DEVELOPMENT OF A REGRESSION MODEL FOR THE FATIGUE LIFE ASSESSMENT OF OPEN-HOLE SPECIMENS WITH DOUBLE THROUGH THE THICKNESS CRACKS A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY BY SEYED SOHRAB HEIDARI SHABESTARI IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN AEROSPACE ENGINEERING JULY 2019