Mateusz Koziol

Experimental study on the effect of stitch arrangement on mechanical performance of GFRP laminates manufactured on a basis of stitched preforms

This article presents the results of interlaminar shear and flexural tests of stitched polyester glass fiber laminates in dependence on stitch density and main geometric stitching parameters: stitch length and stitch spacing. Purpose of the study is to work out guidelines and indications for manufacturers of composite laminates who use or who plan to use stitching technique. It was found that stitching significantly improves interlaminar shear strength which increases with stitch density. However, stitching causes deterioration of in-plane flexural properties – the deterioration progresses when stitch density increases. Obtained results indicate that it is better to achieve increase in stitch density (resulting in improvement of interlaminar shear strength) by reduction of stitch length than by reduction of stitch spacing. Stitched laminate shows higher flexural strength and flexural modulus when bent into direction parallel to the stitch lines than when bent into the transverse direction. The results obtained within the study and their approximation constants may be a base for a new theoretical model simulating behavior of stitched laminate during static bending and enabling prediction of its mechanical performance.

Natural Fiber Textile Structures Suitable for Composite Materials

Natural fiber-reinforced composites have drawn attention of the research community mainly because of their economical and environmental attractiveness. So far, the shortened natural fibers were mainly used as the reinforcement, but textile products like yarn and fabrics can improve mechanical properties of composite in a significant way. The objective of the investigation is to develop a textile material suitable for composite reinforcement based on flax fibers. Pure flax fabric as well as hybrid fabrics containing flax fibers and chemical fibers: Kevlar, glass fibers, and poliimid have been developed in order to meet requirements related to mechanical properties of composites. The results of the mechanical tests confirmed suitability of the flax fabrics as a composite reinforcement. Due to strongly hydrophilic properties of pure flax, composites reinforced by the hybrid fabrics with polyaramide or glass fibers showed evidently better mechanical properties (even by about 80%). Chemical modification of flax fabric results in the improvement in adhesion between fibers and matrix. The results obtained within the work show that chemically treated flax fiber can serve as a valuable reinforcement for polymer matrix composite material ensuring mechanical properties comparable to glass fibers.

Effect of thread tension on mechanical performance of stitched glass fibre-reinforced polymer laminates –experimental study

This article presents the results of interlaminar shear and flexural tests of polyester resin – glass fibre laminates made from the preforms unstitched and stitched by various thread tensions: low, medium and high. The laminates were impregnated using resin transfer moulding method. The interlaminar shear and flexural tests were conducted by loading parallel and transverse to the stitch lines. Interlaminar shear strength, flexural strength and flexural modulus were obtained. Deformation extent of the reinforcing structure of the stitched laminate increases with the increase in the thread tension and causes decrease in the flexural strength and modulus. However, the increase in the thread tension offers improvement in the interlaminar shear strength. Optimization of changes in the flexural properties and the interlaminar shear strength with the thread tension showed that an operation of stitching of the reinforcing preforms should be conducted by low or medium stitching thread tension (approximately 3–9 N).

Repeatability of glass fiber reinforced polymer laminate panels manufactured by hand lay-up and vacuum-assisted resin infusion

The paper presents a comparison of the test results for the fiber volume fraction, static bending and Charpy impact strength of glass fiber reinforced polymer laminates reinforced by 0/90 fabric and chopped strand mat, produced by the hand lay-up and the vacuum-assisted resin infusion (VARI) method. The laminates were produced under equivalent conditions, with polyester matrix and lay-up areal mass 2100 g/m2. In the comparison of the obtained measurement results, similar mechanical performance was stated in the case of the hand lay-up and the VARI production. However, significantly smaller scatter of results and better uniformity of the reinforcement in the matrix with smaller amount of local structure defects was observed for the samples obtained by VARI method. The laminates obtained by VARI method show a much more advantageous coefficient of variation than that of the hand lay-up method, especially in the case of the mat reinforcement.

Influence of surface treatment on the wetting process of jute fibres with thermosetting polyester resin

Influence of surface treatment on the wetting process of jute fibres with thermosetting polyester resin The aim of this study is the evaluation of the effect of chemical treatment of jute fibres on their permeability with polyester resin and determination of optimal treatment parameters. Unmodified jute fabric by LENTEX, Poland and polyester resin ESTROMAL 14 were used in the experiments. NaOH and KOH water solutions (various concentration - from 1 to 15% - and treatment duration - from 0.5 to 6h), vinyl acetate, methanol, toluene diizocyanate were used for the treatment. Determination of the effect of the chemical treatment on fiber-resin interaction was made in: infiltration of resin into reinforcement structure tests and fabric absorbability tests. Three criteria: time of penetration of resin into the reinforcing fabric, fabric absorbability and the area of a part of the specimen cross-section penetrated with the resin were taken into account. The fabrics treated with 1-5% - long-time and 15% short-time NaOH and KOH water solutions showed the best results within the tested criteria. These results do not correspondent directly with the mechanical properties of cured composites manufactured on the base of the treated fabrics. Alkali- and organic treatments significantly improve the mechanical performance of natural fibres reinforced composites, but organic treatments (methanol, toluene diisocyanate) gives better improvement in mechanical properties in comparison with the alkali-treatments. The measurement methods applied in the study may be generally useful in the evaluation of the effect of chemical treatment of natural fibres on their wettability with the liquid matrix.

Evaluation of jute fiber preforms absorbability using optical profilographometer

The article presents an analysis of absorbability of polyester resin by jute fabric – untreated and treated in KOH water solutions (various exposure times), toluene diisocyanate, methanol, vinyl acetate and vinylsilane. A change with time in height of a resin droplet put on the fabric specimen was measured using optical profilographometer. The height of droplet was taken as a criterion of absorbability. Additional profilometric scanning observations, scanning electron microscopic observations and a saturation time in vacuum resin infusion tests were conducted in order to confirm and explain the results of the droplet height measurements. Although scanning electron microscopic analysis showed improvement in wetting between the treated fibers and the resin, the droplet height measurements showed that chemical treatment impairs translaminar absorbability of jute fabric. Deterioration of absorbability is caused by swelling of jute strands. However, technological tests showed that in most cases the treatment shortens the saturation time, which testifies an improvement in in-plane absorbability. Especially treatment with vinylsilane and KOH solution visibly shortened the preform impregnation time.

Influence of wet chemistry treatment on the mechanical performance of natural fibres

Influence of wet chemistry treatment on the mechanical performance of natural fibres The paper presents determination of the effect of various chemical treatment on the strength of 288 tex jute yarn arisen from the plain weave fabric produced by LENTEX, Poland. The yarn was put to alternative treatments in: NaOH and KOH water solutions with various concentration (from 1 to 15%) and treatment duration (from 0.5 to 6 hours), vinyl acetate, methanol and toluene diisocyanate. After the treatment it was put to tensile tests. Yarn diameter and elementary fibre twist angle were also measured using MICRO PROF FRT optical profilographometer. The SEM micro-photographs have also been performed in order to evaluate the structural changes of the yarn after the treatment. Optimal conditions of alcali-treatment are: 5% concentration and 2h duration for NaOH, 3% concentration and 4h duration for KOH. Such treatments give a growth in yarn rupture force up to 10% and they are well applicable in composite materials manufacturing. Also interaction with vinyl acetate and toluene diisocyanate has practically not negative influence on the mechanical performance of the yarn. Two effects were observed which can explain the influence of chemical treatment on mechanical performance of jute yarn: swelling and change in the orientation of elementary fibres.

Mechanical performance of GFRP laminates manufactured from deformed stitched and three-dimensional woven preforms

The aim of the work is an assessment of the effect of the deformation (curvature) of reinforcing fibre preform caused by insertion into a mould on the laminate properties. The basis for the assessment is a comparison with an analogous laminate without deformation, i.e., made in a flat mould. The work includes the production of laminate panels by way of resin transfer moulding, on the basis of three types of preforms: plain woven fabric (classic), stitched plain woven fabric and three-dimensional woven fabric, as well as an evaluation of their mechanical properties in static bending tests (in the radial direction of the curved panels) and tensile tests (in the axial direction of the curved panels). The state of the laminate reinforcement structure was assessed by the means of computer tomography. The strength of the curved laminates, both in the radial and in the axial directions, is lower than in case of the equivalent flat laminates. The strength decrease proceeds together with the deformation degree. The stitched and three-dimensional laminates are much less prone to strength loss resulting from deformation than the classic laminate; they also exhibit lesser strength drops in the direction transverse to the stitch lines or the interweave strands than in the parallel direction. The curved classic laminate shows the biggest deviations of the fibre strands from rectilinearity as well as local structure anomalies, in comparison to the two other laminate types. In the curved three-dimensional laminate, the fibre strands are much more uniformly oriented than in the other two.

Mechanical Performance of Polymer-Matrix Laminate Reinforced with 3D Fabric during Three-Point Impact Bending

The paper presents the analysis of course and the results of impact 3-point bending tests conducted with use of instrumented Charpy hammer (CEAST RESIL 50 hammer + MC101 registrator + CALKA J computer program) on GFRP laminate specimens cut of the panels manufactured by RTM method on base of classic plain-woven fabric preform and 3D fabric. The specimens of the 3D laminate were cut alternatively along (W direction) and transverse (P direction) to the translaminar interweave strands. It was found that maximum force obtained in the tests is comparable for both the classic and the 3D laminates. Deformability of the 3D (W) specimen is by about 20% higher than those of the classic laminate, whereas it is higher by even 75% for 3D (P). The trend of deformability observed for the tested laminates differs from known trends characteristic for static conditions what confirms different material response mechanisms at low and higher load rate. Failure energy in the classic and in the 3D (W) specimen is on comparable level. However, 3D (P) specimen showed slightly lower summary failure energy than the classic one and almost twice a high failure initiation energy (first effects of failure occur before maksimum load is gained).

Failure Progress of 3D Reinforced GFRP Laminate during Static Bending, Evaluated by Means of Acoustic Emission and Vibrations Analysis

The work aimed to assess the failure progress in a glass fiber-reinforced polymer laminate with a 3D-woven and (as a comparison) plain-woven reinforcement, during static bending, using acoustic emission signals. The innovative method of the separation of the signal coming from the fiber fracture and the one coming from the matrix fracture with the use of the acoustic event’s energy as a criterion was applied. The failure progress during static bending was alternatively analyzed by evaluation of the vibration signal. It gave a possibility to validate the results of the acoustic emission. Acoustic emission, as well as vibration signal analysis proved to be good and effective tools for the registration of failure effects in composite laminates. Vibration analysis is more complicated methodologically, yet it is more precise. The failure progress of the 3D laminate is “safer” and more beneficial than that of the plain-woven laminate. It exhibits less rapid load capacity drops and a higher fiber effort contribution at the moment of the main laminate failure.