Interpreting GB/T 3098.1: What Mechanical Properties Are Actually Tested for Bolts, Screws, and Studs
Executive Summary
Procurement hands you a GB/T 3098.1 inspection report with a dozen data points all marked "pass" — but which ones actually determine whether the bolt is usable? This article breaks the standard down: mechanical properties fall into strength, ductility, and functional indicators, each corresponding to a different failure mode, and explains why passing tensile strength does not mean the joint is safe.
Table of Contents
An "All-Pass" Inspection Report
A familiar scenario: a batch of M12 property class 10.9 bolts arrives, and the accompanying inspection report shows a dozen data points all passing, yet after three months in service they fracture. Pulling the report again, you find that the report never performed the wedge-load test, nor checked the unengaged thread length — and these two are precisely the key to judging the quality of this batch.
The problem lies in understanding the standard. GB/T 3098.1-2010 (Mechanical properties of fasteners — Bolts, screws and studs) defines mechanical properties not as a single indicator, but as a set of indicators corresponding to different failure modes. Only by understanding which failure each indicator guards against can you judge whether a report covers the real risk.
Category 1: Strength Indicators
Strength indicators answer: how much force can this bolt bear?
| Indicator | Symbol | Physical Meaning | Corresponding Failure |
|---|---|---|---|
| Tensile strength | Rm | Maximum stress at fracture | Overload fracture |
| Stress at specified non-proportional extension 0.0048d | Rpf | Approximate 0.2% yield strength | Plastic deformation during tightening and service |
| Proof load | — | No permanent elongation after specified load | Preload loss from permanent elongation |
| Minimum tensile load | Fm,min | Minimum tensile load derived from tensile strength | Below this value is non-conforming |
Note the notation of Rpf: it is not the conventional 0.2%, but 0.0048d (d being the nominal thread diameter). For an M10 bolt, that is 48 μm of permanent elongation — the plastic elongation a joint can tolerate is inherently small, which is why the standard defines yield with a stricter criterion than 0.2%.
The meaning of the property-class digits is also worth remembering: the "10" in class 10.9 means one-hundredth of the tensile strength, with a qualifying range of 1040～1240 MPa; the "9" means the yield-to-tensile ratio is about 0.9, i.e. Rpf ≥ 940 MPa. For class 8.8, Rm is 800～965 MPa and Rpf ≥ 640 MPa.
Category 2: Ductility Indicators
Ductility indicators answer: will this bolt suddenly fracture brittlely?
Elongation after fracture A: ratio of gauge elongation to original gauge length
Reduction of area Z: ratio of contracted fracture area to original cross-sectional area
These two indicators are often overlooked during selection, but they directly reflect toughness. GB/T 3098.1 requires for class 8.8 (M16 and above) an elongation after fracture of no less than 12% and a reduction of area of no less than 48%; for class 10.9 the requirements are 9% and 48% respectively. The higher the strength class, the more lenient the elongation requirement, but the absolute value still stays above 9% — this is the safety floor.
Category 3: Functional Indicators
This is the category most often missed in inspection, and also the leading cause of field failures.
Wedge-Load Test
A wedge-angle block is placed under the bolt head before tension is applied, so the bolt bears both tension and bending. The core purposes are twofold:
- Verify the head-to-shank transition zone: fracture must occur in the shank or unengaged thread, never in the head transition zone.
- Expose low toughness: bolts with insufficient toughness are more likely to fracture brittlely under superimposed bending.
Unengaged Thread Length
The standard requires that the thread length leave an unengaged section of no less than 1d. This unengaged thread is an intentionally designed flexible section: it gives the bolt a buffer zone under load, preventing all stress from concentrating at the thread root. If the thread is made too long, bolt flexibility drops and fatigue life shortens markedly.
Decarburized Layer
A decarburized layer on the thread surface greatly reduces surface hardness and fatigue strength. The standard stipulates that on the cross-section of the unengaged thread, a fully decarburized layer is not permitted, and the depth of a partially decarburized layer has a clear upper limit: class 8.8 requires both fully and partially decarburized layers no greater than 1/2 H1; class 10.9 requires the partially decarburized layer no greater than 1/3 H1 (H1 being the thread tooth height); class 12.9 does not allow a fully decarburized layer. The higher the class, the stricter the control, because high-strength bolts' fatigue life is more sensitive to surface condition.
Property-Class Data Comparison
The most commonly used classes are summarized below:
| Property Class | Tensile Strength Rm (MPa) | 0.0048d Stress Rpf (MPa) | Most Common Material |
|---|---|---|---|
| 4.8 | 420～560 | 340 | Low-carbon steel |
| 8.8 | 800～965 | 640 | Medium-carbon steel quench-tempered / low-carbon martensitic steel |
| 10.9 | 1040～1240 | 940 | Medium-carbon alloy steel quench-tempered |
| 12.9 | 1220～1400 | 1100 | Alloy steel quench-tempered |
Mapping Inspection Items to Failure Modes
Connecting standard clauses with failure modes produces a practical inspection planning table:
| Failure Mode of Concern | Indicators to Check | Consequence of Not Checking |
|---|---|---|
| Overload fracture | Tensile strength, minimum tensile load | Insufficient load capacity |
| Plastic deformation during tightening | Rpf, proof load | Preload decay, poor reusability |
| Brittle fracture | Elongation after fracture, reduction of area, wedge load | Fracture without warning |
| Head forming defect | Wedge-load test | Fracture at head transition zone |
| Early fatigue failure | Decarburized layer, surface defects, thread root | Life far below design value |
| Thread stripping | Unengaged thread length | Clamped part scrapped |
A well-done fastener inspection plan is not "copying the standard clauses," but first listing the joint's failure modes and then working backward to the indicators that need verification. The standard is a toolbox, not a checklist.
Conclusion: From "Values Pass" to "Risk Coverage"
The value of GB/T 3098.1 lies not in prescribing a string of numbers, but in its three indicator categories covering three different failure risks: strength guards against overload, ductility guards against brittle fracture, and functional indicators guard against fatigue and manufacturing defects. As long as the reader understands what each indicator guards against, they can judge whether this batch of bolts is suitable for critical positions.
For fastener suppliers, this also means an upgrade in delivery capability: providing complete performance data including wedge load, decarburized layer, and proof load — rather than just a tensile sheet — is the foundation for qualifying high-safety-class projects.
