E399 Standard Test Method

E399 is a standardized procedure used to determine the fracture toughness of metallic materials, specifically how resistant a material is to crack propagation. It involves preparing a sample with a precise crack and applying controlled stress until the crack begins to grow or the material fractures. This test helps engineers assess the durability and safety of metals used in bridges, aircraft, and other critical structures by measuring their ability to withstand stress without cracking further or failing unexpectedly. The standard ensures consistency and reliability in these measurements across different laboratories and applications.