Testing standard
Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM F2516 stretches superelastic nickel-titanium to 6 % strain, unloads it to a low stress, then pulls it to failure. From that one cycle come upper plateau strength, lower plateau strength, residual elongation, tensile strength and elongation — none of which an ordinary monotonic pull would produce. The current edition is ASTM F2516-22.
A specimen of superelastic nickel-titanium — nitinol — is stretched in tension to 6 % strain, unloaded to a low stress, and then pulled to failure. That three-stage cycle is the whole method, and it exists because an ordinary monotonic pull would miss what makes the material interesting.
From it come the properties the standard names: upper plateau strength, lower plateau strength, residual elongation, tensile strength and elongation.
Superelastic nitinol does not behave like an ordinary metal. It has a stress-induced phase transformation from austenite to martensite, which means it deforms enormously — several percent strain — at almost constant stress, and then recovers when the load is removed. The stress–strain curve has a flat loading plateau, a flat and lower unloading plateau, and a hysteresis loop between them.
That behaviour is the product. A self-expanding stent, a guidewire, an orthodontic archwire and a bone staple all work because the material delivers a nearly constant force over a large deflection and then returns. Ordinary yield strength describes none of it.
**Upper plateau strength** is the stress at 3 % strain during loading — the force the device exerts as it deforms. **Lower plateau strength** is the corresponding unloading value, and the gap between them is the hysteresis that lets a stent be compressed for delivery and then push outwards gently but persistently. **Residual elongation** is the difference between the strain at a specified load during loading and during unloading: it says how much of the deformation did not come back, which is the direct measure of whether the material is genuinely superelastic at that temperature.
Test temperature is a specimen variable in disguise. The plateau behaviour depends entirely on how far the temperature sits above the austenite finish temperature.
The stress at 3 % strain during loading
The force the device exerts as it deforms. Published guidance gives about 500 MPa on average for a 2.5 mm diameter specimen, the largest size the standard covers.
The corresponding stress during unloading
The force the device exerts as it recovers. The gap between UPS and LPS is the hysteresis that lets a stent be compressed for delivery and then push outwards gently but persistently.
The difference between strain at a specified load during loading and during unloading
How much deformation did not come back — the direct measure of whether the material is genuinely superelastic at that temperature.
A stress-induced austenite-to-martensite transformation, not plastic yielding
The material deforms several percent at nearly constant stress and then recovers. Conventional yield describes none of that.
Non-contacting strain measurement, which matters more here than on most metals: nitinol is notch-sensitive at these strains, and a clip-on gauge marks the surface at exactly the stresses that will exploit the mark. Optical measurement also follows the specimen through unloading and on to failure without being reset.
Specifications
Rubber-coated jaw faces holding fine wire or thin strip without notching it. The clamping window is narrow — enough to hold at plateau stress, not enough to initiate a failure — and a soft face widens it.
SpecificationsA fine nitinol wire fails in tens of newtons. A cell sized for the specimen is what resolves the plateau, which is a nearly flat region where small force errors look like real material differences.
SpecificationsForces are modest — a fine wire fails in tens of newtons — so a low-capacity frame with a load cell sized for the specimen and force accuracy to ASTM E4 is right.
Strain measurement is the demanding part. The plateau stresses are read at defined strains, so strain has to be measured accurately on the specimen rather than taken from crosshead travel, and it has to be followed through loading, unloading and on to failure. A non-contacting video extensometer avoids marking a notch-sensitive surface, and a long-travel device is needed for the elongation to failure.
The machine must also be able to reverse cleanly at 6 % strain and unload under control to a low stress without overshoot, which is a control-loop requirement rather than a mechanical one.
Temperature control around the specimen is normally required, since the properties are strongly temperature-dependent.
Strain taken from the crosshead is the error that spoils most nitinol data, because it puts the plateau readings at the wrong strains. Grip-initiated failures shorten the elongation and are often reported rather than rejected. Testing at an uncontrolled room temperature makes results irreproducible between laboratories in different climates. And an unloading stage that overshoots into compression invalidates the residual elongation.
The standard metals tension methods are excellent and they measure the wrong things on this material.
| ASTM F2516 | ASTM E8/E8M and ISO 6892-1 | |
|---|---|---|
| Loading | Load to 6 %, unload, pull to failure | Monotonic pull to failure |
| Captures | Plateau stresses and hysteresis | Yield, tensile strength, elongation |
| Reports | UPS, LPS, residual elongation | Proof stress and ultimate strength |
| Temperature | Controlled and reported — it changes everything | Ambient, normally |
| Right for nitinol | Yes | No — it misses the transformation |
A nitinol component qualified only on tensile strength and elongation has not been characterised. The properties that make it work — the plateau stresses and the recovery — are invisible to a single monotonic pull.
ASTM F2516 is the tension test method for superelastic nickel-titanium. The specimen is stretched to 6 % strain, unloaded to a low stress and then pulled to failure, and from that cycle the method determines upper plateau strength, lower plateau strength, residual elongation, tensile strength and elongation. The current edition is ASTM F2516-22.
Because a single monotonic pull would miss the behaviour that matters. Superelastic nitinol undergoes a stress-induced transformation from austenite to martensite, so it deforms several percent at almost constant stress and then recovers when unloaded. Both plateaus and the hysteresis between them only appear if the specimen is unloaded, and residual elongation cannot be measured any other way.
The stress at 3 % strain during loading. It is the force a device exerts as it is deformed — a stent being crimped into a delivery system, an archwire being tied into a bracket. Published guidance gives about 500 MPa on average for a 2.5 mm diameter specimen, which is the largest size the standard covers. Lower plateau strength is the corresponding value during unloading, and it is the force the device applies as it recovers.
Whether the material is actually superelastic at the temperature it was tested at. It is the difference between the strain at a specified load during loading and during unloading — that is, the deformation that did not come back. A small residual elongation means the transformation reversed cleanly. A large one means part of the deformation was ordinary plastic flow, and a device made from that material will not return to shape.
Because the plateau stresses depend on how far the test temperature sits above the material's austenite finish temperature. The same wire tested at 22 °C and at 37 °C gives different plateau strengths, and the difference is not small. Transformation temperatures are set by composition and thermomechanical processing, so the temperature has to be controlled during the test and reported with the result. A plateau strength without a temperature cannot be compared with anything.
Because they measure the wrong properties on this material. Both are excellent monotonic tension methods and both would report a yield stress, an ultimate tensile strength and an elongation — none of which describes what a nitinol device does in service. A component qualified only on tensile strength and elongation has not been characterised, because the plateau stresses and the recovery are invisible to a single pull.
A low-capacity frame with excellent strain measurement and clean control. Forces are modest — a fine wire fails in tens of newtons — so the load cell is sized for the specimen with force accuracy to ASTM E4. Strain must be measured on the specimen, ideally optically because nitinol is notch-sensitive and a clip-on gauge marks it. The control loop must reverse cleanly at 6 % strain and unload to a low stress without overshooting, and the specimen normally needs temperature control around it.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Low — a fine nitinol wire fails in tens of newtons | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Soft-faced grips that hold at plateau stress without notching a notch-sensitive surface | Our vice-action grips or a fixture built for this method, built to the specimen |
| Environment | Test temperature controlled and reported; the plateau stresses depend on how far it sits above the austenite finish temperature | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
