverse relationship was found between IFSS and temperature. The IFSS of neat HDPE, PA6, and PBT decreased by 44% from 20 °C to 80 °C. Over the same temperature range the IFSS of HDPE with the coupling agent decreased by 66% but remained greater than neat HDPE. The effects of glass transition (Tg), measured to be 50 °C for PA6 and 68 °C for PBT, on IFSS were noticeable through analysis of the single fiber pullout data.

The effect of environmental ageing on the interphase in glass fibre – vinyl ester composites

ABSTRACT The results of a study of environmental ageing on the apparent interfacial shear strength of glass fibre–vinyl ester microbond samples are presented. The scale related issues of estimating water absorption levels in microbond samples are reviewed and discussed and a solution based on Fickian diffusion is given. Interphase strength of microdroplets aged in water at 23°C was relatively stable up to 6 days exposure after which strength degradation was fairly rapid. Immersion in water at 50°C resulted in an immediate 20–30% irreversible loss of strength. Further exposure beyond 2 days led to total loss of integrity of the microbond samples. Electron microscope images of aged microdroplets appeared to show the development of a bi-phasic microstructure. The presence of two phases in small scale vinyl ester polymer samples was confirmed by DMA on thin films.

Interfacial adhesion and thermal stability of high-density polyethylene glass fiber composites

The fiber-matrix adhesion and the thermal stability of glass fiber reinforced high-density polyethylene composites with increasing content of maleic anhydride coupling agent and zinc-based thermal stabilizer have been analyzed by the single fiber pullout test and oxidation induction time. High-density polyethylene compounds containing 8% by weight maleic anhydride grafted polyethylene increases the apparent interfacial shear strength to glass fibers by 3.5x; however, the strength was reduced by 45% with the addition of the thermal stabilizer. The thermal stabilizer was found to greatly improve the oxidative resistance. The oxidation induction time of the non-stabilized compounds is less than 1 min compared to over 25 min for the stabilized high-density polyethylene blends.

Interfacial and Interlaminar Shear Strength of Unidirectional Viscose Fibre-Reinforced Epoxy Composites—an Overview of the Comparability of Results Obtained by Different Test Methods

In this study, the apparent interfacial and interlaminar shear strength (IFSS and ILSS) of single fibres and unidirectional (UD) viscose fibre-reinforced epoxy composites were characterised using different test methods. Microbond and pull-out tests were used to analyse the IFSS of single fibres embedded in epoxy and the transverse tensile test was applied to measure the IFSS of UD fibre-reinforced composites. The short beam shear test, single edge notched bending test (SENB), double-notched tensile test and double-notched compression test were applied to characterise the ILSS. The composites were produced from continuous tows with fibre mass fractions of 20%, 30% and 40% and fibres of different fineness (1.7, 3.3 and 28.0 dtex). The results showed that the different test procedures led to different trends of ILSS depending on the fibre mass fraction and fibre fineness used. The transverse tensile test revealed that the IFSS decreased with increasing fibre mass fraction and fibre diameter. A different trend was found with the short beam shear test and the SENB test for the ILSS. Here, higher values were detected with increasing fibre mass content, and the influence of the fineness was less noticeable. The double-notched shear tests (tensile and compression) showed a different trend: the ILSS increased with increasing fibre mass fraction from 20% to 30%. With a further increase to 40%, the ILSS tend to decrease slightly. An influence of the fibre fineness on the ILSS could not be statistically proven. The different trends of the test methods are attributed to the constitution of the composite and the different load application caused by the test procedures.

Carbon nanotube enhanced carbon Fibre-Poly(ether ether ketone) interfaces in model hierarchical composites

Poly (ether ether ketone) (PEEK) has a high continuous service temperature, excellent mechanical properties, and good solvent and abrasion resistance, which can be further improved through the addition of carbon nanotubes (CNTs). CNT-PEEK nanocomposites are promising matrices for continuous carbon fibre composites; powder processing can mitigate the high melt viscosities in these systems. In this study, model single fibre (hierarchical) composites were produced by embedding sized and desized carbon fibres in nanocomposite CNT-PEEK powders followed by single fibre pull-out tests to assess interfacial characteristics. Carbon fibre-PEEK interfacial shear strength is typically 40–45 MPa. Increasing CNT loadings increased fibre-matrix interfacial shear strength linearly up to ∼70 MPa at 5.0 wt%, which was attributed to the CNT-based mechanical modification of the PEEK matrix. Apparent interfacial shear strength was inversely correlated with the embedded fibre length irrespective of carbon fibre sizing or CNT loading, indicating brittle fracture of the fibre-matrix interface. Pulled out carbon fibres were still coated with the matrix, which indicated strong adhesion at the interface in all samples, likely related to a transcrystalline region. Adhesion was, however, negatively affected by the presence of epoxy sizings. Frictional shear strength was independent of embedded fibre length and CNT content for all samples.

Mechanical Properties of Polylactic Acid Resin Composite Filament Reinforced by SUS304 Wire

The purpose of this study is to investigate the mechanical properties of composite filament which made from polylactic acid resin and SUS304 wire for Fused Deposition Modeling. Both of PLA resin and SUS304 wire were supplied to heated extruder for covering the SUS304 wire with melted PLA. The feed rate of both materials was individually controlled by controlling the number of revolutions of stepper motors. In this study, three types of SUS304 wires were used to investigate the effect of surface roughness of wire on the tensile characteristics of composite filament. The surface of SUS304 wire was modified by chemical etching or mechanical grinding to control the surface roughness. For comparing, two modified wires and as received SUS304 wire were used for following experiments. The tensile characteristics of composite filament was investigated by using universal testing machine. Tensile test results showed that the tensile modulus and strength of composite filament was improved compared that of original PLA filament. The pull-out test of composite filament was also revealed that the apparent interfacial shear strength between PLA and SUS304 wire was improved when the surface roughness of SUS304 wire was high. Moreover, when the surface roughness of SUS304 wire was high, the fracture morphology of composite filament was changed.

Influence of covalently-bound graphene oxide on the mechanical properties of carbon fiber composite materials

Graphene was used as an additive in fiber-reinforced plastics. The apparent interfacial shear strength (IFSS) was improved by grafting graphene onto the fibers. In this study, the effect of graphene grafted onto carbon fiber via chemical covalent bonding on the mechanical properties of the fibers was evaluated. This method was used to link graphene and carbon fibers via ester linkages, which are stronger than physical adsorption interactions and cause little damage to the fiber strength. Before grafting through an ester linkage, an electrophoretic deposition was conducted to uniformly coat graphene onto the carbon fibers. The purpose of this study was to establish a fabrication method that could control the density and thickness of the graphene layer covalently bound to the fibers. In addition, the optimal production process and composite form of the reinforced fibers were determined by evaluating the fiber strength and IFSS of various graphene-reinforced fibers. Ultimately, it was determined that graphene could be grafted onto carbon fibers relatively uniformly, and that the graphene layer thickness affected their interfacial shear strength. In terms of optimizing the IFSS, a thin and uniform graphene layer was best, and the IFSS was improved by 42.1% compared with untreated fibers containing sizing agents.

Fiber-matrix adhesion between high-density polyethylene and carbon fiber

Carbon fiber reinforced polyethylene composites have garnered recent interest in many industrial fields owing to their inherent benefits of excellent chemical and corrosion resistance, and processing ease. The interfacial strength between polyethylene and carbon fiber has been understudied to this point but it is critical to the performance of these new composite parts. In this study the apparent interfacial shear strength between three differently sized carbon fibers and high-density polyethylene containing various levels of maleic anhydride grafted polyethylene coupling agent has been evaluated. Overall, the compatibilizer content plays a more significant role than the fiber sizing but it was found that there are competing and complementary effects between them. Additionally, based on examination of the post debonding frictional forces, the mechanics of the pullout test are changed even with a small content of maleic anhydride even where the magnitude of the interfacial shear strength is unchanged.

Role of interfacial adhesion and fiber length on the mechanical performance fiber reinforced thermoplastic elastomers

Thermoplastic elastomers combine inherent benefits such as the processability and recyclability of thermoplastics and the elasticity, toughness, and flexibility of elastomers. In this work, polyester based thermoplastic elastomers have been reinforced with carbon and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers of similar strength and stiffness. Testing of the respective mechanical properties revealed that the PBO reinforced composites yielded higher tensile strength. Single fiber pullout testing was also performed to evaluate the respective interfacial and mechanical properties where it was found that the apparent interfacial shear strength for carbon fibers was two to three times greater than their PBO counterparts. Further analysis revealed that the fiber length of the PBO composites was much greater than for carbon fiber, resulting in superior strength of the PBO reinforced elastomers.

Functionalization of basalt fibers with ZnO nanostructures by electroless deposition for improving the interfacial adhesion of basalt fibers/epoxy resin composites

Basalt fibers were functionalized by ZnO electroless deposition for obtaining a nanostructured interphase for enhancing the interfacial strength with an epoxy resin matrix. The structural, morphological and wetting properties of the pristine basalt fabrics and ZnO-coated basalt fabrics were evaluated. The fabrics were uniformly coated with ZnO nanostructures featuring a wurtzite structure and a twin hexagonal prism morphology. The contact angle measurements revealed that ZnO prisms transformed the hydrophilic basalt fabric into a hydrophobic one (~130°). ZnOs were also grown on the basalt fibers as yarns to evaluate their interfacial adhesion by single fiber pull-out tests. The results emphasize significant improvement in the apparent interfacial shear strength (~42%) with limited degradation of the pristine basalt fiber tensile strength (a reduction of ~ 17%). Therefore, ZnO electroless deposition can be regarded as an effective route to improve the mechanical performance of basalt/epoxy composites expanding their potential range of applications as structural materials.

Decoration of SiO2 and Fe3O4 Nanoparticles onto the Surface of MWCNT-Grafted Glass Fibers: A Simple Approach for the Creation of Binary Nanoparticle Hierarchical and Multifunctional Composite Interphases.

We report on a versatile method for chemically grafting multiwalled carbon nanotubes (MWCNTs) onto the surface of conventional glass fibers (GFs), as well as depositing further silica (SiO2) or superparamagnetic (SPM) magnetite (Fe3O4) nanoparticles (NPs) creating novel hierarchical reinforcements. The CNT-grafted GFs (GF-CNT) were utilized further as the support to decorate nano-sized SiO2 or Fe3O4 via electrostatic interactions, resulting finally into double hierarchy reinforcements. SiO2 NPs were first used as model nano-particulate objects to investigate the interfacial adhesion properties of binary coated GFs (denoted as GF-CNT/SiO2) in epoxy matrix via single fiber pull-out (SFPO) tests. The results indicated that the apparent interfacial shear strength (IFSS or τapp) was significantly increased compared to the GF-CNT. Fe3O4 NPs were assembled also onto CNT-grafted GFs resulting into GF-CNT/Fe3O4. The fibers exhibited a magnetic response upon being exposed to an external magnet. Scanning electron microscopy (SEM) revealed the surface morphologies of the different hierarchical fibers fabricated in this work. The interphase microstructure of GF-CNT and GF-CNT/SiO2 embedded in epoxy was investigated by transmission electron microscopy (TEM). The hybrid and hierarchical GFs are promising multifunctional reinforcements with appr. 85% increase of the IFSS as compared to typical amino-silane modified GFs. It could be envisaged that, among other purposes, GF-CNT/Fe3O4 could be potentially recyclable reinforcements, especially when embedded in thermoplastic polymer matrices.

Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode

Achieving high strength in fiber reinforced structural composites requires effective load transfer between the high-performance fiber reinforcement, e.g., carbon nanotube (CNT) yarn, and the matrix. Various processing approaches to enhance the interaction between CNT fiber and the matrix were investigated. The apparent interfacial shear strengths (IFSS) of pristine CNT yarns, post-treated CNT yarns (cross-linked, functionalized, and polymer incorporation by resistive heating), and pre-infiltrated polymer/CNT composite fibers measured using single fiber pull-out tests were used to screen the efficacy of the processing methods. Pristine CNT yarns had a low apparent IFSS (<5 MPa) due to shear failure within their dry cores. In post-treated CNT yarns which did not exhibit good IFSS, the failure surface consisted of a resin-infiltrated sheath near the surface of the yarn and a dry section within the yarn core; failure occurred in the dry core. This failure mode is unlike those observed in traditional carbon fiber reinforced composites which fail at the fiber/matrix interface. In contrast to the sword-in-sheath failure modes of post-treated CNT yarns, pre-infiltrated polymer/CNT composite fibers displayed high apparent IFSS (>20 MPa). Improved wet-out of the fiber eliminated the dry-core shear failure mode.

Interfacial adhesion of carbon fiber to special engineering plastics: Effect of the functional groups in the matrix

We studied the interfacial shear strength (IFSS) in carbon fiber (CF) and special engineering plastics matrix, with an emphasis on the effect of the functional group in the matrix. The IFSS was analyzed to quantify the interfacial adhesion between the fiber and the matrix. To obtain the apparent IFSS of the composites, microdroplet test was measured at single-fiber composites. Results of the microdroplet test displayed that the apparent IFSS in the composites was directly determined by their inherent surface properties and the functional groups in the matrix. Compared with other matrices, polyetherimide (PEI) exhibited relatively strong mechanical bonding and interfacial adhesion, showing that the imide groups had good interfacial compatibility with the pristine CF surface. Based on the results of this study, polymers containing imide groups were one of the best candidates for sizing agent of CF used as reinforcement of high-temperature thermoplastics.

Micromechanical and spectroscopic characterisation of the curing performance of epoxy resins in the microbond test

In the present work, the microscale curing performance of two epoxy resin systems was investigated. It was shown that the degree to which these resins formed cured axisymmetric droplets suitable for microbond testing was influenced by the inclusion of a room-temperature pre-cure stage. Immediate curing at elevated temperature of these systems resulted in the formation of under-cured, soft droplets that deformed under loading. The introduction of a protracted room temperature standing time increased the apparent interfacial shear strength substantially. Fourier-transform infrared spectra generated by a novel technique allowed for the glass transition temperature of microdroplet samples to be determined and detected deviations from stoichiometry. Good correlation was shown between the two methods, in that an increase in IFSS was commensurate with spectra indicating droplets were closer to the ideal stoichiometric ratio, and had both higher degrees of cure and glass transition temperatures.

Improved interfacial shear strength in polyphenylene sulfide/carbon fiber composites via the carboxylic polyphenylene sulfide sizing agent

The carboxylic polyphenylene sulfide (PPS–COOH) with low molecular weight was synthesized and used as a part of sizing agent to improve the interfacial shear strength (IFSS) of polyphenylene sulfide/carbon fiber (PPS/CF) composites in this study. And the PPS-COOH sizing agent with good heat resistance was homogeneously coated on the carbon fiber surface, introducing the functional groups and causing the improvement of CF surface roughness. The mechanical properties were investigated through the tensile strength of composites and IFSS with the virgin carbon fiber (VCF), the plasma treated carbon fiber (PCF) and the modified carbon fiber coated by PPS-COOH sizing after plasma treatment (ACF). The tensile strength of ACF/PPS composites has enhanced to 115.20 MPa, which is also verified through the tensile fracture surfaces of composites. The IFSS experimented by microbond test suggests that the apparent interfacial shear strength (τapp) of ACF/PPS composites has increased to 37.19 MPa, which is 27.71% and 15.07% higher than VCF/PPS and PCF/PPS composites, respectively. Hence, it is the promising method to prepare and size the great heat resistant sizing agent on the plasma treated CF to effectively improve the interfacial shear strength of CF/PPS composites.

Improving adhesive strength of WC-CoCr coating with novel bimodal roughening substrate: Finite element modeling

The durability of a coating-substrate system is mainly determined by its adhesion strength. However, a major challenge is to effectively evaluate the adhesive effect after design, as measuring the adhesion strength of complex composite coatings is difficult. In this work, a bi-roughened surface of substrate was designed and its effects on the adhesive strength of WC-CoCr coatings were explored by simulation. Initially, a finite element model was established to simulate the substrate-coating system with bi-roughening (sub-roughness and sub-roughening angles) interface. In the case of monomodal Ra 7.5 μm, the adhesive strength was predicted to be only 68 MPa. While, the designed sub-roughening of Ra 0.8 μm enhanced the value to nearly 80 MPa. Moreover, the adhesive strength is further improved to above 100 MPa by tailoring the sub-roughening angle to 60°. The residual stresses along interfacial region decrease significantly for bi-roughening design, which can be responsible for enhanced adhesive strength. Additionally, individual crack growth occurs in almost every bi-roughening, and does not connect with the main crack, which result in a proportional increase in apparent interfacial shear strength. Briefly, adhesion strength is considerably improved for the bi-roughening design, which is beneficial for durability of the coatings during service.

Interfacial adhesion and shear behaviors of aramid fiber/polyamide 6 composites under different thermal treatments

Aramid fiber (AF) reinforced by polyamide (PA) composites are excepted to have good interfacial matching due to their similar chemical interactions of hydrogen bonding. Thus, polarizing optical microscope (POM), transverse fiber bundle (TFB) test, and droplet micro-debonding technique were respectively performed to characterize interfacial crystallization, adhesion and shear behaviors of AF/PA6 composites with different thermal treatments. Both interface adhesion and AF fibrillation are enhanced with decreasing cooling rate or increasing annealing temperature due to the increased interfacial transcrystallization interaction. However, fast cooled interface also presents a high interfacial shear strength (IFSS) due to favorable normal residual stress. The apparent IFSS is believed to be a result of competition between crystallization enhancing interfacial interaction, interfacial mismatching aggravating debonding, and an uncertain residual stress positive or negative for load transfer. TFB failure mechanism including AF fibrillation and kinking are schematically presented. Fibrillation strength of AF is found to follow Weibull distribution evaluated by droplet micro-debonding technique.

Evaluation of the Apparent Interfacial Shear Strength of Nanocellulose/PVA Composites

Nanocellulose in the form of whiskers and nanofibers has become a promising reinforcement material for polymer composites due to its high mechanical characteristics and sustainability. For optimization of composite properties, efficient means of characterization of the stress transfer between fibrous nanofiller and the polymer matrix are needed. In the current work, the apparent interfacial shear strength (IFSS) of cellulose nanofiber/PVA is evaluated by a modified Bowyer and Bader method based on an analysis of the stress–strain curve of a composite film in uniaxial tension. The IFSS is found to exceed shear yield strength of the neat polymer, suggesting good adhesion.