Tensile Strengths of Fasteners
What Is Tensile Strength?
Tensile strength, or Ultimate Tensile Strength (UTS), defines the maximum axial pulling force a fastener can withstand before failure. For bolts, screws, and studs, tensile strength describes resistance to being pulled apart along the fastener's axis. It does not indicate resistance to shear or bending.Tensile strength varies based on several factors, including material, heat treatment, hardness, fastener diameter, and thread. The manufacturing quality standards used to produce a fastener also significantly affect final strength values.
Standards, Grades, and Markings for Fastener Tensile Strength
Fastener tensile strength is defined through standardized grading systems. These systems allow users to select fasteners with predictable mechanical properties.Metric and ISO Grades
Metric fasteners use property class markings to indicate tensile strength in accordance with ISO standards. Common metric grades include 8.8, 10.9, and 12.9. For a metric class like 8.8:- The first number multiplied by 100 gives the approximate ultimate tensile strength in MPa
- 8 × 100 = 800 MPa UTS
- The second number indicates the ratio of yield strength to tensile strength
- .8 ratio means yield strength is about 80 percent of UTS
Imperial, SAE, and ASTM Grades
Imperial fasteners typically follow SAE J429 or ASTM standards, with grades identified by markings on the fastener head. A higher grade number indicates a higher tensile and yield strength. Examples include Grade 2, Grade 5, and Grade 8.Bolt Tensile Strength Chart
Common Grades for Metric Fasteners
| Metric Class | Approx. Tensile Strength | Approx. Yield Strength | Typical Applications | |
|---|---|---|---|---|
| 4.8 | 400 MPa | 320 MPa | Light-duty assemblies | |
| 5.8 | 500 MPa | 400 MPa | General-purpose fastening | |
| 8.8 | 800 MPa | 640 MPa | Machinery and structural use | |
| 10.9 | 1,000 MPa | 900 MPa | High-load and automotive | |
| 12.9 | 1,200 MPa | 1,080 MPa | Heavy-duty and high-stress joints | |
Common Grades for SAE Fasteners
| SAE Grade | Approx. Tensile Strength | Head Marking | Typical Applications |
|---|---|---|---|
| Grade 2 | 74,000 psi | No Marking or Mfr logo | Light-duty fastening |
| Grade 5 | 120,000 psi | 3 radial lines | Automotive and machinery |
| Grade 8 | 150,000 psi | 6 radial lines | Heavy-duty and structural |
Values shown are typical ranges. Always confirm specifications against the relevant governing standard for each use.
Related Mechanical Terms and How They Differ from Tensile Strength
In addition to tensile strength, there are several terms used to measure other aspects of fastener performance characteristics. Understanding these terms helps prevent overloading and fastener failure.Yield Strength
Yield strength defines the load at which a fastener begins to deform permanently. Beyond this point, the fastener will not return to its original length after unloading.Proof Load
Proof load is the maximum load a fastener must withstand without permanent deformation. It is commonly used in design and quality control. Proof load testing provides a safety margin below yield strength.How Fastener Tensile Strength Is Measured
Fastener tensile strength is measured through standardized testing methods. One common method is tensile load testing. The fastener is pulled axially in a calibrated test machine until fracture occurs. The maximum load is recorded.Wedge tensile testing is another method for measuring tensile strength. A wedge is placed under the fastener head during testing. This introduces bending stress and verifies the integrity of the head and shank.
Grade markings are used to indicate whether a fastener has passed these controlled tests.
How to Choose Bolt Strength Grade
General Grade Selection Guidelines
Light-duty and non-structural assemblies often use lower to medium-strength fasteners. Examples include metric 4.8 or 5.8 and SAE Grade 2 or some Grade 5 applications.Machinery, structural frames, and load-bearing joints often require higher strength grades. Metric 8.8, 10.9, and 12.9 are common, as well as SAE Grade 8 for heavy-duty equipment.
Use Case Examples for Fastener Tensile Strength Grades
- Metric 8.8 or SAE Grade 5: Heavy-duty applications like agricultural equipment, heavy machinery, and structural frames
- Metric 10.9 or SAE Grade 8: Mid-to-heavy applications like automotive and general construction components often require
- Metric 5.8 or SAE Grade 2
5: Light-duty applications, like brackets, signage, and other general assemblies, often use
Material and Environment Considerations When Choosing a Fastener Grade
- Stainless fasteners offer corrosion resistance, but usually have lower tensile strength than alloy steel
- Coated or plated fasteners may be needed for outdoor applications
- Certain Temperature applications may require specialty alloys
- Application conditions should always be reviewed before final fastener grade selection