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Monday, 3 October 2016

Pulser - Receivers in ultrasonic testing

Ultrasonic pulser - Receivers are well suited to general purpose ultrasonic testing. Along with appropriate transducer and an oscilloscope, they can be used for flaw detection and thickness gauging in a wide variety of metals, ceramics,and composites. Ultrasonic pulser - Receivers provide a unique, low - cost ultrasonic measurement capability. The pulser section of the instrument generates short, large amplitude electric pulses of controlled energy, which are converted into short ultrasonic pulses when applied to an ultrasonic transducer. Most pulser section have very low impedance out puts to better drive transducers. Control functions associated with the pulser circuit include.
1)   pulse length or damping (The amount of time the pulse is applied to the transducer)
2)    pulse energy (The voltage applied to the transducer. Typical pulser circuits will apply from 100 volts 800volts to a transducer)  in the receiver section the voltage signals produced by the transducer, which represent the received ultrasonic pulses, are amplified. The amplified radio frequency (RF)  signal is available as an output for display or capture for signal processing. Control functions associated with the receiver circuit include.
3)   signal rectification ( The RF signal can be viewed as positive half wave, negative half wave or full wave
4)    Filtering to shape and smooth return signals.
5)     Gain, or signal amplification.
6)      Reject control.
The pulser - Receiver is also used in material characterization work involving sound velocity or attenuation measurements,  which can be correlated to material properties such as elastic modulus. In conjunction with a Stepless gate and a spectrum analyzer, pulser - Receivers are also used to study frequency dependent material properties or to characterize the performance of ultrasonic transducers. 

Transducer modeling in ultrasonic testing.

In high -technology manufacturing, part design and simulation of part inspection is done in the virtual world of the computer.  Transducer modeling is necessary to make accurate predictions of how a part of component might be inspected, prior to the actual building of that part. Computer modeling is also used to design ultrasonic transducers.
As noted in the previous section, an ultrasonic transducer may be characterized by detailed measurements of its electrical and sound radiation properties. Such measurements can completely determine the response of any one individual transducer.
There is ongoing research to develop general models that relate electrical inputs (voltage, current) to mechanical outputs (force,velocity)  and vice - versa. These models can be very robust in giving accurate prediction of transducer response, but suffer from a lack of accurate modeling of physical variables inherent in transducer manufacturing. These electrical -mechanical response models must take into account the physical and electrical components in the figure below.
The Thompson -Gray measurement model, which makes very accurate predictions of ultrasonic scattering measurements made through liquid - solid interfaces, does not attempt to model transducer electrical -mechanical response. The Thompson -Gray measurement model approach makes use reference data taken with the same transducer (s) To deconvolve electro - physical characteristics specific to individual transducer.
The long term goal term goal in ultrasonic modeling is to incorporate accurate models of the transducer themselves as well as accurate models of pulser-receivers, cables, and other components that completely describe any given inspection setup and allow the accurate prediction of inspection signals. 

Sunday, 2 October 2016

TRANSDUCER TESTING IN ULTRASONIC INSPECTION.

Some transducer manufacturers have lead in the development of transducer characterization techniques have participated in developing the AIUM-E. 1065 standard Guide for evaluating characteristics of Ultrasonic search units.
Additionally, some manufacturers perform characterizations according to AWS, ESI, and many other industrial and military standards.often,equipment in test labs is maintained in compliance with MIL-C-45662A calibration system requirements. As part of the documentation process, an extensive database containing records of the waveform and spectrum of each transducer is maintained and can be accessed for comparative or statistical studies of transducer characteristics.
Manufacturers often provide time and frequency domain plots for each transducer. The signals below were generated by a spiked pulser. The waveform image on the shows the test response signal in the time domain (amplitude versus time). The spectrum image on the right shows the same signal in the frequency domain ( amplitude versus frequency).  The signal path is usually a reflection from the back wall ( fused silica)  with the reflection in the far field of the transducer. 

Saturday, 17 September 2016

Angle beam transducers in ultrasonic testing.

Angle beam transducers :-Angle beam transducers and wedges are typically used to introduce a refrected shear wave into the test material. Transducer can be purchased in a variety of fixed angles or adjustable versions where the user determines the angles of incidence and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material,, which is usually steel. The angled sound path allows the sound beam to be reflected from the Blackwell to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of component.
Normal incident shear wave transducer:-are unique because they allow the introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave component is generally below -30dB.
paint brush transducers 

Thursday, 15 September 2016

DUAL ELEMENT & DELAY LINE TRANSDUCER IN ULTRASONIC TESTING.

Dual element transducer :-Contain two independently operated elements in a single housing. One of the elements transmits and the other recives the ultrasonic signal. Active elements can be chosen for their sending and receiving capabilities to provide a transducer with a cleaner signal,and transducer for special applications such as the inspection of course grained material. Dual element transducer are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single element transducer experience (when single -element transducer are operating in pulse echo mode, the element cannot start receiving reflected signals until the element has stopped ringing from its transmit function).Dual element transducer are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed - beam sound path in the test material.
Delay line transducers:- provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its sending function before it starts it's listening function so that near surface resolution is improved. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high - temperature measurement applications sine the delay line provides some insulation to the piezoelectric element from the heat.

Wednesday, 14 September 2016

CONTACT TRANSDUCER &IMMERSION TRANSDUCERS In ULTRASONIC TESTING.

Contact transducer :- are used for direct contact inspections,and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They after have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils,or commercial materials are used  to remove the air gap between the transducer and the component being inspected.
Immersion transducer :-do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Immersion transducers usually in a liquid environment and all connections are watertight. immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with a planer,cylindrically focused or spherically focused lens, A focused transducer can improve the sensitivity and axial resolution by concentrating the sound energy to a smaller area. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications. Contact transducer are available in a variety of configuration to improve their usefulness for a variety of applications. The flat contact transducer shown above is used in normal beam inspection of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may need to be added to the face of the transducer. If near surface resolution is important or if an angle beam inspection is needed, one of the special contact transducer described below might be used.

Monday, 12 September 2016

TRANSDUCER BEAM SPREAD IN ULTRASONIC TESTING

Round transducers are often referred to as piston source transducer because the sound field resembles a cylindrical mass in front of the transducer. However the energy in the beam does not remain in a cylinder, but instead spread out as it propagates through the materials. The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction. It should be noted that there is actually a difference between beam spread and beam divergence. Beam spread is a measure of the whole angel from side to side of the main lobe of the sound beam in the far field. Beam divergence is a measure of the angle from one side of the sound beam to the central axis of the beam in the far field. Therefore, beam spread is the twice the beam divergence.
Although beam spread must be considered when performing an ultrasonic inspection, it is important to note that in the far field, or fraunhofer zone, the maximum sound pressure is always found along the acoustic axis (centerline)of the transducer. Therefore, the strongest are likely to come Frome the area directly in front of the transducer. Beam spread occurs because the vibrating particle of the material (through which the wave is travelling) do not always transfer all of their energy in the direction of wave propagation. Recall waves propagate through the transfer of energy from one particle to another in the medium. If the particles are not directly aligned in the direction of wave propagation, some of the energy will get transferred off at an angle. (picture what happens when one ball hits another ball slightly off centre). In the near field, constructive and destructive wave interference fill the sound field with fluctuation. At the start of the far field, the beam strength is always greatest at the centre of the beam and diminishes as it spreads outward. Beam angle is important consideration in transducer selection for a couple of reasons. First beam spread lower the amplitude of reflection since sound are less concentrated and, thereby weaker. Second, beam spread may result in more difficulty in interpreting signals due to reflection from the literal sides of the test object or other features outside of the inspection area. Characterization of the sound field generated by transducer is a prerequisite to understanding observed signals.

Friday, 9 September 2016

PIEZOELECTRIC TRANSDUCER IN ULTRASONIC TESTING.

The transducer is a very important part of the ultrasonic instrumentation system. The transducer incorporates a piezoelectric element which converts electrical signals into mechanical vibrational (transmit mode) and mechanical vibrations into electrical signals (receive mode) many factors, including material, mechanical and electrical construction, and the external, mechanical and electrical load conditions,influence the behaviour of a transducer. Mechanical construction includes parameter such as the radiation surface area mechanical damping, housing connector type and other variables of physical construction.As of this writing, transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.
A cut a way of typical contact transducer is shown above. It was previously learn that the piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible and impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that it's thickness is 1/4 of the desired wave length. This keeps waves that were reflected with in the matching layer in phase when they exit the layer (as illustrated in the image to the right) for contact transducer, the matching layer is made from a material that has an acoustical impedance between the active element and steel. Immersion transducers have a matching layer with an acoustical impedance between the active element and water.Contact transducer also incorporate a wear plate to protect the matching layer and active element from scratching.
The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with an impedance similar to that of  the active element will produce the most effective damping such a transducer will have a wider bandwidth resulting in higher sensitivity. As the mismatch in impedance between the active element and the backing material increases material penetration increases but transducer sensitivity is reduced.

Wednesday, 31 August 2016

Radiated fields of ultrasonic transducer.

The sound that emanates from a piezoelectric transducer does not originate from a point but instead originates from most of the surface of the piezoelectric element, Round transducers are often referred to as pista source transducer because the sound field resembles a cylindrical mass in front of the transducer. The sound field from a typical piezoelectric transducer is shown below. The intensity of the sound in indicated by color, with tighter indicating higher indicating higher intensity.  Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference. These are sometimes also referred to as diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variation with in a near field, it can be extremely difficult to accurately evaluate flaws in materials when the are positioned with in this area.The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. In the far field, the beam spreads out in a pattern originating from the centre of the transducer the transition between the near field and the far field occurs at a distance. N.and is sometimes referred to as the natural focus of a flat (or un focused)transducer. The near/far field distance. N is significant because amplitude variations that characterize the near field change to a smoothly declining amplitude at this point. The area just beyond the near field is where the sound waves is well behaved and at its maximum strength. Therefore optimal detection results will be obtained when flaws occurs in this area. 

Wednesday, 10 August 2016

Attenuation of sound in ultrasonic testing.

When sound travels through a medium, it's extreme less with distance. In visual material,  sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials,however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material. Attenuation of sound with in material itself  is often not essential interest. However, natural properties and loading condition can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonics intensity.