Phased Array Ultrasonic Testing for Bolt Inspection

Pipe bolts are critical connecting elements in various major structures, widely used in bridges, pressure vessel flanges, wind turbine towers, crane connections, offshore platforms, and steel structure nodes. During service, they are frequently subjected to thermal stress, differential expansion, alternating loads, impact, vibration, and environmental corrosion, which can lead to:

Fatigue cracks;

Stress corrosion cracking (SCC);

Deformation and loosening;

Localized damage to the threaded area;

Corrosion thinning.

 

If these defects are not detected in time, they may lead to major structural failures. Therefore, periodic non-destructive testing (NDT) of pipe bolts is crucial. This article will focus on Phased Array Ultrasonic Testing (PAUT), which has become increasingly widely used in recent years.

 

What is Phased Array Ultrasonic Testing (PAUT)

Phased Array Ultrasonic Testing (PAUT) is a powerful non-destructive testing (NDT) method that uses advanced sound beam manipulation to detect flaws in materials with high precision and flexibility. 

Unlike conventional ultrasonic testing, which relies on fixed-angle probes, PAUT employs an array of small transducer elements that can be individually controlled to steer, focus, and scan ultrasonic beams electronically. This enables rapid inspection, improved detection capability, and detailed imaging of internal structures—especially valuable for complex components or critical welds.

 

Basic Principle of Phased Array Technology

By subdividing the piezoelectric transducer into many small transducer elements and using an ultrasonic instrument capable of individually controlling and evaluating each of these elements temporally, it becomes possible to modulate the interference pattern of the sound beam to achieve a desired sound pressure distribution that is optimal for a specific application.

 

By exciting the elements at different times, the phase of the elementary waves of the individual elements is influenced, thus giving rise to the term “Phased Array.” In German, the term “Gruppenstrahler” (group radiator) is also used, as a group of n transducer elements is always ex- cited with a specific time delay pat- tern, the so called “delay law”. This is not a physical law but rather a sequence of delay times calculated by the instrument, relative to the individual elements of the group, intended to achieve a specific characteristic of the sound beam.

PAUT Bolt Inspection Procedure

1. Pre-inspection Preparation

Confirm pipe bolt size, material, and service environment; select appropriate probes and wedges; create a scanning plan (angle, focus point, inspection area).

 

2. Equipment Calibration

Calibrate the material sound velocity and wedge delay; set the sensitivity (DAC/TCG) to ensure the equipment is in a repeatable and accurate state.

 

3. Surface Treatment

Clean the pipe bolt surface; lightly grind if necessary; apply coupling agent to ensure good acoustic energy coupling.

 

4. PAUT Scan

Sector scan (S-Scan) covers the head, transition area, and threaded area.

E-scan enhances thread inspection.

TFM imaging can be used for higher resolution if needed.

 

5. Defect Analysis

Determine the presence of cracks, corrosion, and inclusions; measure crack length, depth, and direction to determine if they affect operational safety.

 

6. Result Recording and Reporting

Save the S-Scan/TMF images; record parameters and probe position; provide recommendations for repair, replacement, or continued monitoring.

 

Benefits of Phased Array Ultrasonic Testing for Pipe Bolt Inspection

1. PAUT provides excellent visualisation on any fatigue associated defects.

 

2. PAUT has significant advantages over current manual UT techniques: reproducible, fast, full data storage, auditable, interpretable locally or remotely.

 

3. Total focusing method (TFM) can also be used within the PAUT set-up to provide.

 

4. Higher resolution and image quality enhancement.

 

5. Better sizing of defects located on threads.

 

6. Improved probability of the detection of defects.

Summary

Phased Array Ultrasonic Testing (PAUT) has become the preferred technology for monitoring the integrity of pipe bolts in critical structures because it can detect defects that are difficult to find with traditional ultrasonic testing (UT), provides image-based displays for easy understanding and assessment, can detect complex geometric areas such as threads and roots, and the testing process is repeatable and traceable. Combined with TFM (Total Mechanical Fluid Mechanism), it can achieve precise quantification of millimeter-level cracks. PAUT provides a more advanced and reliable structural health assessment method for industries such as bridges, cranes, wind power, marine engineering, and pressure vessels.