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DESTRUCTIVE TESTING
As the name suggests,
destructive testing (DT) includes methods where your material is broken down in
order to determine mechanical properties, such as strength, toughness and
hardness. In practice it means, for example, finding out if the quality of a
weld is good enough to withstand extreme pressure or to verify the properties
of a material.
These properties can’t be
examined with non-destructive methods, as specimens of the material must be
extracted. Destructive testing is generally most suitable and economic for mass
produced objects, as the cost of destroying a small number of pieces is
negligible. The samples are put under different loads and stress. That way we
can analyze in which point your material eventually gives up and cracks. The
results gained are then compared to regulations and/or quality guidelines.
Destructive tests are
best when used together with our non-destructive methods: this combination
gives the best information on materials and welds. Non-destructive tests show
if cracks, corrosion or other faults exist. Destructive tests in turn indicate
how and when the objects are in danger of breaking down or failing.
Inspection destructive
testing services include mechanical testing (tensile, bend and impact tests),
hardness testing, macro and micro testing as well as material analysis and
metallographic examinations. Inspection has its’ own, accredited destructive
testing laboratories in several countries. We are also able to execute your
required tests at our own workshops. This way we can ensure you get the best
service and the most suitable testing methods.
Benefits
of Destructive Testing (DT)
As the name suggests,
destructive testing (DT) includes methods where your material is broken down in
order to determine mechanical properties, such as strength, toughness and
hardness. In practice it means, for example, finding out if the quality of a
weld is good enough to withstand extreme pressure or to verify the properties
of a material.
These properties can’t be
examined with non-destructive methods, as specimens of the material must be
extracted. Destructive testing is generally most suitable and economic for mass
produced objects, as the cost of destroying a small number of pieces is
negligible. The samples are put under different loads and stress. That way we
can analyze in which point your material eventually gives up and cracks. The
results gained are then compared to regulations and/or quality guidelines.
Destructive tests are
best when used together with our non-destructive methods: this combination
gives the best information on materials and welds. Non-destructive tests show
if cracks, corrosion or other faults exist. Destructive tests in turn indicate
how and when the objects are in danger of breaking down or failing.
Inspection destructive
testing services include mechanical testing (tensile, bend and impact tests),
hardness testing, macro and micro testing as well as material analysis and
metallographic examinations. Inspection has its’ own, accredited destructive
testing laboratories in several countries. We are also able to execute your
required tests at our own workshops. This way we can ensure you get the best
service and the most suitable testing methods.
Benefits
of Destructive Testing (DT)
1. Verifies
properties of a material.
2. Determines
quality of welds.
3. Ensures
compliance with regulations
4. Helps
you to reduce failures, accidents and costs.
Why
the metals are tested:
l Ensure
quality
l Test
properties
l Prevent
failure in use
l Make
informed choices in using materials
Factor
of Safety is the ratio comparing the actual stress on a material and the safe
useable stress. Basically there are two forms testing, they are.
l Ensure
quality
l Test
properties
l Prevent
failure in use
l Make
informed choices in using materials
Factor
of Safety is the ratio comparing the actual stress on a material and the safe
useable stress. Basically there are two forms testing, they are.
Two forms of testing:-
1.
Mechanical tests
– the material may be physically tested to destruction. Will normally specify a
value for properties such as strength, hardness, toughness, etc.
2.
Non-destructive
tests (NDT) – samples or finished articles are tested before being used.
Some
of the Destructive tests are as follows;
1.)
Tensile Testing
2.)
Bend
1.
Mechanical tests
– the material may be physically tested to destruction. Will normally specify a
value for properties such as strength, hardness, toughness, etc.
2.
Non-destructive
tests (NDT) – samples or finished articles are tested before being used.
Some
of the Destructive tests are as follows;
1.)
Tensile Testing
2.)
Bend
3.)
Impact Testing
4.)
Hardness Testing
1. Tensile testing:-
Material is
sectioned and edges rounded of to prevent cracking and punches marks are made
to see elongation. Uses an extensometer to apply measured force to a test
specimen. The amount of extension can be measured and graphed. Variables such
as strain, stress, elasticity, tensile strength, ductility and shear strength
can be gauged. The test specimen can be round or flat
Material is
sectioned and edges rounded of to prevent cracking and punches marks are made
to see elongation. Uses an extensometer to apply measured force to a test
specimen. The amount of extension can be measured and graphed. Variables such
as strain, stress, elasticity, tensile strength, ductility and shear strength
can be gauged. The test specimen can be round or flat
2. Bend
These show the bending test of
Physical condition of the weld and also determine the weld efficiency.
These show the bending test of
Physical condition of the weld and also determine the weld efficiency.
• Tensile strength
• Ductility
• Fusion and penetration
Bend test:-
In the bend test it Bend through
180* and the specimen should be a minimum of 30mm wide. The fulcrums diameter
is 3x thickness of the plate and the bottom rollers have a distance of the
diameter of the former + 2.2 times the
thickness of the plate. The Upper and
lower surfaces ground or filed flat and edges rounded off. The tests should be one against the root -another
against the face, and in some cases a side bend.
In the bend test it Bend through
180* and the specimen should be a minimum of 30mm wide. The fulcrums diameter
is 3x thickness of the plate and the bottom rollers have a distance of the
diameter of the former + 2.2 times the
thickness of the plate. The Upper and
lower surfaces ground or filed flat and edges rounded off. The tests should be one against the root -another
against the face, and in some cases a side bend.
Fig: - Root bend
3.
Impact testing:-
Fig: - Impact test equipment
In
these test we have carried out two types of tests namely “CHARPY AND IZOD”. And
here we can find out the toughness and shock loading of the material and weld
at varying temperatures with a notch such as under cut. To the measurement the
energy required to break a specimen with a given notch 2mm depth at a 45obevel
or a “U” notch.
Fig: - The charpy test specimen placing
base
Fig: - Impact test equipment
In
these test we have carried out two types of tests namely “CHARPY AND IZOD”. And
here we can find out the toughness and shock loading of the material and weld
at varying temperatures with a notch such as under cut. To the measurement the
energy required to break a specimen with a given notch 2mm depth at a 45obevel
or a “U” notch.
Fig: - Izod test specimen base
1.
Hardness testing:-
In the
hardness testing we can find out the metals ability to show resistance to
indentation which show it’s resistance to wear and abrasion.
The tests are
1.
Brinell hardness testing:-
l
The indenter is pressed into the metal
l
Softer materials leave a deeper indentation
l
Uses ball indentor.
l
Cannot be used for thin materials.
l
Ball may deform on very hard materials
l
Surface area of indentation is measured.
Fig: -
Hardness testing machine
2. Rockwell harness testing:-
· It gives direct reading
1.Ball
-B 
3. Vickers hardness
test:-
1. It uses square pyramid indentor.
2. Accurate
results.
3. Measures lengths of diagonal on
indentation
Advantages of destructive testing:-
1. Allows
a roughly identify the mechanical properties of the adhesive joint (fracture
strength, elongation, modulus of elasticity ....)
2. The
mechanical properties of the adhesive or adhesive bonding can be defined
according to the different types of stresses that undergo, efforts such as
tension, compression, shear, peel, dynamic forces of impact ...
3. The
costs of equipment for destructive testing are cheaper compare with the
equipment used in nondestructive testing.
4. Ability
to compare adhesives with this type of testing
5. Verification
of surface preparation, curing conditions, working conditions and adhesives
system products (primers, activators, adhesives ...)
6. Predict
and identify the approximate nature of the failure or breakdown that may occur
during the lifetime of the bonded joint in use, when the specimen is previously
submitted to an accelerated ageing.
7. Tests
on a relatively cheaper cost.
Disadvantages of destructive testing:-
NON DESTRUCTIVE TESTING
In
the modern complex of today, metal testing plays a vital role. The prevailing
technology is posing challenges to technically qualified people.
Nondestructive testing (NDT) is
a wide group of analysis techniques used in science and industry to evaluate
the properties of a material, component or system without causing damage.
Because NDT does not permanently alter the article being inspected, it is a
highly valuable technique that can save both money and time in product
evaluation, troubleshooting, and research. Common NDT methods include
ultrasonic, magnetic-particle, liquid Penetrant, radiographic, and eddy-current
testing NDT is a commonly used tool in forensic engineering, mechanical
engineering, electrical engineering, civil engineering, systems engineering,
medicine, and art. Specialist high risk areas such as nuclear and offshore
structures, and gas and oil pipelines, make extensive use of Non-Destructive
Testing of metallic components during manufacture and construction as part of
quality assurance procedures as well as during routine maintenance inspections
to detect cracking and corrosion.
Radiography and Ultrasonics is
most widely used for checking of welds, although eddy current and magnetic
methods are also available. Alternating current field measurement techniques
permit non contacting crack detection and sizing in welded joints both in air
and underwater. The focus of this presentation concentrates on mainstream
engineering activities, where the extent of N.D.T. usage varies considerably.
Nondestructive testing (NDT)
are noninvasive techniques to determine the integrity of a material, component
or structure or quantitatively measure some characteristic of an object. In
contrast to destructive testing, NDT is an assessment without doing harm,
stress or destroying the test object. The destruction of the test object
usually makes destructive testing more costly and it is also inappropriate in
many circumstances. NDT plays a crucial role in ensuring cost effective
operation, safety and reliability of plant, with resultant benefit to the
community. NDT is used in a wide range of industrial areas and is used at
almost any stage in the production or life cycle of many components. The
mainstream applications are in aerospace and civil structures, power
generation, automotive, railway, petrochemical and pipeline markets. NDT of
welds is one of the most used applications. It is very difficult to weld or
mold a solid object that has no risk of breaking in service, so testing at
manufacture and during use is often essential.
While originally NDT was
applied only for safety reasons it is today widely accepted as cost saving
technique in the quality assurance process. Unfortunately NDT is still not used
in many areas where human life or ecology is in danger. Some may prefer to pay
the lower costs of claims after an accident than applying of NDT. That is a
form of unacceptable risk management.
For implementation of NDT it is
important to describe what shall be found and what to reject. A completely
flawless production is almost never possible. For this reason testing
specifications are indispensable. Nowadays there exist a great number of
standards and acceptance regulations. They describe the limit between good and
bad conditions, but also often which specific NDT method has to be used. The
reliability of an NDT Method is an essential issue. But a comparison of methods
is only significant if it is referring to the same task. Each NDT method has
its own set of advantages and disadvantages and, therefore, some are better
suited than others for a particular application.
By use of artificial Flaws, the
threshold of the sensitivity of a testing system has to be determined. If the sensitivity
is to low defective test objects are not always recognized. If the sensitivity
is too high parts with smaller flaws are rejected which would have been of no
consequence to the serviceability of the component. With statistical methods it
is possible to look closer into the field of uncertainly. Methods such as
Probability of Detection (POD) or the ROC-method "Relative Operating
Characteristics" are examples of the statistical analysis methods. Also
the aspect of human errors has to be taken into account when determining the
overall reliability. Personnel Qualification is an important aspect of
non-destructive evaluation. NDT techniques rely heavily on human skill and
knowledge for the correct assessment and interpretation of test results. Proper
and adequate training and certification of NDT personnel is therefore a must to
ensure that the capabilities of the techniques are fully exploited. There are a
number of published international and regional standards covering the
certification of Competence of personnel.
This
emphasizes the need for “NON
DISTRUCTIVE TESTING” of metals. Various techniques are available
for finding the surface internal/external defects of metals without sectioning
or by conventional destruction tests.
NON DESTRUCTIVE
TESTING:
A Non- Destructive test is such a
test or examination of a component in any manner which will not impair or
blemish the metal for its future use.
Although, Non-Destructive Test do
not provide direct measurement of mechanical properties, yet they are extremely
useful in revealing defects in component that could blemish their performance
when put into service.
Non –Destructive
Testing make components more reliable, safe and economical.
There are
different non destructive testing methods namely:
1.Visual Inspection (VI)
2.Magnetic particle testing (MT)
3.Penetrant testing (PT)
4.Ultrasonic testing (UT)
5. Radiographic testing (RT)
6. Thermal infrared testing (TIT)
7. Eddy current testing (ET)
8. Acoustic emission testing (AET)
9. Leak testing (LT)
10.
Neutron radiographic testing (NRT)
The most common NDT Methods are
discussed in this presentation. In order of most used, they are: Ultrasonic
Testing (UT), Radiographic Testing (RT), Liquid Penetrant testing, Magnetic
particle Testing, Electromagnetic Testing (ET) in which Eddy Current testing
(ECT) is well know and Acoustic Emission (AE or AET).
Besides the main NDT methods a lot of other
NDT techniques are available, such as Shearography Holography, Microwave and
many more and new methods are being constantly researched and developed.
In the next sections the
methods are explained and their applications to structures are discussed.
NON-DESTRUCTIVE TESTING
1. Visual testing (VT):-
Fig: microscopic
Fig:
macroscopic
VT is considered to be the primary NDT
method. Since it relies on an evaluation made using the eye, VT is generally
considered to be the primary and oldest method of NDT. With its relative
simplicity and because it does not require sophisticated apparatus, it is a
very inexpensive method thus provides an advantage over other NDT methods. A
further advantage of VT is that it is an ongoing inspection that can be applied
at various stages of construction.
VT
is most effective when it is performed at all stages of any new fabrication,
and is the main method used during the inspection of pressure equipment.
If
applied say after welding has been completed, it is possible that subsurface
flaws may not be detected. Therefore, it is important to appreciate that VT
will only be fully effective if it is applied throughout any fabrication or
inspection. An effective VT programme if applied at the correct time will
detect most defects or discontinuities that may later be found by some other
costly and time consuming NDT method. The economics of VT can be seen in
welding if we consider how much easier and inexpensive a welding problem can be
corrected when found at the right time, i.e. as it occurs.
Dimension:
- length: 260mm and depth: 50mm
Fig: - MPT report of kiln-2 Girth gear
VT will also give the
technician instant information on the condition of pressure equipment regarding
such things as corrosion, bulging, distortion, correct parts, failures, etc.
VT requires three basic
conditions to be in place. These are:
__ Good vision, to be able to
see what you are looking for
__ Good lighting, the correct
type of light is important
__ Experience, to be able to
recognize problems.
As mentioned previously, one of
the advantages of VT is that there is little or no equipment required, which
improves its economy or portability. Equipment, which may be employed to
improve the accuracy, repeatability, reliability, and efficiency of VT, include
various devices. Magnifying glasses may also be utilized for a more detailed
look at some visual feature. However, care must be taken to avoid making
erroneous decisions regarding the size or extent of some discontinuity when its
image is magnified.
Fig: - MPT report on shaft pinion gear
In order to preserve the test results,
various methods can be employed. These include drawing a sketch, describing the
visual appearance using written words, or taking a photograph or video of the
surface conditions noted. It is important to accurately record the location,
extent, and type of any defect so that the owner, designer, principal and
production personnel know what requires repair and where the repair is to be
carried out.
Limitations:-
Advantages:-
Dimension:
- length: 260mm and depth: 50mm
2. MAGNETIC PARTICLE
TESTING (MT):-
It
is often used on ferrous materials as an NDT tool to detect surfaces and sub
surface discontinuities. Magnetic particle inspection processes are
non-destructive methods for the detection of surface and sub-surface defects in
ferrous materials. They make use of an externally applied magnetic field
through the material, and the principle that the magnetic flux will leave the
part at the area of the flaw. The presence of a surface or near surface flaw
(void) in the material causes distortion in the magnetic flux through it, which
in turn causes leakage of the magnetic fields at the flaw. This deformation of
the magnetic field is not limited to the immediate locality of the defect but
extends for a considerable distance; even through the surface and into the air
if the magnetism is intense enough. Thus the size of the distortion is much
larger than that of the defect and is made visible at the surface of the part
by means of the tiny particles that are attracted to the leakage fields. The
most common method of magnetic particle inspection uses finely divided iron or
magnetic iron oxide particles, are held in suspension in a suitable liquid (often
kerosene). This fluid is referred to as carrier. The particles are often
colored and usually coated with fluorescent dyes that are made visible with a
hand-held ultraviolet (UV) light (sometimes referred to as black light). The
suspension is sprayed or painted over the magnetized specimen during
magnetization with a direct current or with an electromagnet, to localize areas
where the magnetic field has protruded from the surface. The magnetic particles
are attracted by the surface field in the area of the defect and hold on to the
edges of the defect to reveal it as a build up of particles. This inspection
can be applied to raw material in a steel mill (billets or slabs), in the early
stages of manufacturing (forgings, castings), or most commonly to machined
parts before they are put into service. It is also very commonly used for
inspecting structural parts (e.g., landing gear) that have been in-service for
some time to find fatigue cracks. Usually tested pieces needs to be
demagnetized before use. Parts are demagnetized by applying AC current through
the part and reducing the current which scrambles the magnetic domains causing
it to demagnetize. It is a quite economic non-destructive test because it is
easy to do and much faster than ultrasonic testing and radiographic testing.
Because of the left hand rule, there are two different ways of magnetizing a
part, Longitudinal and Circular magnetization. Longitudinal Magnetization
passes current through a coil and the magnetic flux lines go through the part.
Circular magnetization passes current through the part and establishes a
magnetic field around the part. The two different methods are used because
cracks can only be seen at 45 to 90 degrees to the magnetic flux lines.
Magnetic Particle Inspection cannot be used for non-ferrous materials and
non-magnetic ferrous materials such as austenitic stainless steels. In such
cases, other methods such as dye Penetrant inspection are used.
Magnetic Flux Leakage
(MFL):-
It is a magnetic method of
nondestructive testing that is used to detect corrosion and pitting in steel
structures, most commonly pipelines and storage tanks. The basic principle is
that a powerful magnet is used to magnetize the steel. At areas where there is
corrosion or missing metal, the magnetic field "leaks" from the
steel. In an MFL tool, a magnetic detector is placed between the poles of the
magnet to detect the leakage field. Analysts interpret the chart recording of
the leakage field to identify damaged areas and hopefully to estimate the depth
of metal loss. This method is best suitable for pipeline examination and tank
floors.
Here we are discussing the
procedure of the magnetic particle testing. Typically, an MFL tool consists of
two or more bodies. One body is the magnetizer with the magnets and sensors and
the other bodies contain the electronics and batteries. The magnetizer body
houses the sensors that are located between powerful "rare-earth"
magnets. The magnets are mounted between the brushes and tool body to create a
magnetic circuit along with the pipe wall. As the tool travels along the pipe,
the sensors detect interruptions in the magnetic circuit. Interruptions are
typically caused by metal loss and which in most cases is corrosion. Mechanical
damage such as shovel gouges can also be detected. The metal loss in a magnetic
circuit is analogous to a rock in a stream. Magnetism needs metal to flow and
in the absence of it, the flow of magnetism will go around, over or under to
maintain its relative path from one magnet to another, similar to the flow of
water around a rock in a stream. The sensors detect the changes in the magnetic
field in the three directions (axial, radial, or circumferential.
Dimension: - length: 260mm and depth: 50mm
Fig: - MPT report of kiln-2 Girth gear
Crack detection:-
After doing the magnetic
particle test we have observed the large non-axial oriented cracks have been
found in a pinion gear and in a surface plate as shown in figs. that was
inspected by a magnetic flux leakage tool. To an experienced MFL data analyst,
a dent is easily recognizable by trademark "horseshoe" signal in the
radial component of the vector field. What is not easily identifiable to an MFL
tool is the signature that a crack leaves.
The
part is magnetized. Finely milled iron particles coated with a dye pigment are
then applied to the specimen. These particles are attracted the magnetic flux
leakage fields and will cluster to form indication directly over the
discontinuity. This indication can be visually detected under proper lighting
condition.
Features:-
The features of the magnetic
particle testing is primarily used to detect corrosion, MFL tools can also be
used to detect features that they were not originally designed to identify.
When an MFL tool encounters a geometric deformity such as a dent, wrinkle or
buckle, a very distinct signal is created due to the plastic deformation of the
pipe wall.
Fig: - MPT report on shaft pinion gear
Fig: - MPT report on shaft pinion gear
Application:-
1.
Examination of ferrite welds
2.
Examination of castings
3.
Examination of shafts, fasteners, rods
4.
Examination of bearing sleeves, hollow forgings
Advantages:-
Limitations:-
1.
Magnetic particle inspection methods will work only on
ferromagnetic materials.
2.
For best results, the magnetic field must be in a
direction that will intercept the principle plane of the discontinuity.
Sometimes this requires two or more sequential inspections. With different
magnetizations.
3.
Demagnetization following magnetic particle testing is
often necessary.
4.
Post cleaning to remove remnants of the magnetic
particle clinging to the surface may be required after testing and demagnetization.
5.
Exceedingly large currents sometimes are required for
very large parts.
6.
Care is necessary to avoid local heating and burning of
finished parts or surface at the points of electric contact.
7.
Although magnetic particle indications are easily seen,
experience and skill in interpreting their significance are needed.
3. LIQUID PENETRANT
TESTING (PT):-
Dye Penetrant test is based
upon capillary action, where low surface tension fluid penetrates into clean
and dry surface-breaking discontinuities. Penetrant may be applied to the test
component by dipping, spraying, or brushing. After adequate penetration time
has been allowed.
The Excess Penetrant is
removed, a developer is applied. The developer helps to draw Penetrant out of
the flaw where a visible indication becomes visible to the inspector.
Inspection is performed under ultraviolet or white light, depending upon the
type of dye used fluorescent or non fluorescent (visible).
Usually, the test is performed at
normal room temperature, but with the help of special fluids the test can be
performed at other temperature. The normal penetrating systems are usable in
the temperature area -5 to 60 degree centigrade. Actually Penetrants are
classified into sensitivity levels. Visible penetrants are typically red in
color, and represent the lowest sensitivity. Fluorescent penetrants contain two
or more dyes that fluorescent when excited by ultraviolet (UV-A) radiation
(also known as black light). Since, Fluorescent penetrant inspection is performed
in a darkened environment, and the excited dyes emit brilliant yellow-green
light that contrasts strongly against the dark background, this material is
more sensitive to small defects. When selecting a sensitivity level one must
consider many factors, including the environment under which the test will be
performed.
The surface finish of the
specimen and the size of defects sought. One must also assure that the test
chemicals are compatible with the sample so that the examination will not cause
permanent staining, or degradation. This technique can be quite portable,
because in its simplest form the inspection requires only 3 aerosol spray cans,
some paper towels, and adequate visible light. Stationary systems with
dedicated application, wash, and development stations, are more costly and
complicated, but result in better sensitivity and higher sample through-put.
Penetrants
ability to flow between surfaces and enter cavities depends on.
·
Cleanliness of surface
·
Cleanliness of cavity
·
Chemical inertness
·
Sufficient brightness
·
Size of surface opening
·
Configuration of cavity
·
Surface tension of Penetrant
·
Viscosity
·
Contact angle
Penetrant
testing can be classified into six categories, they are
Visible- type Fluorescent-type
1.
Solvent removable 1. Solvent removable
2.
Water washable 2. Latent washable
3.
Post – emulsified 3. Post – emulsified
The most
versatile and suitable for site inspection is solvent removable technique.
The three
chemicals involved in penetrant test are:
·
Cleaner
·
Penetrant
·
Non – aqueous developed
Penetrant
testing is a five step process:
1.
Pre cleaning
2.
Application of dye and dwell time
3.
Removal of excess penetrant from surface
4.
Application of developer
5.
Interpretation of evaluation
Fig:-The process of the dye
penetrant testing.
1. Pre-cleaning:-
2. Application
of Penetrant and dwell time:
The penetrant is then applied
to the surface of the item being tested. The penetrant is allowed time to soak
into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon
the penetrant being used, material being testing and the size of flaws sought.
As expected, smaller flaws require a longer penetration time.
4. Application of
developer:
After excess penetrant has been
removed a white developer is applied to the sample. Several developer types are
available, including: non-aqueous wet developer, dry powder, water suspendable,
and water soluble. Choice of developer is governed by penetrant compatibility
(one can't use water-soluble or suspendable developer with water-washable
penetrant), and by inspection conditions. When using non-aqueous wet developer
(NAWD) or dry powder, the sample must be dried prior to application, while
soluble and suspendable developers are applied with the part still wet from the
previous step. NAWD is commercially available in aerosol spray cans, and may
employ acetone, isopropyl alcohol, or a propellant that is a combination of the
two. Developer should form a semi-transparent, even coating on the surface. The
developer draws penetrant from defects out onto the surface to form a visible
indication, a process similar to the action of blotting paper. Any colored
stains indicate the positions and types of defects on the surface under
inspection.
Non-aqueous
developer is nothing but very fine fluffy white absorbent-type power suspended
in solvent developer shall be thoroughly agitated before application developer
shall be applied by spraying so as to give uniform thin coating.
Developer serves
two purposes:
·
As a blotting agent to be accelerate the reverse
capillary action
·
Gives a contrast to make the penetrant easily
visible to our eyes
5.
Interpretation and evaluation:
Interpretation
and evaluation shall be done not earlies than 7 minutes after application of
developer.
Examination shall be done under good
lighting condition or with the help of black light in case fluorescent
penetrant technique. Relevant indication will have to be identified interpreted
and evaluated in terms of acceptance criteria. Lesser used in cement industry
is fluorescent dye penetrant which in principle is same as the above. With
fluorescent dye penetrant a UV –lamp has to be used (ultraviolet light)
COMMON REMARKS:
With both methods it is essential to
remember that surface contaminated with oil is very often not testable with dye
penetrant, because possible cracks are blocked,
Another
important thing is, very deep cracks are able to consume plenty of penetrant in
some cases so much that the penetrant is sucked away from the surface the common results from these two remarks is that no
cracks have been detected although a crack really exists.
Advantages:
1. Inexpensive
to operate
2. Easy to
transport
3. Can identify
the surface cracks.
4. Economical
5. Aid to VT
6. Portable
7. Can inspect a
wide range of materials and components.
.
Sequence of liquid penetrant tests:
Limitations:-
1.
The major limitation of liquid penetrant testing is
that, it can detect only imperfections that are open to the surface.
2.
Another factor
that may inhibit the effectiveness of liquid penetrant testing is the surface
roughness of the object being tested.
3.
Rough or porous surfaces are likely to produce false
indication.
4.
Depth of flaw not indicated.
5.
Very tight and shallow defects difficult to find.
6.
decontamination and precleaning of test surface may be
needed
4. ULTRASONIC
TESTING (UT):-
In
addition to the methods for Non-Destructive Testing of material surface , in
particular for coarse and fine material
separations (Dye-penetration method, Magnetic powder method) up to the
start of the fifties the tester only know the X-ray method as a NDT method for
internal flaws. Then ultrasonics where introduced for measuring thickness as
well as detecting discontinuities in the materials.
In ultrasonic testing, very
short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz
and occasionally up to 50 MHz are launched into materials to detect internal
flaws or to characterize materials. The technique is also commonly used to
determine the thickness of the test object, for example, to monitor pipe work
corrosion. Ultrasonic testing is often performed on steel and other metals and
alloys, though it can also be used on concrete, wood and composites, albeit
with less resolution. It is a form of non destructive testing used in many
industries including aerospace, automotive and other transportation sectors
ULTRASONIC:
Human ear can hear sound
from 20 to 20,000 HZ all sounds above this range is ultrasonic the base to use
ultrasonic in different materials is that sound with very frequency is
transmitted as a beam.
Principle:
High frequency sound
waves in range of million cycles per second (MHZ) is generated when DC current
is applied to piezo - electric crystal (Quartz), Quartz has the property of
converting electrical energy into mechanical energy and vice-versa. This is
called piezo electric effect .when quartz crystal is coupled to the material,
with grease or oil or water.
Electrical
energy travels as mechanical energy inside the material surface to remove the
air which is poor transmitter of sound waves. Sound waves travel through
material causing displacement of particles about its mean position .these waves
are reflected back by discontinuities (voids or foreign material) as well as
boundaries of material reflected waves are received by the crystal converted
into electrical energy amplified and displayed on cathode ray tube.
Since, velocity of sound remains
constant in a medium, say 5900 ms in steel, time taken to travel through a material
can be measured and converted into distance. This is how it is possible to
measure the thickness as well as depth of flaws in material.
Velocity (V) =Frequency (F)*Wavelength
(w).
Higher
the frequency shorter the wavelength and better the detectability. So the
ultrasonic of higher frequency are used to detect the flaws in the material.
Sound waves are made to travel either normal to the surface (straight beam), or
at an angle. When sound waves strike a discontinuity, reflection is maximum.
During
the line -2 kiln shut down. Ultrasonic test was done to the kp-1 tyre and
identified there are 27-no of defects major cracks as shown below snap shot.
Fig: - ultrasonic test for
kiln tyre
And
ultrasonic test report coal apron feeder flight pin in wagon tippler area and
we have find defects and cracks as shown in below snap shot.
Fig: - UT report on coal
apron feeder flight pin in wagon tippler area.
One
more Ultrasonic test report of packing plant wagon loading machine trolley
shaft. We have observed defect indication of length 2016 mm (approx) from shaft
one end face and observed the shaft sheared, when it was dismantling.
Fig: - UT repot on packing plant wagon
loading machine trolley drive shaft.
THE TESTING SYSTEM
CONSISTS OF:
1.
Ultrasonic generator with CRT
2.
Probes or transducer
3.
object to the evaluated
4.
calibration block
5.
couplant
Pulsar in the machine sends a pulse to the probe
as well as across the horizontal time base. Crystal is made to oscillate and generate
sound wave. these waves travel through
the object as mechanical energy and gets reflected by discontinuities or
boundaries. Reflected rays causes voltage difference when they reach the
crystal back. This is amplified and applied across v-plates there by causing a
vertical deflection on CRT
Fig: -Block diagram for
ultrasonic pulse Echo system
Amplitude of these
deflections depends on the energy of reflected waves. Since the some crystal
can transmit and receive the waves these need to be a time gap between two
consecutive pulses. Clock synchronizes the operation. This is called pulse echo
system. Probe houses the piezo electric crystal.
Types of probes:
Probes are mainly three
types: Normal
or straight beam probes and Angle beam probes.
1. Normal or straight
beam probes:
Probes
which transmit a perpendicular sound beams are known as Normal
beam probes. Because they sent sound pulses into the test specimen which are
normal, or so called perpendicular to the surface of test specimen. Most of the
standard normal beam probes transmit and receive LONGITUDIONAL WAVE pulses, they
are PRESSURE WAVE PULSES. Which propagate in material in the form of zones of
compression and rarefaction? The normal beam probe has frequencies from 0.5MHZ
to 15MHZ. for ultrasonic testing of supporting roller shafts we normally use
2MHZ.
Fig: - ultrasonic flaw detector
2. Angle beam probes:
Probes which transmit
their sound beam at an angle are known as angle beam probes. Because, they send
and receive the sound pulses into and form the test specimen at a certain angle
the surface. Most of the standard angle beam probes transmit and receive
transverse waves which are characterized by particle displacement perpendicular
to the propagating direction of transverse waves.
3. Dual probes:
When
detecting discontinuities which are close to the surface angle beam probes
encounter the same difficulties as comparable to normal beam probes. For this
reason there are TR-probes (transmit/receive-probes) contrary to the standard
normal beam probes the TR-probes have two crystals, one of which is purely the
transmitter, and the other purely the receiver. The complete isolation of the
transmitting and receiving function enables the crystal to be mounted in probe
fairly far away from the surface of the test specimen. The long delay path
keeps the transmission pulse away from surface so that it can no longer have an
interfering effect on the screen instead of this problem. There is a “cross
talk echo” caused by orations of sound, which coming from the transmitting
crystal, are reflected at the interface probe/test specimen and reach the
receiving crystal.
As
a rule, the cross talk echoes are very small, and have hardly any adverse
effect on the detectability of discontinuities which lies just under the
surface object under examination shall be free from rust, dirt, loose scale,
unevenness, spatters etc, which may impair beam transmission surface condition
shall be achieved either by grinding or machining. Rolled and extruded products
will be determined based on the knowledge of the product formation and
orientation of discontinuities likely to occur.
Calibration
blocks require basically to setup the depth range of machine as well as to set
sensitivity of the system to detect flaws. Sensitivity is defined as the
ability of the system to detect smallest discontinuity. Applicable
codes/standards/specification will specify the sensitivity requirement.
Accordingly calibration blocks containing holes/notches will simulate flaws. Calibration
blocks or reference blocks are use to have a reproducible result. Various
reference blocks used for calibration, water or grease are oil is used as compliant
to remove air gap between probe and material.
TABLES: - VELOCITY & ACOUSTIC
IMPEDENCE
S.NO
|
MATERIALS
|
VELOCITY(m/s)
LONGITUDIONAL
|
SHEAR
|
ACOUSTIC
IMPEDANCE
|
1
|
ALLUMINUM
|
6320
|
3080
|
17
|
2
|
IRON(STEEL)
|
5900
|
3230
|
46.5
|
3
|
S.STEEL(302)
|
5660
|
3120
|
45.5
|
4
|
S.STEEL(410)
|
7390
|
2990
|
56.7
|
5
|
CAST IRON
|
3500-5600
|
2200-3200
|
25-40
|
6
|
COPPER
|
4700
|
2260
|
42
|
7
|
BRASS
|
3830
|
2050
|
33
|
8
|
NICKLE
|
5630
|
2960
|
50
|
9
|
PERSPEX
|
2730
|
1430
|
32
|
10
|
WATER
|
1483
|
-
|
15
|
11
|
OIL
|
1410
|
-
|
1.12
|
12
|
RUBBER(VVLCANIZED)
|
2300
|
-
|
2.5-3.7
|
13
|
GLYCERION
|
1920
|
-
|
2.4
|
14
|
QUARTZ
|
5730
|
-
|
15.2
|
15
|
NYLON
|
18OO-2200
|
-
|
1.8-2.7
|
16
|
TEFLON
|
1350
|
-
|
3.0
|
17
|
LUCITE
|
2670
|
1120
|
3.2
|
18
|
GLASS
|
5770
|
3430
|
14.4
|
19
|
TUNGSTEN
|
5180
|
2870
|
19.8
|
20
|
TITANIUM
|
6100
|
3120
|
27.5
|
21
|
LEAD(PURE)
|
2160
|
700
|
24.5
|
Scope in industry:-
1.
To detect defects without in any way disturbing the
usefulness of the product.
2.
To save time and money by weeding out defective raw
material/ parts at the incoming inspection stage itself instead of accepting
and paying for it.
3.
To detect defects that may occur during the
manufacturing process before expending time and money an further processing of
the defective parts.
4.
To improve the manufacturing techniques by inspecting
the product during processing operation to maintain uniform quality and
standard.
5.
To ensure prevention of accidents and promote safety
for workmen and equipment during operation.
Since,
UT can detect both surface and inside defect it must ensure continued safe
operation of machinery and structural components during operation and
maintaince.
Advantages of
ultrasonic:-
1. High penetrating power, which allows the detection
of flaws deep in the part.
2. High
sensitivity, permitting the detection of extremely small flaws.
3. Only one
surface need be accessible.
4. Greater
accuracy than other nondestructive methods in determining the depth of internal
flaws and the thickness of parts with parallel surfaces.
5. Some
capability of estimating the size, orientation, shape and nature of defects.
6. Non hazardous to operations or to nearby
personnel and has no effect on equipment and materials in the vicinity.
7. Capable
of portable or highly automated operation on metals, nonmetals and composites
8. Surface and slightly subsurface flaws can be
detected
9. Can be applied to welds, tubing, joints,
castings, billets, forgings, shafts, structural components, concrete, pressure
vessels, aircraft and engine components
10. Used to determine thickness and mechanical
properties
11. Monitoring service wear and deterioration
Disadvantages:-
1. Depending on
operations skill. Therefore experience and integrity are essential testing becomes difficult at
elevated temperature calling for special transducer and couplant.
Less sensitive
for thin weld joints.
2. It requires
theoretical and practical training and skill before it can be effectively
applied.
3. Small or thin
parts are difficult to inspect and interpret by UT.
4. It may give
rise to ambiguous signals as a result of multiple reflection and geometric
complexity, which may cause to inhibit proper interpretations.
5. Special
probes are required for applications.
6. Parts are
rough, irregular in shape, very small or thin, or not homogeneous are difficult
to inspect.
Applications:-
1. Application of materials:-All metals, non-metals,
composites, ceramics, wood, plastics.
2. Process application:-castings, forging, welding,
fabrication, rolling, heat treatment, quality control.
3. Diagnostic
application:-Degradation of materials in-service, monitoring of thermal and
atomic power plants, life assessment of power plants, equipment pipelines and
structures.
4. Manufacturing
process application:-pressure vessels, heat exchangers, aeroplane, railway
rolling stock, nuclear reactors and other component.
5. Research and
development is going to develop methods and techniques for testing of all kinds
of material. Efforts are being made to replace RT for UT especially for thick
welded joints in national and international standards.
5. Radiographic
testing: -
Radiography
is one of the widely used NDT methods for assessing fabrication-welding and
casting. Radiographic Testing (RT), or industrial radiography, is a
nondestructive testing (NDT) method of inspecting materials for hidden flaws by
using the ability of short wavelength electromagnetic radiation (high energy
photons) to penetrate various materials. Since the amount of radiation emerging
from the opposite side of the material can be detected and measured, variations
in this amount (or intensity) of radiation are used to determine thickness or
composition of material. Penetrating radiations are those restricted to that
part of the electromagnetic spectrum of wavelength less than about 10
nanometers. The extra feature of these methods is films of the objects being
tested can be produced.
Fig: - Schematic diagram of radiographic testing
Radiography involves a
source of radiation, object to be evaluated and a sensing medium, usually film.
When radiation passes through the object, differential absorption by the flaw
and the surrounding material causes different amount of radiation to fall on
film, this latent image become visible, when the film is processed in chemicals
and dried.
Radiation
sources:
Two types of
electro-magnetic radiation are used in radiographic inspection
1.
X-rays
2.
Gamma- rays
1. X- ray method:-
In this method x rays of high frequency are used for inspection.
The instrument is as shown in fig.
Fig: - radiographic testing
machime
Benefits
Metals, nonmetals, composites
and mixed materials can be tested. Used on all shapes and forms; castings,
welds, electronic assemblies, aerospace, marine and automotive components.
Limitations
Access to both sides of test
piece needed. Voltage, focal spot size and exposure time critical radiation
hazards. Cracks must be oriented parallel to beam for detection. Sensitivity
decreases with increasing thickness.
The
radiation used in radiography testing is a higher energy (shorter wavelength)
version of the electromagnetic waves that we see as visible light. The
radiation can come an x-ray generator or a radioactive source.
The part is placed between the
radiation source and a piece of film. The part will stop some the radiation.
Thicker and more dense area will stop more of the radiation. The film darkness
(density) will vary with the amount of radiation reaching the film through the
object
2. Gamma Ray method: -
Benefits
Usually used on denser or thick
material. Used on all shapes and forms; castings, welds, electronic assemblies,
aerospace, marine and automotive components. Used where thickness or access
limits X-ray generators.
Limitations:
-
There some radiation chances of
hazards. Cracks must be oriented parallel to beam for detection. Sensitivity
decreases with increasing thickness. Access to both sides of test piece needed.
Not as sensitive as X-rays.
X-rays
and gamma-rays differ from other types of electro-magnetic radiation (such as
visible light, microwaves and radio waves) only in their wavelengths.
The
examination principle is X-rays and radioisotope inspection are the same, but
the radiation is different in Gamma and X-ray darkens a photographic film in
the same as visible light. Gamma and X-ray are electromagnetic radiations just
as rays of light and they differ from visible light only in their respective
wavelength. Because of their shorter wavelengths Gamma rays are able to
penetrate material opaque to visible light.
The
metal object containing internal cracks gas cavities (or) inclusions of slag is
inspected by telling the rays pass through it and form a radiographic (actually
a shadow graph) on a photographic film placed behind the object.
The
principle is identical with that used by the material and dental professions in
the radiography of the body and of the teeth. The bright areas are where the
radiation has moved through the thickest particles (welding). The darken area
show where the rays has passed thinner material thickness as in the case with
gas porosities.
Advantages:
1. Volumetric
inspection.
2. Can detect
surface and subsurface flaws.
3. Permanent
records.
4. Good quality control method.
Disadvantages:-
1. Compared to other NDT
methods, radiography is expensive.
2. Relatively large capital costs
and apace allocations are required for a radiographic laboratory.
3. Field
testing of thick sections is a time consuming process.
4. High activity sources require heavy
shielding for protection of personnel.
5. Minute discontinuities such as
inclusions in wrought material, flakes, micro- porosity and micro-fissures
cannot be detected unless they are sufficiently segregated to yield a detectable
gross effect.
6. It is well known that large
doses of X-rays or gamma rays can damage skin and blood cells, can produce
blindness and sterility, and in massive doses can cause severe disability or
death. Protection of personnel not only those engaged in radiographic work but
also those in the vicinity or radiographic testing is of major importance.
7. Safety requirements impose both
economic and operational constraints on the use of radiography for testing.
8. .The location of the defect is not possible
to determine in the thickness directions.
Applications:-
1.
Radiographic testing is used extensively on castings
and welding.
2.
Radiography is
well suited to the testing of semiconductor devices for cracks, broken wires,
unsoldered connections, foreign material and misplaced components.
3.
The sensitivity of radiography to various types of
flaws depends on many factors, like type of material, type of flaw and product
form.
4.
Both ferrous
alloys can be radio graphed, as can non-metallic materials and composites.
6. Infrared and Thermal Testing: -
Infrared thermography is the
science of measuring and mapping surface temperatures. An Infrared
thermographic scanning system can measure and view temperature patterns based
upon temperature difference as small as a few hundredths of a degree Celsius.
Fig: - A report on infrared & thermal testing on raw mill
motors
Infrared thermographic testing
may be performed during day or night, depending on environmental conditions and
the desired results. Infrared thermography, a nondestructive, remote sensing technique
has proved to be an effective, convenient, and economical method of testing
concrete. It can detect internal voids, delaminations, and cracks in concrete
structures such as bridge decks, highway pavements, garage floors, parking lot
pavements, and building walls.
As a testing technique, some of
its most important qualities are that
(1) It is accurate;
(2) It is repeatable;
(3) It doesn't cause any inconvenience to the public;
and
(4) It is economical.
7. Eddy current
testing:-
Eddy-current testing uses
electromagnetic induction to detect aw in conductive materials. It is used to
detect near-surface cracks and corrosion in metallic objects such as tubes and
air-craft fuselage and structures. ECT is more commonly applied to non
ferromagnetic material. Since, in ferromagnetic materials the depth of
penetration is relatively small.
Fig: - Schematic diagram of eddy
current testing
In a standard eddy current
testing a circular coil carrying current is placed proximity to the test
specimen (electrically conductive).The alternating current in the coil
generates changing magnetic field which interacts with test specimen and
generates eddy current. Variations in the phase and magnitude of these eddy
currents can be monitored using a second 'search' coil, or by measuring changes
to the current owing in the primary 'excitation' coil. Variations in the
electrical conductivity or magnetic permeability of the test object, or the
presence of any flaws, will cause a change in eddy current ow and a
corresponding changes in the phase and amplitude of the measured current. This
is the basis of standard (at coil) eddy current inspection, the most widely
used eddy current technique. However, eddy-current testing can detect very
small cracks in or near the surface of the material, the surfaces need minimal
preparation, and physically complex geometries can be investigated. The testing
devices are portable, provide immediate feedback, and do not need to contact
the item.
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