5.1 Flaw Coordinates
To determine the coordinates of the flaw, the flaw detector measures time between the moment of generation of the probe pulse and the moment of arrival of the echo pulse (TEC), as a consequence of reflection from the flaw. The distance to the flaw along the ray (R) is calculated as a segment of the line coinciding with the acoustic axis and connecting the center of the reflector to the exit point of the beam. The time spent by the wave for traveling through the prism of the PET in the forward and backward directions, the so-called the prism delay time (2TEP) is to be excluded.
The distance to the defect along the ray can be calculated by the formula:
Where C – sound velocity in the controlled object.
To find the depth and the distance to the defect on the input surface, you need to use the value of the angle of entry (a)
Y – the cause of the negligence of the object; X – the structure of the object to the accuracy of the word; R – the process for the object to go.
Fig.6.1 – Controlling the coding of the product
When calculating X and Y, some errors may arise, primarily related to the determination of the wave velocity in the product, the angle of input (α) and the delay time in the prism, since the values of these parameters are determined by the operator when tuning the flaw detector. The calculation of the coordinates of the defect can also be unreliable if we use an erroneous determination of the exit point of the beam or for the maximum echo signal to receive the signal received from the defect by a lateral ray (Fig. 6.2).
Fig.6.2 – Illustration of one of the reasons for the appearance of the error in determining the coordinates
5.2 Amplitude characteristics
The magnitude of the defect, in those cases when it does not exceed the width of the directional diagramm, largely determines the amplitude of the echo signal. For this reason, amplitude characteristics are often used as criteria for product rejection.
The amplitude (N) is the echo-pulse exceeding of the threshold level of the flaw detector.
The coefficient of detectability (CD) is the ratio of the amplitudes of the echo pulses of the defect and the reference reflector. The coefficient of detectability can be expressed both in dimensionless relative units, and in decibels. As a reference, an omnidirectional reflector is used – this is a 6 mm drill bit.
Fig.6.3 – Determination of the amplitude and coefficient of detectability
The positive detection factor expressed in decibels indicates an excess of the signal value from the defect over the signal from the reference reflector, the negative, on the contrary, indicates that the signal from the defect is less than the signal from the reference reflector.
The magnitude of the reflected signal depends not only on the magnitude of the defect, but also on its orientation toward the axis of the directional diagramm.
Fig.6.4 – Relationship between amplitude and detection factor with defect form
The defect characteristic, which combines two parameters, namely the amplitude of the defect and the depth of its occurrence, is called the equivalent defect area.
The equivalent area is the area of the ideal reflector (the flat-bottomed disk perpendicular to the acoustic axis), the depth of occurrence and amplitude of the signal coincide in magnitude with the depth of occurrence and the amplitude of the signal from the defect (Fig. 6.5).
To determine the equivalent area of a defects and to adjust the device for the maximum sensitivity, use ARD diagrams and SKH diagrams. ARD – the diagram connects Amplitude – Distance – Diameter equivalent (Fig. 6.6).
The equivalent diameter is the diameter of the ideal reflector. SKH – the diagram connects the parameters Equivalent area – Detectability factor – Depth.