Kutz M. Handbook of materials selection

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558 PLASTICS TESTING

Fig. 8 Pendulum impact tester. (Courtesy Tinius Olsen Company)

The energy required to fail the specimen is measured by dropping a known

weight from a known height onto a test specimen. The impact energy is normally

expressed in ft-lb and is calculated by multiplying the weight of the projectile

by the drop height.

The biggest advantage of the falling-weight impact test over the pendulum

impact test or high-rate tension test is its ability to duplicate the multidirectional

impact stresses that a part would be subjected to in actual service. The other

obvious advantage is the ﬂexibility to use specimens of different sizes and

shapes, including an actual part. Unlike the notched Izod impact test, which

measures the notch sensitivity of the material and not the material toughness,

falling-weight impact tests introduce polyaxial stresses into the specimen and

measure the toughness. The variations in the test results due to the ﬁllers and

reinforcements, clamping pressure, and material orientation are virtually elimi-

nated in the falling-weight impact test. This type of test is also very suitable for

determining the impact resistance of plastic ﬁlms, sheets, and laminated mate-

rials.

Three basic ASTM tests are commonly used depending upon the application:

1 MECHANICAL PROPERTIES 559

Fig. 9 Drop impact tester. (Courtesy Byk-Gardner USA)

ASTM D5420. Impact resistance of ﬂat rigid plastic specimen by means of

a falling weight

ASTM D1709. Impact resistance of plastic ﬁlm by the free-falling dart

method

ASTM D2444. Test for impact resistance of thermo-plastic pipe and ﬁttings

by means of a tup

Drop Impact Test. This falling-weight impact test is primarily designed to

determine the relative ranking of materials according to the energy required to

break ﬂat rigid plastic specimens under various conditions of impact of a striker

impacted by a falling weight. A free-falling weight, or a tup, is used to determine

the impact resistance of the material.

Many different versions of test equipment exist today. They all basically op-

erate on the same principle. Figure 9 illustrates one such typical commercially

available testing machine. It consists of a cast aluminum base, a slotted vertical

guide tube, a round-nose striker and striker holder, an 8-lb weight, a die and die

560 PLASTICS TESTING

support, and a sample platform. The sample platform is used to position a sheet

of desired thickness for impact testing. The die is removable from the base in

order that the actual parts of complex shapes can be placed onto the base and

impact tested.

The test is carried out by raising the weight to a desired height manually or

automatically with the use of a motor-driven mechanism and allowing it to fall

freely onto the other side of the striker. The striker transfers the impact energy

to the ﬂat test specimen positioned on a cylindrical die or a part lying on the

base of the machine. The kinetic energy possessed by the falling weight at the

instant of impact is equal to the energy used to raise the weight to the height

of the drop and is the potential energy possessed by the weight as it is released.

Since the potential energy is expressed as the product of weight and height, the

guide tube can be marked with a linear scale representing the impact range of

the instrument in in.-lb. Thus, the toughness or the impact resistance of a spec-

imen or a part can be read directly off the calibrated scale in in.-lb. The energy

loss due to the friction in the tube or due to the momentary acceleration of the

punch is negligible.

An alternate method for achieving the same result utilizes an instrument that

employs a free-falling dart dropped from a speciﬁed height onto a test specimen.

The dart with a hemispherical head is constructed of smooth, polished aluminum

or stainless steel. An electromagnetic, air-operated or other mechanical release

mechanism with a centering device is used for releasing the dart. The dart is

also ﬁtted with a shaft long enough to accommodate removable incremental

weights. A two-piece annular specimen clamp is used to hold the specimen.

Instrumented Impact Testing. One of the biggest drawbacks of the conven-

tional impact test methods is that it provides only one value—the total impact

energy—and nothing else. The conventional tests cannot provide additional in-

formation on the ductility, dynamic toughness, fracture, and yield loads or the

behavior of the specimen during the entire impact event.

This effectively limits the application of noninstrumented impact test methods

to quality control and material ranking. Instrumented impact testers are generally

suited for research and development as well as advance quality control.

Instrumented impact testers measure force continuously while the specimen

is penetrated. The resulting data can be used to determine type of failure and

maximum load, in addition to the amount of energy required to fracture the

specimen. One of the most common type of failures occurring from ductile to

brittle transition at low temperatures can only be observed by studying the load–

energy–time curve. The fracture mode of a plastic is sensitive to the changes in

temperature, and can change abruptly at or near the materials transition temper-

ature. Manufacturers of plastic automotive components routinely test materials

at low temperatures (

⫺20 to ⫺40⬚F) to assure that they will not become brittle

in cold weather. By studying the shape of the load–time or load–deﬂection

curve, the type of failure can be analyzed, and important information about its

performance in service can be gathered. The new piezoelectric-equipped strikers

offer increased sensitivity, opening the doors for testing a whole new range of

materials. Applications involving light-weight products such as foam containers

for eggs and ultra-thin ﬁlms used in packaging industry can now be meaningfully

tested.

1 MECHANICAL PROPERTIES 561

Fig. 10 Instrumented impact tester. (Courtesy Instron Corporation)

All standard impact testers can be instrumented to provide a complete load

and energy history of the specimen. Such a system monitors and precisely re-

cords the entire impact event, starting from the acceleration (from rest) to the

initial impact and plastic bending to fracture initiation and propagation to the

complete failure. The instrumentation is done by mounting the strain gauges or

load cell onto the striking bit in the case of pendulum impact tester or onto the

tup in the case of a drop impact tester. During the test, a ﬁber-optic device

triggers an oscilloscope just before striking the specimen. The output of the

strain gauge is recorded by the oscilloscope depicting the variation of the load

applied to the specimen throughout the entire fracturing process. A complete

load–time history of the entire specimen is obtained. The apparent total energy

absorbed by the specimen can be calculated and plotted against time. The spec-

imen displacement can be calculated by the double integration of the load–time

curve and the load–displacement curve can be plotted. With the advent of mi-

croprocessor technology, some manufacturers are now capable of offering a unit

that automatically calculates the sample displacement and provides a load–

displacement curve eliminating the need for calculations. Many other useful data

such as the impact rate, force and displacement at yield, and break, yield, and

failure energies as well as modulus are calculated and printed out. A commer-

cially available instrumented impact tester is shown in Fig. 10.

Interpreting Impact Data

Modern instrumented impact test machines generate a complete record of (1)

force applied to the specimen versus time, (2) displacement of the impactor

562 PLASTICS TESTING

Fig. 11 Load–energy–time curve showing effect of impact on a specimen.

(from the point of specimen contact) versus time, (3) velocity of the impactor

versus time, and (4) energy absorbed by the specimen versus time.

When plotted (Fig. 11), many details of the impact event become clearly

visible in the data. Most systems also analyze the data and provide tabulated

reports of critical values. Results of instrumented impact tests are interpreted

differently depending on the type of material being tested and the failure criteria

applied.

For homogeneous materials, four values are critical:

Maximum (yield) load is simply the highest point on the load–time curve.

Often the point of maximum load corresponds to the onset of material

damage or complete failure.

Energy to maximum (yield) load is the energy absorbed by the specimen up

to the point of maximum load. When maximum load corresponds to failure,

the energy to max load is the amount of energy the specimen can absorb

before failing.

Total energy is the amount of energy the specimen absorbed during the com-

plete test. This number may be the same as energy to maximum load when

the specimen abruptly fails at the maximum load point.

Deﬂection to maximum (yield ) load is the distance the impactor traveled from

the point of impact to the point of maximum load. Figure 12 shows a

typical test data.

High-Speed Impact Tests (ASTM D3763, ISO 6603-2). An ever-increasing

demand for engineering plastics and the need for sophisticated and meaningful

impact test methods for characterizing these materials have forced the industry

into developing new high-speed impact tests. These tests not only provide the

1 MECHANICAL PROPERTIES 563

Fig. 12 Typical test data obtained with instrumented impact tester.

(Courtesy Instron Corporation)

basic information regarding the toughness of the polymeric materials but also

provide other important data of interest, such as the load–deﬂection curve and

the total energy absorption. The high-speed impact test overcomes the basic

limitations of conventional impact testing methods as discussed previously. The

rate of impact can be varied from 30 to 570,000 in./min.

High-speed impact testing has gained considerable popularity in recent years

because of its ability to simulate actual impact failures at high speeds. For ex-

ample, conventional impact testing methods are useless in testing to meet ad-

vanced automotive crash standards that require the impact simulation at 28 mph

(30,000 in./min). High-speed impact testers are able to meet the challenge. As

discussed earlier in this chapter, almost all polymers are strain-rate sensitive.

Two polymers, when impact tested at one strain rate, may show similar impact

strength values. The same two polymers tested at a high strain rate show a

completely different set of values.

Most versatile high-speed impact testing machine is capable of testing every-

thing from the thin ﬁlm that may require as low an impact rate as 30 in./min

to the plastic automotive bumper that may require a high impact rate up to

30,000 in./min. The specimen or product can be tested under a controlled en-

vironment of temperature and humidity. The equipment basically consists of a

tup attached to a motor wound spring or pneumatically powered actuator along

with a plunger displacement measuring system. The force is detected with a fast

responding quartz load cell mounted directly on the actuator. The velocity can

be set digitally from 30 to 30,000 in./min. Some type of clamp assembly to

hold the specimen in place is used. The equipment can also be ﬁtted with an

environmental chamber for specialized testing. The tester is equipped with a

564 PLASTICS TESTING

CRT and x-y plotter that automatically displays load versus displacement data.

A built-in microprocessor provides more useful information, such as modulus,

yield, and failure energies.

High-speed impact testers have been proven very useful in material evalua-

tion. At low strain rates, some polymers fail in ductile manner. The same poly-

mers appear to show brittle failure at high strain rates. The point at which this

ductile-to-brittle transition takes place is of particular importance. A high-rate

impact test can provide such information in a graphical form. Tests can also be

carried out at different temperatures to ﬁnd ductile–brittle transition points at

various temperatures. Other useful applications of the high-speed impact tester

include the process quality control, design evaluation, and assembly evaluation.

Abrasion Resistance Tests

The material’s ability to resist abrasion is most often measured by its loss in

weight when abraded with an abraser. The most widely accepted abraser in the

industry is called the Taber abraser. A variety of wheels with varying degree of

abrasiveness is available. The grade of ‘‘calibrase’’ wheel designated CS-17 with

1000-g load seems to produce satisfactory results with almost all plastics. For

softer materials less abrasive wheels with smaller load on the wheels may be

used. The test specimen is usually a 4-in.-diameter disk or a 4-in.

plate having

both surfaces substantially plane and parallel. A -diameter hole is drilled in

–

-in.

the center. Specimens are conditioned employing standard conditioning practices

prior to testing. To commence testing, the test specimen is placed on a revolving

turntable. Suitable abrading wheels are placed on the specimen under certain set

dead-weight loads. The turntable is started and an automatic counter records the

number of revolutions. Most tests are carried out to at least 5000 revolutions.

The specimens are weighed to the nearest milligram. The test results are reported

as weight loss in mg/1000 cycles. The grade of abrasive wheel along with

amount of load at which the test was carried out is always reported along with

results.

Test methods such as ASTM D1044 (resistance of transparent plastic mate-

rials to abrasion) are also developed for estimating the resistance of transparent

plastic materials to one kind of abrasion by measurement of its optical affects.

The test is carried out in similar manner to that described above, except that

100 cycles with a 500-g load is normally used. A photoelectric photometer is

used to measure the light scattered by abraded track. The percentage of the

transmitted light that is diffused by the abraded specimens is reported as a test

result.

Another method to study the resistance of plastic material to abrasion is by

measuring the volume loss when a ﬂat specimen is subjected to abrasion with

loose abrasive or bonded abrasive on cloth or paper. This method is designated

as ASTM D1242. Figure 13 illustrates a commercially available abrasion tester.

1.6 Fatigue Resistance

The behavior of materials subjected to repeated cyclic loading in terms of ﬂex-

ing, stretching, compressing, or twisting is generally described as fatigue. Such

repeated cyclic loading eventually constitutes a mechanical deterioration and

progressive fracture that leads to complete failure. Fatigue life is deﬁned as the

1 MECHANICAL PROPERTIES 565

Fig. 13 Abrasion tester. (Courtesy Taber Industries)

number of cycles of deformation required to bring about the failure of the test

specimen under a given set of oscillating conditions.

The failures that occur from repeated application of stress or strain are well

below the apparent ultimate strength of the material. Fatigue data are generally

reported as the number of cycles to fail at a given maximum stress level. The

fatigue endurance curve, which represents stress versus number of cycles to

failure, also known as S–N curve, is generated by testing a multitude of speci-

mens under cyclic stress, each one at different stress levels. At high-stress levels,

materials tend to fail at relatively low numbers of cycles. At low stresses, the

materials can be stressed cyclically for an indeﬁnite number of times and the

failure point is virtually impossible to establish. This limiting stress below which

material will never fail is called the fatigue endurance limit. The fatigue endur-

ance limit can also be deﬁned as the stress at which the S–N curve becomes

asymptotic to the horizontal (constant stress) line. For most polymers, the fatigue

endurance limit is between 25 and 30% of the static tensile strength. The fatigue

resistance data are of practical importance in the design of gears, tubing, hinges,

parts on vibrating machinery, and pressure vessels under cyclic pressures.

Two basic types of tests have been developed to study the fatigue behavior

of plastic materials:

1. Flexural fatigue test

2. Tensile fatigue test

Flexural Fatigue Test (ASTM D671)

The ability of a material to resist deterioration from cyclic stress is measured in

this test by using a ﬁxed cantilever-type testing machine capable of producing

566 PLASTICS TESTING

Fig. 14 Flexural fatigue tester. (Courtesy Fatigue Dynamics, Inc.)

a constant amplitude of force on the test specimen each cycle. The main feature

of a fatigue testing machine is an unbalanced, variable eccentric, mounted on a

shaft that is rotated at a constant speed by a motor. This unbalanced movement

of an eccentric produces alternating force. The specimen is held as a cantilever

beam in a vice at one end and bent by a concentrated load applied through a

yoke fastened to the opposite end. A counter is used to record the number of

cycles along with a cutoff switch to stop the machine when the specimen fails.

A typical commercially available fatigue testing machine is illustrated in Fig.

14. The test specimen of two different geometries are used. If machined speci-

mens are used, care must be taken to eliminate all scratches and tool marks from

the specimens. Molded specimen must be stress relieved before using.

The test is carried out by ﬁrst determining the complementary mass and ef-

fective mass of the test specimen. The load required to produce the desired stress

is calculated from these values. The number of cycles required to produce failure

is determined. The test is repeated at varying stress levels. A curve of stress

versus cycles to failure (S–N diagram) is plotted from the test results.

Tensile Fatigue Test

Unlike the ﬂexural fatigue test, which uses the constant deﬂection (strain) prin-

ciple, the tensile fatigue test is conducted under constant-load (stress) conditions.

The specimen is dumbbell-shaped, about 2 in. long with a cylindrical cross

section.

The test is conducted by mounting both ends of the dumbbell specimen in

the testing machine. The specimen is rotated between two spindles, and stress

in the form of tension and compression is applied. The specimen is subjected

to the number of cycles of stress speciﬁed or until fracture occurs.

1 MECHANICAL PROPERTIES 567

Table 1 Typical Hardness Values of Some Common Plastic Materials

Plastic material

Hardness

Rockwell

Durometer

Shore D

Acetal 94 120

Acrylic 85–105

Acrylonitrile butadiene styrene 75–115

Cellulosics 30–125

Polyphenylene oxide 80 120

Nylon 108–120

Polycarbonate 72 118

High-density polyethylene 60–70

Low-density polyethylene 40–50

Polypropylene 75 –85

Polystyrene (general purpose) 68–70

Polyvinyl chloride (rigid) 115

Polysulfone 70 120

1.7 Hardness Tests

Hardness is deﬁned as the resistance of a material to deformation, particularly

permanent deformation, indentation, or scratching. Hardness is purely a relative

term and should not be confused with wear and abrasion resistance of plastic

materials. Polystyrene, for example, has a high Rockwell hardness value but a

poor abrasion resistance. Hardness test can differentiate relative hardness of dif-

ferent grades of a particular plastic. However, it is not valid to compare hardness

of various types of plastics entirely on the basis of one type of test, since elastic

recovery along with hardness is involved. The test is further complicated by a

phenomenon such as creep. Many tests have been devised to measure hardness.

Since plastic materials vary considerably with respect to hardness, one type of

hardness test is not applicable to cover the entire range of hardness properties

encountered. Two of the most commonly used hardness tests for plastics are the

Rockwell hardness test and the Durometer hardness test. Rockwell hardness is

used for relatively hard plastics such as acetals, nylons, acrylics, and polystyrene.

For softer materials such as ﬂexible polyvinyl chloride (PVC), thermoplastic

rubbers, and polyethylene, Durometer hardness is often used. The typical

hardness values of some common plastic materials are listed for comparison in

Table 1.

Rockwell Hardness (ASTM D785)

The Rockwell hardness test measures the net increase in depth impression as

the load on an indenter is increased from a ﬁxed minor load to a major load

and then returned to a minor load. The hardness numbers derived are just num-

bers without units. Rockwell hardness numbers are always quoted with a scale

symbol representing the indenter size, load, and dial scale used. The hardness

scales in order of increasing hardness are R, L, M, E, and K scales. The higher

the number in each scale, the harder the material. There is a slight overlap of

hardness scales and, therefore, it is quite possible to obtain two different dial

readings on different scales for the same material. For a speciﬁc type of material,