The most significant part of an ultrasonic test is an ultrasonic transducer. For any ultrasonic application, the selection of the correct transducer is the most vital step. A number of factors that affect the results are the material properties, the instrument and conditions used for coupling and settings. Depending on the particular application it is required for; the transducer can be selected accordingly according to its sensitivity and resolution. The ability of the transducer to detect the small defects is called its sensitivity whereas the separation to of the two signals produced by the two reflectors when close and either perpendicular or parallel to the beam is called the resolution. A highly damped transducer allows resolving defects that are closely spaced only by helping to shorten the reflected pulse. The maker of this device can also make transducers that are focused for improved resolution and sensitivity, a large selection of polarized ceramic compositions, polymers, piezo composite materials and ceramic compositions so that the performance of the transducer can be altered according to the requirement.
An ultrasound is a sound which is above the level of human hearing range. Although most of the transducers produce ultrasounds that have a frequency above 200 KHz, the ultrasounds begin at only 20 KHz. These sounds are like ordinary sound-waves but have a wavelength which is a lot shorter. This key feature makes these waves more suitable for detecting small flaws. In fact, it is these short wavelengths that make the ultrasound transducers tremendously useful in the testing, detection, inspection and measurement of different materials.
How it works
Ultrasonic transducer is such a device which can generate ultrasounds as well as pick them up as well. It consists of 3 main parts the function of which explained below
- The Piezoelectric Crystal : The core of the transducer which is the ‘active ingredient’ of the device is the Piezoelectric crystal material which undergoes compressions and rarefactions in order to convert the electrical energy to the ultrasonic energy or vice versa. A flaw detector is another instrument which is used to generate the electrical pulse which then goes to the transducer.
- The Backing: It is a very dense material which is also extremely attenuative. The sole purpose of this is to absorb the energy that radiates from the back of the crystal in order to control the vibrations. An excellent resolution highly-damped transducer can be made by matching the acoustic impedance of the backing material with that of the crystal. If the backing material is changed, the difference in this acoustic impedance can be changed. This will definitely affect the transducers as the resolution may become much higher in signal amplitude or sensitivity.
- Wearplate: A wearplate is installed in the transducer to protect the piezoelectric crystal from environment. The environmental factors that it protects from are typicaly the wear and tear and corrosion. The wearplate often functions as an acoustic transformer between the crystal and water, wedge or delay line typically in an immersion type transducer.
Transducer’s Field of Sound
The transducers field of sound generally has two exclusive zones known as ‘the near field’ which is the region directly opposing the transducer and the far field is the area beyond “N. The far field is also the area where pressure of the field of sound drops to zero gradually. It may also become very difficult at times to accurately take readings and assess any flaws because of the variations in the near field.
The characteristics of the transducer can be described in a decent manner using the several different parameters of the sound field. To determine if a transducer is suitable for a certain type of application, the knowhow of the length of the focal, the width of the beam and focal zone is crucial.
The decrease in the diameter of the sound beam is a very important reason why focusing increases the sensitivity of the transducer. That is why even the smallest of the flaws will reflect a much greater part of the transmitted energy. The equation which is used to calculate the diameter of the 6DB pulse echo beam is as below
BD (=6dB) = 1.028 Fc/fD
Where BD is known as the Beam Diameter and F is called the Focal Length.
The points where the signal amplitude of the on-axis pulse echoes drops to about -6DB are known as the initiation and termination points. The following equation is used to calculate the focal zone length.
Fz = NS2F [2/(1 + .5SF)]
The beam spread of the transducers is an important point that must be considered when the flaws that might be close to geometric features of the material that is to be tested are to be inspected. The corners which may cause bogus echoes can easily be considered as flaws or defects so the side walls are the key features that are to be inspected.
The angle of the beam, which is a -6DB pulse echo, are defined for all the flat transducers and are determined by the formula below
Sin (a/2) = .514c/fD
The Different types of transducers
There are a number of different types of transducers in which the straight beam contact ones are the most common and popular ones. These are used to lead longitudinal waves into a specific material. A combination of longitudinal and shear wave transducers or a normal incidence shear wave can be made simply by using some special elements. Such a transducer requires a highly durable wearplate because it is used in contact with a test material.
The basic principle of refraction and mode conversion is utilized by the angle beam transducers to produce the longitudinal waves and the refracted shear in the test material. The required refracted wave is produced using a specific angle of incidence which can be calculated using the Snell’s law.
The wedge angle(represented by Q1) which is required to create the desired mode and refracted angle (represented by Q2) in the material under test.
Sin Q1/Sin Q2 = V1/V2
Where Ø1 is the Wedge Angle and Ø2 is the angle of refracted wave in test material. V1 is the longitudinal velocity of the material used by the wedge and V2 is the material velocity
Separate elements are used by the transducers which have 2 elements for the transmitting and receiving of the signals. Typically, the elements are cut and attached to the delay lines. This is necessary for improving the near surface resolution. The focus is generally created by a cross beam design which makes the dual element transducers more sensitive echoes which come from irregular defects.
There are several advantages of the immersion transducers over the typical contact type ones. Uniform coupling reducing variations in sensitivity is one of them, the second being, and the increased speed of from the ability to perform automated scanning. Another one of the advantages is the focusing capability of immersion type causes the sensitivity to increase to small defects. There are different types of immersion transducers namely, unfocused, spherically focused, and cylindrically focused configurations. A transducer that is unfocused is often used in general applications such as in the measurement of materials that are thick whereas the spherically focused is used to improve the sensitivity to small flaws and defects. The third one, which is the cylindrically focused one, is generally used in the measurement of the tubing of raw stock. The range of lengths of the focal is limited to the transducers near field and is generally a maximum of 0.8N in the case of a cylindrical transducer.