The object of this book is to bring together a series of papers which describe recent experience obtained in the United States and in England with the application of the ASTM E24 Proposed Method of Test for Plane Strain Fracture Toughness of Metallic Materials. This information supplements that which has appeared in ASTM STP 381 on Fracture Toughness Testing and its Application and in STP 410 on Plane Strain Crack Toughness Testing of High Strength Metallic Materials. This publication is a cooperative effort of ASTM and NASA. NASA research personnel have participated in ASTM Fracture Committee activities since their inception in 1959. This participation reflects the strong interest that NASA has maintained in the development of test methods for evaluation of the fracture resistance of engineering materials. This interest arises from the necessity for the use of high-strength alloys in critical parts of aerospace structures. By cooperation with ASTM in publication of this book NASA is helping to fulfill its obligation to provide the widest practicable and appropriate dissemination of results from its activities.