Fibre reinforced polymer (FRP) composites are in high demand for manufacturing structural components of automotive, aerospace, marine and wind energy sector. Despite high investment and manufacturing cost as compared to metallic alloys, the rapid growing interest in the use of FRPs has arisen due to promising properties, such as high strength to weight ratio, high specific stiffness, and excellent resistance to fatigue, creep, rapture and corrosion [1][2][3]. Additionally, the shift towards FRPs as lighter structural material is unavoidable to meet environmental targets of 75% and 90% reduction in the levels of CO2 and NOx by 2050 for which the contribution of airframes is evaluated from 20 to 25% [4]. Conventional composite laminates (2D) are very efficient in distributing in-plane normal and shear loads since the alignment of fibres is along the load-bearing path. However, the lack of through the thickness reinforcing fibres is a disadvantage in terms of delamination toughness and impact damage resistance. Therefore, since the past four decades there has been a growing interest in the development of several methods in order to provide through-thickness strength and stiffness to preforms in the three-dimensional (3D) direction using through the thickness reinforcement techniques [5][6][7][8][9].
Among such developed methods, weaving, kitting, braiding were primarily used for manufacturing three-dimensional preforms. However, these adopted processes from textile industry could not be used for a long period due to lack of flexibility of semi-finished products, costly and complex machinery, and inflexible machine parameters. Furthermore, the choice of adopting a particular process for fabricating 3D preform depends on the end use of product [10][11][12][13][14][15][16][17]. Although Mouritz presented large data base on the in-plane and out of plane mechanical properties of stitched 3D composites manufactured using various textile technologies, yet it is very difficult to conclude the best textile technology to reinforce through the thickness due to various technical, economic and certification issues [18]. In this scenario, stitching has been assessed as a potential technique for sewing through the laminate structure using a high tensile strength yarn, such as glass, carbon and kevlar [19]. Three-dimensional composite structures manufactured using stitched laminates have received considerable attention in high tech industry. Aircraft structure including large parts of airframe, wing panels, fuselages and blade-stiffened components are built using 3D composite materials [10]. It is also used in load bearing structure such as I-beam. In automotive industry it is used for manufacturing doorframe, bumper bar and floor panels [20].
Unfortunately, the manufacturing complexity of accessing the needle from downside to form the stitch and the resulting tension due to formation of loops and knots (chain stich and lockstitch) may degrade the in-plane mechanical properties especially bending and compression due to constrictions and undulations formed in the laminates [21][22]. However, a variety of possible stitching parameters, such as type of needle, stitching material, stitching density, stitching geometry, stitching method, type of stitching, type of laminate, machine parameters, and thread tension makes it a highly flexible technology. Hence, keeping the pros of stitching technology in view, an extensive R&D is pursued to address the drawbacks of this technology to achieve desirable three-dimensional properties of a material without degrading the exceptional mechanical in-place properties.
In this regard, DLR Institute of Structural Mechanics at German Space Centre in Brunswick developed “Single-Sided Tufting Method” as the potential method to meet structural and cost effective efficiency requirements. Tufting was developed as the befitting structural stitching means to bring significant improvement in the energy absorption behaviours particularly the energy release rate, crack propagation, and damage tolerance[23]. In contrast to conventional stitching techniques that employ a dual threading system, “tufting” is a single thread method that makes tension free insertion of the thread needle in dry fabric laminates and removal of the needle from fabric laminates along the same trajectory [24][25]. In this process, loose and tension free loops form on the downward side without adversely affecting the laminates material. Furthermore, tufting is beneficial as compared to conventional stitching methods because the material to be sewn needs to be only accessible from one side therefore there would be no need of complex and costly construction with lift tables [26][27].
It was realised by the authors after making a thorough literature review that there are few results related to the degree to which through-thickness reinforcement degrades in-plane mechanical properties of tufted fibre reinforced composites [1][28][29][30][31] [32][33][34] [35][36][37], and even fewer related to in-plane mechanical properties of tufted green biocomposites. Natural fibre based green biocomposites are the material of future in structural, automotive and aerospace industry due to due to their environmental merits, low density, high specific strength, stiffness, low energy consumption in fabrication, CO2 neutrality and sound proofing characteristics [38]. Since a degradation in the in-plane properties may arise due to local damage to fibres resulting from tufting needle, fibre kink and resin rich areas, this work presents the effect of tufting on the in-plane mechanical properties like tension, shear and compressive strengths of green biocomposites based on flax fibre reinforced with glass tufts.