{"id":29976,"date":"2026-07-15T18:31:51","date_gmt":"2026-07-15T16:31:51","guid":{"rendered":"https:\/\/roder.it\/uncategorized-en-us\/systems-for-elongation-and-tension-testing-of-bolts-and-tie-rods-with-ultrasonic-instruments\/"},"modified":"2026-10-08T18:07:20","modified_gmt":"2026-10-08T16:07:20","slug":"systems-for-elongation-and-tension-testing-of-bolts-and-tie-rods-with-ultrasonic-instruments","status":"publish","type":"post","link":"https:\/\/roder.it\/en\/news-mechanical-engineering-sector\/systems-for-elongation-and-tension-testing-of-bolts-and-tie-rods-with-ultrasonic-instruments\/","title":{"rendered":"Systems for elongation and tension testing of bolts and tie rods with ultrasonic instruments"},"content":{"rendered":"\n

Checking the tension actually applied to a bolt<\/strong> during tightening is one of the most delicate issues in mechanical assembly. The traditional method \u2014 torque control with a torque wrench \u2014 is subject to high intrinsic uncertainty, because the applied torque is converted into axial load only after overcoming thread and under-head friction, two quantities that are difficult to quantify and vary from bolt to bolt. The differential ultrasonic technique<\/strong> for measuring elastic elongation overcomes this limitation by directly measuring the effect of tightening on the bolt. <\/p>\n\n

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RODER SRL distributes Dakota Ultrasonics<\/strong> instruments in Italy for ultrasonic tension testing of bolts and tie rods. For an introduction to the subject, see our earlier article Systems for elongation and tension testing of bolts and tie rods<\/a>. This post instead focuses on the technical and regulatory aspects: measuring principle, ASME PCC-1 compliance, choice of instrument and typical industrial applications. <\/p>\n\n

Principle of differential ultrasonic measurement<\/h2>\n\n

When a bolt is tightened, it elongates elastically in proportion to the applied axial load (Hooke’s law). On standard industrial bolts the elongation is in the order of hundredths of a millimetre, but it can be measured perfectly with the differential ultrasonic technique: <\/p>\n\n

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  1. The time of flight<\/strong> of an ultrasonic pulse is measured in the unloaded bolt (initial length L\u2080).<\/li>\n\n\n\n
  2. The bolt is tightened.<\/li>\n\n\n\n
  3. The time of flight is measured again in the tightened bolt (length L\u2080 + \u0394L).<\/li>\n\n\n\n
  4. The difference \u0394L\/L\u2080 is proportional to the axial load via the elastic modulus of the material and a calibration coefficient specific to the bolt geometry.<\/li>\n<\/ol>\n\n

    The accuracy of Dakota bolt tension monitors is in the order of 0.0001 mm<\/strong> on the time of flight, which translates into a typical uncertainty of \u00b12\u20135%<\/strong> on the axial load \u2014 an order of magnitude better than torque control.<\/p>\n\n

    ASME PCC-1: the reference standard<\/h2>\n\n
    \"Dakota<\/figure>\n\n

    ASME PCC-1<\/strong> (Pressure Boundary Bolted Flange Joint Assembly<\/em>) is the international reference standard for controlled tightening of bolted flanges in pressure boundaries: heat exchangers, pressure vessels, steam generators and critical piping systems. The standard provides detailed procedures to ensure uniform tightening and an axial load that complies with design specifications, drastically reducing the risk of seepage and leaks. <\/p>\n\n

    Appendix K<\/strong> of ASME PCC-1 is dedicated to “Bolt Load Verification using Ultrasonic Measurement” and defines procedures, calibration requirements and acceptance criteria for ultrasonic measurement of bolt load. For a detailed look at the procedure: ASME PCC-1: the reference standard for controlled flange tightening<\/a>. <\/p>\n\n

    Other standards relevant to specific applications:<\/p>\n\n