NDI - Non Destructive Inspection Techniques

Question 1

What are NDI techniques?

Answer 1

NDI techniques are procedures that permit to check the status of a structure without destroying it. They permit to reduce the cost of maintenance and increase reliability of the product.

Question 2

What are NDI techniques?

Answer 2

1) Non Destructive Testing (NDT) are related to the procedures we have to use in order to find a defect,
2) Non Destructive Inspection (NDI) are related to the procedures that permit, once the defect is found, to quantify the damage.
3) Non Destructive Evaluation (NDE) is related to the procedure we have to use in order to evaluate the importance of the damage.

Question 3

What are the classifications for NDT techniques?

Answer 3

1)
- Passive techniques
Techniques based on the observation of the component
- Active techniques
Techniques based on the detection of the change of one or more properties of the energy introduced into the component

2)
- Surface and sub-surface techniques
Techniques which are able to detect only surface or sub-surface defects
- Volumetric techniques
Techniques which are able to detect defects also inside the component

Question 4

What are the techniques for Passive and Active techniques?

Answer 4

1) Passive techniques
- Optical methods
- Dye penetrants (use of liquids)
- Acoustic methods
- Passive thermography (observe natural radiations)

2) Active techniques
- Magnetic and electromagnetic methods such as magnetic particles and eddy currents
- Radiography
- Ultrasounds
- Active thermography

Question 5

What are the techniques for Surface and sub-surface and volumetric techniques?

Answer 5

1) Surface and sub-surface techniques
- Optical methods
- Dye penetrants
- Magnetic and electromagnetic method such as magnetic particles and eddy currents

2) Volumetric techniques
- Acoustic methods
- Radiography
- Ultrasounds
- Thermography (passive and active thermography)

Question 6

Can all methods be used on both metallic and composite materials?

Answer 6

MAgnetic particles and eddy current CAN’T be used on composite materials.

Question 7

Describe the principles of metrology and inspection. (definition of metrology, measurement, measure, accuracy and precision)

Answer 7

Metrology: it is the discipline that deals with the theoretical rules and practical techniques with lead to the measurement of physical quantities.

Measurement: It is the procedure which leads to the determination of physical quantities

Measure: It is the result of the measurement procedure

Accuracy: It defines the deviation between average measure and theoretical value. It is influenced by bias (systematic errors)

Precision: It defines the degree of repeatability of the measurements. It is influenced by random errors.

For example, The measurement has high accuracy and low precision if the mean is at the true value, but has a big standard deviation. It has low accuracy and high precision if it has a small standard deviation but the mean is not at the true value

Question 8

What is the Dye-Penentrants technique based on?

Answer 8

The technique is based on the principle of capillarity (penetration inside the cracks) of liquids having low surface tension and low viscosity.

Question 9

What is the characteristics of the liquids used in dye penetrants?

Answer 9

• Low surface tension (size of particles are very small)
• Low viscosity, even at room temperature
• Easily removable
• Creating high contrast with revealing powder
• Allowing spraying, brushing, and dipping applicationW

Question 10

What is the procedure of dye penetrants?

Answer 10

1) Degreasing (surface cleaning), so that the liquid can act within the crack we want to find
2) Application of dye penetrant.
3) Washing and drying of the component in order to remove the dye penetrant from the surface.
4) Application of revealing powder, which creates a high contrast with the dye penetrant color and the cracks are revealed.

Question 11

What are the different types of dye penetrants used?

Answer 11

1) Red dye penetrant
2) Fluorescent dye penetrant
3) Non-emulsifiable fluorescent dye penetrant

The use of UV light is needed for detecting the defects of the last two types. Also, the last two allow to reveal the presence of the crack without removing the dye penetrant from the surface, which results to reduced inspection times.

Question 12

What are the fluoescent dye penetrants characterized by?

Answer 12

1) Dipping in extra-fine powder which does not adhere to the component surface, but only to the dye penetrant in correspondance of the defects. The powder can be applied without preliminary removal of the fluorescent dye penetrant, which leads to reduced inspection times.

2) Visualization by means of UV light in dark room.

3) Suitability to smooth surfaces; ability to detect defects of small dimensions

Question 13

What are the non-emulsifiable dye penetrants characterized by?

Answer 13

1) The liquid cannot be directly removed by means of water, but must be preliminary treated with an emulsifying agent. This way, it becomes more easility removable than other dye penetrants and it allows to detect defects of very small dimensions
2) It is suited to inspect components already inspected by means of other types of dye penetrants

Question 14

What are the advantages and disadvantages of the dye penetrant technique?

Answer 14

Advantages:
- The technique is suited to detect surface defects
- The result is objective (unbiased) and does not require skilled/experienced operators
- The technique is suited to most types of materials
- The technique is very in-expensive
- The technique is easy to be used
- The technique is easily portable (suited to field inspections)

Disadvantages:
- The technique is unable to measure the depth of the defect
- The tehnique cannot be used in case of very porous materials or very rough-surface materials
- The technique cannot detect internal or sub-surface defects

Question 15

What is the method of magnetic particles based on and what is the procedure?

Answer 15

The technique can only be adopted in case of magnetized ferromagnetic materials. It is based on the appearance of necking of flow lines of the magnetic field inside the component and the release of flow lines in the air in correspondance of voids, flaws and cracks.

The procecure is:
1) Magnetization of the component to be investigates
2) Defects detection
3) Demagnetization of the component

Changes in flow lines distribution can be visualized by appling magnetic particles that will be more attracted to those areas

Question 16

What types of magnetic fields can be used for the method of magnetic particles?

Answer 16

1) Longitudinal magnetic field
Obtained by applying an electric current through a solenoid
2) Circular or transverse magnetic field
Obtained by applying an electric current that can pass within the component

Cracks parallel to flow lines CANNOT be detected, because it doesn’t produce a change in the flow lines! Usually these two magnetic fields are enough.

Question 17

How can the component be magnetized>

Answer 17

The magnetization of the component happens by means of the magnetic field generated by electric current. THe magnetoscope induces the magnetic field due to the electric current.

1) Direct current
The flow lines penetrate in depth

2) Alternate current
The flow lines remain confined on the surface, but a better distribution of the magnetic particles is obtained, due to the frequency of the current.

3) Pulsating current
Has the advantages of direct + alternating current

Question 18

How is the defect detected with the method of magnetic particles??

Answer 18

The defect is detected through the deposition on the surface of the component of MAGNETIC PARTICLES.

Magnetic particles possess a high magnetic pereability and low residual magnetization. This means that they are easily attracted also by small magnetic fields and they immediately deatch as soon as the magnetic field is removed.

The particles are attracted towards the defective regions (due to the increased intensity of magnetic field and to the presence of diffusion effects).

Sometime, pigments of fluorescent agents are used to improve detectability.

A high granulometry increases the contrast but reduced the ability to detect defects of small dimensions

An accurate preliminary degeasing and cleaning of the component is mandatory.

Question 19

What is the procedure for the demagnetization of the component?

Answer 19

The material always retains a certain degree of residual magnetization owing to its hysteretical behaviour.

Procedure:
1) Thermal cycle at T>Tcurie
Tcurie is the temperature beyong which, in ferromagnetic materials, permanent polarization vanishes

2) Progessive reduction of hysteresis cycle
Application of an alternating magnetic field characterized by a magnetization force decreasing up to finally vanishing

If the component is already going to be subjected to thermal treatments, demagnetization is not needed.

Question 20

What are the advantages and disadvantages of the method of magnetic particles?

Answer 20

Advantages
- The technique is particularly suited to detect surface and sub-surface defects
- The result is objective (unbiased) and does not require skilled/experienced operators
- The technique is quite in-expensive
- The technique is easy to be used
- The technique can be easility automated (eg it can be used as on-line control technique during production)

Disadvantages
- The technique provides unsatisfactory results in case of internal defects
- The technique is unable to provide the depth of the defects
- The technique can be adopted only in case of ferromagnetic materials
- The surface must be accurately cleaned
- The component must be demagetized once the procedure is completed

Question 21

How does the Radiography technique work?

Answer 21

A source of highly penetrant X- or γ- rays are needed. The beam is abosrbed by the component under examination to a different degree, depending on its density and thickness.

The radiation beyond the component reaches the film in a different quantity.

Once developed, the film darkens more insensely where it has been reached by a greater amount of radiations.

The presence of defects (porosities, delaminations) induces the appearing of dark spots, which point out the regions of low radiation absorbance.

Question 22

What is a classical source of X-rays?

Answer 22

It the the Coolidge tube.

It is based on a glass tube with vacuum inside. The production of X-rays is obtained by pushing electrons (cathode) against a metallic plate (anode). The metallic plate emits electromagnetic waves, which consist in the converted electrons kinetic energy. The electric field is obtained by applying an electric tension between anode and cathode.

Question 23

Describe the tube of Coolidge?

Answer 23

The cathode is made of an incandescent filament. (Emitted electroned increase with the temperature). 99% of the energy is heat-dissipated.

The anode consists in a copper cylinder which embeds a tungsten plate.

The cathode focuses the electron bean in order to get a point-like source and to obtain high definition and sharpness.

Question 24

What is the intensity and the quantity of the radiation? WHat is the dose?

Answer 24

• The unit of measure of radiation is roentgen
• The intensity of radiation is roentgen/second (Is = Io e^-μs, where Is is the intensity of the emerging beam, Io is the intensity of the colliding beam, μ is the coefficient of absorbance and s is the thickness. An emi-valent layer is the thickness s of the type of material which halves the intensity of the colliding radiation (Is,Io = 0.5)).

When doing an inspection using radiography it is important to define the dose of the radiation (amount of X-rays), defined ast he intensity times the time. At a parity of dose, it must be selected the best combination among tension, current intensity, time of exposition and distance of the object as a function of the material to be investigated.

Question 25

What are the radiographic films and how are they selected?

Answer 25

The radiographic films are:
- Similar to photographic films
- They consist in several layers (gelatine layers embedding the emulsion (silver halides) and transparent polyester thin layer).

The radiographic film is selected on the amount of the dose expected to arrive on the other side of the component. When exposed to radiation, each grain of silver halides is independently hit by electromagnetic waves. When the X-rays arrive to the silver halides, a chemical reaction inside the grains emulsion occurs, which scissions the molecular joints, with the deposit of silver at the ground of the film. Deposited grains darken, while non-reacted halides are removed.

Question 26

What is the densitometric curve?

Answer 26

The densitometric curve shows the characteristic of a radiographic film. At the beginning, before exposing the radiographic film the X-rays have a certain level of darkening valed Veil.

Veil: Light opacity in correspondance of the non-exposed regions

The sensitivity of the film is the quantity of exposure needed for darkening. (in terms of time)

The slope of the curve is the photographic contrast, the film attitude to enhance darkening variation due to exposure variation.

The exposure latitude is the useful field of the film (where the film works).

Ultimately, the limit density or saturation darkening is the maximum density which can be obtained when the film is competely exposed and completely developed.

Question 27

What does the radiographic quality depend on?

Answer 27

It depends on:

1) Contrast
- The absorbance contrast, which is the ratio between intensity of emerging beam from 2 contiguous points and depends on the nature of the material. Can be tuned by setting low KV in order to obtain low penetrant radiation
- Photographic contrast
Depends on the characteristics of the film

2) Definition: It is the inverse of shading
Total shading
- Geometric shading: is due to a non point-like source and increases when film/object distance increases
- Film shading
- Kinetic shading

Question 28

What are the advantages and disadvantages of Radiography?

Answer 28

Advantages
- High penetration capability; it is able to detect voids or presence of foreign materials in depth
- Radiographic techniques are suited for the majority of materials
- In case of composite materials, opaque enhanced dye penetrants are used in order to improve detection capability
- It is possible to detect layers orientation by embedding special boron fibred coated with a fluorescent liquid, which distrincly stood out against radiography background
- The use of dyes which penetrates among the fibers improves the contrast of delaminated regions without influencng their mechanical performance

• Disadvantages
• It is unable to detect defects throgh the thickness position
• Personnel safety concerns arise -> heavy shielding systems are needed

Question 29

What is stereo-radiography?

Answer 29

Two or more radiographies are taken at different inclinations and then overlapped.

This creates the parallax phenomenon, where the subject seems to displace against the background if the point of observation is changed

The depth of the defect is obtained from the measurement of the parallax through simple trigonometric considerations

Question 30

What is Radioscopy? What are the advantages and the disadvantages?

Answer 30

Radioscopy differes from radiography in the type of image detector used. Instead of the X-ray film, a fluorescent screen based on zinc sulphide and cadmium is adopted. Under the action of X-rays, these fluorescent screen illuminates with a yellow-green light that is clearly visible to the human eye

The advantages are:
- No need of developing
- Dynamic acquisitions

The disadvantages are:
- Lower power inspection capabilities. Since image is based on the intensity of the radiation and non on the dose.

Question 31

What are Ultrasounds based on?

Answer 31

The ultrasound technique is based on the propagation of high-frequency mechanical vibrations. They propagate well through solid materials and are reflected in presence of discontuinities.

The analysis of the signal reflected or transmitted through the material allows to obtain information about thickness, physical nature and defectology of the material itself.

The generated waves can propagate through the molecular oscillation of the medium; Different kind of waves can be exploited for defects detection.

Question 32

What are the type of ultrasound waves?

Answer 32

1) Longitudinal waves: waves generated by molecular longitudinal oscillation due to alternate compression and extension actions

2) Transverse waves: waves generated by molecular transversal oscillations due to shear actions

Question 33

What is the acoustic impedance Z?

Answer 33

It characterizes the resistance of the medium to ultrasound waves propagation. It is proportional to the density of the material and the velocity of propagation of ultrasounds within the medium.

Question 34

How to discontuinities affect the acoustic impedance?

Answer 34

A discontuinity within the material causes a change in the acoustic impedance. This leads to a change of the propagation of the US inside the material.

Specifically, in the correspondence of defects, we have a part that will be reflected and a part of the signal that will be transmitted. It is possible to define a coefficient of reflection r and transmission t. Ultrasound inspection technique are based on the reflected or transmitted signal.
(eg: if we consider a component made of Al with a signal propagating inside it, in presence of an internal defect, like air, we will have the total reflection of the signal (r=1))

Question 35

What is the attenuation, the absorption and the diffusion in ultrasounds?

Answer 35

Attenuation = absorption + diffusion of ultrasound beams.

The intensity of US beam decreases along its path owing to two effects:
- Absorption (heat dissipation) due to the medium impedance that creates a damping of the US beam (damping of the molecular oscillations)
- Diffusion (scattering): due to the material heterogeneity that caues the dispersion of the US beam in all the directions

In case of composite materials, the diffusion phenomenon is even more remarkable, since the intrinsic material heterogeneity is increased by the nature of the diffects.

Question 36

Why does high diffusion allow detecting defects?

Answer 36

High diffusion due to the presence of defects allows very good results of transmission US controls in detecting defects such as:
- Diffuse voids and porosities
- Gas entrapments
- Micro-cracks
- Delamination
- Foreing bodies inclusions

Question 37

How are the US waves generated?

Answer 37

The piezo-electric effext is exploited, typical of some kinds of crystals (quartz) or polarized ceramics.

Several types of probes exist:
- Contact or immersion probes
- Straight or inclined probes (longitudinal waves)
- Inclined probes (fixed angle or variable angle)
- Flat or focused probes

Question 38

What are the techniques for US analysis?

Answer 38

• Reflection technique or pulse-echo technique
• Transmission techniques

Question 39

What are the reflection techniques or pulse-echo method?

Answer 39

It is possible to use a single probe, which both emits and receives, or a couple of probes which are locates such a way that one probe can receive the possibly reflected US beam generated by the other beam.
The presence of a defect induces a reduction of the intensity of ultrasonic energy collected by the receiving probe. The front echno is produced by the front surface and the back echo is produces by the back surface.

Question 40

What are the advantages of reflection techniques or pulse-echo method?

Answer 40

• The position of the echo allows to determine defect through-the-thickness location
• The amplitude and shape of the echo define defect type and dimensions

Question 41

What are the main parameters of reflection techniques?

Answer 41

Using a single probe it is necessary to emit ultrasonic pulsed and then wait for receiving the echo back.

The main parameters are:
- Pace of repetition: number of pulses per time nit
(it depends on the time interval between pulse emission and back echo reception; it is important to avoid multiple echo disturbances coming from the previous echo)
- Dead zone: it is the region near the surface which cannot be detected (the defects echoes cannot be detected since the probes is still transmitting)

Question 42

What are the transmission techniques?

Answer 42

In the transmission techniques, one probe transmits and the other one receives. In the presence of a defect it induces a reduction of the intensity of ultrasonic energy collected by the receiving probe.

Question 43

What are the limitations of the transmission method?

Answer 43

• It is unable to detect through-the-thickness position of defects
• It needs high and constant ultrasonic coupling between the probes. The method can be used only as an immersion technique, where we put the material under inspection inside water, since water creates a good coupling.

Question 44

What is ultrasonic coupling?

Answer 44

Ultrasonic coupling is the physical coupling between the proble and the material which avoids the propagation through the air (high attenuation medium r=1 through air)
- Contact probes which make use of coupling liquids (gel or vaseline)
- Immersion probes (where the coupling medium is de-mineralized water)
- Squirter technique: a water jet of a laminar flow

Question 45

What are the advantages of transmission techniques wrt to the pulse-echo techniques?

Answer 45

• It is not affected by the deado zone
• Higher penetration
• Continuos emission is possible
• It crosses the material once a time
• It works thin, thich and high fading materials.

Question 46

What are the methods of presentation of ultrasounds?

Answer 46

1) A-Scan: it’s a time domain representation of the US signal
2) B-Scan: is an A-Scan representation along scanning path
3) C-Scan: With a c-scan we can scan all the surface

Question 47

What are the advantages and disadvantages of ultrasounds?

Answer 47

Advantages
- High penetration capability; it is able to detect voids and presence of foreign materials in depth
- Ultrasonic techniques are suited to the majority of the materials
- Most popular NDI technique, can detect delaminations
- It is widely used to detect de-bondings
- Easily automated and used as on-line quality control method

Disadvantages:
- It needs the interpretation of results (skilled personnel)
- Ultrasonic coupling is required, immersion tank and bulky non portable equipment
- It required a pretty smooth surface
- It is affected by a pretty low scanning velocity