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Non Destructive Testing In The Aerospace Industry

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Published: Mon, 5 Dec 2016

Since several years, technology has been improved tremendously and is still in a need for advancement due to the increasing growth of demands. In today’s economy structures must remain for operation for such longer period than originally anticipated. The ageing effect on these structures is becoming significant. Non-destructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both money and time in product evaluation, troubleshooting, and research. Common NDT methods include ultrasonic, magnetic-particle, liquid-penetrant, radiographic and eddy-current testing. NDT is a commonly-used tool in forensic engineering, mechanical engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art.(3) The development of Non destructive testing examines the techniques to verify the conditions of these structures and these developments are focused on extending the cost of maintenance of the existing structures.

History:

It may be said that NDT has been used from ancient times. The audible ring of a Damascus sword blade would show the indication of the strength of the metal in combat. This technique was also used by the early blacksmiths. They used to listen to the ring of different metals being shaped. The same technique was also used by early bell makers. Visual testing had been used for many years for a wide range of applications. Heat sensing was used to monitor thermal changes in materials and sonic tests were performed years ago(1).

NDT by far is a visual testings which is the oldest mankind testing, this was used for visual checking knifes. The first use of NDT was done by a English man S.H.Saxby . who used compass for finding cracks in gun pipes in 1868.With a long established history in non-destructive testing (NDT), producing its first UV meter in the late 1960s (as Levy West Laboratories), AST is the market leader with the Levy Hill MkVI meter. NDT is well known as a part of industrial procedures, but it is also of importance in examinations of a more general interest in everyday life. In the aerospace industry, NDT can make the difference between life and death. Aircraft components are inspected before they are assembled into the aircraft and then they are periodically inspected throughout their useful life. Aircraft parts are designed to be as light as possible while still performing their intended function. This generally means that components carry very high loads relative to their material strength and small flaws can cause a component to fail. Since aircraft are cycled (loaded and unloaded) as they fly, land, taxi, and pressurize the cabin, many components are prone to fatigue cracking after some length of time. Even parts that are loaded well below the level that causes them to deform can develop fatigue cracks after being cycled for a long time. Cracking can also occur due to other things like a lightning strike. Aircraft have some protection against lightning strikes but occasionally they occur and can results in cracks forming at the strike location. The historical development of NDT is outlined for each technique.

The second oldest method was the Acoustics it has been used by ancient time when man started making the pottery vessels.

Table1. Table below shows a list of some of the key events in the chronology of NDT. (1)

Year

Chronology of Early Key Events in NDT

1800

First thermography observations by Sir William Herschel

1831

First Observation of electromagnetic induction by Michael Faraday

1840

First infrared image produced by Herschel’s son, John

1868

First reference to magnetic particle testing reported by S.H. Saxby, by observing how magnetized gun barrels affect a compass

1879

Early use of eddy currents to detect differences in conductivity, magnetic permeability, and temperature initiated by E. Hughes

1880-1920

”Oil and whiting” technique, forerunner of present-day penetrant test used for railroad axles and boilerplates

1895

X-rays discovered by Wilhelm Conrad Roentgen

1898

Radium discovered by Marie and Pierre Curie.

1922

Industrial Radiography for metals developed by Dr. H.H. Lester.

1927-28

Electric current induction/ magnetic field detection system developed by Dr. Elmer Sperry and H.C. Drake for the inspection of railroad track.

1929

Magnetic particle tests/ equipment pioneered by A.V. deforest and F.B. Doane.

1930

Practical uses for gamma radiography using radiumwere demonstrated by Dr. Robert F. Mehl

1935-40

Penetrant techniques developed by Betz, Doane , and deForest

1935-40

Eddy current developments by H. C. Knerr, C. Farrow, Theo Zuschlag, and Dr. F. Foerster

1940-44

Ultrasonic test metod developed in United States by Dr. Floyd Firestone

1942

First Ultrasound flaw detector using pulse-echo introduced by D.O. Sproule

1946

First portable ultrasonic thickness measuring instrument, the Audigage, was introduced by Branson

1950

Acoustic emission introduced as an NDT method by J. Kaiser

Mid 1950’s

First ultrasonic testing immersion B and C scan instruments developed by Donald C. Erdman

From the late 1950’s to present, NDT has seen many developments, innovation, and growth. The roots of non-destructive testing began to take form prior to the 1920’s. In the 1920’s there was an awareness of some of some of the magnetic particle tests, the visual test and also X-radiography. Prior to World War II, design engineers were content to rely on unusually high safety factors which were usually built or engineered into many products. In addition, there were a significant number of catastrophic failures and other accidents relating to product inadequacies that bought the concern for system and component quality to the forefront. (1)

Background History of Individual Testings:

Magnetic Particle Test: Saxby used a magnetic compass to locate the defects and in homogeneities in gun barrels in 1868. Herring in 1879 obtained a patent in United States for the defection of defects in railway lines using a compass needle. Deliberate studies were carried out by A.V. de Forest in 1928 and 1929 of the use of magnetic particles for non-destructive testing of ferromagnetic materials. In 1936, Unger and Hilpert described in a patent in Germany at where the inspection of aircraft engine parts, engines for fatigue cracks are developed rapidly and the magnetic particle testing was made mandatory. The real breakthrough for magnetic particle testing came, in 1960’s these methods was further greatly developed by C.Betz.

Radiography: Wilhelm.Conrad. Roentgen discovered X rays in 8 November 1895 when it was observed by him that fluorescent glow of crystals on a table near the cathode ray tube. Later the penetrating radiations were characterized systematically which are emitted by cathode ray tubes. These represent the very first radiographs taken in 1896. He conducted a test on his own hunting rifle. Radiograph of the rifle showed some defects in the material and was the start of industrial radiography.

Penetrant Test: It is a simple and effective method of examining surface areas for defects, cracks and discontinuities. Rubbing of carbon black on glazed pottery is one of the early surface inspections. Later on in 1940 magnetic particle method was introduced which is more sensitive. Many of the earlier developments were carried out by Magnaflux, IL USA. In 1942 it introduced Zyglo system where the fluorescent dyes were added to the liquid penetrant. The methods were formerly called “The oil and whiting method” and were used by the huge locomotives in the beginning of the 20th century.(13)

Ultrasonic testing: Richardson proposed Echo ranging detection of objects at sea in 1912. Later, Lavengin developed a source of ultrasonic waves by using the piezoelectric effect with quartz crystals between steel plates. Future on experiments was carried out by Boyle and by Wood by using quartz piezoelectric transducers. The most simple and quick method came up in 1940 by Firestone. The method avoided many of the problems which were associated with standing wave formation. Then in 1942 Firestone was the first to use his own method for the sonar. The two German brothers H and J. Krautkramer did a lot of research of method and contributed for the development of method. Since their time method has gone through several phases of development.

Eddy Current Test: Jean-Bernard Leon Foucault is generally credited with the first clear demonstration of eddy currents, by showing that electrical currents are set up in a copper disk moving in a non uniform magnetic field. Hughes is considered to have been the pioneer in using Eddy currents to inspect the metals. Eddy currents are also known as Induced currents which can only exist in conducting materials.(13)

The use of NDT methods took place during the Second World War started from the testings of submarines and airplanes. During these years the use of NDT is seen in the inspection of nuclear power plant components, pressure vessels and car parts.

Aim: To investigate the use of various NDT methods in industry.

Objectives:

To Conduct research on NDT and its applications

To identify key components of an aircraft, where likely hood of crack is high.

To test the components with appropriate NDT methods.

To perform penetrant test to identify the cracks in a gear.

Initial Plan

As the project initiates with the major task of carrying out the background reading and researching, great amount of time has to be required to collect information about project. A great deal of time has been spent on background reading. Project requires thorough knowledge on NDT in aerospace and its classification. A detailed background reading has been done to understand the concept of NDT. Background reading has also been done on different types of NDT testings and its classifications. A detailed study has been done on the history of NDT.

Background Reading:

Non Destructive Testing (NDT)

Non-destructive testing is the process by which materials, components or assemblies can be inspected without affecting their ultimate serviceability. It offers some important advantages over destructive testing processes such as mechanical testing. A destructive test must necessarily be on a sample basis. This is acceptable in many cases: to check that a batch of material is within specification, for example, or as a sample check on a large number of identical components. But a sample destructive test may not be feasible for a small number of high value components and in critical cases a 100% inspection may be required. Because of this, one key role of NDT is in assuring the quality of many manufacturing processes.

NDT can also be used to test components which have been subjected to service conditions. A welded structure can be inspected to detect fatigue cracking. NDT results can further be used to aid decisions on the need for and timing of repairs. Suitable techniques, correctly applied, can be used to measure the depth of a fatigue crack. This, coupled with knowledge of the system stresses, the toughness of the material and the fatigue crack growth rate, can be used to determine if a cracked component can be left in service until the next planned overhaul period.

These twin roles of NDT ensure that the technology will continue to play a fundamental role in materials testing. Aircrafts are designed to withstand a certain amount of damage from cracking and corrosion without cause for concern, and NDT inspectors are trained to find the damage before it becomes a major problem. The rigorous process used to design aircraft either allows for a certain amount of damage to occur before a part fails, or in many cases, a part can fail completely and performance of the aircraft will not be affected. The job of the NDT inspector is to find the damage while it is within acceptable limits.(5)

Role of NDT

Non destructive testing can be taken place at any transportation industry. The main aim is to consider the over view of all the terms of NDT and its applications and also to consider research on the optical methods. This is considered to be one of the important terms in the project. Another term is to do research on the optical methods which are implemented on the aircraft parts. (12)

Types of methods:

The number of NDT methods that can be used to inspect components and make whether the measurements are large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT methods. However, 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. (12)

Visual and 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, or bore scopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex. The principle behind Visual testing is the use of reflected or transmitted light from test object that is imagined with the human eye or other light-sensing device. VT finds its application in many industries ranging from raw materials to finished products and in-service inspection. VT can be inexpensive and simple with minimal training required. It has broad scope of uses and benefits. However VT can only evaluate surface conditions. Also effective source of illumination is required. (1)

Penetrant Testing (PT)

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 colour contrasts between the penetrant and developer make “bleed 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. (6)

Figure1: Figure illustrating a simple Penetrant testing (8)

The principle behind PT is that a liquid containing visible or fluorescent dye is applied to the surface and the fluid enters the discontinuities by capillary action. PT finds its application in virtually any solid non-absorbent material having uncoated surfaces that are not contaminated. PT is relatively easy and materials are inexpensive. It is extremely sensitive and very versatile. Also minimal training is required. Discontinuities can only be found on the surface. The surface condition must be relatively smooth and free of contaminants. (1)

Magnetic Particle Testing (MT)HYPERLINK “http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/cc_mpi_index.htm” 

This NDE method is accomplished 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. (6)

Figure2. Figure illustrating the basic concept of Magnetic Particle Testing (9)

The principle behind MT is that the test particle is magnetized and fine ferromagnetic particles are applied to the surface, hence aligning at discontinuity. MT finds its application in all ferromagnetic materials, for surface and subsurface discontinuities. It can be used in large and small parts. MT is relatively easy to use. Equipment and material is usually inexpensive. MT is highly sensitive and fast compared to PT. However in MT only surface and a few subsurface discontinuities can be detected. The test can be only done on ferromagnetic materials. (1)

Electromagnetic Testing (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 materials conductive and permeability properties, can be detected with the proper equipment.

Figure3: Figure illustrating Eddy Current Testing (10)

The principle behind ET is that localised electric fields are induced into a conductive test specimen by electromagnetic induction. ET finds its application in virtually all conductive materials. All conductive materials can be examined for flaws, metallurgical conditions, thinning and conductivity. ET is quick, versatile, and sensitive. It can be no contacting. Also it can be easily adaptable to automation. However variables must be understood and controlled. (1)

Radiography (RT)HYPERLINK “http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/cc_rad_index.htm” 

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.

Figure4: Figure illustrating the basic principle of Radiographic Testing (11)

The principle behind RT is that a radiographic film is exposed when radiation passes through the test object. RT finds its application in most material, shapes and structures. RT is the most widely used and accepted volumetric examination. RT can used only on limited thickness based on material density. Also there is a possible threat of radiation hazard.(1)

Ultrasonic Testing (UT)HYPERLINK “http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/cc_ut_index.htm” 

It 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.

Figure5: Figure illustrating the basic concept of Ultrasonic Testing (11)

The principle behind UT is that high frequency sound pulses from a transducer propagate through the test material, reflecting at interfaces. Most materials can be examined if sound transmission and surface finish are good and shape is not complex. UT provides precise, high-sensitivity results quickly. Thickness information, depth, and type of flaw can be obtained from one side of the component. (1)

Work to date

As suggested by the project plan, the entire duration of the project is distributed variably as required by different tasks commencing from the basic research and reading. According to the project plan the research was done on few NDT techniques and some of the techniques are still under research, along with types of methods and its techniques.

Necessary Future Work

A more detailed and precise study of non destructive testings in aero industry is to be done. Project requires identifying various testings in Aerospace industry. A detailed study should be done on techniques and testings of NDT in Aerospace industry. A precise study of industrial applications and research should be done on optical methods. Key components of an aircraft should be identified and finally components may be tested by appropriate NDT methods. The main project work to be done is perform a Dye Penetrant Testing to identify cracks on gear. Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or penetrant testing(PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials.

The main steps in Liquid Penetrant Inspection which are intended to be performed on the gear may be:

Pre-cleaning: The test surface is to be cleaned to remove any dirt, paint, oil, grease or any loose scale that could keep the penetrant out of a defect.

Application of Penetrant: The penetrant is to be then applied to the surface of the item which is to be tested.

 Excess Penetrant Removal: The excess penetrant is to be then removed from the surface.

Application of Developer: After excess penetrant has been removed a white developer is to be applied.

Inspection: The inspection is to be done using visible light with adequate intensity for visible dye penetrant.

Post Cleaning: The test surface is then to be cleaned after inspection and recording of defects.

Summary

It is clear that NDT has advantage from other inspection techniques as it does not make any changes to the article. It is a highly-valuable technique that can save both money and time in product evaluation, troubleshooting, and research. NDT is very useful in order to detect cracks in materials of magnitude in micrometers. (2)

Its aerospace application includes detection of cracks caused due to corrosion, fatigue and their synergistic interactions. Crank shafts, frames, flywheels, crane hooks, shaft, steam turbine blades and fasteners are some of the components which are more vulnerable to fatigue so regular inspection is necessary via NDT method. Typical components which are inspected using NDT method by manufacture in order to maintain quality are turbine rotor disc and blades, aircraft wheels, castings, forged components and welded assemblies. One of the NDT methods is Eddy current technique which can be used to check tube, bar and wire and also this method is automatic and highly sophisticated.(3)

It can be said that NDT method has to be carried out in order for the aircraft to operate safely during service time and avoid any accidental damage due to fatigue and corrosion.(2)

In this report, a detailed explanation of NDT testings and techniques are explained. The uses of NDT in different industrial applications are explained and a precise study on optical methods is done. According to the project plan, next task would be to conduct research on NDT applications.

List of Tables

Table1: Table showing a list of some of the key events in the chronology of NDT

List of figures

Figure1: Figure illustrating a simple Penetrant testing

Figure2. Figure illustrating the basic concept of Magnetic Particle Testing

Figure3: Figure illustrating Eddy Current Testing

Figure4: Figure illustrating the basic principle of Radiographic Testing

Figure5: Figure illustrating the basic concept of Ultrasonic Testing


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