Proof Load vs. Minimum Tensile Load: Distinguishing the Two Core Strength Indicators of Fasteners
Executive Summary
Proof load and minimum tensile load—both show "pass" on the report, but they target completely different failure modes: one guards against permanent deformation, the other against tensile fracture. This article uses test methods and applicable objects to explain the essential difference between these two indicators, and gives a clear conclusion on which one to use for nuts versus bolts.
Table of Contents
The Two Indicators Are Often Confused
On fastener inspection reports, proof load and minimum tensile load often appear together with similar values, and are easily mistaken for "the same thing." But the failures they guard against are completely different:
- Minimum tensile load guards against tensile fracture—it is the minimum load a threaded fastener must withstand without breaking.
- Proof load guards against permanent deformation—it is the load which, after being applied and removed, must not produce permanent elongation exceeding a specified amount.
One is "not breaking," the other is "not permanently deforming." This difference carries enormous weight in real engineering: a nut that passes tensile strength may well exceed the elongation limit under proof load, and the latter is the direct cause of preload loss.
Minimum Tensile Load: Calculated, Not Measured
The formula for minimum tensile load Fm,min is very simple:
Fm,min = Rm,min × As
Rm,min: lower tensile strength limit specified for the property class (MPa)
As: threaded stress cross-sectional area (mm²)
The threaded stress cross-sectional area As is neither the minor-diameter area nor the major-diameter area, but an equivalent area specified by the standard:
As = π/4 × [(d2 + d3) / 2]²
d2: pitch diameter; d3: minor diameter (accounting for root radius effect)
The average of the pitch and minor diameters is used because thread shearing occurs both at the minor diameter (tensile fracture) and relates to the shear capacity at the pitch diameter; the actual fracture surface is an irregular curved surface between the two. Only the load calculated from this equivalent area matches test results.
For common sizes, As and the corresponding loads can be looked up directly:
| Thread Size | As (mm²) | Grade 8.8 Fm,min (kN) | Grade 10.9 Fm,min (kN) | Grade 12.9 Fm,min (kN) |
|---|---|---|---|---|
| M6 | 20.1 | 16.1 | 20.9 | 24.5 |
| M8 | 36.6 | 29.3 | 38.1 | 44.7 |
| M10 | 58.0 | 46.4 | 60.3 | 70.8 |
| M12 | 84.3 | 67.4 | 87.7 | 102.9 |
| M16 | 157 | 125.6 | 163.3 | 191.5 |
| M20 | 245 | 196.0 | 254.8 | 298.9 |
Proof Load: Test-Verified, Focused on Residual Deformation
The proof load test is conducted by applying a specified load to the fastener, holding it for 10–15 s, then unloading and measuring the change in length before and after.
The key point is the limit on "permanent elongation." GB/T 3098.1 specifies for bolts and screws:
when L ≤ 8d, permanent elongation after unloading must not exceed 12.5 µm;
when L > 8d, it must not exceed 25 µm (L is the fastener length).
What does 12.5 µm mean? Roughly one-sixth the diameter of a human hair. This limit is extremely strict, which is precisely what makes it effective at screening out parts that "pass strength but easily deform permanently."
| Item | Minimum Tensile Load | Proof Load |
|---|---|---|
| Criterion object | Whether it fractures | Whether permanent elongation exceeds the limit |
| Load magnitude | Equals the lower tensile strength limit of the material | Generally 0.8–0.9 times the lower tensile strength limit |
| Loading method | Load to fracture | Load and hold, then unload |
| Applicable parts | Bolts, screws, studs | Bolts, screws, studs, nuts |
| Failure guarded against | Overload fracture | Permanent deformation, loss of preload |
Proof Load Is the Primary Criterion for Nuts
For nuts, the standard (GB/T 3098.2) primarily assesses proof load, not minimum tensile load.
The reason is: functionally, the nut is the party "pulled by the bolt"; it experiences shear and compression of the thread flanks, not full-section tension. Assessing a nut with tensile load has no physical meaning. Thus the inspection approach for nuts is—under proof load, there must be no thread stripping, cracking, or excessive permanent elongation.
A nut's property class is usually written as "matched with a certain bolt class," meaning its proof load is sufficient to make the mating bolt fracture first while the nut does not fail. For example, a class 10 nut matches a grade 10.9 bolt, and a class 8 nut matches a grade 8.8 bolt.
Two Pitfalls in Proof Load Testing
Pitfall 1: Loading Speed
Loading too fast introduces dynamic effects, making the measured permanent elongation too large; loading too slowly may introduce creep, likewise distorting the data. The standard specifies the loading rate explicitly, and differences between third-party testing labs often stem from this point.
Pitfall 2: Measurement Reference
Measuring permanent elongation requires measuring the length at the same location before and after applying the test load, and the measuring device itself must not deform under force. In actual inspection, if end-face gauges are used and contact deformation exists, it introduces an error of several micrometers—and the acceptance limit is only 12.5 µm, so the error proportion cannot be ignored.
Therefore, when choosing a testing lab, confirm whether they use a dedicated proof load testing machine (with an equal-length measuring device), rather than a general tensile tester temporarily converted. The impact of such equipment differences on the acceptance result is far greater than many purchasers expect.
Conclusion: Bolts Look at Tensile Load, Nuts Look at Proof Load
The division of labor between these two indicators can be remembered in one sentence:
- Bolts, screws, studs—minimum tensile load determines "no fracture," proof load determines "no deformation"; both must be checked.
- Nuts—proof load is the primary criterion; the core is that threads do not strip or elongate.
- The two must have matching property classes, so the bolt fractures first and the nut fails later, keeping loss on the cheapest replaceable part.
Understanding this division resolves many selection-related confusions naturally: why nuts need not be checked for tensile strength, why both loads must appear on bolt reports, and why matching classes matter more than unit price when replacing parts in the field.
