Proof Load vs Yield vs Tensile Strength in Bolts Explained

Three Numbers, Three Different Questions

When a bolt datasheet lists proof load, yield strength and tensile strength, it is answering three different questions: how much load can this bolt take without changing at all (proof), where does it start to stretch permanently (yield), and where does it finally break (tensile). Confusing them leads to two classic mistakes: designing to tensile strength as if the bolt were usable up to fracture, or treating proof load as the working limit when the joint actually needs preload below yield.

All three are defined by ISO 898-1 for the standard property classes — the background on 8.8 / 10.9 / 12.9 markings is in our strength grades guide.

The Definitions

Proof Load

The axial force the bolt must withstand, in a testing machine, without measurable permanent set. Load it to proof load, hold, release — it returns to its original length. It is a pass/fail verification test, not a design number, and it sits at roughly 90% of yield for the standard classes.

Yield Strength (Rp0.2)

The stress at which the bolt has permanently elongated by 0.2% — the practical end of elastic behaviour. Beyond this, the bolt stretches for good; the joint loses clamp and, in structural terms, has already failed even though nothing is broken.

Tensile Strength (Rm)

The stress at the highest force the bolt carries — the top of the stress-strain curve. Fracture follows shortly after. The property class number is the tensile strength: class 8.8 = 800 MPa minimum, 10.9 = 1,000 MPa, 12.9 = 1,200 MPa. Hence the naming logic in class 12.9 socket screws and class 8.8/10.9 hex bolts.

Reference Values by Property Class

Property Class	Proof Stress (MPa)	Yield min (MPa)	Tensile min (MPa)	Breaks at ~
4.8	310	340	420	~1.2× proof
5.8	380	420	520	~1.2× proof
8.8 (≤M16)	580	640	800	~1.35× proof
10.9	830	940	1,040	~1.2× proof
12.9	970	1,100	1,220	~1.25× proof

Note how narrow the margin is: for 10.9, yield is only ~13% above proof stress. That is why torque tables are built around 70–75% of proof-load preload — details in our metric torque chart.

Which Number Does Your Joint Design Use?

Bolt preload (the clamp you actually design with): 65–75% of proof load for static joints — the de-facto standard across machinery design.
Proof load: the acceptance test for the lot — and the reference point for nuts: a nut’s class is proven by a proof-load test on a matched bolt, which is why high-strength nuts and DIN 934 nuts are graded with the same numbering logic.
Yield: the ceiling for any joint that must survive overload without permanent loss of clamp.
Tensile: for fracture-mechanics, factor-of-safety against collapse, and understanding the class marking — never a working load.

The One-Sentence Rule

Design the preload from proof load, keep yield as your “no permanent damage” ceiling, and read tensile strength as the class label — useful for identification and fracture margins, useless as a design load.

Frequently Asked Questions

Is a higher class always safer?

No. Higher classes trade ductility for strength — a 12.9 bolt is harder and more susceptible to hydrogen embrittlement after electroplating and to brittle fracture under shock. For structural joints, 8.8 is frequently the more forgiving choice; see our grades comparison for the full trade-off.

How are these values verified in production?

By wedge tensile testing and hardness sampling per ISO 898-1 on production lots, documented on the mill test certificate. A verification regime is standard at any quality manufacturer.

What about shear loading?

None of the three numbers above directly cover shear — shear capacity is commonly approximated at ~60% of tensile strength. If your joint loads bolts in shear, that approximation belongs in the calculation with its own safety factor.

Do stainless bolts have the same classes?

A2/A4 stainless use a different system (70/80 suffixes, e.g., A4-80 ≈ 800 MPa tensile). Our 304 vs 316 guide explains the stainless grades and their strength designations.