ISO 8981 and EN 208981 Standards Guide Bolt and Screw Performance

Imagine a skyscraper, a high-speed vehicle, or precision machinery—their stability and safety often depend on seemingly insignificant bolts and screws. These small fasteners bear tremendous tensile, shear, and even torsional forces. How can we ensure they withstand pressure in critical moments and maintain structural integrity? The answer lies in a thorough understanding of their mechanical properties and proper selection.

This article provides a comprehensive guide to the mechanical properties of steel bolts and screws as defined by ISO 898-1 and EN 20898-1 standards, offering engineers and designers a quick reference for making informed decisions during design, manufacturing, and maintenance processes.

ISO 898-1 and EN 20898-1 are internationally recognized standards that specify the mechanical properties of steel bolts, screws, and studs. These standards define requirements for tensile strength, yield strength, hardness, proof stress, and elongation after fracture for different property classes. Understanding these parameters enables professionals to select appropriate fasteners for specific applications, ensuring reliable and secure connections.

The property class serves as an "identity card" for bolts and screws, clearly indicating their mechanical performance characteristics. Common property classes include 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 10.9, and 12.9. These numbers carry specific meanings:

• First number: Represents 1/100 of the fastener's tensile strength (Rm). For example, an 8.8 class bolt has a tensile strength of 800 MPa.
• Second number: Indicates the ratio of yield strength (Rel or Rp0.2) to tensile strength (Rm) multiplied by 10. For an 8.8 class bolt, this ratio is 0.8, meaning the yield strength is 640 MPa.

These numbers provide quick insight into a fastener's fundamental strength characteristics, serving as valuable references during selection.

Beyond property classes, several critical mechanical properties require understanding:

• Tensile strength (Rm): The maximum stress a material can withstand during stretching. Higher tensile strength means greater resistance to breaking under tension.
• Yield strength (Rel or Rp0.2): The stress at which material begins plastic deformation. Higher yield strength indicates better resistance to permanent deformation.
• Vickers hardness (HV): Measures resistance to localized plastic deformation. Higher hardness improves wear resistance and compressive strength.
• Brinell hardness (HB): Similar to Vickers hardness but uses different testing methods.
• Rockwell hardness (HR): Another hardness measurement with different scales (e.g., HRC, HRB) based on indenter and load variations.
• Surface hardness (HV 0.3): Measures surface hardness, typically used to evaluate surface treatment effectiveness.
• Proof stress (Sp): The maximum stress a fastener can endure under specified test conditions, often used to assess fatigue performance.
• Elongation after fracture (A5): The ratio of length increase after fracture to original length. Higher values indicate better plasticity and deformation adaptability.

The following table details mechanical properties for various steel bolt and screw property classes, including tensile strength, yield strength, hardness, proof stress, and elongation after fracture. Note that for some classes (e.g., 3.6), properties differ between diameters ≤16mm and >16mm.

Beyond tensile properties, torsional resistance is equally crucial. Breaking torque refers to the minimum torque required to cause torsional failure in a fastener. This becomes particularly important in applications involving rotational or vibrational loads.

ISO 898-7 and DIN 267 pt25 specify testing methods and requirements for bolt and screw breaking torque. The following table shows minimum breaking torque values for different thread sizes and property classes.

Choosing appropriate bolts and screws requires careful evaluation of multiple factors:

1. Application: Different scenarios demand varying performance characteristics. High-vibration environments require superior fatigue resistance, while high-temperature applications need heat-resistant materials.
2. Load type: Identify whether fasteners will primarily experience tensile, shear, bending, or torsional loads, then select accordingly.
3. Materials being joined: The composition of connected materials (steel, aluminum, plastic, etc.) influences fastener material and surface treatment choices.
4. Environmental conditions: Humidity, temperature, and corrosive elements dictate necessary corrosion resistance levels.
5. Installation method: Preload requirements and tightening techniques affect size and shape selection.

Consider a steel structure connection requiring high tensile strength in humid conditions. An 8.8 class high-strength bolt with zinc plating would be appropriate:

1. Determine load: Calculate maximum tensile load through structural analysis.
2. Select diameter: Choose diameter based on load and tensile strength, applying appropriate safety factors.
3. Choose material/class: Select 8.8 class high-strength steel.
4. Surface treatment: Specify zinc plating for corrosion resistance.
5. Determine length: Calculate based on material thickness and tightening requirements.

Though small, bolts and screws play monumental roles in structural safety and equipment reliability. Proper selection and application, guided by standards like ISO 898-1 and EN 20898-1, ensure optimal performance. By understanding mechanical properties and carefully evaluating application requirements, engineers can make informed decisions that uphold structural integrity across countless industries.