The standard describes the general principles of the TOFD method for defect detection and sizing. The draft incorporates, by reference, provisions from specific editions of other publications. Those are EN 473, EN 583-1, EN 583-2, EN 583-5, EN 1330-4, Characterization and Verification of Testing equipment - Ultrasonic prEN 00138007, 00138058, 00138059.
In the initial description of TOFD it is mentioned that if the work piece is available for testing from both sides and defects are suspected to be in all thickness sections, better results are achieved when performed from both sides, especially defects near the surface .
Since the diffracted signals are very weak, the draft points out that good surface and coupling conditions are essential for the test. Also the material must have a low attenuation, that means the method is not suitable for coarse grained materials.
The draft mentioned that personnel must be certified according to EN 473. The equipment must fulfill at least the above mentioned standard (prEN) for Characterization and Verification of Testing equipment. Additional requirements are cited, which are:
- Amplifier bandwidth
- Shape of the exiting pulse
- Sample rate must be 4 times of the nominal probe frequency
- Minimum one A-scan for each 1mm probe index
- Special demands for gate ranges
- Minimum 64 digitalization steps of gray or color images of the scan
- Memory capability for A or B images for a later display of the results
- The equipment must be able to do signal averaging.
Probes applied for TOFD must meet the following conditions:
- Two probes (transmitter and receiver)
- All probes can be used (see table 1)
- Longitudinal waves should be used; the use of shear waves is more
difficult, however, it can be done.
- The two probes must have frequencies within 20% of each other
- Frequency used (see table 2)
- A maximum signal ring of two wavelengths (10% of maximum)
- The PRF must be low enough to prevent 'ghost echoes'.
Probe manipulators must be used to perform a constant scan. Manual or motorized devices which have a positioning accuracy of 10% in relation to the probe distance can be used .
The figures in tables 1 & 2 are guidelines for achieving good test results on thin as well as thick components. The use of one probe pair is enough for up to 70 mm thickness. Above 70 mm more probes must perform the test in different sections.
| Table 1: Recommended probes for a working range up to 70 mm | |||
| Wall Thickness | Nominal Frequency | Crystal Diameter | Nominal Angle |
| < 10 mm | 10 MHz to 15 MHz | 2 mm to 6 mm | 50° to 70° |
| 10 mm to < 30 mm | 5 to 10 MHz | 2 mm to 6 mm | 50° to 60° |
| 30 mm < 70mm | 2 MHz to 5 MHz | 6 mm to 12 mm | 45° to 60° |
|
Table 2 : Recommended probes for a working range from 70 mm to 300 mm | |||
| Depth Range | Nominal Frequency | Crystal Diameter | Nominal Angle |
| 0 < 30 mm | 5 MHz to 10 MHz | 2 mm to 6 mm | 50° to 70° |
| 30 mm to < 100 mm | 2 to 5 MHz | 6 mm to 12 mm | 45° to 60° |
| 100 mm < 300mm | 1 MHz to 3 MHz | 10 mm to 25 mm | 45° to 60° |
The evaluation of diffracted signals is optimized by use of a 120 ° angle between the probes. A mismatch of - 35° or +45° can cause small echoes. The electronic noise must be 6 dB less than the lateral wave amplitude, the latter must be adjusted on approx. 5% of full screen. The adjusted equipment can be tested on reference blocks (Fig. 1).
Basics on how to analyze the defects are given as well as interpretation of surface breaking cracks - the determination of position, length, depth. A more detailed analysis can be done for defects which are already detected by normal TOFD techniques. Therefore the variation of frequencies and angles can be used as well as advanced algorithms such as SAFT.
The draft describes the TOFD method limitations with respect to resolution and accuracy. For sizing errors and the influence of the dead zone, formulae are provided for error determination.
Regarding test reports, the draft refers to the EN 583-1 and outlines some additional TOFD-specific requirements.
Fig 1. Reference Block |