WELDMENT ANGLE BEAM IN ULTRASONIC INSPECTION.

The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principal of refraction and mode conversion to produce refrected shear or longitudinal waves in the test material. This inspection may include the root,sidewall, Crown, and heat- affected zones of a weld.
The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector ( discontinuity)  in the path of the sound beam. Will proper angle beam techniques, echoes returned from the weld zone may allow the operator to determine the location and type of discontinuity.
To determine the proper scanning area for the welder,the inspector must first calculator the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the Crown, side wall,and root of the weld.

WELDMENTS ( welded joints) in ultrasonic inspection.

The most commonly occurring defects in welded joints are porosity, Slag inclusions, lack of side - wall fusion, lack of inter - run fusion, lack of  root penetration, undercutting, and longitudinal or transverse cracks.
With the exception of signal gas pores all the defects listed are usually well detectable by ultrasonic. Most applications are on low-alloy construction quality steels,however, world's in aluminium can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding application. This safe,accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology.
Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incident into the test piece, is first used to locate any laminations in important because an angle beam transducer may not able to provide a return signal from a lamina flaw. 

RAIL INSPECTION IN ULTRASONIC INSPECTION.

Rail inspections were initially performed solely means. Ofcourse, visual inspections will only detect external defects and sometimes the subtle signs of large internal problems. The need for a better inspection Methley became a high priority because of aderailment at Manchester, NY in 1911, in which 29 people were killed and 60 were seriously injured. In the U.S bureau of safety's ( now the national transportation safety board)  investigation of the accident, a broken rail was determined to be the cause of the derailment. The bureau established that the rail failure was caused by defect that entirely internal and probably could not have been detected by visual means. The defect was called a transverse fissure ( example shown on the left).  The railroads began investigating the prevalence of this defect and found transverse fissures were widespread.
One of the methods used to inspect rail is ultrasonic inspection. Both normal - and angle - beam techniques are used, as are both pulse - echo and pitch - catch techniques transducer arrangements offer different inspection capabilities. Manual contact testing is done to evaluate small section of rail but the ultrasonic inspection has been automated to allow inspection of large amounts of rail.
Fluid filed wheels or sleds are often used to couple the transducer to the rail. Sperry Rail services,  which is one of the companies that perform rail inspection, uses Roller search units ( RSUs)  comprising a combination of different transducer angles to achieve the best inspection possible. A schematic of an RSU is shown below. 

(DC) BLOCK & (RC) BLOCK IN ULTRASONIC INSPECTION.

Distance calibration Block :-The DC AWS block is a metal path distance and beam exit point calibration standard that conforms to the requirements of the American welding society (AWS) and the American association of state highway and transportation officials ( AASHTO).  Instructions on using the DC block can be found in the annex of American society for testing and materials standard E164, standard practice for ultrasonic contact Examination of weldments.

RESOLUTION CALIBRATION BLOCK:-The RC block is used to determine the resolution of angle beam transducers per the requirements of AWS and AASHTO. engraved index markers are provided for 45,60, and 70 degree refracted angle beams.

ANGLE - BEAM CALIBRATION BLOCK IN ULTRASONIC INSPECTION.

The miniature angle-beam is a calibration Block that was designed for the US Air Force for use in the field for instrument calibration. The block is much smaller and lighter than IIW block but performs many of the same functions. The miniature angle-beam block can be used to check the beam angle and exit point of the transducer. The block can be used to make metal - distance and sensitivity calibrations for both angle and normal - beam inspection setups.

A block that closely resembles the miniature angle-beam block and is used in a similar way is the DSC AWS block. This block is used to determine the beam exit point and refracted angle of angle - beam transducers and to calibration distance and set sensitivity for both normal and angle beam inspection setups. Instructions on using the DSC block can be found in the annex of American society for testing and materials standard E164, standard practice for ultrasonic contact Examination of weldment.

The IIW Type Calibration Block in ultrasonic inspection.

The standard shown in the above figure is commonly known in the US as an IIW Type reference block. IIW is an acronym for the international Institute of welding. It is referred to as an IIW Type reference block because it was patterned after the true IIW block but does not conform to IIW requirements in IIS/IIW-23-59. True IIW blocks are only made out of steel (to be precise, killed, open hearth or electric furnace, low - carbon steel in the normalize condition with a grain size of McQuaid-ehn #8) where IIW Type blocks can be commercially obtained in a selection of materials. The dimensions of  true IIW blocks are in metric units while IIW  type blocks usually have English units. IIW type blocks may also include additional calibration and references features such as notches, circular groves, and scales that are not specified by IIW. There are two full -sized and a mini versions of the IIW Type blocks. The mini version is about one - half the size of the full-sized block and weighs only only about one-fourt as much. The IIW type US-1 block was derived the basic true IIW block and is shown below in the figure on the left. The IIW type US-2 block was developed for US air force application and is shown below in the center. The mini version is shown on the right.

IIW type blocks are used to calibrate instruments for both angle beam and normal incident inspections. Some of their uses include setting metal-distance and sensitivity settings,determining the sound exit point and refracted angle of angle beam transducers, and evaluating depth resolution of normal Beam inspection setups. 

Distance Amplitude correction (DAC) in ultrasonic inspection.

Acoustic signals from the same reflecting surface will have different amplitudes at different distances from the transducer. Distance Amplitude correction (DAC)  provides a means of establishing a graphic reference level sensitivity as a function of sweep distance on the A - scan display. The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated. Most often DAC will allow for loss in amplitude over material depth  (time), graphically on the A - scan display but can also be done electronically by certain instruments. Because near field length and beam spread vary according to transducer size and frequency, and materials vary in attenuation and velocity, a DAC cure must be established for each different situation. DAC may be employed in both longitudinal and shear modes of operation as well as either contact or immersion inspection techniques.
A distance Amplitude correction curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same material. A-scan echoes are displayed at their non - electronically compensated height and peak amplitude of each signal is marked on the flaw detector screen or, preferably, on a transparent plastic sheet attached to the screen. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH),or notches whereby the reflectors are located at varying Depths are commonly used.

INTRODUCTION TO THE COMMON STANDARDS IN ULTRASONIC INSPECTION.

Calibration and reference standards for ultrasonic testing come in many shapes and sizes. The type of standard used is dependent on the NDE application and the form and shape of the object being evaluated. The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirements is a major limitations of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more realistic defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size. Computer programs that allows the inspector to creat computer simulated model of the part and flaw may one day lessen this limitation.

CALIBRATION METHODS IN ULTRASONIC INSPECTION.

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a user calibration of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigure for use in a large variety of applications. The user must calibration the system which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually  only used when an absolute value is measured and in many cases,the standards are traceable back to standards at the national institute for standards and technology.
In ultrasonic testing, there is also a need for reference standards. Reference standards are used to establish a general level of consistency in measurements and to help interpret and quantity the information contained in the received signal. Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal.

Attenuation measurements in ultrasonic inspection.

Ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The velocity of the ultrasonic waves is influenced by the elastic moduli and the density of the material, which in turn are mainly governed by the amount of various phases present and the damage in the material. Ultrasonic attenuation, which is the sum of the absorption and the scattering, is mainly dependent upon the damping capacity and scattering from the grain boundary in the material. However, to fully characterize the attenuation required knowledge of a large number of thermo-physical parameters that in practice are hard to quantity..

Relative measurements such as the change of attenuation and simple qualitative tests are easier to make than absolute measure. Relative attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. However, significant variations in microstructure characteristics and mechanical properties often produce only a relatively small change in wave velocity and attenuation.
Absolute measurements of attenuation are very difficult to obtain because the echo amplitude depends on factors in addition to amplitude. The most common method used to get quantitative results is to use an ultrasonic source and detector transducer separated by a known distance. By varying the separation distance, the attenuation can be measured from the changes in the amplitude. To get accurate results, the influence of coupling conditions must be carefully addressed. To overcome the problems related to conventional ultrasonic attenuation measurements, ultrasonic spectral parameters for frequency - dependent attenuation measurements, which are independent from coupling conditions are also used. For example, the ratio of the amplitudes of higher frequency peak to the lower frequency peak, has been used for microstructure characterization of some materials. 

Time measurement technique in ultrasonic inspection.

Fourier Transform - phase - slope determination of delta time between received RF burst (T2-R)-(T1-R),where T2 and T1 EMATs are driven in series to eliminate differential phase shift due to probe liftoff.

Slope of the phase is determined by linear regression of weighted data points within the signal bandwidth and weighted y-intercept. The accuracy obtained with this method can exceed one part in one hundred thousand. 

Precision velocity measurements in ultrasonic inspection.

Electromagnetic - acoustic transducer (EMAT) generate ultrasound in the material being investigated. When a wire or coil is placed near to the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region. If a static magnetic field is also present, these currents will experience Lorentz forces of the form F=J x B
Where F is a body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction. The most important application of EMATs has been in nondestructive evaluation (NDE)applications such as flaw detection or material property characterization. Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarizations that would be difficult to realize with fluid coupled piezoelectric probes. In the inference of material properties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.

Normal Beam inspection in ultrasonic inspection

NORMAL BEAM INSPECTION:- Pulse -echo ultrasonic measurement can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship.
     d=vt/2 or v=2d/t
Where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip time.

Precision ultrasonic thickness gages usually operate at frequencies between 500kHz and 100MHz, by means of piezoelectric transducer that generate bursts of sound waves when excited by electrical pulses. A wide variety of transducer with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non - attenuating, non - scattering materials. 

ANGLE BEAM IN ULTRASONIC INSPECTION.

Angle Beam 1:- Angle Beam Transducers and wedges are typically used to introduce a refrected shear wave into the test material. An angled sound path allows, the sound beam come in from the side,thereby improving detectability of flaws in and around welded areas.

Angle Beam 2:- Angle beam transducers and wedges are typically used to introduce a refrected shear wave into the test material. The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

C-SCAN DATA IN ULTRASONIC INSPECTION.

The C-scan presentations provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time -of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time -of-flight is displayed as a shade of Gray or a color for each of the positions where data was connected. The C-scan presentations provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.
High resolution scans can produce very detailed images. Both images were produced using a pulse-echo technique with the transducer scanned over the head side in an immersion scanning system. For the C-scan image on the left, the gate was setup to capture the amplitude of the sound reflecting from the front surface of the quarter. Light areas in the image indicate areas that reflected a greater amount of energy back to the transducer. In the C-scan image on the right, the gate was moved to record the intensity of the sound reflecting from the back surface of the coin. The details on the back surface are clearly visible but front surface features are also still visible since the sound energy is affected by these features as it travels through the front surface of the coin.

B-SCAN DATA IN ULTRASONIC INSPECTION.

B-SCAN PRESENTATION:-The B-scan presentation is a profile (cross - sectional) view of the test specimen. In the B - scan, the time - of - flight (travel time) of the sound energy is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis. From the B - scan, the depth of the reflector and it's approximate linear dimensions in the scan direction can be determined. The B - scan is typically produced by establishing a trigger gate on the A - scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the Blackwall of the specimen and by smaller reflectors with in the material. In the B-scan image above, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the Blackwall line BW is produced. When the transducer is over flaws B and C, lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B - scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface. 

Data presentation in ultrasonic inspection.

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A-scan,  B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning system can display data in all three presentation forms simultaneously.

A-scan presentation:-The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (Which may be related to the sound travel time within the material) is displayed along the horizontal axis. Most instruments with an A-scan display allow the signal to be displayed in its natural radio frequency form(RF),as a fully rectified RF signal, or as either the positive or negative half of the RF signal. In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep. In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero.As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A,the sound energy reflecting from surface A,will be seen on the trace. As the transducer is scanned to the right, a signal from the Blackwall BW will appear later in time, showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B,signal B will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.

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. 

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.