ASTM E8/E8M

What the test does

A prepared metal coupon — a flat sheet strip or a machined round bar with a reduced central section — is clamped at both ends and pulled along its axis until it breaks. The crosshead moves, the load cell records force, and an extensometer clipped to the marked gauge length records stretch. The result is a continuous force–extension record from first loading through yield, necking and fracture.

What it measures, and why it matters

The methods report yield strength, ultimate tensile strength, elongation at fracture and reduction of area. Yield strength is the design input for any load-bearing member, since it is the stress at which a part deforms permanently rather than springing back. Ultimate strength sets the margin before fracture. Elongation and reduction of area quantify ductility and are the acceptance figures in mill certificates, so they carry lot release for plate, bar and sheet. In failure investigations they separate a material out of specification from a component loaded beyond its design case.

Specimen geometry and gauge length

Both flat and round geometries are defined, and the gauge length is tied to the specimen size rather than fixed — which is why an elongation figure means nothing until you know the gauge it was measured over.

Sheet-type coupon

50 mm gauge lengthThe standard flat specimen for sheet, strip and plate up to the thickness the geometry allows.

Round, SI

62.5 mm gauge, five diameters

Round, inch-pound

Gauge of four diametersThe two systems are not the same specimen. E8 and E8M are one document with two sets of units, and the results are not interchangeable.

Plate-type coupon

200 mm gauge length

Test temperature

10 °C to 38 °CNo conditioning atmosphere. Metals are not hygroscopic, so humidity is not controlled.

Surface of the reduced section

Free of tool marksA machining mark is a stress raiser, and fracture will find it before it finds the true weakest section.

Discard and replace

Any fracture at or inside the grip

Keep preparation cool

No heating, minimal cold workDakCutting a coupon aggressively changes the properties you are about to measure. The specimen must represent the material, not the workshop.

Elongation is a gauge-length-dependent figure. The same material measured over 50 mm and over 200 mm gives two different percentages, both correct — so a certificate that omits the gauge length has not reported ductility at all.

Rate control — two stages, not one

This is where E8/E8M differs most from the plastics methods. The rate is prescribed in two separate stages, because metals are rate-sensitive precisely where yield is determined.

Through the yield region — stress rate

1.15 to 11.5 MPa/sControl by stress, by strain or by crosshead position is permitted, so long as the resulting rate falls in the window.

Through the yield region — strain rate alternative

About 0.015 min⁻¹

After yield, to fracture

0.05 to 0.5 mm/mm of reduced-section length per minuteThe faster stage, once the number that matters most has already been captured.

One speed throughout

Wrong, and it inflates yieldDakRunning the whole test at the post-yield rate is the commonest rate error. It does not merely shorten the test — it shifts the reported yield strength upward.

A yield strength obtained outside the prescribed window is not a slightly less accurate yield strength; it is a different number. If a result is being disputed, the rate record is the first thing to check.

Calculations

How the test runs

1. 01Machine or cut the coupon to the chosen geometry, keeping the reduced section free of tool marks and the material cool.
2. 02Measure the reduced section — diameter for rounds, width and thickness for flats — and record the original area.
3. 03Mark the gauge length on the specimen.
4. 04Select grips to suit: serrated V-faces for rounds, flat serrated faces for sheet, threaded or shouldered holders for machined ends, snubbing grips for wire.
5. 05Mount the specimen with attention to axial alignment; a misaligned load path superimposes bending on tension.
6. 06Fit the extensometer over the marked gauge length, classified to E83 Class B-2 for the yield determination.
7. 07Load through the yield region at a stress rate between 1.15 and 11.5 MPa/s, capturing the elastic slope cleanly.
8. 08Determine yield by the offset method, or by extension under load where the specification directs.
9. 09Increase to the post-yield rate and continue to fracture.
10. 10Remove the extensometer before fracture if it is a clip-on that cannot survive it.
11. 11Fit the fractured halves together, measure the final gauge length and the smallest area, and compute elongation and reduction of area.
12. 12Discard any specimen that fractured at or inside the grip line.

Alignment is the quiet one. A worn wedge or an out-of-square column bends the specimen as it pulls, and the yield strength comes out low with no other sign that anything was wrong.

Watch the test

Grips and fixtures for this method

Upto 100 kNTJ-145

Manually Operated Heavy Duty Wedge Grips

Manually operated heavy duty wedges with interchangeable faces — serrated V-faces for round specimens, flat serrated faces for sheet.

Specifications

Self-tighteningTJ-135

Heavy Duty Circular Hydraulic Wedge Grips

Hydraulic actuation closes the wedges and they tighten further as load rises, which is what high-strength alloys need to avoid slipping before yield.

Specifications

What the report has to contain

• Full designation and edition, and whether E8 or E8M units were used
• Material identification, product form, heat or lot number, and orientation relative to rolling direction
• Specimen type and dimensions of the reduced section
• Gauge length — without it the elongation figure cannot be interpreted
• Test temperature
• Rate through the yield region and after it, and the method of control
• Yield strength, and the offset or extension used to determine it
• Ultimate tensile strength
• Elongation after fracture, with the gauge length restated
• Reduction of area
• Location of the fracture, and any specimen discarded

What the machine must be capable of

Most metals work runs on 50–300 kN frames. Thin sub-size sheet coupons need only a few hundred newtons, while full-thickness plate and high-strength rounds can demand 600 kN, so frame selection follows the thickest product tested, not the average. Force indication must be verified to ASTM E4.

Rate control is prescribed in two stages. Through the yield region the stress rate is held between 1.15 and 11.5 MPa/s, or alternatively a strain rate near 0.015 min⁻¹; control by stress, strain or crosshead position is permitted. After yield, crosshead rate runs between 0.05 and 0.5 mm/mm of reduced-section length per minute to fracture. Metals are rate-sensitive at yield, so a rate outside the window shifts the reported yield strength rather than merely the test duration.

Strain measurement needs an extensometer classified to ASTM E83 Class B-2 over the 0–0.5 % range where yield is determined; Class C or better is permitted above 5 % strain. Total elongation commonly reaches 50 %, so either a long-travel device or a clip-on that is removed and tracked through is needed.

Self-tightening wedge grips are the default — serrated V-faces for rounds, flat serrated faces for sheet, with hydraulic side-loading wedges usual on high-strength alloys. Threaded and shouldered holders suit machined round ends, pin loading suits plate and sheet, and snubbing grips suit wire. Axial alignment is emphasised throughout, because a misaligned load path superimposes bending on tension.

What goes wrong in practice

Grip slippage is the common one: the coupon creeps in the jaws, the recorded extension includes that movement, and modulus and yield read low. Jaw breaks are the opposite — serrations bite too hard and the specimen fractures at the grip line, so the test is void. Off-axis loading from worn wedges or a misaligned column bends the specimen and depresses yield strength. Rate sensitivity catches operators who run one speed throughout: too fast through yield inflates the yield figure.

How it differs from the standards nearest to it

The two are comparable in kind rather than directly interchangeable — the specimen proportions, the rate definitions and even the symbols differ. A mill certificate should always state which method produced the numbers, and a purchaser specifying one should not accept the other without saying so.

Questions we are asked about this test