A tension test produces a page of numbers, and almost the whole of this course lives in four of them. Learn to tell them apart by their UNITS, because on a real report nobody labels them for you.

The load $P$ is what the test frame pulls with, in newtons or kilonewtons — a force, exactly as in first-year mechanics. The stress $\sigma$ (the Greek letter sigma) is that load spread over the area carrying it, in megapascals: working stresses in metals run in the tens and hundreds of MPa. The strain $\varepsilon$ (epsilon) is how much longer the specimen got, divided by how long it was to begin with — a length over a length, so it wears no unit at all and arrives as a small decimal like 0.0012. And Young's modulus $E$ is the stress each unit of strain costs, quoted in gigapascals, a thousand MPa apiece, because materials are stiff.

One habit worth building today: strain is so small that laboratories quote it in microstrain, written µε — millionths. A strain of 0.0012 is 1200 µε. Same number, fewer zeros, and still no unit gained: µ is a scale, not a dimension.