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The tube, which can be rigid or flexible with a wide variety of lengths and diameters, provides the necessary optical connection between the viewing end and an objective lens at the dist

ASM

INTERNATIONAL ®

The Materials Information Company

Publication Information and Contributors

Nondestructive Evaluation and Quality Control was published in 1989 as Volume 17 of the 9th Edition Metals Handbook With the second printing (1992), the series title was changed to ASM Handbook The Volume was prepared

under the direction of the ASM Handbook Committee

Authors and Reviewers

• LAMET UFRGS

• D.A Aldrich Idaho National Engineering Laboratory EG&G Idaho, Inc

• Craig E Anderson Nuclear Energy Services

• Gerald L Anderson American Gas and Chemical Company

• Glenn Andrews Ultra Image International

• Bruce Apgar DuPont NDT Systems

• R.A Armistead Advanced Research and Applications Corporation

• Ad Asead University of Michigan at Dearborn

• David Atherton Queen's University

• Yoseph Bar-Cohen Douglas Aircraft Company McDonnell Douglas Corporation

• R.C Barry Lockheed Missiles & Space Company, Inc

• John Bassart Iowa State University

• George Becker DuPont NDT Systems

• R.E Beissner Southwest Research Institute

• Alan P Berens University of Dayton Research Institute

• Harold Berger Industrial Quality, Inc

• Henry Bertoni Polytechnic University of New York

• R.A Betz Lockheed Missiles & Space Company, Inc

• Craig C Biddle United Technologies Research Center

• Kelvin Bishop Tennessee Valley Authority

• Carl Bixby Zygo Corporation

• Dave Blackham Consultant

• Gilbert Blake Wiss, Janney, Elstner Associates

• James Bolen Northrop Aircraft Division

• Jim Borges Intec Corporation

• J.S Borucki Ardox Inc

• Richard Bossi Boeing Aerospace Division The Boeing Company

• Byron Brendan Battelle Pacific Northwest Laboratories

• G.L Burkhardt Southwest Research Institute

• Paul Burstein Skiametrics, Inc

• Willard L Castner National Aeronautics and Space Administration Lyndon B Johnson Space Center

• V.S Cecco Atomic Energy of Canada, Ltd Chalk River Nuclear Laboratories

• Francis Chang General Dynamics Corporation

• Tsong-how Chang University of Wisconsin, Milwaukee

• F.P Chiang Laboratory for Experimental Mechanics Research State University of New York at Stony Brook

• D.E Chimenti Wright Research & Development Center Wright-Patterson Air Force Base

• P Cielo National Research Council of Canada Industrial Materials Research Institute

• T.N Claytor Los Alamos National Laboratory

• J.M Coffey CEGB Scientific Services

• J.F Cook Idaho National Engineering Laboratory EG&G Idaho, Inc

• Thomas D Cooper Wright Research & Development Center Wright-Patterson Air Force Base

• William D Cowie United States Air Force Aeronautical Systems Division

• L.D Cox General Dynamics Corporation

• Robert Cribbs Folsom Research Inc

• J.P Crosson Lucius Pitkin, Inc

• Darrell Cutforth Argonne National Laboratory

• William Dance LTV Missiles & Electronics Group

• Steven Danyluk University of Illinois

• Oliver Darling Spectrum Marketing, Inc

• E.A Davidson Wright Research & Development Center Wright-Patterson Air Force Base

• Vance Deason EG&G Idaho, Inc

• John DeLong Philadelphia Electric Company

• Michael J Dennis NDE Systems & Services General Electric Company

• Richard DeVor University of Illinois at Urbana-Champaign

• Robert L Ditz GE Aircraft Engines General Electric Company

• Kevin Dooley University of Minnesota

• Thomas D Dudderar AT&T Bell Laboratories

• Charles D Ehrlich National Institute of Standards & Technology

• Ralph Ekstrom University of Nebraska Lincoln

• Robert Erf United Technologies Research Center

• K Erland United Technologies Corporation Pratt & Whitney Group

• J.L Fisher Southwest Research Institute

• Colleen Fitzpatrick Spectron Development Laboratory

• William H Folland United Technologies Corporation Pratt & Whitney Group

• Joseph Foster Texas A&M University

• Kenneth Fowler Panametrics, Inc

• E.M Franklin Argonne National Laboratory Argonne West

• Larry A Gaylor Dexter Water Management Systems

• David H Genest Brown & Sharpe Manufacturing Company

• Dennis German Ford Motor Company

• Ron Gerow Consultant

• Scott Giacobbe GPU Nuclear

• Robert S Gilmore General Electric Research and Development Center

• J.N Gray Center for NDE Iowa State University

• T.A Gray Center for NDE Iowa State University

• Robert E Green, Jr. The Johns Hopkins University

• Arnold Greene Micro/Radiographs Inc

• Robert Grills Ultra Image International

• Donald Hagemaier Douglas Aircraft Company McDonnell Douglas Corporation

• John E Halkias General Dynamics Corporation

• Grover L Hardy Wright Research & Development Center Wright-Patterson Air Force Base

• Patrick G Heasler Battelle Pacific Northwest Laboratories

• Charles J Hellier Hellier Associates, Inc

• Edmond G Henneke Virginia Polytechnic Institute and State University

• B.P Hildebrand Failure Analysis Associates, Inc

• Howard E Housermann ZETEC, Inc

• I.C.H Hughes BCIRA International Centre

• Phil Hutton Battelle Pacific Northwest Laboratories

• Frank Iddings Southwest Research Institute

• Bruce G Isaacson Bio-Imaging Research, Inc

• W.B James Hoeganaes Corporation

• D.C Jiles Iowa State University

• Turner Johnson Brown & Sharpe Manufacturing Company

• John Johnston Krautkramer Branson

• William D Jolly Southwest Research Institute

• M.H Jones Los Alamos National Laboratory

• Gail Jordan Howmet Corporation

• William T Kaarlela General Dynamics Corporation

• Robert Kalan Naval Air Engineering Center

• Paul Kearney Welch Allyn Inc

• William Kennedy Canadian Welding Bureau

• Lawrence W Kessler Sonoscan, Inc

• Thomas G Kincaid Boston University

• Stan Klima NASA Lewis Research Center

• Kensi Krzywosz Electric Power Research Institute Nondestructive Evaluation Center

• David Kupperman Argonne National Laboratory

• H Kwun Southwest Research Institute

• J.W Lincoln Wright Research & Development Center Wright-Patterson Air Force Base

• Art Lindgren Magnaflux Corporation

• D Lineback Measurements Group, Inc

• Charles Little Sandia National Laboratories

• William Lord Iowa State University

• D.E Lorenzi Magnaflux Corporation

• Charles Loux GE Aircraft Engines General Electric Company

• A Lucero ZETEC, Inc

• Theodore F Luga Consultant

• William McCroskey Innovative Imaging Systems, Inc

• Ralph E McCullough Texas Instruments, Inc

• William E.J McKinney DuPont NDT Systems

• Brian MacCracken United Technologies Corporation Pratt & Whitney Group

• Ajit K Mal University of California, Los Angeles

• A.R Marder Energy Research Center Lehigh University

• Samuel Marinov Western Atlas International, Inc

• George A Matzkanin Texas Research Institute

• John D Meyer Tech Tran Consultants, Inc

• Morey Melden Spectrum Marketing, Inc

• Merlin Michael Rockwell International

• Carol Miller Wright Research & Development Center Wright-Patterson Air Force Base

• Ron Miller MQS Inspection, Inc

• Richard H Moore CMX Systems, Inc

• Thomas J Moran Consultant

• John J Munro III RTS Technology Inc

• N Nakagawa Center for NDE Iowa State University

• John Neuman Laser Technology, Inc

• H.I Newton Babcock & Wilcox

• G.B Nightingale General Electric Company

• Mehrdad Nikoonahad Bio-Imaging Research, Inc

• R.C O'Brien Hoeganaes Corporation

• Kanji Ono University of California, Los Angeles

• Vicki Panhuise Allied-Signal Aerospace Company Garrett Engine Division

• James Pellicer Staveley NDT Technologies, Inc

• Robert W Pepper Textron Specialty Materials

• C.C Perry Consultant

• John Petru Kelly Air Force Base

• Richard Peugeot Peugeot Technologies, Inc

• William Plumstead Bechtel Corporation

• Adrian Pollock Physical Acoustic Corporation

• George R Quinn Hellier Associates, Inc

• Jay Raja Michigan Technological University

• Jack D Reynolds General Dynamics Corporation

• William L Rollwitz Southwest Research Institute

• A.D Romig, Jr. Sandia National Laboratories

• Ward D Rummel Martin Marietta Astronautics Group

• Charles L Salkowski National Aeronautics and Space Administration Lyndon B Johnson Space Center

• Thomas Schmidt Consultant

• Gerald Scott Martin Marietta Manned Space Systems

• D.H Shaffer Westinghouse Electric Corporation Research and Development Center

• Charles N Sherlock Chicago Bridge & Iron Company

• Thomas A Siewert National Institute of Standards and Technology

• Peter Sigmund Lindhult & Jones, Inc

• Lawrence W Smiley Reliable Castings Corporation

• James J Snyder Westinghouse Electric Company Oceanic Division

• Doug Steele GE Aircraft Engines General Electric Company

• John M St John Caterpillar, Inc

• Bobby Stone Jr. Kelly Air Force Base

• George Surma Sundstrand Aviation Operations

• Lyndon J Swartzendruber National Institute of Standards and Technology

• Richard W Thams X-Ray Industries, Inc

• Graham H Thomas Sandia National Laboratories

• R.B Thompson Center for NDE Iowa State University

• Virginia Torrey Welch Allyn Inc

• James Trolinger Metro Laser

• Michael C Tsao Ultra Image International

• Glen Wade University of California, Santa Barbara

• James W Wagner The Johns Hopkins University

• Henry J Weltman General Dynamics Corporation

• Samuel Wenk Consultant

• Robert D Whealy Boeing Commercial Airplane Company

• David Willis Allison Gas Turbine Division General Motors Corporation

• Charles R Wojciechowski NDE Systems and Services General Electric Company

• J.M Wolla U.S Naval Research Laboratory

• John D Wood Lehigh University

• Nello Zuech Vision Systems International

Foreword

Volume 17 of Metals Handbook is a testament to the growing importance and increased sophistication of methods used to

nondestructively test and analyze engineered products and assemblies For only through a thorough understanding of modern techniques for nondestructive evaluation and statistical analysis can product reliability and quality control be achieved and maintained

As with its 8th Edition predecessor, the aim of this Volume is to provide detailed technical information that will enable readers to select, use, and interpret nondestructive methods Coverage, however, has been significantly expanded to encompass advances in established techniques as well as introduce the most recent developments in computed tomography, digital image enhancement, acoustic microscopy, and electromagnetic techniques used for stress analysis In addition, material on quantitative analysis and statistical methods for design and quality control (subjects covered only briefly in the 8th Edition) has been substantially enlarged to reflect the increasing utility of these disciplines

Publication of Volume 17 also represents a significant milestone in the history of ASM International This Volume

completes the 9th Edition of Metals Handbook, the largest single source of information on the technology of metals that

has ever been compiled The magnitude, respect, and success of this unprecedented reference set calls for a special tribute

to its many supporters Over the past 13 years, the ASM Handbook Committee has been tireless in its efforts, ASM members have been unflagging in their support, and the editorial staff devoted and resourceful Their efforts, combined with the considerable knowledge and technical expertise of literally thousands of authors, contributors, and reviewers,

have resulted in reference books which are comprehensive in coverage and which set the highest standards for quality To all these men and women, we extend our most sincere appreciation and gratitude

The subject of nondestructive examination and analysis of materials and manufactured parts and assemblies is not new to

Metals Handbook In 1976, Volume 11 of the 8th Edition Nondestructive Inspection and Quality Control provided what

was at that time one of the most thorough overviews of this technology ever published Yet in the relatively short time span since then, tremendous advances and improvements have occurred in the field so much so that even the terminology has evolved For example, in the mid-1970s the examination of an object or material that did not render it unfit for use was termed either nondestructive testing (NDT) or nondestructive inspection (NDI) Both are similar in that they involve looking at (or through) an object to determine either a specific characteristic or whether the object contains discontinuities, or flaws

The refinement of existing methods, the introduction of new methods, and the development of quantitative analysis have led to the emergence of a third term over the past decade, a term representing a more powerful tool With nondestructive evaluation (NDE), a discontinuity can be classified by its size, shape, type, and location, allowing the investigator to determine whether or not the flaw is acceptable The title of the present 9th Edition volume was modified to reflect this new technology

Volume 17 is divided into five major sections The first contains four articles that describe equipment and techniques used for qualitative part inspection Methods for both defect recognition (visual inspection and machine vision systems) and dimensional measurements (laser inspection and coordinate measuring machines) are described

In the second section, 24 articles describe the principles of a wide variety of nondestructive techniques and their application to quality evaluation of metallic, composite, and electronic components In addition to detailed coverage of more commonly used methods (such as magnetic particle inspection, radiographic inspection, and ultrasonic inspection), newly developed methods (such as computed tomography, acoustic microscopy, and speckle metrology) are introduced The latest developments in digital image enhancement are also reviewed Finally, a special six-page color section illustrates the utility of color-enhanced images

The third section discusses the application of nondestructive methods to specific product types, such as one-piece products (castings, forgings, and powder metallurgy parts) and assemblies that have been welded, soldered, or joined with adhesives Of particular interest is a series of reference radiographs presented in the article "Weldments, Brazed Assemblies, and Soldered Joints" that show a wide variety of weld discontinuities and how they appear as radiographic images

The reliability of discontinuity detection by nondestructive methods, referred to as quantitative NDE, is the subject of the fourth section Following an introduction to this rapidly maturing discipline, four articles present specific guidelines to help the investigator determine the critical discontinuity size that will cause failure, how long a structure containing a discontinuity can be operated safely in service, how a structure can be designed to prevent catastrophic failure, and what inspections must be performed in order to prevent failure

The final section provides an extensive review of the statistical methods being used increasingly for design and quality control of manufactured products The concepts of statistical process control, control charts, and design of experiments are presented in sufficient detail to enable the reader to appreciate the importance of statistical analysis and to organize and put into operation a system for ensuring that quality objectives are met on a consistent basis

This Volume represents the collective efforts of nearly 200 experts who served as authors, contributors of case histories,

or reviewers To all we extend our heartfelt thanks We would also like to acknowledge the special efforts of Thomas D

Cooper (Wright Research & Development Center, Wright-Patterson Air Force Base) and Vicki E Panhuise Signal Aerospace Company, Garrett Engine Division) Mr Cooper, a former Chairman of the ASM Handbook Committee, was instrumental in the decision to significantly expand the material on quantitative analysis Dr Panhuise organized the content and recruited all authors for the section "Quantitative Nondestructive Evaluation." Such foresight

(Allied-and commitment from H(Allied-andbook contributors over the years has helped make the 9th Edition of Metals H(Allied-andbook all 17

volumes and 15,000 pages the most authoritative reference work on metals ever published

The Editors

General Information

Officers and Trustees of ASM International

Officers

• Richard K Pitler President and Trustee Allegheny Ludlum Corporation (retired)

• Klaus M Zwilsky Vice President and Trustee National Materials Advisory Board National Academy of Sciences

• William G Wood Immediate Past President and Trustee Kolene Corporation

• Robert D Halverstadt Treasurer AIMe Associates

Trustees

• John V Andrews Teledyne Allvac

• Edward R Burrell Inco Alloys International, Inc

• Stephen M Copley University of Southern California

• H Joseph Klein Haynes International, Inc

• Gunvant N Maniar Carpenter Technology Corporation

• Larry A Morris Falconbridge Limited

• William E Quist Boeing Commercial Airplanes

• Charles Yaker Howmet Corporation

• Daniel S Zamborsky Consultant

• Edward L Langer Managing Director ASM International

Members of the ASM Handbook Committee (1988-1989)

• Dennis D Huffman (Chairman 1986-; Member 1983-) The Timken Company

• Roger J Austin (1984-) ABARIS

• Roy G Baggerly (1987-) Kenworth Truck Company

• Robert J Barnhurst (1988-) Noranda Research Centre

• Peter Beardmore (1986-1989) Ford Motor Company

• Hans Borstell (1988-) Grumman Aircraft Systems

• Gordon Bourland (1988-) LTV Aerospace and Defense Company

• Robert D Caligiuri (1986-1989) Failure Analysis Associates

• Richard S Cremisio (1986-1989) Rescorp International, Inc

• Gerald P Fritzke (1988-) Metallurgical Associates

• J Ernesto Indacochea (1987-) University of Illinois at Chicago

• John B Lambert (1988-) Fansteel Inc

• James C Leslie (1988-) Advanced Composites Products and Technology

• Eli Levy (1987-) The De Havilland Aircraft Company of Canada

• Arnold R Marder (1987-) Lehigh University

• John E Masters (1988-) American Cyanamid Company

• L.E Roy Meade (1986-1989) Lockheed-Georgia Company

• Merrill L Minges (1986-1989) Air Force Wright Aeronautical Laboratories

• David V Neff (1986-) Metaullics Systems

• Dean E Orr (1988-) Orr Metallurgical Consulting Service, Inc

• Ned W Polan (1987-1989) Olin Corporation

• Paul E Rempes (1986-1989) Williams International

• E Scala (1986-1989) Cortland Cable Company, Inc

• David A Thomas (1986-1989) Lehigh University

• Kenneth P Young (1988-) AMAX Research & Development

Previous Chairmen of the ASM Handbook Committee

of Reference Publications

Conversion to Electronic Files

ASM Handbook, Volume 17, Nondestructive Evaluation and Quality Control was converted to electronic files in 1998

The conversion was based on the fifth printing (1997) No substantive changes were made to the content of the Volume, but some minor corrections and clarifications were made as needed

ASM International staff who contributed to the conversion of the Volume included Sally Fahrenholz-Mann, Bonnie Sanders, Marlene Seuffert, Gayle Kalman, Scott Henry, Robert Braddock, Alexandra Hoskins, and Erika Baxter The electronic version was prepared under the direction of William W Scott, Jr., Technical Director, and Michael J DeHaemer, Managing Director

Copyright Information (for Print Volume)

Copyright © 1989 ASM International All rights reserved

No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the written permission of the copyright owner First printing, September 1989

Second printing, May 1992

Third printing, May 1994

Fourth printing, January 1996

Fifth printing, December 1997

ASM Handbook is a collective effort involving thousands of technical specialists It brings together in one book a wealth

of information from world-wide sources to help scientists, engineers, and technicians solve current and long-range problems

Great care is taken in the compilation and production of this Volume, but it should be made clear that no warranties, express or implied, are given in connection with the accuracy or completeness of this publication, and no responsibility can be taken for any claims that may arise

Nothing contained in the ASM Handbook shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered

by letters patent, copyright, or trademark, and nothing contained in the ASM Handbook shall be construed as a defense against any alleged infringement of letters patent, copyright, or trademark, or as a defense against any liability for such infringement

Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International

Library of Congress Cataloging-in-Publication Data (for Print Volume)

Metals handbook

Includes bibliographies and indexes

Contents: v 1 Properties and selection v 2 Properties and selection nonferrous alloysand pure metals [etc.] v 17 Nondestructiveevaluation and quality control

1 Metals Handbooks, manuals, etc

I ASM Handbook Committee

II ASM International Handbook Committee

TA459.M43 1978 669 78-14934

ISBN 0-87170-007-7 (v 1)

SAN 204-7586

Visual Inspection

Introduction

VISUAL INSPECTION is a nondestructive testing technique that provides a means of detecting and examining a variety

of surface flaws, such as corrosion, contamination, surface finish, and surface discontinuities on joints (for example, welds, seals, solder connections, and adhesive bonds) Visual inspection is also the most widely used method for detecting and examining surface cracks, which are particularly important because of their relationship to structural failure mechanisms Even when other nondestructive techniques are used to detect surface cracks, visual inspection often provides a useful supplement For example, when the eddy current examination of process tubing is performed, visual inspection is often performed to verify and more closely examine the surface disturbance

Given the wide variety of surface flaws that may be detectable by visual examination, the use of visual inspection may encompass different techniques, depending on the product and the type of surface flaw being monitored This article focuses on some equipment used to aid the process of visual inspection The techniques and applicability of visual inspection for some products are considered in the Selected References in this article and in the Section "Nondestructive Inspection of Specific Products" in this Volume

The methods of visual inspection involve a wide variety of equipment, ranging from examination with the naked eye to the use of interference microscopes for measuring the depth of scratches in the finish of finely polished or lapped surfaces Some of the equipment used to aid visual inspection includes:

areas

photographs, videotapes, or computer-enhanced images

surface microstructures

(and sometimes near-surface conditions in the case of magnetic particle inspection)

This article will review the use of the equipment listed above in visual inspection, except for dye penetrants and magnetic particles, which are discussed in the articles "Liquid Penetrant Inspection" and "Magnetic Particle Inspection," respectively, in this Volume

Acknowledgements

ASM International would like to thank Oliver Darling and Morley Melden of Spectrum Marketing, Inc., for their assistance in preparing the section on borescopes They provided a draft of a textbook being developed for Olympus Corporation Thanks are also extended to Virginia Torrey of Welch Allyn, Inc., for the information on videoscopes and to Peter Sigmund of Lindhult and Jones, Inc., for the information on instruments from Lenox, Inc

Visual Inspection

Borescopes

A borescope (Fig 1) is a long, tubular optical device that illuminates and allows the inspection of surfaces inside narrow tubes or difficult-to-reach chambers The tube, which can be rigid or flexible with a wide variety of lengths and diameters, provides the necessary optical connection between the viewing end and an objective lens at the distant, or distal, tip of the borescope This optical connection can be achieved in one of three different ways:

• By using a rigid tube with a series of relay lenses

device (CCD) imaging sensor at the distal tip

These three basic tube designs can have either fixed or adjustable focusing of the objective lens at the distal tip The distal tip also has prisms and mirrors that define the direction and field of view (see Fig 2) These views vary according to the type and application of borescope The design of illumination system also varies with the type of borescope Generally, a fiber optic light guide and a lamp producing white light is used in the illumination system, although ultraviolet light can

be used to inspect surfaces treated with liquid fluorescent penetrants Light-emitting diodes at the distal tip are sometimes used for illumination in videoscopes with working lengths greater than 15 m (50 ft)

Fig 1 Three typical designs of borescopes (a) A rigid borescope with a lamp at the distal end (b) A flexible

fiberscope with a light source (c) A rigid borescope with a light guide bundle in the shaft

Rigid Borescopes

Rigid borescopes are generally limited to applications with a straight-line path between the observer and the area to be observed The sizes range in lengths from 0.15

to 30 m (0.5 to 100 ft) and in diameters from 0.9 to 70

mm (0.035 to 2.75 in.) Magnification is usually 3 to 4×, but powers up to 50× are available The illumination system is either an incandescent lamp located at the distal tip end (Fig 1a) or a light guide bundle made from optical fibers (Fig 1c) that conduct light from an external source

The choice of viewing heads for rigid borescopes (Fig 2) varies according to the application, as described in the section "Selection" in this article Rigid borescopes generally have a 55° field of view, although the fields of view can range from 10 to 90° Typically, the distal tips are not interchangeable, but some models (such as the extendable borescopes) may have interchangeable viewing heads

Some rigid borescopes have orbital scan (Fig 1c), which involves the rotation of the optical shaft for scanning purposes Depending on the borescope model, the amount of rotation can vary from 120 to 370° Some rigid borescopes also have movable prisms at the tip for scanning

Rigid borescopes are available in a variety of models having significant variations in the design of the shaft, the distal tip, and the illumination system Some of these design variations are described below

Basic Design The rigid borescope typically has a series of achromatic relay lenses in the optical tube These lenses

preserve the resolution of the image as it travels from the objective lens to the eyepiece The tube diameter of these borescopes ranges from 4 to 70 mm (0.16 to 2.75 in.) The illumination system can be either a distal lamp or a light guide bundle, and the various features may include orbital scan, various viewing heads, and adjustable focusing of the objective lens

Miniborescopes Instead of the conventional relay lenses, miniborescopes have a single image-relaying rod or quartz

fiber in the optical tube The lengths of miniborescopes are 110 and 170 mm (4.3 and 6.7 in.), and the diameters range from 0.9 to 2.7 mm (0.035 to 0.105 in.) High magnification (up to 30×) can be reached at minimal focal lengths, and an adjustable focus is not required, because the scope has an infinite depth of field The larger sizes have forward, side view, and forward-oblique views The 0.9 mm (0.035 in.) diam size has only a forward view Miniborescopes have an integral light guide bundle

Hybrid borescopes utilize rod lenses combined with convex lenses to relay the image The rod lenses have fewer

glass-air boundaries; this reduces scattering and allows for a more compact optical guide Consequently, a larger light guide bundle can be employed with an increase in illumination and an image with a higher degree of contrast

Hybrid borescopes have lengths up to 990 mm (39 in.), with diameters ranging from 5.5 to 12 mm (0.216 to 0.47 in.) All hybrid borescopes have adjustable focusing of the objective lens and a 370° rotation for orbital scan The various viewing directions are forward, side, retrospective, and forward-oblique

Extendable borescopes allow the user to construct a longer borescopic tube by joining extension tubes Extendable

borescopes are available with either a fiber-optic light guide or an incandescent lamp at the distal end Extendable borescopes with an integral lamp have a maximum length of about 30 m (100 ft) Scopes with a light guide bundle have a shorter maximum length (about 8 m, or 26 ft), but do allow smaller tube diameters (as small as 8 mm, or 0.3 in.) Interchangeable viewing heads are also available Extendable borescopes do not have adjustable focusing of the objective lens

Fig 2 Typical directions and field of view with rigid

borescopes

Rigid chamberscopes allow more rapid inspection of larger chambers Chamberscopes (Fig 3) have variable

magnification (zoom), a lamp at the distal tip, and a scanning mirror that allows the user to observe in different directions The higher illumination and greater magnification of chamberscopes allow the inspection of surfaces as much as 910 mm (36 in.) away from the distal tip of the scope

Mirror sheaths can convert a direct-viewing

borescope into a side-viewing scope A mirror sheath is designed to fit over the tip of the scope and thus reflect

an image from the side of the scope However, not all applications are suitable for this device A side, forward-oblique, or retrospective viewing head provides better resolution and a higher degree of image contrast A mirror sheath also produces an inverse image and may produce unwanted reflections from the shaft

Scanning In addition to the orbital scan feature

described earlier, some rigid borescopes have the ability

to scan longitudinally along the axis of the shaft A movable prism with a control at the handle accomplishes this scanning Typically, the prism can shift the direction of view through an arc of 120°

Flexible Borescopes

Flexible borescopes are used primarily in applications that do not have a straight passageway to the point of observation The two types of flexible borescopes are flexible fiberscopes and videoscopes with a CCD image sensor at the distal tip

Flexible Fiberscopes A typical fiberscope (Fig 1b) consists of a light guide bundle, an image guide bundle, an

objective lens, interchangeable viewing heads, and remote controls for articulation of the distal tip Fiberscopes are available in diameters from 1.4 to 13 mm (0.055 to 0.5 in.) and in lengths up to 12 m (40 ft) Special quartz fiberscopes are available in lengths up to 90 m (300 ft)

The fibers used in the light guide bundle are generally 30 m (0.001 in.) in diameter The second optical bundle, called the image guide, is used to carry the image formed by the objective lens back to the eyepiece The fibers in the image guide must be precisely aligned so that they are in an identical relative position to each other at their terminations for proper image resolution

The diameter of the fibers in the image guide is another factor in obtaining good image resolution With smaller diameter fibers, a brighter image with better resolution can be obtained by packing more fibers in the image guide With higher resolution, it is then possible to use an objective lens with a wider field of view and also to magnify the image at the eyepiece This allows better viewing of objects at the periphery of the image (Fig 4) Image guide fibers range from 6.5

to 17 m (255 to 670 in.)

Fig 3 Typical chamberscope Courtesy of Lenox

Instrument Company

Fig 4 Two views down a combustor can with the distal tip in the same position A fiberscope with smaller

diameter fibers and 40% more fibers in the image bundle provides better resolution (a) than a fiberscope with larger fibers (b) Courtesy of Olympus Corporation

The interchangeable distal tips provide various directions and fields of view on a single fiberscope However, because the tip can be articulated for scanning purposes, distal tips with either a forward or side viewing direction are usually sufficient Fields of view are typically 40 to 60°, although they can range from 10 to 120° Most fiberscopes provide adjustable focusing of the objective lens

Videoscopes with CCD probes involve the electronic transmission of color or black and white images to a video

monitor The distal end of electronic videoscopes contains a CCD chip, which consists of thousands of light-sensitive elements arrayed in a pattern of rows and columns The objective lens focuses the image of an object on the surface of the CCD chip, where the light is converted to electrons that are stored in each picture element, or pixel, of the CCD device The image of the object is thus stored in the form of electrons on the CCD device At this point, a voltage proportional to the number of electrons at each pixel is determined electronically for each pixel site This voltage is then amplified, filtered, and sent to the input of a video monitor

Videoscopes with CCD probes produce images (Fig 5) with spatial resolutions of the order of those described in Fig 6 Like rigid borescopes and flexible fiberscopes, the resolution of videoscopes depends on the object-to-lens distance and the fields of view, because these two factors affect the amount of magnification (see the section "Magnification and Field

of View" in this article) Generally, videoscopes produce higher resolution than fiberscopes, although fiberscopes with smaller diameter fibers (Fig 4a) may be competitive with the resolution of videoscopes

Fig 5 Videoscope images (a) inside engine guide vanes (b) of an engine fuel nozzle Courtesy of Welch Allyn,

Inc

Fig 6 Typical resolution of CCD videoscopes with a 90° field of view (a), 60° field of view (b), 30° field of view

(c) Source: Welch Allyn, Inc

Another advantage of videoscopes is their longer working length With a given amount of illumination at the distal tip, videoscopes can return an image over a greater length than fiberscopes Other features of videoscopes include:

eyepiece)

integration with automatic inspection systems

Fig 7 Image from a videoscope (a) and a fiberscope (b) In some fiberscope images, voids between individual

glass fibers can create a honeycomb pattern that adds graininess to the image Courtesy of Welch Allyn, Inc

Special Features

Measuring borescopes and fiberscopes contain a movable cursor that allows measurements during viewing (Fig

8) When the object under measurement is in focus, the movable cursor provides a reference for dimensional measurements in the optical plane of the object This capability eliminates the need to know the object-to-lens distance when determining magnification factors

Working channels are used in borescopes and fiberscopes to pass working

devices to the distal tip Working channels are presently used to pass measuring instruments, retrieval devices, and hooks for aiding the insertion of thin, flexible fiberscopes Working channels are used in flexible fiberscopes with diameters as small as 2.7 mm (0.106 in.) Working channels are also under consideration for the application and removal of dye penetrants and for the passage of wires and sensors in eddy current measurements

Selection

Flexible and rigid borescopes are available in a wide variety of standard and customized designs, and several factors can influence the selection of a scope for a particular application These factors include focusing, illumination, magnification, working length, direction of view, and environment

Focusing and Resolution If portions of long objects are at different planes, the

scope must have sufficient focus adjustment to achieve an adequate depth of field If the scope has a fixed focal length, the object will be in focus only at a specific lens-to-object distance

To allow the observation of surface detail at a desired size, the optical system of a borescope must also provide adequate resolution and image contrast If resolution is adequate but contrast is lacking, detail cannot be observed

In general, the optical quality of a rigid borescope improves as the size of the lens increases; consequently, a borescope with the largest possible diameter should be used For fiberscopes, the resolution is dependent on the accuracy of alignment and the diameter of the fibers in the image bundle Smaller-diameter fibers provide more resolution and edge contrast (Fig 4), when combined with good geometrical alignment of the fibers Typical resolutions of videoscopes are given in Fig 6

Illumination The required intensity of the light source is determined by the reflectivity of the surface, the area of

surface to be illuminated, and the transmission losses over the length of the scope At working lengths greater than 6 m (20 ft), rigid borescopes with a lamp at the distal end provide the greatest amount of illumination over the widest area However, the heat generated by the light source may deform rubber or plastic materials Fiber-optic illumination in scopes with working lengths less than 6 m (20 ft) is always brighter and is suitable for heat-sensitive applications because filters can remove infrared frequencies Because the amount of illumination depends on the diameter of the light guide bundle, it

is desirable to use the largest diameter possible

Magnification and field of view are interrelated; as magnification is increased, the field of view is reduced The

precise relationship between magnification and field of view is specified by the manufacturer

The degree of magnification in a particular application is determined by the field of view and the distance from the objective lens to the object Specifically, the magnification increases when either the field of view or the lens-to-object distance decreases

Working Length. In addition to the obvious need for a scope of sufficient length, the working length can sometimes

dictate the use of a particular type of scope For example, a rigid borescope with a long working length may be limited by the need for additional supports In general, videoscopes allow a longer working length than fiberscopes

Direction of View The selection of a viewing direction is influenced by the location of the access port in relation to the

object to be observed The following sections describe some criteria for choosing the direction of view shown in Fig 2 Flexible fiberscopes or videoscopes, because of their articulating tip, are often adequate with either a side or forward viewing tip

Circumferential or panoramic heads are designed for the inspection of tubing or other cylindrical structures A centrally located mirror permits right-angle viewing of an area just scanned by the panoramic view

The forward viewing head permits the inspection of the area directly ahead of the viewing head It is commonly used when examining facing walls or the bottoms of blind holes and cavities

Fig 8 View through a

measuring fiberscope with

reticles for 20° and 40°

field-of-view lenses Courtesy of

Olympus Corporation

Forward-oblique heads bend the viewing direction at an angle to the borescope axis, permitting the inspection of corners

at the end of a bored hole The retrospective viewing head bends the cone of view at a retrospective angle to the borescope axis, providing a view of the area just passed by the advancing borescope It is especially suited to inspecting the inside neck of cylinders and bottles

Environment Flexible and rigid borescopes can be manufactured to withstand a variety of environments Although

most scopes can operate at temperatures from -34 to 66 °C (-30 to 150 °F), especially designed scopes can be used at temperatures to 1925 °C (3500 °F) Scopes can also be manufactured for use in liquid media

Special scopes are required for use in pressures above ambient and in atmospheres exposed to radiation Radiation can cause the multicomponent lenses and image bundles to turn brown When a scope is used in atmospheres exposed to radiation, quartz fiberscopes are generally used Scopes used in a gaseous environment should be made explosionproof to minimize the potential of an accidental explosion

Applications

Rigid and flexible borescopes are available in different designs suitable for a variety of applications For example, when inspecting straight process piping for leaks rigid borescopes with a 360° radial view are capable of examining inside diameters of 3 to 600 mm (0.118 to 24 in.) Scopes are also used by building inspectors and contractors to see inside walls, ducts, large tanks, or other dark areas

The principal use of borescope is in equipment maintenance programs, in which borescopes can reduce or eliminate the need for costly teardowns Some types of equipment, such as turbines, have access ports that are specifically designed for borescopes Borescopes provide a means of checking in-service defects in a variety of equipment, such as turbines (Fig 9), automotive components (Fig 10), and process piping (Fig 11)

Fig 9 Turbine flaws seen through a flexible fiberscope (a) Crack near a fuel burner nozzle (b) Crack in an

outer combustion liner (c) Combustion chamber and high pressure nozzle guide vanes (d) Compressor damage showing blade deformation Courtesy of Olympus Corporation

Fig 10 In-service defects as seen through a borescope designed for automotive servicing (a) Carbon on

valves (b) Broken transmission gear tooth (c) Differential gear wear Courtesy of Lenox Instrument Company

Fig 11 Operator viewing a weld 21 m (70 ft) inside piping with a videoscope Courtesy of Olympus Corporation

Borescopes are also extensively used in a variety of manufacturing industries to ensure the product quality of reach components Manufacturers of hydraulic cylinders, for example, use borescopes to examine the interiors of bores for pitting, scoring, and tool marks Aircraft and aerospace manufacturers also use borescopes to verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult-to-reach regions

difficult-to-Visual Inspection

Optical Sensors

Visible light, which can be detected by the human eye or with optical sensors, has some advantages over inspection methods based on nuclear, microwave, or ultrasound radiation For example, one of the advantages of visible light is the capability of tightly focusing the probing beam on the inspected surface (Ref 1) High spatial resolution can result from this sharp focusing, which is useful in gaging and profiling applications (Ref 1)

Some different types of image sensors used in visual inspection include: