A polyurethane part can pass hardness and rebound checks and still fail in service if tensile strength was never specified. Rollers crack, seals tear, couplings fatigue — often after commissioning, when the root cause is a property nobody tested. Tensile strength is the number that closes that gap: it tells you how much pulling force the material can survive before it lets go.
What Tensile Strength Actually Measures in Polyurethane
Tensile strength is the maximum tensile stress a polyurethane specimen can withstand while being stretched before it breaks. It is normally reported in megapascals (MPa) or pounds per square inch (psi), and it is calculated by dividing the applied force by the original cross-sectional area of the test specimen [1][2].
That number alone is not enough. Two polyurethane samples can both report 40 MPa and behave very differently in use if one breaks at 80% elongation and the other at 450% elongation. This is why a useful tensile specification pairs tensile strength with elongation at break, and sometimes with modulus and tear strength.
How Tensile Strength Is Tested
Test standards and specimen setup
For cast and thermoplastic polyurethane elastomers, the common standards are ASTM D412 and ISO 37. For rigid or structural polyurethane parts, ASTM D638 and ISO 527 are more common [1][2][3][4].
A standard test uses a dumbbell-shaped specimen cut or molded to the dimensions in the selected standard. The two ends are gripped, and one grip moves at a constant speed while load and extension are recorded. For rubber-like polyurethane, a crosshead speed of 500 mm/min is widely specified, but the exact speed depends on the standard and specimen type.
Preparation has an outsized effect on the result. Bubbles, voids, notches from a dull die, inconsistent thickness, or a specimen cut with visible flaws can all start a premature break. Conditioning also matters: tests are usually run at 23 ± 2 °C and 50 ± 5 % relative humidity unless the part’s service conditions call for something else [1][2].
Typical values across polyurethane systems
In cast elastomer work, I generally see tensile strength targets from about 5 MPa for very soft grades up to roughly 55 MPa for high-hardness, highly crosslinked systems. Thermoplastic polyurethane grades often sit in a similar upper range, while rigid and microcellular foams are much lower because their strength comes from density and cell structure rather than solid elastomer crosslinking. These are screening ranges, not specification limits; the correct value always comes from the specific formulation, cure cycle, and part geometry.
Chemistry and Processing Variables That Move the Number
Tensile strength is not a fixed property of “polyurethane” as a category. It moves with formulation and process.
- Hard segment content. The isocyanate and chain extender form ordered hard segments. Higher hard segment content generally raises tensile strength and modulus, but it can also reduce elongation and make the material more notch-sensitive.
- Polyol backbone. Polyester polyols tend to deliver higher tensile and tear strength than many polyether polyols because of stronger intermolecular forces, but they often trade away hydrolysis resistance. Polyether systems are generally more forgiving in wet environments.
- NCO index and stoichiometry. Running a formulation with insufficient isocyanate leaves soft, under-crosslinked material. Too far the other way can create excessive crosslinks, raising stiffness while making the part brittle.
- Metering and mixing quality. If the A/B ratio drifts or the mixing head leaves unmixed regions, the same formula produces different tensile results from cup to cup and shift to shift. On our casting units, dynamic metering accuracy within ±0.3% and vacuum defoaming at or below −0.095 MPa are used to hold those variables stable.
- Cure and post-cure. A part pulled from the mold too early has not reached its design network. Post-curing can raise tensile strength and improve heat resistance, but the time and temperature must be matched to the system.
In a rail pad line for a South American operator, switching to closed-loop temperature control and high-pressure metering tightened elastic modulus and part-to-part consistency. Tensile testing became more reproducible for the same reason: the reaction conditions stopped drifting.
Even a well-mixed formulation can produce a part that fails tensile requirements in service if the mold was underfilled or the shot size drifted. How to Calculate Shot Weight in Polyurethane covers the fill calculation that connects machine output to repeatable part properties.
Tensile Strength vs. Hardness, Tear Strength, and Modulus
Hardness is the most commonly quoted property, but it is not a shortcut to tensile strength. Shore hardness measures resistance to indentation at the surface. Tensile strength measures resistance to being pulled apart through the whole cross-section. A material can be hard and relatively brittle, or moderately hard and extremely tough.
Modulus describes stiffness at low strain — how quickly the material resists deformation — while tensile strength describes the failure point. Tear strength measures resistance to crack growth from an existing cut or nick. In applications such as rollers, seals, and rail pads, all three properties usually matter, and optimizing one can reduce another.
Hardness is the number most people ask for first, but it says little about how the part fails under stretch. Shore A vs Shore D Hardness in Polyurethane covers how the two scales map to polyurethane formulation choices.
How to Specify Tensile Strength Without Over-Specifying
The most common specification mistake is writing “high tensile strength” with no standard, no specimen, and no acceptance tolerance. That cannot be tested repeatably and leaves both buyer and supplier guessing.
A workable specification should include:
- the test standard, for example ASTM D412 or ISO 37,
- the specimen type and conditioning,
- the minimum tensile strength and minimum elongation at break,
- the test frequency, from first-article approval through periodic production lots.
Do not chase a maximum number at the expense of the property your application actually needs. A high-durometer part may show impressive tensile strength but fail in rebound or compression set. The better question is: what combination of tensile strength, elongation, and tear resistance keeps this part functional over its full service temperature range?
A roller that passes incoming inspection can still fail in service if the cast layer is under-cured or the mix ratio drifted during the run. How Polyurethane Rollers Are Made From Casting to Cure covers the casting variables that decide whether that tensile number holds up on the machine.
If you are comparing polyurethane systems for a rotating or load-bearing part, fix the test method before buying tooling. Send your part drawing, target hardness and elongation, and the failure mode you are seeing to info@chinahaifeng.com or WhatsApp 86 13566296633, and we can help you set a realistic tensile window for the next run.
Get a Tensile Property Review for Your Polyurethane Part
Tensile strength becomes difficult only when it is treated as a vague phrase instead of an engineering value. If you are specifying a cast polyurethane roller, seal, wheel, buffer, or structural component, let us help you define the test standard, realistic acceptance range, and the metering and cure controls needed to hold it in production.
Contact us at info@chinahaifeng.com or WhatsApp 86 13566296633 with the part drawing, required hardness and elongation, and the failure mode you are trying to eliminate.
Frequently Asked Questions
What is the difference between tensile strength and elongation at break?
Tensile strength is the maximum stress the material withstands before breaking. Elongation at break is how much it stretched at that point. Together they describe whether a material is strong and rigid, strong and stretchy, or neither [1][2].
Does higher Shore hardness always mean higher tensile strength?
No. Hardness measures surface indentation resistance, while tensile strength measures resistance to breakage under tension. A hard compound can be brittle and test lower in tensile strength than a softer but tougher compound.
Why do two polyurethane parts with the same formula test differently?
Most variation comes from processing: off-ratio mixing, incomplete degassing, mold temperature differences, short cure time, or inconsistent shot fill. Test-specimen preparation also contributes, especially notches and voids.
What tensile strength should I specify for a polyurethane roller or seal?
There is no universal number. Start with the service load, elongation required for the part to function, and whether the part must resist tearing or rebound. Set a minimum tensile strength and minimum elongation based on the specific formulation, test standard, and production process — not on a generic datasheet value.
References
[1] ASTM D412-16, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension, ASTM International, West Conshohocken, PA, 2016.
[2] ISO 37:2017, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties, International Organization for Standardization, Geneva, 2017.
[3] ASTM D638-14, Standard Test Method for Tensile Properties of Plastics, ASTM International, West Conshohocken, PA, 2014.
[4] ISO 527-1:2019, Plastics — Determination of tensile properties — Part 1: General principles, International Organization for Standardization, Geneva, 2019.
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