The metallurgical transformations from high to low toughness are strength-dependent and can be modified by metal quality. Fracture resistance generally decreases with increase in yield strength of the aluminum. The plane-strain fracture toughness (KIc) generally increases with the material's ductility and decreases with yield strength. However, the value of KIc cannot be deduced from the knowledge of standard mechanical properties. Deflection depends on the modulus of elasticity and geometry, but it is independent of the material's strength. The practical limitation of our ability to predict realistic stresses and to assure the prevention of structural failure depends on a blend of fracture mechanics knowledge with experimental ability to conduct nondestructive testing. The object of a fracture-control program is to minimize the probability of a catastrophic failure due to the presence of undetected flaws. The actual mechanism of structural failure may be opened to some interpretation in terms of what constitutes a critical level of damage in a particular situation.