TOFD detection; ISONIC parameter setting; study
China Library ClassificationNo. TN9 14 Document IdentificationNo. A1673-9671-(2012) 071-0198-01.
Importance of parameter setting in 1 TOFD detection
The main parameters that should be paid attention to before TOFD detection and scanning are: probe real frequency, pulse width, repetition frequency, impedance, inductive reactance, filtering frequency, signal average value, time window, gain and other parameters.
Pulse width is very important, which helps to optimize the shape of the received signal. Changing the pulse width can lead to the weakening or strengthening of different periods. If two ultrasonic pulses are to form a signal with a single frequency, the pulse width should be set to half the frequency period of the probe used (for example,100 ns); Used at 5 MHz); In order to make the signal last for the least number of cycles, the pulse width should be set to one cycle of the used detection frequency (for example, 200 ns at 5 MHz). Wherein that probe frequency must be the actual frequency of the probe, not the nominal frequency of the probe. In practical work, the best pulse width must be obtained through experiments.
If manual data acquisition is used, it is necessary to pay attention to pulse repetition frequency PRF and probe moving speed. Because the computer can't judge and control the probe movement during manual scanning, only the operator can correctly select PRF to ensure the normal acquisition of A-scan data. If it is driven by encoder or motor, PRF is relatively unimportant, because the computer can calculate the probe position and collect data at the specified A-scan sampling rate interval. If the PRF setting is incorrect, a blank A scan will be obtained.
Impedance tuning term matching refers to the working state in which the load impedance and the internal impedance of the excitation source adapt to each other to obtain the maximum power output. The matching conditions are different for circuits with different characteristics. In the pure resistance circuit, when the load resistance is equal to the internal resistance of the excitation source, the output power is the maximum, and this working state is called matching, otherwise it is called mismatch.
The unit of inductive damping term is Europe. Knowing the frequency f(Hz) of alternating current and the inductance L(H) of the coil, the inductive reactance can be calculated. In the actual adjustment of RF wave amplitude, it is necessary to constantly change the inductance value to choose the best amplitude in order to achieve the best map effect.
When selecting the frequency of the high-low pass filter, it is suggested that the minimum range of the bandpass width of the filter is 0.5 to 2 times of the probe center frequency. Select the average signal value to the minimum requirement to obtain a reasonable signal-to-noise ratio, and set the time window to cover the useful part of A scan for digitization. The A-scan data window of the partition should cover the adjacent detection partition in the depth direction. Generally, the time window is set according to direct wave or bottom reflection wave. If there is no direct wave or bottom reflection wave, the time window must be set by calculation and checked on the test block. When the workpiece is less than 50 mm and single-channel inspection is carried out, the starting position of the time window should be set more than 0.5 μs before the direct wave reaches the probe, and the ending position of the time window should be set more than 0.5 μs after the first waveform conversion wave on the bottom surface of the workpiece. When detecting in the thickness direction, the starting position of the time window in the upper area should be set more than 0.5 μs before the direct wave reaches the probe, and the time ending position of the A-scan signal in the lower area should be set more than 0.5 μs after the reflected wave on the bottom surface of the workpiece. The time window of each partition should cover at least 25% of the height of adjacent detection partitions in the thickness direction. The depth parameter input provided by the test equipment can be used, but the coverage of the time window in the thickness direction should be checked by reference block [3].
There are three ways to set the sensitivity: the first way is to set the sensitivity directly on the inspected workpiece, and generally set the direct wave signal to 40% ~ 80% of the full screen height; Secondly, if the direct wave signal is not suitable, the bottom reflection amplitude can be set to 80% of the screen height, and then increased by 20 dB to 32 dB; ; Thirdly, if the transparent wave and the bottom wave are not suitable, the particle noise of the material can be set to 5% ~ 10% of the full screen height as the sensitivity. When the thickness of the inspected workpiece is less than 50 mm, the first method can be directly used for adjustment. After all sensitivities are set, the set sensitivities shall be verified or calibrated on the test block. Personally, I think the height of all the above direct waves is based on the highest amplitude, not the lowest peak or trough. Because in the actual detection, the height difference between the trough and the peak is sometimes half in one and a half periods of the direct wave. If the low amplitude is taken as the standard, the particle noise will be greater than 10% when making thick plates, and the spectrogram effect will be poor, which will affect the evaluation. If conditions permit, it is best to verify the reference block.
Comprehensive judgment in 2 TOFD detection
Qualitative and quantitative analysis is the key to TOFD test. Nb/T 47013.10-2010 Non-destructive Testing of Pressure Equipment Part 65438 +00 Diffraction Time Difference Ultrasonic Testing, the relevant displays are classified as follows:
Obviously, there is no provision in TOFD test that needs to pay attention to whether there are harmful defects such as cracks as in conventional tests. The accurate characterization of defects requires the detection experience of inspectors, and its harmfulness can be determined by combining with conventional ultrasound if conditions permit.
In TOFD detection, the position of defects, especially on the left and right sides of the weld centerline, is complicated. After the defect is found by non-parallel scanning, the specific position of the defect deviating from the center line is determined by parallel scanning. If the width of the weld is large, this method will not work properly. It is suggested to use conventional ultrasound for positioning, which will improve the work.
Efficiency.
Misunderstandings in 3 TOFD detection
TOFD test will exaggerate some defects, especially stomatal defects. If the interval between several smaller air holes is not large enough, these air holes will be more serious on the TOFD diagram, but in fact they can be left untreated, and the inspectors must be cautious in their judgment, as shown in the following figure. If conditions permit, it can be judged by combining with conventional ultrasound, which is helpful to improve the detection rate and accuracy.
refer to
He Sha, Yuan, Yu Jiansheng, Liu Congqing. Research on ultrasonic diffraction time difference detection technology [J]. China Test, March 2009.
Iron and Steel, Yan Xu, Chi Dazhao, Lv Pin. Signal characteristics of ultrasonic TOFD detection of aluminum alloy welds [J]. Welding Journal, August 2005.
Chen Jianyu, Rong Yuan. TOFD detection technology for deep weld of hot wall hydrogenation reactor [J]. Pressure vessel, August 2004.