Strain sensing in polymer/carbon nanotube composites by electrical resistance measurement

In this work multiwall carbon nanotubes (MWCNTs) dispersed in a polymer matrix have been used for strain sensing of the resulting nanocomposite under tensile loading. This was achieved by measuring the relative electrical resistance change (ΔR/R₀) in conductive polyvinylidenefluoride (PVDF)/MWCNTs nanocomposites prepared by melt-mixing with varying filler content from 0.5 wt.% to 8 wt.%. Two main parameters were systematically studied. The PVDF/MWCNTs mixing procedure that results in a successful MWCNTs dispersion, and the effect of MWCNTs content on material’s sensing behaviour. The samples were subjected to tensile loading and the longitudinal strain was monitored together with the longitudinal electrical resistance. The results showed that MWCNTs dispersed in insulating PVDF matrix have the potential to be used as a sensitive network to monitor the strain levels in polymer/carbon nanotube nanocomposites as the deformation level of each sample was being reflected by the resistance changes.     

Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves

Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.     

Low velocity impact damage evaluation in fiber glass composite plates using PZT sensors

In this work PZT sensors are proposed to characterize the impact effects in fiber glass composite plates. To provide effective and reliable results the analysis was accomplished on samples submitted to multiple impacts, then guaranteeing the same testing conditions for different impact levels. The analysis of the impact effect has been made by two parameters; amplitude response and time shift. PZT sensors have been bonded to the samples in a pitch-and-catch configuration and the Lamb wave symmetrical mode (S₀) signal was used. The results demonstrate that the two evaluated parameters are able to characterise the different damage types occurring in these composites, as well as to evaluate their severity. It was also observed that amplitude predict well the defect size, whenever fiber-breakage occurs.     

Mechanical properties of fire-retardant glass fiber-reinforced polymer materials with alumina tri-hydrate filler

Alumina tri-hydrate (ATH) can be effectively used to increase fire resistance of Fiber-Reinforced Polymer (FRP) materials. This paper studies the effect of ATH filler on mechanical properties of Glass FRP (GFRP) material, based on compression, tension, shear and flexural test results from three types of GFRP materials with the amount of 0% (control), 25%, and 50% ATH filler by weight of the resin. It was found that the control was the strongest for all tests except for flexure, which is 3% lower than the flexural strength of 25% ATH sample. The compressive strength dropped 19% and 25% for 25% and 50% ATH loadings, respectively, compared to the control. For shear and tensile strengths, the 25% ATH sample acted similarly to the control, but the 50% ATH sample had a significantly lower strength. For stiffness, changing the additive amount from 0% to 50% had only small changes for compression, tension, and flexure. It can be concluded that adding ATH generally decreases the strength and makes FRP more brittle. The performance of a 25% ATH loading is comparable to the control except compression, while a 50% ATH loading has a more significant effect on the mechanical properties of the GFRP. The data presented in this paper can be used to develop fire-resistant FRP systems.     

Imaging of porosity in fiber-reinforced composites with a fiber-optic pump–probe laser-ultrasound system

Assessing material porosity in composites is critical because pores can greatly affect material strength and lifetime. Ultrasound (US) is one of the primarily methods to quantify porosity, usually based on the relationship between US speed/attenuation and void content. However, most US approaches require a sample with plane parallel and relatively smooth surfaces to correctly measure the attenuation and speed, but such conditions cannot always be fulfilled in practice. In addition, conventional US cannot directly image porosity as X-rays can. Here we present a non-contact US method to directly image porosity that can be easily integrated with US speed/attenuation measurements. The overall approach uses ultra wideband acoustic signals generated at the surface of a composite material with a laser pulse (i.e., pump), and non-contact, point-like detection of backscattered transients (i.e., probe) with spatial resolution better than 1 ply. US-assessed porosity is compared with that measured gravimetrically.     

Mechanical,thermal and morphological characterization of cellulose fiber-reinforced phenolic foams

In this work, the compressive mechanical properties, thermal stability and morphology of cellulose fiber-reinforced phenolic foams were studied. The cellulose fiber-reinforced phenolic foam showed the greatest compressive mechanical properties by incorporating 2 wt.% of the reinforcement. The compressive modulus and strength of 2 wt.% cellulose fiber-reinforced phenolic foam were increased by 21% and 18%, respectively, relative to the unreinforced material. The addition of the cellulose fibers to the phenolic foam slightly decreased the thermal stability of the material. The study on the morphology of the cellulose-reinforced phenolic foams via Scanning electron microscopy (SEM) indicated a strong bonding between the fibers and phenolic matrix. In addition, the incorporation of the cellulose fibers into the foam resulted in a decreased cell size and increased cell density of the material. The incorporation of 2 wt.% of cellulose fibers into the phenolic foam led to obtain the material with the best features.     

Infrared stress measurements of thermal damage to laser-processed carbon fiber reinforced plastics

Recently, the laser processing of carbon fiber reinforced plastics (CFRPs) has attracted attention owing to the high processing speed and less tool wear. A problem in the laser processing of CFRPs is the lower strength than that of CFRPs processed by machines. This is considered to be due to the heat-affected zone (HAZ) generated during laser processing. In this study, the stress distributions of CFRPs processed by a laser obtained was evaluated by using infrared thermography. X-ray CT images were also obtained, which enabled us to discuss the stress distribution in terms of the HAZ. The stress distribution showed that the area with reduced stress generated in the HAZ which was introduced by laser processing. The region of low stress in the HAZ was visualized by infrared thermography. It is shown that the regions with reduced stress induce the conventionally reported decrease in strength of laser-processed CFRPs.     

The effect of interfacial state on the thermal conductivity of functionalized Al2O3 filled glass fibers reinforced polymer composites

Rapidly increasing packaging density of electronic devices puts forward higher requirements for thermal conductivity of glass fibers reinforced polymer (GFRP) composites, which are commonly used as substrates in printed circuit board. Interface between fillers and polymer matrix has long been playing an important role in affecting thermal conductivity. In this paper, the effect of interfacial state on the thermal conductivity of functionalized Al₂O₃ filled GFRP composites was evaluated. The results indicated that amino groups-Al₂O₃ was demonstrated to be effective filler to fabricate thermally conductive GFPR composite (1.07 W/m K), compared with epoxy group and graphene oxide functionalized Al₂O₃. It was determined that the strong adhesion at the interface and homogeneous dispersion of filler particles were the key factors. Moreover, the effect of interfacial state on dielectric and thermomechanical properties of GFRP composites was also discussed. This research provides an efficient way to develop high-performance GFRP composites with high thermal conductivity for integrated circuit packaging applications.     

Passive and SMA-activated confinement of circular masonry columns with basalt and glass fibers composites

Fiber Reinforced Polymer (FRP) composites are widely used for strengthening and conservation of historic masonry, even if research problems are still open. The mechanical behavior of masonry columns having a circular cross section, confined with glass and basalt FRP systems was studied in this paper. An extended experimental investigation is presented in order to show the results of axial compression tests on circular masonry columns built with natural blocks (calcareous stone). Active confinement was also studied by using a novel technique that employs Shape Memory Alloys (SMA). Totally twenty-four masonry columns were built, instrumented and tested. Different fibers, strengthening schemes and matrix/adhesive were used for the confinement of the columns.Unstrengthened columns were tested as reference specimens. Axial strain of the columns and tensile strain of the fibers in the direction perpendicular to the primary axis of the columns were measured with the applied load. Experimental results revealed the effectiveness of the FRP-confinement for masonry columns. Active confinement was found to be effective at early loading stages since an increased stiffness of the SMA/GFRP-confined columns was measured.A prediction of the compressive strength was obtained by using the model of the Italian guidelines CNR DT 200 (National Research Council) in order to compare the experimental results with the design approach, also for new types of fiber like basalt which were not included in the technical codes. Finally, the experimental results were compared with theoretical values calculated according with to two existing analytical models in order to test their effectiveness for the analyzed configurations.     

Subcritical damage mechanisms of bolted joints in CFRP composite laminates

A detailed experimental programme is presented that was conducted in order to establish a data base for strength and subcritical damage mechanisms of bolted joints in CFRP composite laminates. Single fastener double-shear tensile tests for various joint geometries were performed for a range of cross-ply and quasi-isotropic lay-ups of HTS40/977-2 CFRP material system. Penetrant enhanced X-ray radiography was used to define the subcritical damage locations which are of great importance when modelling the failure response of the joints. It is suggested that the subcritical damage planes can be modelled using cohesive zone elements (CZEs) in order to develop physically based strength prediction methods for bolted joints in CFRP laminates.     

Mechanical damage assessment of Polymer–Matrix Composites using active infrared thermography

The present work uses an active form of the infrared (IR) thermography technique for monitoring and characterization of the damage of glass fibers reinforced Polymer–Matrix Composites (PMCs) following an experimental protocol based on a monotonic tensile test with constant stress steps. The active form introduces a new approach using the cooling rate variation of the material after thermal stimulations during different constant stress levels. This study proves that the cooling speed increases when the damage state has accentuated due to the change of the applied stress. This new approach is original because it allows the auscultation of materials and structures at rest and in service under mechanical loadings without destroying them and with no contact.     

Prediction of failure properties of injection-molded short glass fiber-reinforced polyamide 6,6

This study simulates the tensile failure of injection-molded short glass fiber-reinforced polyamide 6,6 (GF/PA66). Tensile tests of unreinforced PA66 are first conducted and the material properties are obtained by fitting a simulated stress–strain curve to the experiment result. Using the obtained material properties, failure simulations of GF/PA66 composites are performed for four types of specimens with various fiber lengths and fiber orientation distributions. In the simulations, multiscale mechanistic model, which can simulate micromechanical damage, and Micromechanics Model (MM), which has very low computational cost, are adapted and the results are compared with experiments. Both models reproduce the experiment results well. Considering the computational cost, MM is the better model for predicting the failure properties of GF/PA66 composites.     

Impact and post-impact damage characterisation of hybrid composite laminates based on basalt fibres in combination with flax,hemp and glass fibres manufactured by vacuum infusion

The impact and flexural post-impact behaviour of ternary hybrid composites based on epoxy resin reinforced with different types of fibres, basalt (B), flax (F), hemp (H) and glass (G) in textile form, namely FHB, GHB and GFB, has been investigated. The reinforcement volume employed was in the order of 21–23% throughout. Laminates based exclusively on basalt, hemp and flax fibres were also fabricated for comparison. Hybrid laminates showed an intermediate performance between basalt fibre reinforced laminates on the high side, and flax and hemp fibre reinforced laminates on the low side. As for impact performance, GHB appears to be the worst performing hybrid laminate and FHB slightly overperforms GFB. In general, an increased rigidity can be attributed to all hybrids with respect to flax and hemp fibre composites. The morphological study of fracture by SEM indicated the variability of mode of fracture of flax and hemp fibre laminates and of the hybrid configuration (FHB) containing both of them. Acoustic emission monitoring during post-impact flexural tests confirmed the proneness to delamination of FHB hybrids, whilst they were able to better withstand impact damage than the other hybrids.     

Application of infrared thermography for the characterization of damage in braided carbon fiber reinforced polymer matrix composites

The focus of this study is to assess, using infrared thermography, the fatigue behavior and the corresponding damage states of a textile polymeric composite plate, as a prerequisite step in the development of damage based life prediction models for such advanced composite materials. Monotonic (quasi-static) loading test results confirmed that the dominant damage mechanism is cracking in the braider yarns, which was monitored using thermographic images and confirmed by edge replication microscopic observations. Fatigue results confirmed that the saturation of braider yarn cracks during cyclic loading corresponded to changes in the stiffness degradation rate as well as the surface temperature profile. This was confirmed by edge replication and scanning electron microscopic analysis. The reported results and observations provide an important step in the validation of thermography as a powerful non-destructive evaluation tool for monitoring the development of fatigue damage as well as predicting the damage states of laminated composite materials in general, and braided polymeric composite materials in particular.     

Fibrous composite matrix characterisation using nanoindentation: The effect of fibre constraint and the evolution from bulk to in-situ matrix properties

The variation of the in-situ matrix properties of a carbon-fibre composite has been investigated using nanoindentation. The aerospace carbon-fibre composite material (HTA/6376) and the bulk matrix (6376) have been co-cured to produce specimens ideal for matrix characterisation. The in-situ matrix has been characterised using fifty indentations in matrix pockets of many different sizes. The fibre constraint effect on in-situ matrix indentations has been characterised experimentally using the continuous stiffness measurement (CSM) technique, showing good correlation with finite element results from a previous study. The co-cured specimens allow the evolution of property change in the matrix material to be observed. The in-situ matrix modulus increases with decreasing matrix pocket size, and is up to 19% greater than the bulk matrix. This property change occurs outside the normal range of the interphase region for CFRP materials, and is statistically significant relative to the experimental scatter associated with the nanoindentation technique.     

Processing and mechanical properties evaluation of glass fiber-reinforced PTFE laminates

A specific manufacturing process to obtain continuous glass fiber-reinforced PTFE laminates was studied and some of their mechanical properties were evaluated. Young’s modulus and maximum strength were measured by three-point bending test and tensile test using the Digital Image Correlation (DIC) technique. Adhesion tests, thermal analysis and microscopy were used to evaluate the fiber–matrix adhesion, which is very dependent on the sintering time. The composite material obtained had a Young’s modulus of 14.2 GPa and ultimate strength of 165 MPa, which corresponds to approximately 24 times the modulus and six times the ultimate strength of pure PTFE. These results show that the PTFE composite, manufactured under specific conditions, has great potential to provide structural parts with a performance suitable for application in structural components.     

Modification of montmorillonite by novel geminal benzimidazolium surfactant and its use for the preparation of polymer organoclay nanocomposites

A new benzimidazolium derivative, the benzimidazolium-N,N′-hexadecane-2-hydroxy-ethyl bromide (Bz) featuring two geminal hexadecyl hydrophobic buttress has been synthesized and used for the functionalization of sodium montmorillonite (MMT-Na) via cationic exchange process. The resulting benzimidazolium-modified MMT (MMT-Bz) exhibits a large d-spacing of 3 nm between silicate layers and shows a high thermal stability compared to the commonly used clay modified alkyl ammonium salts (cloisite 20A and cloisite 20B). MMT-Bz was incorporated in high density polyethylene (HDPE) matrix via melt mixing method to produce HDPE/MMT-Bz nanocomposites. The microstructure and the morphology of these nanocomposites were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The dispersion state of the organoclay within HDPE was monitored by UV–Vis spectroscopy and melt rheology. A more homogeneous dispersion or a greater content of the MMT-Bz in the matrix produced stronger solid-like and non-terminal behavior in the nanocomposites. Tensile properties and thermal stability were evaluated and discussed on the basis of the amount of clay incorporated within the nanocomposites. The intercalated structure in the nanocomposites, resulting from both the better dispersion/distribution of clay nano-platelets and their strong interaction with the polymer chains, provides the driving force to significantly enhance the HDPE properties.     

Thermomechanical behavior of PA6.6 composites subjected to low cycle fatigue

In this study, manifold experiments were conducted to investigate the thermomechanical behavior of short E-glass fiber-reinforced polyamide 6.6 composites subjected to low cycle fatigue loadings. Different hygrometric states, fiber configurations and loading rates were considered. Mechanical, thermal and energy responses of composite specimens were recorded using photomechanic techniques. The influence of water content, fiber orientation and loading rate on these thermomechanical responses was systematically analysed.The mechanical findings indicated that the ratcheting phenomenon was more pronounced for humid composites reinforced with fibers oriented transversely and subjected to a low loading rate. Moreover, the order of magnitude in self-heating was greater for transversal fiber composites conditioned at high relative humidity and subjected to a 10 Hz loading rate. From a thermodynamic standpoint, we also noticed that high proportions of the mean stored energy rate were obtained at a high loading rate, with values exceeded 64%. These values were noticeably altered by the water content and fiber angles, i.e. lower as the relative humidity increased and higher as the fiber angles increased.     

The effect of graphene presence in flame retarded epoxy resin matrix on the mechanical and flammability properties of glass fiber-reinforced composites

The effect of adding graphene in epoxy containing either an additive (MP) or reactive-type (DOPO) flame retardant on the thermal, mechanical and flammability properties of glass fiber-reinforced epoxy composites was investigated using thermal analysis; flexural, impact, tensile tests; cone calorimetry and UL-94 techniques. The addition of MP or DOPO to epoxy had a thermal destabilization effect below 400 °C, but led to higher char yield at higher temperatures. The inclusion of 10 wt% flame retardants slightly decreased the mechanical behavior, which was attributed to the poor interfacial interactions in case of MP or the decreased cross-linking density in case of DOPO flame retarded resin. The additional graphene presence increased flexural and impact properties, but slightly decreased tensile performance. Adding graphene further decreased the PHRR, THR and burning rate due to its good barrier effect. The improved fire retardancy was mainly attributed to the reduced release of the combustible gas products.