Kutz M. Handbook of materials selection

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516 INFORMATION FOR MATERIALS PROCUREMENT AND DISPOSAL

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PART 4

TESTING AND INSPECTION

HandbookofMaterialsSelection.EditedbyMyerKutz

519

CHAPTER 19

TESTING OF METALLIC MATERIALS

Peter C. McKeighan

Southwest Research Institute

San Antonio, Texas

1 MECHANICAL TEST

LABORATORY 520

1.1 Test Machines 520

1.2 Sensors and Instrumentation 522

2 TENSILE AND COMPRESSIVE

PROPERTY TESTING 524

3 CREEP AND STRESS

RELAXATION TESTING 526

4 HARDNESS AND IMPACT

TESTING 528

5 FRACTURE TOUGHNESS

TESTING 531

6 FATIGUE TESTING 535

7 OTHER MECHANICAL

TESTING 540

8 ENVIRONMENTAL

CONSIDERATIONS 540

REFERENCES 543

One of the daunting challenges that face a designer when making a material

selection is knowing what properties are the most critical for a given application.

Assuming that the appropriate properties can be chosen, the designer next needs

to be able to ﬁnd the properties for each metallic material of interest (a challenge

in itself) and then interpret the values relative to the design requirements. This

requires that the designer have rudimentary knowledge of both the properties

and test methods as well as an understanding of how to interpret the properties

in the design context.

The purpose of this chapter is to provide some of the basic background for

how metallic materials are mechanically tested. The goal is not to provide enor-

mous detail for each different type of property; other sources, for instance Ref.

1, can provide this. Rather, the intent is to provide sufﬁcient basic information

so that the reader understands what the purpose of the mechanical test is, what

mechanical property typically results, and what the corresponding design impli-

cation is.

The complexity of metallic materials testing and the speciﬁc technical lan-

guage required can be confounding, especially to engineers not intimately fa-

miliar with the ﬁeld. Those of us in the ﬁeld also can confuse each other as new

methods, terminologies, and approaches develop and are adopted. Luckily, there

exists an organization whose mission in part is to minimize this type of mis-

understanding.

HandbookofMaterialsSelection.EditedbyMyerKutz

520 TESTING OF METALLIC MATERIALS

The American Society for Testing and Materials (ASTM) is one of the world’s

largest standards development organizations with over 34,000 members respon-

sible for more than 10,000 standards. The breadth of these standards is enormous

(74 volumes), covering not only test/evaluation procedures but also standardi-

zation approaches for designating metals, paints, plastics, and the like. Originally

developed in 1898, this independent, not-for-proﬁt organization serves as a fo-

rum for producers, users, consumers, and others in developing voluntary, con-

sensus standards. In practice, a hierarchical committee organization develops the

standards initially with a group of experts providing a draft standard. This draft

is subsequently balloted by ASTM and does not become a standard until all

concerns raised during the voting process have been fully addressed.

The test-related standards that ASTM publishes help to ensure that mechan-

ical properties generated in one laboratory are consistent with those generated

in another laboratory. This consistency is critical from a design point of view

since speciﬁc property values are typically extracted from numerous sources,

and knowing that consistent methods were used in generating these properties

is subsequently crucial. The majority of the ASTM standards related to metals

test methods and discussed in this chapter are contained within Ref. 2.

Knowing where to look for the standard does not necessarily imply that it is

easy to know which property is the most germane for a given design. For in-

stance, under the classiﬁcation ‘‘fracture testing,’’ there are 16 separate standards

in Ref. 2, all very different and evaluating disparate quantities such as crack-tip

opening displacement, plane-strain fracture toughness, surface crack toughness,

dynamic tear properties, and the like. The fundamental characteristics of the

design scenario involved and the candidate metallic materials considered will

dictate which fracture property is suitable. In a case such as this where many

different test methods are available, selection of the appropriate test standard is

complex and beyond the scope of this elementary treatise.

Finally, ASTM also provides other resources to ensure that the properties

required are available. In particular, the Directory of Testing Labs

provides a

state-by-state listing of laboratories that are available to perform materials test-

ing. The laboratories are identiﬁed by the type of test performed, with an index

cross-referenced to speciﬁc test methods. Moreover, a brief synopsis of the lab,

including scope of testing, facilities, and staff, is also provided to allow differ-

entiating between the different available choices.

1 MECHANICAL TEST LABORATORY

During mechanical testing, a component or material is loaded with either dis-

placement or force, causing a deformation and associated mechanical response

to occur. The loading occurs at a design relevant rate (impact, static, or cyclic),

and the test is typically performed in the most suitable environment for the end

use of the component. The two key components in this process that make up

the primary constituents in a mechanical test lab (Fig. 1) are the test machine

required to apply the speciﬁed loading as well as the sensors and instrumentation

used to measure the behavior of the test article.

1.1 Test Machines

The test machine, or loading frame, is used to apply stress (or strain) resulting

in axial tension or compression, bending, shear, torsion, or pressure. Although

1 MECHANICAL TEST LABORATORY 521

Fig. 1 Typical material test lab containing numerous servohydraulic test frames.

(Courtesy of Fracture Technology Associates)

a variety of different loading systems are available, the most common is the

universal testing machine, which allows all of these different types of loading

types by use of different test ﬁxtures and load train components. Load is applied

through a hydraulic piston and cylinder (servohydraulic machine) or with pre-

cision machine screws (screw machine) driven by an electric motor and gear

box system. Although screw machines have lower load capacities than servo-

hydraulic systems (generally 125 kips or less), their displacement stability, ac-

curacy, and precision is better than typically observed with servohydraulic

systems. Nevertheless the advantage of the more common servohydraulic sys-

tems is the rapid rate of response necessary for many types of tests (e.g., any

type of cyclic testing, including fatigue and fracture applications).

For the case of servohydraulic machines, the actuators are typically double-

ended allowing tension and compression of the load train. The displacement rate

of these machines range from nearly static to approximately 350–1000 in./s.

The actual highest rate possible for a given machine/specimen combination is

difﬁcult to assess since it depends upon the actuator size, ﬂow rating of the

servovalve, as well as the supply pump, plumbing conﬁguration, and control

system parameters. Servohydraulic test machines range in size but are typically

available from 1 kip to 1000 kips or more.

The control of screw and servohydraulic machines is accomplished through

dedicated microprocessor-based electronics. Controllers generally fall into two

categories: the older generation (but stable and cost effective) analog controller

and the newer digital controller (Fig. 2). In the strictest sense, an analog con-

troller needs a signal source to drive it, typically either a built-in function gen-

erator or an external computer for complex waveforms or more stringent control

requirements. Whereas machine control in this case is with analog electronics,

in the fully digital system closed-loop control is achieved with a computer run-

ning software designed to perform the test of interest. The digital systems are

typically provided with a suite of programs designed to perform a wide variety

of standard test protocols.

A test machine is incomplete without a range of grips and ﬁxtures suitable

for inserting a test specimen into the load train of the frame. Although the

522 TESTING OF METALLIC MATERIALS

Fig. 2 Servohydraulic test frame and digital controller. (Courtesy of MTS Corporation)

majority of the ASTM test standards address ﬁxtures and grips in some manner,

inevitably the hardware available in a given laboratory is diverse and varied,

consisting of both standard and hybrid ﬁxtures whose functionality has been

proven through repeated use. Versatile pneumatically or hydraulically actuated

mechanical clamp grips constitute one of the more popular types available.

1.2 Sensors and Instrumentation

The sensors and instrumentation in a lab are designed to measure mechanical

behavior. Perhaps the most commonly used sensor is the load measuring device,

also called a load cell. Load cells are placed in series with the specimen in the

loading train and can come in a wide variety of sizes and conﬁgurations. Nor-

mally, threaded couplings are available on either side of the load cell to attach

ﬁxtures. Inside the load cell is an instrumented transducer, typically using strain

gauge technology to relate applied load to a resistance change suitable for sens-

ing.

Although numerous nontraditional methods and systems exist for measuring

displacement or strain,

4,5

the most common method is based on a sensor that

provides output based upon strain gauge technology. An example of this is the

highly reliable modern material test extensometer (Fig. 3). This device, fastened

to the specimen by mechanical clamping, is used to accurately measure displace-

ment between a pair of short knife edges afﬁxed to the gauge length of the

specimen. Another type of displacement gauge is the clip gauge, which is

basically a spring-loaded extensometer with longer arms. These devices are

1 MECHANICAL TEST LABORATORY 523

Fig. 3 Extensometer (displacement measurement device) mounted on a tensile specimen.

extremely accurate and robust, but they do have environmental limitations to

some extent due to their reliance on strain gauge technology.

Sometimes it is also useful to mount strain gauges directly on the surface of

the test specimen to infer localized behavior. Furthermore, linear variable dif-

ferential transformers (LVDTs) and capacitive coupling-based displacement de-

vices are commercially available for use in the most aggressive environments.

In addition, noncontact optical systems (laser or charge coupled device, CCD,

camera based) can also be used to obtain displacement measurements. Recent

improvements in computer processor speed, CCD camera size, as well as more

affordable random-access memory (RAM) and disk storage makes these vision-

based technology methods more desirable now more than ever before. The pre-

ponderance of optical systems and lack of any standardization in the core

technology has resulted in recent ASTM standards activity and a draft standard

that deﬁnes the relevant terminology.

Data acquisition tools have also beneﬁted greatly from the recent advances

in personal computers. Newer data acquisition systems tend to be more ﬂexible

than the older, device-dependent applications. The most popular platform for

application building currently appears to be the LabVIEW (National Instruments,

Austin, TX) programming environment. The LabVIEW platform provides a

graphical, icon-based environment to build and modify applications that are more

device/board independent and well suited to typical complex process, and data

acquisition requirements.

524 TESTING OF METALLIC MATERIALS

Fig. 4 Several sample specimen geometries for tensile testing in accordance with ASTM E8.

(Courtesy of the Instron Corporation)

Some of the most complex tests performed in a mechanical test laboratory

are fatigue and fracture evaluations of metallic components and materials. These

tests usually require highly specialized, custom software to control the test ma-

chine, take the data, and analyze the results. It is always difﬁcult to know a

priori, without extensive experience performing these tests, what data acquisition

parameters are the most critical. For this reason a recent guide, ASTM E1942,

has been developed that assists a user to understand the limitations of instru-

mentation and data acquisition systems, especially as related to cyclic fatigue

and fracture mechanics testing. Furthermore, this standard is also useful for the

segment of the test community not familiar with fatigue and fracture tests, since

it provides a practical guide for understanding data acquisition performance.

2 TENSILE AND COMPRESSIVE PROPERTY TESTING

The tension test, deﬁned for metallic materials in ASTM E8, is perhaps one of

the simplest and most common tests of mechanical behavior. The test is con-

ducted by gripping each end of a reduced section specimen and slowing pulling

it until catastrophic failure occurs. Several sample specimen geometries (round

and ﬂat, threaded and smooth shank), conventionally described as ‘‘dogbone’’

specimens by virtue of their reduced section geometry, are shown in Fig. 4.

Moreover, tension test specimens are sometimes notched in the center of the

gauge length as described in ASTM E338 and E602. These tests are useful for

determining how the given metallic material behaves in the presence of a stress

concentration. Furthermore, results from these tests have also been related to

fracture mechanics parameters for a given material.

In a tensile (or compressive) test, the applied load and displacement (or strain)

in the gauge section is used to develop a plot of the stress–strain behavior using

the specimen geometry and calibration constants of the transducers employed

during the test. One challenge during tensile testing is measuring elongation to

2 TENSILE AND COMPRESSIVE PROPERTY TESTING 525

pipe material

325

Strain, in/in

0.000 0.005 0.010 0.015

Stress, ks

100

125

150

extensometer

strain gage (average)

strain in/in

0.00e+0 1.00e-1 2.00e-1 3.00e-1

Stress, ksi

100

125

150

extensometer

Fig. 5 Example stress–strain data from a tensile test from a pipe material tested at 325⬚F.

high strain levels while still having sufﬁcient sensitivity in the elastic region. It

is therefore not uncommon to utilize two strain transducers as shown in Fig. 5.

Special high-gain strain gauges or transducers are typically required to provide

sufﬁcient sensitivity and resolution in the elastic region to derive a Young’s

modulus value. This is contrasted with conventional extensometry, which is usu-

ally used to measure higher levels of strain, even 20% or higher. Alternatively,

two extensometers, one measuring behavior over a small range while the other

conﬁgured for a large range, can be mounted on a single specimen. One advan-

tage with this approach is that strain differences as a consequence of different

gauge lengths, due to issues such as strain localization, is greatly reduced since

each extensometer can have the same basic gauge length but be calibrated to

different ranges.

During tensile testing, the ultimate strength is the largest applied stress mea-

sured in the specimen section. The proportional limit, often called the yield

strength, indicates the onset of plasticity where the assumption of linear stress–

strain behavior is violated. Since there are many deﬁnitions of proportional limit

(e.g., the conventional 0.2% strain offset method or the API approach

using the

stress corresponding to a speciﬁc strain level), it is imperative that the physical

meaning of the limit be clearly deﬁned when presenting the results. Moreover,

post-test measurements made on the gauge length and fracture surface of the

specimen are used to quantify the ductility of the specimen. Ductility is the

ability of a material to deform plastically without failure occurring. Ductility

parameters include percent elongation and percent reduction of area.

The mode of failure can also sometimes inﬂuence the strain results obtained.

For instance, if the metallic material tested tends to exhibit highly localized

plastic deformation, the region of conﬁned necking results in either higher or

lower effective strain if the gauge length is short or long, respectively. On the

other hand, if deformation is equally distributed along the gauge section, the

526 TESTING OF METALLIC MATERIALS

strain data exhibits no gauge length dependence. Obviously, the potential exists

for misinterpretation when comparing stress–strain behavior from one sized

specimen to another. Therefore, it is important to ensure that specimen size is

constant when performing a relative comparison of properties from one condition

to another or between metallic materials.

Compression testing is typically performed on short cylindrical specimens

(ASTM E9). Care must be taken during testing to minimize the inﬂuence of

both buckling and barreling, two modes of behavior that can corrupt compression

test results. Buckling is prevented by minimizing the ratio of specimen length

to diameter (typically in the range of 1–2). Load train alignment is also a critical

feature of these tests. Alignment is often practically achieved by loading the

specimen through a spherical joint so that slight, nearly unmeasurable misalign-

ment will not cause buckling. Barreling results from the friction between the

end faces of the specimen and the loading platens. Friction is minimized by

using lubricant and machining concentric rings on the end of the specimen to

trap the lubricant and keep it from squeezing out.

3 CREEP AND STRESS RELAXATION TESTING

Creep and stress relaxation properties are critically important for high-

temperature applications. Without knowledge of these properties, both air- and

land-based jet turbines, internal combustion engines, and even some electronic

equipment could not be adequately designed. Furthermore, there is always in-

creasing pressure to develop new metallic materials with better performance at

high temperature so that operating temperature in these applications can be in-

creased to yield higher efﬁciencies.

Creep deformation is examined by applying either a constant load or a con-

stant true stress to a material held at a speciﬁed temperature. When the material

is ﬁrst loaded, a small permanent loading strain occurs and the creep strain rate

gradually decreases. This portion of the creep strain versus time plot is called

primary creep (stage I). This is subsequently followed by secondary (or also

steady-state or stage II) creep with the highest rates observed in the ﬁnal tertiary

creep (stage III) immediately before failure. These three regions of the creep

curve are shown schematically in Fig. 6. Whereas the test methods are described

in detail in ASTM E139, the primary components of the test include a static

load frame, furnace, and high-temperature extensometry.

The test specimens are similar to those used in a quasi-static tensile test,

although the actual geometry used is often controlled by the metallic material

form available. Static dead-weight machines are typically employed for this type

of test, as shown in Fig. 7. The frames usually have a balance beam to magnify

the load applied to the specimen by a factor of between 2 and 25. All of the

pull-rod and coupling ﬁxtures employed are manufactured from high-

temperature alloys impervious to the environment used during the test. Tubular,

electric resistance furnaces are usually used with the number of zones typically

dependent upon the size of the specimen. Special attention must be made to

ensure minimal thermal gradients and uniform temperatures applied to the spec-

imen.

The measurement of gauge length deformation is typically critical during a

creep experiment. If temperatures are sufﬁciently low (

⬍400⬚F), conventional