Ultrasonic Inspection
It is employed to detect and locate defects such as internal cracks, porosity and large non-metallic inclusion. Ultrasonic vibration can be used to locate defect in ferrous and non-ferrous metallic object as well in plastic and ceramics.Ultrasonic inspection for floor detection make use of acoustic wave with frequencies in the range between 20 kHz and 20 MHz, which can be transmitted through solid [even liquid, and air as well] and get reflected by the sub-surface defect. Ultrasonic wave form a bases for a detection, location and size estimation of defect.
Principle of operation:-
► Ultra sonic wave are usually generated by the piezoelectric effect, which convert electric energy to mechanical energy. A quartz crystal is used for the purpose.
► When a high frequency alternating electric current [of about one million cycles/second] is impressed across the faces of the quartz crystal, the crystal will expand during the first half of the cycle and contract when the electric field is reversed. In this manner, the mechanical vibrations (sound waves) are produced in a crystal.
► Ultrasonic inspection employs separate probes (or search units), one for transmitting the wave and other to receive them.After passage through the casting (fig A), alternatively since the ultrasonicwave are transmitted as a series of intermittent pulses the same crystal may be employed both as the transmitter and the receiver (fig B)
► Before transmitting ultrasonic waves, an oil film is provided between the probe and the casting surface, this ensures proper contact between them and better transmission of wave from the probe into the surface of the object to be tested
► For operation, ultrasonic wave is introduced into the metal and time interval between the transmission of the outgoing and reception of the incoming signals are measured with a cathode ray oscilloscope (CRO)
► The time base of CRO is so adjusted that the full width of the trace represents the section being examined.
► To start with as the wave is sent from the transmitter probe it strikes the upper surface of casting and makes a sharp [peak] of pip [echo] at the left hand side of the CRO screen. If the casting is sound this wave will strike the bottom surface of the casting get reflected and indicated by a PIP towards the right hand end of the CRO screen.
In case a defect exist in between top and bottom casting surfaces most of the beam striking this defect will get reflected from the defect reach the receiver prop and indicate a PIP (echo) on CRO screen before PIP given by the wave striking the for end of the casting and returning.
The distance of the defect from the surface where transmitted probe applied, can be determined with the help of a time distance scale in the form of square wave constantly shown on the oscilloscope. The distance scale may be change as per convenience indicate 1 cm or 25 cm etc
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