Due to the complex nature of such composite structures, an understanding of the guided wave propagation mechanism in honeycomb composite panels with different frequencies inherently imposes many challenges. In this paper, a numerical simulation is first conducted to investigate the wave propagation mechanism in honeycomb sandwich structures using piezoelectric actuators/sensors. In contrast to most of the previous work, elastic wave responses based on the real geometry of the honeycomb core are obtained by using the finite element method (FEM). Based on the simulation, the global guided waves in the composite can be observed when the loading frequency is low and the leaky guided waves in the skin panel are found when the loading frequency is sufficiently high. The applicability of the homogenization technique for a celled core is discussed. The effects of cell geometry on the wave propagation are also demonstrated. Experimental testing is finally conducted to validate the results of numerical simulation and very good agreement is observed. Specifically, some guided wave propagation characteristics such as group velocity dispersion and mode tuning capabilities with the presence of a honeycomb core are discussed.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHard rock applications continue to pose serious challenges for PDC bits. Although improvements 1,2,3 have been achieved in these environments, PDC bit performance is highly inconsistent and leaves much to be desired.Operational costs in such environments, due to reduced footage and/or low penetration rates (ROP), are enormously high. This paper will evaluate the challenges of hard rock drilling. It will discuss the effects formation hardness has on drilling efficiency, and the energy level requirements associated with the different rock failure mechanisms.The definition and relationships between WOB requirements, drilling efficiency, ROP, wear flat generation and on-bottom drilling time will also be discussed.An advanced cutting structure (ACS), which improves PDC bit effectiveness and consistency in hard rock drilling environments, will be presented. ACS improves drilling performance by redefining and optimizing the relationships between ROP, durability and stabilization. It maximizes bit life, by slowing the PDC cutter deterioration process, without compromising ROP. This paper will describe ACS's functionality, and show the positive effects it is having on operational costs. Laboratory and field data, supporting ACS's effectiveness will also be presented. Minimizing PDC Cutter DeteriorationAccelerated PDC cutter deterioration, either through wear or impact damage, reduces durability and drilling efficiency. To improve PDC bit performance, especially in hard rock drilling environments, cutter deterioration must be drastically delayed.

Hard rock applications continue to pose serious challenges for PDC bits. Although improvements1,2,3 have been achieved in these environments, PDC bit performance is highly inconsistent and leaves much to be desired. Operational costs in such environments, due to reduced footage and/or low penetration rates (ROP), are enormously high. This paper will evaluate the challenges of hard rock drilling. It will discuss the effects formation hardness has on drilling efficiency, and the energy level requirements associated with the different rock failure mechanisms. The definition and relationships between WOB requirements, drilling efficiency, ROP, wear flat generation and on-bottom drilling time will also be discussed. An advanced cutting structure (ACS), which improves PDC bit effectiveness and consistency in hard rock drilling environments, will be presented. ACS improves drilling performance by re-defining and optimizing the relationships between ROP, durability and stabilization. It maximizes bit life, by slowing the PDC cutter deterioration process, without compromising ROP. This paper will describe ACS's functionality, and show the positive effects it is having on operational costs. Laboratory and field data, supporting ACS's effectiveness will also be presented. Background To improve PDC bit effectiveness in such hard rock environments, certain performance and/or behavioral relationships (PBR) must be understood and optimized. The conditions and levels of durability, required to enhance PDC bit longevity, must be defined. In addition, the appropriate operational medium needed to enhance drilling efficiency must be established4,5,6. These conditions must be achieved, while maintaining efficient relationships between ROP, rate of wear flat generation and on-bottom drilling time. These characteristics will improve PDC bit performance in hard rock applications. To be durable and/or exhibit high ROP characteristics, PDC bits must be stable. This requirement, in ensuring effective use of available mechanical energy, also minimizes diamond table degradation through impact damage - spalling, chippage and delamination (Figure 1). The benefits of stabilization can be summarized as follows:Efficient energy use --- improves ROPReduced diamond degradation --- enhances durability ACS achieves the performance and/or behavioral requirements listed in this section. Its effectiveness, in hard rock applications, is based on its unique ability to establish the appropriate energy efficiency and durability characteristics. Performance and Behavioral Relationships (PBR) Improving PDC bit performance, especially in hard formations, primarily requires the development of products and/or processes that will extend bit life. In addition, these products must be consistently effective in such applications. The poor performances of PDC bits in hard rock applications are due to the wrongful interpretation of the requirements needed to make these bits durable. As such, most of the solutions advocated have not had the desired effects. To address the hard rock applications problem, PDC bit durability7 - in terms of its definition, optimization and relationship to ROP - must be clearly understood. In addition, technologies and/or processes must be developed that drastically slow down the rate of PDC cutter deterioration. Solutions developed to address this requirement must not compromise ROP. Minimizing PDC Cutter Deterioration Accelerated PDC cutter deterioration, either through wear or impact damage, reduces durability and drilling efficiency. To improve PDC bit performance, especially in hard rock drilling environments, cutter deterioration must be drastically delayed.