The Ultrasonic Extensometer

The Ultrasonic Extensometer

When You Need to Know the Actual Bolt Load

Most industrial bolted joints are tightened using torque. That is standard practice and, when applied correctly, it works.

However, torque is an indirect method. It measures rotational effort, not actual bolt tension.

An ultrasonic extensometer measures bolt elongation. Bolt elongation is directly proportional to preload.

That distinction is critical when preload accuracy matters.

What an Ultrasonic Extensometer Does

An ultrasonic extensometer sends a high-frequency sound pulse through the length of a bolt. The pulse travels to the far end of the fastener and reflects back to the probe.

The instrument measures the time required for that round trip.

Before tightening, a baseline measurement is recorded.

After tightening, a second measurement is taken.

If the bolt has stretched, the sound travel time changes.

That change represents elongation.

Elongation correlates directly to tensile load in the bolt.

This is not an estimate. It is a physical measurement of stretch.

Why Torque Alone Has Limits

Torque tightening depends heavily on friction.

Friction under the bolt head, friction in the threads, surface condition, lubrication, coating type, washer hardness, and temperature all influence the final preload.

A significant portion of applied torque is consumed overcoming friction rather than generating tension.

Two identical bolts tightened to the same torque value can produce different preload results.

This variation is known as torque scatter.

In many applications, torque scatter falls within acceptable tolerance.

In highly critical joints, it can introduce unacceptable risk.

Ultrasonic measurement evaluates the result of tightening rather than the effort applied.

Where Ultrasonic Measurement Is Appropriate

Ultrasonic extensometry is not necessary for routine structural bolting.

It becomes valuable in applications where preload must be verified with high confidence.

Examples include:

• Power generation turbine casings

• Pressure boundary flanges

• Nuclear energy systems

• Large-diameter critical fasteners

• Research and validation testing

• High-temperature or cyclic loading environments

In these settings, documentation and verification are often required. Engineering teams may need confirmation that calculated preload targets were achieved.

Ultrasonic measurement provides that confirmation.

Requirements for Accurate Measurement

Ultrasonic systems require discipline and preparation.

Reliable results depend on:

• Known bolt material properties

• Accurate baseline readings prior to tightening

• Clean and consistent probe contact surfaces

• Calibrated equipment

• Repeatable probe positioning

The system measures extremely small changes in sound travel time. Surface quality and measurement consistency are essential.

Improper technique can introduce error. Proper technique produces highly repeatable results.

What Ultrasonic Extensometry Reveals

Ultrasonic measurement allows technicians and engineers to:

• Verify preload after torque tightening

• Validate hydraulic tensioning operations

• Measure load loss after thermal cycling

• Monitor bolt relaxation

• Document compliance with engineering specifications

It shifts bolting from an assumed process to a measured outcome.

That shift significantly improves confidence in critical joints.

Relationship to Hydraulic Tensioning

Hydraulic tensioning stretches the bolt directly by applying axial force. This reduces reliance on thread friction and improves preload consistency.

Ultrasonic measurement does not replace tensioning. It verifies results.

Tensioning applies controlled stretch.

Ultrasonics confirm the amount of stretch achieved.

In high-precision bolting programs, both methods may be used together to maximize accuracy and documentation.

Practical Perspective

Most industrial bolting operations do not require ultrasonic verification.

However, when joint failure would result in major financial, safety, or operational consequences, direct measurement of bolt elongation becomes a logical step.

Ultrasonic extensometers provide measurable evidence of preload.

They move the discussion from applied torque to confirmed tension.

In high-consequence environments, that difference matters.