Kutz M. Handbook of materials selection

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548 PLASTICS TESTING

capable of indicating total tensile load with an accuracy of Ⳳ1% of the indicated

value or better is used. Lately, the inclination is toward using digital-type load

indicators, which are easier to read than the analog-type indicators. An extension

indicator, commonly known as the extensometer, is used to determine the dis-

tance between two designated points located within the gauge length of the test

specimen as the specimen is stretched. Figure 1 shows a commercially available

tensile testing machine. The advent of new microprocessor technology has vir-

tually eliminated time-consuming manual calculations. Stress, elongation, mod-

ulus, energy, and statistical calculations are performed automatically and

presented on a visual display or hard-copy printout at the end of the test.

1.2 Flexural Properties (ASTM D790, ISO 178)

The stress–strain behavior of polymers in ﬂexure is of interest to a designer as

well as a polymer manufacturer. Flexural strength is the ability of the material

to withstand bending forces applied perpendicular to its longitudinal axis. The

stresses induced due to the ﬂexural load are a combination of compressive and

tensile stresses. This effect is illustrated in Fig. 2. Flexural properties are reported

and calculated in terms of the maximum stress and strain that occur at the outside

surface of the test bar. Many polymers do not break under ﬂexure even after a

large deﬂection that makes determination of the ultimate ﬂexural strength im-

practical for many polymers. In such cases, the common practice is to report

ﬂexural yield strength when the maximum strain in the outer ﬁber of the spec-

imen has reached 5%. For polymeric materials that break easily under ﬂexural

load, the specimen is deﬂected until a rupture occurs in the outer ﬁbers.

There are several advantages of ﬂexural strength tests over tensile tests. If a

material is used in the form of a beam and if the service failure occurs in

bending, then a ﬂexural test is more relevant for design or speciﬁcation purposes

than a tensile test, which may give a strength value very different from the

calculated strength of the outer ﬁber in the bent beam. The ﬂexural specimen is

comparatively easy to prepare without residual strain. The specimen alignment

is also more difﬁcult in tensile tests. Also, the tight clamping of the test speci-

mens creates stress concentration points. One other advantage of the ﬂexural test

is that at small strains, the actual deformations are sufﬁciently large to be mea-

sured accurately.

There are two basic methods that cover the determination of ﬂexural prop-

erties of plastics. Method 1 is a three-point loading system utilizing center load-

ing on a simple supported beam. A bar of rectangular cross section rests on two

supports and is loaded by means of a loading nose midway between the supports.

The maximum axial ﬁber stresses occur on a line under the loading nose. A

closeup of a specimen in the testing apparatus is shown in Fig. 3. This method

is especially useful in determining ﬂexural properties for quality control and

speciﬁcation purposes.

Method 2 is a four-point loading system utilizing two load points equally

spaced from their adjacent support points, with a distance between load points

of one-third of the support span. In this method, the test bar rests on two supports

and is loaded at two points (by means of two loading noses), each an equal

distance from the adjacent support point. Method 2 is very useful in testing

materials that do not fail at the point of maximum stress under a three-point

1 MECHANICAL PROPERTIES 549

Fig. 1 Tensile testing machine. (Courtesy Tinius Olsen Corporation)

Fig. 2 Forces involved in bending a simple beam. (Reprinted by permission

of McGraw-Hill Company)

550 PLASTICS TESTING

Fig. 3 Close-up of a specimen shown in ﬂexural testing apparatus.

(Courtesy DuPont Company)

loading system. The maximum axial ﬁber stress occurs on a line under the

loading nose.

Either method can be used with the two procedures. Procedure A is designed

principally for materials that break at comparatively small deﬂections. Procedure

B is designed particularly for those materials that undergo large deﬂections dur-

ing testing. The basic difference between the two procedures is the strain rate,

procedure A being 0.01 in./in./min, and procedure B being 0.10 in./in./min.

Modulus of Elasticity (Flexural Modulus)

The ﬂexural modulus is a measure of the stiffness during the ﬁrst or initial part

of the bending process. This value of the ﬂexural modulus is, in many cases,

equal to the tensile modulus.

The ﬂexural modulus is represented by the slope of the initial straight-line

portion of the stress–strain curve and is calculated by dividing the change in

stress by the corresponding change in strain.

1 MECHANICAL PROPERTIES 551

Fig. 4 Generalized creep curve

1.3 Creep Properties

Today, plastics are used in applications that demand high performance and ex-

treme reliability. Many components, conventionally made of metals, are now

made of plastics. The pressure is put on the design engineer to design the plastic

products more efﬁciently. An increasing number of designers have now recog-

nized the importance of thoroughly understanding the behavior of plastics under

long-term load and varying temperatures. Such behavior is described in terms

of creep properties.

When a plastic material is subjected to a constant load, it deforms quickly to

a strain roughly predicted by its stress–strain modulus, and then continues to

deform slowly with time indeﬁnitely or until rupture or yielding causes failure.

This phenomenon of deformation under load with time is called creep. All plas-

tics creep to a certain extent. The degree of creep depends upon several factors,

such as type of plastic, amount of load, temperature, and time.

The short-term stress–strain data is of little practical value in actual designing

the part, since such data does not take into account the effect of long-term

loading on plastics. Creep behavior varies considerably among types of plastics;

however, under proper stress and temperature conditions, all plastics will exhibit

a characteristic type of creep behavior. One such generalized creep curve is

shown in Fig. 4. The total creep curve is divided into four continuous stages.

552 PLASTICS TESTING

The ﬁrst stage (OP) represents the instantaneous elastic deformation. This initial

strain is the sum of the elastic and plastic strain. The ﬁrst stage is followed by

the second stage (PQ) in which strain occurs rapidly but at a decreasing rate.

This stage, where creep rate decreases with time, is sometimes referred to as

creep or primary creep. The straight portion of the curve (QR) is characterized

by a constant rate of creep. This process is called ‘‘cold ﬂow.’’ The ﬁnal stage

(RS) is marked by increase in creep rate until the creep fracture occurs.

If the applied load is released before the creep rupture occurs, an immediate

elastic recovery, substantially equal to elastic deformation followed by a period

of slow recovery is observed. The material in most cases does not recover to

the original shape and a permanent set remains. The magnitude of the permanent

set depends upon length of time, amount of stress applied, and temperature.

The creep values are obtained by applying constant load to the test specimen

in tension, compression, or ﬂexure and measuring the deformation as a function

of time. The values are most commonly referred to as tensile creep, compressive

creep, and ﬂexural creep.

Tensile Creep

Tensile creep measurements are made by applying the constant load to a tensile

test specimen and measuring its extension as a function of time. The extension

measurement can be carried out several different ways. The simplest way is to

make two gauge marks on the tensile specimen and measure the distance be-

tween the marks at speciﬁed time intervals. The percent creep strain is deter-

mined by dividing the extension by initial gauge length and multiplying by 100.

The percent creep strain is plotted against time to obtain a tensile creep curve.

The tensile stress values are also determined at speciﬁed time intervals to facil-

itate plotting a stress rupture curve. The more accurate measurements require

the use of a strain gauge, which is capable of measuring and amplifying small

changes in length with time and directly plotting them on a chart paper. Figure

5 illustrates a typical setup for tensile creep testing. The test is also carried out

at different stress levels and temperatures to study their effect on tensile creep

properties.

Flexural Creep

Flexural creep measurements are also made by applying a constant load to the

standard ﬂexural test specimen and measuring its deﬂection as a function of

time. A typical test setup for measuring creep in ﬂexure is shown in Fig. 6. As

illustrated, the deﬂection of the specimen at midspan is measured using a dial

indicator gauge. The electrical resistance gauges may also be used in place of

a dial indicator. The deﬂections of the specimen are measured at a predetermined

time interval.

1.4 Stress Relaxation

Stress relaxation is deﬁned as a gradual decrease in stress with time, under a

constant deformation (strain). This characteristic behavior of the polymers is

studied by applying a ﬁxed amount of deformation to a specimen and measuring

the load required to maintain it as a function of time. This is in contrast to creep

measurement, where a ﬁxed amount of load is applied to a specimen and re-

sulting deformation is measured as a function of time.

1 MECHANICAL PROPERTIES 553

Fig. 5 Typical test setup for tensile creep testing. (Courtesy Applied Test Systems Inc.)

Fig. 6 Flexural creep testing. (Courtesy Ceast U.S.A. Inc.)

554 PLASTICS TESTING

Fig. 7 Stress–time curve. (Courtesy Instron Corporation)

Stress relaxation behavior of the polymers has been overlooked by many

design engineers and researchers, partly because the creep data is much easier

to obtain and is readily available. However, many practical applications dictate

the use of stress relaxation data. For example, extremely low-stress relaxation is

desired in the case of a threaded bottle closure, which may be under constant

strain for a long period. If the plastic material used in the closures shows an

excessive decrease in stress under this constant deformation, the closures will

eventually fail. Similar problems can be encountered with metal inserts in

molded plastics, belleville or multiple cantilever springs used in cameras, appli-

ances, and business machines.

Stress relaxation measurements can be carried out using a tensile testing ma-

chine such as that described earlier in this chapter. However, the use of such a

machine is not always practical because the stress relaxation test ties up the

machine for a long period of time. The equipment for a stress relaxation test

must be capable of measuring very small elongation accurately, even when ap-

plied at high speeds. Many sophisticated pieces of equipment now employ a

strain gauge or a differential transformer along with a chart recorder capable of

plotting stress as a function of time. A typical stress–time curve is schematically

plotted in Fig. 7. At the beginning of the experiment, the strain is applied to the

specimen at a constant rate to achieve the desired elongation. Once the specimen

reaches the desired elongation, the strain is held constant for a predetermined

amount of time. The stress decay, which occurs due to stress relaxation, is ob-

served as a function of time. If a chart recorder is not available, the stress values

at different time intervals are recorded and the results are plotted to obtain a

stress versus time curve. The stress relaxation experiment is often carried out at

various levels of temperature and strain. The stress data obtained from the stress

relaxation experiment can be converted to a more meaningful apparent modulus

data by simply dividing stress at a particular time by the applied strain. The

curve may be replotted to represent apparent modulus as a function of time. The

use of logarithmic coordinates further simpliﬁes the stress relaxation data by

1 MECHANICAL PROPERTIES 555

allowing us to use standard extrapolation methods such as the one used in creep

experiments.

1.5 Impact Properties

The impact properties of the polymeric materials are directly related to the over-

all toughness of the material. Toughness is deﬁned as the ability of the polymer

to absorb applied energy. The area under the stress–strain curve is directly pro-

portional to the toughness of a material. Impact strength is a measure of tough-

ness. The higher the impact strength of a material, the higher the toughness and

vice versa. Impact resistance is the ability of a material to resist breaking under

a shock loading or the ability to resist the fracture under stress applied at high

speed.

The theory behind toughness and brittleness of the polymers is very complex

and therefore difﬁcult to understand. The molecular ﬂexibility plays an important

role in determining the relative brittleness or toughness of the material. For

example, in stiff polymers such as polystyrene and acrylics, the molecular seg-

ments are unable to disentangle and respond to the rapid application of me-

chanical stress and the impact produces brittle failure. In contrast, ﬂexible

polymers such as plasticized vinyls have high-impact strength due to the ability

of the large segments of molecules to disentangle and respond rapidly to me-

chanical stress.

Impact properties of the polymers are often modiﬁed simply by adding an

impact modiﬁer such as butadiene rubber or certain acrylic compounds. The

addition of a plasticizer also improves the impact strength at the cost of rigidity.

Material such as nylon, which has relatively fair impact strength, can be oriented

by aligning the polymer chains to improve the impact strength substantially.

Another way to improve the impact strength is to use ﬁbrous ﬁllers that appear

to act as stress transfer agents.

Most polymers, when subjected to the impact loading, seem to fracture in a

characteristic fashion. The crack is initiated on a polymer surface due to the

impact loading. The energy to initiate such a crack is called the crack initiation

energy. If the load exceeds the crack initiation energy, the crack continues to

propagate. A complete failure occurs when the load has exceeded the crack

propagation energy. Thus, both crack initiation and crack propagation contribute

to the measured impact strength. There are basically four types of failures en-

countered due to the impact load.

Brittle Fracture. In this type of failure the part fractures extensively without

yielding. A catastrophic mechanical failure such as the one in the case of

general-purpose polystyrene is observed.

Slight Cracking. The part shows evidence of slight cracking and yielding

without losing its shape or integrity.

Yielding. The part actually yields showing obvious deformation and stress

whitening but no cracking takes place.

Ductile Failure. This type of failure is characterized by a deﬁnite yielding of

material along with cracking. Polycarbonate is considered a ductile mate-

rial.

The distinction between the four types of failures is not very clear and some

overlapping is quite possible.

556 PLASTICS TESTING

Impact strength is one of the most widely speciﬁed mechanical properties of

the polymeric materials. However, it is also one of the least understood prop-

erties. Predicting the impact resistance of plastics still remains one of the most

troublesome areas of product design. One of the problems with some earlier

Izod and Charpy impact tests was that the tests were adopted by the plastic

industry from metallurgists. The principles of impact mechanisms as applied to

metals do not seem to work satisfactorily with plastics because of the plastics’

complex structure.

Types of Impact Tests

In the 1980s and 1990s, a tremendous amount of time and money have been

spent on research and development of various types of impact tests by organi-

zations throughout the world. Attempts have been made to develop different

sizes and shapes of specimens as well as impact testers. The specimens have

been subjected to a variety of impact loads including tensile, compression, bend-

ing, and torsion impacts. Impact load has been applied using everything from a

hammer, punches, and pendulums to falling balls and bullets. Unfortunately, very

little correlation exists, if any, between the types of tests developed so far. Nu-

merous technical papers and articles have been written on the subject of the

advantages of one method over the other. To this date, no industrywide consensus

exists regarding an ideal impact test method. In this chapter, an attempt is made

to discuss as many types of impact tests as possible along with the respective

advantages and limitations of each test. Impact testing is divided into three major

classes and subdivided into several classes as follows:

Pendulum Impact Test: IZOD–CHARPY (ASTM D-256, ISO 179). The

objective of the Izod–Charpy impact test is to measure the relative susceptibility

of a standard test specimen to the pendulum-type impact load. The results are

expressed in terms of kinetic energy consumed by the pendulum in order to

break the specimen. The energy required to break a standard specimen is actually

the sum of energies needed to deform it, to initiate its fracture, and to propagate

the fracture across it, and the energy expended in tossing the broken ends of the

specimen. This is called the toss factor. The energy lost through the friction and

vibration of the apparatus is minimal for all practical purposes and usually ne-

glected.

The specimen used in Izod test must be notched. The reason for notching the

specimen is to provide a stress concentration area that promotes a brittle rather

than a ductile failure. A plastic deformation is prevented by such type of notch

1 MECHANICAL PROPERTIES 557

in the specimen. The impact values are seriously affected because of the notch

sensitivity of certain types of plastic materials.

The Izod test requires a specimen to be clamped vertically as a cantilever

beam. The specimen is struck by a swing of a pendulum released from a ﬁxed

distance from the specimen clamp. A similar setup is used for the Charpy test

except for the positioning of the specimen. In the Charpy method, the specimen

is supported horizontally as a simple beam and fractured by a blow delivered in

the middle by the pendulum. The obvious advantage of the Charpy test over the

Izod test is that the specimen does not have to be clamped and, therefore, it is

free of variations in clamping pressures.

Apparatus and Test Specimens. The testing machine consists of a heavy base

with a vise for clamping the specimen in place during the test. In most cases,

the vise is designed so that the specimen can be clamped vertically for the Izod

test or positioned horizontally for the Charpy test without making any changes.

A pendulum-type hammer with an antifriction bearing is used. Additional

weights may be attached to the hammer for breaking tougher specimens. The

pendulum is connected to a pointer and a dial mechanism that indicates the

excess energy remaining in a pendulum after breaking the specimen. The dial

is calibrated to read the impact values directly in in.-lb or ft-lb. A hardened steel

striking nose is attached to the pendulum. The Izod and Charpy tests use dif-

ferent types of striking noses. A detailed list of requirements is discussed in the

ASTM standards book. Figure 8 illustrates a typical pendulum-type impact test-

ing machine. The test specimens can be prepared either by molding or cutting

them from a sheet. Izod test specimens are 2

⫻⫻in. size. The most

111

–––

228

common specimen thickness is but in. is preferred since it is less sus-

––

-in.

ceptible to bending and crushing. A notch is cut into a specimen very carefully

by a milling machine or a lathe. The recommended notch depth is 0.100-in.

Test Procedures

Izod Test. The test specimen is clamped into position so that the notched

end of the specimen is facing the striking edge of the pendulum. The pendulum

hammer is released, allowed to strike the specimen, and swing through. If the

specimen does not break, more weights are attached to the hammer and the test

is repeated until failure is observed. The impact values are read directly in

in.-lbf or ft-lbf from the scale. The impact strength is calculated by dividing

the impact values obtained from the scale by the thickness of the specimen. For

example, if a reading of 2 ft-lbf is obtained using an -in.-thick specimen, the

–

impact value would be 16 ft-lbf/in. of notch. The impact values are always

calculated on the basis of 1-in.-thick specimens even though much thinner spec-

imens are usually used. The reversed notched impact strength is obtained by

reversing the position of a notched specimen in the vise. In this case, the notch

is subjected to compressive rather than tensile stresses during impact. As dis-

cussed earlier in this chapter, the energy required to break a specimen is the

sum of the energies needed to deform it, initiate and propagate the fracture, and

toss the broken end (toss factor).

Falling-Weight Impact Test. The falling-weight impact test, also known as

the drop impact test or the variable-height impact test, employs a falling weight.

This falling weight may be a tup with a conical nose, a ball, or a ball-end dart.