Mechanical performance optimization through interface strength gradation in PP/glass fibre reinforced composites

A new design for thermoplastic composites based on the gradation of the interlaminar interface strength (IGIS) has been developed with the aim of coupling high impact resistance with high static properties. IGIS laminates have been prepared by properly alternating layers of woven fabric with layers of compatibilized or not compatibilized polymeric films. To prove the new concept, polypropylene (PP) and glass fibres woven fabrics have been used to prepare composites by using the film stacking technique. Maleated PP, able to compatibilize polypropylene with glass fibres, has been used to manage the interface strength layer by layer.The flexural and low-velocity impact characterizations have shown that the presence of the coupling agent in conventional composite structures (prepared with fully compatibilized polymeric layers) improves the static flexural properties through the strengthening of the matrix/fibre interface but considerably lowers the low velocity impact resistance of the composite, in terms of maximum load before fibre breakage and recovered energy after impact. The use of the IGIS design, that grade the interface strength through the laminate thickness, allows to prepare composites with both high flexural properties and high impact resistance, without affecting the balance and type of the reinforcement configuration.     

Biocomposites from Musa textilis and polypropylene: Evaluation of flexural properties and impact strength

Abaca (Musa textilis)-reinforced polypropylene composites have been prepared and their flexural mechanical properties studied. Due to their characteristic properties, M. textilis has a great economic importance and its fibers are used for specialty papers. Due to its high price and despite possessing very distinctive mechanical properties, to date abaca fibers had not been tested in fiber-reinforced composites. Analysis of materials prepared showed that, in spite of reduced interface adhesion, flexural properties of the PP composites increased linearly with fiber content up to 50 wt.%. Addition of a maleated polypropylene coupling agent still enhanced the stress transfer from the matrix to the reinforcement fiber. As a result, composites with improved flexural properties were obtained. The mechanical properties of matrix and reinforcing fiber were evaluated and used for modelling both the flexural strength and modulus of its composites. In addition, the impact strength of materials was evaluated. Comparison with mechanical properties of composites reinforced with fiberglass points out the potentiality of abaca-reinforced polypropylene composites as suitable substitutes in applications with low impact strength demands.     

Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites

Natural fiber reinforced polymer composites became more attractive due to their light weight, high specific strength, and environmental concern. However, some limitations such as low modulus, poor moisture resistance were reported. This study aimed to investigate the effect of glass fiber hybridization on the physical properties of sisal–polypropylene composites. Polypropylene grafted with maleic anhydride (PP-g-MA) was used as a compatibilizer to enhance the compatibility between the fibers and polypropylene. Incorporating glass fiber into the sisal–polypropylene composites enhanced tensile, flexural, and impact strength without having significant effect on tensile and flexural moduli. In addition, adding glass fiber improved thermal properties and water resistance of the composites.     

Mechanical and thermal characterization of epoxy composites reinforced with random and quasi-unidirectional untreated Phormium tenax leaf fibers

The present work investigates tensile and flexural behavior of untreated New Zealand flax (Phormium tenax) fiber reinforced epoxy composites. Two series of laminates were produced using the same reinforcement content (20 wt%), arranged either as short fibers or quasi-unidirectional ones. Composites reinforced using quasi-unidirectional fibers showed higher modulus and strength both in tensile and flexural loading, when compared to neat epoxy resin. Short fiber composites, although still superior to epoxy resin both for tensile and flexural moduli, proved inferior in strength, especially as concerns tensile strength. These results have been supported by scanning electron microscopy (SEM), which allowed characterizing fiber–matrix interface, and by acoustic emission (AE) analysis, which enabled investigating failure mechanisms. In addition, thermal behavior of both untreated phormium fibers and composites has been studied by thermogravimetric analysis (TGA), revealing the thermal stability of composites to be higher than for phormium fibers and epoxy matrix alone.     

Developing rigid gliadin based biocomposites with high mechanical performance

We aim to produce unidirectional fiber composites with high mechanical performance based on flax fibers and a rigid gliadin matrix. As a fraction from wheat gluten, gliadin is soluble in alcohol containing media. The fabrication process did not involve any further solvents or plasticizers. Finally, samples were cooled at different rates. Overall, the cooling rate does not strongly affect the mechanical properties although slowly cooled materials contain a higher amount of non-disulfide cross-links, next to disulfide bonds within the gliadin matrix. At 40% fiber volume fraction, flax/gliadin composites with a flexural modulus and strength of respectively 21.5 GPa and 240 MPa were obtained when loaded in the longitudinal direction. These high values demonstrate that in this composite fabrication process, a good impregnation of the polymer matrix in between the fiber bundles has been achieved. However, the fiber–matrix adhesion, as measured by transverse flexural and tensile tests, was still relatively modest.     

Studies on the characterization of piassava fibers and their epoxy composites

This paper presents morphology, physical and strength properties of piassava fiber, a very rigid fiber having a potential to be used as composite reinforcement. Composites of continuous and aligned piassava fibers with and without alkali treatment dispersed in epoxy matrix were subjected to three point bend, tensile, and Izod impact tests. Composites with fibers above 20 vol.% showed an effective reinforcement behavior both in flexural and tensile tests, while the impact energy linearly increased for the amount of piassava fibers used in this study. Fractographic study revealed a relatively weaker fiber/matrix adhesion acting as preferential site for crack nucleation. Evidence was also found for crack arrest by the fiber above 20 vol.%. This, together with spiny surface protrusion in the piassava fibers, was found to be responsible for the reinforcement of the epoxy composites.     

Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling

Additive manufacturing (AM) technologies have been successfully applied in various applications. Fused deposition modeling (FDM), one of the most popular AM techniques, is the most widely used method for fabricating thermoplastic parts those are mainly used as rapid prototypes for functional testing with advantages of low cost, minimal wastage, and ease of material change. Due to the intrinsically limited mechanical properties of pure thermoplastic materials, there is a critical need to improve mechanical properties for FDM-fabricated pure thermoplastic parts. One of the possible methods is adding reinforced materials (such as carbon fibers) into plastic materials to form thermoplastic matrix carbon fiber reinforced plastic (CFRP) composites those could be directly used in the actual application areas, such as aerospace, automotive, and wind energy. This paper is going to present FDM of thermoplastic matrix CFRP composites and test if adding carbon fiber (different content and length) can improve the mechanical properties of FDM-fabricated parts. The CFRP feedstock filaments were fabricated from plastic pellets and carbon fiber powders for FDM process. After FDM fabrication, effects on the tensile properties (including tensile strength, Young's modulus, toughness, yield strength, and ductility) and flexural properties (including flexural stress, flexural modulus, flexural toughness, and flexural yield strength) of specimens were experimentally investigated. In order to explore the parts fracture reasons during tensile and flexural tests, fracture interface of CFRP composite specimens after tensile testing and flexural testing was observed and analyzed using SEM micrograph.     

Influence of fiber/matrix interfacial adhesion on composite fracture behavior

A systematic study has been conducted to identify the effect of fiber/matrix interface strength on various composite properties. A new fiber treatment technique was developed to allow fibers to be treated and then made into prepregs and composites of acceptable quality. T500 carbon fibers were treated with release agent to establish the extreme case of poor fiber/matrix interface. Composite systems made of toughened epoxy R6376 and T500 fibers with and without such a treatment were subjected to a number of fracture and impact tests. For tests involving propagating pre-existing delamination cracks, such as double cantilever beam (DCB), end notched flexural (ENF) and crack lap shear (CLS) methods, the material properties were not appreciably affected by the release agent-treated fiber surfaces. For tests that had to initiate cracks in specimens without pre-introduced cracks, such as impact and edge delamination, the material variables and failure modes were highly sensitive to the fiber/matrix interface. The critical role of the fiber/matrix interface in crack initiation was demonstrated in this study.     

A study of the mechanical properties of short natural-fiber reinforced composites

The degree of fiber–matrix adhesion and its effect on the mechanical reinforcement of short henequen fibers and a polyethylene matrix was studied. The surface treatments were: an alkali treatment, a silane coupling agent and the pre-impregnation process of the HDPE/xylene solution. The presence of Si–O–cellulose and Si–O–Si bonds on the lignocellulosic surface confirmed that the silane coupling agent was efficiently held on the fibres surface through both condensation with cellulose hydroxyl groups and self-condensation between silanol groups.

The fiber–matrix interface shear strength (IFSS) was used as an indicator of the fiber–matrix adhesion improvement, and also to determine a suitable value of fiber length in order to process the composite with relative ease. It was noticed that the IFSS observed for the different fiber surface treatments increased and such interface strength almost doubled only by changing the mechanical interaction and the chemical interactions between fiber and matrix.

HDPE-henequen fiber composite materials were prepared with a 20% v/v fiber content and the tensile, flexural and shear properties were studied. The comparison of tensile properties of the composites showed that the silane treatment and the matrix-resin pre-impregnation process of the fiber produced a significant increase in tensile strength, while the tensile modulus remained relatively unaffected. The increase in tensile strength was only possible when the henequen fibers were treated first with an alkaline solution. It was also shown that the silane treatment produced a significant increase in flexural strength while the flexural modulus also remained relatively unaffected. The shear properties of the composites also increased significantly, but, only when the henequen fibers were treated with the silane coupling agent. Scanning electron microscopy (SEM) studies of the composites failure surfaces also indicated that there is an improved adhesion between fiber and matrix. Examination of the failure surfaces also indicated differences in the interfacial failure mode. With increasing fiber–matrix adhesion the failure mode changed from interfacial failure and considerable fiber pull-out from the matrix for the untreated fiber to matrix yielding and fiber and matrix tearing for the alkaline, matrix-resin pre-impregnation and silane treated fibers.     

Assessment of flexural toughness and impact resistance of bundle-type polyamide fiber-reinforced concrete

This study compares the fiber/matrix bonding strength and flexural properties of bundle-type polyamide fibers to those of hooked-end steel fibers. Their fracture behavior upon impact with a high-velocity projectile is also assessed in terms of penetration depth, crater diameter and rear-side scabbing. The results obtained demonstrate that the bundle-type polyamide fibers undergo fracture without fiber pullout because of the increased interfiber gap and specific surface area for bonding, but exhibit poorer flexural fracture behavior with a lower flexural strength and fracture energy when compared to hooked-end steel fibers. Yet despite this, concrete reinforced with bundle-type fibers is shown to more effectively suppress scabbing during high-velocity impact, which is attributed to a more efficacious dispersion of shock stress due to the increased number of individual fibers.     

Characterization and biodegradability of polypropylene composites using agricultural residues and waste fish

The objective of this research was to study the potential of waste agricultural residues such as rice-husk fiber (RHF), bagasse fiber (BF), and waste fish (WF) as reinforcing and biodegradable agents for thermoplastic composites. Addition of maleic anhydride grafted polypropylene (MAPP) as coupling agent was performed to promote polymer/fiber interfacial adhesion. Several composites with various polypropylene (PP) as polymer matrix, RHF, BF, WF, and MAPP contents were fabricated by melt compounding in a twin-screw extruder and then by injection molding. The resulting composites were evaluated through mechanical properties in terms of tensile, flexural, elongation at break and Izod notched impact following ASTM procedures. Biodegradability of the composites was measured using soil burial test in order to study the rates of biodegradation of the composites. In general, the addition of RHF and BF promoted an increase in the mechanical properties, except impact strength, compared with the neat PP. According to the results, WF did not have reinforcing effect on the mechanical properties, while it could considerably improve the biodegradation of the composites. It was found that the composites with high content of WF had higher degradation rate. Except impact strength, all mechanical properties were found to enhance with increase in cellulosic fiber loading In addition, mechanical properties and biodegradability of the composites made up using RHF was superior to those of the composites fabricated with BF, due to its morphological (aspect ratio) characteristics.     

The effect of the fiber/matrix interface on the flexural fatigue performance of unidirectional fiberglass composites

Fatigue tests were conducted on oriented fiberglass-reinforced polymer matrix composites using four-point bending with a stress ratio of −0·8. Composites in which the fiberglass was treated with a commercial diaminofunctional silane coupling agent were found to possess a relatively high flexural fatigue performance compared with composites without coupling agents. Using the interlaminar shear strength as an indication of the interface strength, it was found that composites having a high interface strength possess a high fatigue performance. The failure sequence of the flexural (tensile) fatigue was identified as: nucleation and growth of superficial damage (including fiber ridging, transverse matrix cracking, longitudinal matrix cracking, fiber breaking and local delamination), sudden fiber-bundle breakage and, finally, macroscopic delamination. A strong interface between fiber and matrix delayed the occurrence of fiber ridging and longitudinal matrix cracking, thus improving the fatigue performance of the unidirectional composites.     

Direct fluorination of Twaron fiber and the mechanical, thermal and crystallization behaviour of short Twaron fiber reinforced polypropylene composites

An experimental study of the incorporation of non-fluorinated and fluorinated Twaron fibers in polypropylene (PP) is presented. Surface modifications were made to Twaron fiber by direct fluorination technique using elemental fluorine in order to improve the interfacial adhesion between the fiber and matrix. Composites of PP/Twaron fiber (both Fluorinated and non-fluorinated) with 0.6%, 1.25%, 5% and 10% of Twaron fibers (w/w) were prepared by a solution method. Mechanical behaviour was estimated by the measurement of the tensile strength. The mechanical properties of PP improve significantly with the incorporation of Twaron fibers and fluorinated fiber composites show superior mechanical properties compared to the non-fluorinated system. The morphology was determined by scanning electron microscopy (SEM), showing good dispersion of the fibers. The thermal and crystallization behaviour of PP/Twaron fiber composites were studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). The effect of fiber content and fiber surface treatments on the thermal properties was evaluated. DSC analysis exhibited an increase in the crystallization temperature and crystallinity, melting temperature upon the addition of fluorinated fibers to the PP matrix. This is attributed to the nucleating effects of the fiber surfaces. Also the thermal stability (from TG) and surface energy (determined from contact angle measurement) increased for fluorinated fiber composites. Surface modification of Twaron fibers leads to improved adhesion with the PP matrix and hence an improvement in properties of the Twaron fiber-PP composites.     

Enhanced interfacial properties of domestic aramid fiber-12 via high energy gamma ray irradiation

The high energy irradiation was used to modify domestic aramid fiber-12 (DAF12) in epoxy chloropropane. The properties of the interphase between DAF12 and epoxy matrix systems were optimized. Scanning electron microscopy was employed to characterize the surface morphology of DAF12 and high energy irradiated fibers (HEI-DAF12) and composites de-bonding section. Atomic force microscopy showed the original smooth surface disappeared. Single fiber pull-out tests revealed that the interfacial shear strength of HEI-DAF12/epoxy composite was substantially improved by 45.17% after irradiation in 400 kGy dose. Dynamic contact angle analysis indicated the increased total surface free energy. The changes of elemental composition investigated via X-ray Photoelectron Spectrometer verified the increase polar groups on fiber surface caused by high energy irradiation.     

A methodology to measure the interface shear strength by means of the fiber push-in test

A methodology is presented to measure the fiber/matrix interface shear strength in composites. The strategy is based on performing a fiber push-in test at the central fiber of highly-packed fiber clusters with hexagonal symmetry which are often found in unidirectional composites with a high volume fraction of fibers. The mechanics of this test was analyzed in detail by means of three-dimensional finite element simulations. In particular, the influence of different parameters (interface shear strength, toughness and friction as well as fiber longitudinal elastic modulus and curing stresses) on the critical load at the onset of debonding was established. From the results of the numerical simulations, a simple relationship between the critical load and the interface shear strength is proposed. The methodology was validated in an unidirectional C/epoxy composite and the advantages and limitations of the proposed methodology are indicated.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

Mechanical properties of basalt fibers and their adhesion to polypropylene matrices

This work is aimed to study the mechanical properties of basalt fibers, and their adhesion to polypropylene (PP) matrices. Single filament tensile tests were used to calculate the strength of different types of fibers, characterized by different providers and surface treatment. Single fiber fragmentation tests (SFFT) were used to calculate the critical length of the fibers, in a homopolymer PP matrix and in a maleic anhydride modified PP matrix. It was shown that the tensile strength of the fibers is not significantly influenced by the origin or the surface treatment. Only fibers without any sizing show very reduced mechanical properties. On the other hand, the tensile strength was shown to be severely dependent on the filament length. Weibull theory was used in order to calculate the fitting parameters σ₀ and β, which were necessary in order to extrapolate the tensile strength to the critical length determined by SFFT. This allowed calculating the adhesion properties of the basalt fibers. It was shown that fiber–matrix adhesion is dependent on both the presence of sizing on the fiber surface, as well as on the modification of the matrix.     

Combined effects of fiber/matrix interface and water absorption on the tribological behaviors of water-lubricated polytetrafluoroethylene-based composites reinforced with carbon and basalt fibers

Tribological behaviors of two PTFE-based composites reinforced with carbon fibers and basalt fibers sliding against stainless steel under water lubrication were investigated and compared with those of pure PTFE. Results showed that carbon fibers were well bonded with PTFE matrix by dendritic PTFE nano-ribbons in a Boston ivy-like manner, but the basalt fibers were poorly bonded with the matrix. Due to the great accelerating effect of poor fiber/matrix interfacial adhesion on water absorption, BF/PTFE with the highest crystallinity unexpectedly showed the highest water absorption, resulting in serious matrix plasticization and degradation of fiber/matrix interfacial adhesion. As a result, as the reinforcement failure of basalt fibers occurred, BF/PTFE exhibited the highest wear rate. Instead, because good fiber/matrix interfacial adhesion was favor of the resistance to water intrusion, CF/PTFE composite was not dominated by remarkable matrix plasticization and fiber/matrix interface degradation, and showed the lowest wear rate.     

Interfacial shear strength estimates of NiTi–Al matrix composites fabricated via ultrasonic additive manufacturing

The purpose of this study is to understand and improve the interfacial shear strength of metal matrix composites fabricated via ultrasonic additive manufacturing (UAM). NiTi–Al composites can exhibit dramatically lower thermal expansion compared to aluminum, yet blocking stresses developed during thermal cycling have been found to degrade and eventually cause interface failure in these composites. In this study, the strength of the interface was characterized with pullout tests. Since adhered aluminum was consistently observed on all pullout samples, the matrix yielded prior to the interface breaking. Measured pullout loads were utilized as an input to a finite element model for stress and shear lag analysis. The aluminum matrix experiences a calculated peak shear stress near 230 MPa, which is above its ultimate shear strength of 150–200 MPa thus corroborating the experimentally-observed matrix failure. The influence of various fiber surface treatments and consolidation characteristics on bond mechanisms was studied with scanning electron microscopy, energy dispersive X-ray spectroscopy, optical microscopy, and focused ion beam microscopy.     

Expanded graphite nanoplatelets as coupling agents in glass fiber reinforced polypropylene composites

The interfacial adhesion between E-glass fibers and various types of nanomodified polypropylene (PP) matrices have been investigated on single-fiber model composites. In particular, an evaluation of the fiber–matrix interfacial shear strength was performed by the fragmentation tests on model composites prepared by using PP matrices containing various amounts (up to 7 wt%) of expanded graphite nanoplatelets (xGnP).The presence of xGnP in the polymer matrix resulted in a remarkable increase of the interfacial shear strength values (up to a factor of about 6 for a 7 wt% content of xGnP) if compared to neat PP. Moreover, wettability measurements in various liquids evidenced that the work of adhesion of the polymer matrix with respect to glass fiber, was improved by the presence of xGnP.