Manufacturing Operation	Product OR Process	Possible Defects Produced
Foundry	Cast Ingots	Center line pipe, Inclusion voids, Porosity.
	Sand Or Dye Casting	Shrink cracks, Inclusion voids, Porosity, Cold shuts etc.
Forge shop	Billet Forging	Burts, Laps, Cracks, Inclusions, Flash line cracks etc.
Mill	Extrusions	Inclusions, Pipes, Seams, Laps, Die drag etc.
	Plate or Sheet	Segregation, Laminations, Shearing separation of edges.
	Bar, Wire, Rod	Inclusions, Pipes, Seams, Laps, Cracks.
	Tubing & Pipe	Inclusions, Seams, Cracks.
Assembly OR Finishing	Welding	Porosity, Voids, Cracks, Incomplete fusion, and Penetration.
	Grinding	Cracks.
	Machining	Tears, Cracks, Thickness variation.
	Heat Treating	Cracks,Grain size or metallurgic variations, Surface oxidation etc.
	Plating,	Cracks, Embrittlement, Peeling.
	Bonding(adhering, Braze, diffusion)	Lack of bond, Voids, Cracks.
	Forming, Pickling	Cracks, Folds, Stress cracks.
In Service Cycle	All Products	Fatigue cracks, Overload cracks, Stress corrosion, Galling pits, Debends, Torsional overload etc.

 

NDT tests are made directly upon the objects to be in service and hence there is no doubt that the tests were made on representative test objects.
Tests can be made on every unit to be used in service (If Economically Justified); consequently they may be used validly even when great differences from unit to unit occur in production lots.
Tests are made if desired on the entire production parts; consequently the evaluation applies to the part as a whole.
Many Non Destructive tests, each sensitive to different properties of the material or part, may be applied simultaneously or sequentially to measure as many properties correlated with service performance as may be desired.
NDT may be applied to parts in service often without interruption of service and with no loss of serviceable parts.
Acceptable parts of very high fabrication costs are not lost in testing, consequently, extensive testing during service is possible.
Little or no specimen preparation is required in many NDT tests. Several forms of NDT equipments are portable and are capable of rapid sorting and testing. The cost of NDT in most cases is far less than the cost of adequate Destructive tests except X-Ray inspection of low fabrication cost items.
Most NDT methods are much more rapid and require far fewer man hours than typical Destructive tests and hence economical.

Visual Testing is very important, though neglected by many NDT Personnel.
Visual Inspection is a nondestructive testing technique that provides a means of detecting and examining a variety of surface flaws, such as corrosion, contamination, surface finish, and surface discontinuities on joints(for example, welds, seals, solder connections, and adhesive bonds). Visual inspection is also the most widely used method for detecting and examining surface cracks, which are particularly important because of their relationship to structural failure mechanisms.
Even when other nondestructive techniques are used to detect surface cracks, visual inspection often provides a useful supplement. For example, when the eddy current examination of process tubing is performed, visual inspection is often performed to verify and more closely examine the surface disturbance.
Given the wide variety of surface flaws that may be detectable by visual examination, the use of visual inspection may encompass different techniques, depending on the product and type of surface flaw being monitored.
The methods of visual inspection involve a wide variety of equipment, ranging from examination with the naked eye to the use of interference microscopes for measuring the depth of scratches in the finish of finely polished or lapped surfaces.
Some of the equipments used to aid visual inspection includes :
Flexible or rigid borescopes for illuminating and observing internal, closed or otherwise inaccessible areas.
Image sensors for remote sensing or for the development of permanent visual records in the form of photographs, videotapes, or computer-enhanced images.
Magnifying systems for evaluating surface finish, surface shapes(profile and contour gagging), and surface microstructures.
Dye and fluorescent penetrants and magnetic particles for enhancing the observation of surface cracks (and sometimes near-surface conditions in the case of magnetic particle inspection).

Flaw detection process utilizing dye penetrants are extensively used in Aircraft industry. This is a logical development of the oil and chalk method popular in the early days. Briefly the method consists of immersing the part to be tested in a liquid penetrant wiping the excess panetrant and then applying a developer to bring back the dye which seeped into the flaw open to surface.
The basic principle of penetrant inspection is that when a penetrant is applied over a clean surface to be inspected by the combined action of surface tension and capillary action penetrant seeps into defect which when developed, by the blotting action of the developer powder clearly shows up on a white back ground and can be conveniently inspected.
The following are the basic stages on dye-penetrant method:
Surface preparation: The surface on the part to be tested is thoroughly cleaned to remove dirt.
Application of penetrant: The penetrant is a liquid of a very low viscosity, carrying a dye - generally red or a fluorescent penetrant which easily inters into flaws because of low surface tension and capillary action.
The dye - penetrant may be applied on the parts by the following methods:
a. Dipping or immersing,
b. Spraying,
c. Brushing,
d. Pouring.
Excess penetrant removal: After the parts stand in the penetrant for desired time it is washed in clean running water & dried with air.
Developer application: The developer (dry or wet) is applied by evenly spreading or dusting over the part.
The developer may be chalk powder of very fine grade or wet developer which is a fine chalk powder suspended in alcohol or spirit.
Inspection: As the alcohol or spirit in the developer evaporates, the chalk powder draws back penetrant to the surface because of blotting properties of the chalk. This shows up as a red line on a white background.
In case of fluorescent penetrant inspection, the part is inspected in a dark enclosure under ultra violet or black light. The blotted out fluorescent particles will give a visible glow under ultra violet light.
Fluorescent penetrant inspection is very effective for machined machined and finished parts.
Dye penetrant inspection is suitable for rough cast surfaces and finished surfaces.

Magnetic Particle Inspection is a method of locating surface and subsurface discontinuities in ferromagnetic materials.
It depends on the fact that when the material or part under test is magnetized, magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The presence of this leakage field, and therefore the presence of the discontinuity, is detected by the use of finely divided ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field.
This magnetically held collection of particles forms an outline of the discontinuity and generally indicates its location, size, shape, and extent. Magnetic particles are applied over a surface as dry particles, or as wet particles in a liquid carrier such as water or oil.
Ferromagnetic materials include most of the iron, nickel, and cobalt alloys. Many of the precipitation-hardening steels, such as 17-4 PH, 17-7 pH, and 15-4 pH stainless steels, are magnetic after aging. These materials lose their ferromagnetic properties above a characteristic temperature called the Curie point. Although this temperature varies for different materials, the Curie point for most ferromagnetic materials is approximately 760 degree C (1400 degreeF).
Advantages :
The magnetic particle method is a sensitive means of locating small and shallow surface cracks in ferromagnetic materials. Indications may be produced at cracks that are large enough to be seen with the naked eye, but exceedingly wide cracks will no produce a particle pattern if the surface opening is too wide for the particles to bridge.
Discontinuities that do not actually break through the surface are also indicated in many cases by this method, although certain limitations must be recognized and understood. If a discontinuity is fine, sharp, and close to the surface, such as a long stringer of nonmetallic inclusions, a clear indication can be produced. If the discontinuity lies deeper, the indication will be less distinct. The deeper the discontinuity lies below the surface, the larger it must be to yield a readable indication and the more difficult the discontinuity is to find by this method.
Magnetic particle indications are produced directly on the surface of the part and constitute magnetic pictures of actual discontinuities. There is no electrical circuitry or electronic readout to be calibrated or kept in proper operating condition. Skilled operators can sometimes make a reasonable estimate of crack depth with suitable powders and proper technique. Occasional monitoring of field intensity in the part is needed to ensure adequate field strength.
There is little or no limitation on the size or shape of the part being inspected. Ordinarily, no elaborate precleaning is necessary, and cracks filled with foreign material can be detected.

Ultrasonic denotes high frequency sound waves beyond the audible range generally above 20,000 cycles per second.
Generation of Ultrasonic waves is done by means of Magnetostriction, or Piezoelectric Effect.
The waves are made to pass the material under checking through a coupling media. The waves propagate into the material in the form of bean resembling a solid cone diverging steadily from the source of generation.
If the material is perfectly homogenous, the energy travels up back to the surface of specimen and is reflected and returns to the transducer. The transducer at once changes this mechanical energy into electrical energy which after amplification is routed through different electronic circuits. It ultimately reaches a vertical plate of cathode-ray tube in the form of unidirectional voltage. This is indirected by a pipe on the time base which is incorporated in between the horizontal plates of CPT to reckon the time of travel of the Ultrasonic energy into the material.
A part of the electrical pulse is fed into the vertical plates of the CRT simultaneously with the same being applied to the transducer. This gives the transmission echo on the time base which the starting point for measuring time of interval on the calibrated scale.
If now a discontinuity exists on the path of the Ultrasonic energy, part of it will be reflected and part of it will be transmitted depending upon the acoustic impendances of the material and the flaw. The energy reflected from the flaw and that from the back surface, having traveled different path lengths shall be indicated on the CRT screen by the two different echoes on the time base, the space between the echoes giving the difference of path traversed.
Graduations on the time base scale when calibrated with reference to the material under checking shall therefore readily give us the depth of flaw as well as thickness of the material.

Radiographic examination involves using the property of rectilinear propagation of electro magnetic waves such as both X - rays and Gamma Rays.
In aeronautic industry only X - ray examination find good application. The quality or penetrating power of X - rays increases with kilovoltage applied across the filament and target and this depends purely on the density and thickness of the material to be penetrated.

Principle of (X - ray) Radiographic Examination :
The intensity of a beam of X - rays or rays undergoes local attenuation as it passed through a heterogeneous body, due to the absorption and scattering of radiation by the object concerned.
As a result the beam emerging from the object forms at the surface of the film areas of differing intensity which together make up the radiation image.
The image obtained when the film is developed is a negative reproduction of the radiation image. The film produces a two dimensional shadow graphic projection of three dimensional part under examination.

Interpretation of X - ray film is only distinguishing between areas of different density or film blackness.
Black areas represent less absorption of X - rays due to presence of defects compared to adjacent homogeneous areas of less black areas.
Whereas light areas in a homogeneously back area show some foreign metals of high absorption.
The shadow graph is purely due to selective absorption of X - rays by matters in its path.

Radiographic examination finds many application in inspecting casting, welding, structures etc. for internal soundness, process control checks, development of new casting and process technology, in evaluation of internal assemblies like electronic circuitry, injection nozzle continuity of filament inside assay.
X - rays inspection plays vital role in inspecting areas without disassembly which otherwise warrants disassembly involving labour and time.

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