Non Destructive Testing
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Transcript of Non Destructive Testing
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Non Destructive Testing Lunch Time Training in Mechanical Department – June 2006
Madhukar Srivastava- Mechanical
BecRel Engineering
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Introduction
The field of Nondestructive Testing (NDT) is a very broad & interdisciplinary field.
It plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion.
These tests are performed in a manner that does not affect the future usefulness of the object or material.
In other words, NDT allows parts and materials to be inspected and measured without damaging them.
It allows inspection without interfering with a product's final use.
NDT provides an excellent balance between quality control and cost-effectiveness.
Generally speaking, NDT applies to industrial inspections.
Technologies are used in NDT that are similar to those used in the medical industry, typically nonliving objects are the subjects of the inspections.
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NDT – A Brief Perspective
Nondestructive testing (NDT) has been defined as comprising those test methods used to examine an object, material or system without impairing its future usefulness.
The term is generally applied to non medical investigations of material integrity.
Strictly speaking, this definition of nondestructive testing does include noninvasive medical diagnostics.
Ultrasound, X-rays and endoscopes are used for both medical testing and industrial testing.
In the 1940s, many members of the American Society for Nondestructive Testing (then the Society for Industrial Radiography) were medical X-ray professionals.
Medical nondestructive testing, however, has come to be treated by a body of learning so separate from industrial nondestructive testing that today most physicians never use the word nondestructive.
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Nondestructive testing is used to investigate the material integrity of
the test object.
A number of other technologies - for instance, radio astronomy, voltage and amperage measurement and rheometry (flow measurement) - are nondestructive but are not used to evaluate material properties specifically.
Nondestructive testing is concerned in a practical way with the performance of the test piece - how long may the piece be used and when does it need to be checked again?
Radar and sonar are classified as nondestructive testing when used to inspect dams, for instance, but not when they are used to chart a river bottom.
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Purposes of NDT
To ensure product integrity, and in turn, reliability
To avoid failures, prevent accidents and save human life
To make a profit for the user
Ensure customer satisfaction and maintain the manufacturer's reputation
To aid in better product design
To control manufacturing processes
To lower manufacturing costs
To maintain uniform quality level
To ensure operational readiness.
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NDT/NDE Methods
Visual and Optical Testing (VT)
Dye Penetrant Testing ( DPT)
Magnetic Particle Testing ( MPT)
Electromagnetic Testing (ET) or Eddy Current
Testing
Ultrasonic ( UT )
Radiography ( RT)
Acoustic Emission Testing ( AE)
Leak Testing ( LT)
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Visual Testing
Visual inspection involves using an inspector's eyes to look for defects.
The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area.
Visual examiners follow procedures that range from simple to very complex.
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Dye Penetrant Testing
Test objects are coated with visible or fluorescent dye solution.
Excess dye is then removed from the surface, and a developer is applied.
The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface.
With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see.
With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.
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DPT Sequence
1) PENETRANT IS APPLIED 2 ) EXCESS DYE IS REMOVED
3) DEVELOPER APPLIED 4) VISUALLY INSPECTED
Remember to Clean and dry the surface before test
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DPT - Advantages & Disadvantages
Primary Advantages
The method has high sensitivity to small surface discontinuities.
The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.
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.
Penetrant materials and associated equipment are relatively inexpensive.
Primary Disadvantages
Only surface breaking defects can be detected.
Only materials with a relative nonporous surface can be inspected.
Pre-cleaning 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
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Magnetic Particle Test
This NDE method is accomplished by inducing a
magnetic field in a ferromagnetic material
After that dusting the surface with iron particles
(either dry or suspended in liquid).
Surface and near-surface imperfections distort the
magnetic field and concentrate iron particles near
imperfections, previewing a visual indication of
the flaw.
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Magnetic Particle Test-Contd..
Any place that a magnetic line of force exits or
enters the magnet is called a pole. A pole where a
magnetic line of force exits the magnet is called a
north pole and a pole where a line of force enters the
magnet is called a south pole.
If the magnet is just cracked but not broken
completely in two, a north and south pole
will form at each edge of the crack. The
magnetic field exits the north pole and
reenters the at the south pole. The magnetic
field spreads out when it encounter the
small air gap. When the field spreads out, it
appears to leak out of the material and, thus,
it is called a flux leakage field.
If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not
only at the poles at the ends of the magnet but also at the poles at the edges of the crack. This cluster
of particles is much easier to see than the actual crack and this is the basis for magnetic particle
inspection.
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Eddy Current Inspection
Electrical currents are generated in a conductive material by an induced alternating magnetic field.
The electrical currents are called eddy currents because they flow in circles at and just below the surface of the material.
Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the proper equipment.
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Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the
conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the
alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought
into the close proximity to this changing magnetic field, current will be induced in this second conductor. Eddy currents are
induced electrical currents that flow in a circular path. They get their name from “eddies” that are formed when a liquid or gas
flows in a circular path around obstacles when conditions are right.
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Eddy Current Applications
Crack Detection
Material Thickness Measurements
Coating Thickness Measurements
Conductivity Measurements For:
Material Identification
Heat Damage Detection
Case Depth Determination
Heat Treatment Monitoring
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Eddy Current - Advantages & Disadvantages
Primary Advantages
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
Primary Disadvantages
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
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Example-Tube Inspection
Eddy current inspection is often used to detect corrosion, erosion,
cracking and other changes in tubing. Heat exchangers and steam
generators, which are used in power plants, have thousands of tubes
that must be prevented from leaking.
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Example – Thickness Measurement
Eddy current techniques can be used to perform a number of dimensional
measurements. The ability to make rapid measurements without the need
for couplant or, in some cases even surface contact, makes eddy current
techniques very use.
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Ultrasonic Testing
Ultrasonics use transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties.
The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.
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Basic Principles of UT
A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices.
A pulser/receiver is an electronic device that can produce high voltage electrical pulse. Driven by the pulser, the transducer generates high frequency ultrasonic energy.
The sound energy is introduced and propagates through the materials in the form of waves.
When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface.
The reflected wave signal is transformed into electrical signal by the transducer and is displayed on a screen.
The reflected signal strength is displayed versus the time from signal generation to when a echo was received.
Signal travel time can be directly related to the distance that the signal traveled.
From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.
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Transducer
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UT-Advantages & Disadvantages
Primary Disadvantages
Surface must be accessible to transmit
ultrasound. Skill and training is more extensive than
with some other methods. It normally requires a coupling medium
to promote transfer of sound energy into test specimen.
Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
Linear defects oriented parallel to the sound beam may go undetected.
Reference standards are required for both equipment calibration, and characterization of flaws.
Primary Advantages
It is sensitive to both surface and subsurface discontinuities.
The depth of penetration for flaw detection or measurement is superior to other NDT methods.
Only single-sided access is needed when the pulse-echo technique is used.
It is high accuracy in determining reflector position and estimating size and shape.
Minimal part preparation required. Electronic equipment provides
instantaneous results. Detailed images can be produced with
automated systems. It has other uses such as thickness
measurements, in addition to flaw detection.
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Radiography
Radiography involves the use of penetrating gamma or X-radiation to examine parts and products for imperfections.
An X-ray generator or radioactive isotope is used as a source of radiation.
Radiation is directed through a part and onto film or other imaging media.
The resulting shadowgraph shows the dimensional features of the part.
Possible imperfections are indicated as density changes on the film in the same manner as a medical X-ray shows broken bones.
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Set-Up
Gamma -Ray X-Ray
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Radioactive Sources for Gamma Rays
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Example – Cold Lap
Cold lap is a condition where the weld filler metal does not properly fuse
with the base metal or the previous weld pass material (interpass cold
lap). The arc does not melt the base metal sufficiently and causes the
slightly molten puddle to flow into base material without bonding
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Example- Porosity
Porosity is the result of gas entrapment in the solidifying metal. Porosity can
take many shapes on a radiograph but often appears as dark round or irregular
spots or specks appearing singularly, in clusters or rows. Sometimes porosity is
elongated and may have the appearance of having a tail This is the result of gas
attempting to escape while the metal is still in a liquid state and is called
wormhole porosity. All porosity is a void in the material it will have a
radiographic density more than the surrounding area.
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Example – Slag Inclusion
Slag inclusions are nonmetallic solid material entrapped in weld
metal or between weld and base metal. In a radiograph, dark,
jagged asymmetrical shapes within the weld or along the weld joint
areas are indicative of slag inclusions.
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Example -Incomplete Penetration
Incomplete penetration (IP) or lack of penetration (LOP) occurs
when the weld metal fails to penetrate the joint. It is one of the most
objectionable weld discontinuities. Lack of penetration allows a natural
stress riser from which a crack may propagate. The appearance on a
radiograph is a dark area with well-defined, straight edges that follows
the land or root face down the center of the weldment
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Example - Cracks
Cracks can be detected in a radiograph only when they are propagating
in a direction that produces a change in thickness that is parallel to the
x-ray beam. Cracks will appear as jagged and often very faint irregular
lines. Cracks can sometimes appear as "tails" on inclusions or porosity.
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Example- Offset or Mismatch
Offset or mismatch are terms associated with a condition where two
pieces being welded together are not properly aligned. The
radiographic image is a noticeable difference in density between the
two pieces. The difference in density is caused by the difference in
material thickness. The dark, straight line is caused by failure of the
weld metal to fuse with the land area.
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Acoustic Emission Testing
When a solid material is stressed, imperfections
within the material emit short bursts of acoustic
energy called "emissions.“
As in ultrasonic testing, acoustic emissions can be
detected by special receivers.
Emission sources can be evaluated through the
study of their intensity, rate, and location.
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Leak Testing
Several techniques are used to detect and
locate leaks in pressure containment parts,
pressure vessels, and structures.
Leaks can be detected by using electronic
listening devices, pressure gauge
measurements, liquid and gas penetrant
techniques, and/or a simple soap-bubble
test.
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Thank You !