Kutz M. Handbook of materials selection

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780 FAILURE ANALYSIS OF PLASTICS

(FT-IR) analysis and gel permeation chromatography (GPC) must be employed.

These methods can not only positively identify the basic material but also point

out the type and level of impurities in most cases.

3.3 Stress Analysis

Once the part failure resulting from poor molding practices or improper material

usage through visual examination and material identiﬁcation is ruled out, the

next logical step is to carry out an experimental stress analysis. Experimental

stress analysis is one of the most versatile methods for analyzing parts for pos-

sible failure. The part can be externally stressed or can have residual or molded-

in stresses. External stresses or molded-in stresses or a combination of both can

cause a part to fail prematurely. Stress analysis is an important part of failure

identiﬁcation. Detection of residual stresses has a different meaning than eval-

uation of stresses due to applied forces. It is possible of course to see failure

due to poor design or underestimating of forces. These failures are usually de-

tected in proof testing or in early production. Residual stresses are altogether

different: A molding process can generate residual stress just about anywhere,

anytime. Here, ongoing photoelastic inspection can prove extremely helpful,

allowing detection of defective molded parts or identiﬁcation of failures in clear

plastic products. Experimental stress analysis can be conducted to determine the

actual levels of stress in the part. Five basic methods are used to conduct stress

analysis: photoelastic method, brittle-coating method, strain gauge method,

chemical method, and heat reversion technique.

1. Photoelastic Method. The photoelastic method for experimental stress

analysis is quite popular among design engineers and has proved to be an ex-

tremely versatile yet simple technique.

If the parts to be analyzed are made out of one of the transparent materials,

stress analysis is simple. All transparent plastics, being birefringent, lend them-

selves to photoelastic stress analysis. The transparent part is placed between two

polarizing mediums and viewed from the opposite side of the light source. The

fringe patterns are observed without applying external stress. This allows the

observer to study the molded-in or residual stresses in the part. High fringe order

indicates the area of high-stress level whereas low fringe order represents an

unstressed area. Also, close spacing of fringes represents a high-stress gradient.

A uniform color indicates uniform stress in the part. Next, the part should be

stressed by applying external force and simulating actual-use conditions. The

areas of high-stress concentration can be easily pinpointed by observing changes

in fringe patterns brought forth by external stress. Figure 1 illustrates a typical

stress pattern in a part. This type of evaluation is useful as a regular part of

product inspection for quality control of transparent parts for any manufacturer

to maintain product quality and consistency and to prevent failures. The method

often reveals problems associated with many other process control parameters

such as temperature, material, ﬁll rate, design, etc.

Another technique known as the photoelastic coating technique can be used

to photoelastically stress-analyze opaque plastic parts. The part to be analyzed

is coated with a photoelastic coating, service loads are applied to the part, and

3 ANALYZING FAILURES 781

Fig. 1 Typical photoelastic stress pattern. (Courtesy Measurements Group, Inc.)

coating is illuminated by polarized light from the reﬂection polariscope. Molded-

in or residual stresses cannot be observed with this technique. However, the

same part can be fabricated using one of the transparent plastic materials. In

summary, photoelastic techniques can be used successfully for failure analysis

of a defective product.

2. Brittle-Coating Method. The brittle-coating method is yet another tech-

nique of conveniently measuring the localized stresses in a part. Brittle coatings

are specially prepared lacquers that are usually applied by spraying on the actual

part. The part is subjected to stress after air-drying the coating. The location of

maximum strain and the direction of the principal strain is indicated by the small

cracks that appear on the surface of the part as a result of external loading. Thus,

the technique offers valuable information regarding the overall picture of the

stress distribution over the surface of the part. The data obtained from the brittle-

coating method can be used to determine the exact areas for strain gauge location

and orientation, allowing precise measurement of the strain magnitude at points

of maximum interest. They are also useful for the determination of stresses at

stress concentration points that are too small or inconveniently located for in-

stallation of strain gauges. The brittle-coating technique, however, is not suitable

for detailed quantitative analysis such as photoelasticity. Sometimes it is nec-

essary to apply an undercoating prior to the brittle coating to promote adhesion

and to minimize compatibility problems.

782 FAILURE ANALYSIS OF PLASTICS

3. Strain Gauge Method. The electrical resistance strain gauge method is

the most popular and widely accepted method for strain measurements. The

strain gauge consists of a grid of strain-sensitive metal foil bonded to a plastic

backing material. When a conductor is subjected to a mechanical deformation,

its electrical resistance changes proportionally. This principle is applied in the

operation of a strain gauge. For strain measurements, the strain gauge is bonded

to the surface of a part with a special adhesive and then connected electrically

to a measuring instrument. When the test part is subjected to a load, the resulting

strain produced on the surface of the part is transmitted to the foil grid. The

strain in the grid causes a change in its length and cross section, and produces

a change in the resistivity of the grid material. This change in grid resistance,

which is proportional to the strain, is then measured with a strain gauge record-

ing instrument. In using strain gauges for failure analysis, care must be taken to

test the adhesives for compatibility with particular plastics to avoid stress-

cracking problems.

Residual or molded-in stresses can be directly measured with strain gauges

using the hole drilling method. This method involves measuring a stress at a

particular location, drilling a hole through the part to relieve the frozen-in

stresses, and then remeasuring the stress. The difference between the two mea-

surements is calculated as residual stress.

4. Chemical Method. Most plastics, when exposed to certain chemicals

while under stress, show stress cracking. This phenomenon is used in stress

analysis of molded parts. The level of molded-in or residual stress in a part

molded in ABS (acrylonitrile butadiene styrene) material can be determined by

this method. The part is immersed in a mixture of glacial acetic acid and water

for 2 min at 73

⬚F and later inspected for cracks that occur where tensile stress

at the surface is greater than the critical stress. The part may also be externally

stressed to a predetermined level and sprayed on with the chemical to determine

critical stresses. Stress cracking curves for many types of plastics have been

developed by material suppliers. If a defective product returned from the ﬁeld

appears to have stress-cracked, similar tests should be carried out to determine

molded-in stresses as well as the effect of external loading by simulating end-

use conditions. Failures of such types are seen often in parts where metal inserts

are molded-in or inserted after molding. Three other tests, stain resistance test,

solvent stress-cracking resistance, and environmental stress-cracking resistance

(ESCR) are also employed to analyze failed parts. The acetone immersion test,

to determine the quality of rigid polyvinyl chloride (PVC) pipe and ﬁttings as

indicated by their reaction to immersion in anhydrous acetone, is very useful.

An unfused PVC compound attacked by an anhydrous acetone causes the ma-

terial to swell, ﬂake, or completely disintegrate. A properly fused PVC com-

pound is impervious to anhydrous acetone and only a minor swelling, if any, is

observed. Defective PVC pipe or ﬁttings returned from the ﬁeld are subjected

to this test for failure analysis.

3.4 Heat Reversion Technique (ASTM F1057)

All plastics manufacturing process introduce some degree of stress in the ﬁn-

ished product. The stresses in molded parts are commonly referred to as molded-

in (residual) stresses. By reversing the process, by reheating the molded or

3 ANALYZING FAILURES 783

extruded product, the presence of stress can be determined. The test is conducted

by simply placing the entire specimen or a portion of the specimen in thermo-

statically controlled, circulating air oven and subjecting it to a predetermined

temperature for a speciﬁed time. The specimens are visually examined for a

variety of attributes. The degree and severity of warpage, blistering, wall sepa-

ration, ﬁsh-scaling, and distortion in the gate area of molded parts indicate stress

level. Stresses and molecular orientation effects in the plastic material are re-

lieved, and the plastic starts to revert to more stable form. The temperature at

which this begins to occur is important. If changes start below the heat distortion

temperature of the material, high levels of stress and ﬂow orientation are indi-

cated. The test has been signiﬁcantly improved by new methods including the

attachment of strain gauges to critical regions of the part to carefully monitor

initial changes in the shape. ASTM F1057 describes the standard practice for

estimating the quality of extruded PVC pipe by the heat reversion technique.

3.5 Microtoming

Microtoming is a technique of slicing an ultrathin section from a molded plastic

part for microscopic examination. This technique has been used by biologists

and metallurgists for years, but only in the last decade has this technique been

used successfully as a valuable failure analysis tool.

Microtoming begins with the skillful slicing of an 8- to 10-

␮

m-thick section

from a part and mounting the slice on a transparent glass slide. The section is

then examined under a light transmission microscope equipped with a polarizer

for photoelastic analysis. A high-power (1000

⫻) microscope that will permit

photographic recording of the structure in color is preferred. By examining the

microstructure of a material, much useful information can be derived. For ex-

ample, microstructural examination of a ﬁnished part that is too brittle may show

that the melt temperature was either nonuniform or too low. The presence of

unmelted particles is usually evident in such cases. Another reason for frequent

failures of the injection-molded part is failure to apply sufﬁcient time and pres-

sure to freeze the gates. This causes the parts to be underpacked, which creates

center-wall shrinkage voids. Figure 2 illustrates such shrinkage voids. Voids tend

to reduce the load-bearing capabilities and toughness of a part through the con-

centration of stress in a weak area.

Contamination, indicated by abnormality in the microstructure, almost always

creates some problems. Contamination caused by the mixing of different poly-

mers can be detected through such analysis by carefully studying the differences

in polymer structures. Quite often, a poor pigment dispersion also causes parts

to be brittle. This is readily observable through the microtoming technique. To

achieve optimum properties, additives such as glass ﬁbers and ﬁllers must dis-

perse properly. Microtoming a glass-ﬁber-reinforced plastic part reveals the de-

gree of bonding of the glass ﬁber to the resin matrix as well as the dispersion

and orientation of glass ﬁbers. Molded-in stresses as well as stresses resulting

from external loading are readily observed under cross-polarized light because

of changes in birefringence when the molecular structure is strained. Microtom-

ing technique can also be applied to check the integrity of spin and ultrasonic

or vibration welds.

784 FAILURE ANALYSIS OF PLASTICS

Fig. 2 Shrinkage voids created by insufﬁcient time and pressure to freeze the gates during

injection-molding process. (Courtesy BASF Corporation)

3.6 Mechanical Testing

Defective product returned from the ﬁeld is often subjected to a variety of me-

chanical tests to determine the integrity of the product. Two basic methods are

employed. One method involves conducting mechanical tests such as tensile,

impact, or compression on the actual part or a small sample cut out from the

part. The test results are then compared to the test results obtained from the

retained samples. The second method requires grinding up the defective parts

and either compression or injection-molding standard test bars and conducting

mechanical tests. The test results are compared to the published data for the

virgin material. The amount of material available for molding the test bars quite

often precludes injection molding. The test data obtained from compression-

molded samples are generally lower than injection-molded samples. Fatigue fail-

ure tests such as ﬂexural fatigue or tensile fatigue can be employed to determine

premature failure from cyclic loading.

3.7 Thermal Analysis

Thermal analysis techniques are used extensively in failure analysis. A detailed

discussion on the use of thermogravimetric analysis (TGA), thermomechanical

analysis (TMA), and differential scanning calorimetry (DSC) techniques is in a

chapter on material characterization tests.

3.8 Nondestructive Testing Techniques

NDT techniques are useful in determining the ﬂaws, discontinuities and joints.

BIBLIOGRAPHY 785

BIBLIOGRAPHY

Durelli, A. J., Phillips, E. A., and Tsao, C. H., Introduction to the Theoretical and Experimental

Analysis of Stress and Strain, McGraw-Hill, New York, 1958.

Ezrin, M., Plastics Failure Guide Causes and Prevention, Hanser Gardner Publications, 1996, p. 154.

Holman, J. P., Experimental Methods for Engineers, McGraw-Hill, New York, 1971, pp. 333–334.

‘‘Introduction to Stress Analysis by the Photoelastic Coating Technique,’’ Tech. Bull. IDCA-I, Pho-

toelastic Division, Vishay Intertechnology, Inc.

Levys, ‘‘Product Testing, Insurance against Failure,’’ Plastics Design Forum, July–August, 83 (1984).

Measurements Group, Tech. Bull., Vishay Intertechnology, Inc.

Morita, D. R. ‘‘QC Tests That Can Help Pin Point Material or Design Problems,’’ Plastics Design

Forum, May–June, 51–55 (1980).

‘‘Reﬂection Polariscope,’’ Tech. Bull. S-103-A, Photoelastic Division, Vishay Intertechnology, Inc.

Sessions, M. L., Microtoming, Engineering Design with Dupont Plastics, E. I. Dupont Co., Wil-

mington, DE, Spring 1977, p. 12.

Spier, I. M., ‘‘The Most Common Mistakes Made by Design Engineer Working in Plastics,’’ Plastic

Design Forum, March / April, 24 (1986).

Tobin, W., Plastic Design Forum, Jan / Feb, 48 (1987).

Vogt, J. P., ‘‘Testing for Mechanical Integrity Assures Service Life of Plastic Parts,’’ Plast. Design

and Processing, March, 13 (1976).

787

CHAPTER 27

FAILURE MODES: PERFORMANCE

AND SERVICE REQUIREMENTS FOR

CERAMICS

Dietrich Munz

Universita¨t Karlsruhe

Institut fu¨r Zuverla¨ssigkeit und Schadenskunde im Maschinenbau

Karlsruhe, Germany

1 OVERVIEW 787

2 FLAWS 789

3 FRACTURE MECHANICS 790

4 STRENGTH 792

5 DELAYED FAILURE 792

6 SCATTER OF STRENGTH AND

LIFETIME 794

6.1 Scatter of Strength 794

6.2 Scatter of Lifetime 797

7 DESIGN APPLYING MULTIAXIAL

WEIBULL STATISTICS 798

7.1 Strength under Compression

Loading 798

7.2 Global Multiaxial Fracture

Criterion 799

7.3 Local Multiaxiality Criterion 799

8 MATERIALS SELECTION FOR

THERMAL SHOCK CONDITIONS 803

9 FAILURE AT HIGH

TEMPERATURES 805

9.1 Creep Strain 805

9.2 Creep Rupture 806

REFERENCES 808

1 OVERVIEW

A reliable design of components requires the knowledge of the possible failure

modes under service conditions and knowledge of the material properties de-

scribing these failure modes. These material properties are characterized by spe-

ciﬁc material parameters, such as strength, fracture toughness, hardness, and

relations between loading parameters and materials response, e.g., the stress–

strain curve or the S–N curve for fatigue loading. A reliable design has to

incorporate the uncertainties in the loading parameters and in the material re-

sponse. Therefore, a correct description of the scatter of the material properties

is an important part in any design procedure. Material properties usually are

determined under simple loading conditions: uniaxial loading, homogeneous

stress state, constant temperature, simple load–time variations, such as constant

load, cyclic loading, or constant loading rate. The loading conditions of com-

ponents are more complex: multiaxial and inhomogeneous stress state, complex

HandbookofMaterialsSelection.EditedbyMyerKutz

788 FAILURE MODES

load, and temperature variation. Therefore, methods to transform results from

simple loading conditions to complex situations in components have to be de-

veloped and applied.

For metals, a good knowledge of failure modes, design rules following from

these failure modes, and criteria for materials selection exists. Ceramics are

brittle materials that fail without preceding plastic deformation. Only at high

temperatures may creep deformation occur before fracture. The brittle failure

mode requires speciﬁc design rules and criteria for material selection. Whereas

in metals high local stresses lead to local plastic deformation only and not to

failure of the component in ceramics, these high local stresses can lead to failure

at relatively low global stresses. High local stresses can occur under different

situations: sharp notches, thermal shock, contact loading. Tension stresses under

these conditions are dangerous because of the lower tensile strength compared

to compressive strength. From this behavior general design rules follow:

●

Avoid sharp notches.

●

Avoid surface damage, e.g., scratches.

●

Avoid sudden changes in temperature.

●

Avoid point or line loading of ceramic components.

The failure in ceramics starts from ﬂaws introduced during fabrication or

surface treatment. These ﬂaws may be pores, inclusions, cracks, or surface

scratches. Due to the scatter in the size of the ﬂaws, also the strength shows a

large scatter, which is much larger than in metals. This requires special statistical

design criteria. This large scatter is associated with a large effect of the size of

the component on the strength, which has to be included in the statistical design

criteria.

Under mechanical or thermal loading, different failure modes are possible:

●

Unstable fracture

●

Subcritical extension of ﬂaws under constant loading (static fatigue)

●

Subcritical extension of ﬂaws under cyclic loading (cyclic fatigue)

●

Creep deformation and creep fracture at high temperatures

●

Corrosion, especially oxidation

●

Wear in components under sliding contact

The different failure modes are characterized by material properties and relations

between the applied loading in terms of stresses or stress intensity factors and

the materials response in terms of lifetime or plastic deformation. The material

properties are:

●

Tensile strength

␴

or, to be more speciﬁc,

␴

●

Compressive strength

␴

●

Fracture toughness K

●

Hardness H

2 FLAWS 789

Fig. 1 Flaw origin in a silicon carbide.

The failure relations are:

●

Relation between stress

␴

and lifetime t

●

Relation between stress amplitude

␴

or stress range ⌬

␴

⫽ 2

␴

and num-

ber of cycles to failure N

●

Relation between creep strain and time

The ﬂaws leading to failure can be described as cracks. Therefore, the methods

of fracture mechanics can be applied to describe the ﬂaw extension behavior.

The corresponding relations are:

●

Relation between ﬂaw size and strength

●

Relation between crack growth rate and stress intensity factor for static

fatigue

●

Relation between crack extension for one load cycle and the range of the

stress intensity factor for cyclic fatigue

Because of the scatter in strength and in the lifetime, the failure cannot be

described by a unique strength or a ﬁxed lifetime. For a given load, only the

probability of failure can be speciﬁed. This failure probability depends not only

on the maximum load in the component, it also depends on the size of the

component and the stress distribution in the component.

2 FLAWS

Looking on a fracture surface of a ceramic, one very often can ﬁnd the origin

of the fracture. An example is shown in Fig. 1. It can distinguished between

intrinsic and extrinsic ﬂaws. Intrinsic ﬂaws are generated during the fabrication

process. They can be pores, shrinkage cracks, or areas of low density. Inclusions

of other materials and speciﬁc grain conﬁgurations, especially large grains may

790 FAILURE MODES

Table 1 Fracture Toughness and Strength of Different Ceramics

Fracture Toughness

(MPa )

兹m

Tensile Strength

(MPa)

Compressive Strength

(MPa)

AlN 3–4.5 300–500

3–4.5 300–500 2500

TiO

10–50

C 3.5 300–500 2900

BeO 200–400 1500

MoSi

300 2500

SiC hot pressed 4.6 500–850 460

dense reaction bonded 4–8

1.5–2.8

500–1200

150–350

SiO

fused quartz 0.8 50–100 1300

2000

TiB

300–400 1600

TiC 400

ZrB

300

ZrO

8–12 800–1000 2000

Glass 0.6–1.0

lead to high microstructural residual stresses after cooling from the sintering

temperature. These stresses may cause cracks during the application of a small

mechanical load. Phase transformation during cooling or under an applied load,

accompanied by a change in volume, e.g., in zirconia, is another cause of crack

nucleation. All these ﬂaws are primarily volume ﬂaws that, however, may extend

to the surface. Extrinsic ﬂaws are surface ﬂaws caused by surface treatment,

such as machining or grinding. Oxidation at high temperature may introduce

surface pits.

3 FRACTURE MECHANICS

The ﬂaws that cause failure are described as cracks. Consequently, the methods

of linear elastic fracture mechanics can be applied. The relation between the

size of the ﬂaw a and the strength

␴

⫽ (1)

兹aY

where K

is the fracture toughness and Y a constant that depends on the shape

of the crack. For a semicircular surface ﬂaw there is Y

⫽ 1.28.

The fracture toughness is a material property. In Table 1 typical values of

fracture toughness are listed. They range from less than 1 MPa for glass to

兹m

8 MPa for silicon nitride and 12 MPa for zirconia. For a given material

兹m 兹m

the fracture toughness depends on the speciﬁc microstructure and density. There-

fore, a large variation of K

is observed.

In Fig. 2 the strength

␴

according to Eq. 1 is plotted versus the crack size a

for different values of K

. A ﬂaw of a size of 100

␮

m leads to a strength of

391 MPa for a material with K

⫽ 5 MPa (e.g., alumina) and to a strength兹m