Strength of Common Bolting Materials

Selecting the appropriate fastener material and strength grade is one of the most critical steps in bolted joint design. Fasteners must safely withstand clamping preloads, external tensile loads, shear forces, and fatigue without yielding or fracturing. Engineers specify bolt strength using standard grading systems: SAE J429 (imperial automotive and mechanical), ASTM (structural and heavy construction), and ISO 898-1 (metric property classes).

Understanding Bolting Strength Properties

Proof Strength (Proof Load): The maximum tensile stress a bolt can withstand without permanent plastic deformation (typically 90–95% of yield strength). Bolt preloads are designed based on proof load.
Yield Strength: The stress at which the fastener material exhibits a specified permanent elongation (usually 0.2% offset).
Tensile Strength (Ultimate Tensile Strength): The maximum engineering stress sustained prior to necking and ultimate fracture.

Fastener Grading Comparison Overview

SAE Grade 2 / ISO Class 4.6 – 5.8: Low to medium carbon steels for general utility and non-critical assemblies.
SAE Grade 5 / ASTM A325 / ISO Class 8.8: Medium carbon steel, quenched and tempered. Standard workhorse grade for structural framing, machinery, and automotive joints.
SAE Grade 8 / ASTM A490 / ISO Class 10.9: Medium carbon alloy steel, quenched and tempered. High-strength applications requiring high clamp loads in compact envelopes.
ISO Class 12.9: High-strength alloy steel commonly used in socket head cap screws for precision tooling and high-fatigue machinery.

The comprehensive reference table below provides detailed chemical compositions, minimum proof strength, yield strength, and tensile strength values for standard bolting grades: