和田 明浩

Nowadays, fiber reinforced plastic (FRP) has been widely used in many areas such as auto mobile, airplane and marine vessel due to its high specific strength, good corrosion resistance, relatively low cost and so on. However, it still remains unknown that what kind of damage will happen in the internal structure when an automobile, which is made from FRP, has a slight impact with something such as a wall. Then the following road safety of the automobile cannot be guaranteed because certain parts may be exposed to damage in what seems even like a slight impact. In addition, it is well known that initial fracture can bring damage and great effect to the mechanical properties of the FRP material. The novelty of this paper is that the object of this research is micro crack such as transverse crack. While, almost previous report is aimed at delamination. Actually, before the delamination happens, micro crack has already occurred. The mechanical property of FRP is beginning to decrease by delamination. However, when the delamination occurred in the FRP is examined, it is already too late because the delamination can bring great influence to the safety of the FRP products. Therefore, it is important to investigate and detect the presence of micro crack with ultrasonic wave. In this way, some accidents might be avoided. While, because of the variety of the constraints in the fracture mechanism, the damage behavior is very difficult to evaluate and there are rarely researches and data on it. Although damage assessment by visual observation and the durable service life on FRP has become a general tendency in these recent years, the appropriate way of non-destructive examination has not been confirmed yet. The purpose of this study is to investigate the possibility of non-destructive examination with ultrasonic wave testing for mechanical damage of glass fiber reinforced plastics (GFRP). The possibility of dividing of Lamb wave modes by reducing the thickness of samples was confirmed and the variance of distribution of frequency of So mode wave by micro fracture in GFRP.

近年のインフラ整備における合理的設計や工期短縮，工事費節減などの社会要請の高まりは消波構造物の分野も例外ではない．日本国内では設計条件により2層積被覆工法が採用となるケースがあるが，海外では経済性に優れた１層積被覆工法が主流である．本研究では2層積被覆工法に準じる1層積消波ブロックの研究開発をSamsung C&Tと共同で試みた．基本形状は現存する消波ブロック特性を把握した上で方向性を見極め，FEM解析と落下試験により構造強度を高める検証を重ねた．また水理模型実験により安定性，機能性を照査し，施工性・経済性・自然共生機能についても検証を重ねた結果，海外で広く一般的に普及している１層積消波ブロックに準じる新型１層積消波ブロックが完成した．

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.

An analytical model to predict the constitutive behavior of laminated composites with cracking layers is developed in the framework of continuum damage mechanics. A new concept that a cracked lamina can be replaced with: an equivalent uniform work-softening layer is proposed. With this new concept, the constitutive equations for a cracking layer are constructed according to the modern plasticity theory. A lamina damage surface, a threshold of crack evolution, is defined in the stress space, and the constitutive equations for a cracking layer are constructed along with the associated flow rule. Furthermore, the derived constitutive equations for a Cracking layer are applied to the classical lamination theory to describe laminate constitutive behavior in the cracking process The validity of the constitutive model formulation is also verified comparing the analytical predictions to experimental results for some laminate configurations.

An analytical model to predict the constitutive behavior of laminated composites with cracking layers is developed in the framework of continuum damage mechanics. A new concept that a cracked lamina can be replaced with an equivalent uniform work-softening layer is proposed. With this new concept, the constitutive equations for a cracking layer are constructed according to the modem plasticity theory. A lamina damage surface, a threshold of crack evolution, is defined in the stress space, and the constitutive equations for a cracking layer are constructed along with the associated flow rule. Furthermore, the derived constitutive equations for a cracking layer are applied to the classical lamination theory to describe laminate constitutive behavior in cracking process. The validity of the constitutive model formulation is also verified comparing the analytical predictions to experimental results for some laminate configurations.

A damage mechanics model to predict the nonlinear behavior of laminated composites due to crack evolution is developed. We propose a new concept that a cracking layer can be replaced with an equivalent uniform work-softening layer. With this new concept, the constitutive equations for a cracking layer are constructed according to modern plasticity theory. A lamina damage surface, i.e. a threshold of crack evolution, is defined in the stress space of each lamina, and the constitutive equations for a cracking layer are constructed by applying the defined damage surface to the associated flow rule. Then, the derived constitutive equations for a cracking layer are introduced with classical lamination theory to predict laminate constitutive behavior in the damage process. Comparisons are made between the predictions and experimental results for some laminate configurations, and reasonable agreements are shown for all laminates.

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.