Tensile strength.

The top of the curve.

Pull a ductile metal past yield and it keeps getting stronger as it stretches — until it doesn't. Tensile strength, or ultimate tensile strength (UTS), is the highest stress it reaches.

After yield, every increment of stretch tangles more dislocations together, which makes the next increment harder. That is work hardening, and it is why the stress–strain curve keeps rising. At the same time the specimen gets thinner. At the peak, the material's hardening can no longer keep up with the shrinking cross-section; one spot thins faster than the rest, and a neck forms. From there the load falls until the specimen breaks.

Note the A₀: tensile strength is engineering stress, calculated on the original area. The true stress in the neck keeps rising right up to fracture. UTS is the useful design number because it tells you the largest load a given cross-section can carry.

How it is measured.

UTS comes out of the same tensile test as yield strength — ASTM E8/E8M for metals, ASTM D638 or ISO 527 for plastics. The machine records the maximum force before fracture; divide by the starting cross-section and you have tensile strength.

Because it is a single peak, UTS is more repeatable than yield and easier to measure — it needs no extensometer. That is why mill test reports always list it and why some specifications, such as A36 at 58–80 ksi, give tensile strength as a range rather than a minimum.

Plastics and composites

For many plastics the "tensile strength" reported on a datasheet is the yield peak; for brittle plastics, glass-filled grades and composites it is the stress at break. Results depend strongly on test speed and temperature, and on build direction for 3D-printed parts — HP's MJF values, for example, are reported separately for XY and Z.

How materials compare.

The tensile chart has the same shape as the yield chart, but the bands sit higher and are compressed differently. Austenitic stainless is the striking case: 304 yields at about 215 MPa but reaches about 505 MPa before necking, so its UTS is more than twice its yield.

How engineers use it.

The yield-to-tensile ratio

Divide yield by tensile strength. The ratio tells you how much the material hardens after it starts to yield:

Structural codes use the ratio too: steels with a low ratio can redistribute load after local yielding, which is what allows plastic design of steel frames.

Fasteners and joints

Bolts, rivets, welds and press-in hardware are rated against tensile and shear strength because their job is to not break. Shear strength of ductile metals is commonly estimated at about 0.6 × UTS. Hardware pull-out and torque-out values, such as those for PEM-style self-clinching fasteners, are tested numbers for a given sheet hardness and thickness, not something to derive.

Fatigue and hardness

For steels, the endurance limit is roughly half the tensile strength (lower for rough or notched surfaces); aluminum has no true endurance limit. And because tensile strength correlates with hardness, a quick Brinell or Rockwell test on a part can estimate its UTS — see the hardness guide.

What it means in fabrication.

• Press force. Air-bending tonnage formulas are written in terms of tensile strength: force rises in proportion to UTS and with thickness squared. Stainless at the same gauge needs roughly 1.5× the force of mild steel.
• Laser cutting. UTS has little to do with laser cutting speed — that is set by thickness, absorption and thermal properties. A 1,700 MPa AR500 plate cuts at about the same speed as A36 of the same thickness.
• Tooling and punching. Hard, high-UTS sheet wears punches and dies faster and limits the smallest hole that can be punched (laser cutting avoids that).
• Edge cracking. Grades with a high yield ratio are more likely to crack on the outside of tight bends or at sheared edges.

Common mistakes.

• Using UTS as the working stress. A part that reaches tensile strength is necking. Design to yield with a safety factor; use UTS for fracture checks and fastener sizing.
• Comparing numbers from different tests. Plastic tensile values swing with test speed, temperature and print orientation. Compare like with like.
• Assuming strength survives welding. Heat-treated aluminum loses a large share of its tensile strength in the heat-affected zone; welded 6061-T6 is usually designed to the as-welded strength in AWS D1.2.