Yield strength vs tensile strength: how big is the gap?

Yield strength, tensile strength and elongation as stored. (minimums) marks specified minimums; ranges run across tempers and forms. The A36 tensile top is the upper limit of the 58 to 80 ksi mill-plate band, not a capability. Ti-6Al-4V yield spans annealed and aged material, but its tensile and elongation ranges are annealed only (aged elongation falls to 6 to 10 %). PA 66 is the Zytel 101L pair only.

What is the difference between yield strength and tensile strength?

Yield strength is the stress at which a material stops springing back and starts to deform for good, and tensile strength is the highest stress the test bar carries before it breaks. Up to the yield point the bar returns to its length when unloaded; beyond it the stress keeps rising as the metal strain-hardens, peaks at the tensile strength, and the bar necks and breaks (general engineering knowledge, not in our data).

Many alloys, including stainless steels, aluminium, copper and titanium, have no sharp yield point, so yield is read by convention, and many of our profiles print the 0.2 percent offset. The C360 rod data defines yield as 0.5 percent extension under load, and the 304L minimums do not state an offset.

Elongation, the third number on a datasheet row, is the permanent stretch at fracture over a stated gauge length. The test piece changes the number. Gauge length: A36 is specified at a minimum of 21 % in a 2 in gauge but 18 % in an 8 in gauge. Specimen: 6061 is 17 % on a 12.7 mm round but 12 % on a 1.6 mm sheet specimen.

Why do engineers design with yield strength and not tensile strength?

Because a part that has yielded has already changed shape, and a bent shaft has failed in service without breaking (general engineering knowledge, not in our data). Structural design stress is therefore usually set below the specified minimum yield. A36 has a minimum yield of 250 MPa under 8 in thick and 220 MPa over 8 in, next to a tensile range of 400 to 552 MPa (58 to 80 ksi; 80 ksi is the upper limit of the band).

Tensile strength still matters. It is stored for every metal here, so it is the common basis for comparing grades. Read with elongation it shows how much warning a metal gives: a wide gap with high elongation means it bends long before it breaks, a narrow gap with low elongation that it fails with little visible deformation. Some rules also use tensile strength, such as allowable stress, ultimate-load factors, bolts and welds; the limits sit in the governing code (general engineering knowledge, not in our data).

Choosing when the gap matters (general engineering knowledge, not in our data). A narrow gap is the price of precipitation hardening, not a reason to pick a grade: choose 6061 T6 or 17-4 PH H1025 for strength and corrosion resistance, then check elongation and fracture toughness and do not count on warning before failure. If visible deformation before failure matters, pick annealed austenitic stainless steel such as 316, or a copper alloy.

How big is the gap between yield and tensile strength in real metals?

The gap is narrow in precipitation-hardened steels, nickel alloys, T6 aluminium, annealed titanium alloy bar and cold-worked duplex tube, and wide in soft or annealed austenitic metals and in brass at every temper. The gap here is the tensile figure compared with the yield figure of one pair (tensile over yield, the inverse of the yield ratio in the terms).

At the narrow end, 17-4 PH cold-finished bar in condition H900 is typically 1,262 MPa yield and 1,365 MPa tensile, and the ASTM A564 bar minimums for that condition are 1,170 and 1,310 MPa. 2205 cold-worked seamless tube has minimums of 895 and 965 MPa, though annealed 2205 plate is far softer. Inconel 718 is 1,179 MPa (171 ksi) yield and 1,386 MPa (201 ksi) tensile for one aged hot-rolled bar, a typical value and not a minimum. 6061 T6 is typically 276 and 310 MPa and 7075 T6 tube 503 and 572 MPa. Ti-6Al-4V annealed bar to AMS 4928 lists 862 and 931 MPa as minimums.

At the wide end, in ASTM A240 plate, sheet and strip, 316 has a 205 MPa minimum yield against a 515 MPa minimum tensile and 304L 170 against 485 MPa. Annealed Copper (Cu) (C10200) is a nominal 69 MPa yield against 179 to 262 MPa tensile. C360 rod minimums are 130 and 330 MPa in the O60 temper, 170 and 390 MPa in half-hard H02 and 240 and 480 MPa in hard H04, so cold work narrows the gap in brass but never to the narrow band. The A36 minimums (250 MPa yield against 400 MPa tensile) are wide as well.

The basis can matter as much as the metal: Grade 2 titanium falls between the two bands on either its minimums or its typical values. Only typical values are stored for 6061, 7075 and Inconel 718, so those pairs show the gap but cannot be used to design; specified minimums are usually lower than typical figures (general engineering knowledge, not in our data).

Which alloys reach the highest yield and tensile strength, and why does the order change?

Among the like-for-like figures here, the highest yields at room temperature are 1,262 MPa for 17-4 PH (typical, H900 bar), 1,179 MPa for Inconel 718 (typical, aged bar) and 1,125 MPa for 4340 (a minimum, condition Z* at a 30 mm ruling section). The highest tensile figures belong to the same three in reverse order: 4340 above 1,550 MPa as a minimum, then Inconel 718, then 17-4 PH. 4340 moves from third to first because its condition Z* pair is wider than the other two. One hydrogen-study specimen of 4140 reaches 1,235 MPa yield, but it is a single research test and is left out.

Specified minimums, the figures to design with, are lower: 4340 runs from 665 MPa for condition T at a 150 mm ruling section to 1,125 MPa for Z* at 30 mm, and 4140 guarantees 740 MPa for 10 to 100 mm and 665 MPa for 101 to 250 mm.

Two metals cannot be paired. Annealed Ti-6Al-4V has a tensile range of 900 to 1,000 MPa, but its solution-treated and aged yield of 1,000 to 1,100 MPa has no tensile value beside it. AZ31B has a tensile strength of 290 MPa (as-rolled foil and sheet) and no yield strength.

How do temper and heat treatment change the gap and the elongation?

Strengthening usually costs stretch. In 5052 the annealed O temper has the lowest yield, 89.6 MPa (13 ksi), a tensile strength more than double that, and 25 % elongation. In H38 the yield is 255 MPa (37 ksi), the tensile strength only 290 MPa (42 ksi), and elongation 7 %. Yield rose faster than tensile strength, so the gap closed.

1018 shows a similar shift, but only suggestively. Hot rolled it is 240 MPa yield, 430 MPa tensile and 27 % elongation; cold drawn 370 MPa, 440 MPa and 15 %. The two rows come from different sources with the basis of both unstated, so the shift is not a controlled comparison.

Heat-treated alloys follow strength but not always the gap. For 4340 the tensile range rises from 850 to 1,000 MPa in condition T to above 1,550 MPa in Z*, and minimum elongation falls from 13 % to 5 %. For 17-4 PH bar typical elongation falls from 20 % at H1150 to 16 % at H1025 and 15 % at H900, but yield and tensile sit closest at H1025: 1,117 and 1,158 MPa typical. 1045 as-rolled bar is typically 410 MPa (59,000 psi) yield and 621 MPa (90,000 psi) tensile with 26 % elongation, against 331 and 552 MPa with 30 % annealed.

What about polymers: do yield and tensile strength mean the same thing?

Partly. Yield still marks the start of permanent flow, but in many plastics it is a stress peak after which the material stretches a long way. For ductile polymers measured to ISO 527, the reported tensile strength is often the yield stress itself (both points general engineering knowledge, not in our data). No polymer profile here stores a tensile strength for a named grade, so the gap cannot be measured; we hold yield and strain. PC yields at 65 MPa with a strain at break of 120 %. PA 66 yields at 85 MPa dry and 54 MPa conditioned as BASF Ultramid A3K, and at 82 and 55 MPa as DuPont Zytel 101L NC010. Polyetheretherketone (PEEK) grade 450G yields at 98.0 MPa with a strain at break of 25 %.

Form and test move polymer numbers too. A PEEK film flown on the ISS had a pristine yield of about 85 MPa and an elongation of about 200 %, against 98.0 MPa and 25 % for the 450G datasheet. A printed PEEK part measured 78.0 MPa tensile strength (ASTM D638), which is not comparable to the 98.0 MPa yield.

Terms used

Yield strength

the stress where a material stops springing back and starts to deform for good.

Tensile strength

the highest engineering stress a tensile test reaches before the bar breaks. Our profiles store it as ultimate tensile strength.

Yield ratio

the yield strength divided by the tensile strength of one pair (Y/T), the usual engineering measure of the gap; near one is narrow. This page states the gap the other way round, tensile over yield (T/Y), and calls it narrow when tensile is within about a fifth above yield and wide when it is at least half as much again.

Elongation

the permanent stretch at fracture, as a percentage of a stated gauge length.

0.2 percent offset

the usual way to read yield when a metal has no sharp yield point: shift the elastic line by 0.2 percent strain and read where it meets the curve.

Minimum and typical values

a minimum is a figure a standard or supplier specifies; a typical value is an average that single lots can fall below (general engineering knowledge, not in our data).

Temper

the strength state of a metal, set by cold work or heat treatment and written as codes such as O, H32, T6, H900 or half hard.

Strain hardening

the rise in strength that comes with cold work. It raises yield strength faster than tensile strength (general engineering knowledge, not in our data).

Ruling section

the thickness that a strength guarantee applies to.

What to check before you order

1. Ask for yield strength, tensile strength and elongation from the same datasheet row or mill certificate, with grade, temper, form, size and test temperature.
2. Ask whether each figure is a minimum, a typical value or one test, and for elongation the gauge length.
3. Ask how yield is defined. The 0.2 percent offset is the usual convention; the brass C360 data here uses 0.5 percent extension under load.
4. For heat-treatable alloys, quote the condition code (T6, H900, condition Z*) and the section size the guarantee covers. For 6061, 7075 and Inconel 718, order against the specified minimums of the governing standard (such as AMS 5662 and 5663 for 718), not the typical values stored here.

What do we not know yet?

• No polymer profile pairs yield with a room-temperature tensile strength for a named grade. Polycarbonate and PA 66 store yield only; PEEK stores yield for grade 450G and a tensile strength only from an unnamed-grade cryogenic preprint and a printed part.
• Ti-6Al-4V has no tensile value for the aged condition, AZ31B has no yield, and 7075 and 6061 hold one typical T6 value each, so neither shows how the gap changes with temper.
• Temperature data is thin: 316 and 2205 have yield at several temperatures without tensile strength; only Inconel 718 has both.
• No stress-strain curve, uniform elongation or strain-hardening exponent is stored for any profile used here.
• Several rows (1018, the second 304L source) do not say minimum or typical. 6061, 7075 and Inconel 718 hold typical values only, with no specified minimum (no AMS 5662 or 5663 for 718).
• The A36 thickness split rests on a secondary web page, not the standard or a mill datasheet. No profile used here stores welded or heat-affected-zone strength or specimen orientation.

Questions people ask