Apparent Shear Strength Research Articles

Effects of flame‐retardant additives on the manufacturing, mechanical, and fire properties of basalt fiber‐reinforced polybenzoxazine

AbstractBasalt fiber‐reinforced polybenzoxazines (BFRP) were manufactured through vacuum infusion using resorcinol bis (diphenyl phosphate) and poly‐(m‐phenylene methylphosphonate) together with bisphenol‐F and aniline based benzoxazine. Different types and loadings of flame‐retardant additives showed to have catalysis or dilution effects in viscosity measurements. BFRPs show well‐penetrated fibers and near‐zero porosity. Additive addition did not influence tensile properties, while apparent interlaminar shear strength decreased indicating a lower adhesion between fiber and matrix. BFRP's heat and smoke release properties increased, though time to ignition increased and flammability behavior improved by decreasing delamination yielding oxygen indices in between 72 and 91%.

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.

Foaming performance and bonding strength of a novel urea-formaldehyde foaming resin facilely prepared with thermo-expandable microspheres

A novel urea-formaldehyde (UF) foaming resin to be used in low-density wood-based materials was facilely fabricated by mixing thermo-expandable microspheres (TEMs) with UF resin. The foaming performance and bonding strength of the new resin were investigated with specially designed paper/UF samples and bonded veneers joints, respectively. The developed resin had a good expansion effect, with an apparent density of less than 300 kg/m3 and expansion ratios of 4.5–9.8, and a uniform cellular structure inside the resin was demonstrated by scanning electron microscope observations. However, the addition of TEMs resulted in a 47.5%–91.4% decrease in the shear strength of the resins. As the concentration of TEMs increased from 2.5% to 10.0%, the apparent density and shear strength showed similar decreasing trends. The introduction of glutaraldehyde (GA) increased the bonding strength of the resins to 69.8%–199.7%, while significantly increasing their apparent density. Increasing the concentration of TEMs would be an effective way to reduce the apparent density of the UF foaming resins, but there will be an accompanying degradation in their strength. GA could be a potential chemical modifier for TEMs/UF resins in terms of improving their bonding strength.

Intense pulsed light surface treatment for improving adhesive bonding of aluminum and carbon fiber reinforced plastic (CFRP)

Carbon fiber reinforced plastic (CFRP) is increasingly considered for various industrial applications. CFRP has been applied to the automotive industry to increase fuel efficiency due to its excellent strength and lightweight. Therefore, the demand for dissimilar material joining techniques is gradually increasing. In this study, we investigated the intense pulsed light (IPL) as surface treatment for CFRP. The IPL surface treatment requires a very short processing time and can be easily automated compared with other processes such as chemical and mechanical treatments. The change in morphology and chemical composition of the CFRP-treated surfaces was analyzed using scanning electron microscopy, profiler, and X-ray photoelectron spectroscopy. The bonding strength due to varying IPL exposure time was evaluated by the lap shear test. The apparent shear strength increased by up to 159% in this study. The IPL treatment is very simple and requires short processing time to produce enhanced adhesive joints.

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.

Durability of GFRP reinforcing bars in seawater concrete

This paper presents an experimental study that investigated the durability performance of unstressed glass fiber-reinforced polymer (GFRP) bars embedded in concrete mixed with seawater (seawater concrete). GFRP bars were extracted from concrete elements made with two different seawater concrete mix designs and exposed to different environmental conditions for 1, 6, 12, and 24 months. The concrete samples’ exposure environments consisted of typical field conditions of a subtropical region and seawater at 60 °C as an accelerated aging method. The mechanical test results of GFRP bars are reported in residual capacities of tensile strength, longitudinal elastic modulus, transverse shear strength, and apparent horizontal shear strength. Furthermore, the physical evaluations are in terms of glass transition temperature (Tg) and microstructural integrity through scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis. Among all tested properties, tensile strength was the most affected by environmental conditioning. Based on an exponential degradation model, the long-term prediction of the tensile strength capacity was on average 92% under typical field exposure and 72% under the more aggressive conditioning (seawater at 60 °C).

Ytterbium disilicate-based glass-ceramic as joining material for ceramic matrix composites

A key aspect of ceramic matrix composites integration is related to a reliable joining technique. An ytterbium disilicate based glass-ceramic material is processed by reactive viscous flow sintering between a barium aluminium borosilicate glass and ytterbium oxide and it is used to join SiC/SiC and C/SiC composites. The joining temperature and the in situ formation of the Yb2Si2O7 is optimised at 1200°C without pressure, on the basis of the sintering and crystallisation mechanisms. The mechanical characterization of SiC/SiC and C/SiC joined with the ytterbium disilicate-based glass-ceramic, tested by single-lap offset at RT, exhibits an apparent shear strength of 35 MPa, similar to their interlaminar shear strength.The proposed system displays self-healing behaviour at 1000 °C and 1150 °C, as demonstrated by the partial and complete sealing of induced cracks by Vickers indentation on its surface at different loads, thus suggesting that it can effectively be used as promising joining material for CMCs.

Enhancing functionalities in carbon fiber composites by titanium dioxide nanoparticles

Carbon fiber composite's high specific strength makes it incredibly useful for structural applications. However, their low strain-to-failure can be problematic in structural applications that can potentially see high strain conditions or high fatigue cycles resulting in sudden, catastrophic failure. This is further complicated by damage manifesting within the composite thus not showing damage indicators on the surface. Therefore, monitoring the structural integrity and strain history of the composite in application by itself―while maintaining positive composite performance―is vital to ensure safe operation. Here, a homogeneous dispersion of TiO2 nanoparticles on carbon fiber is demonstrated to generate a piezoresistive carbon fiber polymer matrix composite with enhanced self-sensing capabilities. The nanoparticle-embedded composites also exhibited superior strength and damping potential compared to the nanoparticle-free composites. The apparent interlaminar shear strength increased by up to 15% and the damping loss factor increased by an average of 150% while the piezoresistive sensitivity increased by up to 180% with the addition of a small weight fraction of nanoparticles to the fiber surface. These results demonstrate an approach to simultaneously improve the sensing capabilities, damping behavior and mechanical strength of carbon fiber composites by simply adding nanoparticles in the fiber sizing using a commercially scalable deposition method.

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.

Understanding interlaminar toughening of unidirectional CFRP laminates with carbon nanotube veils

The introduction of nanostructured interlayers is one of the most promising strategies for interlaminar reinforcement in structural composites. In this work, we study the failure mechanism and interlayer microstructure of aerospace-grade structural composites reinforced with thin veils of carbon nanotube produced using an industrialised spinning process. Samples of unidirectional carbon fiber/epoxy matrix composites interleaved with different composition CNT veils were prepared using hot press method and tested for interlaminar fracture toughness (IFT), measured in Mode-I (opening) and Mode-II (in-plane shear), and for interlaminar shear strength (ILSS), evaluated by the short beam shear (SBS) test. The crack propagation mode could be directly determined through fractography analysis by electron microscopy and resin/CNT spatial discrimination by Raman spectroscopy, showing a clear correlation between interlaminar reinforcement and the balance between cohesive/adhesive failure mode at the interlayer region. Composites with full resin infiltration of the CNT veils give a large increase of Mode II IFT (88%) to 1500 J/m2 and a slight enhancement of apparent interlaminar shear strength (6.5%), but a decrease of Mode I IFT (−21%). A toughening factor, defined as the relative increase in toughness per interleaf-to-ply thickness ratio, give a record-high value of 15.8 for Mode II IFT. The results help establish the role of interlayer infiltration, interlaminar crossings and formation of a carbon fiber bridgings, for interlaminar reinforcement with interleaves.

Elasto-plastic behavior and failure of thick GLARE laminates under bending loading

Mechanical properties of thick GLARE1-9/8 laminates with unidirectional structure are investigated. Three-point bending tests are conducted to determine the elasto-plastic behavior and flexural strength of samples with different length. Effective longitudinal and interlaminar shear moduli of GLARE are measured based on various span method. Failure loads and failure mechanisms are determined within the range of the samples’ span-to-thickness ratio L/h=3.5...27. Micro/meso-mechanical models are involved for the analysis of experimental data. It is shown, that found values of elastic moduli agree well with simple analytical models, while the non-linear deformations of GLARE under high loading levels should be described accounting for plastic effects that arise in aluminum sheets as well as in GFRP layers. Change of failure mechanisms is accurately predicted based on the developed elasto-plasticity model involving maximum stress and maximum strain criteria. Size effects for the apparent interlaminar shear strength (ISS) of GLARE laminates are explained accounting for anisotropy and inelastic deformations of GFRP layers. It is shown, that true ISS evaluated based on the proposed model is closed to those one determined by the double-notched shear test.

Ultrasonic Welding of Glass Fiber Reinforced PP Thermoplastic Composites: An Investigation of the Outer Layer Orientation and the Fiber Volume Fraction

This paper presents an experimental study of the influence of the orientation of the outer layer of polypropeylene (PP) reinforced with E-glass fiber laminate (GF/PP) and the influence of the fiber volume fraction on the quality of the welded joint using an ultrasonic welding process. An orthogonal L 16 array (OA) design of experiment was conducted in this paper based on the Taguchi method to evaluate the effect of the orientation of the outer layer and the fiber volume fraction, on the welding process parameters; the welding energy, the amplitude of vibration, the welding pressure, the holding pressure and the holding time were considered in order to achieve a high weld quality. The experiments were carried out using a 15 kHz ultrasonic welding unit with a maximum supplied power of 4000-Watt. GF/PP laminates with fiber volume fraction of 36% and 46% were used in this paper, and the GF/PP laminates were either unidierctional or had a 90 degree outer layer orienation. A 0.127 mm thick polypropeylene film was used as a flat energy director (ED). The evaluation of the weld quality was measured by the apparent shear strength of the single lap welded joints, and by using laser shearography as a non-destructive inspection technique . The failure mechanism of the single lap joint was monitored, using a high speed digital imaging system. A combination of the highest selected level of welding energy, lowest level of amplitude, lowest level of welding pressure, and the lowest level of both hold time and hold pressure of a unidirectional GF/PP with the lowest fiber volume fraction, were found to achieve a higher apparent shear strength of the welded adherends, as compared with the apparent shear strength obtained with the presence of the flat energy director for the same level of factors. A confirmation experiment was conducted to measure the predicted apparent shear strength and compare it with the measured apparent shear strength from the test.

Microstructure optimizations for improving interlaminar shear strength and self-healing efficiency of spread carbon fiber/epoxy laminates containing microcapsules

The purpose of this study is to improve interlaminar shear strength and self-healing efficiency of spread carbon fiber (SCF)/epoxy (EP) laminates containing microcapsules. Microencapsulated healing agents were embedded within the laminates to impart a self-healing functionality. Self-healing was demonstrated on short beam shear specimens, and the healing efficiency was evaluated by strain energies of virgin and healed specimens. The effects of microcapsule concentration and diameter on apparent interlaminar shear strength and healing efficiency were discussed. Moreover, damaged areas after short beam shear tests were examined by an optical microscope to investigate the relation between the microstructure and the healing efficiency of the laminates. The results showed that the stiffness and the apparent interlaminar shear strength of the laminates increased as the microcapsule concentration and diameter decreased. However, the healing efficiency decreased with decreasing the microcapsule concentration and diameter.

Pseudo-ductile behaviour in fibre reinforced thermoplastic angle-ply composites

Pseudo-ductile behaviour has previously been demonstrated in a [±45n]s carbon fibre composite via fibre rotation. To further develop the pseudo-ductile response, the high strain properties of the matrix should be considered to avoid strain localisation and potentially exploit strain-hardening in an amorphous thermoplastic matrix. For the first time, polycarbonate, a high strain-to-failure matrix is utilised in a pseudo-ductile composite design and compared to a high performance thermoset fibre composite containing an epoxy resin. The use of polycarbonate leads to enhanced pseudo-ductile strain (49%), longitudinal strength (24%) and apparent in-plane shear strength (26%), albeit with a reduction in elastic modulus (26%), shear chord modulus (22%) and yield strength (26%).

Mechanical response of saturated and unsaturated gravels of different sizes in drained triaxial testing

During the construction of pavements, gravelly soil is used as the main material in the base and sub-base layers. Predicting the deformation behaviour of gravelly soil has been increasingly important, and this may require comprehensive knowledge about both the mechanical and microstructural behaviours of gravelly materials. This study evaluates the saturated and unsaturated behaviours of 200-mm-high remoulded gravelly soil samples under a large-scale consolidated drained triaxial test. The laboratory tests included physical, mechanical, and microstructural testing on gravelly soil and different sizes of gravel. A series of drained triaxial tests were conducted on gravelly soil under both saturated and unsaturated conditions and on 5 mm, 10 mm, 14 mm, 20 mm, and 25 mm sizes of gravel. The results indicated an increase in the mechanical properties with increasing gravel size. The shear strength parameters were also improved with increasing gravel size. When comparing the results of the gravelly soil under both saturated and unsaturated conditions, the soil had an apparent shear strength that decreased beyond the residual suction during the unsaturated condition. The deviator stress and axial and volumetric strains were significantly dependent on the confining pressure. The microstructural studies confirmed the presence of calcite minerals with granular structures and crystalline textures as the main mineral in the gravelly soil. Therefore, the particle breakage when confining pressure is applied will be minimal, which enables the use of this soil in the base and sub-base layers of the pavement.

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.

Effect of surface functionalization of molybdenum disulfide nanosheets with polysulfone on cyanate ester nanocomposites

The aggregation of two-dimensional (2D) layered nanofillers restricts their use as reinforcements in polymers. Herein, the bisphenol E cyanate ester resin (CE) nanocomposites based on surface functionalization of molybdenum disulfide (MoS2) nanosheets with polysulfone (PSF) are prepared. In the structures, PSF segment were sandwiched between adjacent MoS2 nanoplatelets, which restrained the restacking of the MoS2 nanoplatelets. The compatibility between PSF modified MoS2 and CE is improved as compared with MoS2 and CE. The tensile strength and apparent shear strength of the nanocomposite show that modified MoS2 nanosheets are very effective at increasing the adhesive and mechanical properties of cyanate esters at very low quality fractions. Pulling out effects, crack bridging, crack deflection, and crack pinning are the main strengthening mechanisms of mechanical properties. The glass transition temperature increases from 245 °C for the unfilled cyanate ester to 265 °C at 0.15 % MoS2 loading. The results are consistent with the nanoconfinement effect including a nanoscale interfacial layer that can support the effective reinforcements of functionalized MoS2 nanoplatelets over pure MoS2. The excellent properties of MoS2/PSF/CE nanocomposites indicate that the approach in this study is effective to prepare high-performance molybdenum disulfide/cyanate ester polymer 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.

Long-term shear performance of bare and cement mortar-coated BFRP bars in corrosive environments

For the safe design of a basalt fibre-reinforced polymer (BFRP) bar for use as a reinforcement in concrete structures, it is essential to investigate the durability of the BFRP bars. This work focuses on the physicochemical properties of BFRP bars coated with cement mortar after immersion in distilled water, alkaline solution and seawater. The effects of the immersion medium, cement mortar coating size, immersion time and immersion temperature on the shear resistance of the BFRP bar as well as the microstructure and chemical composition of the interface between the basalt fibre and concrete substrate were examined. The cross-linking density of the resin matrix was used to illustrate the decomposition of BFRP bars. Then, the degradation evolution and mechanism of cement mortar-coated BFRP under various immersion environments were revealed. The results show that all the immersions have a negative influence on the shear performance of BFRP bars, and the effect of the alkaline solution is the most significant. The alkalinity played an important role for the BFRP bars immersed in distilled water and seawater, and the apparent shear strengths of the specimens coated with 0, 10, and 20 mm cement mortar immersed in 40 °C distilled water decreased by 27%, 32% and 57%, respectively, after one year. The alkali-silica reaction (ASR) of the SiO2 in basalt fibre was found at the cement mortar-BFRP interface, and the ASR contributed to this coating size effect.

Shear strength of bonded joints of carbon fiber reinforced plastic (CFRP) laminates enhanced by a two-step laser surface treatment

The apparent shear strength of single-lap bonded joints of carbon fiber reinforced plastic (CFRP) can be enhanced by laser surface treatment. In this work, a two-step laser surface treatment was used to improve the single-lap bonded joint performance of CFRP-laminates. STEP-1, the resin at the outer surface of laminates was completely removed by an optimal laser etching process to provide neat carbon fiber fabric. STEP-2, the as-obtained exposed carbon fiber fabric was further irradiated to produce a series of mini-grooves on its surface. Laser scanning parameters, i.e., spot distance, scanning angles, groove distance and patterns, were investigated to analyze their effects on the morphologies of generated grooves. Single-lap shear tests were also carried out to assess the mechanical performance of these laser processing bonded joints. Experimental results indicated that crossed grooves significantly enhanced the shear strength of the CFRP-laminates. Failure morphologies of shear tests were observed to analyze the strengthening mechanism of grooved-CFRP by laser surface treatment. Using optimal processing parameters during laser surface treatment, the apparent shear strength of single-lap bonded joints of CFRP-laminates reached 18.58 MPa, implying a 40.8% enhancement compared with smooth laminates.