NDT a seminar Report

Non-Destructive Testing

Non-Destructive Testing

CHAPTER 1

INTRODUCTION

1.1 GENERAL

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 oshore 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 are most widely used for checking of welds, although eddy current and magnetic methods are also available. Alternating current eld measurement techniques permit non- contacting crack detection and sizing in welded joints both in air and underwater. [1]

1.2. DESTRUCTIVE TESTING

In destructive testing, tests are carried out to the specimens failure, in order to understand a specimens structural performance or material behavior under dierent loads. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing. Destructive testing is most suitable, and economic, for objects which will be mass produced, as the cost of destroying a small number of specimens is negligible. It is usually not economic to do destructive testing where only one or very few items are to be produced (for example, in the case of a building). Some of the destructive testing are:

Stress testing: It is used to determine the stability of a given system or entity. It involves testing beyond normal operational capacity, often to a breaking point, in order to observe the results. Stress testing may have a more specic meaning in certain industries, such as fatigue testing for materials.

Crash testing: This testing usually performed in order to ensure safe design standards in crash worthiness and crash compatibility for automobiles or related components. Some of the examples are Frontal-Impact Tests, Oset Tests, and Side-Impact Tests, Roll over Tests, Roadside hardware crash tests etc.

Hardness testing: Hardness refers to various properties of matter in the solid phase that gives it high resistance to various kinds of shape change when force is applied. Macroscopic hardness is generally characterized by strong intermolecular bonds. However, the behavior of solid materials under force is complex, resulting in several dierent scientic denitions of what might be called hardness in everyday usage.

1.3 NON-DESTRUCTIVE TESTING

Non-Destructive 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 eective operation, safety and reliability of plant, with resultant benet 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 dicult 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 awless production is almost never possible. For this reason testing specications 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 specic NDT method has to be used. The reliability of an NDT Method is an essential issue. But a comparison of methods is only signicant 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 articial aws, 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 aws 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 eld 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 reli- ability. Personnel Qualication 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 certication 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 certication of competence of personnel.

The most common NDT Methods are discussed in this presentation. In order of most used, they are: Visual inspection, Ultrasonic Testing (UT), Radiographic Testing (RT), Liquid penetrant Testing, Magnetic particle Testing, Electromagnetic Testing (ET) in which Eddy Current Testing (ECT) is well known 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.

CHAPTER 2

VISUAL INSPECTION

Visual testing is probably the most important of all non-destructive tests. It can often provide valuable information to the well trained eye. Visual features may be related to workmanship, structural serviceability, and material deterioration and it is particularly important that the engineer is able to differentiate between the various signs of distress which may be encountered. These include for instance, cracks, pop-outs, spalling, disintegration, colour change, weathering, staining, surface blemishes and lack of uniformity. Extensive information can be gathered from visual inspection to give a preliminary indication of the condition of the structure and allow formulation of a subsequent testing programme. The visual inspection however should not be confined only to the structure being investigated. It should also include neighbouring structures, the surrounding environment and the climatic condition.

2.1 TOOLS AND EQUIPMENT FOR VISUAL INSPECTION

An engineer carrying out a visual survey should be well equipped with tools to facilitate the inspection. These involve a host of common accessories such as measuring tapes or rulers, markers, thermometers, anemometers and others. Binoculars, telescopes, borescopes and endoscopes or the more expensive fibre scopes may be useful where access is difficult. A crack width microscope or a crack width gauge is useful, while a magnifying glass or portable microscope is handy for close up examination. A good camera with the necessary zoom and micro lenses and other accessories, such as polarized filters, facilitates pictorial documentation of defects, and a portable colour chart is helpful in identifying variation in the colour of the concrete.

Fig. 2.11 Videoscope Fig. 2.12 Borescope

Fig. 2.13 Magnifying glass

2.2 GENERAL PROCEDURE OF VISUAL INSPECTION

Before any visual test can be made, the engineer must peruse all relevant structural drawings, plans and elevations to become familiar with the structure. Available documents must also be examined and these include technical specification, past reports of tests or inspection made, construction records, details of materials used, methods and dates of construction, etc.

The survey should be carried out systematically and cover the defects present, the current and past use of the structure, the condition of adjacent structures and environmental condition. All defects must be identified, the degree classified, similar to those used for fire damaged concrete and, where possible, the causes identified. The distribution and extent of defects need to be clearly recognized. Visual comparison of similar members is particularly valuable as a preliminary to testing to determine the extent of the problems in such cases.

Segregation or excessive bleeding at shutter joints may reflect problems with the concrete mix, as might plastic shrinkage cracking, whereas honeycombing may be an indication of a low standard of construction workmanship. Lack of structural adequacy may show itself by excessive deflection or flexural cracking and this may frequently be the reason for an in situ assessment of a structure. Long term creep defections, thermal movements or structural movements may cause distortion of doorframes, cracking of windows, or cracking of a structure or its finishes.

Material deterioration is often indicated by surface cracking and spalling of the concrete and examination of crack patterns may provide a preliminary indication of the cause. Systematic crack mapping is a valuable diagnostic exercise when determining the causes and progression of deterioration.

Visual inspection is not confined to the surface but may also include examination of bearings, expansion joints, drainage channels and similar features of a structure. Any misuse of the structure can be identified when compared to the original designed purpose of the structure.

A careful and detailed record of all observations should be made as the inspection proceeds. Drawings can be marked, coloured or shaded to indicate the local severity of each feature.

Fig. 2.21 Cracks due to differential settlement of central column.

Fig. 2.22 Cracks due to bending and shear stresses.

Fig. 2.23 Sketch of severe rusting of reinforcing bars due to chemical action

CHAPTER 3

ULTRA SONIC PULSE VELOCITY TEST

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 aws or to characterize materials. The technique is also commonly used to determine the thickness of the test object, for example, to monitor pipework 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

3.1 HOW IT WORKS

In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing.

Fig. 3.11 Typical UPV testing equipment

There are two methods of receiving the ultrasound waveform, reection and attenuation. In reection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the sound is reected back to the device. Reected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with amplitude representing the intensity of the reection and the distance, representing the arrival time of the reection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence.

Table 3.1 Velocity criterion for concrete quality grading

Sr. No.

Ultrasonic Pulse Velocity by Cross probing (Km/Sec)

Concrete quality grading

1

Above 4.5

Excellent

2

3.5 to 4.5

Good

3

3.1 to 3.5

Medium

4

Below 3.0

Doubtful

Fig. 3.12 UPV Apparatus

CHAPTER 4

SCHMIDT/REBOUND HAMMER TEST

4.1 FUNDAMENTAL PRINCIPLE

The Schmidt rebound hammer is principally a surface hardness tester. It works on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. There is little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties and the rebound number.

4.2 EQUIPMENT FOR SCHMIDT/REBOUND HAMMER TEST

The Schmidt rebound hammer is shown in Figure. The hammer weighs about 1.8 kg and is suitable for use both in a laboratory and in the field.

Fig. 4.11 Schematics of Rebound Hammer

A schematic cutaway view of the rebound hammer is shown in Figure. The main components include the outer body, the plunger, the hammer mass, and the main spring. Other features include a latching mechanism that locks the hammer mass to the plunger rod and a sliding rider to measure the rebound of the hammer mass. The rebound distance is measured on an arbitrary scale marked from 10 to 100. The rebound distance is recorded as a rebound number corresponding to the position of the rider on the scale.

4.3 GENERAL PROCEDURE FOR SCHMIDT REBOUND HAMMER TEST

The hammer pushed hard against the concrete, the body is allowed to move away from the concrete until the latch connects the hammer mass to the plunger.The plunger is then held perpendicular to the concrete surface and the body pushed towards the concrete. This movement extends the spring holding the mass to the body. When the maximum extension of the spring is reached, the latch releases and the mass is pulled towards the surface by the spring. The mass hits the shoulder of the plunger rod and rebounds because the rod is pushed hard against the concrete. During rebound the slide indicator travels with the hammer mass and stops at the maximum distance the mass reaches after rebounding. A button on the side of the body is pushed to lock the plunger into the retracted position and the rebound number is read from a scale on the body.

CHAPTER 5

COVER METER TESTING

5.1 PRINCIPLE

The basic principle is that the presence of steel affects magnetic field. An electromagnetic search probe is swept over the surface of the concrete under test. The presence of reinforcement within the range of the instrument is shown by movement of the indicator needle. When the probe is moved until the deflection of the needle is at a maximum, the bar in question is then parallel to the alignment of the probe and directly beneath it. The needle indicates the cover on the appropriate scale for the diameter of the reinforcing bar.

5.2 MAIN APPLICATION

It is used for determining the presence, location and depth of rebars in concrete and masonry components. Advanced versions of covermeter can also indicate bar diameter when cover is known. It is moderately easy to operate. However, some training or experience is required to interpret the results.

5.3 ADVANTAGES

The presence of closely spaced reinforcing bar, laps, transverse steel, metal tie, wires or aggregates with magnetic properties can give misleading results. The meter has several scales for different bar sizes, therefore the bar diameter must be known if a true indication of cover is to be obtained.

5.4 LIMITATIONS

The maximum range of the instrument for practical purposes is about 100 mm. It does not give indication of the quality of concrete cover or the degree of protection afforded to the reinforcement.

Fig. 5.1 Covermeter testing

CHAPTER 6

CARBONATION DEPTH MEASUREMENT TEST

6.1 FUNDAMENTAL PRINCIPLE

Carbonation of concrete occurs when the carbon dioxide, in the atmosphere in the presence of moisture, reacts with hydrated cement minerals to produce carbonates, e.g. calcium carbonate. The carbonation process is also called depassivation. Carbonation penetrates below the exposed surface of concrete extremely slowly. The time required for carbonation can be estimated knowing the concrete grade and using;

t= (d/k) 2

Where, t is the time for carbonation,

d is the concrete cover,

k is the permeability.

Table 6.1 Permeability values versus concrete grade

Concrete Grade

Permeability Value

15

17

20

10

25

6

30

5

35

4

40

3.5

6.2 EQUIPMENT FOR CARBONATION DEPTH MEASUREMENT TEST

If there is a need to physically measure the extent of carbonation it can be determined easily by spraying a freshly exposed surface of the concrete with a 1% phenolphthalein solution. The calcium hydroxide is coloured pink while the carbonated portion is uncoloured.

6.3 GENERAL PROCEDURE FOR CARBONATION DEPTH MEASUREMENT TEST

The 1% phenolphthalein solution is made by dissolving 1gm of phenolphthalein in 90 cc of ethanol. The solution is then made up to 100 cc by adding distilled water. On freshly extracted cores the core is sprayed with phenolphthalein solution, the depth of the uncolored layer (the carbonated layer) from the external surface is measured to the nearest mm at 4 or 8 positions, and the average taken. If the test is to be done in a drilled hole, the dust is first removed from the hole using an air brush and again the depth of the uncolored layer measured at 4 or 8 positions and the average taken. If the concrete still retains its alkaline characteristic the color of the concrete will change to purple. If carbonation has taken place the pH will have changed to 7 (i.e. neutral condition) and there will be no color change.

Fig. 6.1 Coring of concrete surface for testing

Fig. 6.2 Testing on walls

6.4 RANGE AND LIMITATIONS OF CARBONATION DEPTH MEASUREMENT TEST

The phenolphthalein test is a simple and cheap method of determining the depth of carbonation in concrete and provides information on the risk of reinforcement corrosion taking place. The only limitation is the minor amount of damage done to the concrete surface by drilling or coring.

CHAPTER 7

LIQUID PENETRATION TEST

7.1 GENERAL

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (Can be applied to welds, tubing, castings, forgings, aluminum parts, turbine blades and disks, gears, metals, plastics). The penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic-particle inspection is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components. The merits of this technique are, Limited training is required for the operator (although experience is quite valuable), Low testing costs.

7.2 PRINCIPLE

DPI is based upon capillary action, where low surface tension uid 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 aw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used - uorescent or non uorescent (visible).

7.3 PROCEDURE

1. Pre-cleaning: The test sur- face is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor degreasing. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination.

2. Application of Penetrant: The penetrant is then applied to the surface of the item being tested. The penetrant is allowed time to soak into any aws (generally 5 to 30 minutes). The dwell time mainly depends upon the penetrant being used, material being testing and the size of aws sought. As expected, smaller aws require a longer penetration time

3. Excess Penetrant Removal: 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-emulsiable or hydrophilic post-emulsiable are the common choices. Emulsiers 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 this can the remove the penetrant from the aws. 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 cant 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.

5. Inspection: The inspector will use visible light with adequate intensity (100 foot-candles or 1100 lux is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for uorescent penetrant examinations. Inspection of the test surface should take place after a 10 minute development time. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye. Also of concern, if one waits too long after development, the indications may bleed out such that interpretation is hindered.

6. Post Cleaning: The test surface is often cleaned after inspection and recording of defects, especially if post-inspection coating processes are scheduled. The aws are more visible, because the defect indication has a high visual contrast (e.g. red dye against a white developer background, or a bright uorescent indication against a dark background). The developer draws the penetrant out of the aw over a wider area than the real aw, so it looks wider.

Fig. 7.11 Stages of test procedures

CHAPTER 8

HALF-CELL ELECTRICAL POTENTIAL METHOD

8.1 FUNDAMENTAL PRINCIPLE

The method of half-cell potential measurements normally involves measuring the potential of an embedded reinforcing bar relative to a reference half-cell placed on the concrete surface. The half-cell is usually a copper/copper sulphate or silver/silver chloride cell but other combinations are used. The concrete functions as an electrolyte and the risk of corrosion of the reinforcement in the immediate region of the test location may be related empirically to the measured potential difference. In some circumstances, useful measurements can be obtained between two half-cells on the concrete surface. ASTM C876 - 91 gives a Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete.

8.2 GENERAL PROCEDURE FOR HALF-CELL ELECTRICAL POTENTIAL METHOD

Measurements are made in either a grid or random pattern. The spacing between measurements is generally chosen such that adjacent readings are less than 150 mV with the minimum spacing so that there is at least 100 mV between readings. An area with greater than 150 mV indicates an area of high corrosion activity. A direct electrical connection is made to the reinforcing steel with a compression clamp or by brazing or welding a protruding rod. To get a low electrical resistance connection, the rod should be scraped or brushed before connecting it to the reinforcing bar. It may be necessary to drill into the concrete to expose a reinforcing bar. The bar is connected to the positive terminal of the voltmeter. One end of the lead wire is connected to the half-cell and the other end to the negative terminal of the voltmeter. Under some circumstances the concrete surface has to be pre-wetted with a wetting agent.

This is necessary if the half-cell reading fluctuates with time when it is placed in contact with the concrete. If fluctuation occurs either the whole concrete surface is made wet with the wetting agent or only the spots where the half-cell is to be placed. The electrical half-cell potentials are recorded to the nearest 0.01 V correcting for temperature if the temperature is outside the range 22.2 5.5oC.

Measurements can be presented either with a equipotential contour map which provides a graphical delineation of areas in the member where corrosion activity may be occurring or with a cumulative frequency diagram which provides an indication of the magnitude of affected area of the concrete member

Fig. 8.1 Half-Cell Electric potential testing on concrete surface

.

Half-Cell potential (mV) reading

Percentage chance of active

corrosion

< -350

90%

-200 to 350

50%

> -200

10%

8.3 APPLICATIONS OF HALF-CELL ELECTRICAL POTENTIAL TESTING METHOD

This technique is most likely to be used for assessment of the durability of reinforced concrete members where reinforcement corrosion is suspected. Reported uses include the location of areas of high reinforcement corrosion risk in marine structures, bridge decks and abutments. Used in conjunction with other tests, it has been found helpful when investigating concrete contaminated by salts.

8.4 RANGE AND LIMITATIONS OF HALF-CELL ELECTRICAL POTENTIAL INSPECTION METHOD

The method has the advantage of being simple with equipment also simple. This allows an almost non-destructive survey to be made to produce isopotential contour maps of the surface of the concrete member. Zones of varying degrees of corrosion risk may be identified from these maps.

The limitation of the method is that the method cannot indicate the actual corrosion rate. It may require to drill a small hole to enable electrical contact with the reinforcement in the member under examination, and surface preparation may also be required. It is important to recognize that the use and interpretation of the results obtained from the test require an experienced operator who will be aware of other limitations such as the effect of protective or decorative coatings applied to the concrete.

CHAPTER 9

INFRARED THERMOGRAPHY

9.1 GENERAL

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 dierences as small as a few hundredths of a degree Celsius. 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 eective, 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 oors, parking lot pavements, and building walls. As a testing technique, some of its most important qualities are;

(1) It is accurate

(2) It is repeatable

(3) It doesnt cause any inconvenience to the public and

(4) It is economical.

9.2 IR THERMOGRAPHY IN CIVIL ENGINEERING STRUCTURES

Examples are inspections of bridge decks and of paving in general. For locating delaminations at bridge decks, an ASTM standard, published in 2007 with the title Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography [4], exists. The method is intended for use on exposed and overlaid concrete bridge decks, asphalt or concrete overlays as thick as 100 mm. The standard has no Precision and Bias statement and should not be used for acceptance or rejection of a material because comparative data is not available. According to the test procedure, bridge deck should be dry for a minimum of 24 hours prior to the test and the temperature difference must be at least 0.5 C between the delaminated or deboned area and the adjacent solid concrete.

9.3 ACTIVE THERMOGRAPHY TESTING PROCEDURES

If a thermal gradient between the scene and the object of interest exist, the target can be inspected using the passive approach. However, when the object or feature of interest is in equilibrium with the rest of the scene, it is possible to create a thermal contrast on the surface using a thermal source; this is known as the active approach in infrared thermography. Energy brought to the object of interest will cause the change of thermal gradient compared to the bulk material thus witnessing the presence of subsurface anomalies.

9.3.1 Pulsed thermography

Pulsed thermography (PT) is one of the most common thermal stimulation methods used in thermography for nondestructive testing. One reason for this is the quickness of the inspection, in which a short thermal stimulation pulse lasting from a few milliseconds for high-conductivity material, such as metal, to a few seconds for low conductivity specimens, such as plastics, is used. Basically, PT consists of heating the specimen briefly and then recording the temperature decay curve, Figure

Fig. 9.1 Schematics of the pulsed thermography test procedure

Qualitatively, the phenomenon is as follows, the temperature of the material changes rapidly after the initial thermal pulse because the thermal front propagates by diffusion under the surface and also because of radiation and convection losses. The presence of a subsurface defect modifies the diffusion rate so that when observing the surface temperature, a different temperature with respect to the surrounding sound area appears over a subsurface defect once the thermal front has reached it. As for the detection depth, it is limited since thermography for nondestructive testing is a border technique, but often, anomalies such as cracks start close to the surface.

9.3.2 Lock-in thermography

Lock-in thermography (LT) is based on thermal waves generated inside a specimen and detected remotely. Wave generation, for example is performed by periodic deposition of heat on a specimens surface while the resulting oscillating temperature field in the stationary regime is recorded remotely through thermal infrared emission. Lock-in refers to the necessity to monitor the exact time dependence between the output signal and the reference input signal, the modulated heating. This is done with a locking amplifier in point-by-point laser heating or by computer in full-field (lamp) deployment so that both phase and magnitude images become available. Phase images are related to the propagation time, and since they are relatively insensitive to local optical surface features such as nonuniform heating. The depth range of images is inversely proportional to the modulation frequency, so that higher modulation frequencies restrict the analysis in a near surface region.

Fig. 9.2 Schematics of the Lock-In Thermography test procedure

Table9.1 Comparative characteristics of Pulsed and Lock-In Thermography

Pulsed Thermography

Lock-In Thermography

Heat source

Heat pulse

Regime

Transitory

Advantages

Fast

A single experience launches a series of thermal waves at several frequencies.

Little impact of nonuniform heating, environmental reflections, emissivity variations and nonplanar surfaces.

Low power thermal waves.

Depth inversion is straightforward

Disadvantages

Inversion techniques are complex

Affected by nonuniform heating.

Requires a test for every inspected depth.

Slow: a permanent regime has to be reached

CHAPTER 10

CONCLUSIONS

The present report aimed at explaining the methods of NDT and their techniques. Engineering is not always complete, and further research works are needed. To set up a good system for maintenance of existing concrete structures, there are still many things to be done. Dierent methods can be applied to the same problem, but the best method is chosen based on the features of the problem.

BIBLIOGRAPHY

[1] J H BUNGEY, Non-destructive testing in U K.,Seiken Symposium, 2000.

[2] Guidebook on non-destructive testing of concrete structures - INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 2002.

[3] Concrete Technology- A.M. Neville, J.J. Brooks, 2nd edition, Trans-Atlantic Publications.

[4] ASTM D 4788-03; Standard Test Method for Detecting Delaminations in Bridge Decks Using Infrared Thermography; 2007.

Dept. of Civil Engineering, JNNCE, ShivamoggaPage 11