3.3. Discussion
In order to verify the accuracy of ultrasonic test based on wavelet packet analysis, four groups of test value and the actual metallographic measured value were compared, as shown in Fig. 11(a). A refers to actual measured the value of nugget diameter; B refers to the test value. Obviously, actual measured value fits the test value very well. Fig. 11(b) is the statistical chart of error statistics. Mean value of normal distribution curve is 0.00187, and the standard deviation is 0.1392. Mean value of relative error and variance are small, which shows that under the condition in this test, precision and stability of ultrasonic test based on wavelet packet analysis in time–frequency domain are high.