TOFD was originally developed during the 1970s, and was utilized for weld inspections and crack sizing in the nuclear industry.  Typically it uses two wide angle longitudinal wave transducers on either side of the weld in a pitch-catch arrangement (see Figure 1). The scan covers the full volume of the weld (or component) with a single probe pair.

TOFD records signals diffracted from defect tips for both detection and sizing. Full waveform data is collected, and this is displayed in a grey-scale B-scan. Since diffracted signals are weak, TOFD usually runs at much higher gain than pulse echo inspections.

Figure 1: TOFD set-up (left), and typical TOFD display (right)

ADVANTAGES

• Fast, economical, high speed scanning
• Good mid-wall defect detection
• Accurate sizing of defects using the times of arrival of diffracted signals
• Non-amplitude scanning and detection
• Set-up independent of weld configuration
• Safety No special safety requirements as in radiography. 
• ASME Code Compliant, TOFD can be used in lieu of radiography, as per ASME code case 2235

APPLICATIONS

TOFD can be used during fabrication or for in-service inspections within the Power and Petrochemical industries.  Typical components inspected would be piping and vessel welds.  It is a reliable tool in finding defects from production welding, as well as service induced defects. Defects such as inside surface breaking defects, near surface breaking defects, mid-wall defects, lack of root penetration, lack of sidewall fusion, porosity, transverse cracks, and interpass lack of fusion are all easily detected.

TYPICAL EXAMPLES OF TOFD INDICATIONS

Porosity

Lack of Sidewall Fusion

Lack of Root Penetration