f NASA / TM--2002-2 t 1153 / PART1 Slow Crack Growth of Brittle Materials With Exponential Crack-Velocity Formulation_ Part 1: Analvsis Sung R. Choi Ohio Aerospace Institute, Brook Park, Ohio Noel N. Nemeth and Jol in P. Gyekenyesi Glenn Research Center, Cleveland, Ohio June 2002