MA-1004

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Introduction to NDT for Metals

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1 NDT Method and Application in Materials Field| PDHengineer.com©2012 Animesh Talapatra. Portions ©2012 Decatur Professional Development, LLC All rights reserved

IntroductiontoNDTforMetalsAnimeshTalapatra

INTRODUCTION Non-Destructive Testing (NDT) is defined by the American Society for Non-destructive Testing (ASNT) as:

�The determination of the physical condition of an object without affecting that object�s ability to fulfill its intended function. Non-destructive testing techniques typically use a probing energy form to determine material properties or to indicate the presence of material discontinuities (surface, internal or concealed).�

For the purpose of this course, the terms non-destructive testing, Non-Destructive Inspection (NDI), and Non-Destructive Evaluation (NDE) will be considered to be equivalent.

NDT can be broken into several categories where it plays an important role:

Material property measurements

Inspection of Raw Products

Inspection Secondary Processing

In-Services Damage Inspection (Corrosion, Fracture, Fatigue and Creep etc.)

Quality controls as various stages of manufacturing are completed.

Characteristics of NDT are given below:

Applied directly to the product

Tested parts are not damaged

Various tests can be performed on the same product

Specimen preparation not required

Can be performed on parts that are in service

Low time consumption

Low labour cost

Major types of NDT are:

(a) Detection of surface flaws (Visual, Magnetic Particle Inspection, Dye Penetrant Inspection and Eddy current Testing).

(b) Detection of internal flaws (Radiography, Ultrasonic Testing, Thermography).


The application of non-destructive testing methods can be summarized in the following three groups:

1) Defectology of materials: Allows for the detection of discontinuities, assessment of corrosion and deterioration caused by environmental agents; determination of tensions; detection of leaks.

2) Characterization of materials: Assessment of the chemical, structural, mechanical and technological features of materials; physical properties (elastic, electrical and electromagnetic); heat transference and isotherm pathways.

3) Metrology of materials: Control of thicknesses; measurements of thicknesses on a single side, measurements of coating thicknesses; filling levels.

Notable events in early NDT

1854 Hartford, Connecticut: a boiler at the Fales and Gray Car works explodes, killing 21 people and seriously injuring 50. Within a decade, the State of Connecticut passes a law requiring annual inspection (in this case visual) of boilers.

1880 - 1920 The �Oil and Whiting� method of crack detection is used in the railroad industry to find cracks in heavy steel parts. (A part is soaked in thinned oil, and then painted with a white coating that dries to a powder. Oil seeping out from cracks turns the white powder brown, allowing the cracks to be detected.) This was the precursor to modern liquid penetrant tests.

1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In his first paper he discusses the possibility of flaw detection.

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

1924 � Lester uses radiography to examine castings to be installed in a Boston Edison Company steam pressure power plant.

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.1929 Magnetic particle methods and equipment pioneered (A.V. DeForest and F.B. Doane.)

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 The Schmidt Hammer (also known as �Swiss Hammer�) is invented. The instrument uses the world�s first patented non-destructive testing method for concrete.

1950 J. Kaiser introduces acoustic emission as an NDT method

NON DESTRUCTIVE TECHNIQUES Visual/Optical NDT: Using visual inspection methods for enclosed systems can be challenging and possibly ineffective. Methods of visual/optical NDT methods include:

1. Infrared Thermography,

2. Passive Thermography,

3. Laser Stereography,

4. Optical Holography

To enable a technician or engineer to inspect these difficult-to-see areas, a device known as a borescope is often used. There are a variety of enhanced visual/optical NDT methods available. In terms of corrosion these NDT methods are generally used to detect and measure deformations on surfaces. These deformations may be caused by pitting on the exposed surface, or by subsurface corrosion damage in built-up structure.

Magnetic particle Testing (MT): When a ferromagnetic (steel or iron) component is magnetized-by another magnet or by a coil carrying electrical current and fine iron particles are spread onto the component's surface, the particles will cling to and outline any discontinuity, since the discontinuity breaks the magnetic field. Surface corrosion cracks can be readily, and at very little expense, outlined in this way. Unless the cracks are very small, they can be detected even in the presence of corrosion products and paint. A disadvantage of MT is that it can only be used on ferrous materials and alloys, such as iron, nickel and carbon steel.

Before and after inspection pictures of cracks emanating from a hole

Dye Penetrant Testing (DPT): When a dye with low surface tension is applied to a component, it will penetrate into surface cracks and discontinuities. If the surface dye is then washed off and


the part is subsequently treated to draw the dye out of the discontinuities, the configurations of the discontinuities will be outlined, Surface corrosion cracks can readily evaluated by using dye penetrant techniques However, corrosion products tend to absorb the dye, thereby masking the cracked areas. It is therefore necessary to clean surfaces thoroughly for dye penetrant inspections. Inspection can be performed using visible (or red dye) or fluorescent penetrant materials.

Ultrasonic Testing (UT): The ultrasonic method normally utilized in non destructive testing evaluations of corrosion damage is the pulse-echo technique. Here, the distance between the surface (to which the ultrasonic - transducer is applied) and sub surface interface is determined by the time (in microseconds) required for the pulse to be reflected. The technique is very sensitive and accordingly very small interfaces can be detected by this method.

In corrosion evaluation, ultrasonic has the important advantage that access to only one side of the component is required. Also, the cost of the testing procedure is relatively low, although capable, experienced ultrasonic technicians are required if reliable results are to be expected. The major disadvantage of ultrasonic testing for corrosion damage is that the evidence is normally assessed in field by the ultrasonic technician, and the engineer must therefore rely heavily on the technician�s judgment or must accompany him on the job in order to gain experience in applying and interpreting ultra-sonic signal displays.

Most ultrasonic techniques employ frequencies in the range of 1 to 10 MHz. The velocity of ultrasonic waves traveling through a material is a simple function of the material's modulus and density, and thus ultrasonic methods are uniquely suited to materials characterization studies. In addition, ultrasonic waves are strongly reflected at boundaries where material properties change and thus are often used for thickness measurements and crack detection. Recent advances in ultrasonic techniques have largely been in the field of phased array ultrasonic, now available in portable instruments. The timed or phased firing of arrays of ultrasonic elements in a single transducer allows for precise tailoring of the resulting ultrasonic waves introduced into the test object.

Full record of examination is done in A-Scan, B-Scan, C-Scan, P-Scan, S-scan, Zip or TOFD Scan. Due to its sensitivity and sizing accuracy, TOFD (Time of Flight Diffraction) is also an excellent tool for in-service material and flaw monitoring. There are various other types of advance ultrasonic instruments such as SAFT (Synthetic Aperture Focusing Technique), ALOK and Phased array. Ultrasonic phased arrays are a novel technique for generating, receiving & imaging ultra-sound. Instead of a single transducer & beam, phased arrays use multiple ultrasonic elements & electronic time delays to create beams by constructive & destructive interference. SAFT as an imaging method offers the benefit of high-resolution defect detection accompanied by simple interpretation of the signals. However, due to the large amount of raw data, SAFT has only limited suitability as a search method. In the event of large components and unknown defect orientation, TOFD is the best choice.

Eddy current Testing (UCT): In eddy current testing, the probe (a coil with alternating current normally in the KHz frequency) is applied to the component and the effect of the presence of the component in its resistance to the field created by the coil is indicated. The


eddy current effects are sensitive to compositional and metallurgical changes as well as to thickness and discontinuities, and the results may, therefore, be difficult to evaluate unless a number of the variables are standardized. In corrosion studies, eddy current tests are devised in order to evaluate specific problems: corrosion in condenser tubes of known composition, corrosion under aircraft skin of a known composition and heat treatment, crack in the surface of a neoprene-coated maraging steel component. Once the instrument has been suitably standardized, this inexpensive testing can in some instances be performed by relatively inexperienced personnel; however, more frequently, the interpretation of the results is open to considerable question.

Radiographic Testing (RT): Radiography is also used in the fight against corrosion and especially for corrosion under insulation. Today digital radiography, and specifically computerized radiography (CR), is being increasingly specified. Radiography works on the shadow picture principle. The radiation used-X-rays, as produced by electric apparatus, or gamma rays, as emitted by radio isotopes--is partially absorbed by the object being inspected, and the shadow image it recorded on a photographic film. Accordingly, differences in metal thickness result in darkness (density) differences on the film. In corrosion studies, these density differences indicate relative amounts of metal loss.

In corrosion evaluation, radiography has the very important feature of giving the corrosion engineer a general impression of the nature, the distribution and the extent of the areas of wastage. It may be possible then, from the radiographs, to assess both the corrosion mechanism and also the amount of damage. The radiograph is a permanent record; the engineer can, therefore, assess the results of the NDT himself and not rely completely on the interpretation of his technician. The testing technique has no inherent calibration requirements and one does not really have to look for possible sources of error in evaluating results. Radiography has the additional value of giving information about the configuration of hidden components (sleeves, spindles, springs) as well as showing non-metallies which may be associated with the corrosion problem (shellfish growth, deposits, paint). The major limitation of radiography is that access to both sides of the component is required. Also, the costs of inspection may be high relative to other NDT techniques. In addition, X-rays and gamma rays are inherently dangerous and must therefore be used at inconvenient hours to avoid working personnel.

Thermographic testing (TT): Infrared and thermal testing methods are characterized by the use of thermal measurements of a test object as it undergoes a response to a stimulus. Thermal imaging cameras are the most common sensing method. Passive imaging of machinery or electronics may be used to detect hot spots indicative of problems. Imaging of test objects after the application of energy can be used to monitor the flow of heat in the object, which is a function of material properties as well as boundaries. Flash thermography techniques have been very successful in imaging disbonds and delaminations in composite parts, for example. The high cost of quality thermal cameras was previously a drawback of the IR method, but recently these have become significantly less expensive. Another significant recent advancement is the use of mechanical energy to stimulate localized heating at sub-surface discontinuities, such as cracks in metals, opening up a new field of application for the IR method.


Acoustic emission testing:

Acoustic emission is the technical term for the noise emitted by materials and structures when they are subjected to stress. Types of stresses can be mechanical, thermal or chemical. This emission is caused by the rapid release of energy within a material due to events such as crack initiation and growth, crack opening and closure, dislocation movement, twinning, and phase transformation in monolithic materials and fiber breakage and fiber-matrix debonding in composites. Acoustic emission monitoring is the analysis of the ultrasonic waves generated by dynamic events, such as deformation and cracking, occurring in the material under investigation. Thus, stress corrosion cracking, hydrogen embrittlement and corrosion fatigue can be detected.

However, acoustic emission cannot indicate the size of a defect, only whether a defect is growing or not. Acoustic emission sensors can be located at some distance from the defect, because the acoustic waves propagate easily in metallic components. Structures can therefore be monitored from relatively few fixed sensors. The location of the defect can be revealed if several sensors are fixed to the structure in a special pattern and the times of arrival of an acoustic wave recorded at the different sensors. The method has fewer problems with access and scanning than other inspection methods.

Until about 1973, acoustic emission technology was primarily employed in the non-destructive testing of such structures as pipelines, heat exchangers, storage tanks, pressure vessels, and coolant circuits of nuclear reactor plants. However, this technique was soon applied to the detection of defects in rotating equipment bearings.

Other testing: There are a number of other NDT methods that have been used for corrosion NDT. These include the Magneto-Optic Imager (MOI), a commercial device that images magnetic fields induced by a sheet current. Microwave NDT methods have been used to find corrosion under paint layers. Terahertz imaging has been used to find corrosion damage under thermal insulation tiles on the space shuttle. A number of other non-destructive testing techniques are used in corrosion evaluation: vibration analysis, spectrometric oil analysis, Infra-red testing and various visual methods. The Microwave Corrosion Detector (MCD) is a handheld battery-powered tool that operates much like an electronic stud finder. As the operator passes the MCD over a painted surface (organic coating) under which corrosion is present its display lights up to indicate the severity of the corrosion detected. Moisture and fluid detection, and composite inspection are also possible with MCD.


Selection of NDT methods for different materialsNDT Methods

Materials Clad Plate Weld Castings Forgings

weld plate T-joint Partial

T-Joint

Butt Fillet

VT All X X X X X X X X X

MT Fenomagnetic C and C-Mn/Alloy/Duplex 1)

- - X X X X X X X

PT Aluminum /Cu-Alloys/SS/Duplex2)

X - X X X X X X X

UT5) Aluminum C and C-Mn/Alloy/SS/Duplex

X X X 3) X X - X X

RT Aluminum/C and C-Mn/Alloy/SS/Duplex

- - - - - X4) - 3) 3)

ET3) All X - X X X X X 3) 3)

1) Methods is applicable with limitations for Duplex, shall be approved case by case 2) May be used for other materials also after special approval in each case 3) May be used after special approval in each case 4) Recommended for t ≤ 40mm 5) Only applicable for welds with t ≥ 10mm


Non-Destructive Testing Methods & Applications (Defectology /Characterization/Metrology of materials)(R.T. - X or Gamma Radiography M.T. - Magnetic Particle Inspection P.T. - Dye Penetrant U.T. � Ultrasonic E.T. - Eddy Current)


Common Application of NDT in Industry

Dividing into three divisions

a) Inspection of Raw Products (Forgings, Castings, Extrusions etc.)

b) Inspection Following Secondary Processing (Machining, Welding, Grinding, Heat treating)

c) In-Services Damage Inspection (Fracture, Fatigue, Creep, Corrosion)

Aerospace Industry;

Testing components including aero-engine, Landing gear and air frame parts during production

Aircraft Overhaul

Testing components during overhaul including aero-engine and landing gear components

Automotive Industry

Testing Brakes-Steering and engine safety critical components for flaws introduced during manufacture. Iron castings � material quality. Testing of diesel engine pistons up to marine engine size.

Petrochemical & Gas Industries

Pipe-Line and tank internal corrosion measurement from outside. Weld testing on new work. Automotive LPG tank testing

Railway Industry

Testing locomotive and rolling stock axles for fatigue cracks. Testing rail for heat induced cracking. Diesel locomotive engines and structures.

Mining Industry

Testing of pit head equipment and underground transport safety critical components.

Agricultural Engineering

Testing of all fabricated, forged and cast components in agricultural equipment including those in tractor engines.

Power Generation

Boiler and pressure vessel testing for weld and plate defects both during manufacturing and in subsequent service. Boiler pipe work thickness measurement and turbine alternator component testing.

Iron Foundry

Testing ductile iron castings for metal strength on 100% quality control basis.


Shipbuilding Industry

Structural and welding testing. Hull and bulkhead thickness measurement. Engine components testing.

Steel Industry

Testing of rolled and re-rolled products including billets, plate sheet and structural sections.

Pipe & Tube Manufacturing Industry

Raw plate and strip testing. Automatic ERW tube testing. Oil line pipe spiral weld testing.

Material Degradation during Service

Detection of fatigue damage

Early detection of creep damage

Hydrogen attack in low alloy steel

Hydrogen damage in zircaloy � 2

Intergranular Corrosion attack in Austenitic Stainless Steel

Thermal Embitterment of Duplex Stainless Steel


Description of frequencies of transducers and their application


Codes standards, specifications and procedure for NDT methods

EN 1435 Non-destructive examination of welds� Radiographic examination of welded joints

ISO 17636 Non-destructive examination of welds� Radiographic testing of fusion-welded joints

EN 444 Non-destructive testingGeneral principles for radiographic examination of materials by X and gamma-rays

EN 462 Non-destructive testing � Image quality of radiographs

EN 584 Non-destructive testing � Industrial radiographic film

EN 970 Non-destructive examination of fusion welds - Visual examination

ISO 11699 Non-destructive testing � Industrial radiographic film

ISO5580 Non-destructive testing � Industrial radiographic illuminators

EN 1711 Non-destructive examination of welds�Eddy Current Examination of welds by complex plane analysis.

EN ISO 17635 Non-destructive examination of welds� General rules for fusion welds in metallic materials

EN-ISO 23277 Non-destructive examination of welds� Penetrant testing of welds �Acceptance levels.

EN 1289 Non-destructive examination of welds� Penetrant testing of welds �Acceptance levels.

EN-ISO 23278 Non-destructive examination of welds� Magnetic particle testing -Acceptance levels

EN-ISO5817Arc-welded joints in steels � Guidance on quality levels for imperfections.

EN-ISO 6520 Classification of imperfections in metallic fusion welds with explanations.

EN-ISO 10042 Arc-welded joints in aluminium and its weldable alloys� Quality levels for imperfections.

EN 1330 Non-destructive testing � Terminology

EN-ISO 15548 Non-destructive testing � Equipment for eddy current examination.

IACS Rec. No. 47 International Association of Classification societies-Recommendation No. 47, Shipbuilding and repair Quality Standard

IACS Rec. No. 68 International Association of Classification societies � Rec. No. 68,Guidelines for non-destructive examination of hull and machinery steel forgings

IACS Rec. No. 69 International Association of Classification societies � Rec. No. 69,Guidelines for non-destructive examination of marine steel castings


EN 473 Qualification and certification of NDT personnel – General principles

ASNT SNT-TC-1A American Society for Non-destructive testing- Recommended Practice

EN-ISO 17638 Non-destructive testing of welds, Magnetic particle testing

EN-ISO 9934-1 Non-destructive testing – Magnetic particle testing – General Principles

EN 10160 Ultrasonic testing of steel and flat product of thickness equal or greater than 6 mm (reflection method)

ISO 9712 Non-destructive testing – Qualification and certification of personnel

EN-ISO 11666 Non-destructive examination of welded joints – Acceptance levels

EN-ISO 23279 Non-destructive testing of welds – Ultrasonic examination–Characterization of indications in welds

EN-ISO 17640Non-destructive examination of welds –Ultrasonic examination of welded joints

ASME V ASME Boiler and Pressure Vessel Code; Non-destructive Examination

ASTM A 609Standard Practice for Castings, Carbon, Low-Alloy, and Martensitic Stainless Steel, Ultrasonic Examination

ASTM 388 Standard Practice for Ultrasonic - Examination of Heavy Steel Forgings

EN 12668 Characterization and verification of ultrasonic of ultrasonic equipment;Part 1 – Instruments, Part 2 – Probes, Part 3 – Combined equipment

EN-ISO 3059 Non-destructive testing – Penetrant testing− Magnetic Particle testing– Viewing conditions

ASTM E-1316 Standard Terminology for Non-destructive Examinations

EN 571 Non-destructive testing – penetrant testing

EN 956 Non-destructive testing – penetrant testing – Equipment

EN ISO 12706 Non-destructive testing – Terminology – Terms used in penetrant testing

EN14096-1 Non-destructive testing - Qualification of radiographic film digitalisation systems- Part 1: Definitions, quantitative measurements of image quality parameters, standard reference film and qualitative control

ISO 3452-1 Non-destructive testing -- Penetrant testing -- Part 1: General principles

Abbreviations

ET- Eddy current testing MT- Magnetic particle testing

PT- Penetrant testing RT- Radiographic testing

UT- Ultrasonic testing VT- Visual testingHAZ- Heat affected zone WPS- Welding Procedure

SpecificationTMCP- Thermo mechanically controlled processed

NDT- Non-destructive testing.


The following definitions apply

Testing: Testing or examination of a material or component in accordance with this Classification Note, or a standard, or a specification or a procedure in order to detect, locate, measure and evaluate flaws

Defect: One or more flaws whose aggregate size, shape, orientation; location or properties do not meet specified requirements and are rejectable.

Discontinuity: A lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component

Flaw: An imperfection or discontinuity that may be detectable by non-destructive testing and is not necessarily rejectable.

Indication: Evidence of a discontinuity that requires interpretation to determine its significance

False indication:

An indication that is interpreted to be caused by a discontinuity at a location where no discontinuity exists.

Non relevant indication:

An indication that is caused by a condition or type of discontinuity that is not rejectable. False indications are non-relevant

Imperfections: A departure of a quality characteristic from its intended condition.

Internal imperfections: Imperfections that are not open to a surface or not directly accessible.

Quality level: Fixed limits of imperfections corresponding to the expected quality in a specific object. The limits are determined with regard to type of imperfection, their amount and their actual dimensions.

Acceptance level: Prescribed limits below which a component is accepted.

Planar discontinuity: Discontinuity having two measurable dimensions

Non-planar discontinuity: Discontinuity having three measurable dimensions.


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