Published on 15-Apr-2024

Time of Flight Diffraction (TOFD): An Overview

Time of Flight Diffraction (TOFD): An Overview

Sources - TUV Rheinland

Table of Content

The primary goal of NDT of engineering structures and systems is to verify the absence of flaws during the component's service life as well as during construction. Liquid penetrant testing, ultrasonic testing, magnetic particle testing, and radiography testing are the main non-destructive testing techniques used on welded components. New methods like Ultrasonic Time of Flight Diffraction are leading the industry thanks to advancements in automated inspection technology and the expansion of Fitness For Purpose (FFP) inspection. With a level of accuracy attained never before by previous Ultrasonic Techniques, the TOFD approach is a completely computerized system that can scan, store, and assess indications regarding height, length, and position.

What are the Basic Principles of TOFD?

Basic Principles of TOFD

The Time of Flight Diffraction test is an NDT technique widely used in various industries for evaluating the integrity of welds and detecting flaws or defects. This method relies on the principles of Ultrasonic Wave Propagation and diffraction phenomena to provide accurate and reliable results. The TOFD test is a type of ultrasonic testing, a widely utilized NDT Technique. A variety of NDT methods that use ultrasonic waves to pass through a substance or object is referred to as ultrasonic testing. These high-frequency sound waves are emitted into materials to assess their properties or find flaws. While frequencies as high as 50 MHz are possible, the majority of Ultrasonic Testing Inspection applications employ brief pulse pulses with frequencies between 0.1 and 15 MHz. 

TOFD method involves the generation of high-frequency ultrasonic waves through a transducer. These waves travel through the tested material and interact with its internal structure. When a wave encounters a discontinuity or flaw, such as a crack or void, a portion of the wave energy is diffracted, or redirected, away from its original path. This diffraction phenomenon provides valuable information about the size, shape, and location of the flaw. The depth of the crack can be analyzed by measuring the wave’s time of flight.

What is the TOFD Procedure?

TOFD Procedure

Time of Flight Diffraction is a widely used NDT method that offers accurate and reliable results for detecting flaws in welds. The procedure involves sending ultrasonic waves through the material and analysing the diffracted signals to identify any defects present. Here’s the detailed procedure for conducting the Time of Flight Diffraction test:

1. Preparation of the Test Surface

Before conducting the TOFD test, the surface of the material under examination is cleaned and prepared to ensure optimal contact with the ultrasonic transducers.

2. Placement of Transducers

Two ultrasonic transducers are used in the TOFD test - a transmitting transducer and a receiving transducer. The transmitting transducer emits Ultrasonic Waves into the material, while the receiving transducer detects the diffracted waves. Both transducers are placed on opposite sides of the weld or area of interest.

3. Calibration

Calibration is a crucial step in ensuring accurate results. Known reference notches or artificial reflectors are used to establish a baseline for the TOFD equipment. This allows for precise measurement and sizing of defects.

4. Scanning Process

The transducers are systematically moved along the surface of the material, maintaining a constant distance between them. As the Ultrasonic Transducers move, they emit ultrasonic waves and receive diffracted signals. The time taken for the diffracted signals to travel from the flaw to the receiving transducer is recorded.

5. Data Analysis

The recorded signals are processed and analyzed to create a visual representation of the internal structure of the material. This representation, often displayed as a TOFD image, provides detailed information about the location and size of any detected flaws.

Applications of TOFD

The TOFD test is widely applied in industries where the integrity of welded structures is of critical importance. Some of the key applications include:

1. Weld Inspection

TOFD is extensively used for evaluating weld inspection in various industries, including construction, Aerospace Industry, and manufacturing. It provides highly accurate information about the quality of welds, helping to identify and address potential issues.

2. Pressure Vessel Inspection

TOFD is employed to assess the Integrity of Pressure Vessels, ensuring they meet safety and regulatory standards. This is particularly crucial in industries such as oil and gas, petrochemicals, and power generation.

3. Pipeline Inspection

TOFD is used for inspecting pipelines, both onshore and offshore. It helps detect and characterize defects in Pipelines, reducing the risk of failure and ensuring the reliability of transportation systems.

4. Aerospace Industry

TOFD is applied in the aerospace sector to evaluate critical components like aircraft engines and structural elements. It aids in identifying potential defects that could compromise the safety and performance of aircraft.

When to use the Time of Flight Diffraction Test?