30079_25.pdf

CHAPTER 25

NONDESTRUCTIVE TESTING

Robert L. Crane

Theodore E. Matikas

Air Force Wright Laboratory

Materials Directorate

Nondestructive Evaluation Branch

WL/MLLP

Wright Patterson Air Force Base

Dayton, Ohio

25.1 INTRODUCTION

729

25.5.2 The Impedance Plane

746

25.5.3 Liftoff of the Inspection

Coil from the Specimen

25.2 LIQUIDPENETRANTS

730

747

25.2.1 The Penetrant Process

730

25.2.2 Categories of Penetrants

730

25.6 THERMAL METHODS

750

25.2.3 Reference Standards

730

25.6.1 Infrared Cameras

750

25.2.4 Limitations of Penetrant

Inspections

25.6.2 Thermal Paints

751

730

25.6.3 Thermal Testing

751

25.3 ULTRASONIC METHODS

732

25.7 MAGNETIC PARTICLE

METHOD

25.3.1 Soundwaves

733

751

25.3.2 Reflection and

Transmission of Sound

25.7.1 The Magnetizing Field

751

733

25.7.2 Continuous versus

Noncontinuous Fields

25.3.3 Refraction of Sound

735

752

25.3.4 The Inspection Process

737

25.7.3 The Inspection Process

753

25.7.4 Demagnetizing the Part

753

25.4 RADIOGRAPHY

738

25.4.1 The Generation and

Absorption of

X Radiation 739

25.4.2 Neutron Radiography 740

25.4.3 Attenuation of X Radiation 741

25.4.4 Film-Based Radiography

APPENDIX A ULTRASONIC

PROPERTIES OF

COMMON

MATERIALS

754

742

APPENDIX B ELECTRICAL

RESISTIVITIES AND

CONDUCTIVITIES

OF COMMERCIAL

METALS AND

ALLOYS

25.4.5 The Penetrameter

743

25.4.6 Real-Time Radiography

744

25 A.I Computed Tomography

744

25.5 EDDYCURRENTINSPECTION 746

25.5.1 The Skin Effect

759

746

25.1 INTRODUCTION

Nondestructive evaluation (NDE) encompasses those physical and chemical tests that are used to

determine if a component or structure can perform its intended function without the test methods

impairing the component's performance. Until recently, NDE was relegated to detecting physical

flaws and estimating their dimensions. These data were used to determine if a component should be

scrapped or repaired, based on quality-acceptance criteria. Such traditional definitions are being ex-

panded as requirements for high-reliability, cost-effective NDE tests are increasing. In addition, NDE

techniques are changing as they become an integral part of the automated manufacturing process.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

This chapter is but a brief review of the more commonly used NDE methods. Those who require

more detailed information on standard NDE practices should consult Refs. 1-6 at the end of the

chapter. For information on recent advances in NDE research the reader is referred to Refs. 7-14.

The NDE methods reviewed here consist of the five classical techniques—penetrants, ultrasonic

methods, radiography, magnetic particle tests, and eddy current methods. Additionally, we have briefly

covered thermal-inspection methods.

25.2 LIQUIDPENETRANTS

Liquid penetrants are used to detect surface-connected discontinuities in solid, nonporous materials.

The method uses a brightly colored penetrating liquid that is applied to the surface of a clean part.

The liquid in time enters the discontinuity and is later withdrawn to provide a surface indication of

the flaw. This process is depicted schematically in Fig. 25.1. A penetrant flaw indication in turbine

blade is shown in Fig. 25.2.

25.2.1 The Penetrant Process

Technical societies and military specifications have developed classification systems for penetrants.

Society documents (typically ASTM E165) categorize penetrants into two methods (visible and flu-

orescent) and three types (water washable, post-emulsifiable, and solvent removable). Penetrants, then,

are classified by type of dye, rinse process, and sensitivity. See Ref. 1, Vol. 2, for a more detailed

discussion of penetrant testing.

The first step in penetrant testing (PT) or inspection is to clean the part (Fig. 25. Ia and 25.Ib).

Many times this critical step is the most neglected phase of the inspection. Since PT detects only

flaws that are open to the surface, the flaw and part surface must, prior to inspection, be free of dirt,

grease, oil, water, chemicals, and other foreign materials. Typical cleaning procedures use vapor

degreasers, ultrasonic cleaners, alkaline cleaners, or solvents.

After the surface is clean, a penetrant is applied to the part by dipping, spraying, or brushing.

Step 2 in Fig. 25.1c shows the penetrant on the part surface and in the flaw. 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 minutes to enhance the probability of complete flaw filling. Fluorescent dye pene-

trants are used for many inspections where high sensitivity is required.

At the conclusion of the minimum dwell time, the penetrant on the surface of the part is removed

by one of three processes, depending on the characteristics of the inspection penetrant. Ideally, only

the surface penetrant is removed and the penetrant in the flaw is left undisturbed (Fig. 25.Ic).

The final step in a basic penetrant inspection is the application of a developer, wet or dry, to the

part surface. The developer aids in the withdrawal of penetrant from the flaw and provides a suitable

background for flaw detection. The part is then viewed under a suitable light source; either ultraviolet

or visible light. White light is used for visible penetrants while ultraviolet light is used for fluorescent

penetrants. A typical penetrant indication for a crack in a jet engine turbine blade is shown in Fig.

25.2.

25.2.2 Categories of Penetrants

Once the penetrant material is applied to the surface of the part, it must be removed before an

inspection can be carried out. Penetrants are often categorized by their removal method. There are

generally three methods of removing the penetrant and thus three categories. Water-washable pene-

trants contain an emulsifier that permits water to wet the penetrant and carry it from the part, much

as a detergent removes stains from clothing during washing. The penetrant is usually removed with

a water spray. Post-emulsifiable penetrants require that an emulsifier be applied to the part to permit

water to remove the excess penetrant. After a short dwell time, during which the emulsifier mixes

with the surface penetrant, a water spray cleans the part. For solvent-removable penetrants, the excess

material is usually removed with a solvent spray and wiping. This process is generally used in field

applications where water-removal techniques are not applicable.

25.2.3 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 chromium-cracked panels, which are avail-

able in sets containing fine, medium, and coarse cracks. The panels are capable of classifying pen-

etrant materials by sensitivity and identifying changes in the penetrant process.

25.2.4 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 methods are used for detecting subsurface flaws. Another factor that may inhibit

the effectiveness of liquid-penetrant inspection is the surface roughness of the part being inspected.

Very rough surfaces are likely to produce excessive background or false indications during inspection.

Although the liquid-penetrant method is used to inspect some porous parts, such as powder metallurgy

Table 25.1 Capabilities of the Common NDE Methods

Typical Flaws Detected

Voids, porosity, inclusions, and

cracks

Typical Application

Castings, forgings, weldments,

and structural assemblies

Advantages

Detects internal flaws; useful on a

wide variety of geometric

shapes; portable; provides a

permanent record

Disadvantages

High cost; insensitive to thin

laminar flaws, such as tight

fatigue cracks and

delaminations; potential health

hazard

Flaw must be open to an

accessible surface, level of

detectability operator-dependent

Method

Radiography

Cracks, gouges, porosity, laps,

and seams open to a surface

Castings, forgings, weldments,

and components subject to

fatigue or stress-corrosion

cracking

Tubing, local regions of sheet

metal, alloy sorting, and

coating thickness measurement

Castings, forgings, and extrusions

Inexpensive; easy to apply;

portable; easily interpreted

Liquid

penetrants

Eddy current

testing

Cracks, and variations in alloy

composition or heat treatment,

wall thickness, dimensions

Cracks, laps, voids, porosity, and

inclusions

Moderate cost, readily automated;

portable

Detects flaws that change in

conductivity of metals; shallow

penetration; geometry-sensitive

Useful for ferromagnetic materials

only; surface preparation

required, irrelevant indications

often occur; operator-dependent

Difficult to control surface

emissivity; poor discrimination

Simple; inexpensive; detects

shallow subsurface flaws as

well as surface flaws

Magnetic

particles

Voids or disbonds in both

metallic and nonmetallic

materials, location of hot or

cold spots in thermally active

assemblies

Cracks, voids, porosity, inclusions

and delaminations and lack of

bonding between dissimilar

materials

Laminated structures, honeycomb,

and electronic circuit boards

Produces a thermal image that is

easily interpreted

Thermal

testing

Ultrasonic

testing

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

is often difficult

Fig. 25.1 (a) Schematic representation of a part surface before cleaning for penetrant inspec-

tion; (b) part surface after cleaning and before penetrant application; (c) part after penetrant ap-

plication; (d) part after excess penetrant has been removed.

parts, the process generally is not well suited for the inspection of porous materials because the

background penetrant from pores obscures flaw indications.

25.3 ULTRASONICMETHODS

Ultrasonic methods utilize sound waves to inspect the interior of materials. Sound waves are me-

chanical or elastic waves and are composed of oscillations of discrete particles of the material. The

process of inspection using sound waves is quite analogous to the use of sonar to detect schools of

fish or map the ocean floor. Both government and industry have developed standards to regulate

ultrasonic inspections. These include, but are not limited to, the American Society for Testing and

Materials Specifications 214-68, 428-71, and 494-75, and military specification MIL-1-8950H.

Acoustic and ultrasonic testing takes many forms, from simple coin-tapping to transmission of sonic

waves into a material and analyzing the returning echoes for the information they contain about its

internal structure. Reference 15 provides an exhaustive treatment of this inspection technique.

Instruments operating in the frequency range between 20 and 500 kHz are usually defined as

sonic instruments, while above 500 kHz is the domain of ultrasonic methods. In order to generate

and receive the ultrasonic wave, a piezoelectric transducer is usually used to convert electrical signals

Fig. 25.2 Penetrant indication of a crack running along the edge of a jet engine turbine blade.

Ultraviolet light causes the extracted penetrant to glow.

to sound wave signals and vice versa. This transducer usually consists of a piezoelectric crystal

mounted in a waterproof housing that facilitates its electrical connection to a pulsar (transmitter)

receiver. In the transmit mode, a high-voltage, short-duration pulse of electrical energy is applied to

the crystal, causing it to change shape rapidly and emit a high-frequency pulse of acoustic energy.

In the receive mode, any ultrasonic waves or echoes returning from the acoustic path, which includes

the coupling media and part, compress the piezoelectric crystal, producing an electrical signal that

is amplified and processed by the receiver.

25.3.1 Sound Waves

Ultrasonic waves have several characteristics, such as wavelength (A), frequency (/), velocity (i>),

pressure (P), and amplitude (a). The following relationship between wavelength, frequency, and sound

velocity is valid for all types of waves

/XA = u

For example, the wavelength of longitudinal ultrasonic waves of frequency 2 MHz propagating in

steel is 3 mm and the wavelength of shear waves is 1.6 mm.

The sound pressure is related to the particles' amplitude by the relation, where the terms were

defined in the previous paragraph.

P = 277/

XpXvXa

Ultrasonic waves are reflected from all interfaces/boundaries that separate media with different

acoustic impedances, a phenomenon quite similar to the reflection of electrical signals in transmission

lines. The acoustic impedance Z of any medium capable of supporting sound waves is defined by

Z = p X v

where p = the density of the medium in g/cm 3

v = the velocity of sound along the direction of propagation

materials. For example, steel (Z = 7.7 g/cm 3 x 5.9 km/sec = 45.4 x 10 6 kg/m 2 sec) is sonically

Materials with high acoustic impedance are called (sonically) hard in contrast with (sonically) soft

harder than aluminum (Z = 2.7 g/cm 3 x 6.3 km/sec = 17 x 10 6 kg/m 2 sec). An extensive list of

acoustic properties of many common materials is provided in Appendix A.

25.3.2 Reflection and Transmission of Sound

Since very nearly all the acoustic energy incident on air/solid interfaces is reflected because of the

large impedance mismatch of these two media, a coupling medium with an impedance closer to that

of the part is needed to transmit ultrasonic energy into the part under examination. A liquid couplant

has obvious advantages for components with complex external geometries, and water is the couplant

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

also time-gates echoes that return between the front surface and rear surfaces of the component. Thus,

any unusually occurring echo can either be displayed separately or used to set off an alarm.

A schematic diagram of a typical ultrasonic pulse echo setup is shown in Fig. 25.3. This display

of voltage amplitude versus time or depth (if acoustic velocity is known) at a single point of the

specimen is known as an A-scan. In the setup shown in Fig. 25.3, the first signal corresponds to the

reflection of the ultrasonic wave from the front surface of the sample (FS), the last signal corresponds

to the reflection of the ultrasonic wave from the back surface of the sample (BS), and the signal in

between corresponds to the defect echo from inside the component.

The portion of sound energy that is reflected from or transmitted through each interface is a

function of the impedances of media on each side of the interface. The reflection coefficient R (ratio

of the sound pressures or intensities of the reflected and incident waves) and transmission coefficient

T (ratio of the sound pressures or intensities of the transmitted and incident waves) for an acoustic

wave normally incident onto an interface are

R = PL = Z i ~ Z n

z i + Z n

pwr I 1 u + Z n ;

= I L= (Z 1 - Z 11 Y

Likewise, the transmission coefficients, T and T pwr , are defined as

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