FATIGUE PRE-CRACKING LIFE ESTIMATION FOR FRACTURE TOUGHNESS TEST SPECIMENS A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATRUAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY BY AMIR ALIPOUR GHASABI IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCINENCE IN MECHANICAL ENGINEERING JUNE 2018