The thread elongation gauge is used to determine if a bolt’s thread has undergone plastic deformation due to excessive tension, which commonly arises from significant temperature fluctuations or dynamic stresses. The gauge measures the stretch in the middle section of the thread, providing both the percentage extension and a clear indication of acceptance or rejection. All gauges are supplied according to ASME and DIN standards.

When using standard-quality bolts and nuts up to M30 (1 inch) diameter, it is advisable to replace them with new ones if force multipliers—such as hydraulic torque or tensioning equipment—are required. For larger diameters, weigh the cost of cleaning, deburring, and reconditioning against the cost of replacement when dealing with critical flange assemblies.

The 0.2% yield strength (or proof stress) is the stress at which 0.2% plastic deformation occurs, as defined by ASTM. Industry variations include 0.1% and 0.5%. Essentially, the 0.2% yield strength method provides practical, measurable values that mark the transition from elastic behavior to the onset of plastic deformation.