How can someone determine how brittle a metal is?
Brittleness
A material is brittle if, when subjected to stress, it breaks without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses (which do not deform plastically) and some polymers, such as PMMA and polystyrene. Many steels become brittle at low temperatures (see ductile-brittle transition temperature), depending on their composition and processing.
Constance Tipper
Tipper established that the fractures were not caused by welding, but were due to the properties of the steel itself. She demonstrated that there is a critical temperature below which the fracture mode in steel changes from ductile to brittle. Because ships in the North Atlantic were subjected to low temperatures, they were susceptible to brittle failure. While these fatigue cracks would not propagate beyond the edges of riveted steel plates, they were able to spread across the welded joints in the Liberty ships. She developed what is now known as the "Tipper Test" to help ensure that the metal used in ship construction was sufficiently sound.