Dynamic Responses of Composite Structures with Fluid-Structure Interaction

Young W. Kwon and Angela C. Owens

Naval Postgraduate School USA

1. Introduction

As composite materials have been used for marine structures such as boats, ships, offshore structures, etc., the effect of FSI should be understood. In particular, FSI effect is expected to be significant for polymer composites because the water density is very comparable to the composites’ densities. For example, the density of a carbon composite is approximately 50% greater than the water density. Sandwich composites consisting of very low densities of core materials have lower densities than that of water. As a result, the hydrodynamic mass associated with FSI would be very critical to composite structures under water. The growing use of composites in ship masts, superstructures, deck grates, piping, ducting, rudders, propellers, stacks, and various submarine structures requires extensive modeling and testing to help designers, builders and operators better understand composite response [Mouritz, et al., 2001].

These materials are subjected to a wide spectrum of loads during manufacturing and service life. Dynamic loadings, in particular, impact type event, represent a serious design concern for use of composite. Composite structures are more susceptible to impact damage than similar metallic structures which are more ductile in nature and can absorb typically large amounts of energy without failure. Furthermore, the damage in composites from impact can go undetected even when the mechanical properties may be drastically reduced from an impact. For these reasons, numerous experimental and analytical studies have been conducted to study the dynamic response of composites subjected to impact loading [Abrate, 1994; Aslan, et al., 2003; Kwon & Wojcik, 1998].

According to the review of past works, most of the research effort has been focused on low velocity impact damage, specifically, the damage predictions, and the evaluation and prediction of residual properties of damaged laminates. All of the research completed thus far has focused on damage in composites under impact loading in dry environments to support development of composites in aircraft structures.

As far as dynamic response of structures under water is concerned, a great deal amount of analytical and experimental studies have been conducted on the effect of fluid force on the natural frequencies, damping ratios and mode shapes of vibrating structures in contact with fluid. This is commonly known as the Fluid Structure Interaction (FSI) problem. FSI investigations have supported many problems in submarine signaling, offshore oil structure stability, and ship structure vibrations. Through these studies, many numerical and analytical methods have been developed in order to predict the added mass and the

resulting changes in natural frequency of a structure in contact with fluid. It has been determined and widely proven that the effect of fluid surrounding a structure decreases the natural frequency of a structure due to the increase in total kinetic energy of the vibrating structure and fluid from the addition of kinetic energy of the fluid. This effect can be interpreted as an added mass to the vibrating structure in the analysis of the dynamic response. Essentially as the structure vibrates, its mass is increased by the mass of the vibrating fluid with which it is in contact, consequently decreasing its natural frequency. Studies of fluid structure interaction and the added mass effect, also known as virtual mass effect, hydrodynamic mass, and hydroelastic vibration of structures, started with Lamb [Lamb, 1921] who calculated the first bending mode of a submerged circular plate. In response to a problem of submarine signaling, Lamb investigated the vibrations of a thin elastic circular plate in contact with water. In his investigation he discovered that the natural frequencies for structures in contact with fluid are lower than the frequencies in air, based on the assumption that the modes shapes are virtually the same in water as in a vacuum. The resonant frequency was determined using Rayleigh’s method. Lamb’s theortical results were verified experimentally [Powell & Roberts, 1923]. Much later, more research was conducted for measuing and calculating natural frequencies of free vibartions of beams and plates under water [Lindholm, et al., 1965; Fu & price, 1979; Kwak, 1996]. Another branch of FSI studies is underwater explosion. Both experimental and numerical studies have been conducted for metallic structures [Kwon & Fox, 1993; Kwon, et al., 1994; Kwon & McDermott, 2001]. On the other hand, a much limited studies were undertaken for composite structures subjected to underwater explosion [Rasmussen, 1992; Rousseau, 1993; Mouritz, 1995, 1996; McCoy & Sun, 1997; Gong & Lam, 1998; and Lam, et al., 2003].

As far as impact loading on composite structures under water is concerned, the author’s research team conducted the research for the first time, to our best knowledge [Kwon, 2009; Kwon & Kendall, 2009; Owen, et al., 2010]. This chapter presents both experimental work and numerical modeling and simulation of dynamics responses of composites subjected to impact loading as well as under water. The next section decsribes the fabrication of composite samples and the testing setup and procedure. Subsequently, experiental results are presented and discussed, followed by computational modeling and simulation of FSI to explain the experimental findings as well as to provide a series of parametric studies so that any important property or parameter can be identified in terms of the FSI effect. Conclusions are provided at the end.