Mechanical Behavior of Time-Dependent, Viscoelastic, Materials

Following basic definitions of tensile and shear stresses and strains, the range of materials’ mechanical behavior (elastic, viscoelastic and viscous) is illustrated with examples and graphs, including how it changes with temperature and the timescale or speed of testing. The time-dependent behavior of viscoelastic materials is then qualitatively presented and physically justified in uniaxial tensile stress relaxationModulusstress relaxation and creep (with full descriptions of both types of excitations), for amorphousAmorphous and semi-crystallineSemi-crystalline polymersPolymers, with graphical reference to the effects of crosslinking and crystallinity, respectively. The classical phenomenological representations are recalled of the elastic (spring) and viscous (Newtonian damper or “dashpot”) components of the mechanical behavior (to be combined in Chap. 3 in simple models of linearLinear, constant-parameter, viscoelasticity, namely the standard linear solid and liquid) and the Boltzmann’s superposition principleBoltzmann’s superposition principle justified in that linear domain. The formulation and physical analysis are then presented of the response of elastic and viscoelastic materials to cyclic (sinusoidal) strain and stress excitations, with reference to its  storage and lossModulusloss or dissipative components and including qualitative plots of their variation with frequency and with the crosslinks’ density and crystallinity, for amorphous and semi-crystalline polymers, respectively. The chapter ends with the formal interrelationships of the various linear viscoelastic functions.