Java Applet of Radiography Principles

1) Java Simulation 
(Please remember to allow running of scripts/ActiveX controls for viewing the applet)

• INTENSITY-DISTANCE INVERSE SQUARE LAW

Applet> Intensity-Distance Law

The intensity-distance inverse square law is given by

where I₁ and I₂ are the intensities at 2 points and  d₁ and d₂ are their respective distances from the source. The law assumes a fixed source.

• SFD-EXPOSURE FORMULA

Applet> SFD-Exposure Formula

The sfd-exposure formula is given by

where E₁ and E₂ are the respective exposures required to produce the same quality of image when the film is at distances SFD₁ and SFD₂ from the source.  An exposure can be changed in 3 ways:


1) Varying the source intensity, I_s, keeping the exposure time, t, fixed.
2) Varying the exposure time, t, keeping the source intensity, I_s, fixed.
3) Varying both the source intensity, I_s, and the exposure time, t.
                             
In the Java simulation, a constant source intensity(I_s1 = I_s2) is assumed and since

The respective exposure times, t₁ and t₂ , required at the source-to-film distances, SFD₁ and SFD₂,  are given by the formula,

                                    


2) Theory
                                

• A Conventional X-Ray Unit

Figure 1  Conventional design of glass envelope X-ray tube

X-rays are a form of electromagnetic radiation, of the same physical nature as visible light, radiowaves, etc. However, they have a wavelength which allows them to penetrate most materials with partial absorption during transmission. Their wavelength varies from about 10 nm for low energy radiation to about 10^-4 nm for high energy x-rays which will penetrate up to 500 mm in steel.

X-rays are generated when a beam of high energy electrons is stopped suddenly by a metal target. The essentials of an X-ray tube are shown in Figure 1.

To produce x-rays, the filament is heated with a current until it is hot enough to emit electrons. The tungsten target or anode is charged positively with respect to the filament and therefore the negatively charged electrons are attracted to the anode. At the anode, the electrons are stopped and they then produce x-rays which are directed towards the object to be radiographed.

Three variables which can be adjusted using the instrument panel connected to the x-ray tube are energy (in kV), intensity (in mA) and the exposure time (in min) which the x-rays are left on for during an x-ray shot. These variables need to be adjusted accurately in order to produce a satisfactory radiograph.

The high voltage difference between the filament and the target is termed the kV used for the radiograph. Typical values in industrial radiography are 50 to 300 kV although higher and lower values are used. If the kV is higher, the x-rays produced have more energy and  therefore they can penetrate a thicker component. The general practice is to increase the kV used as the thickness of the component to be radiographed increases. The kV can be considered as the quality of the x-rays.

The mA is a measure of the electron flow striking the target. The focusing cup can control the mA and also focus the electrons onto the target. The greater the mA value, the greater the quantity of x-rays produced at the target. Therefore the mA is a measure of the amount of x-rays leaving the target. This is also called the intensity.

The Exposure (in mA.min), E, given to a film is equal to the product of the intensity, I, and the exposure time, t, i.e., E = I x t.

At the anode, only a small proportion (1-10%) of the energy of the electrons is converted to x-rays and most becomes heat energy. The tungsten target therefore needs to be air, water or oil cooled.

The effective width of the source of the x-rays is considerably smaller than the area of the target on which the electrons are incident. Effective source widths vary up to 5 mm in diameter.

•  Production of A Radiograph

Figure  2  Arrangement for film radiography

A radiograph is a photographic image produced by a beam of penetrating ionised radiation after passing through a specimen and radiography is the production of radiographs.

The usual arrangement for producing a radiograph is shown in Figure 2, using a small diameter source G, and a sheet of film as a detector.

The cavity in the specimen, as shown at B, causes a lower absorption along the path GBF. More radiation reaches the film at point F, compared with, say, point  N. Therefore an x-ray "image" of the cavity is produced.

To produce a radiograph, the x-rays are allowed to reach the film for an appropriate exposure time, which depends on the intensity of the x-rays, the thickness of the specimen, and the characteristics of the film. The film is then processed (developed, fixed, washed and dried) and the defects can be seen as blackened areas. The film is then placed on an illuminated screen so that the image can be examined and interpreted.  

•  Intensity-Distance Law 

Figure 3  Intensity varies inversely to the distance from source

As ionised radiation travels in straight lines outward from a source, i.e., it is not focused, the simple "inverse square law" applies since when the distance between the film and the source is increased, the radiation has to cover a larger area and so is reduced in intensity as shown in Figure 3.  Assuming a fixed source,  intensities I₁ and I₂ at two points which are at distances d₁ and d₂ from the source, are related as,

Therefore,

A Java simulation of the inverse square law can be found at this link> Intensity-Distance Law.   .  

•  SFD-Exposure Formula  

If a film is exposed at a distance "D" from the x-ray tube and a certain density produced, then to produce the same density on a similar film at a distance of "2D", a greater exposure is required. The exposure required to produce the same quality of image is related to the source-to-film distance(sfd) by, 

where E₁ and E₂ are the respective exposures required when the film is at distances SFD₁ and SFD₂ from the source.  

An exposure can be changed in 3 ways:
1) Varying the source intensity, I_s, keeping the exposure time, t, fixed.
2) Varying the exposure time, t, keeping the source intensity, I_s, fixed.
3) Varying both the source intensity, I_s, and the exposure time, t.


In the Java simulation, a constant source intensity(I_s1 = I_s2) is assumed and since         

The respective exposure times, t₁ and t₂ , required at the source-to-film distances, SFD₁ and SFD₂,  are given by the formula,

Therefore,

The Java simulation of the sfd-exposure formula can be found at this link> SFD-Exposure Formula.  

Ultrasonic Testing - Probe Systems (Theory)

The operator's wish to accurately know the actual discontinuity size is understandable, therefore it is expected that a nondestructive testing method, such as ultrasonic testing, gives this information.  However, due to the fact that on the screen of the Ultrasonic Flaw Detector only the reflected sound (echo) coming from the discontinuity can be interpreted, it is therefore very difficult to reliably assert the size of the discontinuity.  In fact, the echo height plays the decisive part when evaluating discontinuities during manual ultrasonic testing.

It is also important to know the signal characteristic of different types of defect so that upon detection of the defect, the type of defect can be identified.  The signal characteristics can be found in the section below "Defect Characterization".


There are generally 3 types of probe systems.  Regardless of which probe system is used in the detection of discontinuity, if the reflected portion of the sound wave is not received by the probe, then it is unlikely that the discontinuity will be detected.  The possibilities of detection only increase when the plane discontinuity is hit vertically by the sound beam.  The 3 types of probe systems are:

• Straight-Beam Probe
• Angle-Beam Probe
• Immersion Probe

Java simulations of the probe systems and the defect characterization can be found in their respective section below.  The type of defect in the Java simulations of the probe systems is the fine linear slag inclusion, the symbols in the applets are as follow:


IP  = Initial Pulse
BE = Backwall Echo
R   = Reflector(flaw) Echo
(In through-transmission technique, R simply symbolizes the signal detected by the receiver)
F   = Front Echo in Immersion Probe System
B   = Backwall Echo in Immersion Probe System

1) Straight-Beam Probe

Figure 1  A single crystal longitudinal wave probe

Probes whose beams are normal to the surface are called straight-beam probes.  Most standard straight-beam probes transmit and receive longitudinal waves.

The typical design of a single crystal longitudinal wave probe is shown in Figure 1. Such probe is called a 0⁰ longitudinal wave probe.  If a metal wedge is placed between the piezoelectric element and the specimen surface, of the same material as the specimen, longitudinal waves can be propagated into the specimen at an angle.

The piezoelectric element (the crystal, or ceramic plate), of a suitable thickness to produce the resonant frequency required, is usually circular in shape, and typical diameters are 6 to 30 mm (1/4 to 1 inch), with frequencies in the range 1-15 MHz. The crystal faces are metallized, either by coating them with electro conductive ink which gives a deposit of silver or copper after baking.

The piezoelectric crystal is backed with a damping backing as shown in Figure 1. This material must have a similar acoustic impedance to that of the crystal, so the back wave travels into it without reflection. It should be highly absorbent, and obviously well bonded to the piezoelectric element. Nowadays, the acoustic backing is one of the two kinds, (1) a scattering, diffusing backing, made of tungsten powder in epoxy resin or some form of sintered metal; (2) a quarter- wavelength layer.

There are a number of basic straight-beam probe configurations which are applicable to a range of testing problems.  3 configurations will be discussed as follow:

• Basic Longitudinal Wave Pulse Echo System

Figure 2  Basic longitudinal wave pulse echo system

Figure 2 shows the basic longitudinal wave pulse echo system using a normal (0⁰) longitudinal wave combined transmitter and receiver (T/R) probe.

A Java simulation of the probe configuration can be found at this link>Basic Pulse Echo System. 

• Through-Transmission Technique

Figure 3  Through-transmission technique

Figure 3 shows the through-transmission technique with the transmitter and receiver separately on opposite sides of the specimen.  If a flaw is detected, the signal R is lost or reduced as shown in cases B and C of Figure 3.

A Java simulation of the probe configuration can be found at this link>Through-Transmission Technique. 

• Double-Probe Longitudinal Wave System

Figure 4  Double-probe system

For a double-probe longitudinal wave system, normally there is no input signal shown on the display.

A Java simulation of the probe configuration can be found at this link> Double-Probe System. 

2) Angle-Beam Probe

Figure 5  A single crystal transverse wave probe

Probes whose beams enter at an angle are called angle-beam probes because they transmit and receive the sound waves at an angle to the surface of the test specimen.  Most standard angle-beam probes transmit and receive, due to technical reasons, transverse waves.

The transverse wave probe shown in Figure 5 is the most-widely-used for weld inspection. The piezoelectric element is cemented to the sloping face of a Perspex block, the angle of this face to the base being chosen so that when the Perspex flat face is placed on a metal specimen, the longitudinal wave in the Perspex is mode converted into a transverse wave in the specimen, at a chosen angle. The angle of the transverse wave beam, for any particular probe, will of course depend on the velocity of ultrasound in the specimen material. Thus a 70⁰ probe for use on steel is not a 70⁰ probe when used in aluminum.  Such probes are commonly sold in terms of the nominal angle of the transverse wave beam in steel.

Materials other than Perspex have been proposed for the probe wedge in a transverse wave probe. For example, for use on copper and cast iron specimens, Nylon wedges have been used.

The most important points about the transverse wave probe design are to make sure that the probe angle (nominal value in steel) of the transverse waves and the point of entry of the centerline of the beam into the specimen are known. These are two very important characteristics of any probe, which the ultrasonic equipment user needs to determine in the calibration procedure for each individual probe.

The usable range for the nominal probe angle is determined from the 2 critical angles as illustrated in Figure 6.  For more information on critical angle, pls refer to this link> Critical Angle.

Figure 6  Usable range for nominal probe angle

There are a number of basic angle-beam probe configurations which are applicable to a range of testing problems.  3 configurations will be discussed as follow:

• Basic Transverse Wave Pulse Echo System

Figure 7  Basic transverse wave pulse echo system

Figure 7 shows the basic transverse wave pulse echo system using the half-skip technique.

A Java simulation of the probe configuration can be found at this link> Half-Skip Technique.

• Full-Skip Technique

Figure 8  Full-skip technique

Figure 8 shows the full-skip technique where the transverse wave can be reflected off the lower surface.

A Java simulation of the probe configuration can be found at this link>Full-Skip Technique. 

• Tandem Probe

Figure 9  Tandem probe

Often in thick-walled test specimens, in which there are vertical discontinuities, a T/R probe cannot be used since the reflected sound waves from the discontinuity and the surface of the test specimen do not return to the T/R probe.  In this case, a second probe is used for receiving the reflected portions of the sound wave, thus enabling detection of the discontinuity.  In the Tandem technique, one probe is used as a transmitter and the other probe is used as the receiver.  Both the probes are mechanically-linked at a fixed distance apart.  Scanning is made for vertical discontinuities at different depths of the test specimen, depending on the probe spacing.

A Java simulation of the probe configuration can be found at this link> Tandem Probe.

3) Immersion Probe

1 configuration of the immersion probe will be discussed as follows:

• Water-Immersion Technique

Figure 10  Water-immersion technique

Figure 10 shows the water-immersion technique.  The path length in water is large, so the distance between the front and backwall echoes in the specimen may be rather small, unless "beam expansion" is used on this part of the display. "R" in Figure 10 represents the 3 flaw echoes resulting from the reverberations between the front of the specimen and the flaw.

A Java simulation of the probe configuration can be found at this link> Water-Immersion Technique.

4) Defect Characterization

The common defects in welds and their signal characteristics are listed in Table 1.  For the following echo patterns shown in Table 1, the sensitivity is adjustable to produce a full screen height echo from a 1.5mm horizontal hole at the same range as the defects discussed.

A Java simulation of the defect characterization can be found at this link> Characteristics Simulation.

Table 1 Flaw Characterization from Welded Defects

Defects	Pulsed Shape
Gas Pore Echo amplitude depends on pore size but usually between 1/5 and 3/5 screen height	Single range reflector with a narrow pulse display at time base
Group Porosity Echo amplitude usually less than 1/5 screen height	Multi-range reflector with a wide pulse display at time base
Isolated Slag Inclusion Echo amplitude generally about 2/3 screen height with reflection from more than one range	Forked type display with some pulse width at time base
Fine Linear Slag Inclusion Echo amplitude usually about 1/2 to 3/4 screen height	Usually narrow pulse width at time base
Cracks Echo amplitude tends to be high with numerous peaks	Due to the multi-faceted nature and range of cracks, the pulse will have multiple peaks and usually wide at time base

Ultrasonic Testing - Probe Systems (Java Simulation)

(Please remember to allow running of scripts/ActiveX controls for viewing the applet)


Symbols in applets:   
IP  = Initial Pulse
BE = Backwall Echo
R   = Reflector(flaw) Echo
(In through-transmission technique, R simply symbolizes the signal detected by the receiver)
F   = Front Echo in Immersion Probe System
B   = Backwall Echo in Immersion Probe System


                                                                
1) Straight-Beam Probe

• Basic longitudinal wave pulse echo system using the transmitter-receiver (T/R) probe

Applet> Basic Pulse Echo System

• Through-transmission technique with transmitter and receiver on opposite sides of the specimen

Applet> Through-Transmission Technique 

• Double-probe system using the transmitter- receiver (T/R) probe

Applet> Double-Probe System

2) Angle-Beam Probe

• Basic transverse wave pulse echo system using the half-skip technique with the transmitter-receiver (T/R) probe

Applet> Half-Skip Technique

• Signal characteristic of different types of defect using the half-skip technique

Applet> Defect Characterization

• Full-skip technique using the transmitter-receiver (T/R) probe where the transverse wave can be reflected off the lower surface

Applet> Full-Skip Technique

• Tandem probe in which a mechanically-linked transmitter and receiver probe system is used to detect vertical defects

Applet> Tandem Probe

3) Immersion Probe

• Basic immersion probe system using the water-immersion technique

Applet> Water-Immersion Technique

An Introduction to Ultrasonic Method for Non-Destructive Testing (NDT)

• Non-Destructive Testing (NDT)

Non-destructive tests (NDT) are inspection methods which are usually used to search for the presence of defects in components, without causing any effects on the properties of the components. The type of defects detectable are cracks, porosity, voids, inclusions, etc.

Modern NDT is used by manufacturers to: ensure product integrity and reliability; prevent failure, accidents and saving lives; make profit for users; ensure customer satisfaction; aid in better product design; control manufacturing process; lower manufacturing costs; maintain uniform quality level; ensure operational readiness.

 The table below shows the types of NDT methods used:

Commonly Used Methods	Other Methods
Ultrasonics	Visual Methods
Radiography	Acoustic Emission
Dye Penetrant	Thermography
Magnetic Particle Inspection	Holography
Eddy Current	Potential - drop

• Basic Principles of Ultrasonic Testing

Mechanical vibrations can be propagated in solids, liquids and gases. The actual particles of matter vibrate, and if the mechanical movements of the particles have a regular motion, the vibration can be assigned a frequency in cycles per second, measured in hertz (Hz), where 1 Hz =  1 cycle per second. If this frequency is within the approximate range 10 to 20,000 Hz, the sound is audible; above about 20 kHz, "the sound" waves are referred to as ultrasound or ultrasonics.

The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves.  The waves are not only reflected at the interfaces but also by internal flaws (material separations, inclusions,etc.). 

As an example of a practical application, if a disc of piezoelectric materials is attached to a block of steel (Figure 1a), either by cement or by a film of oil, and a high- voltage electrical pulse is applied to the piezoelectric disc, a pulse of ultrasonic energy is generated in the disc and is propagated into the steel. This pulse of waves travels through the metal with some spreading and some attenuation and will be reflected or scattered at any surface or internal discontinuity such as an internal flaw in the specimen. This reflected or scattered energy can be detected by a suitably-placed second piezoelectric disc on the metal surface and will generate a pulse of electrical energy in that disc. The time- interval between the transmitted and reflected pulse is a measure of the distance of the discontinuity from the surface, and the size of the return pulse can be a measure of the size of the flaw. This is the simple principle of the ultrasonic flaw detector and the ultrasonic thickness gauge. The piezoelectric discs are the "probes" or "transducers"; sometimes it is convenient to use one transducer as both transmitter and receiver. In a typical ultrasonic flaw detector the transmitted and received pulses are displayed in a scan on a timebase on an oscilloscope as shown in Figure 1b.

Figure 1  Basic principle of ultrasonic testing with a compressional probe (a) set-up (b) standard A-scan display

Java Applet for Transverse Wave Reflection

1) Java Simulation 

(Please remember to allow running of scripts/ActiveX controls for viewing the applet)

•     AT THE FREE END OF A STRETCHED STRING

Reflection in 3-D spatial-time domain

Applet>Wave Reflection in Spatial-Time Domain(3D Free End)


In this applet, the wave is reflected at 4s at one end of the string (position 100m) and then reflected again at 8s at the other end of the string (position 0m).


Reflection in 2-D spatial domain
Applet>Wave Reflection in Spatial Domain(Free End)


In this applet, the wave is again reflected at one end of the string (position 100m) and then reflected at the other end of the string (position 0m). The monitoring of the wave propagation at any position (position 0m to 100m) can be found at
Applet>Wave Reflection in Time Domain(Free End)

• AT THE FIXED END OF A STRETCHED STRING

Reflection in 2-D spatial domain
 Applet> Wave Reflection in Spatial Domain(Fixed End)


In this applet, the wave is reflected at one end of the wall (position 100m) and then reflected again at the other end of the wall (position 0m). Note that the wave is inverted upon each reflection.  The monitoring of the wave propagation at any position (position 0m to 100m) can be found at
Applet> Wave Reflection in Time Domain(Fixed End)

2) Theory

A transverse wave has particles moving perpendicularly to the direction of travel of the wave.  Each wave particle has the same amplitude and frequency.  Examples of transverse waves are waves on plucked strings, water waves and electromagnetic waves. 


When a transverse wave travels down a stretched string and reaches the end of the string, the entire wave or part of it will be reflected. The reflected wave may be inverted, depending whether the end of the string is free to move or the string is fixed at the end. 

•  Reflection At The Free End 

Figure 1  Reflection of a transverse wave at the free end of a stretched string

The reflection of a transverse wave at the free end of a stretched string is shown in Figure 1. The ends of the string are attached to light rings which are free to slide without friction along the rods. When the wave arrives at the free end, it exerts a force on the element of string there.  This element is accelerated and its motion carries it past the equilibrium points; it "overshoots" and exerts a reaction force on the string.  This generates a reflected wave as shown in Figure 1d and Figure 1e.  It can be seen that  the reflected wave is not inverted compared to the incident wave.  Hence, at the free end, a transverse wave is reflected without a phase change.

A Java simulation in the 3-D spatial-time domain of wave reflection at the free end can be found at this link> Wave Reflection in Spatial-Time Domain(3D Free End). In this applet, the wave is reflected at 4s at one end of the string (position 100m) and then reflected again at 8s at the other end of the string (position 0m).  

A more familiar 2-D spatial domain representation can be found at this link> Wave Reflection in Spatial Domain(Free End). In this applet, the wave is again reflected at one end of the string (position 100m) and then reflected at the other end of the string (position 0m). The monitoring of the wave propagation at any position (position 0m to 100m) can be found at this link> Wave Reflection in Time Domain(Free End). 

•  Reflection At The Fixed End 

  

Figure 2  Reflection of a transverse wave at the fixed end of a stretched string

The reflection of a transverse wave at the fixed end of a stretched string is shown in Figure 2. When the wave reaches the end of the string that is fixed at the wall, the string exerts an upward force on the rigid wall. By Newton's third law, the wall exerts an equal but opposite reaction force on the string. This reaction force causes the wave to invert upon reflection as shown in Figure 2d and Figure 2e.  Hence, at the fixed end, a transverse wave is reflected with a phase change of 180⁰.

A Java simulation in the 2-D spatial domain of wave reflection at the fixed end can be found at this link> Wave Reflection in Spatial Domain(Fixed End). In this applet, the wave is reflected at one end of the wall (position 100m) and then reflected again at the other end of the wall (position 0m).  Note that the wave is inverted upon each reflection. The monitoring of the wave propagation at any position (position 0m to 100m) can be found at this link> Wave Reflection in Time Domain(Fixed End).