K

ULTRASONIC FLAW DETECTORS ( UFDs ) It is a major UTE which detects ( and also locates and evaluates ) any discontinuities / irregularities like cracks , laminations, slag inclusions, porosity, cavities etc in the material which can be metal or non-metal. It is a major Material Testing tool. Any major discontinuity in the material existing from the production stage or developed during the service life of the part can affect the serviceability of the part or equipment. Depending on the use and application of the part some discontinuity could be harmless or disastrous. UFD detects the discontinuity and also helps the operator to evaluate the same whether it can be considered as a defect.

UFD

an

be

either

portable

or

stationary.

Portable is used on site, on shop floor or even in the laboratory for carrying out ultrasonic testing, involving manual / contact scanning of the test piece. It could be a production stage testing, in-service examination or even developmental or investigative inspection of test objects. Stationary type normally involves automatic scanning using immersion coupling method. It is used where either the test-piece is intricate in shape or where on-line/off-line inspection is required for very high through-put like that used for pipes, tubes, bars / rods, plates, wires, automobile parts, rails, wheels and axles etc. Like UTGs portable UFDs consist of the following three components : • • •

Main Unit : It houses the display unit, transmitter( Tx ), receiver( Rx ), power supply and other processing circuit boards. Search Unit : Probe or transducer as also they are called.\ Connecting cable : To connect the probe with the main-unit.

Portable UFDs available to-day in the market are of two types.

One is ANALOGUE type. It uses conventional electronics and has CRT ( Cathode Ray Tube) as a display medium. In this type in some of the models the controls may be microprocessor based but the display output is essentially in the analogue domain. These UFDs are comparatively inexpensive and though can fulfill most of the testing requirements they do not have memory facility for storage of the test -results and do not have computer connectivity. DIGITAL type, in common man's language ,is like a computer. The outstanding features can be enlisted as under: • • •

The display medium is LCD or its variant. It is always light in weight and compact. Many convenience features which are not possible in ANALOGUE type like freezing of the display at any time, automatic DAC plotting and many more.

•

On-board Memory for storage and recall of calibration set-ups and test-results. Direct print-out of the wave forms and other relevant data is also possible

In time to come DIGITAL type will replace the ANALOGUE just like the transistorized gadgets replaced the valve type ones. But still people have preference for the ANALOGUE foremost due to low price and also due to its simple operation. ( resistance to change as usual will also apply to this as well ).

APPLICATIONS OF ULTRASONIC FLAW DETECTOR 1. Detection and evaluation of discontinuities : UFD can not only detect the discontinuities but also evaluate them. Operator can precisely determine the place and depth of the discontinuity. Its plane of orientation can also be found out. An experienced operator can tell the type of discontinuity ( i.e. crack, animation, slag inclusion, porosity etc. from the shape of the signal on the display ). Sizing of the discontinuity is possible using different methods like DGS scales and diagrams, using reference blocks for comparison etc. Eventual diagnosis and decision about acceptance or rejection is taken depending on the end use of the test object ,standards being followed and the agreement between the concerned people. Following are some of the practical applications: • Examination of all types of welding ( butt weld, T weld, SAW, spot weld etc) in all types of structures.

• • • • •

•

This is one of the most extensive applications of UFD. During construction stage and then in-service weld examinations, portable machines are used .For on-line and off-line inspection during production of pipes (longitudinal and spiral) semiautomatic and automatic ultrasonic test systems are in use for many years. Plate testing for laminations is also another major application. Here also it is done manually or using a trolley or on automatic test systems on production lines for higher output. In Railways in-service inspection of rails and axles is also major and critical application. Fracture of rails and breakage of axles due to stress induced cracks is common and must be detected in good time. Application of UFD in foundries is immense. Detection of casting related defects like porosity, shrinkage cavities ,blow holes is done using UFDs, which gives instant results compared to radiographic inspection. Similarly forged components like those used in power generation equipment , artillery in military ,earth moving equipment etc. are extensively examined using UFD. Rolled products like rods, bars etc.is other area where the defects like piping can be examined with manual scanning or using automatic systems on the production line. Besides the above, UFD finds major applications for maintenance inspection in programmer, aviation and shipping etc

all types of power plants, space

2. Examination of bonds : UFD provides a quick and reliable method for examination of bonds e.g. between white metal and bronze used in critical bearings. It can provide go and no-go test and also if required severity of dis-bond can also be assessed. 3. Thickness measurements : Though UTG is a dedicated device for this purpose, UFD can also be used for thickness measurement with only one side accessible. Though direct reading is not available and requires interpretation skill on the part of the operator sometime the result is considered more reliable with UFD than with UTG due to the available wave form verification facility. 4.

Velocity

measurement

:

Like

UTG,

velocity

measurement

can

also

be

done

using

UFD.

5. Sound attenuation measurement : UFD is a simple device to measure sound attenuation which is effectively used for material characterization.

LIMITATIONS OF ULTRASONIC FLAW DETECTOR

It is also important for the buyer to know and understand the limitations of UFD, which are enlisted below: • Discontinuities very close to the surface go undetected due to "dead-zone" of the equipment. Due to the same reason very thin test objects can not be examined using at least the contact method of coupling. Sometime it is also difficult to detect the discontinuity very close to the back surface due to "resolution" problem. • Similarly small and intricately shaped test-objects can not be tested due to inadequate contact area resulting in insufficient sound energy entering the material. • Conversely if the test-object is too thick then also the discontinuity at the far end may not be detectable owing to ( a ) increase in the sound attenuation with distance, ( b ) decrease in the sensitivity with the distance, ( c ) decrease in Signal/Noise ratio with distance. • There is a limitation to the minimum size of the discontinuity which can be detected ( which is called sensitivity of the equipment ). It is a function of the frequency of the probe and its theoretical value is half of the wave length of the probe. • If the discontinuity is oriented unfavorably to the probe positioning then the chances are that it may not be detected. This is because the sound beam must strike the discontinuity normally for the maximum response. • As we will see later, selection of the probe to suit the requirement is very important. So if the right probe is not used then also there could be possibility of discontinuity remaining undetected. • In hazardous areas like some places in oil refineries most of the UFDs can not be used as they are not "intrinsically safe". •

Hard copy documentation of test results is not available for the full test as in radiographic inspection.

UNDERSTANDING THE SPECIFICATIONS OF UFD ( 1 ) Test Range:10 mm to 5 meters ( in steel ) : Meaning :The x-axis of the display can be calibrated to read minimum 0 to 10mm upto maximum 0 to 5 meters. These values are typically for steel and may vary from material to material. It should be noted that these are the values of range to which the UFD can be "calibrated" and does not relate to the "testability" of the material in that range. ( 2 ) Frequency Range:0.5 MHz to 15 MHz. : Meaning : Probes of frequency 0.5 MHz on lower side to 15 MHz on upper side may be used with good performance. Probes of frequency outside this range may suffer in performance. ( 3 ) Gain:100dB : Meaning : The calibrated gain of the machine is 100 dB. Normally the overall gain of the machine is 140dB. So in that case the builtin gain is 40dB.In case where the machine has calibrated gain of 80dB then the built-in gain in that machine becomes 60dB. ( 4 ) Non-Linearity of deflections : ( a ) Horizontal axis: ± 1% ( b ) Vertical axis: ± 5% Meaning : The deflection is linear to the extent of 99% and 95% respectively. ( 5 ) Mode of operation : ( Single and double probe ) Meaning : It is switch able for use with single or double probe ( it could be dual type probe or separate two probes for "through transmission" method ). This is a standard feature available in all the machines. ( 6 ) Reject control:0% to 80% of FSH. : Meaning : With reject control ( or suppression control as it is called) unwanted ( or NRI-Non Relevant Indications) indications can be suppressed for better visibility of defect signals and increased signal/noise ratio Suppression of signals from 0% height to 80% height (full scale height ) is possible. ( 7 ) Flaw Monitor Gate : ( Single or dual gates available with all the three ) Meaning :There can be single or double gates available on the screen, which are user adjustable. Any signal appearing within the gate gives out Audio and Visual alarm for the attention of the operator. This helps in faster scanning of the test-object without the risk of missing any flaw signal .

HISTORY AND LATEST TRENDS IN UFDs After world-war II within a couple of years first UFDs were introduced in the the market almost simultaneously by American and European manufacturers. As it is expected these devices were valve type , heavy and bulky nonetheless very efficient and reliable for that time. Advent in electronics over a period of almost half a century brought remarkable transformations in UFDs. The sequence was from valve type to transistorized to Integrated circuits based to the latest digital flaw detectors. The size and weight reducing to less than one tenth and capabilities increasing to ,may be, hundred times. As discussed earlier in the category of portable UFDs there are now digital flaw detectors taking the market by storm. They are as one manufacturer claims "size of a book and power of a PC". These are the UFDs of to-day. Another trend is the add-on cards with software to convert your PC or even a laptop into a powerful UFD. Varieties of standard and customized software's are available which impart great power and versatility to an UFD. On the automatic systems side most of them to-day are PC based. Speed, reliability and repeatability of tests performed are constantly improving to match the stringent standards.

ACCESSORIES OF ULTRASONIC FLAW DETECTORS There are more number of accessories used with UFD than with UTG and for the right selection of these ,the buyer need to have some basic information about them. The information given below will be useful to the buyer. (1)Probes:It is very important part of UTE and overall performance of the equipment largely depends on it. The function of the probe is to generate and receive the ULTRASONIC waves of specied frequency and transmit them in the test object at a specified angle. Generation and reception of ULTRASONIC waves by the probe is done using piezp-elements which exhibit the properties of PIEZOELECTRICITY as explained earlier. Any probe has following six parameters. (i) Number of piezo-elements used, (ii) Frequency of sound waves, (iii) Size of piezo-elements, (iv) Angle of refraction ,(v) Type of connector used, (vi) Type of sound waves. On the basis of the above,the probes are divided into three major categories, viz, (I) Normal Probes, (ii) Angle Beam Probes, (iii) TR or dual Probes. Following are the standard configurations of these probes. Sr PARA - METERS No

NORMAL PROBES

ANGLE PROBE

TR (DUAL ) PROBES

1

Number of piezo - elements

One

One to Two

Two

2

Frequency ( MHz)

0.5,1.0, 2.0, 4.0, 6.0, 10.0

2.0 and 4.0

.0.5,1.0, 2.0, 4.0, 6.0

3

Sizeof probes ( mm )

5.0, 10.0, 25.0 ( dia. )

8 X 9 and 20 X 22

5.0, 10.0 and 20.0 ( dia.)

4

Angle of Refractions (Degrees) 0

37, 45, 60, 70, 80, & 90 0 ( Roof angle of 6 degrees)

5

Type of Connector

LEMO "00" and BNC

LEMO "00" and BNC

LEMO "00"

6

Type of Waves

Longitudinal

Shear

Longitudinal

Uses of different probes:

( 1 ) Normal Probes : They emit the ULTRASONIC waves normally into the material. They are used for all general purpose applications like inspection of rods, shafts, plates, castings etc where the discontinuities are expected to be oriented parallel to the surface and which are not very close to the surface.

( 2 ) Angle Beam Probes : Here there is a delay line called wedge, made of perspex material .Hence the sound waves undergo refraction upon entering the material and travel at an angle. Because of this it is useful for detecting discontinuity which is oriented at an angle to the surface. Examination of welding is a major application of angle beam probe. Stress cracks developed in the journal portion of the railways axle can only be detected by angle probe due to its inclined orientation.

( 3 ) TR ( Dual ) Probe : This probe has separate elements for transmission and reception. Here the sound waves travels at a slight angle ( due to roof angle of the elements ) which gives focusing effect to the sound beam. Also due to separate transmitter and receiver the "dead zone" is very small making TR probes ideal for checking thin walled parts and for detecting discontinuities close to the surface.

Frequency of probe v/s Applications Following are the two important considerations while selecting the frequency of the probe for any application. • The sensitivity (ability of the equipment to detect the smallest of the discontinuities) keeps improving as frequency goes higher.

•

Sound attenuation (sound absorption) also increases as the frequency goes higher.

Following guide-lines may be used for selection of the probes: FREQUENCY OF PROBE

APPLICATION

0.5 MHz

Very coarse grained materials like C.I., S. G. Iron, austenitic Stainless Steel, soft plastics, rubber, composites etc.

1.0 MHz

For coarse grained materials like steel castings and those with very high thickness.

2.0 MHz

For large sized components with fair sensitivity requirement like testing of forgings.

4.0 MHz

For optimum sensitivity, resolution and penetration. For inspection of fine grained material and those involving low thickness.

6.0 MHz

For very high sensitivity or checking thin walled components used in critical space and nuclear applications.

10.0 MHz

For obtaining exceptionally high sensitivity and resolution. For inspection of materials like titanium, managing steel etc.

RECORDER ATTACHMENT It is a device to be attached with UFD for recording CRT wave-form ( with or without the associated calibration data ) on the built-in printer. The wave forms and other data can also be stored on the on-board memory for later-on printing -out or down loading to a computer. Documentation of the test-results has become very essential part of any inspection assignment. Proper documentation of the test results makes the whole test reproducible which is very much required for many applications. It must be noted by the buyer that such Recorder Attachments are not "Real Time / On-line" recorders but only an event recorder, where the operator chooses to record any CRT trace for archival purpose. It does not provide continuous recording.

Probe Cables Probe cable connects the probe with the main unit. The cable used is shielded co-axial type to prevent EMI ( electro-magnetic interference ). The normal length is 2 meters but can be increased to as long as 100 meters for special application. ( the energy loss due to extra length must be compensated ). On the two ends of the cable special RF co-axial connectors ( male type ) are fitted. Normally the type of connectors used are : ( i ) LEMO size '1', ( ii ) LEMO size "00" ( also called mini LEMO ), ( iii ) BNC, ( iv ) Microdot. Probe cables with various combinations of connectors are available. Buyer must check the type of connector fitted by the manufacturer on the main unit. The other end goes to the probe and the connector on the probes are different hence the buyer must ascertain that the suitable cables have been selected for each probe to be purchased. ( without a suitable cable a probe can not be put to use). It also must be noted by the buyer that TR ( or double) probe requires two cables of the same type or a dual type of cable specially meant for TR probe. Your attention is also brought to the fact that M/S LEMOSA S.A, Switzerland are the original and the oldest manufacturer of these special LEMO connectors. Some Indian manufacturers of UTE use sub-standard locally made LEMO look-alike connectors ,which last only for a few weeks and due to impedance mis-match the test-results are also not accurate and reliable. So it is advisable for the buyers to insist on ORIGINAL SWISS made LEMO connectors on the main units, probes and probe cables.

Reference and Calibration Blocks : Ultrasonic Testing is a comparative method and these blocks are always required for preparing any test set-up. Following are some of the important blocks. • One of the most important reference and calibration blocks is IIW ( International Institute of Welding ) block. It provides various thickness required for calibration of the Test Range of UFD. Also it has provisions to allow the user to check various critical parameters of UFD like dead zone, resolution power, penetration power, angle and exit point of angle probe etc. IT IS A MUST BUY WITH UFD. • Another important block is V2.It is a handy block useful for calibration of test range using angle probes. A side-drilled hole ( SDH )of 1.5 mm or 3.0 mm. dia on this block is also used to perform sensitivity setting of the machine. • VW is a step block with 10 steps from 1 mm. to 10 mm. thickness. It is used for test range calibration with TR probe. •

There are other blocks as per ASTM standards made of aluminum, steel or other material as per buyer's choice. They have FBH ( flat bottom holes ) of various options. They are available in three main types viz, ( i ) Area Amplitude blocks, ( ii ) Distance Amplitude blocks and ( iii ) Area Distance Amplitude blocks. These blocks are used for plotting DAC curves and beam profiles of the probes. Other blocks are available as per buyer's design e.g. for plotting DAC curves for using with angle probes required for weld inspection.