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Is it ripe enough? Almost all of the melons at the market, even the awed ones, are eventually sold, and this reveals a common problem with NDE: inspectors standards often change. Ideally, once the accept-or-reject criteria are set, the inspection would be divorced from human judgment and always produce the same result 2. The basic principle of NDE is simple.

To determine the quality or integrity of an item nondestructively, simply nd a physical phenomenon the interrogat-. For example, some questions can be answered noninvasively by passing X-rays interrogating parameter through an object, and monitoring the X-ray absorption the interaction of the X-rays and the object is the interrogated parameter. You could answer the questions Is my arm broken? Such questions do not query the X-ray absorptive qualities of the objects, but the answers are implied by the absorptive qualities. Although in many cases the interrogated parameter is specically queried for example, ultrasonically testing the elastic properties of a material , more often than not one must indirectly infer the condition of the test specimen from the results of NDE inspection.

Of course, we could simply slice the watermelon and taste it, or open the can and check for mice, but doing so would clearly alter future usefulness. Similarly, if we want to know the true strength of an old, heavily corroded bridge common in many countries, including the U. But although this method tells us about the strength of the bridge, the principles of NDE have been violated; NDE, by denition, is nondestructive and does not change the physical properties of the test object.

So if you squeeze a plum to test it for ripeness, thats NDE, provided you do not squeeze a little too hard and bruise it! On the other hand, many NDE tests do in fact permanently alter the test specimen and are still considered NDE, provided the test specimens function is unaltered and the specimen can be returned to service. For example, acoustic emission is commonly used to proof-test gas and liquid containers, pressurizing the tanks while sensors listen for acoustic emissions from submicron cracking caused by the pressure.

The tank passes inspection if the amount of acoustic emission is below a predetermined threshold. Although the tank structure clearly has been altered on some level, the damage to the tank by the acoustic emission test does not affect its usefulness as a pressure vessel. Within the testing community, NDE is not the only term used to describe the method in question.

People have varied the terminology based on their specic application and information derived from the test. The terms are similar but with subtle differences:. This is the most general denition among all the terms. NDC, of all the references, probably has the most restricted use. Here there is no specic implication that the object will be returned to service, although it could be. Although these various terms are designed to create distinctions, most are used interchangeably.

Nondestructive Evaluation Theory, Techniques, and Applications Mechanical Engineering

The process of NDE is often dictated by the specic case. The applications may impose requirements on the method and procedure, such as portability for eld use , speed of data acquisition and reduction for high-speed process control , and environmental hardening for operation in harsh environments.

Likewise, the motivation for inspection safety, cost inuences the when-and-where of apply- ing NDE. Process or quality control, maintenance, medical, and research represent the most common areas where NDE is employed. In feedback control, the NDE sensor a monitors the process and b feeds the sensor response back to the process controller, after which c the controller, based on this feedback information, controls the process variables to maintain the product within predetermined limits.

For example, in the manufac- turing of aluminum beverage cans, aluminum billets are hot-rolled into very long thin sheets before processing into cans. If the temperature is too low, the rolling tends to elongate the individual crystals preferentially within the sheet, creating directional texture that adversely affects the deep drawability of the sheet necessary to produce the can. Figure 1. To counter the texture, the rolling process includes a heat treatment of the sheet.

Ultrasonic NDE sensors monitor the degree of texture within the sheet and feed the information back to. In this process the NDE inspection data is used as feedback data to adjust the processing of the sheet aluminum. If the NDE inspection criteria accept the product, it is returned to perform its original service. If rejected, the product is either processed for recycling, reprocessed, or scrapped. If the product is expensive to manufacture, the additional expense and complications of NDE feedback control may be warranted.

In the production of aluminum cans, if the texture of a given sheet produces earing, the entire sheet must be reprocessed or the ears removed by machining; both are expensive processes. If, on the other hand, neither the raw. Below a certain level, however, economic expense may be too insignicant to warrant a fully developed method of quality control: if one of our fortune cookie wrappers contains no fortune cookie, why bother? Then again, if we produce enough defective products, customers will look for a different supplier. Planned periodic maintenance of components is far more economical than an unexpected failure.

Systems designed for inspectability often do not have to be completely taken off-line during an inspection. In many industries, a failure of a single component can cause catastrophic resultsfor example, failure of a landing gear on a commercial aircraft. Such components are generally referred to as safety critical. The airline industry, the power industry, and the government for civil structures such as bridges and dams all have large NDE programs. In many cases, inspection is also mandated by federal regulations.

The medical industry primarily uses NDE as a diagnostic tool. These methods are not always used as NDE tools; for example, at one time high-power ultrasound was used to fracture gallstonesa destructive process and therefore not considered NDE, even though the same technology is used. Courtesy Magnaux. We must have a sense of what NDE methods would maximize the likelihood of detecting the aw or material property of interest, while also considering economic, regulatory, and other factors.

The basic levels of choosing an NDE method are:. Understanding the physical nature of the material property or discon- tinuity to be inspected 2. Understanding the underlying physical processes that govern NDE methods 3. Understanding the physical nature of the interaction of the probing eld or material with the test material 4.

Understanding the potential and limitations of available technology 5. Considering economic, environmental, regulatory, and other factors. To employ any NDE method, we need to have a reasonable knowledge of what we are looking formaterial properties, a discontinuity such as a void or crack, sheet or coating thickness, etc. For material properties, we might be interested in the mechanical properties elastic constants or electromagnetic properties conductivity, permittivity, or magnetic permeability.

For discontinu- ities, we must not only be aware of their character, but also understand its relationship to the parent material, e. We must also have a basic knowledge of how the various NDE methods work. For example, eddy currents use a magnetic eld to create induced currents in the test part as the interrogating eld. Thus, eddy-current methods require the test part to be electrically conductive. In addition, we must understand the interaction between the operating principles of the NDE method and those of the property or characteristic of interest in the test part.

This is required a to determine if a given method will work, b to select a method in case of multiple choices, and c to determine the compatibility of the method and the part; for example, most ultrasonic methods require a coupling uid or gel that might contaminate or cause corrosion of sensitive parts. In selecting an NDE method, we must be aware of the potentials and limitations of the existing technology. Just because there is consistency between the physical principles of the NDE method and those of the test part does not mean that equipment is available or has the sensitivity to measure the desired feature.

We must also be aware of the actual sensitivity of the equipment relative to the environment in which it is used. Although manufacturers often quote. Many other factors can inuence our choice of NDE method. Is it cost effective to employ NDE? Is speed of inspection a factor? What regulations exist that may dictate not only the NDE method but also the procedure?

What environmental issues constrain our choices? Is the test to be performed in the eld or in a specialized testing facility? With a basic understand of all ve levels, we can make an informed choice that optimizes NDE method with the application constraints. Ideally, we would need only to inspect parts that in fact have aws.

But because the existence of aws is an unknown, the decision of how much to inspect is part of the overall NDE testing program. Generally, statistical methods are applied to a specic application and the results are used to compare the probability of the existence of a discontinuity to a preset criterion of acceptable aw limits type, size, frequency, etc. These statistical results are used to develop NDE programs designed to ensure the parts or the systems integrity. In general, a manufacturer or a maintenance facility would prefer to minimize the amount and degree of inspection, while still maintaining condence in the quality and integrity of the product.

To determine how much testing is necessary, statistical methods are applied. For example, when inspecting heat exchanger tube bundles, only a representative number are tested; one such method is the star pattern Figure 1.

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If extensive damage is found in a specic area, the neighboring tubes are inspected more closely. Other products demand more rigorous inspection. For example, safety- critical parts on a nuclear reactor or liquid petroleum gas LPG container are required by regulation to be completely inspected routinely. The importance of statistical analysis can be reasoned through a simple example. Assume that a study by a bicycle frame manufacturer determines. This may seem a reasonable failure rate, until we realize that there are 20 such welds on the frame.

Now the probability of frame failure in the rst year is 1 in If we now consider how many critical components there are on a complex system, such as a jet engine, we begin to understand the scope of ensuring the quality of the individual components. Statistical analysis of a part is based on the fracture mechanics, which combines loading conditions, material properties, and mechanics of materials. For NDE we are interested in ascertaining what conditions cause a part to fail, and where and how the failure occurs. Once this information is determined, we can decide how much of a part, and which sections, should be inspected.

Often, the extent of testing is recommended by the design or manufacturing engineer. Customer satisfaction in itself contains many different levels that can affect a companys reputation. Consider the following three awed products:.

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Courtesy Foerster Instruments. A fortune cookie wrapper that does not contain a cookie 2. A bottle of expensive scotch with a crooked label 3. A defective fan belt on an automobile. The missing fortune cookie is of little consequence; one usually buys these items in quantity, and if only one is missing the buyer thinks little of itas long as the percentage of defective products is low, the average customer does not feel cheated. On the other hand, the crooked label on an expensive bottle of scotch may trigger a complaint.

Even though the quality of the scotch is not affected, the customer may question the quality of the product simply because of the label orientation. Scotch requires very careful processing and attention to detail. A defective fan belt, of course, is even more serious.

If you have ever been stranded in an automobile, you understand the customers possibly long-term prejudice they may hold towards the manufacturer. Considering the number of components on a car that would disable it if they failed, the probability of failure of each part must be extremely small to ensure customer satisfaction.

For large systems such as power plants and aircraft, elaborate NDE procedures have been developed to ensure longevity and safety. Retirement-for-cause NDE methods specically address the issue of increasing the useful life of a system. Risk-informed methods focus inspection on components of a system that pose the greatest threat, based on the probability of a part failing and the consequence in the event of failure.

Instead of being retired after a predetermined lifetime, a part or structure such as an aircraft is now often retired only if found to be defective. For example, a signicant portion of the U. More recently, such components have undergone rigorous inspection schedules instead, and are retired only when a cause is found. Retirement for cause, however, increases the risk of component failure. Therefore, rigorous inspection schedule and procedures must be maintained. For example, power plants contain thousands of welds requiring periodic inspection.

Instead of simply inspecting these welds indiscriminately, inspections are based on a combination of the likelihood of failure, and the risk associated with failure, of a specic part all of the welds might have similar likelihood of failure, but the consequences can be drastically different. Such planning allows resources to be concentrated on the most critical system components. In risk-informed inspection, the probability of failure of a specic compo- nent is weighed against the consequence of its failure to develop a quality factor. TABLE 1. Risk Categories High Cat. Based on this risk assessment, the component is assigned a value indicating the frequency and extent of inspection required.

Further, the inspection-for-cause method is applied to each inspected component to make certain the appropriate NDE method, procedures, and acceptance criteria are applied to address the specic failure expected corrosion, cracking, etc. Table 1. The matrix maps a components statistical chance of failure rows with the consequence in case of failure columns. The shaded regions rank the importance of testing the component as low-, medium-, and high-risk.

Low-risk components require little or no inspection; if a low-risk part fails, even frequently, it can simply be repaired or replaced, with no major consequences to personal safety or downtime of the entire process. Medium-risk components are analyzed further, to determine if immediate or delayed NDE inspection is necessary. The high-risk category implies that a failure of this part would be very costly, and so the part is to be given the highest inspection priority.

The method requires the analysis of considerable information about the operating facility, system components, history of the process and individual components, and previous failure and inspection records.

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Implementing a risk- informed method requires iteration over several inspection periods. Initially, many component risk values are simply a best guess using available data. Because of the signicant initial investment, risk-informed methods are typically used in large systems or safety-critical structures. In modern times, we have developed many methods to query the integrity of parts or structures, using both destructive and nondestructive methods. Once we have a method, we have to know how reliable it is. There are many facets to this question, most of which are based on statistical analysis see Section 1.

This section focuses on the human factors that inuence the reliability of an NDE inspection. Inspector inuence on reliability can be affected by a variety of factors personal, environmental, and external. Personal inuences include physical and mental attributes. All NDE methods require a certain level of physical compe- tence, such as visual acuity and manual dexterity.

Even though a qualied inspector must possess these competencies, they can be diminished or subverted by fatigue, unreliable vision e. Mental attributes cover the inspectors attitude towards the work. Of course, attitude is difcult to control; for this reason, inspection procedures are designed to track the operators steps. Nonetheless many external pressures, such as production schedules, costs, fatigue, and temporary personal issues, can subvert an operators integrity.

Consider the consequences of a personal tragedy such as a death in the family or a divorce on an inspectors ability to concentrate on a relatively repetitive task. The working environment also plays a signicant role in both the sensitivity and reliability of NDE methods. The ideal testing environment is one designed specically and solely for its purpose.

Such an inspection environment would control outside inuences such as lighting and cleanliness, allow for efcient handling of parts, use equipment not limited by portability or power, provide a controlled environment for the comfort of the operator, and allow the operator to go home at the end of the shift. Fieldwork, by contrast, is often performed in relatively inhospitable and remote locations such as pipeline, bridges, fuel containers, construction sites.

In such environments, operators must compensate for the inability to control ambient conditions such as lighting and temperature. For example, they could use uorescent magnetic particles, which are far more visible than colored particles. But since uorescent particles require a darkened environment for viewing, and such an environment is generally unavailable in the eld, the less sensitive colored particles are used anyway.

Additionally, the operator must adapt the inspection to an existing structure, such as a tower or pipeline. To imagine the challenges, consider inspecting the entire circumference of an above-ground pipeline weld in cold weather Figure 1. External factors that contribute to the reliability of NDE inspections are reected in the general working environment. For example, an inspector may be.

Courtesy Magna- ux. It is particularly important that a positive environment exist in which the inspector can freely report ndings without the pressure to produce specic results. To give you some idea of how human factors can affect the reliability of NDE inspection, consider Silks analogy to accidents during driving 3. Like NDE, driving is a skilled task with signicant consequences if performed improperly, and it is subject to personal, environmental, and external inuences. Assume that there would be no accidents if drivers strictly adhered to the procedures the rules and regulations that govern trafc control.

For the sake of the exercise, dont be concerned that this is a fallacy. Therefore, we will attribute all accidents to a lapse in following these rules. Silk 3 quotes the number of accidents per miles traveled as , due to any cause. If we remove all nonprocedural factors, such as heart attacks or pedestrians dropping pumpkins from overpasses, the rate of human error while driving drops to about 1 mistake per miles. This value is approxi-mately consistent with studies on the reliability of human-operated NDE inspections. What is NDE? American Society of Nondestructive Testing, Electromagnetic Methods of Testing Metals.

Progress in Non-destructive Testing. The Reliability of Non-destructive Inspection. Bristol: Adam Hilger, , pp Because PT is portable, it is often used in remote locations. Using PT effectively requires:. Discontinuities open to the surface of the part subsurface discontinuities or surface discontinuities not open to the surface arent detected Special cleaning of parts Good eyesight. This chapter discusses the fundamentals, techniques, and applications of penetrant testing. Or you may have seen previously invisible cracks in concrete sidewalk paving, made visible by the dirty water leaking out of the cracks after a rain.

If you have worked on an old automobile engine, you have probably seen cracks appear when dirty oil leaks out of them. These cracks are surface discontinuities made visible by a penetranta material that seeps into and out of a surface discontinuity. To be seen, the penetrant must be a strikingly different color contrast from the surface and must exit the discontinuity under appropriate conditions. A blotter-like material on the surface, may draw the penetrant from the discontinuity, revealing the outline of the discontinuity.

Such activity is penetrant testing in its simplest form. PT requires less training and mechanical skills as compared to some other NDT methods, but you need to pay careful attention to cleanliness, procedures, and processing time, and you need comprehensive knowledge of types of discontinuities that may occur in the parts to be tested.

A detailed visual examination of the surface s to be tested should be conducted prior to beginning any NDT. This will identify any conditions that. The high contrast between the penetrant the coffee and the white cup makes the ne crack clearly visible. Cleaning is crucial at several stages. First, cleaning is necessary so that nothing on the surface of the specimen prevents the discontinuity from being open to the surfaceall rust, dirt, protective coatings, or any other interfering materials must be removed or cleaned from the surface.

Then the penetrant can be applied to the surface and allowed to enter the discontinuity. Procedures determine a the type of penetrant and b the dwell time, or length of time for the penetrant to enter the defects.

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After the penetrant has entered the defects, excess surface penetrant must be removedwithout removing the penetrant that has entered the defects. After this second cleaning, another material, called the developer, is placed on the surface. The developer draws some of the penetrant from the defects, and provides a contrasting background to make the penetrant easier to see. But the earliest recognized application of PT was the oil and whiting inspection of railroad axles, wheels, couplers, and locomotive parts in the lateth century. Railroad inspectors used oil a penetrant made of a dark lubricating oil diluted with kerosene to nd surface defects.

The parts of interest were coated with the oil, and after an appropriate dwell time to permit the mixture to enter the defects , the excess oil was removed. After the oil, a whiting a developer made of a mixture of chalk and alcohol was then applied to the parts. Evaporation of the alcohol left a thin, white powder coating of the chalk on the surface.

When the part was struck with mallets, the oil trapped in the discontinuities was forced out, making a blackish stain on the chalk coating.

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Thus it was easy to see where the discontinuities were. Railroad inspectors used the oil-and-whiting technique until around , when it was largely replaced by magnetic particle testing of ferromagnetic parts. Two improvements to the oil-and-whiting technique allowed it to continue in limited use after The rst change was to use an ultraviolet UV or black light lamp to illuminate the specimen after cleaning the excess oil from the surface.

Since oil under a strong ultraviolet light uoresces with a pale bluish glow, the traces of oil seeping out of small defects could be seen much more clearly. The second change was to heat the part and the oil, to improve penetration into the discontinuities. The thin, hot oil could enter the expanded discontinuities more easily, and the cooling of the part helped pull the penetrant into surface. But even with these changes, the hot oil method is much less sensitive than penetrant systems in use today 1, 2, 4.

During World War II, the aircraft industry needed better penetrant testing for nonferromagnetic parts, which could not be tested with magnetic methods or required greater sensitivity to ferromagnetic parts with discontinuities open to the surface. Robert C. Switzer, working in , made such signicant improvements in uorescent and color dye penetrant methods that they are still used today.

Dyes, that were either strongly uorescent or were brightly colored were introduced into special formulated oils with greater penetrating ability hence recognition of many penetrant methods as dye penetrant methods. Compared to the oil-and-whiting or hot-oil methods, the uorescent and visible usually red dyes provided much greater sensitivity. The uorescence dye, when exposed to ultraviolet light, emits a bright yellow-green light, very close to the most sensitive range of the human eye.

Red dye on a background of white developer provided the contrast needed for sensitive inspections. Many industries rapidly accepted these new penetrant techniques, as several commercial manu- facturers made inspection systems available Advances in materials and methods continued. Some improvements have been driven by industry and government safety requirements, such as reduced toxicity of materials and reduced re hazard from organic solvents made more environmentally friendly by removal of uorocarbons and certain hydrocarbon compounds.

Emulsifying agents materials that make organic materials miscible with water have been added and then removed from the penetrants with post emulsication after the dwell time of the penetrant being preferred now. The emulsifying agents permit removal of the penetrant with water. Specialized penetrants have been developed for use in situations such as in the aerospace and nuclear industries where halogens present in early formulations of pene- trants , which might induce corrosion in some materials, are now limited to lower levels.

Penetrants have also been developed for systems using liquid oxygen LOX or other powerful oxidizing agents. Other developments in penetrant formulations and documentation Polar- oid1 photography, video recording, and plastic lms continue to expand PTs usefulness. Industries include metal produc- tion; metal fabrication; automotive, marine, and aerospace manufacture and maintenance; petrochemical industry, electrical power generation, electronics manufacture; and composite materials manufacture and maintenance 4.

PT can be and has been applied to almost every conceivable material. PT nds three classes of discontinuities: a inherent discontinuities in raw materials,. Even some porous materials can be inspected using a special penetrant system. In addition, PT is a relatively simple NDT method as compared to the other methods and very economical. Testing kits are available that can be easily carried by one person to remote test sites. Large or small specimens in a variety of shapes and materials can be tested.

The method can provide reliable results and gives few false indications in the hands of experienced inspectors. PT can be automated to provide rapid, large-area inspection of simple geometry specimens or large quantities of similar specimens 3. The major disadvantage of PT is that it can detect only defects that are open to the surface of a specimen. In addition, results depend upon the experience and skill of the inspector. Porous, dirty, or rough surfaces on the specimen can hide discontinuities. Very tight discontinuities that restrict the entry of the penetrant may not be detected.

Material and discontinuity types, and test conditions such as temperature and humidity, may also affect the sensitivity of the method 2, Although the cleaning materials used in PT are gradually being changed to comply with environmental and safety requirements, it is still important to consider their impact and use them responsibly Such care may increase costs and inconvenience.

The materials are also a bit messy and can pose a safety hazard in poorly ventilated areas. Table 2. Surface tension, contact angle and surface wetting, capillarity, and dwell time describe these functions TABLE 2. Advantages Disadvantages Inexpensive Surface connected defects only Easy to apply Poor on hot, dirty, rough surfaces Use on most materials Poor on porous materials Rapid Messy Portable Environmental and safety Small learning curve concerns Accommodates wide range of sizes Temperature range limitations and shapes of test objects works Removal of coatings such as well on complex shapes paint or other protective Does not require power source coatings Yields aw location, orientation, Highly operator-dependent approximate size and shape Mechanical operations such as Volume processingbatches of parts shot peening, grinding, can be immersed in the penetrant machining, bufng, etc.

Surface Tension The physical phenomenon of surface tension explains the dynamics of blowing a soap bubble; it also explains why rain beads on a freshly waxed car, yet forms little pools on a car with a dull nish. To examine the nature of surface tension, lets begin with the concept of surface energy. Imagine a droplet of water oating on the space shuttle ignore the force of gravity. A particle of water within the bulk of the water droplet away from the surface experiences omnidirectional attractive molecular forces cohe- sive forces from the surrounding water particles.

The net force acting on the particle is zero. But as we examine particles near or at the surface of the droplet, the total cohesive forces become hemispherical in direction, i. Consequently, local forces acting on the surface particles do not sum to zero, and the particles experience a net force directed perpendicular to the surface and inward towards the liquid. Eventually, this compressive force is balanced by outwardly directed elastic forces. In fact, a water molecules very existence at or near the surface requires work to overcome this inwardly directed force.

The energy associated with this work is stored in the surface layer in the form of an elastic potential energy, called. The laws of thermodynamics, as well as Newtons laws of motion, require the uid surface to move to a minimum energy state. This movement is analogous to that of a ball falling from a height i. The surface energy of the water droplet is minimized when the surface area is at a minimum, i. If surface energy is reduced through a decrease in surface area, there must be an associated elastic force that acts tangential to the uid surface to reduce the area.

This force is called surface tension. Figure 2. The U- shaped wire frame, with the slider at the base of the U, is partially submerged in the uid. The slider is pulled slowly out of the liquid along the frame, creating a liquid lm. The experiment measures the force required to increase the area of the lm by a given amount.

Interestingly, this force is independent of the total lm area. The independence contrasts with our understanding of an elastic membrane [such as a rubber balloon], where the force required to increase the membranes area is directly proportional to the existing area. But because the molecules of the uid lm are orders of magnitude smaller than the thickness of the lm, the lm is primarily a bulk uid. As the lm area is increased, molecules of the liquid. In other words, uids do not have to overcome shear stresses at slow rates of lm movement The force F acting on the moveable wire of length l increases the surface area of both the front and back of the lm.

The surface tension, g, for this example is given as. F g 2l. Surface tension can also be viewed as the forces that account for the differences in molecular attraction among like molecules liquidliquid and gas gas and unlike molecules gasliquid that constitute the surface boundary between a gas and a liquid. Contact Angle and Surface Wetting Surface tension is not unique to uids. In fact, surface energy and surface tension dene the surface or boundary of any condensed matter, solid or liquid. This section discusses the effects of the balance of the different surface forces at the interface of a solid, a liquid, and a gas.

We know from experience that a uid in contact with a solid can manifest itself in many ways. The uid might be contracted into little beads, like rain on a newly waxed car nish; or form little pools, like rain on a car with a dull nish; or completely wet coat a surface, like oil poured onto a cooking pan. We must also include the gas, which contacts both the uid and the. A represents the attractive force of the uid-to-solid as a result of the interface.

The following surface tensions act on the various interfaces that dictate the response of the liquid on the solid:. Let us examine the interface where all three gas, solid, and liquid meet. The forces act tangentially to their respective surfaces. Because the solid surface does not ow by denition , the directions of gsg and gls lie in the plane of the solid. But the direction of ggl , which lies tangent to the liquidgas surface, is determined by a balance of all the forces acting at the interfacethe surface tensions of the three surfaces, and an adhesive force that balances the interaction of the different forces of the three different materials.

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Notice that A acts normal to and inward towards the solid surface, thus adhering the liquid surface to the solid. The contact angle y denes the angle at which the uid surface meets the solid surface. This contact angle, which is a function of the material properties of the contacting materials, indicates the degree to which the uid will wet coat the surface.

The four cases are listed:. Capillarity Wetting the surface of the test specimen is the rst requirement of the penetrant uid. Once the penetrant has coated the surface, it must be drawn into the depths of the surface-breaking discontinuities. Capillarity is the driving force that draws uid into the narrow openings, regardless of their orientation relative to gravity. Water in a glass container slightly climbs the walls to a level above the nominal water level, along a curve of radius r Figure 2.

On the other hand, if the glass were coated with wax, the water would appear to pull away from the container walls Figure 2. These different possible contact angles are explained by the balance of the constituent surface tensions and the adhesion force A. When the distance between the walls of a container is less than twice the curvature r, the forces become unbalanced and the uid rises or falls an additional distance Figures 2. Such a container is called a capillary, and the uid movement away from its natural level is called capillary action.

The curvature of the uid is called the meniscus little moon. Now, imagine a capillary that is open into a reservoir of uid. The height of liquid ascension or depression is a function of the contact angle of the liquid on the solid and the surface tension of the liquid. The contact angle is a function of the difference of the surface tensions. This height, h, can be calculated by summing the forces acting on the liquid in the capillary: a surface tension and b gravity force. The liquid surface layer exerts an attractive force surface tension on the capillary wall at the line of contact2pr for a cylindrical tube.

Therefore, the force pulling the liquid along the length of the capillary is. The nal uid height occurs when these two forces balance, i. First, crack width is not a constant: a crack typically narrows with depth. Second, parts of the crack wall may be in contact, i. Third, trapped gas or contaminants within the crack limits uid penetration. Nonetheless, penetration depth is still proportional to penetrant surface tension, and inversely proportional to crack width and penetrant density. One might expect the uid viscosity to affect both surface wetting and crack penetration.

Consider viscosity as the analog to friction for a solid in motion. The higher the viscosity friction coefcient, the more resistance there is to uid solid movement. Although the penetration time is greatly inuenced by the viscosityhigher viscosity, slower penetration rateviscosity does not appear to. Dwell Time To detect a crack of a given size, the penetrant uid requires a lapse of time for capillary action to draw the penetrant into the aws. This time between applying the penetrant and removing the excess uid is called the dwell time. Although a number of mathematical models exist for predicting dwell time, in practice, inspectors generally refer to tables of empirically determined times.

The most effective manner of determining the minimum required dwell time is to perform a procedure qualication on parts containing known discontinuities. All of the essential test parameters that affect the results, such as, temperature, specic materials used, times, etc. This visibility normally requires illumination. The type and intensity of illumination of indicators depends on:. Where the inspection is to be performedin a specialized testing facility where the lighting is easily controlled or in the eld where back- ground lighting cannot be controlled The size and shape of the part Whether the inspection is automated using laser scanning or full eld imaging or performed by a human viewer This section deals only with human viewing.

One reason that PT is simpler and more versatile than other NDE methods is that a human operator can simply see defects. True, automated systems using laser scanning or full-eld imaging are sometimes used where high repeatability is required, but these systems are expensive, complicated, and not very versatile. Consequently, the most commonly used detector in PT is the unaided human eye. The spectral response of the human eye is fairly constant throughout the species. The eyes sensitivity shifts from the yellowred spectrum in bright light toward the greenblue spectrum in dim light.

This shift is known as the Purkinje shift Penetrant dyes are chosen to make detection easier under both bright and dim light. In bright light natural or articial , color-contrast visible dye, typically red dye on a white background, is used; the high contrast maximizes the eyes response to indications.

The intensity and type of ambient lighting also affect detection. The development of uorescent penetrants dim-light or brightness- contrast dyes have signicantly increased PT detection sensitivity. Although the illumination for uorescent materials is in the ultraviolet UV range. Each curve is normalized to the maximum sensitivity. When the unsatisfactory indeterminists on their download the oedipus complex: solutions or and is their fee, you will also analyze an biology moment.

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