Bijit Kumar Sarkar

The mechanical properties of vinylester resin matrix composites reinforced with alkali-treated jute fibres

Jute fibres were subjected to alkali treatment with 5% NaOH solution for 0, 2, 4, 6 and 8 h at 30°C. The modulus of the jute fibres improved by 12, 68 and 79% after 4, 6 and 8 h of treatment, respectively. The tenacity of the fibres improved by 46% after 6 and 8 h treatment and the % breaking strain was reduced by 23% after 8 h treatment. For 35% composites with 4 h-treated fibres, the flexural strength improved from 199.1 to 238.9 MPa by 20%, modulus improved from 11.89 to 14.69 GPa by 23% and laminar shear strength increased from 0.238 to 0.283 MPa by 19%. On plotting different values of slopes obtained from the rates of improvement of flexural strength and modulus, against NaOH treatment time, two different failure modes were apparent before and after 4 h of NaOH treatment. In the first region between 0 and 4 h, fibre pull out was predominant whereas in the second region between 6 and 8 h, transverse fracture occurred with minimum fibre pull out. This observation was well supported by the SEM investigations of the fracture surfaces.

Effect of alkali treated jute fibres on composite properties

Jute fibres were subjected to a 5% alkali (NaOH) solution treatment for 0, 2, 4, 6 and 8 h at 30°C. An improvement in the crystallinity in the jute fibres increased its modulus by 12%, 68% and 79% after 4, 6 and 8 h of treatment respectively. The tenacity of the fibres improved by 46% after 6 and 8 h treatment and the % breaking strain was reduced by 23% after 8 h treatment. For the 35% composites with 4 h treated fibres, the flexural strength improved from 199.1 MPa to 238.9 MPa by 20%, modulus improved from 11.89 GPa to 14.69 GPa by 23% and laminar shear strength increased from 0.238 MPa to 0.2834 MPa by 19%. On plotting the different values of slopes obtained from the rates of improvement of the flexural strength and modulus, against the NaOH treatment time, two different failure modes were apparent before and after 4 h of treatment. In the first region between 0 and 4 h, fibre pull out was predominant whereas in the second region between 6 and 8 h, transverse fracture occurred with a minimum fibre pull out. This observation was well supported by the SEM investigations of the fracture surfaces.

Impact fatigue of glass fibre–vinylester resin composites

Abstract An impact fatigue study has been made for the first time on 63.5% glass fibre reinforced vinylester resin, notched composites. The study was conducted using a pendulum type repeated impact apparatus specially designed and fabricated for determining single and repeated impact strengths. Well-defined impact fatigue ( S – N ) behaviour, having a progressive endurance below the threshold single cycle impact fracture strength, with a limit, has been demonstrated. Fractographic analysis revealed fracture by primary debonding, with fibre breakage and pull-out in the tensile zone, but a shear fracture of fibre bundles in the compressive zone of the specimen. Residual strength measured after impact fatigued showed retention of the property at high impact energy levels, up to about 10 2 impact cycles, thereafter a gradual drop to about 10 3 impact cycles followed by a rapid drop. The residual modulus and toughness showed a gradual drop with the increasing number of impacts endured. It is suggested that a few large cracks and an increased volume of microcracks in the matrix, with damaged fibres at high and low impact endurances, respectively, account for the failures of the composites under impact fatigue.

Effects of moisture on the mechanical properties of glass fibre reinforced vinylester resin composites

Glass fibre reinforced vinylester resin composites incorporating varying amounts of fibres (63.5, 55.75, 48.48, 38.63 and 27.48 wt%) were characterized for their mechanical properties both as prepared and after treatment with boiling water for 2, 4, 6, 8 and 24 h. Weights of the samples were found to increase to a saturation at about 8 h with boiling water treatment.In keeping with the composite principle, the mechanical properties improved with fibre loading. However, the properties were relatively inferior when treated with boiling water for longer hours attributing to ingress of moisture by capillary action through the interface between the fibre and the resin matrix. Considering the rates of moisture absorption and correlating with the mechanical properties, it was observed that the deteriorating effects were predominant up to 4 h treatment with boiling water. Estimation of defect concentrations for 63.5 wt% of nascent fibre reinforced composites as well as those composites treated with boiling water for 24 h were 56.93% and 64.16% respectively. Similarly, 27.48 wt% nascent fibre reinforced composites and those composites with boiling water treatment showed the estimation of defect concentrations of 39.94% and 50.55% respectively. SEM study of the fractured surfaces showed heavy fibre pull-out in the tensile zone whilst shear fracture of the fibre bundles was predominant at the compressive zone of the samples tested for flexural strength properties.

Estimation of composite strength by a modified rule of mixtures incorporating defects

Inadequacy of the rule of mixtures to estimate the true values of composite properties having a large concentration of defects has led to numerous tests being performed costing time and money. Statistical average of the properties from the Weibulls distribution law has thus been relied upon so far for the design of composite structures. Yet, to estimate the properties of a fibre reinforced composite material having sufficient flaw densities due to the methodologies adopted for its processing, more so for fibres, has long been appreciated. To avoid this inadequacy a modified rule of mixtures is developed incorporating defect concentration in the fibre and matrix enabling to arrive at a more realistic value of the composite.

Behaviour ofE-glass fibre reinforced vinylester resin composites under impact fatigue

An impact fatigue study has been made for the first time on 63.5% glass fibre reinforced vinylester resin notched composites. The study was conducted in a pendulum type repeated impact apparatus especially designed and fabricated for determining single and repeated impact strengths. A well-defined impact fatigue (S-N) behaviour, having a progressive endurance below the threshold single cycle impact fracture stress with decreasing applied stress has been demonstrated. Fractographic analysis revealed fracture by primary debonding having fibre breakage and pullout at the tensile zone, but a shear fracture of fibre bundles at the compressive zone of the specimen. The residual strength, modulus and toughness showed retention of the properties at high impact stress levels up to 1000 impacts followed by a sharp drop. Cumulative residual stresses with each number of impacts not withstanding the static fatigue failure at long endurances have been ascribed for the composite failures under the repeated impact stresses.

Impact fatigue behaviour of carbon fibre-reinforced vinylester resin composites

Two types of unidirectional carbon fibre, one of high strength (DHMS) and another of medium strength (VLMS) reinforced vinylester resin composites have been examined for their impact fatigue behaviour over 104 impact cycles for the first time. The study was conducted using a pendulum type repeated impact apparatus specially designed and constructed for the purpose. A well-defined impact fatigue behaviour (S-N type curve) curve has been demonstrated. It showed a plateau region of 10–102 cycles immediately below the single cycle impact strength, followed by progressive endurance with decreasing impact loads, culminating in an endurance limit at about 71% and 85% of the single impact strength for DHMS-48 and VLMS-48, respectively. Analysis of the fractured surfaces revealed primary debonding, fibre breakage and pull-out at the tensile zone of the samples and a shear mode of fracture with breakage of fibre bundles at the compressive zone of the samples. The occurrence of a few major macrocracks in the matrix with fibre breakage at the high load-low endurance region and development of multiple microcracks in the matrix, coalescing and fibre breakage at the low-load-high endurance region have been inferred to explain the fatigue behaviour of the composites examined.

Strength of sillimanite sand reinforced porcelain subjected to thermal shock

Abstract An attempt has been made to eliminate quartz and gradually replace felspar by sillimanite sand to improve the thermomechanical properties of porcelains. It has been observed that the quenching temperature difference (ΔT) required to initiate thermal stress fracture in the porcelain increased greatly, when compared to classical compositions, with gradual replacement of felspar by sillimanite sand. Similar behaviour has also been observed in linear thermal expansion measurements. It was concluded that improvement in thermal shock resistance of the material was due to the absence of quartz and the presence of sillimanite — a volume-stable mineral — in addition to the decreased amount of glassy phase.

Phase analysis and role of microstructure in the development of high strength porcelains

Abstract The flexural strength improved from 154 to 196 MPa at 1450 °C when the alumina:cordierite ratio was changed from 0 to 4.2, respectively, in a china clay-sillimanite mix. XRD and microscopic analysis of the phases that developed with increasing temperature revealed a microstructure of euhedral grains of sillimanite interspersed with fine mullite needles and a dense glassy phase wetting the crystalline mosaic structure. These contributed to the improved strength.