Amit Rawal

Geometrical modelling and control of a triaxial braiding machine for producing 3D preforms

Abstract Braiding is a relatively less explored textile process for producing composite preforms. Biaxial braids can be produced as hoses and subsequently be draped over different three-dimensional surfaces. However, triaxial braids are relatively stable structures and should be produced to the desired shape during the braiding process. This is achieved by over-braiding on mandrels that either form part of the finished composite or removed before the moulding process. Triaxial braided composites have superior mechanical properties due to fibre orientations along three directions. Geometry of a braided structure depends on the number of yarn carriers, rotational speed of the carriers, take-up speed and the effective perimeter of the cross-section of a mandrel. In the present work, a VRML based geometrical visualisation tool has been developed to simulate a braid structure on any predefined mandrel geometry, and using a predefined yarn cross-section. Braid angles, cover factors and yarn volume fractions can be computed from these simulations. A triaxial braiding machine has been developed with an independent servo control of the carrier movement and the take-up mechanism; geometrical simulation is used as an input to the control system to continuously vary the braid structure along the length of a mandrel. Flexible tooling is important for rapid product development. A flexible mandrel has been developed that can be mechanically adjusted to change the cross-section and the taper. This system enables rapid development of braided preforms.

Geometrical Modeling of the Yarn Paths in Three-dimensional Braided Structures:

A circular braiding process is adapted to produce three-dimensional shapes by braiding over a contoured mandrel. The present work deals with the simulation of various braided structures. The simulated structures are based on geometrical models developed for yarns on different shapes of mandrels. The geometry of a braided structure depends upon the machine parameters, such as number of yarn carriers, rotational speed of the carriers, take-up speed, and the effective perimeter of the mandrel cross section. Virtual reality modeling language (VRML) has been employed as a visualization tool to simulate these geometrical models on a predefined mandrel geometry. Models relate to the machine speeds and a number of machine settings. The path of the yarn on the mandrel is dependent upon the shape of the mandrel, i.e., if it is circular then it will be in the form of a helix or if it is a cubical body then it will be in the form of straight lines. An expert system has been generated to take into account all the parameters interfacing with VRML to simulate the braid geometry. A comparison is also made between the experimental and simulated structures.

Mechanical Behavior of Thru-air Bonded Nonwoven Structures

Thermally bonded structures are amongst the most widely used nonwoven structures, with applications ranging from baby diapers to high performance geotextiles. These structures undergo various modes of deformation during their performance. In this study, the mechanical properties namely, tension, shear, bending and compression of thermal bonded polyester fabrics, have been investigated, including directional dependence of the tensile and bending tests, revealing anisotropic characteristics. In addition, the fiber orientation distribution was determined using two-dimensional (2D) image analysis of fabric cross-sections, a technique that is generally used for measuring fiber orientation in short-fiber reinforced composites. The ambiguities, errors and corrections employed in the measurements of fiber orientation are discussed. The initial tensile response of thermal bonded nonwovens has been predicted and compared with the experimental stress-strain curves obtained in various test directions. Bending and compression measurements were done on KES-F and shear measurements were carried out using picture frame test up to a shear angle of 45°.

Tensile mechanics of braided sutures

Sutures are the materials primarily used for closing wounds, and their performance is significantly dependent on their mechanical characteristics. Thus, their tensile property is a key parameter responsible for the success of a suture. In this paper, a simple analytical tensile model of braided sutures has been developed based on braid geometry, braid kinematics, and constituent monofilament properties. The model has accounted for the changes in the braid geometry, including braid angle, diameter, and Poisson’s ratio. The kinematics of the braided suture is analyzed pertaining to monofilament locking or jamming in the braid. The model of jamming state of monofilaments has been presented, and both braid angle and diameter are found to be critical design parameters. The experimental results have been compared to the theoretical stress–strain curves of braided sutures, and an excellent agreement has been observed between them.

Structural analysis of pore size distribution of nonwovens

Pore size distribution is a prerequisite to investigate any transport phenomena, especially in a porous structure such as nonwovens. The pores inside the nonwovens are highly complex in terms of the sizes, shapes and the capillary geometries. The majority of existing theories/models of pore size distribution of nonwovens do not account for the fibre orientation distribution characteristics. In this research work, the model for predicting the pore size distribution of nonwoven structures has been developed by combining the stochastic and stereological or geometrical probability approaches. These techniques have incorporated the effects of fibre orientation characteristics in nonwoven structures. The analytical model formulated is compared with the existing theories to predict the pore size distribution of nonwoven structures. A comparison is also made between the experimental and theoretical pore size distributions of spun-bonded and needle-punched nonwovens. The effect of various fibre and fabric parameters including fibre volume fraction, fibre orientation distribution characteristics and number of layers on pore size distribution of nonwoven structures has been investigated.

A Modified Micromechanical Model for the Prediction of Tensile Behavior of Nonwoven Structures

Nonwoven structures have a range of applications which include geotextiles, interlinings, carpet backing and as reinforcements in composites. In this study, the tensile behavior of nonwoven structures including needlepunched and thermal bonded has been predicted by modifying the micromechanical model based upon the geometrical and mechanical properties of the constituent fibers (Adanur, S. and Liao, T. (1999). Fiber Arrangements Characteristics and Their Effects on Nonwoven Tensile Behavior, Textile Research Journal, 69(11): 816-824). In the modified model, the effects of fiber orientation in addition to the fiber curl factor are incorporated. The theoretical models have been validated with experimental results. The effects of lapping angle and curl factor have also been investigated on the thermal bonded nonwoven structures. Furthermore, the distribution of fiber orientation within a nonwoven structure is related to the lapping and fiber orientation angles using Fourier series.

Prediction of Yarn Paths in Braided Structures Formed on a Square Pyramid

A circular braiding process is used for producing three-dimensional (3D) textile preforms using 3D mandrels. In our previous study, we have predicted and simulated the yarns paths on various 3D mandrels, including cylinder, square prism, and cones with circular and elliptical cross-sections (Rawal, A., Potluri, P. and Steele, C. (2005). Geometrical Modeling of the Yarn Paths in Three-Dimensional Branded Structure, Journal of Industrial Textiles, 34(2): 115–135.). This study predicts the yarn paths on a square pyramid in which the yarns are modeled as straight lines varying in length from the top to the bottom of the square pyramid. The 3D coordinates of the yarn paths on a square pyramid have been mathematically related to the braid angle.

Relationship between Process Parameters and Properties of Multifunctional Needlepunched Geotextiles

Geotextiles are commonly produced by needlepunching technology and generally used for various civil engineering applications. Some of these applications require geotextiles to perform more than one function including separation, drainage, and filtration. In this study, the effect of process parameters, namely, feed rate, stroke frequency, and depth of needle penetration has been investigated on various properties of needlepunched geotextiles. These process parameters are then empirically related with the properties of geotextiles. Subsequently, an expert system has been developed to predict the properties of geotextiles for any desired application.

Geotextiles: production, properties and performance

The monograph critically reviews most commonly used geotextile structures, their properties and performance characteristics. In general, both natural and synthetic fibres are used for the production of geotextiles, and the advantages and disadvantages of each type of fibre are discussed for various applications of geotextiles. The important functions of geotextiles, i.e. filtration, drainage, separation and reinforcement have been identified and have been related to several properties and major applications of geotextiles. Various geotextile properties, namely mechanical, hydraulic and chemical and their test methods have been critically discussed. A process–structure–property relationship for most commonly used geotextiles is also analysed. Furthermore, the design of a geotextile is of paramount importance for any civil engineering application. Thus, the design criteria for various functions of geotextiles have been addressed. Subsequently, the durability characteristics of geotextile have been introduced for analysing the performance over its lifetime.

Optimization of Parameters for the Production of Needlepunched Nonwoven Geotextiles

Needlepunched nonwoven geotextiles are widely used for various civil engineering applications. These applications are required to perform more than one function, i.e., filtration, separation, protection, drainage, and reinforcement. Reinforcement is a complex phenomenon and strongly depends upon the fabrics dimensional and mechanical properties in addition to the soil—geotextile interaction. In this study, the effect of process parameters including web area density, punch density, and depth of needle penetration has been investigated on dimensional (area density and thickness) and mechanical (puncture resistance and tensile strengths in the machine and cross-machine directions) properties of needlepunched nonwoven geotextiles. These process parameters are then empirically related with the fabric properties using multiple regression technique. The anisotropic characteristics of needlepunched nonwoven geotextiles tensile properties have also been discussed.