REVIEW OF FABRICATION TECHNOLOGY
WHAT IS FABRICATION ?
Manufacturing process in which an item is made
(fabricated) from raw or semi-
finished materials instead of being assembled
from ready-made components or parts.
In Fabrication technology, there are 2 main parts :-
1.DT ( Destructive Technique )
2.NDT ( Non Destructive Technique)
DESTRUCTIVE TESTING
In destructive testing, tests are carried out to the specimen's
failure, in order to understand a specimen's structural
performance or material behaviour under different loads.
These tests are generally much easier to carry out, yield
more information, and are easier to interpret
than nondestructive testing.
Destructive testing is most suitable, and economic, for
objects which will be mass-produced, as the cost of
destroying a small number of specimens is negligible.
Itis usually not economical to do destructive testing where
only one or very few items are to be produced (for example, in
the case of a building).
Some types of destructive
testing:
Stress tests
Crash tests
Hardness tests
Metallographic tests
STRESS TESTING :-
Stress testing (sometimes called torture testing) is a form of
intense or thorough testing used to determine the stability of a given
system or entity.
It involves testing beyond normal operational capacity, often to a
breaking point, in order to observe the results.
Tension test, compression test, shear test, bending test, torsional test
are the stress testing methods where values of tensile stress,
compressive test, shear stress, bending stress, torsional stress are
evaluated.
Reasons can include:
To determine breaking points or safe usage limits
To confirm intended specifications are being met
To determine modes of failure
(how exactly a system fails)
To test stable operation of a part or system outside standard
usage.
CRASH TESTING :-
To test (a new product) for safety and reliability by finding
out its breaking point under pressure, heat, etc
The act of crashing under controlled conditions in order to
assess its safety for passengers
For Eg.: New Car
HARDNESS TESTING :-
Hardness is the property of a material that enables it to resist
plastic deformation, usually
by penetration. However, the term hardness may also refer to
resistance to bending,
scratching, abrasion or cutting.
Hardness is a measure of how resistant solid matter is, to various kinds of
permanent shape change when a force is applied.
METALLOGRAPHIC TESTS :-
A test to determine the structural composition of a
metal at low and high magnification and by x-ray diffraction
methods; tests include macroexamination,
microexamination, and x-ray diffraction studies.
Ceramic and polymeric materials may also be prepared
using metallographic techniques
NON DESTRUCTIVE TESTING :-
Quality control method that does not damage or
destroy the material or product being tested.
NDT is examination of an object in any manner
which will not damage in future.
It is carried out to locate the internal defects of
material. Thus NDT give reliability accuracy location
and appearance of defects.
Detection of surface flaws
Visual
Magnetic Particle Inspection
Fluorescent Dye Penetrant Inspection
Detection of internal flaws
Radiography
Ultrasonic Testing
Eddy current Testing
Visual Inspection
Defects which are easily located by visual inspection are
surface cracks, roughness swells, tears etc.
Visual Inspection is carried out with the naked eye or using a
magnifying glass.
An inspector carrying out visual examination identifies costing
defects and assign their cause to same foundry operation.
Visual inspection is simplest, fastest & almost commonly
employed, but it needs greater skill on the part of the inspector
to locate and identify defects.
Only visual inspection may be sufficient for many castings
such as manhole covers, drains, counter balance weight etc.
RADIOGRAPHIC TEST :-
High frequency radiations of constant intensity are made to fall
on the component to be tested.
Some of radiations which passing through the component are
absorbed by defects & remaining radiations emerges out from
the other side of the metal component.
The intensity of transmitted radiations are usually recorded on
photographic film.
It may be noted that the less darker region represents the
perfect portion.
The image developed on film is called Radiography.
This technique is know as radiographic technique.
The part is placed between the radiation
source and a piece of film. The part will
stop some of 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
X-ray film test object.
•Defects, such as voids, cracks,
inclusions, etc., can be detected.
= less exposure
= more exposure
Top view of developed film
This method is used to detect defects in material which
with stand high pressure & temperature.
Eg. REACTOR, PRESSURE VESSEL Etc.
Advantages :-
1) High sensitivity
2) Permanent record on film
3) Adjustable energy level
Disadvantages :-
1) High initial cost
2) Power source required
ULTRASONIC TESTING :-
Ultrasonic inspection is employed to detect and locate defects
such as cavities ,internal bursts or cracks etc.
For operation ultrasonic wave is introduced into the metal and
the time interval between transmission of outgoing waves and
incoming waves are measured with the help of CRO.
In case of defects exists between the top & bottom casting
surface, most of the waves striking this defect will get reflected
from defect, and will reach the receiver probe & indicate a echo
on CRO screen.
This technique is know as Ultrasonic testing.
Advantages:-
1) It is fast & reliable method of NDT
2) This method of locating flaw is more sensitive than radiography.
3) Low speed and high speed of operation
Disadvantages :-
1) Very sensitive to rough surfaces
2) In complex casting the interpretation of oscilloscope traces may not be
easy.
MAGNETIC PARTICLE INSPECTION (MPI) (ASTM E 709-80) :-
This method of inspection used an magnetic ferrous
casting for detecting invisible surface or subsurface
defects.
When a material is placed within a magnetic field, the
magnetic forces of the material's electrons will be affected. This
effect is known as Faraday's Law of Magnetic Induction.
Deeper subsurface defects are not satisfactorily detected
because the influence of the distorted lines of magnetic
flux on the magnetic particles spread over the casting
surface becomes weaker with the distance.
The defects commonly are thermal crack, grinding
cracks, overlaps, fatigue cracks etc.
This method is very simple and easy technique.
Diamagnetic, Paramagnetic, and
Ferromagnetic Materials
Diamagnetic metals: very weak and negative susceptibility
to magnetic fields. Diamagnetic materials are slightly
repelled by a magnetic field and the material does not retain
the magnetic properties when the external field is removed.
Paramagnetic metals: small and positive susceptibility to
magnetic fields. These materials are slightly attracted by a
magnetic field and the material does not retain the magnetic
properties when the external field is removed.
Ferromagnetic materials: large and positive susceptibility
to an external magnetic field. They exhibit a strong
attraction to magnetic fields and are able to retain their
magnetic properties after the external field has been
removed.
Unmagnetized material
Magnetized material
Advantages :-
1. Economical & Portable
2. Simple and easy to perform
3. Fast process
Disadvantages :-
1. Material tested must be magnetic
2. Demagnetization is necessary
Applications :-
1. Applicable to all ferromagnetic material
Surface penetrate liquid method (LPI) (ASTM E165-
80 ))
liquid penetrant inspection (LPI) or penetrant testing (PT), is a
widely applied and low-cost inspection method used to locate
surface-breaking defects in all non-porous materials (metals,
plastics, or ceramics).
The penetrant may be applied to all non-ferrous materials and
ferrous materials, although for ferrous components magnetic-
particle inspection is often used instead for its subsurface
detection capability.
LPI is used to detect casting, forging and welding surface
defects such as hairline cracks, surface porosity, leaks in new
products, and fatigue cracks on in-service components.
Dye penetrants Fluorescent penetrants
The liquids are coloured so that Liquid contain additives to
they provide good contrast give fluorescence under UV
against the developer Object should be shielded from
Usually red liquid against white visible light during inspection
developer Fluorescent indications are
Observation performed in easy to see in the dark
ordinary daylight or good
indoor illumination
Standard: Aerospace Material
Specification (AMS) 2644.
Basic processing steps of LPI
1. Surface Preparation: One of the most critical steps of a liquid penetrant
inspection is the surface preparation. The surface must be free of oil,
grease, water, or other contaminants that may prevent penetrant from
entering flaws. The sample may also require etching if mechanical
operations such as machining, sanding, or grit blasting have been
performed. These and other mechanical operations can smear the surface
of the sample, thus closing the defects.
2. Penetrant Application: Once the surface has been thoroughly cleaned
and dried, the penetrant material is applied by spraying, brushing, or
immersing the parts in a penetrant bath.
3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time
to allow as much penetrant as possible to be drawn from or to seep into a
defect. The times vary depending on the application, penetrant materials
used, the material, the form of the material being inspected, and the type
of defect being inspected. Generally, there is no harm in using a longer
penetrant dwell time as long as the penetrant is not allowed to dry.
Excess Penetrant Removal: This is the most delicate part of the
inspection procedure because the excess penetrant must be removed
from the surface of the sample while removing as little penetrant as
possible from defects. Depending on the penetrant system used, this
step may involve cleaning with a solvent, direct rinsing with water,
or first treated with an emulsifier and then rinsing with water.
Developer Application: A thin layer of developer is then applied to
the sample to draw penetrant trapped in flaws back to the surface
where it will be visible. Developers come in a variety of forms that
may be applied by dusting (dry powdered), dipping, or spraying
(wet developers).
. Indication Development: The developer is allowed to stand on the
part surface for a period of time sufficient to permit the extraction of
the trapped penetrant out of any surface flaws. This development
time is usually a minimum of 10 minutes and significantly longer
times may be necessary for tight cracks.
Inspection: Inspection is then performed under appropriate lighting to
detect indications from any flaws which may be present.
Clean Surface: The final step in the process is to thoroughly clean
the part surface to remove the developer from the parts that were
found to be acceptable.
ADVANTAGES :-
1. The method has high sensitivity to small surface
discontinuities.
2. The method has few material limitations, i.e. metallic and
nonmetallic, magnetic and nonmagnetic, and conductive and
nonconductive materials may be inspected.
3. Large areas and large volumes of parts/materials can be
inspected rapidly and at low cost.
4. Parts with complex geometric shapes are routinely inspected.
5. Indications are produced directly on the surface of the part
and constitute a visual representation of the flaw.
Disadvantages :-
1. Only surface breaking defects can be
detected.
2. Only materials with a relatively nonporous
surface can be inspected.
3. Precleaning is critical since contaminants
can mask defects.
4. Post cleaning of acceptable parts or
materials is required.
5. Chemical handling and proper disposal is
required.
Eg. Plastics, vessels, tanks etc.
FATIGUE ASSESSMENT OF VESSELS
IMPORTANT DUE TO VESSELS MAY BE SUBJECTED TO
CYCLIC LOADS AND STRESSES.
CYCLING CAUSES DUE TO :
FLUCTUATIONS IN OPERATING CONDITIONS
VIBRATIONS
LOADING DUE TO FLOW OF LIQUIDS AND SOLIDS
AND FLUCTUATIONS
FLUCTUATIONS DUE TO EXTERNAL LOADING
FATIGUE ASSESSMENT
PRESSURE TEST
Test pressure=1.30 [P*fa/fd* t/(t-c)]
WHERE: P= Design pressure, N/mm2
Fa, fd= Maximum allowable stress at test and design
coditions, N/mm2
c= Corrossion allowance, mm
t= actual thickness of vessel, mm
TYPES:
HYDRO PRESSURE TEST (WATER)
PNEUMATIC PRESSURE TEST (AIR)
