The thread elongation gauge is used to determine whether a bolt’s threads have undergone plastic deformation as a result of excessive tension. Such deformation often occurs due to significant temperature variations or dynamic loading. The gauge measures the elongation in the central portion of the threaded section, providing both the percentage of extension and a clear pass/fail indication. All gauges are manufactured in accordance with ASME and DIN standards.

When using standard-grade bolts and nuts up to M30 (1 inch) in diameter, it is recommended to replace them with new ones if force multipliers—such as hydraulic torque wrenches or tensioning tools—are required. For larger diameters, the decision should balance the cost of cleaning, deburring, and reconditioning against the cost of replacement, particularly for critical flange assemblies.

The 0.2% yield strength (or proof stress) represents the stress level at which 0.2% plastic deformation occurs, as defined by ASTM. Industry variations may use 0.1% or 0.5%. Essentially, the 0.2% yield strength provides a practical and measurable value that marks the transition from purely elastic behavior to the onset of plastic deformation.

It should be noted that Shell applies a non-standard rejection criterion of 2% elongation, which significantly exceeds the conventional 0.2% yield-based threshold and is therefore considered technically unjustified and difficult to rationalize.