Time-of-Flight Diffraction Inspection

Schedule expert consultation

Precise defect detection, location, and sizing for welds, pressure vessels, and piping

Detecting a defect is only part of the inspection challenge. Operators making run/repair/replace decisions on critical pressure systems need more than confirmation that a crack exists. They need to know exactly where it is, how deep it runs, and how fast it is growing. Visual methods and even conventional UT can identify anomalies, but often cannot provide the through-wall sizing accuracy needed to support Fitness-for-Service assessments or remaining life calculations.

MISTRAS’ TOFD delivers that level of precision. By analyzing the time it takes for diffracted ultrasonic waves to travel from a defect to a receiver, TOFD provides accurate defect detection, location, and through-wall sizing on welds, pressure vessels, heavy-wall piping, and other critical assets. With no shutdown required and the speed and coverage of an automated scanning system, our TOFD services keep programs on schedule.

How TOFD works

TOFD is a form of automated ultrasonic testing (AUT) that uses two transducers positioned on either side of a weld or inspection zone. The transmitter generates longitudinal waves that travel through the material. Where a defect is present, the wavefront diffracts at the defect tips. TOFD analyzes these diffracted signals, which arrive at the receiver at times determined by the defect's position and through-wall extent, to characterize the flaw.


Unlike conventional pulse-echo techniques, TOFD relies on diffraction rather than reflection, making it sensitive to defect tips regardless of their orientation. This is a significant advantage where planar defects are not perpendicular to the beam, and pulse-echo methods can miss them.  Results are displayed as B-scan images with measurements of defect height, length, and position recorded and stored for reporting and future comparison.

Core benefits

Accurate through-wall sizing
TOFD provides reliable height, length, and positional measurements of defects, giving operators the level of detail required to support FFS assessments and remaining life calculations.

Sensitivity to all weld flaw types
Diffraction-based detection identifies defects regardless of orientation, including planar flaws that pulse-echo techniques may miss.

In-service capability
TOFD inspections can be performed on operating assets without shutdown, reducing the cost and disruption of compliance and integrity inspections.

Rapid area coverage
Automated scanning covers large weld volumes quickly compared to manual techniques, making TOFD practical for both turnaround and in-service inspection programs.

Permanent digital record
B-scan images and defect measurements are stored digitally, providing a comparison baseline and a documented record for regulatory and client reporting requirements.

Applications

TOFD is most commonly used to inspect heavy-wall welds in storage tanks, pressure vessels, and piping, where through-wall sizing accuracy is critical for Fitness-for-Service evaluation and inspection interval management. It also functions as a rapid survey tool across a wide range of in-service and new construction applications.

TOFD rapidly scans heat-affected zones and weld volumes on in-service and newly-fabricated pressure vessels and piping. It detects and sizes fabrication defects, in-service cracking, and corrosion-related damage without requiring internal access or asset shutdown. Its sensitivity to all types of weld flaws makes it a comprehensive solution for both new vessel code inspection and in-service damage assessment. 

TOFD is a key component of MISTRAS' HTHA special emphasis program for pressure vessels operating in high-temperature hydrogen service. HTHA degradation causes deep, through-wall cracking that can be characterized accurately using TOFD's through-wall depth measurement capability. The result is a detailed B-scan image of damage in affected components and supporting well-informed decisions on continued safe operation.

For newly-fabricated assets, TOFD provides rapid, code-compliant weld examination during construction quality assurance programs. Documented results support acceptance or rejection decisions against applicable fabrication standards. 

TOFD results include accurate positioning data and through-wall measurements, allowing successive TOFD scans of the same asset to be compared to track defect growth rates over time. This data supports remaining life calculations, risk-based inspection planning, and Fitness-for-Service assessments.

Defect types detected

TOFD inspection is sensitive to a wide range of damage mechanisms and fabrication defects. These include corrosion, cracking, fatigue damage, hydrogen-induced cracking, stress corrosion cracking, and weld fabrication flaws such as lack of fusion, lack of penetration, and porosity.

TOFD Inspection

Frequently as a quick survey tool for heavy wall pipes and pressure vessels, Time-of-Flight Diffraction (TOFD) can examine assets for welding fabrication defects and provide accurate through-wall depths of existing cracks.

Find your solution

Whatever the challenge, we have the depth of expertise and breadth of solutions to deliver certainty.

Select your industry
Select a specialty
Select a method

Solve your most complex challenges

Partner with MISTRAS Field Services to inspect, access, and repair your critical assets. Contact our experts to define the integrated solution that maximizes your safety and operational uptime.

Schedule expert consultation