Bolt Strength Explained: Tensile Strength, Yield Strength, Proof Load and Hardness
Ask three suppliers what makes a bolt "strong" and you may get three different numbers — because "strength" is not one property. A bolt's behaviour under load is described by several distinct values: tensile strength, yield strength, proof load and hardness. Confusing them is how joints get under-specified, over-specified, or verified against the wrong test.
This is the concept guide behind the numbers stamped on a bolt head. Once the four terms are clear, the property class system — 4.8, 8.8, 10.9, 12.9 — reads as simple arithmetic. For choosing between those classes, see our property class selection guide; this article covers what the underlying terms actually mean.
Tensile strength: the breaking point
Tensile strength (ultimate tensile strength) is the maximum pulling stress the bolt material can withstand before it fractures. Pull a bolt in a test machine, and the load climbs until the bolt breaks; divide that peak load by the bolt's threaded stress area and you have the tensile strength, expressed in MPa (N/mm²).
It is the headline number of the property class system: the first digit of the class, multiplied by 100. A class 8.8 bolt has a nominal tensile strength of 800 MPa; a 10.9 bolt, 1,000 MPa; a 12.9 bolt, 1,200 MPa. These are the nominal values defined by the designation system in ISO 898-1 — not a measurement of any individual bolt.
But tensile strength alone is a poor design number, because a bolt that has stretched permanently is already a failed bolt, long before it breaks. That is what the next value captures.
Yield strength: where permanent damage starts
Yield strength is the stress at which the material stops springing back. Below it, a loaded bolt behaves like a stiff spring — release the load and it returns to its original length. Above it, the bolt permanently elongates: the joint loses clamp load, loosens under vibration, and is on the path to failure even though nothing has visibly broken.
This is why yield, not tensile, governs real joint design. The second digit of the property class is the yield ratio: the ".8" in 8.8 means the yield strength is 0.8 × 800 = 640 MPa nominal. The four classes we manufacture:
| Property class | Nominal tensile strength | Yield ratio | Nominal yield strength |
|---|---|---|---|
| 4.8 | 400 MPa | 0.8 | 320 MPa |
| 8.8 | 800 MPa | 0.8 | 640 MPa |
| 10.9 | 1,000 MPa | 0.9 | 900 MPa |
| 12.9 | 1,200 MPa | 0.9 | 1,080 MPa |
Note the pattern: as the class rises, the yield point moves closer to the breaking point. A 12.9 bolt yields at 90% of its tensile strength — extremely strong, but with a thinner margin of warning stretch before fracture. That trade-off is built into the system, and it is why the highest class is not automatically the safest choice.
Proof load: the working ceiling you can verify
Proof load sits between the two. It is the maximum load a bolt can carry and still return to its original dimensions afterwards — in effect, a tested "do not exceed" line set slightly below the yield point (in round terms, around nine-tenths of it for the common classes). Its purpose is verification: a bolt can be loaded to its proof load in a test and measured; if it has not permanently elongated, it passes. Tensile and yield values describe the material; proof load is the one you can actually check a finished bolt against without destroying it.
Hardness: the convenient proxy
Hardness — measured on the Rockwell or Vickers scale — is the material's resistance to indentation. It correlates with tensile strength, which makes it the fastest shop-floor check that a bolt received the heat treatment its class requires: a "10.9" bolt that tests soft was not properly quenched and tempered, whatever the paperwork says.
Two boundaries matter, though. Hardness is a proxy, not a substitute: it does not directly measure yield behaviour or ductility, and a hardness value says nothing about thread quality, dimensional accuracy or surface defects. It is one check among several — in our own production, hardness testing sits alongside composition, metallography, dimensional, contour and surface roughness inspection, with reports available to accompany an order. And the class those checks certify is decided earlier in production: by the material chosen, by how the blank is formed — cold heading in our M5–M24 range, machining in M8–M64 — and by the heat treatment that follows.
A worked example: what an M10 class 8.8 bolt actually carries
The class numbers become loads when multiplied by the bolt's tensile stress area. For a standard M10 coarse-thread bolt, that area is 58.0 mm²:
- Nominal tensile capacity: 800 MPa × 58.0 mm² ≈ 46.4 kN — the theoretical breaking load
- Nominal yield load: 640 MPa × 58.0 mm² ≈ 37.1 kN — the load at which permanent stretch begins
These are designation-based calculations, useful for understanding scale — a joint designer works from the full standard, including safety factors and the share of external load the bolt actually sees, not from a blog table. But they expose the key point: an M10 4.8 bolt in the same joint yields at roughly 18.6 kN — half the working capacity — with no visible difference on the shelf.
When the class is too low: how under-specified bolts fail
- Fracture. The service load crosses the tensile limit, or repeated loading fatigues a bolt that was stretched past yield on installation. The joint fails suddenly, not gradually.
- Thread stripping. If the bolt (or the tapped hole it screws into) lacks the strength and engagement to carry the load, the threads shear off and the bolt spins or pulls out instead of clamping. Class is part of this equation; thread engagement length and the mating material are the other parts, which is why stripping is a joint problem, not only a bolt problem.
- Silent loosening. A bolt tightened beyond its yield point does not hold its clamp load. Nothing breaks on day one; the joint works loose weeks later, and the investigation usually blames vibration.
When the class is too high: the waste nobody budgets for
Over-specifying feels safe and is not free:
- You pay for strength you cannot use. Higher classes need alloy steel and controlled quenching and tempering — in our production, 40Cr is the standard material route to the higher classes, as our guide to 40Cr steel explains — and the clamped parts often cannot accept higher clamp loads anyway.
- Ductility shrinks. As the table above shows, high-class bolts yield closer to their fracture point. The joint loses its stretch-based warning and its tolerance for imperfect tightening.
- Finishing gets harder. High-strength fasteners are sensitive to hydrogen embrittlement in electroplating; the stronger the bolt, the more carefully its surface treatment must be chosen.
- Verification costs rise. A claimed 12.9 must be proven with material and hardness evidence; if the design only needed 8.8, that proof burden bought nothing.
Common mistakes
- Quoting tensile strength as the working load. The bolt must live below yield, with margin — tensile is the breaking point, not a service limit.
- Comparing bolts by hardness alone. Two bolts can share a hardness reading and differ in ductility and thread quality; hardness verifies heat treatment, not the whole part.
- Treating an unmarked bolt as "probably strong enough". No head marking means unknown class, which means unknown yield — the value that actually governs the joint.
- Judging strength by weight or appearance. Classes 4.8 and 10.9 look identical. Only the marking, the documentation and the test data separate them.
FAQ
Which number should I design to — tensile, yield, or proof load?
Joint design is built around keeping the bolt below its yield point under all service conditions, with proof load as the verifiable ceiling for the finished bolt. Tensile strength tells you where catastrophic failure sits; it is not a design target.
Is a harder bolt always a stronger bolt?
Within the same material and heat treatment, hardness tracks tensile strength closely — that is why it works as a production check. But hardness alone does not certify a class: composition, yield behaviour and dimensions must also conform, which is why inspection covers all of them.
Why do bolt heads carry two numbers instead of one strength figure?
Because the pair encodes both anchor points of the bolt's behaviour: tensile strength (first number × 100 MPa) and the yield ratio (second number ÷ 10). Together they tell you where the bolt starts to deform permanently and where it breaks — the two facts a joint designer actually needs.
How is the class of a custom bolt verified before shipment?
By testing the things the class is made of: material composition, hardness after heat treatment, and metallographic structure, alongside dimensional, contour and roughness checks. We run these in-house and can supply inspection reports with the order — state the class on your drawing via the custom fasteners page and the verification follows the specification.
HyHaven manufactures custom fasteners to your drawing or sample, in property classes 4.8–12.9, from 2,000 pieces. Request a quote.