1. Introduction: Understanding Breaking Torque in Fasteners
In the fastener industry, breaking torque (also known as failure torque or torsional strength) is one of the most critical mechanical properties that determines the reliability and safety of bolted connections. Breaking torque refers to the maximum torsional load that a bolt or screw can withstand before failure occurs—typically manifesting as thread stripping, torsional fracture, or plastic deformation of the fastener body
While tensile strength and proof load are well-understood parameters for most fasteners, there is a specific category of bolts and screws for which traditional tensile testing is not feasible or applicable:
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Fasteners with nominal diameters below M3 (for which minimum tensile loads are not specified in standards such as ISO 898-1)
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Fasteners with diameters of 3–10 mm that are too short to undergo standard tensile testing
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Fasteners where torsional loading is the primary service condition
For these products, breaking torque testing serves as the essential mechanical property verification method.This article provides a comprehensive technical analysis of breaking torque—covering definitions, applicable standards, test methods, numerical data for both carbon steel and stainless steel fasteners, influencing factors, and practical solutions to common industry problems.
2. What Is Breaking Torque?
2.1 Definition and Mechanism
Breaking torque is the torsional load applied to a fastener that causes failure of the threaded section or the fastener body. When a torsional load is applied to a bolt or screw, shear stresses develop across the cross-section of the fastener. As the torque increases, the shear stress eventually exceeds the material’s shear strength, resulting in:
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Thread stripping — the threads are sheared off or plastically deformed to the point where they can no longer transmit load–
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Torsional fracture — the fastener body twists and fractures across the cross-section
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Plastic torsional deformation — permanent twisting of the fastener without complete fracture
2.2 Breaking Torque vs. Other Torque Parameters
It is essential to distinguish breaking torque from other torque-related parameters:
| Parameter | Definition | Application |
|---|---|---|
| Breaking Torque (Failure Torque) | Maximum torque before fastener failure | Mechanical property verification, design limit |
| Tightening Torque (Installation Torque) | Torque applied during assembly | Assembly process control |
| Stripping Torque | Torque at which threads begin to strip | Assembly limit, preventing over-tightening– |
| Prevailing Torque | Torque required to rotate a locking fastener | Locking feature verification |
The ratio of breaking torque to tightening torque is a critical design parameter—industry practice typically recommends that tightening torque should not exceed 60–80% of the breaking torque to provide a safety margin against failure.
3. International Standards for Breaking Torque
3.1 GB/T 3098.13-1996 (China)
GB/T 3098.13-1996 is the Chinese national standard titled “Mechanical properties of fasteners — Torsional test and minimum breaking torques for bolts and screws with nominal diameters 1 mm to 10 mm.”
Scope and Application:
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Nominal diameters: 1 mm to 10 mm–
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Property classes: 8.8 to 12.9 per GB/T 3098.1–
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Applicable to bolts and screws with thread sizes below M3 (for which no minimum tensile loads are specified in GB/T 3098.1)–
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Also applicable to bolts and screws with diameters 3–10 mm that are too short for tensile testing
3.2 ISO 898-7:1992 (International)
ISO 898-7:1992 is the international equivalent standard, titled “Mechanical properties of fasteners — Part 7: Torsional test and minimum torques for bolts and screws with nominal diameters 1 mm to 10 mm.”–
Key Provisions:
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Specifies a torsional test for determining breaking torque–
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Applies to bolts and screws of property classes 8.8 to 12.9 in accordance with ISO 898-1–
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Applicable to thread sizes below M3 for which no breaking and proof loads are indicated in ISO 898-1–
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Not applicable to hexagon socket set screws
3.3 EN 20898-7:1995 (European)
EN 20898-7:1995 is the European harmonized standard, adopting ISO 898-7:1992
3.4 ASTM Standards
ASTM F606/F606M provides standard test methods for measuring the mechanical properties of threaded fasteners, including torque-related tests
3.5 Standard Summary
| Standard | Region | Title | Scope |
|---|---|---|---|
| GB/T 3098.13-1996 | China | Torsional test and minimum breaking torques for bolts and screws | 1–10 mm, 8.8–12.9 |
| ISO 898-7:1992 | International | Mechanical properties of fasteners — Part 7 | 1–10 mm, 8.8–12.9 |
| EN 20898-7:1995 | Europe | Mechanical properties of fasteners — Part 7 | 1–10 mm, 8.8–12.9 |
| ASTM F606/F606M | USA | Mechanical properties of threaded fasteners | General torque testing |
4. Breaking Torque Data for Carbon Steel Fasteners
4.1 Data Source and Standard
The following breaking torque data for carbon steel fasteners is derived from GB/T 3098.13, applicable to bolts and screws with 6g, 6f, and 6e thread tolerances.
4.2 Minimum Breaking Torque Values — Carbon Steel (8.8 to 12.9 Grades)
| Thread Size | Pitch (mm) | Min. Breaking Torque (N·m) — 8.8 | Min. Breaking Torque (N·m) — 9.8 | Min. Breaking Torque (N·m) — 10.9 | Min. Breaking Torque (N·m) — 12.9 |
|---|---|---|---|---|---|
| M1 | 0.25 | 0.033 | 0.036 | 0.040 | 0.045 |
| M1.2 | 0.25 | 0.075 | 0.082 | 0.092 | 0.100 |
| M1.4 | 0.30 | 0.120 | 0.130 | 0.140 | 0.160 |
| M1.6 | 0.35 | 0.160 | 0.180 | 0.200 | 0.220 |
| M2 | 0.40 | 0.370 | 0.400 | 0.450 | 0.540 |
| M2.5 | 0.45 | 0.820 | 0.900 | 1.000 | 1.100 |
| M3 | 0.50 | 1.500 | 1.700 | 1.900 | 2.100 |
| M3.5 | 0.60 | 2.400 | 2.700 | 3.000 | 3.300 |
| M4 | 0.70 | 3.600 | 3.900 | 4.400 | 4.900 |
| M5 | 0.80 | 7.600 | 8.300 | 9.300 | 10.000 |
| M6 | 1.00 | 13.000 | 14.000 | 16.000 | 17.000 |
| M7 | 1.00 | 23.000 | 25.000 | 28.000 | 31.000 |
| M8 | 1.25 | 33.000 | 36.000 | 40.000 | 44.000 |
| M8×1 (fine) | 1.00 | 38.000 | 42.000 | 46.000 | 52.000 |
| M10 | 1.50 | 66.000 | 72.000 | 81.000 | 90.000 |
| M10×1 (fine) | 1.00 | 84.000 | 92.000 | 102.000 | 114.000 |
| M10×1.25 (fine) | 1.25 | 75.000 | 82.000 | 91.000 | 102.000 |
4.3 Key Observations from the Data
1. Strength Grade Impact: For the same thread size, the breaking torque increases proportionally with the strength grade. For example, an M6 bolt has a breaking torque of 13 N·m at 8.8 grade, increasing to 17 N·m at 12.9 grade—a 30.8% increase.
2. Thread Size Impact: Breaking torque increases approximately with the cube of the diameter (D³), consistent with torsional strength theory–. An M10 bolt (10 mm) has a breaking torque of 66 N·m at 8.8 grade, compared to 13 N·m for M6 (6 mm)—a 5× increase for a 1.67× diameter increase.
3. Pitch Impact: For the same nominal diameter, fine pitch threads generally provide higher breaking torque values than coarse pitch threads. For M10, the coarse pitch (1.5 mm) gives 66 N·m at 8.8 grade, while the fine pitch M10×1 gives 84 N·m—a 27.3% increase.
5. Breaking Torque Data for Stainless Steel Fasteners
5.1 Data Source and Standard
The following breaking torque data for stainless steel fasteners is derived from GB/T 3098.6-2000, the Chinese national standard for mechanical properties of stainless steel fasteners
Stainless steel fasteners are classified by property classes based on their tensile strength:
| Property Class | Material | Minimum Tensile Strength (MPa) | Typical Grade |
|---|---|---|---|
| 50 | Austenitic stainless steel | 500 | A2-50, A4-50 |
| 70 | Austenitic stainless steel | 700 | A2-70, A4-70 |
| 80 | Austenitic stainless steel | 800 | A2-80, A4-80 |
5.2 Minimum Breaking Torque Values — Stainless Steel (50, 70, 80 Classes)
| Thread Size | Breaking Torque (N·m) — Class 50 | Breaking Torque (N·m) — Class 70 | Breaking Torque (N·m) — Class 80 |
|---|---|---|---|
| M1.6 | 0.15 | 0.20 | 0.24 |
| M2 | 0.30 | 0.40 | 0.48 |
| M2.5 | 0.60 | 0.90 | 0.96 |
| M3 | 1.10 | 1.60 | 1.80 |
| M4 | 2.70 | 3.80 | 4.30 |
| M5 | 5.50 | 7.80 | 8.80 |
| M6 | 9.30 | 13.00 | 15.00 |
| M8 | 23.00 | 32.00 | 37.00 |
| M10 | 46.00 | 65.00 | 74.00 |
| M12 | 80.00 | 110.00 | 130.00 |
| M16 | 210.00 | 290.00 | 330.00 |
5.3 Key Observations from Stainless Steel Data
1. Class 70 vs. Class 50: For M8, Class 70 provides 32 N·m compared to Class 50’s 23 N·m—a 39.1% increase.
2. Class 80 vs. Class 70: For M8, Class 80 provides 37 N·m compared to Class 70’s 32 N·m—a 15.6% increase.
3. Size Sensitivity: The breaking torque increases significantly with diameter—M16 at Class 70 provides 290 N·m, more than 26× the M4 value of 3.8 N·m.
4. Stainless vs. Carbon Steel Comparison: For equivalent sizes, carbon steel fasteners generally have higher breaking torque values than stainless steel. For example, an M8 carbon steel 8.8 grade bolt has a breaking torque of 33 N·m (GB/T 3098.13), while an M8 stainless steel Class 70 bolt has 32 N·m (GB/T 3098.6)—a modest difference. However, for M10, the 8.8 carbon steel bolt provides 66 N·m versus Class 70 stainless steel’s 65 N·m
6. Breaking Torque Testing Methods
6.1 Test Methods
| Method | Description | Application |
|---|---|---|
| Direct Torsion Method | Fastener clamped at one end, torque applied at the other until failure | Most common |
| Torque-Rotation Method | Torque and rotation angle are continuously recorded until failure | Provides torque-angle curve for failure analysis– |
| Static Torque Test | Gradually increasing torque to predetermined failure criteria | Quality control, verification testing |
6.2 Interpretation of Results
The test result is the maximum torque value recorded before failure. Failure is typically defined as:
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Complete fracture of the fastener
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Thread stripping where the fastener can no longer transmit torque
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A specified drop in torque from the peak value (typically 10–20% drop)
7. Factors Affecting Breaking Torque
7.1 Material Grade and Strength
As clearly demonstrated by the data in Sections 4 and 5, higher strength grades directly correlate with higher breaking torque values–. For a given thread size, upgrading from 8.8 to 12.9 grade can increase breaking torque by 30–40%.
7.2 Thread Size and Pitch
Breaking torque is proportional to the cube of the diameter. This means that even small increases in diameter yield significant improvements in torsional strength.
Fine pitch threads generally provide higher breaking torque than coarse pitch threads for the same nominal diameter due to the larger stress area and reduced helix angle.
7.3 Surface Finish and Lubrication
Surface conditions significantly affect the torque-tension relationship–:
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Lubricated threads reduce friction, allowing higher preload for the same applied torque
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Excessively oily surfaces can reduce friction coefficient and increase the risk of thread stripping–
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Coatings such as zinc plating or black oxide can alter the friction characteristics
8. Common Problems and Solutions
8.1 Problem: Thread Stripping During Assembly
Symptoms: Threads are sheared or deformed during tightening, resulting in a loose connection that appears tight
Root Causes:
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Applied torque exceeds the breaking torque of the fastener
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Insufficient thread engagement length
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Cross-threading during installation
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Excessive tightening speed (impact tools)
Solutions:
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Use torque-limited tools — always use torque wrenches for critical applications–
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Verify torque specifications — ensure the specified tightening torque is ≤60–80% of the breaking torque
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For stainless steel fasteners — reduce tightening torque by 10–15% compared to carbon steel equivalents, as stainless steel has lower shear strength–
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Use proper installation techniques — start threads by hand before using power tools
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Consider thread-locking compounds — these can provide additional security without requiring higher torque
8.2 Problem: Fastener Fracture During Tightening
Symptoms: The bolt or screw breaks before reaching the specified torque.
Root Causes:
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Tightening torque exceeds the breaking torque
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Torque wrench calibration error (typical errors: ±7–15%)
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Material defect or improper heat treatment
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Stress concentration at thread root or under-head fillet
Solutions:
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Calibrate torque tools regularly — ensure accuracy within ±5%–
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Verify breaking torque values against the standard data for the specific grade and size
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Reduce applied torque for stainless steel fasteners (10–15% reduction recommended)–
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Perform torque-to-failure studies to establish safe tightening windows–
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Inspect for surface defects such as cracks, laps, or seams
8.3 Problem: Inconsistent Breaking Torque Results
Symptoms: Test results show wide variation for supposedly identical fasteners.
Root Causes:
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Variation in material properties (batch-to-batch)
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Inconsistent heat treatment
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Variation in thread quality
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Improper test setup
Solutions:
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Standardize test procedures per GB/T 3098.13 or ISO 898-7
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Use statistical process control to monitor manufacturing consistency
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Request mill test certificates verifying material grade and properties
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Implement incoming inspection to verify breaking torque values
9. Summary
Breaking torque is a critical mechanical property for bolts and screws—particularly for smaller diameters (below M3) and short fasteners where tensile testing is not feasible. Key takeaways:
| Key Point | Explanation |
|---|---|
| Breaking torque is the maximum torsional load before failure | Critical for preventing assembly failures |
| Standards define minimum values | GB/T 3098.13 (carbon steel), GB/T 3098.6 (stainless steel), ISO 898-7 |
| Higher strength grades = higher breaking torque | 12.9 > 10.9 > 8.8; Class 80 > 70 > 50 |
| Fine pitch threads provide higher breaking torque | Up to 27% increase for M10×1 vs. coarse pitch |
| Stainless steel has lower breaking torque than carbon steel | Use 10–15% lower tightening torque– |
| Breaking torque ∝ D³ | Small diameter increases yield significant strength gains |
| Never exceed 60–80% of breaking torque in assembly | Provides safety margin against failure |
For international buyers, understanding breaking torque data—and the standards that define them—is essential for selecting the right fastener for each application and preventing costly assembly failures.
