Kutz M. Handbook of materials selection

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648 CERAMICS TESTING

for material selection and design with brittle materials are still being developed.

As these methodologies mature and become widely accepted, additional test

methods related to the required properties and performance will be proposed,

developed, and approved.

REFERENCES

1. Richerson and Associates and Energetics, Inc., ‘‘Advanced Ceramics Technology Roadmap—

Charting Our Course’’ USACA Pub., 2000, pp. 1–3.

2. ‘‘Market Environment and Objectives of the ISO/ TC’’ in 7th Plenary Meeting of ISO/ TC 206—

Report of the Secretariat, 2000, pp. 2–6.

3. M. G. Jenkins, S. M. Wiederhorn, and R. K. Schiffer, ‘‘Creep Testing of Advanced Ceramics’’

in Mechanical Testing Methodology for Ceramic Design and Reliability, D. C. Cranmer and

D. W. Richerson (Eds.), Marcel Dekker, New York, 1998.

4. P. K. Khandelwhal and D. L. Vaccari, ‘‘Life Prediction Methodology of Ceramic Engine Com-

ponents’’ in Proceedings of the Annual Automotive Technology Development Contractors’ Meet-

ing, P-256, Dearborn, MI, October 28–31, 1991, Society of Automotive Engineers, Warrendale,

PA, pp. 253–260, 1992.

5. G. D. Quinn and F. Baratta, ‘‘Flexure Data Can It Be Used for Ceramics Part Design?’’ Adv.

Mat. Proc., 129(12) 31–35 (1985).

6. F. I. Baratta, ‘‘Requirements for Flexure Testing of Brittle Materials’’ in Methods for Assessing

the Structural Reliability of Brittle Materials, ASTM STP 884, S. W. Freiman and C. M. Hudson

(Eds.) ASTM, W. Conshohocken, PA, 1984, pp. 194–222.

7. G. D. Quinn, ‘‘Flexure Strength of Advanced Structural Ceramics: A Round Robin,’’ J. Am.

Ceram. Soc., 73(8) 2374–2384 (1990).

8. G. D. Quinn, ‘‘Indentation Hardness Testing of Ceramics’’ in ASM Handbook, Vol. 8, ASM

International, Materials Park, OH, 2000, pp. 243–251.

9. D. M. Butterﬁeld, D. J. Clinton, and R. Morrell, ‘‘The VAMAS Hardness Tests Round-Robin on

Ceramic Materials,’’ Report No. 3, Versailles Advanced Materials and Standards/ National Phys-

ical Laboratory, April 1989.

10. J. H. Miller and P. K. Liaw, ‘‘Fracture Toughness of Ceramics and Ceramic Matrix Composites’’

in ASM Handbook, Vol. 8, ASM International, Materials Park, OH, 2000, pp. 654–64.

11. G. Subhash and G. Ravichandran, ‘‘Split-Hopkinson Pressure Bar Testing of Ceramics’’ in ASM

Handbook, Vol. 8, ASM International, Materials Park, OH, 2000, pp. 497–504.

12. H. Kolsky, ‘‘An Investigation of the Mechanical Properties of Materials at Very High Rates of

Loading,’’ Proc. R. Soc. (London) B, 62, 676–700 (1949).

13. K. Breder and A. A. Wereszczak, ‘‘Fatigue and Slow Crack Growth,’’ in Mechanical Testing

Methodology for Ceramic Design and Reliability, D. C. Cranmer and D. W. Richerson (Eds.),

Marcel Dekker, New York, 1998. p. 223–227.

14. J. A. Salem and M. G. Jenkins, ‘‘Fatigue Testing of Brittle Solids,’’ in ASM Handbook, Vol. 8,

ASM International, Materials Park, OH, 2000, pp. 768–778.

15. William F. Hammetter, ‘‘Thermophysical Properties,’’ in Engineered Materials Handbook, Vol.

4, Ceramics and Glasses, ASM International, Materials Park, OH, 1991, pp. 610–612.

16. William F. Hammetter, ‘‘Thermophysical Properties,’’ in Engineered Materials Handbook, Vol.

4, Ceramics and Glasses, ASM International, Materials Park, OH, 1991, pp. 612–614.

17. William F. Hammetter, ‘‘Thermophysical Properties,’’ in Engineered Materials Handbook, Vol.

4, Ceramics and Glasses, ASM International, Materials Park, OH, 1991, pp. 614–615.

18. L. A. Lott and D. C. Kunerth, ‘‘NDE Testing and Inspection,’’ in Engineered Materials Hand-

book, Vol. 4, Ceramics and Glasses, ASM International, Materials Park, OH, 1991, pp. 617–

626.

649

CHAPTER 24

NONDESTRUCTIVE INSPECTION

Robert L. Crane

Air Force Wright Laboratory

Materials Directorate

Nondestructive Evaluation Branch

Wright Patterson Air Force Base

Dayton, Ohio

Ward D. Rummel

D&W Enterprises

Littleton, Colorado

1 INTRODUCTION 650

1.1 Information on Inspection

Methods 651

1.2 Additional References 651

1.3 Electronic References 651

1.4 Future NDE Capabilities 652

2 LIQUID PENETRANTS 652

2.1 Penetrant Process 654

2.2 Reference Standards 656

2.3 Limitations of Penetrant

Inspections 656

3 ULTRASONIC METHODS 656

3.1 Sound Waves 657

3.2 Reﬂection and Transmission

of Sound 657

3.3 Refraction of Sound 660

3.4 Inspection Process 662

4 RADIOGRAPHY 663

4.1 Generation and Absorption of

X-Radiation 664

4.2 Neutron Radiography 666

4.3 Attenuation of X-Radiation 667

4.4 Film-Based Radiography 668

4.5 The Penetrameter 669

4.6 Real-Time Radiography 670

4.7 Computed Tomography 671

5 EDDY CURRENT INSPECTION 672

5.1 Skin Effect 673

5.2 Impedance Plane 673

5.3 Lift-Off of Inspection Coil from

Specimen 676

6 THERMAL METHODS 678

6.1 Infrared Cameras 678

6.2 Thermal Paints 679

6.3 Thermal Testing 679

7 MAGNETIC PARTICLE METHOD 680

7.1 Magnetizing Field 680

7.2 Continuous versus

Noncontinuous Fields 681

7.3 Inspection Process 682

7.4 Demagnetizing the Part 682

8 CONSIDERATIONS FOR

INSPECTABILITY IN

MATERIALS SELECTION 683

8.1 Conﬁdence in Materials

Properties 683

8.2 Structural Integrity 683

8.3 Beneﬁts of Quantiﬁed Crack

Detection 686

8.4 Quantiﬁcation of NDE

Capabilities 690

8.5 Probability of

Detection 691

, 8.6 General Process Control Is

Required in All NDI

Applications 693

9 SUMMARY 693

APPENDIX A: ULTRASONIC

PROPERTIES OF COMMON

MATERIALS 694

APPENDIX B: ELECTRICAL

RESISTIVITIES AND

CONDUCTIVITIES OF METALS

AND ALLOYS 699

REFERENCES 700

HandbookofMaterialsSelection.EditedbyMyerKutz

650 NONDESTRUCTIVE INSPECTION

1 INTRODUCTION

This chapter deals with the nondestructive inspection of materials, components,

and structures. The term nondestructive inspection (NDI) or nondestructive eval-

uation (NDE) is deﬁned as that class of physical and chemical tests that permit

the detection and/or measurement of the signiﬁcant properties or the detection

of defects in materials, components, or structures without impairing their use-

fulness. The NDI process is often complicated by the fact that many modern

materials are by nature anisotropic. Most of the current NDI techniques were

developed for isotropic materials such as metals. The added complication due

to the anisotropy usually means that an inspection is more complicated and

requires more analysis than had been the case previously.

Inspection of complex materials or structures is often carried out by compar-

ing the expected NDI data with a standard and noting unexpected deviations.

This means a high-quality standard must be available for calibration of the in-

spection instrumentation. Furthermore, high-quality standards also must contain

special implanted ﬂaws that mimic those that naturally occur in these materials

or structures. Without a high quality standard to test and calibrate the inspection

process, the analysis of NDI results can be signiﬁcantly in error. For example,

to estimate the amount of porosity in a cast component from ultrasonic mea-

surements, standards with calibrated levels of porosity must be available to cal-

ibrate the instrumentation. Without such standards, estimation of porosity from

ultrasonic data is a highly speculative process.

There are many methods available to conduct the NDE or NDI. This chapter

covers some of the important and some less well known NDI tests. Since in-

formation on the less often used tests is not generally in standard texts, additional

sources of information are listed in the references at the end of the chapter.

Inspection instrumentation must possess four qualities to receive widespread

acceptance in the NDI community. These qualities are

1. Accuracy. The instrument must accurately measure some property of the

material or structure that can be used to infer either properties or the

presence of ﬂaws.

2. Reliability. The instrument must be highly reliable. That is, it must con-

sistently detect and quantify ﬂaws or a property with a high degree of

reliability. If an instrument is not reliable, then it will misdiagnosed prop-

erties or ﬂaws can lead to failure of the component, property damage,

and potential loss of life.

3. Simplicity. The instruments that are most useful are those that may be

used by factory or repair-level technicians. Instrumentation that requires

highly skilled operators are very rarely used by the inspection community.

4. Low Cost. An instrument need not be low cost in an absolute sense.

Instead, it must be inexpensive relative to the value of the component

under test or the cost of a failure or aborted mission. For ﬂight-critical

aircraft components, as much as 12% of the value of the component may

be spent in inspection.

While this list covers the desirable characteristics for instrumentation, those

that apply to the inspection process are enumerated in the Section 8 on reliability.

1 INTRODUCTION 651

1.1 Information on Inspection Methods

To the engineer or scientist confronted by a new inspection requirement there

arises the question of where to ﬁnd pertinent information regarding an inspection

procedure and its interpretation. Fortunately there are many potential sources of

information about instrumentation and techniques available for nondestructive

inspection. A brief examination of the literature is presented next. Many of these

references were generated as a result of the demands on materials used in aero-

space structures. There was an attempt to provide traditional scientiﬁc and en-

gineering books and journals that would be available in any well-stocked library.

However, with the rise of the Internet there are now many electronic sources of

information available on the World Wide Web. These include such information

sources as: the library catalog, societal home pages, an on-line journal devoted

to NDI, manufacturers of instrumentation with on-line demonstrations of ser-

vices, inspection software, and on-line chat rooms devoted to solving inspection

problems. The references include the most commonly used sources. However,

with new electronic sources appearing daily this is only a brief snap-shot of the

sources available at the beginning of the twenty-ﬁrst century.

General NDE Reference Books

Some general references to the NDE techniques are given in Refs. 1–22. The

reader will note that some of these citations are not recent; they were included

because of their value to the engineer who does not possess formal training in

NDE. Additionally, some older works were chosen because of the clarity of

presentation, completeness, or their usefulness to the inspection of complex

structures.

NDE Journals

The periodical literature is a source of the latest research results of new or

modiﬁed inspection methodologies.

23–31

Some journals are no longer available,

but are included because of their archival value or because the may contain data

available nowhere else. Whenever possible, World Wide Web addresses are pro-

vided to give the readers ready access to the publisher.

1.2 Additional References

There are many potentially useful references for the reader interested in using

or modifying NDE techniques.

4,7,11,13,32

For those new to the ﬁeld the American

Society for Testing and Materials (ASTM) standards is particularly valuable

because it gives very detailed directions on many NDE techniques. More im-

portantly, it is widely accepted as a standard reference for standard inspections.

The other references are given for those situations where standard inspection

methods are not sufﬁcient to detect the material condition of interest.

1.3 Electronic References

There are many useful electronic references for those working in NDE technol-

ogy. Of the many useful sites on the World Wide Web (www), only a few are

included here. Many sites contain links to other sites that may contain infor-

mation on a special topic of interest to the reader. Because the web is growing

at a fantastic rate, the list in the bibliography represents a very brief snapshot

of the information available to the NDE community. Even some useful www

652 NONDESTRUCTIVE INSPECTION

sites associated with government agencies were not included due to space lim-

itations. The list of sites is divided into sections associated with NDE

societies,

33–37

institutes

38–42

government agencies,

43–46

publishers,

29,45,47–49

and

general-interest sites.

37,50–54

1.4 Future NDE Capabilities

At this point the reader might be tempted to ask if there are new technologies

on the horizon that will enable more cost-effective, anticipatory inspections, or

monitoring of materials and structures? This answer to this is an emphatic yes.

There are new developments in solid-state detectors that should signiﬁcantly

impact both the capability of inspection and its cost as well. For example, optical

and X-ray detectors will give the inspector the ability to rapid scan large areas

of structures for defects. By coupling this technology to computer algorithms

that search image information, the inspection of large areas can be automated

providing more accurate inspections with much less operator fatigue.

The area of data fusion is just being explored in the NDE ﬁeld. This means

that data collected with one technique can be combined with another technique

to detect a range of ﬂaws not detected with either independently. The data from

several techniques that are coupled at the basic physics level provide a more

complete description of the microstructural details of a material than is now

possible.

Finally, with the rapid development of new silicon-based devices [microelec-

tromechanical systems (MEMS)] that are implantable into a material at the time

of manufacture offer real-time, health-monitoring capabilities. This new capa-

bility is permitting inspectors to detect and quantify material or structural deg-

radation in time to prevent failure, thus avoiding unexpected down time. Many

of these devices can be interrogation through a wireless radio frequency (RF)

link. This reduces the burden of inspection while giving the inspector the ability

to inspect areas of structure that are now termed ‘‘hidden.’’ For more information

about this rapidly evolving area the reader is referred to the literature for more

detailed data.

29,54–62

This chapter is just a brief review of the more commonly used NDI methods.

Table 1 provides a brief review of the common NDI methods, types of ﬂaws

that each detects, and their advantages and disadvantages. Those who require

more detailed information on any method should consult one or more of the

references at the end of the chapter. For information on the state-of-the-art in-

spection technology, the reader is referred to the scientiﬁc and engineering lit-

erature or the Internet. A good place to start any search for the latest NDE

technology is the home page of The American Society for Nondestructive Test-

ing.

2 LIQUID PENETRANTS

The liquid penetrants are used to detect surface connected discontinuities, such

as cracks, porosity, and laps in solid, nonporous materials.

The method uses a

brightly colored visible or ﬂuorescent penetrating liquid, which is applied to the

surface of a cleaned part. After a ‘‘dwell time’’ the liquid enters the discontinuity

and then is removed from the part in a cleaning process. The penetrant is drawn

from the ﬂaw to the surface by a developer to provide an indication of surface-

2 LIQUID PENETRANTS 653

Table 1 Capabilities of the Common NDI Methods

Method

Typical Flaws

Detected

Typical

Application Advantages Disadvantages

Radiography Voids, porosity,

inclusions, and

cracks

Castings, forging,

weldments, and

structural

assemblies

Detects internal

ﬂaws; useful

on a wide

variety of

geometric

shapes;

portable;

provides a

permanent

record

High cost;

insensitive to

thin laminar

ﬂaws, such as

tight fatigue

cracks and

delaminations;

potential health

hazard

Liquid

penetrants

Cracks, gouges,

porosity, laps,

and seams open

to a surface

Castings, forging,

weldments, and

components

subject to

fatigue or

stress–

corrosion

cracking

Inexpensive; easy

to apply;

portable; easily

interpreted

Flaw must be

open to an

accessible

surface, level

of delectability

operator

dependent

Eddy current

inspection

Cracks, and

variations in

alloy

composition or

heat treatment,

wall thickness,

dimensions

Tubing, local

regions of

sheet metal,

alloy sorting,

and coating

thickness

measurement

Moderate cost,

readily

automated;

portable

Detects ﬂaws that

change

conductivity of

metals;

shallow

penetration;

geometry

sensitive

Magnetic

particles

Cracks, laps,

voids, porosity,

and inclusions

Castings, forging,

and extrusions

Simple;

inexpensive;

detects shallow

subsurface

ﬂaws as well

as surface

ﬂaws

Useful on

ferromagnetic

materials only;

surface

preparation

required,

irrelevant

indications

often occur;

operator

dependent

Thermal testing Voids or disbonds

in both metallic

and nonmetallic

materials,

location of hot

or cold spots in

thermally active

assemblies

Laminated

structures,

honeycomb,

and electronic

circuit boards

Produces a

thermal image

that is easily

interpreted

Difﬁcult to

control surface

emissivity and

poor

discrimination

between ﬂaw

types.

Ultrasonic

methods

Cracks, voids,

porosity,

inclusions and

delaminations

and lack of

bonding

between

dissimilar

materials

Composites,

forgings,

castings, and

weldments and

pipes

Excellent depth

penetration;

good

sensitivity and

resolution; can

provide

permanent

record

Requires acoustic

coupling to

component;

slow;

interpretation

of data is often

difﬁcult

654 NONDESTRUCTIVE INSPECTION

Fig. 1 Schematic representation of a part surface before cleaning for penetrant inspection.

Fig. 2 Part surface after cleaning and before penetrant application.

connected defects. This process is depicted schematically in Figs. 1–4. A pen-

etrant ﬂaw indication in turbine blade is shown in Fig. 5.

2.1 Penetrant Process

Both technical societies and military speciﬁcations require a classiﬁcation system

for penetrants. Society documents (typically ASTM E165) categorize penetrants

into visible and ﬂuorescent, depending on the type of dye used. In each category

there are three types, depending on how the excess penetrant is removed from

the part. These are water washable, postemulsiﬁable, and solvent removable.

The ﬁrst step in penetrant testing (PT) or inspection is to clean the part (Figs.

1 and 2). Sometimes this critical step is the most neglected phase of the pro-

cedure. Since PT only detects ﬂaws that are open to the surface, the ﬂaw and

part surface must be free of dirt, grease, oil, water, chemicals, and other foreign

materials that might block the penetrant’s entrance to a defect. Typical cleaning

procedures use vapor degreasers, ultrasonic cleaners, alkaline cleaners, or sol-

vents.

After the surface is clean, a liquid penetrant is applied to the part by dipping,

spraying, or brushing. In this step the penetrant on the surface is wicked into

the ﬂaw—see Fig 3. In the case of tight surface openings, such as fatigue cracks,

the penetrant must be allowed to remain on the part for a minimum of 30 min

2 LIQUID PENETRANTS 655

Penetrant

Fig. 3 Part after penetrant application.

Fig. 4 Schematic representation of part after excess penetrant has been removed and

developer applied.

Fig. 5 Penetrant indication of a crack running along the edge of a jet engine turbine blade.

Ultraviolet illumination causes the extracted penetrant to ﬂuoresce.

656 NONDESTRUCTIVE INSPECTION

to enhance the probability of complete ﬂaw ﬁlling. High-sensitivity ﬂuorescent

dye penetrants are used for this type of inspection.

At the conclusion of the dwell time on the part, the excess penetrant is re-

moved by one of three processes, depending on the type penetrant. Ideally, only

the surface penetrant is removed with the penetrant in the ﬂaw left undisturbed—

see Fig. 4.

The ﬁnal step in a basic penetrant inspection is the application of a ﬁne

powder developer. This may be applied either wet or dry. The developer aids in

wicking the penetrant from the ﬂaw and provides a suitable background for easy

ﬂaw detection. The part is then viewed under a suitable light source—either

ultraviolet or visible. A typical ﬂuorescent penetrant indication for a crack in a

jet engine turbine blade is shown in Fig. 5.

2.2 Reference Standards

Several types of reference standards are used to check the effectiveness of liquid-

penetrant systems. One of the oldest and most often used methods involves

applying penetrant to hard chromium-plated brass panels. The panel is bent so

as to place the chromium in tension, producing a series of cracks in the plating.

These panels are available in sets containing ﬁne, medium, and coarse cracks.

The panels are used to classify penetrant materials by sensitivity and to detect

degrading changes in the penetrant process.

2.3 Limitations of Penetrant Inspections

The major limitation of liquid-penetrant inspection is that it can only detect ﬂaws

that are open to the surface. Other inspection methods must be used to detect

subsurface defects. A factor that may inhibit the effectiveness of liquid-penetrant

inspection is surface roughness. Rough surfaces are likely to produce false in-

dications by trapping penetrant. Penetrant testing is also not suited for the in-

spection of porous materials because the penetrant in the pores obscures any

ﬂaw indications. Other penetrant-like methods are available for porous compo-

nents—see ﬁltered particle inspection in Nondestructive Testing Handbook, Vol.

3 ULTRASONIC METHODS

Ultrasonic methods utilize high-frequency sound waves to inspect the interior of

parts. Sound waves are mechanical or elastic waves that propagate in ﬂuid and

solid media. Ultrasonic inspection is analogous to the use of sonar to detect

schools of ﬁsh or map the ocean ﬂoor.

Both the government and industry have

developed standards for ultrasonic inspections. These include but are not limited

to the ASTM speciﬁcations 214-68, 428-71, and 494-75 and military speciﬁca-

tion MIL-1-8950H. Acoustic and ultrasonic testing can take many forms. Ultra-

sonic testing (UT) ranges from simple coin tapping to the transmission and

reception of very high frequency waves into a part in order to analyze its internal

structure.

UT instruments operating in the frequency range between 20 and 500 kHz

are referred to as ‘‘sonic’’ instruments, while above 500 kHz operate in the

domain of ‘‘ultrasonic’’ methods. To generate and receive ultrasonic waves, a

piezoelectric transducer is usually used to convert electrical signals to sound

3 ULTRASONIC METHODS 657

wave signals and vice versa. The usual form of this transducer consists of a

piezoelectric crystal mounted in a waterproof housing that facilitates its electrical

connection to a pulsar (transmitter) and a receiver. In the transmit mode a high-

voltage, short-duration electrical pulse is applied to the crystal, causing it to

rapidly change shape and emit an acoustic or mechanical pulse. In the receive

mode, any sound waves or echoes returning from along the path of the emitted

acoustic pulse compress the piezoelectric crystal producing an electrical signal

that is ampliﬁed and processed by the receiver.

3.1 Sound Waves

Ultrasonic waves have several characteristics such as wavelength

␭

, frequency

ƒ, velocity

v, pressure P, and amplitude a. The following relationship between

wavelength, frequency, and sound velocity is valid for all waves, including sound

waves.

␭

⫽ v

For example, the wavelength of longitudinal ultrasonic waves with a fre-

quency 2 MHz propagating in steel is 3 mm and the wavelength of shear waves

is 1.6 mm.

The formula that relates the sound pressure to the particle amplitude, with

the terms as deﬁned in the previous paragraph, is

⫽ 2

␲

␳

Ultrasonic waves are reﬂected from boundaries or interfaces that separate a

different materials or media. Each media has characteristic acoustic impedance,

and reﬂections occur in a phenomenon quite similar to those observed with

electrical signals. The acoustic impedance, Z, of any media capable of supporting

sound waves is deﬁned by

⫽

␳

where

␳

is the density of the media in g/cm

, and v is the velocity of sound

along the direction of propagation. Materials with high acoustic impedance are

often referred to as sonically hard in contrast to sonically soft materials with

low impedances. For example, steel (Z

⫽ 7.7 g/cm

⫻ 5.9 km/s ⫽ 45.4 ⫻ 10

kg/m

s) is sonically harder than aluminum (Z ⫽ 2.7 g/cm

⫻ 6.3 km/s ⫽ 17

⫻ 10

kg/m

s). A list of acoustic properties of many common materials is

provided in Appendix A.

3.2 Reﬂection and Transmission of Sound

Almost all the acoustic energy incident on air/solid interfaces is reﬂected be-

cause of the large impedance mismatch between air and most solids. For this

reason, a coupling media, with impedance close to that of the part, is used to

couple the sonic energy from the transducer into the part. A liquid couplant has

obvious advantages for parts with a complex geometry, and water is the couplant

of choice for most inspection situations. The receiver, in addition to amplifying