In-plane Shear Response of a Flax Fiber-epoxy Resin Composite Subjected to Repeated Loading and Creep-recovery Cycles

The present paper is aimed at studying the in-plane shear response of a flax fiber - epoxy resin composite laminate. Rectangular specimens, with �45� laminate orientation with respect to loading direction were used for the experimental procedure. Tensile testing up to failure allowed to extract the shear strain-shear stress curve, which have shown a linear domain, up to approximately 25 MPa, where a shear modulus was calculated, of 1.67 GPa and a Poisson ratio of 0.7, value which is typical for off axis laminates. Strain measurement during these tests, using Digital Image Correlation, have shown that, at high stress levels, concentrators occur in the specimen in the region of failure. Repeated loading tests have shown that the material stiffens approximately 9% when increasing loading speed, leading to conclude that a viscoelastic component of the deformation is present during loading. Repeated creep-recovery tests showed that, for longer periods of time, viscoplastic deformations appear as well, with an exponential evolution with respect to the creep duration.

COMPREHENSIVE ASSESSMENT OF A NEW FLAX VARIETY "CHANCE" OF THE PSKOV SELECTION ACCORDING TO THE MAIN ECONOMICALLY VALUABLE CHARACTERISTICS

Целью исследований являлась комплексная оценка нового сорта льна-долгунца Шанс по основным хозяйственно ценным признакам и параметрам адаптивности в условиях Северо-Западного региона на основе конкурсного сортоиспытания, проводимого в 2016-2018 годах и производственной проверки в 2019 году. ГТК за период вегетации был равен 1,98 в 2016 г., в 2017 г. – 1,60 и в 2018 г. – 0,95. Как показали расчеты индекса среды, наиболее благоприятные условия для роста и развития льна-долгунца сложились в 2017 г. (Ij = +3,98), и худшие – в 2018 г. (Ij = -3,68). Сорт Шанс раннеспелый (период вегетации 71-85 дней), высокорослый (82-88 см), относительно устойчив к фузариозу (89-94%) и полеганию (4-5 баллов), отличается высоким содержанием волокна в соломе (всего 36,0-36,5%, в том числе длинного 27,3-28,1%). По содержанию всего волокна он превышал стандарт на 2,4% (абс.), а длинного – на 0,8% (абс.) В среднем за годы исследований урожайность льносоломы составила 52,1 ц/га, семян 6,0 ц/га, что незначительно превышало стандарт соответственно на 3,6 и 7,1%. По урожайности всего льноволокна (18,9 ц/га) он достоверно превысил стандарт на 1,9 ц/га при НСР0,95 или на 10,4%. По прочности волокна, которая измеряется по разрывной нагрузке, сорт Шанс в среднем за 2 года имел равный стандарту Восход показатель (10,5 кгс), а по гибкости превосходил последнего на 2,5 мм (50,5 мм). Сорт Шанс в сравнении со стандартом в большей степени адаптирован к условиям внешней среды. Он характеризуется более низкой вариабельностью урожайности волокна по годам исследований (Cv – 4,75%), более высокой стрессоустойчивостью (-5,3) и генетической гибкостью. Сорт Шанс пластичный (bi – 0,91), относится к сортам нейтрального типа, которые слабо отзываются на улучшение условий среды, но более эффективны на более низких агрофонах и в зонах рискованного земледелия, к которым относится и Псковская область. В производственных условиях урожайность льноволокна у нового сорта составила 22,5 ц/га, что на 2,0 ц или 12,1% больше стандарта (НСР0,95 – 0,92 ц/га). В настоящее время сорт находится в государственном сортоиспытании. The purpose of the research was a comprehensive assessment of a new variety of flax «Chance» according to the main economically valuable characteristics and adaptability parameters in the conditions of the North-Western region on the basis of a competitive variety testing conducted in 2016-2018 and a production inspection in 2019. The SCC for the growing season was equal to 1.98 in 2016, 1.60 in 2017 and 0.95 in 2018. As the calculations of the environment index showed, the most favorable conditions for the growth and development of flax were formed in 2017 (Ij = +3.98), and the worst-in 2018 (Ij = -3.68). The «Chance» variety is early-maturing (the growing season is 71-85 days), tall (82-88 cm), relatively resistant to fusarium (89-94%) and lodging (4-5 points), has a high fiber content in straw (only 36.0-36.5%, including 27.3- 28.1% long). In terms of the content of the entire fiber, it exceeded the standard by 2.4% (abs.), and the long fiber -by 0.8% (abs.). On average, over the years of research, the yield of flax straw was 52.1 c/ha, seeds 6.0 c/ha, which slightly exceeded the standard by 3.6 and 7.1%, respectively. In terms of the yield of all flax fiber (18.9 c/ha), it significantly exceeded the standard by 1.9 c/ha at NSR0.95 or by 10.4%. According to the strength of the fiber, which is measured by the breaking load, the «Chance» grade on average for 2 years had an indicator equal to the standard of the Sunrise (10.5 kgf), and in terms of flexibility it exceeded the latter by 2.5 mm (50.5 mm). The «Chance» variety is more adapted to the environmental conditions in comparison with the standard. It is characterized by a lower variability of fiber yield over the years of research (Cv-4.75), higher stress resistance (-5.3) and genetic flexibility. The plastic «Chance» variety (bi – 0.91) belongs to neutral-type varieties that respond poorly to improving environmental conditions, but are more effective at lower agricultural zones and in risky farming zones, which include the Pskov region. Under production conditions, the yield of flax fiber in the new variety was 22.5 c/ha, which is 2.0 c or 12.1% more than the standard (NSR0,95-0.92 c/ha). Currently, the variety is in the state variety testing.

Effectiveness of Sodium Acetate Treatment on the Mechanical Properties and Morphology of Natural Fiber-Reinforced Composites

This paper aims to investigate the ability of an eco-friendly and cheap treatment based on sodium acetate solutions to improve the mechanical properties of flax fiber-reinforced composites. Flax fibers were treated for 5 days (i.e., 120 h) at 25 °C with mildly alkaline solutions at 5%, 10% and 20% weight content of the sodium salt. Quasi-static tensile and flexural tests, Charpy impact tests and dynamical mechanical thermal (DMTA) tests were carried out to evaluate the mechanical properties of the resulting composites. Fourier transform infrared analysis (FTIR) was used to evaluate the chemical modification on the fibers surface due to the proposed treatment, whereas scanning electron microscope (SEM) and helium pycnometry were used to get useful information about the morphology of composites. It was found that the treatment with 5% solution of sodium acetate leads to the best mechanical performance and morphology of flax fiber-reinforced composites. SEM analysis confirmed these findings highlighting that composites reinforced with flax fibers treated in 5% sodium acetate solution show an improved morphology compared to the untreated ones. On the contrary, detrimental effects on the morphology as well as on the mechanical performance of composites were achieved by increasing the salt concentration of the treating solution.

Influence of the hydrodynamic degumming process on the quality of decorticated flax fibers

The study explored the impact of the hydrodynamic degumming process applied for decorticated monomorphic flax on fiber quality. The experiment was designed as the first stage of research leading to the development of a method for decorticated flax fiber elementarization and cottonization; in particular, effectively dividing the fiber bundles to ensure low linear density and reducing impurities in the content, to make the fibers suitable for cotton spinning systems. The degumming process of the decorticated fibers covered hydrodynamic disposal of the gluing substances, mainly pectins from the fibers, with use of a specially designed lab-scale Model Device for Physical Degumming of the Flax Fibers. The degummed fibers were tested for linear density, length, impurity content and chemical composition by thermogravimetric analysis combined with the analysis of evolved gases (Fourier transform infrared spectroscopy) and analysis of images of fiber cross-sections and longitudinal views from a scanning electron microscope. The study outcomes allowed us to determine the optimal parameters of the degumming process applied for decorticated flax fibers, in which the obtained fibers were of the highest quality. It was found that the optimal parameters of the process were a bath temperature of 30°C and a degumming process duration of 24 hours. These lab-scale process conditions were used in further work on the degumming process of flax fiber carried out on a semi-technical scale, followed by a mechanical cottonization of the fiber, at the final stage of the technological chain.