Atsushi Yokoyama

A quasi-three-dimensional lateral compressive analysis method for a composite cylinder

This paper presents a lateral compressive analytical method for CFRP cylinders using the quasi-three-elements which represent fiber and resin respectively. Using this model, the behavior of a [θ/θ]sym (θ = 15, 30, 45, 60) CFRP cylinder subjected to the lateral compressive load were simulated, and compared with experimental results. The quasi-three-dimensional model is found to be effective for the prediction of the load-displacement curve. For the damage propagation, the transverse crack and interlaminar delamination could be simulated independently. Moreover, the critical strength law using this model was proposed.

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.

Damage tolerance of glass mat/epoxy laminates hybridized with flexible resin under static and impact loading

This study deals with the impact property and damage tolerance of matrix hybrid composite laminates with different laminate constitution. The matrix hybrid composite laminates consisted of the laminae with a conventional epoxy resin and the laminae with a flexible epoxy resin modified from the conventional resin to avoid the interlaminar delamination. The impact energy absorption ratio greatly depended on the matrix resin placed at the impact face. The energy absorption was almost constant if the conventional resin was placed at the impact surface layer, while it increased exponentially with the increasing fraction of the flexible resin if the flexible resin was placed at the impact face. The impact energy was absorbed by the damage development and propagation in the laminate with conventional resin laminae as the impacted face, while it was absorbed by both the recoverable deformation of the flexible resin and the damage propagation in the laminate with flexible resin laminae as the impacted face.

A simplified tensile damage analysis method for composite laminates using a quasi-three-dimensional model

Abstract Measurement of the strength of laminated composites is very difficult because their failure processes imply various failure modes, which are, for example, an interlaminar delamination, a destruction of matrix and an interfacial fracture between fiber and matrix. However, that strength is one of the most important characteristics in structural design using laminated composites. Hence we try a fractural progress analysis of laminated composites using a quasi-three-dimensional analysis method under a tensile load. The quasi-three-dimensional model is constructed of shell elements and beam elements which represent fiber and matrix respectively. The fractural progress analyses of the laminated composites are carried out to evaluate this proposed model. The precision is very good. Therefore we confirm that this proposed model can simulate a transverse crack and an interlaminar delamination.

A new numerical modeling for laminated composites

Recently, with increasing interest in the performance of fiber reinforced laminated composites, various behaviors of these materials have been simulated by the finite element method (FEM). However, conventional models are not good enough to simulate behaviors of laminated composites. The main reason is that the laminated composite is modeled by assuming it to be a homogeneous object though it has a heterogeneous nature. In this paper, the new numerical modeling for laminated composites was proposed. In order to check the validity of the proposed model, elementary simulations were performed and compared with theoretical results. Moreover the application of the proposed model to the laminated composites with interlaminar delamination was discussed.

Optimum design of weaving structure of 3-D woven fabric composites by using genetic algorithms

This paper discusses the optimum design method of the weaving structure of three-dimensional (3-D) reinforced composites. We propose the design method which combines the genetic algorithms (GA) and the finite element analysis. GA is one of the optimization techniques for the combinatorial optimization problem. In the finite element analysis, we used the original structure model which can express the fiber arrangement state in the 3-D composites faithfully. In this study, the original weaving structure model is constructed by combining the basic structure which has the fiber bundle and the cubic grid of resin. From analysis results, in the small design region, we can obtain the optimum weaving structure. Moreover, we proposed a new genetic operation, to design the weaving structure at the larger design region. These operations aim to prevent the failure of the partial weaving structure in the analytical model as much as possible. From the analysis results, the optimum weaving structure is obtained at the large design region, similar to above results. Consequently, it seems that the proposed method enables the design of the optimum weaving structure in the 3-D composites.

Strength analysis for three-dimensional fiber reinforced composites

Three-dimensional fiber reinforced composite materials produced by impregnating resin into woven fabric have superior interlaminar and impact strength and are capable of being formed into complex shapes. Consequently it is expected in the future that they will be used for various structural members which have to date been difficult to make with conventional composite materials. With the growth in their fabrication technoloy, the development of a strength analysis method is being demanded. This paper describes a strength analysis method for three-dimensional composite materials on the basis of a micro-mechanical analysis of a unit cell. The unit cell is a small geometrical unit of fiber architecture. A feature of the present analysis method is to represent a unit cell as a rigid frame structure constructed of fiber-beam elements and matrix-beam and matrix-rod elements. Strength analyses are made for orthogonal weave and 5-axial weave three-dimensional carben/epoxy composite materials; the tensile, compressive, and shear moduli and strengths, and Poissons ratio are calculated. The analytical results show fairly good agreement with experimental results; 11%, 21%, and 20% differences between them on the average for elastic moduli, strengths, and Poissons ratios, respectively. It is also understood that the present idealized analysis model cannot accurately predict the characteristics of undulated fiber composites, especially in respect to the compressive strength.

A quasi-three-dimensional elastic wave propagation analysis for laminated composites

Abstract Localized impact problems for composite structures have recently become important. In this study, some elastic wave velocities in 7-ply GFRP laminate with [02/903/02] ply orientation after low speed impact was investigated by using both experimental methods and finite element methods. For the finite element simulation, the quasi-three-dimensional model was used. Comparing the results, the validity for the application of this model to the dynamic problem was estimated. Moreover the quasi-three-dimensional model is applied to the GFRP plates with interlaminar delamination. The relationship between the elastic wave velocities and delaminated states is discussed.

Damage Mechanics on Hydrothermal-Aged Fiber-Reinforced Plastics (FRP)

The elastic modulus of randomly oriented, glass fiber mat reinforced plastics after hydrothermal aging was estimated introducing finite element analyses and the damage mechanics. The debonding between fiber and matrix of the hydrothermal aged fiber-reinforced plastics (FRPs) was dealt with as the internal damage, and the damage mechanics were introduced to the finite element analyses. The damage angle around the fiber greatly affected the modulus reduction of the aged FRPs independent of the damage thickness. The internal damage of the aged FRPs was determined quantitatively by the weight loss due to water immersion, and the modulus of the aged FRPs was calculated by the finite element analysis introducing the damage parameter. The calculated results corresponded well to the experimental results, and it was analytically clarified that the modulus reduction of the aged FRPs was caused by the occurrence of the debonding between fiber and matrix.

Assessment of the influence of interfacial properties on stress transfer in composites by a numerical approach

Abstract To investigate the effect of interfacial properties on stress transfer by a numerical approach, a finite element model has been used where the fiber, the matrix and the interphase were treated separately. When the interphase was assumed to be orthotropic, an influence of the interfacial shear stiffness on stress transfer was evaluated from the axial tensile stress transferred from the matrix to the fiber by passing through the interphase. The influence of fiber breakage and interfacial debonding or matrix cracking on the stress distribution has also been studied. When the shear stiffness of the interphase is greater than that of the matrix, a high stress concentration appears near the fiber break point. However this stress concentration disappears when the fiber breakage occurs together with interfacial debonding or matrix cracking. It was therefore suggested that the fiber fracture propagation mode might be changed by altering the shear stiffness of the interphase and by the existence of interfacial debonding or matrix cracking. Thus it was obvious that the proposed FE numerical approach and the concept of material constants of the composite interphase might be appropriate for investigating the influence of the interfacial properties on stress transfer.