I. INTRODUCTION:

The manufacturing company structure has changed from a labour intensive industry to a technology-intensive, i.e. capital intensive, industry. Many changes in the internal environment of the companies are taking place: increased use of mechanisation and automation of operations such as flexible manufacturing systems (FMS), robots, automatic warehousing, and automatic guided vehicles (AGVs); increased trends of using just-in-time (JIT), NDT (Non Destructive testing) etc.

It is fact that there are inherent flaws in materials due to crystal lattice imperfections and dislocations however microscopic they may be. Manufacturing processes such as welding, casting, forging, surface treatment, etc. may cause further flaws or defects[1]. Materials are used under various conditions of stress, fatigue and corrosion, which may create additional defects or aggravate present ones. It has been established that most material failures occur because these defects reach dangerous proportions such that remaining parts of the materials could not withstand the stress they are subjected to, thus become ductile or brittle. There is, therefore, a need to detect these flaws and evaluate them in terms of their nature, size and location. Further steps should be to assess:

(a) How severe and dangerous the flaws are in their present state,

(b) Whether they need to be removed by repairing the tested component,

(d) With known flaws, if the product can be allowed to go into service.

These are done through inspection and testing. One method of inspection is to subject the material or weld to destructive tests, which would provide information about the performance of that test object. The disadvantage of destructive testing is that, as the name implies, the test object is destroyed in the process. Therefore, testing methods have been developed to provide the information required of the test object without rendering it unfit for service. These methods are referred to as non-destructive tests because they permit evaluation of the material or component without destroying it. Destructive testing of parts can be expensive and assumes that the untested parts are of the same quality as those tested. Non-destructive tests give indirect yet valid results and, by definition, leave the test object fit for its intended use.

By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suit ability for service. Whilst being a high technology concept, evolution of the equipment has made it robust enough for application in any industrial environment at any stage of manufacture – from steel making to site inspection of components already in service[2]. A certain degree of skill is required to apply the techniques properly in order to obtain the maximum amount of information concerning the product, with consequent feed back to the production facility.

II. Historical Events in NDT.

NDT techniques are being used for very early in industries [3]. Some of the historical events in NDT are as follows.

• 1880 - 1920 The "Oil and Whiting" method of crack detection is used in the railroad industry to find cracks in heavy steel parts. This was the precursor to modern liquid penetrant tests.

• 1920 Dr. H. H. Lester begins development of industrial radiography for metals.

• 1926 The first electromagnetic eddy current instrument is available to measure material thicknesses.

• 1927 - 1928 Magnetic induction system to detect flaws in railroad track developed by Dr. Elmer Sperry and H.C. Drake.

• 1930s Robert F. Mehl demonstrates radiographic imaging using gamma radiation from Radium, which can examine thicker components than the low-energy X-ray machines available at the time.

• 1935 - 1940 Liquid penetrant tests developed (Betz, Doane, and DeForest)

• 1935 - 1940s Eddy current instruments developed (H.C. Knerr, C. Farrow, Theo Zuschlag, and Fr. F. Foerster).

• 1940 - 1944 Ultrasonic test method developed in USA by Dr. Floyd Firestone.

• 1950 J. Kaiser introduces acoustic emission as an NDT method.

III. Classification of NDT Techniques.

The technique uses a variety of principles; there is no single method around which a black box may be built to satisfy all requirements in all circumstances. The various techniques are as follows [4]:

§ Acoustic emission testing (AE or AT)

§ Dye penetrant inspection Liquid penetrant testing (PT or LPI)

§ Electromagnetic testing (ET)

o Alternating current field measurement (ACFM)

o Alternating current potential drop measurement (ACPD)

o Barkhausen testing

o Direct current potential drop measurement (DCPD)

o Eddy-current testing (ECT)

o Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope

o Magnetic-particle inspection (MT or MPI)

o Remote field testing (RFT)

§ Ellipsometry

§ Guided wave testing (GWT)

§ Hardness testing

§ Impulse excitation technique (IET)

§ Infrared and thermal testing (IR)

§ Laser testing

o Electronic speckle pattern interferometry

o Holographic interferometry

o Low coherence interferometry

o Profilometry

o Shearography

§ Leak testing (LT) or Leak detection

o Absolute pressure leak testing (pressure change)

o Bubble testing

o Halogen diode leak testing

o Hydrogen leak testing

o Mass spectrometer leak testing

o Tracer-gas leak testing method Helium, Hydrogen and refrigerant gases

§ Magnetic resonance imaging (MRI) and NMR spectroscopy

§ Metallographic replicas [5] [6]

§ Near-infrared spectroscopy (NIRS)

§ Optical microscopy

§ Positive Material Identification (PMI)

§ Radiographic testing (RT) (see also Industrial radiography and Radiography)

o Computed radiography

o Digital radiography (real-time)

o Neutron radiographic testing (NR)

o SCAR (Small Controlled Area Radiography)

o X-ray computed tomography (CT)

§ Scanning electron microscopy

§ Surface Temper Etch (Nital Etch)

§ Ultrasonic testing (UT)

o ART (Acoustic Resonance Technology)

o Electro Magnetic Acoustic Transducer (EMAT) (non-contact)

o Laser ultrasonics (LUT)

o Internal rotary inspection system (IRIS) ultrasonics for tubes

o Phased array ultrasonics

o Time of flight diffraction ultrasonics (TOFD)

o Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)

§ Vibration Analysis

§ Visual inspection (VT)

§ Corroscan/C-scan

§ IRIS - Internal Rotary Inspection System

§ 3D Computed Tomography

§ Heat Exchanger Life Assessment System

§ RTJ Flange Special Ultrasonic Testing

Few of the important techniques are described below [7, 8, 9, 10]:

NDT Techniques	Advantages
Visual Inspection- Visual inspection (VT) relies upon the detection of surface imperfections using the eye. Normally applied without the use of any additional equipment, VT can be improved by using aids such as a magnifying glass to improve its effectiveness and scope. 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.	• Relative simplicity • It does not require sophisticated apparatus, it is a very inexpensive method • A further advantage of VT is that it is an ongoing inspection that can be applied at various stages of construction.
Radiography - X and Gamma- X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes, are penetrating radiation which is differentially absorbed by the material through which it passes; the greater the thickness, the greater the absorption. Furthermore, the denser the material the greater the absorption.	• A permanent record is provided which may be viewed at a time and place distant from the test. • Useful for thin sections. • Suitable for any material.
Magnetic Particle Inspection-The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity they will stay out in to the air at the mouth of the crack. The crack edge becomes magnetic attractive poles North and South. These have the power to attract finely divided particles of magnetic material such as iron fillings.	• Simplicity of operation and application. • Quantitative. • Can be automated, apart from viewing.
Dye Penetrant Testing- The subject to be examined is first of all chemically cleaned, usually by vapour phase, to remove all traces of foreign material, grease, dirt, etc. from the surface generally, and also from within the cracks. Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrant into the crack during this period. The surplus penetrant on the surface is then removed completely and thin coating of powdered chalk is applied. After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background.	• Simplicity of operation. • Best method for surface breaking cracks in nonferrous metals. • Quantative.
Ultrasonic Flaw Detection- This technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials. The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezo electric crystal, which vibrates for a very short period at a frequency related to the thickness of the crystal.	• Thickness and lengths up to 30 ft can be tested. • Position, size and type of defect can be determined. • Instant test results. • Portable. • Extremely sensitive if required.
Eddy Current and Electro-Magnetic Methods- ET can be applied on coated and uncoated objects and the testing can be carried out on all accessible surfaces on welds of almost any configuration. Usually, it can be applied in the as-welded condition. However, a very rough surface may prevent an efficient testing.	• Suitable for the determination of a wide range of conditions of conducting material, such as defect detection, composition, hardness, conductivity, permeability etc. in a wide variety of engineering metals. • Extremely compact and portable units are available. • No consumables (except probes – which can sometimes be repaired). • Suitable for total automation.
Acoustic emission testing - When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. With the right equipment and setup, motions on the order of picometers (10 -12 m) can be identified.	• The ability to discern between developing and stagnant defects is significant. • AE testing usually provides an immediate indication relating to the strength or risk of failure of a component. • Fast and complete volumetric inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and clean a specimen.
Scanning electron microscopy - The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample.	• There is arguably no other instrument with the breadth of applications in the study of solid materials that compares with the SEM. • Most SEM's are comparatively easy to operate, with user-friendly "intuitive" interfaces. Many applications require minimal sample preparation. • For many applications, data acquisition is rapid (less than 5 minutes/image for SEI, BSE, spot EDS analyses.) Modern SEMs generate data in digital formats, which are highly portable.

IV. Broad Applications of NDT.

Application with respect to product and Industries are practically unlimited. NDT Techniques can inspect a small pin to big dam structures. Some industries in which NDT plays a key role include the following Aerospace, Aircraft, Automotive, Casting & Forging, Chemical & Petroleum, Construction, Electronics, Food Processing, Marine, Fabrication, Air port Security, Nuclear, Defence, Transportation and Utilities. NDT services [11] are not only integrated with Asset Integrity Management (AIM) solutions, but also with Material Testing laboratories and seamlessly fit into Supply Chain services.

S. No	Applications	Specific Area
1.	Automotive	Engine parts Frame
2.	Aviation / Aerospace	Airframes Space frames Rocketry
3.	Powerplants	Propellers Reciprocating Engines Gas turbine engines
4.	Construction	Structures Bridges Cover Meter
5.	Maintenance, repair and operations	Bridges
6.	Manufacturing	Machine parts Castings and Forgings Fabrication Inspection
7.	Industrial plants such as Nuclear, Petrochemical, Power, Refineries, Pulp and Paper, Fabrication shops, Mine processing and their Risk Based Inspection programmes.	Pressure vessels Storage tanks Welds Boilers Heat exchangers Turbine bores In-plant Piping Full Storage tank Assessment Shutdown Inspections In-service Equipment Inspections.
8.	Miscellaneous	Pipelines · In-line Inspection using "pigs" · Pipeline integrity management · Leak Detection · Pipeline Open Data Standard · ASME Pressure Vessel and Piping as-built Inspections · Piping and Pressure Vessel Corrosion Monitoring. Railways · Rail Inspection · Wheel Inspection Tubular NDT, for Tubing material Corrosion Under Insulation (CUI) Amusement park rides Submarines and other Naval warships Medical imaging applications

V. Case Study

A process plant contained two stainless steel vessels which had been operating for 21 years. The contents of the vessels were non- toxic and contained 500 ppm of chlorides. The vessels were operated from full vacuum up to 15 psi for 20 cycles per day. They contained an agitator which was used in part of the process. Both vessels had been hydraulically tested to 70 psi when new but had not been subjected to a test since. The company philosophy was 'Leak before break' but they didn't think that stainless steel would break. No leak detection equipment had been installed and reliance was placed on plant operators noticing the smell or observing drips & External visual examination supplemented by a hammer test every 2 years. The combination of stainless steel and chlorides immediately raises concerns regarding the possibility of stress corrosion cracking. Whilst the cracks were likely to initiate on the inner surface an external examination could detect the presence of through wall cracks. However, stress corrosion cracks can be very tight and difficult to see with the naked eye. The hammer test offers no benefit because we don’t know exactly how a vessel sounds. During a thorough examination of one of the vessels the Competent Person called for a small welded repair to an external weld and for this to be followed by a hydraulic test. The vessel developed leaks at 40 psi. Further investigation of the vessel found thousands of through wall cracks. The vessel had not leaked in service because the contents were too viscous to pass through the tight stress corrosion cracks.

The maintenance personnel modified the maintenance schedule for the 2nd vessel:

· Yearly examination instead of 2 yearly.

· Addition of internal examination from the access way.

· Addition of internal Dye Penetrant examination using red dye on 10% of welds.

The Internal inspection would be carried out from the small access way with agitator still in place. The failed vessel had shown most through wall cracks in base. This region could not be inspected on the second vessel from the access way. The failed vessel showed through cracks on parent plate and most welds. There was no justification for limiting the inspection to welds only and for just inspecting 10% of them. Dye Penetrant Inspection using red dye. With the cracking on the internal surface there was a chance that the cracks may have been filled with product and if this had been the case dye Penetrant inspection would not have been effective. Stress corrosion cracking can be tight and if so the Dye Penetrant indications would not reveal the defects. Fluorescent dyes give a higher sensitivity and would give much better results in the confined, dark space of the vessel. When the maintenance personnel were consulted they estimated that the probability of detection, using the method stated, was less than 30%. In limiting the inspection to just 10% of the welds then the overall probability of detecting a crack in a weld was just 3%. This is unacceptably low. A probability of detection of only 50% may be acceptable for a regularly applied, non-critical inspection whilst for a highly critical inspection the probability of detection would need to be up near 95%. So frequency of examination raised from yearly to twice in a year than the maintenance personnel were finally consulted they estimated that the probability of detection was now become nearly 58% which was acceptable.

Role of Non Destructive Testing: NDT plays a vital role [12] in this case in the following ways

· Development of material and process for fabrication.

· Determining quantitative influence of flaws and properly variables.

· Determining Critical spots in manufacturing operations.

· Establishing and measuring quality and acceptance limits

· Failure analysis.

VI. Improving Productivity and Profitability-

It is clear from above case study the NDT is useful tool for failure prevention & analysis. It helps in determining defects in advance which if not identified results in failure of complete system hence NDT helps in improving productivity & profitability.

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 [13]. 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.

VII. CONCLUSION:

Non-destructive testing can be an effective tool in the inspection and condition assessment of industrial machinery/equipments. It can provide knowledge that may not be possible to deduce from visual observation alone. The integration of both visual and non-destructive inspection methods is key to complete industrial machinery/equipments condition assessment and management. Some simple non-destructive techniques, such as hammer sounding, rebound hammer testing, dye penetration and magnetic particle testing, and can be easily integrated into visual inspections. The results of these integrated inspections will improve maintenance data, and will yield more technically based recommendations for further inspection and maintenance, and more accurate estimations of remaining service life. Once a full representation of the overall machine condition is determined, appropriate and economical decisions regarding the possible rehabilitation or replacement of equipments can be made. Recent advances in NDE techniques have improved the functional characteristics of many NDE methods and have led to systems that are more reliable. Increased use of NDE methods will depend on several factors including the ability of the systems to accurately detect deteriorated conditions, the ease of use and portability of the systems, and the total cost of completing the NDE based inspections. Since large industries are consist of almost hundreds of different kind of machine/equipments and using so many different materials of supporting components, so that it is not possible to use just one NDT method for all tasks. Also there are many tasks which need further research to make NDT methods suitable. A lot of application reports of NDT methods for machine testing are there. Several methods are available, some are in research and some are used for further inspection after regularly inspections indicates their needs.

VII. ACKNOWLEDGEMENT:

We are thankful to Department of Mechanical Engineering of MANIT Bhopal India for providing the required facilities needed for the successful completion of this paper.

VIII. REFERENCES:

[ 1 ] “Guidebook for the Fabrication of Non-Destructive Testing (NDT) Test Specimens”, http://www-pub.iaea.org,International Atomic Energy Agency, 2001, Austria.

[ 2 ] Willcox Mark, Downes George, “A Brief Description of NDT Techniques”, Insight NDT Equipment Limited, England, (2003) pp.3.

[ 3 ] Chuck Hellier (2003), Handbook of Nondestructive Evaluation, Second Edition McGraw-Hill Companies, Incorporated 2012, ISBN 2012 0071777148, 9780071777148.

[ 4 ] “Non-Destructive Testing”, http://en.wikipedia.org/wiki/ Nondestructive_testing, 2008.

[ 5 ] ASTM E1351: "Standard Practice for Production and Evaluation of Field Metallographic Replicas" (2006).

[ 6 ] BS ISO 3057 "Non-destructive testing - Metallographic replica techniques of surface examination" (1998)

[ 7 ] Willcox Mark, Downes George, “A Brief Description of NDT Techniques”, Insight NDT Equipment Limited, England, (2003) 4-20.

[ 8 ] “Introduction to Acoustic Emission Testing”, http://www.ndt-ed.org

[ 9 ] Swapp Susan, “Scanning electron microscopy”, http://serc.carleton. edu, University of Wyoming

[ 10 ] “Non-Destructive Testing”, Det Norske Veritas, Norwey. http://exchange.dnv.com, (2011).

[ 11 ] Non-Destructive Testing, http://www.sgs.com

[ 12 ] Dr Prabhat Kumar ‘Non-Destructive Evaluation plays a vital role in industry' http://www.thehindubusinessline.com Chennai, Dec.

[ 13 ] Uday B. Kale, Advantages of NDT, http://www.kqsndt.com

Received on 17.12.2012 Accepted on 29.12.2012

Int. J. Tech. 2(2): July-Dec. 2012; Page 49-54