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. 

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.

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.

HISTORY OF ULTRASONIC

HISTORY OF ULTRASONIC:   prior to world war 2, sonar,the technique of sending sound waves through water and observing the returning to characterize submerged objects,inspired early ultrasound investigators to explor ways to apply the conceptto medical in 1920 and 1935,sokolov studied the use of ultrasonic waves in detecting metal object mulhausar,in 1931,obtained a patent for using ultrasonic waves,using two transducers to detect flaws in solid.Firestone(1940) and simons (1945) developed pulsed ultrasonic testing using a pulse-echo technique.                

                shortly after the close of world war 2,researchers in japan began to explor the medical diagnostic capabilities of ultrasound.The first ultrasonic instruments used an A-mode presentation with blips on oscilloscope screen. that was followed by a B-mode presentatoin with a two dimensional,gray scale image.

                japans work ultrasound was relatively unknown in the united states and Euroup until the 1950s.Researchers then presented their finding on the use of ultrasound to detect gallstone,breast masses,and tumors to the international medical community.japan was also the firsr country to apply Doppler ultrasound,an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation.                                                                                  

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.

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. 

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. 

BEGINNING OF NON DESTRUCTIVE EVALUATION (NDE).

           Beginnings of nondestructive evaluation:

• Nondestructive testing has been practiced for many decades, with initial rapid developments in instruments in instrumentation spurred by the technological advances that occured during world war2 and the subsequent effort .during the earlier days ,the primary purpose was the detection of defects.As a part of safe life design,it was intended that a structure should not develop macroscopic defect during its life,with the detection of such defect being a cause for removal of the component from service.in response to this need ,increasingly sophisticated techniques using ultrasonic eddy currents,x-rays,dye penetrants,magnetic particles,and other forms of interrogating energy emerged.                                                                                                                                                                          In the early 1970s two event occurredwhich caused the major change in the NDT feild first improvement in the technology led to the ability ti defect small flaws.which caused more parts to be rejected even though the probability of component failure had not change .however the discipline of fracture mechanics emerged which enabledone to predict whether a crack of a given size will fail under a particular load when a materials fracture toughness properties are known other laws were developed to predict the growth rate of crack under cyclic loading with the advent of thes tools it become possible to accept structure containing defect if the sizes of those defects were known This formed the basis for the new philosophy of damage tolerent design component having known defects could continue in service as long as it could be established that those defects would not grow to a critical,failure producing size.
  
        A new challenge was thus presented to the nondestructive testing community.detection was not enough.one needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics based prediction of remaining life. The need for quantitative information was particularly strongly in the defense and nuclear power industries and led to the emergence of quantitative nondestructive evaluation .as a new engineering research discipline.A number of research programs around the world were started.such as the center for nondestructive evaluation at lowa state university (growing out of a major research effort at the Rockwell international science center): the electric power research institute in charlotte, North carolina; the fraunhofer institute for nondestructive testing in saarbrucken,Germany, and the nondestructive testing center in Harwell,England.                                                                                                                                                 

PRESENT STATE OF ULTRASONICS.

PRESENT STATE OF ULTRASONICS:-Ultrasonic testing (UT) has been practiced for many decades.initial rapid developments in instrumentation spurred by the technological advances from the 1950s continue today. through the 1980s and continuing through the present, computers have provided technicians with smaller and rugged instruments with greater capabilities.Thickness gauging is an example application where instruments have been refined make data collection easier and better.Built in data logging capabilities allow thousand of measurements to be recorded and eliminate the need for a scribe some instruments have the capability to capture waveforms as well as thickness readings.The waveform option allows an operator to view or review the A-scan signal of thickness measurement long after the completion of an inspection. Also some instrument are capable of modifying the measurement based on a surface condition of the material.For example , the signal from a pitted or eroded inner surface of pipe would be treated differently then a smooth surface. This has led to more accurate and repeatable field measurements.

Many ultrasonic flaw detector have a trigonometric function that allows for fast and accurate location determination of flaws when performing shear wave inspections cathode ray tubes,for the most part, have been replaced with LED or LCD screens.These screens, in most cases, are extremely easy to view in a wide range of ambient lighting, Bright or low light working conditions encountered by technician have little effect on the technicians ability to view the screen. screens can be adjusted for brightness,contrast and on some instruments even the color of the screen and signal can be selected.Transducers can be programmed with predetermined instrument settings.The operator only has to connect the transducer and the instrument will set variables such as frequency and probe drive.                                                                                                
Along with computers,motion control and robotic have contributed to the advancement of ultrasonic inspections Early on the advantage of a stationery platform was recognized and used in industry. computers can be programmed to inspect large,complex shaped components, with one or multiple transducers collecting information automated system typically consisted of an immersion tank scanning system and recording system, for a printout of the scan. The immersion tank can be replaced with a squirter systems.which allows the sound to be transmitted through a water column.The resultant C-scan provides a plan or top view of the component.scanning of components is considerably faster than contact hand scanning the coupling is much more consistent.The scan information is collected by a computer for evaluation,transmission to a customer and archiving.

Today,quantitative theories have been developed to describe the interaction of the interrogating fields with flaws.models incorporating the results have been integrated with solid model descriptions of real-part geometries to simulate practical inspections. Related tools allow NDE to be considered during the design process on an equal footing with  other failure-related engineering disciplines.Quantitative description of NDE performance,such as the probability of detection (POD),have become an integral part of statistical risk assessment.measurement procedure initially developed for metals have been extended to engineered materials such as composites,where anisotropy and inhomogeneity have become important issues.The rapid advances in digilization and computing capabilities have totally changed the faces of many instruments and the type of algorithms that are used in processing the resulting data.High-resolution imaging systems and multiple measuerment modalities for characterizing a flaw have emerged.Interest is increasing not only detecting.characterizing,and sizing defects,but also characterizing the materials.Goals range from the determination of fundamental microstructural characteristics such as grain size,porosity and texture (preferred grain orientation),to material properties related to such failure mechanisms as fatigue,creep and fracture toughness.as technology continues to advance,application of ultrasound also advance.The high-resolution imaging systems in the laboratory today will be tools of the technician tomorrow.... 

Future Direction of ultrasonic inspection.

Future Direction of ultrasonic inspection:- Looking to the future those in the field of NDE see an exciting  new set of opportunities.The defense and nuclear power industries have played a major role in the emergence of NDE, increasing global competition has led to dramatic changes in product development and business cycles.At the same time,aging infrastructure,from roads to buildings and aircraft,present a new set of measurement and monitoring challenges for engineers as well as technicians.

Among the new application of NDE spawned by these changes is the increased emphasis on the use of NDE to improve the productivity of manufacturing processes.Quantitative nondestructive evaluation (QNDE) both increases the amount of information about failure modes and the speed with which information can be obtained and facilitates the development of in-line measurements for process control.                                       The phrase,you cannot inspect in quality,you must build it in,exemplifies the industry focus on avoiding the formation of flaws.Nevertheless,manufacturing flaws will never be completely eliminated and material damage will continue to occur in-service so continual development of flaw detection and characterization techniques is necessary.                                                                                                                                                Advanced simulation tools that are designed for inspectability and their integration into quantitative strategies for life management will contribute to increase the number and types of engineering application of NDE.with growth in engineering application for NDE,there will be a need to expand the knowledge base of technicians performing the evaluations.Advanced simulation tools used in the design for inspecatbility may be used to provide technical students with a greater understanding of sound behavior in materials. UTSIM, developed at lowa state university,provides a glimpse into what may be used in the technical classroom as an interactive laboratory tool.