Factorial study of material and process parameters on the mechanical properties of extruded kenaf fibre/polypropylene composite sheets

Thin kenaf/polypropylene (PP) composite sheets were manufactured via extrusion. The effects of kenaf and maleated PP (MAPP) proportions, fibre length, PP melt flow index (MFI) and die temperature on tensile, flexural, in-plane and out-of-plane shear properties were analysed by conducting experiments through ‘design of experiments’ methodology. Higher kenaf content and lower die/barrel temperatures resulted in composite sheets with higher average mechanical properties in various modes of testing. Matrix MFI appeared to significantly affect all mechanical properties. It is interesting to note that the properties of the very short-fibre composites produced are comparable to those reinforced with longer discontinuous fibres and long-fibre mats.     

Influence of the filament winding tension on physical and mechanical properties of reinforced composites

In this experimental investigation the influence of the applied tow tension during filament winding on the physical and mechanical properties of glass-fibre reinforced polymeric composite tubulars, was studied. Pressure retaining tubular products used in the transportation/storage of fluids are generally subjected to a variety of loading conditions during their service life; thus tubular specimens were tested under different biaxial loading ratios. The stress/strain response was recorded and functional and structural failure envelopes were developed. These envelopes indicate the leakage and final failure characteristics of the components, respectively. The mechanical properties were analysed in conjunction with the measured physical properties: ‘fibre volume fraction’ and ‘effective wall thickness’. Experimental findings demonstrate that the component strength depends on the degree of fibre tensioning. Under fibre-dominated loading conditions, higher winding tension leads to an improved resistance against failure of tubular components, whereas under matrix-dominated loading failure is delayed by reduced fibre tensioning.     

Development of a new generation of filament wound composite pressure cylinders

A new generation of composite pressure vessels for large scale market applications has been studied in this work. The vessels consist on a thermoplastic liner wrapped with a filament winding glass fibre reinforced polymer matrix structure. A high density polyethylene (HDPE) was selected as liner and a thermosetting resin used as matrix in the glass reinforced filament wound laminate.     

All-poly(ethylene terephthalate) composites by film stacking of oriented tapes

Self-reinforced polymer or all-polymer composites have been developed to replace traditional fibre reinforced plastic (FRP) with good interfacial adhesion and enhanced recyclability. Poly(ethylene terephthalate) (PET) is one of the most attractive polymers to be used in these fully recyclable all-polymer composites, in terms of cost and properties. In this work, all-PET composites were prepared by film stacking of oriented PET tapes. A processing temperature window was determined by a series of tests on PET tapes and co-PET films, including DSC and T-peel tests. Tensile properties of PET tape, co-PET film and all-PET composites are reported and compared with a commercial co-extruded PURE^® polypropylene tape. The effect of compaction temperatures and pressures on tensile properties of all-PET composites was investigated to explore the optimum processing parameters for balancing good interfacial adhesion between tapes and residual tensile properties of PET tapes.     

Strength development versus process data in ultrasonic welding of thermoplastic composites with flat energy directors and its application to the definition of optimum processing parameters

Ultrasonic welding of thermoplastic composites is a very interesting joining technique as a result of good quality joints, very short welding times and the fact that no foreign material, e.g. a metal mesh, is required at the welding interface in any case. This paper describes one further advantage, the ability to relate weld strength to the welding process data, namely dissipated power and displacement of the sonotrode, in ultrasonic welding of thermoplastic composite parts with flat energy directors. This relationship, combined with displacement-controlled welding, allows for fast definition of optimum welding parameters which consistently result in high-strength welded joints.     

Investigation of damage mechanism of flax fibre LPET commingled composites by acoustic emission

Tensile failure and fracture behaviour of parallel laid twisted flax fibre reinforced low melting polyethylene terephthalate (LPET) composites were investigated. The tensile failure results of the model specimens were compared with AE results in terms of amplitude, energy and counts. The failure results of the flax fibre LPET composites exhibited mainly matrix crack initiation as a brittle failure for low, medium and high fibre contents. Since the composites at high fibre contents have higher porosity content, they show higher strain to failure, higher variation in the tensile results and have different appearances on their fracture surfaces than those of the composites at low and medium fibre contents.     

Enhancing the thermomechanical behaviour of poly(phenylene sulphide) based composites via incorporation of covalently grafted carbon nanotubes

The thermal and thermomechanical properties of poly(phenylene sulphide) (PPS) based nanocomposites incorporating a polymer derivative covalently anchored onto single-walled carbon nanotubes (SWCNTs) were investigated. The grafted fillers acted as nucleating agents, increasing the crystallization temperature and degree of crystallinity of the matrix. They also enhanced its thermal stability, flame retardancy, glass transition (T_g) and heat deflection temperatures while reduced the coefficient of thermal expansion at temperatures below T_g. A strong rise in the thermal conductivity, Young’s modulus and tensile strength was found with increasing filler loading both in the glassy and rubbery states. All these outstanding improvements are ascribed to strong matrix-filler interfacial interactions combined with a compatibilization effect that results in very homogeneous SWCNT dispersion. The results herein offer useful insights towards the development of engineering thermoplastic/CNT nanocomposites for high-temperature applications.     

Tensile properties of kenaf fiber and corn husk flour reinforced poly(lactic acid) hybrid bio-composites: Role of aspect ratio of natural fibers

Kenaf fiber and corn husk flour were used as reinforcement in a novel biodegradable hybrid bio-composite system. It was investigated how the aspect ratios of kenaf fibers measured before and after passing through extrusion process influence the mechanical properties and the improvement of predicted values obtained by the Halpin–Tsai equation. It was found that considering of the aspect ratio of reinforcement obtained after the extrusion process, the difference between theoretical and experimental values of the tensile modulus was not significant, indicating that the aspect ratio determined after extrusion did not influence the predicted values. Therefore it was pointed out that the initial values of aspect ratio determined before extrusion can be used directly. It was also found that a scale ratio between reinforcements of different aspect ratios may play a role as a controlling factor in optimizing the mechanical properties of a hybrid bio-composite.     

Influence of process parameters on the thermostamping of a [0/90]12 carbon/polyether ether ketone laminate

In this study, an experimental evaluation of the thermostamping process was made for a 3D part molded with a [0/90]₁₂ laminate composed of unidirectional carbon fiber/polyether ether ketone (CF/PEEK) plies. Using the Taguchi method, the effect of four operational parameters on the part thickness, interlaminar shear strength and the degree of crystallinity were investigated. These parameters are the preheating temperature in the oven, the mold temperature, the oven to mold transfer time and the stamping pressure. The results show that the mold temperature and stamping pressure have a significant effect on part consolidation. In addition, the interlaminar shear strength, measured at the base of the molded part, was higher for thinner parts compared to those having a greater thickness. These results were also confirmed by Differential Scanning Calorimetry analysis, which show that the degree of crystallinity is higher for thinner parts.     

Enhanced properties of lignin-based biodegradable polymer composites using injection moulding process

Composites from polybutylene succinate (PBS) and lignin-based natural material were fabricated using a melt mixing process. The effects of lignin material and polymeric methylene diphenyl diisocyanate (PMDI) compatibilizer on the properties of composites were investigated. Incorporation of 65% lignin material into PBS was achieved with an improvement in the tensile and flexural properties of composites. Incorporation of 1% PMDI in 50% lignin filled composites enhanced the tensile, flexural and impact strength simultaneously. Heat deflection temperature (HDT) of the virgin plastic also increased with lignin and PMDI incorporation. Improved interfacial adhesion was observed from SEM micrographs of the compatibilized composites.     

Reviewing some design issues for filament wound composite tubes

This article discusses important aspects of the design of composite tubes manufactured by filament winding. The work was divided into three parametric studies. The first study was conducted to determine the minimum length that can represent an infinite tube in hydrostatic testing. The second study was conducted in order to find the optimum wind angle of composite tubes subjected to internal pressure under different end conditions. The purpose of the last one was to study the influence of diameter and thickness on the failure pressure during tube burst tests. A progressive failure analysis was performed using ABAQUS software employing a damage model implemented by a user subroutine (UMAT). The models used were validated using experimental data obtained from tube burst tests in previous studies. The results provide a better understanding of the behavior of composite tubes under internal pressure thereby making possible to improve design practices used in industry.     

Investigation of sub-melt temperature bonding of carbon-fibre/PEEK in an automated laser tape placement process

Automated placement of thermoplastic-based composite tape is a highly non-isothermal process with temperature gradients often exceeding 1000 °C/s. Models for the process often assume that bonding ceases below the melting point, however the extreme cooling rates combined with high placement velocities can result in a highly amorphous polymer during consolidation. Bonding below the melting point is generally limited due to the presence of crystallites which impede reptation of the polymer molecules, however in the case of an amorphous state, autohesion should proceed. This paper investigates bonding of carbon-fibre(CF)/PEEK with experimental trials performed where sub-melting point temperatures occur at the nip point. The thermal history was recorded with thermocouples embedded in the substrate. Bond predictions are compared with experimental strengths determined by lap shear measurements. Bonding below the melting point was shown to occur, indicating the processing window could be wider than previously estimated by bonding models, particularly for higher placement rates.     

A displacement-detection based approach for process monitoring and processing window definition of resistance welding of thermoplastic composites

The evolution of weld displacement, or the thickness of welding stack, with welding time during resistance welding of thermoplastic composites was characterised, and based on this the possibility of using displacement data for process monitoring and processing window definition was investigated. Resistance welding of glass fabric reinforced polyetherimide using a metal mesh as the heating element was studied, and weld displacement was detected using a laser sensor. The effect of welding parameters on the displacement curve was studied. Welding defects, such as voids and squeeze flow, could be detected by monitoring the weld displacement. Fast definition of the welding processing window was found to be possible using displacement curves, and the predicted processing window showed good agreement with the processing window determined from mechanical tests.     

Induced forming modes in a pre-consolidated woven polypropylene composite during stretch forming process at room temperature: I. Experimental studies

In the current study, rectangular specimens of pre-consolidated woven Self-Reinforced Polypropylene (SRPP) possessing different fibre orientations and aspect ratios were stretch formed in an open die. Induced displacements were recorded by an in-situ 3D photogrammetric measurement system. Resultant principal strains were investigated to clarify the role of different deformation modes during stamp forming. The dependency of induced deformation modes to the specimens’ geometries was studied. A novel path/deformation dependent failure criterion was established to distinguish between safe and failed regions of SRPP in a stamping process and to elucidate the dependency between failure and induced forming modes in a woven composite. The experimental results highlighted the suitability of consolidated SRPP to be formed into complex doubly curved geometries by the stamp forming process at room temperature. It was found that required forming depths could be achieved if a proper combination of specimen size, boundary condition, and fibre orientation was selected.     

Influence of filament winding parameters on composite vessel quality and strength

An experimental design investigation of manufacturing and design variables that affect composite vessel quality, strength, and stiffness was conducted. Eight 20-in. cylinders (with one additional cylinder as a replicate) were manufactured and tested for hoop strength, hoop stiffness, fiber and void volume fraction distribution through thickness, residual stress, and interlaminar shear strength. Material and processing variables were divided into five categories: (a) resin, (b) fiber, (c) fabrication process, (d) design, and (e) equipment. Five variables were selected (from a list of 12) for study using a 1/4 fractional factorial design of experiment setup. The five variables were: (a) winding tension, (b) stacking sequence, (c) winding-tension gradient, (d) winding time, and (e) cut-versus-uncut helicals.

Statistical analysis of the data shows that the composite vessel strength was affected by the manufacturing and design variables. In general, it was found that composite strength was significantly affected by the laminate stacking sequence, winding tension, winding-tension gradient, winding time, and the interaction between winding-tension gradient and winding time. The mechanism that increased composite strength was related to the strong correlation between fiber volume in the composite and vessel strength. Cylinders with high fiber volume in the hoop layers tended to deliver high fiber strength.     

Manufacturing and testing of a CFRC sandwich cylinder with Kagome cores

Filament winding and twice co-curing processes were proposed to make a carbon fiber reinforced composite (CFRC) sandwich cylinder with Kagome cores. Axial compression was carried out to reveal the stiffness and load capacity of the fabricated sandwich cylinder. Compared with the stiffened cylinder with similar dimensions and mass, the sandwich cylinder is shown to be stiffer and stronger by several times. Restrained by the double skins, the buckling of lattice ribs is effectively suppressed, leading to the elimination of the dominant failure mode. Skin crippling and strength failure were observed in the testing to be the competing failure mechanisms of the lattice sandwich cylinder. The prediction based on the weakest link of these mechanisms only overestimates the testing load capacity by 30%. The novel sandwich structure shows the promise of lattice composites in upgrading the mechanical properties.     

A comparative study of the effect of different rigid fillers on the fracture and failure behavior of polypropylene based composites

Polypropylene (PP) composites with 5 wt% of different rigid particles (Al₂O₃ nanoparticles, SiO₂ nanoparticles, Clay (Cloisite 20A) nanoparticles or CaCO₃ microparticles) were obtained by melt mixing. Composites with different CaCO₃ content were also prepared. The effect of fillers, filler content and addition of maleic anhydride grafted PP (MAPP) on the composites fracture and failure behavior was investigated. For PP/CaCO₃ composites, an increasing trend of stiffness with filler loading was found while a decreasing trend of strength, ductility and fracture toughness was observed. The addition of MAPP was beneficial and detrimental to strength and ductility, respectively mainly as a result of improved interfacial adhesion. For the composites with 5 wt% of CaCO₃ or Al₂O₃, no significant changes in tensile properties were found due to the presence of agglomerated particles. However, the PP/CaCO₃ composite exhibited the best tensile behavior: the highest ductility while keeping the strength and stiffness of neat PP. In general, the composites with SiO₂ or Clay, on the other hand, displayed worse tensile strength and ductility. These behaviors could be probably related to the filler ability as nucleating agent. In addition, although the incorporation of MAPP led to improved filler dispersion, it was damaging to the material fracture behavior for the composites with CaCO₃, Al₂O₃ or Clay, as a result of a higher interfacial adhesion, the retardant effect of MAPP on PP nucleation and the lower molecular weight of the PP/MAPP blend. The PP/MAPP/SiO₂ composite, on the other hand, showed slightly increased toughness respect to the composite without MAPP due to the beneficial concomitant effects of the presence of some amount of the β crystalline phase of PP and the better filler dispersion promoted by the coupling agent which favor multiple crazing. From modeling of strength, the effect of MAPP on filler dispersion and interfacial adhesion in the PP/CaCO₃ composites was confirmed.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

Effects of raw materials and process parameters on the physical and mechanical properties of flat pressed WPC panels

For the production of WPC, the flat pressing technology can be considered as an alternative to common techniques, particularly when manufacturing large-dimensioned panels. The aim of the present study was to identify and analyze the main influencing parameters on sheets made of wood flour (WF) imbedded in a polypropylene matrix. Test panels were made to measure the effects of panel density, polymer melt flow rate (MFR), WF content, coupling agent, WF size and press temperature on water absorption (WA), thickness swelling (TS), internal bond strength, MOE and MOR. Density, WF content and MFR were identified as main influencing parameters. The increase of density typically leads to an improvement of properties. Lower WF contents cause reduced WA and TS, while the MOE seems to have a maximum at a WF level of 50-70%, depending on raw material used. As an explanation for improving properties when using a high-MFR polymer, a better distribution on the wood surface is hypothesized.     

Effect of seawater on compressive strength of concrete cylinders reinforced by non-adhesive wound hybrid polymer composites

Seawater absorption in concrete structures can be a serious problem in humid and marine environments. It was demonstrated that non-adhesive (separated by aluminum foil) filament wound tubes could better reinforce concrete cylinders. Furthermore, composites have ever proved to be resistant to detrimental environment. Therefore, in this study four kinds of non-adhesive composite materials are used to reinforce concrete cylinders, and then composite/concrete systems are subjected to six environmental conditions. The composites include hand-wrapped woven cloth glass/epoxy, filament wound glass/epoxy, glass-kevlar-glass hybrid, and glass-carbon-glass hybrid. These allow comparing reinforcement effect from various processing methods, fibers, and interface. The six environmental conditions consist of soaking in live or dead seawater, soaking before or after winding, and air aging before or after winding. The first and second conditions can verify the accuracy of the experiment in laboratory (dead seawater) without bothering to test near coast (live seawater). The third to sixth conditions can provide a good designing criterion regarding what the best sequence is for reinforcing seawater or air attacked concrete structure by composites.