Chapter 08 - Non Destructive Test | Basic of NDT and NDT Methods Flashcards
Definition of NDT
The use of non-invasive techniques to: – determine the integrity of a material, component or structure – quantitatively measure some characteristic of an object.
Inspect or measure
without doing harm
What are Some Uses of
NDE Methods?
Flaw Detection and Evaluation
• Leak Detection
• Location Determination
• Dimensional Measurements
• Structure and Microstructure Characterization
• Estimation of Mechanical and Physical Properties
• Stress (Strain) and Dynamic Response Measurements
• Material Sorting and Chemical Composition
Determination
Why Nondestructive?
Test piece too precious to be destroyed • Test piece to be reuse after inspection • Test piece is in service • For quality control purpose • Something you simply cannot do harm to
Major types of NDT
Detection of surface flaws – Visual – Magnetic Particle Inspection – Fluorescent Dye Penetrant Inspection • Detection of internal flaws – Radiography – Ultrasonic Testing – Eddy current Testing
What is Visual Inspection and the tools used?
• Most basic and common
inspection method.
• Tools include fiberscopes,
borescopes, magnifying glasses and mirrors.
• Portable video inspection unit with zoom allows inspection of
large tanks and vessels,
railroad tank cars, sewer lines.
• Robots allow inspection of
hazardous or tight areas, such as air ducts, reactors, pipelines.
What two main factors of Dye Penetrant Inspection can be attributed to its popularity
– relative ease
– flexibility
On what kind of surface can LPI be used and what can of materials are commonly inspected
• LPI can be used to inspect almost any material provided
that its surface is not extremely rough or porous
• Materials that are commonly inspected using LPI include
metals (aluminium, copper, steel, titanium, etc.), glass,
many ceramic materials, rubber, and plastics.
Procedure of Dye Penetrant Inspection
• A liquid with high surface wetting characteristics is
applied to the surface of the part and allowed time to
seep into surface breaking defects.
• The excess liquid is removed from the surface of the
part.
• A developer (powder) is applied to pull the trapped
penetrant out the defect and spread it on the surface
where it can be seen.
• Visual inspection is the final step in the process.
Penetrant Types
Dye penetrants
Fluorescent penetrants
Why are Dye penetrants used?
– The liquids are colored so that they provide
good contrast against the developer
– Usually red liquid against white developer
– Observation performed in ordinary daylight
or good indoor illumination
Why are Fluorescent penetrants used?
– Liquid contain additives to give fluorescence under UV – Object should be shielded from visible light during inspection – Fluorescent indications are easy to see in the dark
Dye Penetrant Inspections Primary Advantages
• The method has high sensitive to small surface discontinuities.
• The method has few material limitations, i.e. metallic and nonmetallic,
magnetic
• Large areas and large volumes of parts/materials can be inspected
rapidly and at low cost.
• Parts with complex geometric shapes are routinely inspected.
• Indications are produced directly on the surface of the part and
constitute a visual representation of the flaw.
• Aerosol spray cans make penetrant materials very portable.
Dye Penetrant Inspections
Primary Disadvantages
• Only surface breaking defects can be detected.
• Only materials with a relative nonporous surface can be inspected.
• Precleaning is critical as contaminants can mask defects.
• Metal smearing from machining, grinding, and grit or vapor blasting
must be removed prior to LPI.
• The inspector must have direct access to the surface being inspected.
• Surface finish and roughness can affect inspection sensitivity.
• Multiple process operations must be performed and controlled.
• Post cleaning of acceptable parts or materials is required.
• Chemical handling and proper disposal is required.
Why is Magnetic Particle Inspection (MPI) used?
• Fast and relatively easy to apply and part surface preparation
is not as critical as for some other NDT methods. – MPI one of
the most widely utilized nondestructive testing methods.
How is MPI used and what requirement is needed to it to be used?
• MPI uses magnetic fields and small magnetic particles, such
as iron filings to detect flaws in components.
• The only requirement from an inspectability standpoint is that
the component being inspected must be made of a
ferromagnetic material such as iron, nickel, cobalt, or some of
their alloys.
• The method is used to inspect a variety of product forms such
as castings, forgings, and weldments.
(MPI) - Explain the ways you can magnetizing an object
Direct or Indirect
Explain Direct magnetizing
Direct magnetization: current is passed directly
through the component.
Explain Indirect magnetizing
Indirect magnetization: using a strong external
magnetic field to establish a magnetic field within the
component
Advantages of MPI
• One of the most dependable and sensitive methods for
surface defects.
• fast, simple and inexpensive
• direct, visible indication on surface
• unaffected by possible deposits, e.g. oil, grease or other
metals chips, in the cracks
• can be used on painted objects
• surface preparation not required
• results readily documented with photo or tape impression
Disadvantages of MPI
• Only good for ferromagnetic materials
• sub-surface defects will not always be indicated
• relative direction between the magnetic field and the
defect line is important
• objects must be demagnetized before and after the
examination
• the current magnetization may cause burn scars on the item examined
How is Radiography used?
Radiography involves the use of penetrating
gamma- or X-radiation to examine material’s
and product’s defects and internal features.
In Radiography what will the shadowgraph show?
- The resulting shadowgraph shows the internal features and soundness of the part.
-Material thickness and density changes are
indicated as lighter or darker areas on the
film. The darker areas in the radiograph below
represent internal voids in the component.
Explain absorption of x-ray in Radiography
• All x-rays are absorbed to some extent in passing through matter
due to electron ejection or scattering.
Limitations of Radiography
There is an upper limit of thickness through which the
radiation can penetrate, e.g. γ-ray from Co-60 can
penetrate up to 150mm of steel
• The operator must have access to both sides of an
object
• Highly skilled operator is required because of the
potential health hazard of the energetic radiations
• Relative expensive equipment
What is Ultrasonic Testing?
• High-frequency sound waves are transmitted
into a material to detect imperfections or to
locate changes in material properties
What is the equation for absorption of x ray
• The absorption follows the equation
I = I0e^(-μx)=I = I0e-^(μ/p)px
Where I is the transmitted intensity;
x is the thickness of the matter;
μ is the linear absorption coefficient (element dependent);
p is the density of the matter;
(μ/p) is the mass absorption coefficient (cm2
/gm)
What is the most commonly used ultrasonic (US) testing technique
Echo method
– sound is introduced into a test object
– reflections (echoes) from internal imperfections or
part geometry are returned to receiver
– time interval between transmission and reception of
pulses provide information about the internal
material structure
What is Pulse Echo (in US)
Ultrasonic Inspection (Pulse-Echo)
• High frequency sound waves are introduced into a material and they are reflected
back from surfaces or flaws.
• Reflected sound energy is displayed versus time, and inspector can visualize a
cross section of the specimen showing the depth of features that reflect sound.
What is the equation of Pulse Echo
• Measurement of time required for wave to
– travel from transducer through thickness of material
– reflect from back or surface of discontinuity and
return to transducer
• Required time is in microsecond range
𝑑 =𝑣𝑡/2
or
𝑣 =2𝑑/𝑡
d: distance from surface to discontinuity
v: velocity of sound waves in material
t: measured transit time (in and out)
What is Longitudinal waves
– Similar to audible sound waves
– the only type of wave which can
travel through liquid
What is Shear waves
– generated by passing the ultrasonic beam through the material at an angle – Usually a plastic wedge is used to couple the transducer to the material
Angle Beam Transducers
Introduction of refracted shear wave into test material • angled sound path allows sound wave to come in from the side – improvement of detectability of flaws in and around welded areas
What can Angle Beam Transducers be used to analysed
– flat sheets
– plates
– pipes and tubing
Eddy currents
• electrical currents which are generated in
conductive materials by inducing an alternating
magnetic field
• current flows in the surface of the material and
its close proximity
What can changes in the material can interrupt the flow of eddy currents?
– imperfections
– dimensional changes
– changes of conductivity and permeability
What can Eddy Current Testing be used on
Eddy current testing can be used on all
electrically conducting materials with a
reasonably smooth surface.
What can Eddy Current Testing be used for
Used for crack detection, material thickness measurement (corrosion detection), sorting materials, coating thickness measurement, metal detection, etc.
Principle of Eddy Current Testing
• The strength of the secondary field depends on electrical and magnetic properties, structural integrity, etc., of the test object • If cracks or other inhomogeneities are present, the eddy current, and hence the secondary field is affected. • The changes in the secondary field will be a ‘feedback’ to the primary coil and affect the primary current. • The variations of the primary current can be easily detected by a simple circuit which is zeroed properly beforehand
Eddy Current Testing; Depth of penetration
Eddy currents concentrate near the surface adjacent to an excitation coil The depth at which the eddy current density has decreased to 1/e, or approx. 37% of the surface density, is called the standard depth of penetration 𝛿.
Eddy Current Testing; Probe types
• Three major types
– surface probe
– internal bobbin probe
– encircling probe
Eddy Current Testing; Applications
- Crack Detection
- Material Thickness Measurements
- Coating Thickness Measurements
Eddy Current Testing; Applications - Conductivity measurements for
Material Identification
– Heat Damage Detection
– Case Depth Determination
– Heat Treatment Monitoring
Advantages of Eddy Current testing
- Sensitive to small cracks and other defects
- Detects surface and near surface defects
- Inspection gives immediate results
- Equipment is very portable
- Method can be used for much more than flaw detection
- Minimum part preparation is required
- Test probe does not need to contact the part
- Inspects complex shapes and sizes of conductive materials
Limitations of Eddy Current testing
• Only conductive materials can be inspected
• Surface must be accessible to the probe
• Skill and training required is more extensive than other techniques
• Surface finish and and roughness may interfere
• Reference standards needed for setup
• Depth of penetration is limited
• Flaws such as delaminations that lie parallel to the probe coil winding and
probe scan direction are undetectable
