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Used to locate cracks,porosity, and other

defects that break the

surface of a material

and have enoughvolume to trap and

hold the penetrantmaterial. Liquidpenetrant testing is

used to inspect large

areas very efficiently

and will work on most

nonporous materials.

Used to inspectferromagnetic

materials (those that

can be magnetized)

for defects that resultin a transition in the

magnetic permeabilityof a material.Magnetic particle

inspection can detect

surface and near

surface defects.

Used to locate surfaceand subsurface defects

in many materials

including metals,

plastics, and wood.Ultrasonic inspection

is also used tomeasure the thicknessof materials and

otherwise characterize

properties of material

based on sound

velocity andattenuation

measurements.

Used to detect surfaceand near-surface flaws

in conductive

materials, such as the

metals. Eddy currentinspection is also used

to sort materials basedon electricalconductivity and

magnetic

permeability, and

measures the

thickness of thinsheets of metal and

nonconductive

coatings such as paint.

Used to inspect almostany material for

surface and subsurface

defects. X-rays can

also be used to locatesand measures internal

features, confirm thelocation of hiddenparts in an assembly,

and to measure

thickness of materials.

Main Advantages

Large surface areas or

large volumes of

parts/materials can beinspected rapidly and

at low cost.

Parts with complex

geometry are routinelyinspected.

Indications are

produced directly on

surface of the part

providing a visualimage of the

discontinuity.

Equipment investment

is minimal.

Large surface areas of

complex parts can be

inspected rapidly.

Can detect surface andsubsurface flaws.

Surface preparation is

less critical than it is

in penetrantinspection.

Magnetic particle

indications are

produced directly onthe surface of the part

and form an image of

the discontinuity.

Equipment costs are

relatively low.

Depth of penetration

for flaw detection or

measurement issuperior to other

methods.

Only single sided

access is required.

Provides distanceinformation.

Minimum part

preparation is

required.

Method can be usedfor much more than

just flaw detection.

Detects surface and

near surface defects.

Test probe does not

need to contact thepart.

Method can be used

for more than flaw

detection.

Minimum partpreparation is

required.

Can be used to inspect

virtually all materials.

Detects surface and

subsurface defects.

Ability to inspectcomplex shapes and

multi-layered

structures withoutdisassembly.

Minimum part

preparation is

required.

Disadvantages

Detects only surface

breaking defects.

Surface preparation is

critical as

contaminants canmask defects.

Requires a relatively

smooth and nonporous

surface.

Post cleaning isnecessary to remove

Only ferromagnetic

materials can be

inspected.

Proper alignment of

magnetic field anddefect is critical.

Large currents are

needed for very large

parts.

Requires relatively

Surface must be

accessible to probe

and couplant.

Skill and training

required is moreextensive than other

technique.

Surface finish and

roughness caninterfere with

inspection.

Only conductive

materials can be

inspected.

Ferromagnetic

materials requirespecial treatment to

address magnetic

permeability.

Depth of penetrationis limited.

Flaws that lie parallel

Extensive operator

training and skill

required.

Access to both sides

of the structure isusually required.

Orientation of the

radiation beam to non-

volumetric defects iscritical.

Field inspection of

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chemicals.

Requires multiple

operations under

controlled conditions.

Chemical handlingprecautions are

necessary (toxicity,

fire, waste).

smooth surface.

Paint or other

nonmagnetic

coverings adversely

affect sensitivity.

Demagnetization and

post cleaning isusually necessary.

Thin parts may bedifficult to inspect.

Linear defects

oriented parallel to the

sound beam can go

undetected.

Reference standardsare often needed.

to the inspectionprobe coil winding

direction can go

undetected.

Skill and training

required is moreextensive than other

techniques.

Surface finish and

roughness mayinterfere.

Reference standards

are needed for setup.

thick section can betime consuming.

Relatively expensive

equipment investment

is required.

Possible radiation

hazard for personnel.

Penetrant

Testing

Magnetic Particle

Testing

Ultrasonic

Testing

Eddy Current

Testing

Radiographic

Testing

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NONDESTRUCTIVE TESTING

IN THE AEROSPACE INDUSTRY

How essential is nondestructive testing (NDT) to

airplanes?

In the aerospace industry, as with other transportation

industries, NDT can make the difference between lifeand 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 loadsrelative 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 tofatigue cracking after some length of time. If you are unfamiliar with the term "fatigue cracking" think

about what happens when you bend a paper clip or piece of wire back and forth ...eventually it will

break. Even parts that are loaded well below the level that causes them to deform can develop fatiguecracks after being cycled for a long time. This is what happens in aircraft. After they are used for a

while, fatigue cracks start growing in some of their parts. Cracking can also occur due to other things

like a lightning strike. Aircraft have some protection against lightning strikes but occasionally theyoccur and can results in cracks forming at the strike location like the one shown in the picture.

Another problem that aircraft have is that they are under the constant attack of corrosion. When an

aircraft lands and the door is opened, the inside of the plane often fills with warm moist air. When the

plane takes flight, and reaches altitude, the skin of the aircraft becomes very cold due to the temperature

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of the outside air. This cause the moisture held by the air inside the cabin to condense on the inside of

the aircraft skin. The water will collect at low areas and serve as the electrolyte needed for corrosion to

occur.

The good news is that aircraft 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 allow

for a certain amount of damage to occur before a part fails, or in many cases, a part can fail completelyand 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.

What kind of NDT techniques are used to ensure the safety of the airplane?

Over 80 percent of the inspections done to an aircraft are visual inspections. At regular intervals

inspectors look at various component of the aircraft for signs of damage. During heavy maintenancework, much of the interior of the aircraft is stripped out so inspectors can look for damage on the inside

surface of the fuselage. However, not all areas of the aircraft can be accessed for visual inspection and

not all damage can be detected by visual means. This is were NDT plays a critical role in thoroughly

inspecting airplanes.

NDT methods allow inspectors to inspect areas of the plane that would

otherwise be uninspectable without disassembling structure to gainaccess to the internal areas. NDT methods also allow inspectors to

detect damage that is too small to be detected by visual means. Eddy

current and ultrasonic inspection methods are used extensively to

locate tiny cracks that would otherwise be undetectable. Thesetechniques are also used to measure the thickness of the aircraft skin

from the outside and detect metal thinning from corrosion on the

inside surface of the skin. X-ray techniques are used to find defects

buried deep within the structure and to locate areas were water haspenetrated into certain structure. Obviously, this task requires trained

professionals who are capable of performing a variety of different

NDT techniques to get a complete and accurate status of the airplane.

There is no question that the success of the airplane industry is

dependent on NDT. Without NDT, the cost of maintaining and flying

in airplanes would increase dramatically, while the safety of flyingwould decrease. When people step into an airplane they trust that it

will get them to their destination with as little turbulence as possible.

NDT plays a vital role in keeping air travel one of the safest modes of transportation.

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BRIDGE INSPECTION

The US has 578,000 highway bridges, which are the lifelines of US commerce. The average life span ofhighway bridges is about 70 years and the majority of

bridges currently in use were built after 1945. However,

significant environmental damage requiring repair typically

occurs before the average bridge reaches mid-life.Corrosion, cracking and other damage can all affect a

bridge's load carrying capacity. Therefore, all of theelements that directly affect performance of the bridge

including the footing, substructure, deck, and

superstructure must be periodically inspected or monitored.

Visual inspection is the primary NDE method used toevaluate the condition of the majority of the nation's

highway bridges. Inspectors periodically (about every two

years) pay each bridge a visit to assess its condition. However, it is not uncommon for a fisherman,canoeist and other passerby to alert officials to major damage that may have occurred between

inspections.

The potential penalties for ineffective inspection of bridges canbe very severe. Instances of major bridge collapse are very rare,

but the results are truly catastrophic. The collapse of the Silver

Bridge in 1967 resulted in loss of 47 lives. The bridge connected

Point Pleasant, West Virginia and Gallipolis,Ohio over the OhioRiver. The cost of this disaster was 175 million dollars but some

experts estimate the same occurrence today would cost between

2.1 and 5.6 billion dollars. Furthermore, these cost figures do nottake into account factors such as loss of business resulting from

loss of access or detours, the cost resulting from blockage of a

major river shipping channel, and potential environmental

damage due to hazardous materials being transported over the bridge at the time of collapse.

The consequences of ineffective bridge inspection are usually not as severe as those at Silver Bridge.

However, repair and retrofit costs on bridges represent a very significant portion of a state's

transportation budget. In the future, replacement of a bridge will become an increasingly unattractivealternative. Growing construction costs, increased losses due to traffic disruption during repair or

replacement, and continuing tight budgets will force life extension to be the only viable alternative for

our aging bridges.

Fatigue cracking and corrosion will become

increasingly important considerations as we go

beyond the 75 year life expectancy and currentvisual inspection techniques will not suffice. The life

extension approach will require increased use of

NDE in a coordinated effort to obtain reliability

assurance for these structures. NDE techniques suchas magnetic particle inspection and ultrasonic

inspection are being used with greater frequency.

One of the newer NDE technologies being used is

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acoustic emission (AE) monitoring. Some bridges are being fitted with AE instruments that listen to the

sounds that a bridge makes. These sophisticated systems can detect the sound energy produced when a

crack grows and alert the inspector to the cracks presence. Sensors can be permanently fixed to the

bridge and the data transmitted back to the lab so that continuous bridge condition monitoring ispossible. The image provided here shows field engineers installing an AE monitoring system on the lift

cables of the Ben Franklin Bridge in Philadelphia, PA

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Pipeline Inspection

In the United States, millions of miles of pipeline carrying everythingfrom water to crude oil. The pipe is vulnerable to attack by internal

and external corrosion, cracking, third party damage and

manufacturing flaws. If a pipeline carrying water springs a leak

bursts, it can be a problem but it usually doesn't harm theenvironment. However, if a petroleum or chemical pipeline leaks, it

can be a environmental disaster. More information on recent USpipeline accidents can be found at the,National Transportation

Safety Board's Internet site. In an attempt to keep pipelines operating

safely, periodic inspections are performed to find flaws and damage before they become cause for

concern.

When a pipeline is built, inspection personnel may use visual, X-

ray, magnetic particle, ultrasonic and other inspection methods to

evaluate the welds and ensure that they are of high quality. Theimage to the left show two NDT technicians setting up equipment

to perform an X-ray inspection of a pipe weld. These inspections

are performed as the pipeline is being constructed so gaining accessthe inspection area is not problem. In some areas like Alaska,

sections of pipeline are left above ground like shown above, but in

most areas they get buried. Once the pipe is buried, it is undesirable

to dig it up for any reason.

So, how do you inspect a buried pipeline?

Have you ever felt the ground move under your feet? If you're standing in New

York City, it may be the subway train passing by. However, if you're standing inthe middle of a field in Kansas it may be a pig passing under your feet. Huh???

Engineers have developed devices, called pigs, that are sent through the buried

pipe to perform inspections and clean the pipe. If you're standing near a pipeline,vibrations can be felt as these pigs move through the pipeline. The pigs are about

the same diameter of the pipe so they range in size from small to huge. The pigs

are carried through the pipe by the flow of the liquid or gas and can travel and

perform inspections over very large distances. They may be put into the pipe lineon one end and taken out at the

other. The pigs carry a small

computer to collect, store andtransmit the data for analysis.

In 1997, a pig set a world record when it completed a

continuous inspection of the Trans Alaska crude oilpipeline, covering a distance of 1,055 km in one run.

Click here to read more about this record setting

inspection.

Pigs use several nondestructive testing methods toperform the inspections. Most pigs use a magnetic flux

leakage method but some also use ultrasound to

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perform the inspections. The pig shown to the left and below uses magnetic flux leakage. A strong

magnetic field is established in the pipe wall using either magnets or by injecting electrical current into

the steel. Damaged areas of the pipe can not support as much magnetic flux as undamaged areas so

magnetic flux leaks out of the pipe wall at the damaged areas. An array of sensor around thecircumference of the pig detects the magnetic flux leakage and notes the area of damage. Pigs that use

ultrasound, have an array of transducers that emits a high frequency sound pulse perpendicular to the

pipe wall and receives echo signals from the inner surface and the outer surface of the pipe. The toolmeasures the time interval between the arrival of a reflected echos from inner surface and outer surface

to calculate the wall thickness.

On some pipelines it is easier to use remote visual inspection equipment to assess the condition of thepipe. Robotic crawlers of all shapes and sizes have been developed to navigate the pipe. The video

signal is typically fed to a truck where an operator reviews the images and controls the robot.

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Rail Inspection

One of the major problems that railroads have facedsince the earliest days is the prevention of service

failures in track. As is the case with all modes of high-

speed travel, failures of an essential component can have

serious consequences. The North American railroadshave been inspecting their most costly infrastructure

asset, the rail, since the late 1920's. With increasedtraffic at higher speed, and with heavier axle loads in the

1990's, rail inspection is more important today than it has

ever been. Although the focus of the inspection seems

like a fairly well-defined piece of steel, the testingvariables present are significant and make the inspection

process challenging.

Rail is manufactured in different weights; there aredifferent rail conditions (wear, corrosion etc) present;

there are a significant number of potential defects

possible; and the task has to be performed with somespeed to reliably inspect the thousands of miles of track

stretching across the land. Sperry Rail Service, one of the

country's leading inspector of railroad tracks, has been

using specialized test equipment mounted on self-propelled rail cars for over seventy years to protect the

safety of passengers and freight. This information

provides a brief look at rail inspection.

The history of railroading is rooted in the production of

the first metal rails near the city of Sheffield, England in 1776. The rail improved the transportation of

materials in industries such as mining. In 1803 the first railroad intended for public use was opened foroperation between the London docks and Croyden. This first railway, the Surrey Iron Railway, offered a

smoother ride than a wagon, but offered no real advantage in speed since draft animals were used for

locomotion. However, the first steam locomotive was soon to arrive on the scene. In 1804, a steam

locomotive pulled a train of cars carrying several tons of ore for the iron works at Merthyr Tydfil inSouth Wales. The first American locomotive, the Best Friend of Charleston, was placed in operation on

the South Carolina Railroad in 1831.

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The rails have evolved from cast iron plates to specially alloyed steels, which are rolled to a standardshape and specially heat-treated to obtain the desired properties. The figures above show the progression

of rail development. Present day steel rails are vastly superior to their predecessors in both strength and

wear qualities, however defects still develop. The heavy loads and high speed of today's trains can causerails to fail in service unless regular inspections are performed.

Rail inspections were initially performed solely by visual means. Of course,

visual inspections will only detect external defects and sometimes the subtle

signs of large internal problems. The need for a better inspection method

became a high priority because of a derailment at Manchester, NY in 1911, inwhich 29 people were killed and 60 seriously injured. In the U.S. Bureau of

Safety's (now the National Transportation Safety Board) investigation of theaccident, a broken rail was determined to be the cause of the derailment. The

bureau established that the rail failure was caused by a defect that was entirely

internal and probably could not have been detected by visual means. Thedefect was called a transverse fissure (example shown on the left). The railroads began investigating the

prevalence of this defect and found transverse fissures were widespread.

In 1915, the Bureau of Standards began research to determine if magnetic testing could be used to detect

transverse fissures. The inspection technique involved passing a magnetizing solenoid along the rail to

establish a flux in the rail. Flux leakage caused by a defect was detected with search coils. The techniquewas successful in the laboratory but was unable to differentiate between defects and non-relevant rail

features in the field.

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In 1923, Dr. Elmer Sperry, started to develop and build a

rail inspection car with the capability of detecting

transverse fissures in railroad rails. In 1927 Sperry built an

inspection car (shown on the right) under contract with theAmerican Railway Association. The small flatbed in front

of the cab contained the inspection equipment. The

operator and recording devices were housed in the cab.

In 1928, a Sperry built inspection car, SRS 102, was testing

rail on the Wabash Railway in Montpelier, Ohio. The

inspection technique Sperry used established a strongmagnetic field in the rail by passing a large amount of low voltage current through it. A pair of search

coils, fixed at a constant distance from the rail, detected any changes in the magnetic field around the

rail. This magnetic induction flux leakage technique became the foundation of early rail inspection.

This drawing on the left shows the basic operation ofrail inspection using the induction method. Brushes are

used to contact the rail and "inject" electrical current.

The current creates a strong magnetic field in the rail.Where there is a defect in the rail, the steel material

will not support magnetic flux and some of the flux is

forced out of the part. The sensing coil detects a changein the magnetic field and the defect indication is

recorded on the strip chart. Computers are now being

used to record and evaluate the date.

Unfortunately, transverse fissures are not the only types

of defects found in rail. Other manufacturing and service-related defects that can occur includeinclusions, seams, shelling, and corrosion. Fatigue cracks can initiate from these defects, as well as

normal features of the rail such as bolt-holes. If these defects go undetected, they can lead to rail headand web separations. Many of these defects are not detectable with the flux leakage method because the

flaws run parallel to the magnet flux lines or the flaws are too far away from the sensing coils to detect.

The induction technique inspects mainly the railhead.

To complement the flux leakage method, and detect additional flaw types, ultrasonic inspection has

become common. High-frequency sound is transmitted into the metal rail and reflections from rail jointsand surface conditions, as well as internal defects, are displayed on a screen or cause movement of a pen

on a recording tape. Both normal- and angle-beam techniques are used, as are both pulse-echo and pitch-

catch techniques. The different transducer arrangements offer different inspection capabilities. Manualcontact testing is done to evaluate small sections of rail but the ultrasonic inspection has been automated

to allow inspection of large amounts of rail, like the electromagnetic technique previously discussed.The first all-ultrasonic inspection car was introduced in 1959. This car was developed specifically to

meet the needs of the New York City Transit Authority (NYCTA).

Fluid filled wheels or sleds are often used to couple the transducers to the rail. Sperry Rail Services has,

over the years, developed and made use of Roller Search Units (RSU's) comprising a combination of

different transducer angles to achieve the best inspection possible. A schematic of an RSU is shownbelow.

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At Sperry, there are two primary inspection units. The SperryRail Detector Car, referred to as the "big" car, uses both

ultrasonic and electromagnetic technologies to identify defects.

The inspection equipment on a Sperry test car is carried in a

carriage slung between the axles.

The Hi-Rail trucks

currently use only

ultrasonics becausethe electromagnet

equipment is too large

for this vehicle. The detector car will test rail between 6.5 and13 miles per hour. However, higher speed units are in

development.

The data from the inspection

equipment is fed to the operatorinside the car. A picture of the

operator station is shown on the right. Federal Railroad Administration (FRA)

rules require that any indication considered suspect by the test equipment onthe test car are hand verified immediately. This leads to a stop-start test mode.

When the operator sees something on the tape indicating a problem, he uses a

buzzer signal system to tell the driver up front to stop. The car then backs upto the point of examination where the operator gets out to hand test the rail

with an ultrasonic test set mounted on the rear of the car. If a defect isconfirmed, it is marked and a railroad work crew following the Sperry car will change the rail. If they

can't get to it right away, the section of track is assigned a slow order (slower speed) until the crew canrepair it. The amount of rail being tested can be increased by the use of chase cars following the testing

vehicles. The chase cars will receive a radioed signal of the test being done by the lead truck and will

stop to do the necessary hand testing. This elimination of the need to back up to hand test, allows thetesting vehicle to move forward, continuously testing, with the results being sent and recorded for

examination by the chase car.

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Wire Rope (Cable) Inspection

Most skiers and snowboarders rate ski resorts by their average powder base and theoverall quality of the slopes. Few likely give serious consideration to the safety of

chair lifts at their favorite mountains. It is likely, however, that many have looked

up at that relatively small cable that they are dangling from high in the air and

hoped that someone had check to see that it was in good working condition.Luckily, ski resort operators and governing authorities perform regular inspections

and maintenance of chair lifts to ensure passenger safety . One of the components

that gets close scrutiny is the cable.

Wire rope or cable is made by weaving many

individual wires together to produce a product that is both strong andflexible. Wire rope is used in many safety critical applications in

addition to chair lift and gondola systems. Some of these applications

include hoisting systems, such as cranes and winches; guy wires used intall antennas and towers; and mooring lines of oil drilling platforms at

sea. A cable failure in one of these applications could have very serious

consequences.

All wire rope eventually wears out making periodic inspections necessarythroughout the service life of the rope. Wire rope is prone to damage and wear due

to abrasion, fatigue, corrosion, and improper handling. NDT personnel look for

localized flaws or loss of metallic cross-sectional area using a variety of inspectionmethods. The least sophisticated method is visual inspection. Inspector simply look

for broken strands, wear and corrosion on the surface of the cable. However, for a

more thorough evaluation, a number of instruments have been developed that allow

inspectors to assess the internal areas of the cable.

One of the more widely used of

these instruments uses magnetism

to inspect the rope. The inspectioninstrument is placed around the

wire rope and moved along the rope or the rope is

pulled through the instrument. Strong permanentmagnets or electromagnets are used to create a strong

magnetic field within the rope. The rope is said to be

magnetically saturated because it is caring all the

magnet flux that it possibly can. In areas where the ropeis damaged, it can not support as much of the magnet

flux and some of it "leaks" out of the rope. Sensors in

the inspection head detect the magnetic flux leakagecaused by the internal or external defects in the rope.

Defects as small as 0.05 % of the rope's cross-sectional area can often be detected.