Asami Nakai

Initial fracture behavior of satin woven fabric composites

Abstract Final fractures of composites is considered to be caused by cumulation of the microfractures, so that, the initiation of microfracture, namely, initial fracture is important factor to know the mechanical properties. Microfracture behaviors in textile composites were regarded to be decided by the geometry of textile fabric quantitatively. In this study, initial fracture in plain and satin woven fabric composites was investigated and the effect of weaving structure on initial fracture behavior was clarified. First, in order to investigate the geometry of textile fabric, crimp ratio and aspect ratio were measured. Tensile testing was performed and knee point on the stress–strain curve was identified. Fracture process of composites was observed by replica method. Initial fracture in plain woven fabric composite was confirmed as transverse crack in weft fiber bundle, on the other hand, in satin woven fabric composites both transverse crack and filament fracture at the same time was observed. The effects of changes in crimp ratio and aspect ratio on the initial fracture of woven fabric composites were discussed.

Mechanical properties and micro-fracture behaviors of flat braided composites with a circular hole

In this study, attention is paid to braided fabrics as the form of reinforcing fiber in composite materials and the influence of a circular hole on the mechanical properties of a flat braided composite is investigated. Two types of specimens were prepared: a flat braided bar with a braided hole which fiber bundle continuously oriented around the hole and with a machined hole. From the results of static tensile test, the strength of the flat bar with a braided hole was higher than that of the one with the machined hole. Furthermore, difference of damage propagation in the two types of flat braided composites with a circular hole was confirmed by micro-damage observation and factors that caused initial micro-fracture obtained by experiments were identified by numerical approaches.

Axial crushing performance of braided composite tubes

Abstract In this study, the energy absorption characteristics of braided composite tubes with different fiber architectures were investigated. It is clarified that the main fracture mechanism of braided composite tubes under progressive crushing was the central crack dominant. Moreover, the prediction of the mean crush load under progressive crushing was carried out by applying fracture mechanics. Es value could be predicted within 5 % error.

Design methodology for a braided cylinder

In this study, the design methodology for a braided cylinder was presented. The concept of an analytical model which involved both micro model and macro model was proposed. The analytical method was applied to estimate the rigidity and elastic limit of a braided cylinder subjected to bending and torsional load independently and as combined load. Using the analysis method, the design of a braided cylinder would be carried out in consideration of the factors that decide the mechanical properties of braided cylinder. This method has the possibility to be useful for structural design of braided composites and can be included as a unit in CAE systems for braided composites.

Prediction Method for Temporal Change in Fiber Orientation on Cylindrical Braided Preforms

An advantage of braided fabrics is that the fiber bundle orientation angle, called the “braiding angle,” can be changed. Because the braiding angle affects the fabric’s mechanical properties, changing the angle is an important means of adjusting the stiffness distribution as required. However, when the braiding angle is changing from an initial braiding angle to a targeted braiding angle designated by the longitudinal velocity of the mandrel, some delay occurs before the actual braiding angle reaches the targeted braiding angle. Previously, several models have been developed in order to predict braiding angles, although a prediction method for a temporal change in braiding angle caused by the mandrel velocity change has not been presented. In order to obtain the temporal change in braiding angle under unsteady-state conditions, this paper presents a step response model in braiding angle on a cylindrical braided fabric. Furthermore, the method is verified with the experimental data. As a consequence, the model has proved effective for predicting fiber orientation on a cylindrical braided preform under unsteady-state conditions.

Strain monitoring of braided composites by using embedded fiber-optic strain sensors

Recently, fiber optic strain sensors have been applied to internal strain and damage monitoring of composites because of their small size, light weight and flexibility. Braided fiber reinforced plastics (FRP) are compatible with fiber optic sensors because optical fibers can be integrated directly and easily into fabrics. In the present paper, the strain monitoring of braided glass fiber reinforced plastics (GFRP) was conducted by using embedded fiber Bragg grating (FBG) and extrinsic Fabry–Perot interferometric (EFPI) sensors during the cure process, tensile tests and fatigue tests. From the experimental results of cure monitoring, it was found that both sensors can be used only for monitoring of thermal residual strain during cooling process. From the results of tensile tests, it was found that both sensors could measure strain correctly until damage initiation of braided GFRP. It also appeared that FBG sensors could monitor damage to FRP by observing the reflected spectral shape. From the fatigue tests, it appeared that the strain measured by embedded FBG sensors was affected by fatigue damage. Therefore, it is concluded that internal strain monitoring of braided FRP using fiber optic strain sensors is very useful for cure and health monitoring.

Energy-absorption capability of multi-axial warp-knitted FRP tubes

With fibre-reinforced plastics (FRPs) being adopted widely in many fields, low manufacturing cost with high mechanical property is urgently required. Unfortunately, traditional composite structures and manufacturing methods are very labour-intensive and not cost-effective for wide commercial applications. Therefore, multi-axial warp knitting (MWK) is attractive as a new reinforcement form in composite structures because it incorporates textile structural property and the automated pultrusion process. Additionally, this fabric, made by connecting several fibre layers by stitching yarns, is considered to enhance the property through the thickness, which also raises questions as to its effect on energy absorption. In this study, several forms of glass and carbon MWK FRP tubes fabricated by pultrusion process, which have circular or square cross-section geometries, were axially crushed by quasi-static, high compressive speed or impact crash tests to determine the energy-absorption capabilities. It is found that the energy-management capacities of FRP tubes involved in different MWK fabrics are different both in the typical load/displacement response and in terms of specific energy absorption. The cross sections were observed microscopically to clarify the effect of the constitution and the position of MWK fabric.

A method to improve the energy absorption capability of fibre-reinforced composite tubes

Till now, most of the existing works on energy absorption have been focused on the probability of using composite materials instead of metals. With the answer to this problem becoming more and more clear, researchers are altering the emphasis of their studies to another issue, i.e. applicability of the composite materials. Therefore, some experiments were elaborated on this aspect. An inner type jig, which restricts crushed tube wall to be bent towards only the inner side of the tube, is employed as a new way to improve the energy absorption capability of fibre-reinforced polymer tubes which are assumed to be an energy absorption component applied in a vehicle. Quasi-static and high-speed compressions were carried out on the unidirectional carbon square tubes to test the viability of the jig.

Effect of the braiding angle on the energy absorption properties of a hybrid braided FRP tube

Abstract Energy absorption is achieved by the combination of various fracture mechanisms such as fibre fracture, delamination, and central crack. However, serious problems would arise if this energy absorption ability were compromised by brittle crack propagation of the cross-sectional central part. In a previous study, the use of flexible resin with lower stiffness and higher toughness than the resin generally used was suggested as a method to restrain brittle crack propagation. In this study, hybrid braided fibre reinforced plastic (FRP) tubes were fabricated according to the previous study involving FRP rods. In this case, the flexible resin was applied to middle-end-fibre. The energy absorption characteristics and crushing mechanisms based on precise cross-sectional observation of the crush zone of the braided FRP tubes with or without the presence of flexible resin in middle-end-fibre were investigated. It was found that braided FRP tubes with or without the presence of flexible resin in middle-end-fibre were investigated. It was found that braided FRP tube with a 30° braiding angle, together with the presence of flexible resins, shows significant improvement in terms of energy absorption ability. The added flexibility of the tubes owing to the addition of flexible resin in turn causes short cracks, more fibre breakage, and consequently enhanced energy absorption properties.

The crushing performance of a braided I-beam

Braided composite beams with an I-shaped cross-section, i.e. braided I-beams, possess high bending stiffness because fibers constituting the braiding structure are continuously oriented. Axial compression loading of a braided I-beam leads to progressive crushing when a chamfer is cut on one end. This configuration results in high energy-absorption performance. It is therefore possible to retain the space of a vehicle compartment in frontal and lateral collisions so that, in the automotive industry, it is useful for the side members of vehicles. To examine the energy-absorption mechanism of braided I-beams, observations have been made of the crush zone of a partially crushed I-beam. The crushing mechanism of a braided I-beam is similar to that of a composite cylinder in which reinforcing fibers are oriented in axial and hoop directions.