tailieunhanh - Ebook Principles of composite material mechanics: Part 2

(BQ) Principles of composite material mechanics a unique blend of classical and contemporary mechanics of composites technologies. That book includes frequent references to current state-of-the-art composites technology and research findings. | CHAPTER 8 ANALYSIS OF VISCOELASTIC AND DYNAMIC BEHAVIOR INTRODUCTION In the analyses of Chaps. 1 to 7 it has been assumed that the applied loads are static in nature and that the composite and its constituents exhibit time-independent linear elastic behavior. However composite structures are often subjected to dynamic loading caused by vibration or wave propagation. In addition many composites exhibit time-dependent viscoelastic behavior under load this is particularly true for composites having polymeric constituents. This chapter contains the basic information needed for the analysis of both viscoelastic and dynamic behavior of composites and their constituents. The word viscoelastic has evolved as a way of describing materials which exhibit characteristics of both viscous fluids and elastic solids. Polymeric materials which are known to be viscoelastic may behave like fluids or solids depending on the time scale and or the temperature. For example polycarbonate a thermoplastic polymer is a liquid during molding at processing temperatures but is a glassy solid at 270 ANALYSIS OF VISCOELASTIC AND DYNAMIC BEHAVIOR 271 service ambient temperatures. It will deform like a rubber at temperatures just above the glass transition temperature Tg. At temperatures below Tg however it will deform just as much and in the same way if the test time is long enough. We know that ideal Hookean elastic solids are capable of energy storage under load but not energy dissipation whereas ideal newtonian fluids under nonhydrostatic stresses are capable of energy dissipation but not energy storage. Viscoelastic materials however are capable of both storage and dissipation of energy under load. Another characteristic of viscoelastic materials is memory. Perfectly elastic solids are said to have only simple memory because they remember only the unstrained state and the current strains depend only on the current stresses. Viscoelastic materials have what is often referred to as fading .