DESTRUCTIVE & NON DESTRUCTIVE

                                  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.

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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.

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 Testing

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

                           Fig: - Tensile testing specimens and their parts

 

2. Bend testing:-

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.

                               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: - Izod test specimen base

                

                                          

                             Fig: - The charpy test specimen placing 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.

In the hardness test there are different types of tests are carried out.

     

 The tests are

1. Brinell hardness testing.

2. Rockwell harness testing.

3. Vickers diamond pyramid

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

·   Rockwell B (ball) used for soft materials.

·   Rockwell C (cone) uses diamond cone for hard materials.

·   It is flexible, quick and easy to use.

·   Here, two common indenters are used, they are.

1.Ball  -B

2.Cone  -C

 

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:-

1. You cannot identifies internal defectology (bubbles, delaminating, pores, wrong thickness ...) of the real bonded joint, preventing repairs before being put in use or during their lifetime.

2. Need to make specimens simulating the same process (surface preparation, environmental conditions, and adhesives system products) which cannot be reused once have been tested again.

3. Not directly identifies the status of the adhesion area in the bonded joint.

 

 

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):-

The oldest of all the methods. Components are scanned visually, sometimes with the aid of low or high power lenses, fiberscope, cameras and video equipment, to determine surface condition.

                    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.

                                 

The primary limitation of VT is it is only capable of evaluating discontinuities, which can be seen on the surface of the material or part. Sometimes there may be some visual indication of a subsurface imperfection that may need an additional NDT method to provide verification of the subsurface discontinuity.

 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

For example, a flaw, such as incomplete fusion at the weld root, can be repaired easily and quickly right after it is produced, saving on expense and time required repairing it after the weld has been completed and inspected using some other NDT technique.

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

As mentioned before, the primary limitation of VT is that it will only detect surface

discontinuities. It is also limited to the visual acuity and knowledge of the technician.

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:-

(a) Restricted to surface inspection

(b) Good eyesight required

(c) Good lighting required

(d) Person performing the inspection must know and be able to recognize what he/she is looking for.

 Advantages:-

(a) Primary method of inspection

(b) On-going inspection

(c) Most economical inspection method

(d) Applicable at any stage of fabrication.

                                                      

 

 

 

 

 

 

 

 

                                                                                                                       

           

 

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.

 

                        Fig: - cracks observed on plate area by MPT

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

An MFL tool can take sensor readings based on either the distance the tool travels or on increments of time. The second body is called an Electronics Can. This section can be split into a number of bodies depending on the size of the tool.  As the name suggests, contains the electronics or "brains" of the instrument. The Electronics Can also contains the batteries and is some cases an IMU (Inertial Measurement Unit) to tie location information to GPS coordinates.

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

Application:-

1.      Examination of ferrite welds

2.      Examination of castings

3.      Examination of shafts, fasteners, rods

4.      Examination of bearing sleeves, hollow forgings

Advantages:-

1.      It can find internal and external cracks up to depth of 3mm thickness.

2.      It is easy to handle.

3.      It is having less cost compared to other tests.

4.       Aid to VT.

5.   Instant repeatable results

6.   Effective inspection method

7.  Contrast or fluorescent consumables

8.  Easy to transport.

9.  Inexpensive to operate.

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:-

Depending on surface condition pre cleaning methods has to be chosen. Surface under examination shall be free from oil, grease, rust, loose scales, paint, coating etc… which may prevent the entry of penetrant.  vapour degreasing, detergent cleaning, solvent cleaning, paint strippers chemical etching are some of the popular pre cleaning methods used. Surface shall be cleaned with sufficient amount of cleaner before application of penetrant

 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.

 3. Removal of excess Penetrant from surface:

The excess penetrant is then removed from the surface. The removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable or hydrophilic post-emulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrant to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because these can remove the penetrant from the flaws. This process must be performed under controlled conditions so that all penetrant on the surface is removed (background noise). But, penetrants trapped in real defects remains in place.

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:

1. Application of penetrant

2. Application of penetrant and dwell time

3. Removal of excess penetrant

4. Application of developer

5. Examination of surface

6. Post cleaning

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: -

In this method Gamma rays emitted from radio active source are used for inspection.

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.

Limitations:-

1. Requires highly skilled operator

2. Applicable to conductive materials only

3. Depth of penetration in limited

4. Its application to ferromagnetic materials is difficult.

Advantages:-

1. Gives instantaneous response

2. Can be easily automated

3. Versatile

4. No contact between the probe and the test specimen is essential its equipment can be made portable.

 

 

 

 

 

8. Acoustic Emission Testing (AET):-

When a solid material is stressed, growing imperfections, if any within the material emit short bursts of acoustic energy called "emissions". As in ultrasonic testing, acoustic emissions can be detected by special receivers. Emission sources can be evaluated through the study of their intensity, rate and other characteristics. The growing defects can be located by triangulation technique (similar to earthquake epicenter location)

9. Leak Testing (LT):-

Leaks can be detected by using electronic listening devices, pressure gauge measurements, liquid and gas penetrant techniques or a simple soap-bubble test. Several techniques are used to detect and locate leaks in pressure retaining components such as pressure vessels and pipelines

Conclusion:-

The present report aimed to explaining the methods of “NON DESTRUCTIVE TESTING” and their techniques used in science and industry to evaluate the properties of a material, component or system with out causing damage to the component.  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.  Engineering is not always complete, and further research works are needed. To set up a good system for maintenance of existing components and concrete structures, there are still many things to be done. Different methods can be applied to the same problem, but the best method is chosen based on the features of the problems.