Akihiro Wada

 A new single layer breakwater armour unit has been developed by Samsung C&T Corp., Chisui Corp., et al over the past two years. This development has been empirically tested and verified by running a drop test, as well as performing structural analysis and physical model tests. The structural and hydraulic stability has also been thoroughly verified. In addition, favourable results in terms of economics, environment and constructability have been gathered. Given this excellence, this new single layer armour unit is expected to be used for overseas bidding projects and execution sites around the world including Japan.

In this study, partially debonded spherical particles in a particulate composite are analyzed by three-dimensional finite element method to investigate the effect of debondings on the composite stiffness, and the way to replace a debonded particle with an equivalent inclusion is examined. The variation in Young's modulus and Poisson's ratio of a composite with the increase of the debonded angle was evaluated for different particle arrangements and particle volume fractions, which in turn compared with the results derived from the equivalent inclusion method. Consequently, it was found that by replacing a debonded particle with an equivalent othotropic one, the macroscopic behavior of the damaged composite could be reproduced so long as the interaction between neighboring particles is negligible.

A temperature-insensitive damage evaluation parameter for composite laminates is proposed. Ultrasonic wave velocity is directly related to material stiffness, so that it is a quantitative parameter that gives information about the mechanical state of the material. However, ultrasonic velocities vary with temperature therefore environmental factors might cause non-negligible error in inspected results. In this work, the angular dependence of Lamb wave propagation, i.e., the acoustic anisotropy, is exclusively focused on and the change in the acoustic anisotropy is used as a damage evaluation parameter. It is experimentally confirmed that the acoustic anisotropy is significantly altered by matrix cracks formation although it is insensitive to temperature variation. It is also shown that the combination of different Lamb modes is a more promising method for predicting the effective stiffness of damaged laminates.

An analytical model for FRP crossply laminates subjected to a bending load is developed in the framework of continuum damage mechanics. Constitutive equations for a cracked ply are firstly formulated under an assumption of equivalence between a cracked ply and a work-softening material. After dividing a laminated beam into small beam elements, the derived constitutive equations for a cracked ply are introduced into the lamination theory to predict the damage evolution in the laminated beam with a cracked ply. Each beam element is analyzed independently to compute the stress state and curvature of the element, and they are reconnected with each other to predict the deflection of the overall laminate beam. Two kinds of crossply laminate beams having different cracked ply thickness were tested in bending, and corresponding theoretical predictions were made. The prediction for crack distribution as well as load-displacement relations were in good agreement with the experimental results for each laminate.

In loading process of FRP laminates, two kinds of nonlinearity are observed. One of them is concerned with damage, the other is not. The nonlinearity unrelated with damage is mostly due to the nonlinear property of lamina in shear. On the other hand, the nonlinearity related with damage is caused by matrix cracking evolution in each ply. In the previous works, we presented the theory for predicting a laminate stress-strain response in damage process on the basis of continuum damage mechanics. This paper extends the theory to include the damage unrelated nonlinearity using Hahn & Tsai nonlinear elasticity theory. Comparisons are made between the analytical predictions and the experimental results. The analytical method formulated in this paper can be used for general composite laminates under the plane stress condition.

In Part I of this study, the constitutive equation of a cracking layer was formulated introducing the new concept that a cracked lamina was replaced by an equivalent uniform work-softening layer. Part II formulates laminate constitutive equation in damage process using the results obtained in Part I and the laminate theory. In this formulation, influence of lamina stacking sequence is discussed through the magnification of an initial damage surface. It is shown that the damage evolution law in Part I can be used for general laminates even if the initial damage surface is enlarged, as long as cracking layer energy release rate is adopted as a cracking criterion. Present model is shown to reproduce experimental results with good accuracy. Finally, discussions about laminate stiffness reduction and stress path in damage process are given.

Matrix cracking evolution in composite laminates is formulated in the framework of the continuum damage mechanics. In this paper, the damage evolution of a cracking layer is exclusively focused on. We propose a new concept that a cracked lamina is replaced by an equivalent uniform worksoftening layer, and the analytical method of plasticity is introduced to describe the behavior of the work-softening layer. The most important element of this model is a definition of work-softening modulus which is concerned with the damage evolution law. The elastic analysis proposed by Hashin is shown to be available for formulation of the damage evolution law, and it is also shown that adoption of the cracking layer energy release rate as a cracking criterion leads to a reasonable damage evolution law. The model formulated in this paper can be used for general composite laminates.