the ultimate tensile strength

The ultimate tensile strength is the biggest engineering stress a material can take in a pull test — the very peak of the stress-strain curve. Below it the bar can carry more and more load; at the peak it is holding the most it ever will. It is the number people usually mean when they call a material 'strong', though for design the yield strength often matters more.

It is measured as the maximum force during the test divided by the ORIGINAL cross-sectional area, so it is an engineering stress, and it is reported in MPa. Up to the peak, deformation is spread evenly along the gauge length; at the peak a ductile metal starts to neck — deformation localises into one thinning waist. Past that point the shrinking neck can no longer carry the previous force, so the engineering stress falls even though the true stress in the neck keeps rising, and the bar finally parts inside the neck. A structural steel might have a UTS around 400 to 550 MPa; a high-strength alloy steel well over 1500 MPa.

For a brittle material there is no necking and little warning: the UTS and the fracture stress are essentially the same, and the bar snaps at the peak. This is why UTS alone can mislead — two materials with the same UTS can behave completely differently, one stretching a lot first (ductile) and one shattering (brittle). UTS pairs naturally with yield strength (onset of permanent set) and ductility (how much stretch before breaking) to tell the full story.