Fracture Toughness anyone help?

Does anyone know the fracture toughness equation or any help with finding the fracture toughness of a specimen equivalent of finite plate W=50mm and is made of aluminium with a yield strength of 500MN m-2 and has an edge crack of 6mm. During the fracture toughness test the failure stress is measured to be 280MN-2, calculate the fracture toughness Kic