NDT

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Prepared by: DN Reviewed by: RPG Approved By: AKS

Rev: 01 Date: 09-11-2004 Pages: 1 of 83

INDIAN PETROCHEMICAL S CORPORATION LTD

NAGOTHANE

TRAINING MODULE

FOR

NON DESTRUCTIVE TESTINGS

LEARNING CENTRE

NC

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OBJECTIVE:

Non destructive testing (NDT) is one of the important topic in day today life. Though NDT techniques are used in industries, certain techniques like X-ray, ultrasonic testing is used in medical field. It is very interesting to know that X-rays were first used in medical field, later in industry. In this module various NDTs / NDE are listed out but NDTs, which are most commonly used are explained in little detail to familiar with NDTs.

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MODULE IMPLEMENTATION PLAN

TOPIC: NON-DESTRUCTIVE TESTING CODE NO: IPCLDSMEC173

FOR: NDT DATE : 09-11-2004

REV:0 SITE: IPCL-NC

SR NO CONTENTS AUTH OR RESOURCES AVAILABLE ( Y/N) LEARNING VALIDATION 1 Introduction DN ASM NDT Handbook Y 2 Techniques of NDT DN ASM NDT Handbook / ASME hand book Y 3 Liquid Penetrant Test DN ASM NDT Handbook / ASME hand book Y 4 Magnetic Particle Testing DN ASM NDT Handbook / ASME hand book Y 5 Radiography DN ASM NDT Handbook / ASME hand book Y 6 Ultrasonic Testing DN ASM NDT Handbook / ASME hand book Y 8 Hrs. Quiz

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INDEX

CHAPTER NO.

DESCRPTION PAGE NO.

1 INTRODUCTION TO NDT TECHNIQUES 6

2 LIQUID PENETRANT TESTING 2.1 Introduction

2.2 Principle

2.3 Basic steps of Liquid Penetrant Testing 2.4 Quality control of Penetrant

2.5 Quality control of Developer 2.6 Selection of Penetrant Technique 2.7 Process control of Temperature

2.8 Common uses of Liquid Penetrant Testing 2.9 Nature of Defects

2.10 Advantages & Disadvantages of LPT 2.11 Health & Safety Precautions in LPT

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3 RADIOGRAPHIC TESTING

3.1 History of Radiography 3.2 Natural Radioactivity 3.3 Inverse Square Law 3.4 Absorption

3.5 Radiographic Technique

3.6 Sharpness of Radiographic Images 3.7 Filters in Radiography

3.8 Controlling Radiographic Quality 3.9 Film Processing 3.10 Viewing Radiographs 3.11 Image considerations 3.12 Radiographic Interpretation 3.13 Discontinuities 21

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4 MAGNETIC PARTICLE TESTING 4.1 Introduction

4.2 Principle

4.3 Magnetising Current 4.4 Lighting

4.5 Particle Concentration & Condition 4.6 Magnetic Field Indicators

4.7 Quantitative Quality Indicators 4.8 Pie Gage 4.9 Slotted Strips 48 5 ULTRASOINIC TESTING 5.1 Introduction 5.2 Wave Propagation

5.3 Wavelength Frequency & Velocity 5.4 Sound Propagation in Elastic Material 5.5 Material Affect on Speed & Sound 5.6 Acoustic Impedance

5.7 Ultrasonic Wave Generation 5.8 Refraction & Snell’s Law 5.9 Calibration Methods

5.10 Introduction to Common Standards 5.11 The IIW Type Calibration Blocks 5.12 Couplant

5.13 Normal Beam Inspections 5.14 Angle Beams Inspection 5.15 Weldments (Weld Joints)

5.16 Distance Amplitude Correction (DAC) 5.17 Wavelength & Defect Detection

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CHAPTER – 1

INTRODUCTION

NONDESTRUCTIVE TESTING

The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT techniques that locate and characterize material conditions and flaws that might otherwise result in failure of pressure vessels, pipelines or machinery components. These tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and measured without damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost-effectiveness. Generally speaking, NDT applies to industrial inspections. While technologies are used in NDT that are similar to those used in the medical industry, typically nonliving objects are the subjects of the inspections.

NONDESTRUCTIVE EVALUATION

Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material properties such as fracture toughness, formability, and other physical characteristics.

NDT / NDE METHODS

The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. There are six NDT methods that are used most often. These methods are visual inspection, penetrant testing, magnetic particle testing, electromagnetic or eddy current testing, radiography, and ultrasonic testing. These methods and a few others are briefly described below.

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1. VISUAL OR OPTICAL TESTING (VT)

Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, boroscopes or fibroscopes to gain access and more closely inspect the subject area. Visual examination involves procedures that range from simple to very complex.

2. LIQUID PENETRANT TESTING (LPT)

Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface. With visible dyes, vivid color contrasts between the penetrant and developer make "out" easy to see. With fluorescent dyes, ultraviolet light is used to make the bleed-out fluoresce brightly, thus allowing imperfections to be readily seen.

3. MAGNETIC PARTICLE TESTING (MPT)

This NDT method is accomplish by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near-surface imperfections distort the magnetic field and concentrate iron particles near imperfections, previewing a visual indication of the flaw.

4. ELECTROMAGNETIC (ET) OR EDDY CURRENT TESTING

Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because they flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the proper equipment.

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5. RADIOGRAPHIC TESTING (RT)

Radiography involves the use of penetrating gamma or X-radiation to examine parts and products for imperfections. An X-ray generator or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other imaging media. The resulting shadowgraph shows the dimensional features of the part. Possible imperfections are indicated as density changes on the film in the same manner as a medical X-ray shows broken bones.

6. ULTRASONIC TESTING (UT)

Ultrasonic testing uses transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.

7. ACOUSTIC EMISSION TESTING (AET)

When a solid material is stressed, imperfections 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 location.

8. LEAK TESTING (LT)

Several techniques are used to detect and locate leaks in pressure containment parts, pressure vessels, and structures. Leaks can be detected by using electronic listening devices, pressure gauge measurements, liquid and gas penetrant techniques, and / or a simple soap-bubble test.

In this module most commonly and widely used NDTs explained in detail as under: 1. Liquid Penetrant Testing

2. Radiographic Testing 3. Magnetic Particle Testing 4. Ultrasonic Testing

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CHAPTER – 2

LIQUID PENETRANT TESTING (LPT)

2.1 INTRODUCTION:

Liquid Penetrant testing (LPT) is one of the Non Destructive Testing (NDT) methods of inspection to locate discontinuities those are open to the surface. LPT can be used on any material except those are extremely porous & irregular surface. Discontinuities such as cracks, porosities etc. those are open to the surface are detected by `blotting action' after the surface has been treated with penetrant. This method is used as an effective NDT in welding fabrication / maintenance / condition monitoring / quality control.

2.2 PRINCIPLE:

In LPT, a liquid penetrant (contrast colour dye or fluorescent) is applied over the thoroughly cleaned and dry surface, which is having flows(discontinuities) those are open surface due to capillary action. Sufficient time is allowed so that the penetrant can enter in narrow discontinuities. Excess penetrant is removed by cleaning and developer (a fluffy chalk like powder) is applied over the surface. Due to blotting nature of the developer, entrapped penetrant in the discontinuities flows out and gives an indication, which can be viewed either in normal light for contrast dye or in “black light” (UV light) for fluorescent dye. The indication is always greater than the discontinuity due to diffusion of the penetrant in the developer.

2.3 BASIC STEPS OF A LIQUID PENETRANT INSPECTION

2.3.1 SURFACE PREPARATION

One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting

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have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

2.3.2 PENETRANT APPLICATION

Once the surface has been thoroughly cleaned and dried, penetrant material is applied either by spraying, brushing or immersing the parts in a penetrant bath.

2.3.3 PENETRANT DWELL

The penetrant is left on the surface for a sufficient time, to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and is often very specific to a particular application.

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DWELL TIME FOR SOME OF THE MATERIALS

(As per ASTM E 165, Table 2)

Minimum Dwell time (minutes)

Material Form Type of discontinuity Penetrant Developer Aluminium, Magnesium, Steel, Steel, Brass and Bronze, Titanium and High temp. alloys Cast- castings and welds Wrought- Extrusions, forgings, Plate

Cold shuts, Porosity, Lack of fusion, Laps, Cracks all forms)

5 10 7 7 Carbide tipped tools Lack of fusion, Porosity, Cracks 5 7

Plastics All forms Cracks 5 7

Glass All forms Cracks 5 7

Ceramics All forms Cracks 5 7

2.3.4 EXCESS PENETRANT REMOVAL

This is a most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water.

2.3.5 DEVELOPER APPLICATION

A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

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2.3.6 INDICATION DEVELOPMENT

The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

2.3.7 INSPECTION

Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

2.3.8 CLEAN SURFACE

The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

2.4 QUALITY CONTROL OF PENETRANT

The quality of a penetrant inspection is highly dependent on the quality of the penetrant materials used. The performance of a penetrant can be affected by contamination and aging. Contamination by another liquid will change the surface tension and contact angle of the solution, and virtually all organic dyes deteriorate over time resulting in a loss of color or fluorescent response. Therefore, regular checks must be performed to insure that the material performance has not degraded.

When the penetrant is first received from the manufacturer, a sample of the fresh solution should be collected and stored as a standard for future comparison. The standard specimen should be stored in an opaque glass or metal, sealed container. Penetrants that are in-use should be compared regularly against the standard specimen to detect changes in color, odor and consistency. When using fluorescent penetrants, a brightness comparison per the requirements of ASTM E 1417 is also often required. This check involves placing a drop of the standard and the in-use penetrants on a piece of Whatman #4 filter paper and making a side by side comparison of the brightness of the two spots under UV light.

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Additionally, the water content of water washable penetrants must be checked regularly. When water contaminates oil-based penetrants, the surface tension and contact angle of the mixture will increase since water has a higher surface tension than most oil-based penetrants in self-emulsifiable penetrants, water contamination can produce a gel break or emulsion inversion when the water concentration becomes high enough. The formation of the gel is an important feature during the washing processes but must be avoided until the stage in the process. Data indicates that the water contamination must be significant (greater than 10%) for gel formation to occur. Most specification limit water contamination to around 5% to be conservative. Non-water-based, water washable penetrants are checked using the procedure specified in ASTM D95 or ASTM E 1417. Water-based, water washable penetrants are checked with a refractometer. The rejection criteria are different for different penetrants so the requirements of the qualifying specification or the manufacturer's instructions must be consulted.

2.5 QUALITY CONTROL OF DEVELOPER

The function of the developer is very important in a penetrant inspection. It must draw out of the discontinuity a sufficient amount of penetrant to form an indication, and it must spread the penetrant out on the surface to produce a visible indication. In a fluorescent penetrant inspection, the amount of penetrant brought to the surface must exceed the dye's thin film threshold of fluorescence of the indication will not fluoresce. Additionally, the developer makes fluorescent indications appear brighter than indications produced with the same amount of dye but without the developer.

In order to accomplish these functions, a developer must adhere to the part surface and result in a uniform, highly porous layer with many paths for the penetrant to be moved due to capillary action. Some developers are applied wet and other dry, but the desired end result is always a uniform, highly porous, surface layer. Since the quality control requirements for each of the developer types is slightly different, they will be covered individually.

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2.5.1 DRY POWDER DEVELOPER

A dry powder developer should be checked daily to ensure that it is fluffy and not caked. It should be similar to fresh powdered sugar and not granulated like powered soup. It should also be relatively free from specks of fluorescent penetrant material from previous inspection. This is checking is performed by spreading out a sample of the developer and examining it under UV light. If there are ten or more fluorescent specks in a 10 cm diameter area, the batch should be discarded.

Apply a light coat of the developer by immersing the test component or dusting the surface. After the development time, excessive powder can be removed by gently blowing on the surface with air not exceeding 35 kPa or 5 psi.

2.5.2 WET SOLUBLE / SUSPENDIBLE DEVELOPER

Wet soluble developer must be completely dissolved in the water and wet suspendible developer must be thoroughly mixed prior to application. The concentration of powder in the carrier solution must be controlled in these developers. The concentration should be checked at least weekly using a hydrometer to make sure it meets the manufacturer's specification. To check for contamination, the solution should be examined weekly using both white light and UV light. If a scum is present or the solution fluoresces, it should be replaced. Some specification require that a clean aluminum panel be dipped in the developer, dried, and examined for indications of contamination by fluorescent penetrant materials.

These developers are applied immediately after the final wash. A uniform coating should be applied by spraying, flowing or immersion of the component. They should never be applied with a brush. Care should be taken to avoid a heavy accumulation of the developer solution in crevices and recesses. Prolonged contact of the component with the developer solution should be avoided in order to minimize dilution or removal of the penetrant from discontinuities.

2.5.3 SOLVENT SUSPENDIBLE

Solvent suspendible developers are typically supplied in an sealed aerosol spray can. Since the developer solution is in a sealed vessel, direct check of the solution

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are not possible. However, the way that the developer is dispensed must be monitored. The spray developer should produce a fine, even coating on the surface of the part. Make sure the can is well shaken and apply a thin coating to a test article. If the spray produces spatters or other an uneven coating the can should be discarded.

When applying a solvent suspendible developer, it is up to the inspector to control the thickness of the coating. When a visible penetrant system, the developer coating must be thick enough to provide a white contrasting background but not heavy enough to mask indications. When using a fluorescent penetrant system, a very light coating should be used. The developer should be applied under white light condition and should appear evenly transparent.

2.5.4 DEVELOPMENT TIME

Part should be allowed to develop for a minimum of 10 minutes and no more than 2 hours before inspecting.

2.6 SELECTION OF A PENETRANT TECHNIQUE

When sensitivity is the primary consideration for choosing a penetrant system, the first decision that must be made is whether to use fluorescent dye penetrant, or visible dye penetrant. Fluorescent penetrants are generally more capable of producing a detectable indication from a small defect because the human eye is more sensitive to a light indication on a dark background and the eye is naturally drawn to a fluorescent indication. When a dark indication on a light background is further reduced in size, it is no longer detectable even though contrast is increased. Furthermore, with a light indication on a dark background, indications down to 0.003 mm (0.0001 inch) were detectable when the contrast between the flaw and the background was high enough.

Since visible dye penetrants do not require a darkened area for the use of an ultraviolet light, visible systems are more easy to use in the field. Solvent removable penetrants, when properly applied can have the highest sensitivity and are very convenient to use but are usually not practical for large area inspection or in high-volume production settings.

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Another consideration in the selection of a penetrant system is whether water washable, post-emulsifiable or solvent removable penetrants will be used. Post-emulsifiable systems are designed to reduce the possibility of over-washing, which is one of the factors known to reduce sensitivity. However, these systems add another step, and thus cost, to the inspection process.

2.7 PROCESS CONTROL OF TEMPERATURE

The temperature of the penetrant materials and the part being inspected can have an effect on the results. Temperatures from 27 to 49oC (80 to 120oF) are reported in the literature to produce optimal results. Many specifications allow testing in the range of 4 to 52oC (40 to 125oF). A tip to remember is that surfaces that can be touched for an extended period of time without burning the skin are generally below 52oC (125oF).

Since the surface tension of most materials decrease as the temperature increases, raising the temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of course, the converse is also true and lowing the temperature will have a negative effect on the flow characteristics. Raising the temperature will also raise the speed of evaporation of penetrants, which can have a positive or negative effect on sensitivity. The impact will be positive if the evaporation serves to increase the dye concentration of the penetrant trapped in a flaw up to the concentration quenching point and not beyond. Higher temperatures and more rapid evaporation will have a negative effect if the dye concentration is caused to exceed the concentration quenching point or the flow characteristics are changed to the point where the penetrant does not readily flow.

The method of processing a hot part was once commonly employed. Parts were either heated or processed hot off the production line. In its day, this served to increase inspection sensitivity by increasing the viscosity of the penetrant. However, the penetrant materials used today have 1/2 to 1/3 the viscosity of the penetrants on the market in the 1960's and 1970's. Heating the part prior to inspection is no longer necessary and no longer recommended.

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2.8 COMMON USES OF LIQUID PENETRANT TESTING

Liquid penetrant Testing (LPT) is one of the most widely used nondestructive evaluation (NDE) method. Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility. LPT can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using LPT include the following:

• Metals (aluminum, copper, steel, titanium, etc.) • Glass

• Many ceramic materials • Rubber

• Plastics

LPT offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant material can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas.

Liquid penetrant inspection is used to inspect of flaws that break the surface of the sample. Some of these flaws are listed below:

• Fatigue cracks • Quench cracks • Grinding cracks

• Overload and impact fractures • Porosity

• Laps • Seams

• Pin holes in welds

• Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface.

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2.9 NATURE OF THE DEFECT

The nature of the defect can have a large affect on sensitivity of a liquid penetrant inspection. Sensitivity is defined as the smallest defect that can be detected with a high degree of reliability. Typically, the crack length at the sample surface is used to define size of the defect. A survey of any probability-of-detection curve for penetrant inspection will quickly lead one to the conclusion that crack length has a definite affect on sensitivity. However, the crack length alone does not determine whether a flaw will be seen or go undetected. The volume of the defect is likely to be the more important feature. The flaw must be of sufficient volume so that enough penetrant will bleed back out to a size that is detectable by the eye or that will satisfy the dimensional thresholds of fluorescence.

2.10 ADVANTAGES AND DISADVANTAGES OF LPT

Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and disadvantages. The primary advantages and disadvantages when compared to other NDE methods are summarized below.

PRIMARY ADVANTAGES

• The method has high sensitive to small surface discontinuities.

• The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.

• Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

• Parts with complex geometric shapes are routinely inspected.

• Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

• Aerosol spray cans make penetrant materials very portable.

• Penetrant materials and associated equipment are relatively inexpensive.

PRIMARY DISADVANTAGES

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• Only materials with a relative nonporous surface can be inspected. • Precleaning is critical as contaminants can mask defects.

• Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPT.

• The inspector must have direct access to the surface being inspected. • Surface finish and roughness can affect inspection sensitivity.

• Multiple process operations must be performed and controlled. • Post cleaning of acceptable parts or materials is required. • Chemical handling and proper disposal is required.

2.11 HEALTH AND SAFETY PRECAUTIONS IN LPT

When proper health and safety precautions are followed, liquid penetrant inspection operations can be completed without harm to inspection personnel. However, there is a number of health and safety related issues that must be addressed. Since each inspection operation will have its own unique set of health and safety concerns that must be addressed, only a few of the most common concerns will be discussed here.

CHEMICAL SAFETY

Whenever chemicals must be handled, certain precautions must be taken as directed by the material safety data sheets (MSDS) for the chemicals. Before working with a chemical of any kind, it is highly recommended that the MSDS be reviewed so that proper chemical safety and hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore, should be used and stored in small quantities. They should only be used in a well-ventilated area and ignition sources avoided. Eye protection should always be worn to prevent contact of the chemicals with the eyes. Many of the chemicals used contain detergents and solvents that can dermatitis. Gloves and other protective clothing should be warn to limit contact with the chemicals.

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ULTRAVIOLET LIGHT SAFETY

Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths ranging from 180 to 400 nanometers. These wavelengths place UV light in the invisible part of the electromagnetic spectrum between visible light and X-rays. The most familiar source of UV radiation is the sun and is necessary in small doses for certain chemical processes to occur in the body. However, too much exposure can be harmful to the skin and eyes. Excessive UV light exposure can cause painful sunburn, accelerate wrinkling and increase the risk of skin cancer. UV light can cause eye inflammation, cataracts, and retinal damage.

Because of their close proximity, laboratory devices, like UV lamps, deliver UV light at a much higher intensity than the sun and, therefore, can cause injury much more quickly. The greatest threat with UV light exposure is that the individual is generally unaware that the damage is occurring. There is usually no pain associated with the injury until several hours after the exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter which is well below the 365 nm wavelength, where penetrants are designed to fluoresce. Therefore, UV lamps sold for use in LPT application are almost always filtered to remove the harmful UV wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they be used with the proper filter in place and in good condition.

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CHAPTER – 3

RADIOGRAPHIC TESTING

3.1 HISTORY OF RADIOGRAPHY

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wuerzburg University in Germany. Working with a cathode-ray tube in his laboratory, Roentgen observed a fluorescent glow of crystals on a table near his tube. He concluded that a new type of ray was being emitted from the tube. This ray was capable of passing through the heavy paper covering and exciting the phosphorescent materials in the room. He found the new ray could pass through most substances casting shadows of solid objects. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects.

Prior to 1912, X-rays were used little outside the realms of medicine, and dentistry, though some X-ray pictures of metals were produced. The reason that X-rays were not used in industrial application before this date was because the X-ray tubes (the source of the X-rays) broke down under the voltages required to produce rays of satisfactory penetrating power for industrial purpose.

In 1922, industrial radiography took another step forward with the advent of the 200,000-volt X-ray tube that allowed radiographs of thick steel parts to be produced in a reasonable amount of time. In 1931, General Electric Company developed 1,000,000 volt X-ray generators, providing an effective tool for industrial radiography. That same year, the American Society of Mechanical Engineers (ASME) permitted X-ray approval of fusion welded pressure vessels that further opened the door to industrial acceptance and use.

3.2 NATURAL RADIO ACTIVITY

Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896, French scientist Henri Becquerel discovered natural radioactivity.

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It was Henri Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. Becquerel was researching the principles of fluorescence, certain minerals glow (fluoresce) when exposed to sunlight. He utilized photographic plates to record this fluorescence.

While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitchblende contained other radioactive elements. Marie and her husband, a French scientist, Pierre Curie started looking for these other elements. In 1898, the Curies discovered another radioactive element in pitchblende, they named it 'polonium' in honor of Marie Curie's native homeland. Later that year, the Curie's discovered another radioactive element, which they named 'radium', or shining element. Both polonium and radium were more radioactive than uranium. Since these discoveries, many other radioactive elements have been discovered or produced.

Radium became the initial industrial gamma ray source. The material allowed radiographing castings up to 10 to 12 inches thick. During World War II, industrial radiography grew tremendously as part of the Navy's shipbuilding program. In 1946, manmade gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and were much less expensive. The manmade sources rapidly replaced radium, and use of gamma rays grew quickly in industrial radiography.

X-rays and Gamma rays are electromagnetic radiation of exactly the same nature as light, but of much shorter wavelength. Wavelength of visible light is of the order of 6000 angstroms while the wavelength of x-rays is in the range of one angstrom and that of gamma rays is 0.0001 angstrom. This very short wavelength is what gives x-rays and gamma x-rays their power to penetrate materials that light cannot. These electromagnetic waves are of a high energy level and can break chemical bonds in materials they penetrate.

Strength of source is measured in Curie (Ci). 1 Curie is equivalent to 3.7x1010 disintegrations (nuclear decays) per second. Intensity of radiation is expressed in

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roentgens meter hour (RHM). It is the amount of received by the material at distance of 1 meter from 1curie source.

Half-life is time required to reduce the source strength to half of its original value.

3.3 INVERSE SQUARE LAW

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius (d) is the source strength divided by the area of the sphere.

Where, I1 & I2 are intensities of sources at distance d1 & d2 .

All measures of exposure will drop off by the inverse square law.

Sources Used in Industrial Radiography and its properties are given below: Source Half Life Energy(MeV) RHM Useful thickness

range(mm) Ir-192 74 Days 0.4 0.5 12 - 65 Co-60 5.26 Years 1.17, 1.33 1.3 50 - 200 Cs-137 30 Years 0.66 0.32 20 - 90 Tu-170 127 Days 0.08 0.009 2.5 – 12.5

3.4 ABSORPTION

Absorption characteristics of materials are important in the development of contrast in a radiograph. Absorption characteristics will increase or decrease as the energy of the x-ray is increased or decreased. A radiograph with higher contrast will provide greater probability of detection of a given discontinuity. An understanding of the relationship between material thickness, absorption properties, and photon energy is fundamental to producing a quality radiograph. An understanding of absorption is also necessary when designing x- and gamma ray shielding, cabinets, or exposure vaults.

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Attenuation of x-rays in solids takes place by several different mechanisms, some due to absorption, others due to the scattering of the beam. Thompson scattering and Compton Scattering were introduced in the material titled "Interaction Between Penetrating Radiation and Matter" and "Compton Scattering." This needs careful attention because a good radiograph can only be achieved if there is minimum x-ray scattering.

1. Thomson scattering (R) (also known as Rayleigh, coherent, or classical scattering) occurs when the x-ray photon interacts with the whole atom so that the photon is scattered with no change in internal energy to the scattering atom, nor to the x-ray photon.

2. Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is absorbed resulting in the ejection of electrons from the outer shell of the atom, resulting in the ionization of the atom. Subsequently, the ionized atom returns to the neutral state with the emission of an x-ray characteristic of the atom. 3. Compton Scattering (C) (also known a incoherent scattering) occurs when

the incident x-ray photon ejects an electron from an atom and a x-ray photon of lower energy is scattered from the atom.

4. Pair Production (PP) can occur when the x-ray photon energy is greater than 1.02 MeV, when an electron and positron are created with the annihilation of the x-ray photon (absorption).

5. Photodisintegration (PD) is the process by which the x-ray photon is captured by the nucleus of the atom with the ejection of a particle from the nucleus when all the energy of the x-ray is given to the nucleus (absorption).

3.5 RADIOGRAPHIC TECHNIQUE

Radiographs shall be made with a single source of radiation centered as near as practical with respect to the length and width of the portion of the weld being examined. The source to subject distance shall not be less than the total length of film being exposed in a single plane. This provision does not apply to panoramic exposures.

The source to subject distance shall not be less than seven times the thickness of weld plus reinforcement and backing ,if any , then the radiation shall penetrate any of

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the weld represented in the radiograph at an angle greater than 26.5 deg from a line normal to the weld surface. Welded joints shall be radiographed and the film indexed by methods that will provide complete and continous inspection of the joint within the limits specified to be examined. Joints limits shall show clearly in the radiographs. Short film, short screen, excessive undercut by scattered radiation, or any other process that obscures portions of the total weld length shall render the radiograph unacceptable. Film shall have sufficient length and shall be placed to produce at least 0.5" film, exposed to direct radiation from the source, beyond each free edge where the weld is terminated.

3.5.1 SINGLE WALL TECHNIQUE

In the single wall technique, the radiation passes through only one wall of the weld which is viewed for acceptance on the radiograph .A single-wall technique shall be used for radiography whenever practical. When it is not practical to use a single wall technique, a double wall technique shall be used. An adequate number of exposures shall be made to demonstrate that the required coverage had been obtained.

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.

3.5.2 DOUBLE WALL TECHNIQUE

For materials and welds in pipe and tube 3.5" or less in nominal outside diameter, a technique may be used in which the radiation passes through two radiation walls and the weld in both walls is viewed for acceptance on the same film. For welds, the radiation beams may be offset from the plan of the weld at an angle sufficient to

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separate the images of the source side and film side portion of the weld so that there is no overlap of the areas to be interpreted, in which case a minimum of two exposures taken at 90deg to each other shall be made for each joint. As an alternate, the weld may be radiographed with the radiation beam positioned so that the image of both walls are superimposed, in which case at least three exposure shall be made at60deg to each other.

Double wall technique, single wall viewing –for material and welds in pipe and tubes with a nominal outside diameter greater than 3.5" radiographic examination shall be performed for single wall viewing only. An adequate number of exposures shall be taken to ensure complete coverage.

For welds in pipe and tubes with a nominal outside diameter 0.5 or less, single wall viewing may be used provided the source is offset from the welds. As a minimum, three exposures 120 degrees apart shall be required.

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3.6 SHARPNESS OF RADIOGRAPHIC IMAGE:

Geometric unsharpness limitation - geometric unsharpness of radiograph shall not exceed the following.

Geometric unsharphness of the radiograph shall be determined in accordance with:

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Where

• Ug = geometrical unsharpness

• F = source size in mm

• D = distance in mm from the source of the radiation to the weld or object being

radiographed

• d = distance in inches from the source side of the weld or object being radiographed

to the film.

3.7 FILTERS IN RADIOGRAPHY

At radiation energies, filters consist of material placed in the useful beam to absorb, preferentially, radiations based on energy level or to modify the spatial distribution of the beam. The use of filters produces a cleaner image by absorbing the lower energy x-ray photons that tend to scatter more.

For industrial radiography, the filters added to the x-ray beam are most often constructed of high atomic number materials such as lead, copper, or brass. The thickness of filter materials is dependent on atomic numbers, and the desired filtration factor.

Gamma radiography produces relatively high energy levels at essentially monochromatic radiation, therefore filtration is not a useful technique and is seldom used.

3.8 CONTROLLING RADIOGRAPHIC QUALITY

One of the methods of controlling the quality of a radiograph is through the use of image quality indicators (IQI). IQIs provide a means of visually informing the film interpreter of the contrast sensitivity and definition of the radiograph. The IQI indicates that a specified amount of material thickness change will be detectable in the radiograph, and that the radiograph has a certain level of definition so that the density changes are not lost due to unsharpness. Without such a reference point, consistency and quality could not be maintained and defects could go undetected.

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Image quality indicators take many shapes and forms due to the various codes or standards that invoke their use. In the United States two IQI style are prevalent; the placard, or hole-type and the wire IQI. IQIs come in a variety of material types so that one with radiation absorption characteristics similar to the material being radiographed can be used.

3.8.1 HOLE-TYPE IQIS

ASTM Standard E1025 gives detailed requirements for the design and material group classification of hole-type image quality indicators. E1025 designates eight groups of shims based on their radiation absorption characteristics. A notching system is incorporated into the requirements allowing the radiographer to easily determine if the penetrameter is the correct material type for the product. The thickness in thousands of an inch is noted on each pentameter by a lead number 0.250 to 0.375 inch wide depending on the thickness of the shim. Military or Government standards require a similar penetrameter but use lead letters to indicate the material type rather than notching system as shown on the left in the image above.

Image quality levels are typically designated using a two part expression such as 2-2T. The first term refers to the IQI thickness expressed as a percentage of the region of interest of the part being inspected. The second term in the expression refers to the diameter of the hole that must be revealed and it is expressed as a multiple of the IQI thickness. Therefore, a 2-2T call-out would mean that the shim thickness should be two percent of material thickness and that a hole that is twice the IQI thickness must be detectable on the radiograph. This presentation of a 2-2T IQI in

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the radiograph verifies that the radiographic technique is capable of showing a material loss of 2% in the area of interest.

It should be noted that even if 2-2T sensitivity is indicated on a radiograph, a defect of the same diameter and material loss might not be visible. The holes in the penetrameter represent sharp boundaries, and a small thickness change. Discontinues within the part may contain gradual changes, and are often less visible. The penetrameter is used to indicate quality of the radiographic technique and not to be used as a measure of size of cavity that can be located on the radiograph.

3.8.2 WIRE PENETRAMETERS

ASTM Standard E747 covers the radiographic examination of materials using wire penetrameters (IQIs) to control image quality. Wire IQIs consist of a set of six wires arranged in order of increasing diameter and encapsulated between two sheets of clear plastic. E747 specifies four wire IQIs sets, which control the wire diameters. The set letter (A, B, C or D) is shown in the lower right corner of the IQI. The number in the lower left corner indicates the material group. The same image quality levels and expressions (i.e. 2-2T) used for hole-type IQIs are typically also used for wire IQIs.

The wire sizes that correspond to various hole-type quality levels can be found in a table in E747 or can be calculated using the following formula.

3.8.3 PLACEMENT OF IQIS

IQIs should be placed on the source side of the part over a section with a material thickness equivalent to the region of interest. If this is not possible, the IQI may be placed on a block of similar material and thickness to the region of interest. When a block is used, the IQI should the same distance from the film as it would be if placed directly on the part in the region of interest. The IQI should also be placed slightly away from the edge of the part so that atleast three of its edges are visible in the radiograph.

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3.9 FILM PROCESSING

Processing film is a science governed by rigid rules of chemical concentration, temperature, time, and physical movement. Whether processing is done by hand or automatically by machine, excellent radiographs require the highest possible degree of consistency and quality control.

3.9.1 MANUAL PROCESSING & DARKROOMS

Manual processing begins with the darkroom. The darkroom should be located in a central location, adjacent to the reading room and a reasonable distance from the exposure area. For portability darkrooms are often mounted on pickups or trailers.

Film should be located in a light, tight compartment, which is most often a metal bin that is used to store and protect the film. An area next to the film bin that is dry and free of dust and dirt should be used to load and unload the film. While another area, the wet side, will be used to process the film. Thus protecting the film from any water or chemicals that may be located on the surface of the wet side.

Each of step in film processing must be excited properly to develop the image, wash out residual processing chemicals, and to provide adequate shelf life of the radiograph. The objective of processing is two fold. First to produce a radiograph adequate for viewing, and secondly to prepare the radiograph for archival storage. A radiograph may be retrieved after 5 or even 20 years in storage.

One must bear in mind that radiographic contrast and definition are not dependent upon the same set of factors. If detail in a radiograph is originally lacking, then attempts to manipulate radiographic contrast will have no effect on the amount of detail present in that radiograph

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To understand how the image on a radiograph is formed, the characteristics of the film are very important. There are three important parts to a radiographic film. These include the base, the emulsion, and the protective coating.

3.9.2 THE BASE

All radiographic film consists of a base for which the other materials are applied. The film base is usually made from a clear, flexible plastic such as cellulose acetate. This plastic is similar to what you might find in a wallet for holding pictures. The principle function of the base is to provide support for the emulsion. It is not sensitive to radiation, nor can it record an image.

The clarity or transparency of the film base is an important feature. Radiographic film must be capable of transmitting light. Once a film has been processed chemically, it is subject to interpretation. This is commonly done by using a film illuminating device, which is usually a high intensity light source.

3.9.3 THE EMULSION

The film emulsion and protective coating comprise the other two components and are essentially made from the same material. They are applied to the film during manufacturing and usually take on a pale yellow color with a glassy appearance. Although they are made from the same material, they offer two distinct features to the film. These features are separated into the image layer of the emulsion, and the protective layer.

3.9.4 THE PROTECTIVE LAYER

The protective layer has the important function of protecting the softer emulsion layers below. It is simply a very thin skin of gelatin protecting the film from scratches during handling. It offers very important properties to film manufacturers, which include shrinkage (during drying that forms glassy protective layers) and dissolving in warm water. It will absorb the water and swell if it is dissolved in cold water.

The softer layers of the gelatin coating are technically known as the emulsion. An emulsion holds something in suspension. It is this material in suspension that is

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sensitive to radiation and forms the latent image on the film. During manufacturing of the film, silver bromide is added to the solution of dissolved gelatin. When the gelatin hardens the silver bromide crystals are held in suspension throughout the emulsion. Upon exposure of the film to radiation, the silver bromide crystals become ionized in varying degrees forming the latent image. Each grain or crystal of silver bromide that has become ionized can be reduced or developed to form a grain of black metallic silver. This is what forms the visible image on the radiograph. This visible image is made up of an extremely large number of silver crystals each is individually exposed to radiation but working together as a unit to form the image.

Once a film has been exposed to radiation and possesses the latent image, it requires chemical development. The purpose of developing the film is to bring the latent image out so that it can be seen visibly. There are three processing solutions that must be used to convert an exposed film to a useful radiograph. These are the developer, stop bath, and the fixer. Each of these solutions is important in processing the image so that it may be viewed and stored over a period of time.

3.9.5 THE PROCESS OF DEVELOPING FILM

1. To begin the process of converting the latent image on the radiograph to a useful image we first expose the film to the developer solution. The developer’s purpose is to develop, and to make the latent image visible. A special chemical within the developer solution acts on the film by reducing the exposed silver bromide crystals to black metallic silver. This process of developing is actually a multi-step process. Recall the characteristics of the film manufacturing mentioned earlier, they become important in the development process. Before the developer can change the silver crystals it must penetrate the protective coating of the film. Keep in mind that the protective coating of the film is made of gelatin and is sensitive to temperature and water. The developer solution is comprised of a combination of chemicals, consisting of alkali and metol or hydroquinone mixed with water. The purpose of the alkali is to penetrate the protective coating allowing the metol to reduce the exposed silver bromide to black metallic oxide.

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2. The second step in the development process is the stop bath. This bath is comprised of a glacial acetic acid and water. It is important to recognize that alkali’s and acid’s neutralize each other. The function of the stop bath is to quickly neutralize any excessive development of the silver crystals. Over development of the silver crystals results in a radiographic image that is virtually impossible to interpret.

3. The third step in development is the fixer. Its function is to permanently fix the image on the film. This is also a multi-step process. The fixer must first remove any unexposed silver crystals and then harden the remaining crystals in the emulsion. It is this process that is used to preserve the radiographic image over time.

Once the film has been properly developed, it is then rinsed in water and dried so that it may be visually examined.

3.10 VIEWING RADIOGRAPHS

Radiographs (developed film exposed to x-ray or gamma radiation) are generally viewed on a light-box. However, it is becoming increasingly common to digitize radiographs and view them on a high resolution monitor. Proper viewing conditions are very important when interpreting a radiograph. The viewing conditions can enhance or degrade the subtle details of radiographs.

Before beginning the evaluation of a radiograph, the viewing equipment and area should be considered. The area should be clean and free of distracting materials. Magnifying aids, masking aids, and film markers should be close at hand. Thin cotton gloves should be available and worn to prevent fingerprints on the radiograph. Ambient light levels should be low. Ambient light levels of less than 2 fc are often recommended, but subdued lighting, rather than total darkness, is preferable in the viewing room. The brightness of the surroundings should be about the same as the area of interest in the radiograph. Room illumination must be arranged so that there are no reflections from the surface of the film under examination.

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Film viewers should be clean and in good working condition. There are four groups of film viewers. These include: strip viewers, area viewers, spot viewers, and a combination of spot and area viewers. Film viewers should provide a source of defused, adjustable, and relativity cool light as heat from viewers can cause distortion of the radiograph. A film having a measured density of 2.0 will allow only 1.0 percent of the incident light to pass. A film containing a density of 4.0 will allow only 0.01 percent of the incident light to pass. With such low levels of light passing through the radiograph the delivery of a good light source is important.

The radiographic process should be performed in accordance with a written procedure or code, or as required by contractual documents. The required documents should be available in the viewing area and referenced as necessary when evaluating components. Radiographic film quality and acceptability, as required by the procedure, should first be determined. It should be verified that the radiograph was produced to the correct density on the required film type, and that it contains the correct identification information. It should also be verified that the proper image quality indicator was used and that the required sensitivity level was met. Next, the radiograph should be checked to ensure that it does not contain processing and handling artifacts that could mask discontinuities or other details of interest. The technician should develop a standard process for evaluating the radiographs so that details are not overlooked.

Once a radiograph passes these initial checks it is ready for interpretation. Radiographic film interpretation is an acquired skill combining, visual acuity with knowledge of materials, manufacturing processes, and their associated discontinues. If the component is inspected while in service, an understanding of applied loads and history of the component is helpful. A process for viewing radiographs, left to right top to bottom etc., is helpful and will prevent overlooking an area on the radiograph. This process is often developed over time and individualized. One part of the interpretation process, sometimes overlooked, is rest. The mind as well as the eyes need to occasionally rest when interpreting radiographs.

When viewing a particular region of interest, techniques such as using a small light source and moving the radiograph over the small light source, or changing the

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intensity of the light source will help the radiographer identify relevant indications. Magnifying tools should also be used when appropriate to help identify and evaluate indications. Viewing the actual component being inspected is very often helpful in developing an understanding of the details seen in a radiograph.

Interpretation of radiographs is an acquired skill that is perfected over time. By using the proper equipment and developing consistent evaluation processes, the interpreter will increase his or her probability of detecting defects.

3.11 IMAGE CONSIDERATIONS

The most common detector used in industrial radiography is film. The high sensitivity to ionizing radiation provides excellent detail and sensitivity to density changes when producing images of industrial materials. Image quality is determined by a combination of variables: radiographic contrast and definition. Many variables affecting radiographic contrast and definition are summarized below and addressed in following sections.

3.11.1 RADIOGRAPHIC CONTRAST

Radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector or film contrast.

Subject contrast is determined by the following variables: - Absorption differences in the specimen

- Wavelength of the primary radiation - Scatter or secondary radiation

Film contrast is determined by the following: - Grain size or type of film

- Chemistry of film processing chemicals - Concentrations of film processing chemicals - Time of development

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- Temperature of development

- Degree of mechanical agitation (physical motion)

Exposing the film to produce higher film densities will generally increase contrast. In other words, darker areas will increase in density faster than lighter areas because in any given period of time more x-rays are reaching the darker areas. Lead screens in the thickness range of 0.004 to 0.015 inch typically reduce scatter radiation at energy levels below 150, 000 volts. Above this point they will emit electrons to provide more exposure of the film to ionizing radiation thus increasing the density of the radiograph. Fluorescent screens produce visible light when exposed to radiation and this light further exposes the film.

3.11.2 DEFINITION

Radiographic definition is the abruptness of change in going from one density to another. There are a number of geometric factors of the X-ray equipment and the radiographic setup that have an effect on definition. These geometric factors include:

- Focal spot size, which is the area of origin of the radiation. The focal spot size should be as close to a point source as possible to produce the most definition.

- Source to film distance, which is the distance from the source to the part. Definition increases as the source to film distance increase. - Specimen to detector (film) distance, which is the distance between the

specimen and the detector. For optimal definition, the specimen and detector should be as close together as possible.

- Abrupt changes in specimen thickness may cause distortion on the radiograph.

- Movement of the specimen during the exposure will produce distortion on the radiograph.

- Film graininess, and screen mottling will decrease definition. The grain size of the film will affect the definition of the radiograph. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film

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will increase. Also, increased development of the film will increase the apparent graininess of the radiograph.

3.11.3 RADIOGRAPHIC DENSITY

Film speed, gradient, and graininess are all responsible for the performance of the film during exposure and processing. As these combine with processing variables a final product or the radiograph is produced. In viewing the radiograph, requirements have been established for acceptable radiographs in industry. The density of a radiograph in industry will determine if further viewing is possible.

Density considerations date back to early day radiography. Hurder and Drifield have been credited with developing much of the early information on the characteristic curve and density of a radiograph. Codes and standards will typically require densities of a radiograph to be maintained between 1.8 to 4.0 H&D (Hurder and Drifield) for acceptable viewing. As density increases, contrast will also increase. This is true above 4.0 H&D, however as density reaches 4.0 H&D an extremely bright viewing light is necessary for evaluation.

Density, technically should be stated "Transmitted Density" when associated with transparent-base film. This density is the log of the intensity of light incident on the film to the intensity of light transmitted through the film. A density reading of 2.0 H&D is the result of only 1 percent of the transmitted light reaching the sensor. At 4.0 H&D only 0.01% of transmitted light reaches the far side of the film.

3.12 RADIOGRAPH INTERPRETATION - WELDS

Interpretation of radiographs takes place in three basic steps, which are (1) detection, (2) interpretation, and (3) evaluation. All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. The following material was developed to

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help students develop an understanding of the types of defects found in weldments and how they appear in a radiograph.

3.13 DISCONTINUITIES

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specification used to invoke and control an inspection, are referred to as defects.

3.13.1 GENERAL WELDING DISCONTINUITIES

The following discontinuities are typical of all types of welding.

Cold lap is a condition where the weld filler metal does not properly fuse with the

base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding.

Porosity is the result of gas entrapment in the solidifying metal. Porosity can take

many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters or rows. Sometimes porosity is elongated and may have the appearance of having a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material it will have a radiographic density more than the surrounding area.