Abstract

The article describes the main TOFD principles and applications. With use of TOFD the Probability of Detection (POD) increases greatly. Advantages and drawbacks of the method are listed. It is mentioned that AEA Sonomatic succeeded in reducing the dead zone to 2 mm.

Table of contents

1. Introduction
2. Applications for TOFD
3. Advantages of TOFD
4. Drawbacks of TOFD

1. Introduction

The Time Of Flight Diffraction (TOFD) is a NDT method developed in the 70's by AEA. AEA Sonomatic specializes in this method.

The following main principles describe TOFD: 1 = transmitted wave 2 = reflected wave 3 = through transmitted wave 4 = diffracted wave at upper crack tip 5 = diffracted wave at lower crack tip 1- lateral wave 2 - diffraction signal at upper crack tip 3 - diffraction signal at lower crack tip 4- back wall reflection

This method differs from traditional pulse echo technique in that it monitors diffracted signals at the edges of defects which are directly related to the true position and size of the defect, as opposed to the reflection on defects according to a reference reflector.

The TOFD technique uses two probes in a transmitter-receiver arrangement. When sound is introduced into the material via the transmitter the defect will oscillate. Each defect edge works as a source point of ultrasound signals. These very weak signals are called diffracted waves and their appearance does not relate to the orientation of flat or spherical defects. These diffracted signals are received via the receiver probe. The diffracted signals are evaluated with the Microplus-Systems to clear gray scale B-scan or D-scan images (transversal- or longitudinal projection of the object being tested).

The amplitude of the signal is not displayed , but the position of the signals on the time scale are. Thus it is possible to determine the defect location exactly - length, and defect height.

Therefore the Probability of Detection (POD) increases greatly (up to 90 % !) for flat or spherical defects when compared to traditional techniques.

By use of today's advanced computer techniques it is possible to evaluate signals very rapidly. That makes it possible to perform scans with a speed of hundreds of millimeters per second. In practice speed is limited only by the mechanic.

Applications for TOFD

The main TOFD applications are:
• In-service defect monitoring.
  
• Defect detection, documentation and evaluation during the production.

The dead zone under the outside surface has always been a limitation of TOFD. Defects close to the surface could not be detected (surface breaking cracks are detectable).. AEA Sonomatic succeeded in reducing this zone to 2 mm! For that reason TOFD can be applied down to 6 mm wall thickness. On the upper end it is usual to inspect up to 350 mm wall thickness.

Advantages of TOFD

• TOFD defect detection does not depend on the defect orientation, in contrast to the pulse echo technique .
  
• In contrast to the radiography method, planar defects and cracks, which are not perpendicular to the measured surface can be detected .
  
• Defect height can be exactly determined.
  
• Higher POD improves risk reduction and calculation.
  
• The evacuation of areas because of radiation is not necessary. That means less interruption in the production process less during pre-service or in-service inspections and fewer logistical problems for the manufacturer.
  
• The inspection results are immediately available, as is a permanent record and a permanent print as longitudinal or transversal projection of the weld is available.
  
• When Engineering Critical Assessment (ECA) is applied, only the relevant defect has to be cut, thereby preventing needless repairs which could harm the integrity of the weld.
  
• Because of the high test speed the costs are less than those for radiography for wall thickness above 25 mm.
  
• The inspection can be performed above 200° C.
  
• Since the Microplus-System is easy to transport, it is possible to perform test on all feasibly accessible areas.
  
• TOFD saves costs, if applied during construction, since it is possible to distinguish pre-service and in-service defects. That means the unit can stay longer in production, and is safe.

Drawbacks of TOFD

• The weld must be feasibly accessible from both sides.
  
• There is a dead zone for defect detection under the surface.

Author

Ing. Norbert Trimborn
Norbert Trimborn studied Physics, Economics and certificated as NDT Level 3.
1986-1993 DSM Development department for NDT.
Started in 1993 with AEA SONOMATIC, Oosterhout (NL),
as application engineer and works since 1994 as technical manager.
Since 1995 he gives lectures for UT at the PTU.

The paper was presented at the DGZfP workshop "Bildhafte Darstellung und Auswertung der Ergebnisse der ZfP", Studensee, 27.-28. November 1995. The original title: Die Time-of-Flight-Diffraction-Technik
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For more information see: TOFD in UTonline 09/97