Kutz M. Handbook of materials selection

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678 NONDESTRUCTIVE INSPECTION

Fig. 27 Schematic representation of an (a) absolute versus (b) a differential coil conﬁgurations

used in eddy current testing.

6 THERMAL METHODS

Thermal nondestructive inspection methods involve the detection of infrared en-

ergy emitted from the surface of a test object.

This technique is used to detect

the ﬂow of thermal energy either into or out of a specimen and the effect of

anomalies in the surface temperature distribution. Other material properties that

can inﬂuence this method are heat capacity, density, thermal conductivity, and

emissivity of the test specimen. Defects that are usually detected include poros-

ity, cracks, and delaminations. The sensitivity of any thermal method is greatest

for near-surface ﬂaws and degrades rapidly for ﬂaws buried in thermally con-

ductive materials. Materials with lower thermal conductivity yield better reso-

lution because they produce larger thermal gradients.

6.1 Infrared Cameras

All objects emit infrared (IR) radiation with a temperature above absolute zero.

At room temperature, the thermal radiation is predominately infrared, and mea-

6 THERMAL METHODS 679

surements are concentrated in the region near 10

␮

m. IR cameras are available

that view large areas by scanning over a single, liquid-nitrogen-cooled detector

using rotating mirrors. Since the IR images can be stored in digital form, image

processing is easily performed and thermal images can be stored on magnetic

or optical media. For many applications, a thermal image that shows the relative

temperature across the surface of a specimen is sufﬁcient to detect ﬂaws. How-

ever, if absolute temperatures are required, the IR instrumentation must be cal-

ibrated to account for the surface emissivity of the test subject.

Thermography’s ability to detect ﬂaws is often affected by the type of ﬂaw

and its orientation with respect to the surface of the object. To have a maximum

effect on the surface temperatures, the ﬂaw must interrupt heat ﬂow to the sur-

face. Since a ﬂaw can occur at any angle to the surface, the important parameter

is its projected area to the ﬁeld of view of the camera. Subsurface ﬂaws such

as cracks parallel to the surface of the object, porosity, and debonding of a

surface layer are easily detected. Cracks that are perpendicular to the object

surface can be very difﬁcult to ﬁnd using thermography.

Most thermal NDE methods do not have excellent spatial resolution due to

spreading of thermal energy as it diffuses to the surface. The greatest advantage

of thermography is that it can be a noncontact, remote-viewing technique re-

quiring only line-of-sight access to one side of a test specimen. Large areas can

be viewed rapidly, since scan rates for IR cameras run between 16 and 30 frames

per second. Temperature differences of 0.02

⬚C or less can be detected in a con-

trolled environment.

6.2 Thermal Paints

A number of contact thermal methods are available to give the temperature

distribution for surfaces. These methods involve applying a coating to the sample

and observing a color change as the specimen is thermally cycled. Several dif-

ferent types of coatings are available that cover a wide temperature range. Tem-

perature-sensitive pigments in the form of paints have been made to cover a

temperature range from 40 to 1600

⬚C. Thermal phosphors are organic com-

pounds that emit visible light when exposed to ultraviolet (UV) radiation. (The

amount of visible light is inversely proportional to temperature.) Thermochromic

compounds and cholesteric liquid crystals change color over large temperature

ranges. The advantages of these approaches are the simplicity of the test and

the relatively low cost if only small areas are scanned.

6.3 Thermal Testing

Excellent results may be achieved if thermography is performed in a dynamic

environment where the transient effects of a heat ﬂow in test object can be

monitored. This enhances detection of areas where different heat transfer rates

occur. Applications involving steady-state conditions are more limited. Ther-

mography has been successfully used in several different area of testing. In

medicine it is used to detect tumors. In aircraft manufacture or maintenance it

is used to detect debonding in layered structures. In the electronics industry it

is used to detect poor thermal performance of circuit board components. Re-

cently, it has been sometimes used to detect stress-induced thermal gradients

around defects in dynamically loaded test samples. For more information on

680 NONDESTRUCTIVE INSPECTION

Fig. 28 Schematic representation of the magnetic lines of ﬂux in a ferromagnetic metal near a

ﬂaw. Small magnetic particles are attracted to the leakage ﬁeld associated with the ﬂaw.

thermal NDE methods the reader is referred to Nondestructive Testing Hand-

book,

Special Nondestructive Testing Methods,

and to Metals Handbook:

Nondestructive Evaluation and Quality Control.

7 MAGNETIC PARTICLE METHOD

The magnetic particle method of nondestructive testing is used to locate surface

and subsurface discontinuities in ferromagnetic materials.

An excellent short

reference for this NDE method is Ref. 9, especially Chapters 10–16. Magnetic

particle inspection is based on the principle that magnetic lines of force, when

present in a ferromagnetic material, are distorted by changes in material conti-

nuity, such as cracks or inclusions, as shown schematically in Fig. 28. If the

ﬂaw is open or close to the surface, the ﬂux lines escape the surface at the site

of the discontinuity. Even near-surface ﬂaws, such as nonmagnetic inclusions,

cause the same bulging of the lines of force above the surface. This distorted

ﬁeld, usually referred to as a leakage ﬁeld, is used to reveal the presence of the

discontinuity when ﬁne magnetic particles are attracted to it. If these particles

are ﬂuorescent, their presence at a ﬂaw will be visible under ultraviolet light

much like penetrant indications—see Fig. 28. Magnetic particle inspection is

used for the inspection of steel components because it is fast, easily imple-

mented, and has rather simple ﬂaw indications. The part is usually magnetized

with an electric current, and then a solution containing ﬂuorescent particles is

applied by ﬂowing it over the part. The particles that stick to the part form the

indication of the ﬂaw.

7.1 Magnetizing Field

The magnetizing ﬁeld may be applied a component in any one of a number of

methods. Its function is to generate a residual magnetic ﬁeld in the surface of

the part. The application of a magnetizing force (H) generates a magnetic ﬂux

(B) in the component as shown schematically in Fig. 29. In this ﬁgure, the

magnetic ﬂux density, B, has units of newtons/ampere or webers/m

, and the

strength of the magnetic ﬁeld or magnetic ﬂux intensity, H, has units of Oersted

or amperes/meter. Starting at the origin, a magnetizing force is applied and the

magnetic ﬁeld internal to the part increases in a nonlinear fashion along the path

shown by the arrows. If the force is reversed, then the magnetic ﬁeld does not

return to zero, but follows the arrows around the curve as shown. The reader

will note that once the magnetizing force is removed, the ﬂux density does not

return to zero, but remains at an elevated value called the material’s remanence.

This is the point at which most magnetic particle inspections are performed. The

reader will also note that an appreciable reverse magnetic force H must be

7 MAGNETIC PARTICLE METHOD 681

Fig. 29 Magnetic ﬂux intensity, H, versus magnetic ﬂux density, B, hysteresis curve for a typi-

cal steel. Initial magnetization starts at the origin and progresses as shown by the arrows.

Demagnetization follows the arrows of the smaller hysteresis loops.

applied before the internal ﬁeld density is again zero. This point ⫺H

is referred

to as the coercivity of the material. If the magnetizing force is applied and

reversed, the material will respond by continually moving around this hysteresis

loop.

Selection of the type of magnetizing current depends primarily on whether

the defects are either open to the surface or are wholly below it. Alternating

current (ac) magnetization is best for the detection of surface discontinuities

because the current is concentrated in the near-surface region of the part. Direct

currents (dc) magnetization is best suited for subsurface discontinuities because

of the deeper penetration of the part. While dc can be obtained from batteries

or dc generators, it is usually produced by half-wave or full-wave rectiﬁcation

of commercial power. Rectiﬁed current is classiﬁed as half-wave direct current

(HWDC) or full-wave direct current (FWDC). Alternating current ﬁelds are usu-

ally obtained from conventional power mains, but it is supplied to the part at

reduced voltage for reasons of safety and the high-current requirements of the

magnetizing process.

Two general types of magnetic particles are available to highlight ﬂaws. One

type is a low-carbon steel with high permeability and low retentivity particle,

which is used dry and consists of different sizes and shapes to respond to both

weak and strong leakage ﬁelds. The other type of inspection media is very ﬁne

particles of magnetic iron oxide that are suspended in a liquid (either a petroleum

distillate or water). These particles are smaller and have a lower permeability

than the dry particles. Their small mass permits them to be held by the weak

leakage ﬁelds at very ﬁne surface cracks. Magnetic particles are available in

several colors to increase their contrast against different surfaces or backgrounds.

Dry powders are typically gray, red, yellow, and black, while wet particles are

usually red, black, or ﬂuorescent.

7.2 Continuous versus Noncontinuous Fields

Because the ﬁeld is always stronger while the magnetizing current is on, the

continuous magnetizing method is generally preferred. Additionally for speci-

mens with low retentivity the continuous method is often used. In the continuous

method, the current can be applied in short pulses, typically 0.5 s. The magnetic

particles are applied to the surface during this interval and are free to move to

682 NONDESTRUCTIVE INSPECTION

the site of the leakage ﬁelds. Using liquid suspended ﬂuorescent particles pro-

duces the most sensitive indications. For ﬁeld inspections, the magnetizing cur-

rent is often continuously applied during the test to give time for the powder to

migrate to the defect site.

In the residual method, the particles are applied after the magnetizing current

is removed. This method is particularly suited for production inspection of mul-

tiple parts.

The choice of direction of the magnetizing ﬁeld within the part involves the

nature of the ﬂaw and its direction with respect to the surface and the major

axis of the part. In circular magnetization, the ﬁeld runs circumferentially around

the part. It is induced into the part by passing current through it between two

contacting electrodes. Since ﬂaws perpendicular to the magnetizing lines are

readily detectable, circular magnetization is used to detect ﬂaws that are parallel

or less than 45

⬚ to the surface of the long, circular specimens. Placing the spec-

imen inside a coil to create a ﬁeld running lengthwise through the part produces

longitudinal magnetization. This induction method is used to detect transverse

discontinuities to the axis of the part.

7.3 Inspection Process

The surface of the part to be examined should be clean, dry, and free of con-

taminants such as oil, grease, loose rust, loose sand, loose scale, lint, thick paint,

welding ﬂux, and weld splatter. Cleaning of the specimen may be accomplished

with detergents, organic solvents, or mechanical means, such as, scrubbing or

grit blasting.

Portable and stationary equipment are available for this inspection process.

Selection of the speciﬁc type of equipment depends on the nature and location

of testing. Portable equipment is available in lightweight units (35–90 lb), which

can be readily taken to the inspection site. Generally, these units operate off of

115, 230, or 460 V ac and supply current outputs of 750–1500 A in half-wave

or ac.

7.4 Demagnetizing the Part

Once the inspection process is complete, the part must be demagnetized. This

is done by one of several ways depending on the subsequent usage of the com-

ponent. A simple method of demagnetizing to remove any residual magnetism

from small tools is performed as follows: Place the tool in the coil of a soldering

iron and slowly withdrawn it. This has the effect of retracing the hysteresis loop

a large number of times, each time with a smaller magnetizing force. When

completely withdrawn, the tool will then have a very small remnant magnetic

ﬁeld, which is for all practical purposes zero. This same process is accomplished

with industrial part by slowly reducing and reversing the magnetizing current

till it is essentially zero. This process of reducing the residual magnetic ﬁeld is

shown schematically by following the arrows in Fig. 29. Another method of

demagnetizing a part is to heat it above its Curie temperature (about 550

⬚C for

iron), where all residual magnetism disappears. This last process is the best

means of removing all residual magnetism, but it does require the expense and

time of an elevated heat treatment.

8 CONSIDERATIONS FOR INSPECTABILITY IN MATERIALS SELECTION 683

8 CONSIDERATIONS FOR INSPECTABILITY IN MATERIALS

SELECTION

The reader will note that reliability of the inspection process is a very important

parameter. This is so important that the description of any NDI procedure is

incomplete without a thorough evaluation of its reliability. For this reason a

special section of this chapter is devoted to inspection reliability. One might be

tempted to assume that inspection reliability is related to the speciﬁc inspection

method, but this is unfortunately not true. The reliability of each inspection

process is a function of the technique used, the material, and its structural or

geometric conﬁguration, as well as the methodology of interpreting the data.

Considerations for inspectability in materials selection ranges from visual

inspection

for esthetic applications to sophisticated inspection by a variety of

nondestructive methods and processes.

8,10–12,14,16–19,21

The use of a material is a

primary consideration in establishing requirements for inspection. Materials are

initially selected on the basis of their properties. Much of the realization of

modern technology implementation is due to increased reliability and conﬁdence

level in the ‘‘ﬁtness for purpose’’ of the materials, components, structures, and

assemblies. A signiﬁcant factor in those increased is the use and improvements

in nondestructive inspection capabilities and conﬁdence level.

8.1 Conﬁdence in Materials Properties

For engineering applications, expectations and conﬁdence that materials can be

produced with consistent properties is essential to the reliability and conﬁdence

level in the end product. Process control in producing materials is required to

assure that consistent properties are produced and inspection may be an essential

element in both process control and ﬁnal product acceptance. For example, heat

treatment of structural materials is often monitored by eddy current conductivity

and indentation hardness measurements to provide assurance that desired prop-

erties have been produced.

In some cases, the practical use of an excellent engineering material may be

critically dependent on in-process application of nondestructive testing proce-

dures to detect and remove anomalies that are inherent to the materials produc-

tion process. An example is the X-radiography of titanium sponge material to

detect and remove tungsten inclusions that may be introduced during the melting

and alloying process.

8.2 Structural Integrity

The structural integrity of engineering materials, components, assemblies, sys-

tems, and structures is also a primary element in successful designs and appli-

cations. Historically, structural integrity was provided by judgment and

experience in initial materials selection, and assurance was provided by a com-

bination of experience in process control and production and a safety margin to

reduce risk of failure. A design was often proof tested to validate its structural

integrity, and subsequent units were assumed to be identical to the validated

component, and variance in properties was assumed to be accounted for by the

design margin. An article was said to be ‘‘proof tested’’ if it was used in an

environment that was sufﬁciently extreme for some deﬁned period of time. The

684 NONDESTRUCTIVE INSPECTION

extreme conditions were deﬁned to be more severe than any that the component

could be reasonably expected to see in service.

The companion consideration in assuring structural integrity is to provide an

alternate load path such that the load/function of the failed element is carried

by companion structure until shutdown/maintenance/repair can be effected.

‘‘Fail-safe’’ design is always a basic consideration but is not always possible for

primary structure such as pressure vessels.

Safe-Life Design

The structural/performance integrity of a system for a ‘‘safe life’’ was histori-

cally assured by a combination of experience (prior art), design margin, sub-

element tests, and full-scale ‘‘fatigue test articles.’’ Test components were as-

sumed to be either ‘‘ﬂaw free’’ or to contain small (but undetectable) ﬂaws of

an assumed size at the time of production, and ﬂaw growth was calculated from

that assumed small ﬂaw size. Crack growth rate and life were based on materials

properties testing (fatigue life). Various measures were used to assure that the

production components were ‘‘identical’’ to the test article. These included the

application of nondestructive inspection methods and procedures.

Periodic Inspection and Maintenance

Continuing conﬁdence in structural integrity is provided by periodic (often cal-

endar date programmed) inspection and maintenance to address undetected var-

iances in production, design short falls, functional use, and environmentally

induced anomalies. The design and application risks were once unquantiﬁed but

were driven to conservatism by experience in applications (engineering art).

Technology Advancement

The historic methods of design and life-cycle management were successful in

providing functionality and an unquantiﬁed conﬁdence level in inherent and

continuing structural integrity. In like manner, the inspection methods applied to

such designs were based on experience, and the availability of inspection

method(s), which provided a perceived conﬁdence in design. These are the prin-

cipal elements of the experience-based design/decision process. The advent and

development of fracture mechanics as an engineering tool offers a method of

quantifying the effects of material ﬂaws on structural integrity. A key factor in

application is detection and quantiﬁcation of anomalies (primarily cracks)

throughout the component’s service life.

Damage-Tolerant Design

Fracture mechanics provides a method of predicting the capability of material

to perform in a speciﬁc service environment with a crack of a given size under

a given load (environment, etc.). It also deﬁnes a new material property termed

fracture toughness, which is roughly the ability of a material to resist the rapid

growth of a crack from a small size to failure of the material. Quantiﬁcation of

crack detection capability and sizing have new requirements for nondestructive

inspection and life-cycle management with corresponding increases in design

efﬁciency. High efﬁciency, high conﬁdence, and reduced life-cycle cost beneﬁts

8 CONSIDERATIONS FOR INSPECTABILITY IN MATERIALS SELECTION 685

Proof

Test

Fig. 30 Crack size as a function of stress level.

from damage-tolerant design generated a new scenario for materials selection

and design usage.

Use of fracture mechanics as a design tool signiﬁcantly changes the design

process by replacing a part of the engineering art (assumptions of material ﬂaw

content at the time of component production and considerations of ﬂaw behavior

in various environments) with quantiﬁable engineering parameters. Furthermore,

the design life and life-cycle maintenance can now be quantiﬁed using new

design parameters.

Static Considerations

A typical design sequence includes:

1. Selection of a structural material

2. Calculation of the maximum service load

3. Calculation of a ‘‘critical crack size’’ that could result in fracture using

fracture mechanic design tools

Figure 30 shows a simple case relating the critical crack size in a selected

material at various loads. If the calculated critical crack size is very large (low-

stress level) in the selected application, the design process may proceed with

other considerations. Useful stress levels often require consideration for small

cracks and a method of verifying structural integrity may be in order. In a

traditional design, proof test was often used. Proof testing has many advantages

and is often used in conjunction with other veriﬁcation methods. In some cases,

the crack size that may be successfully screened by proof testing is larger than

that screened by nondestructive evaluation (NDE) methods, and a larger design

margin may be attained by use of inspection methods. This advantage is shown

schematically in Fig. 30.

Dynamic Considerations

The next step in the design process is to consider growth of a crack during the

service life of the design component. At a given load (environment, etc.), crack

686 NONDESTRUCTIVE INSPECTION

Fig. 31 Fracture control plan logic.

growth rate may be calculated and a safe life predicted by assuring that a pre-

existing crack (screened on entry into service) does not grow to a critical size

during the service life. A part of that safe-life assurance may include scheduled

maintenance/inspections at various service cycles to remove cracks that could

grow to a critical size before the next scheduled maintenance inspection point.

A design logic sequence incorporating considerations for fracture control is

shown in Fig. 31. Special designation of fracture-critical components is often

required on engineering drawings. Notes may include requirements for special

handling, environmental control, special inspection methods, or special testing

methods. Special inspection methods include quantiﬁed crack detection capabil-

ity assessment and validation.

8.3 Beneﬁts of Quantiﬁed Crack Detection

In design and life-cycle management considerations, the beneﬁts of quantiﬁed

crack detection capabilities are multiplied in both risk analysis/reduction and in

life-cycle cost of ownership. These beneﬁts are manifested in:

Material Quality Selection

The selection of a material quality/grade for a given application is often driven

by the availability and/or cost of inspection that is required to provide the re-

quired conﬁdence level for use. For example, the grade of titanium used in

rotating gas turbine engine (jet) components is considerably different from that

used in a submarine hull. The material quality or grade is often determined by

the defect size that can be detected with conﬁdence in that material.

Impact on Structural Integrity

Damage tolerance is based on the ability of a component, system or structure

to maintain structural integrity in the presence of an assumed included ﬂaw of

8 CONSIDERATIONS FOR INSPECTABILITY IN MATERIALS SELECTION 687

a given size. The design efﬁciency (load levels), margin, etc. are based in part

on the ﬂaw size, which can be detected prior to entry into service and at various

programmed inspection maintenance intervals throughout the service life.

Inspection and Maintenance

Inspection and maintenance requirements for a given design/application can be

quantiﬁed and periodic maintenance can be scheduled on the basis of design

requirements and use rather than on an arbitrary calendar schedule. Additional

conﬁdence/safety margin can be incorporated and maintenance can be adjusted

to account for design short falls, unanticipated service usage, unanticipated en-

vironmental degradation, and risk reduction. The economics of quantifying

inspection, maintenance, and overhaul may drive incorporation of damage tol-

erance design/analysis to traditional applications where tradition and historical

conﬁdence have provided high conﬁdence in structural integrity in past appli-

cations.

Safe Life

The ability to quantify safe life for an engineering structure also has tremendous

economic impact. When the initial assumed ﬂaw size is quantiﬁed, the time for

that ﬂaw to grow to a critical size may be quantiﬁed based on fatigue life and

design load analyses. In many cases, the initial cost of application may be much

lower than the life-cycle cost of ownership. Safe-life analyses provides a capa-

bility to provide better selection of material for both structural/functional integ-

rity and for cost of ownership during that life.

Life Extension

Demands continually increase for life extension of engineering systems and

structures and systems. Indeed many commercial aircraft in service are being

used well beyond their design life. Within limits, service life extension can be

justiﬁed by analysis when the basis for the design is quantiﬁed and the life-cycle

inspection/maintenance cycles are quantiﬁed. Life extension may be effected

with no change in risk or conﬁdence level by consideration of fatigue life be-

havior and the incorporation of quantiﬁed inspection procedures.

Life Extensions Considerations. Consider the initiation and growth of a

ﬂaw in service. Service life will include nucleation and stable ﬂaw growth until

it reaches a critical size and precipitates fracture. This process is shown sche-

matically in Fig. 32. Failure can be avoided by detection of that ﬂaw before it

grows to a critical size. The interaction of inspection is thus to apply an inspec-

tion method that has the capability to detect ﬂaws before they grow to a critical

size.

Material behavior is such that there is a signiﬁcant variance between the time

that a fatigue crack initiates for a given material lot and the time when the all

components will initiate a ﬂaw. If inspection is applied, those components that

have early ﬂaw initiation may be removed from service and the remainder of

the lot may continue in service with extended life. Figure 33 shows a typical

spread in crack initiation for a material lot in service and periodic inspections

to remove those components that have initiated a ﬂaw.