10 INTRODUCTION Chap. 1 (1) Maxwell model: FF u=-+- Prl (2) Voigt model: F = PU + qU, u(0) = 0, (3) Kelvin model: F + T~F = ER(U + T,I~), T,F(O) = ERT,u(O), where T~, T~ are two constants. When these equations are to be integrated, the initial conditions at t = 0 must be prescribed as indicated above. The creep functions c(t), which are the displacement u(t) in response to a unit-step force F(t) = l(t) defined in Eq. (7) below, are the solution of Eqs. (1)-(3). They are: (4) Maxwell solid: c(t) = (1 + 't) l(t) , (5) Voigt solid: (6) Kelvin solid:
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