In engineering and construction, selecting the correct screw requires a precise understanding of the forces it must withstand. Tensile and shear strength are two fundamental mechanical properties that dictate fastener performance and joint integrity. This guide clarifies their differences and application in professional contexts.
Defining the Core Strengths
Tensile Strength: Measures a fastener's resistance to forces that pull it apart along its axis (axial loading). Failure occurs when the screw stretches and ultimately fractures, typically at the thread root.
Measured in: MPa or psi.
Example: A bolt clamping two flanges together is under tensile stress.
Shear Strength: Measures a fastener's resistance to forces that attempt to cut or slide it across its cross-section. Failure occurs when the screw shears off.
Measured in: MPa or psi.
Estimation: For many steels, shear strength is approximately 60% of tensile strength, though this varies.
Example: A bolt securing a bracket to a beam primarily experiences shear force from the load.
Key Differences & Design Implications
| Aspect | Tensile Strength | Shear Strength |
|---|---|---|
| Force Direction | Axial (pulling apart) | Transverse (sliding/cutting) |
| Critical Zone | Threaded section | Shank section (preferably unthreaded) |
| Primary Concern | Clamping force, joint separation | Lateral movement, sliding failure |
A critical design principle is to position the shear plane on the unthreaded shank where possible. Threads act as stress concentrators and significantly reduce effective shear strength.
Screw Selection Based on Dominant Load
For Tensile-Dominated Applications: Prioritize high tensile property classes (e.g., 8.8, 10.9, 12.9). Ensure sufficient thread engagement and use washers to distribute load.
For Shear-Dominated Applications: Specify fasteners with a long, unthreaded shoulder (shoulder bolts) or smooth shank. Design for "double shear" where applicable to enhance stability.
Material & Combined Loading Considerations
The base material (e.g., medium carbon steel, alloy steel, stainless steel) fundamentally determines strength potential. Always consult technical datasheets for certified values.
In practice, many joints experience combined shear and tension. Engineers use interaction equations to ensure the combined stress state does not exceed the fastener's capacity. Proper installation torque and joint design are essential to mitigate these complex stresses.
Conclusion
Correctly distinguishing between shear and tensile strength is not academic—it is essential for specifying safe and reliable fasteners. By analyzing the load type, selecting the appropriate screw grade and geometry, and applying sound engineering principles, professionals can ensure the longevity and integrity of their assemblies. Tianyu provides a range of fasteners with certified mechanical properties to meet these diverse application demands.
Contact us now for samples: support1@tianyufasteners.com.
Explore our offerings at www.tianyufasteners.com.
