Kutz M. Handbook of materials selection
Подождите немного. Документ загружается.
8 CORROSION AND STRESS CORROSION 769
rosive attack on buried steel and concrete pipe lines. There are also iron bacteria,
which ingest ferrous iron and precipitate ferrous hydroxide to produce local
crevice corrosion attack. Other bacteria oxidize ammonia to nitric acid, which
attacks most metals, and most bacteria produce carbon dioxide, which may form
the corrosive agent carbonic acid. Fungi and mold assimilate organic matter and
produce organic acids. Simply by their presence, fungi may provide the site for
crevice corrosion attacks, as does the presence of attached barnacles and algae.
Prevention or minimization of biological corrosion may be accomplished by
altering the environment or by using proper coatings, corrosion inhibitors, bac-
tericides or fungicides, or cathodic protection.
8.2 Stress Corrosion Cracking
Stress corrosion cracking is an extremely important failure mode because it
occurs in a wide variety of different alloys. This type of failure results from a
field of cracks produced in a metal alloy under the combined influence of tensile
stress and a corrosive environment. The metal alloy is not attacked over most
of its surface, but a system of intergranular or transgranular cracks propagates
through the matrix over a period of time.
Stress levels that produce stress corrosion cracking are well below the yield
strength of the material, and residual stresses as well as applied stresses may
produce failure. The lower the stress level, the longer is the time required to
produce cracking, and there appears to be a threshold stress level below which
stress corrosion cracking does not occur.
75
The chemical compositions of the environments that lead to stress corrosion
cracking are highly specific and peculiar to the alloy system, and no general
patterns have been observed. For example, austenitic stainless steels are suscep-
tible to stress corrosion cracking in chloride environments but not in ammonia
environments, whereas brasses are susceptible to stress corrosion cracking in
ammonia environments but not in chloride environments. Thus, the ‘‘season
cracking’’ of brass cartridge cases in the crimped zones was found to be stress
corrosion cracking due to the ammonia resulting from decomposition of organic
matter. Likewise, ‘‘caustic embrittlement’’ of steel boilers, which resulted in
many explosive failures, was found to be stress corrosion cracking due to sodium
hydroxide in the boiler water.
Stress corrosion cracking is influenced by stress level, alloy composition, type
of environment, and temperature. Crack propagation seems to be intermittent,
and the crack grows to a critical size, after which a sudden and catastrophic
failure ensues in accordance with the laws of fracture mechanics. Stress corro-
sion crack growth in a statically loaded machine part takes place through the
interaction of mechanical strains and chemical corrosion processes at the crack
tip. The largest value of plane-strain stress intensity factor for which crack
growth does not take place in a corrosive environment is designated K
ISCC
.In
many cases, corrosion fatigue behavior is also related to the magnitude of K
ISCC
.
8
Prevention of stress corrosion cracking may be attempted by lowering the
stress below the critical threshold level, choice of a better alloy for the environ-
ment, changing the environment to eliminate the critical corrosive element, use
of corrosion inhibitors, or use of cathodic protection. Before cathodic protection
is implemented care must be taken to ensure that the phenomenon is indeed
770 FAILURE MODES
stress corrosion cracking because hydrogen embrittlement is accelerated by ca-
thodic protection techniques.
9 FAILURE ANALYSIS AND RETROSPECTIVE DESIGN
In spite of all efforts to design and manufacture machines and structures to
function properly without failure, failures do occur. Whether the failure conse-
quences simply represent an annoying inconvenience or a catastrophic loss of
life and property, it is the responsibility of the designer to glean all of the
information possible from the failure event so that similar events can be avoided
in the future. Effective assessment of service failures usually requires the intense
interactive scrutiny of a team of specialists, including at least a mechanical
designer and a materials engineer trained in failure analysis techniques. The team
might often include a manufacturing engineer and a field service engineer as
well. The mission of the failure analysis team is to discover the initiating cause
of failure, identify the best solution, and redesign the product to prevent future
failures. Although the results of failure analysis investigations may often be
closely related to product liability litigation, the legal issues will not be addressed
in this discussion.
Techniques utilized in the failure analysis effort include the inspection and
documentation of the event through direct examination, photographs and eye-
witness reports; preservation of all parts, especially failed parts; and pertinent
calculations, analyses, and examinations that may help establish and validate the
cause of failure. The materials engineer may utilize macroscopic examination,
low-power magnification, microscopic examination, transmission or scanning
electron microscopic techniques, energy-dispersive X-ray techniques, hardness
tests, spectrographic analysis, metallographic examination, or other techniques
of determining the failure type, failure location, material abnormalities, and po-
tential causes of failure. The designer may perform stress and deflection anal-
yses, examine geometry, assess service loading and environmental influences,
reexamine the kinematics and dynamics of the application, and attempt to re-
construct the failure scenario. Other team members may examine the quality of
manufacture, the quality of maintenance, the possibility of unusual or uncon-
ventional usage by the operator, or other factors that may have played a role in
the service failure. Piecing all of this information together, it is the objective of
the failure analysis team to identify as accurately as possible the probable cause
of failure.
As undesirable as service failures may be, the results of a well-executed
failure analysis may be transformed directly into improved product reliability by
a designer who capitalizes on service failure data and failure analysis results.
These techniques of retrospective design have become important working tools
of the profession and are likely to continue to grow in importance.
REFERENCES
1. J. A. Collins, Failure of Materials in Mechanical Design; Analysis, Prediction, Prevention, 2nd
ed., Wiley, New York, 1993.
2. K. Hellan, Introduction to Fracture Mechanics, McGraw-Hill, New York, 1984.
3. H. Tada, P. C. Paris, and G. R. Irwin, The Stress Analysis of Cracks Handbook, 2nd ed., Paris
Productions, St. Louis, 1985.
4. T. L. Anderson, Fracture Mechanics, Fundamentals and Applications, 2nd ed., CRC Press, Boca
Raton, FL, 1995.
REFERENCES 771
5. ‘‘Progress in Measuring Fracture Toughness and Using Fracture Mechanics,’’ Materials Research
and Standards, March, 103–119 (1964).
6. X. Wu and A. J. Carlsson, Weight Functions and Stress Intensity Factor Solutions, Pergamon
Press, Oxford, 1991.
7. E 399-90, ‘‘Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials,’’
Annual Book of ASTM Standards, Vol. 03.01, ASTM, Philadelphia, 1991.
8. R. W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 3rd ed., Wiley,
New York, 1989.
9. J. P. Gallagher (Ed.), Damage Tolerant Design Handbook, 4 Vols., Metals and Ceramics Infor-
mation Center, Battelle Columbus Labs., Columbus, OH, 1983.
10. Metallic Materials and Elements for Aerospace Vehicle Structures, MIL-HDBK-5F, 2 Vols., U.S.
Dept. of Defense, Naval Publications and Forms Center, Philadelphia, 1987.
11. W. T. Matthews, Plane Strain Fracture Toughness (K
IC
) Data Handbook for Metals, Report No.
AMMRC MS 73-6, U. S. Army Materiel Command, NTIS, Springfield, VA, 1973.
12. C. M. Hudson and S. K. Seward, ‘‘A Compendium of Sources of Fracture Toughness and Fatigue
Crack Growth Data for Metallic Alloys,’’ Int. J. Fracture 14, R151 (1978).
13. C. M. Hudson and S. K. Seward, ‘‘A Compendium of Sources of Fracture Toughness and Fatigue
Crack Growth Data for Metallic Alloys—Part II,’’ Int. J. Fracture 20, R59 (1982).
14. C. M. Hudson and S. K. Seward, ‘‘A Compendium of Sources of Fracture Toughness and Fatigue
Crack Growth Data for Metallic Alloys—Part III,’’ Int. J. Fracture 39, R43 (1989).
15. C. M. Hudson and J. J. Ferrainolo, ‘‘A Compendium of Sources of Fracture Toughness and
Fatigue Crack Growth Data for Metallic Alloys—Part IV,’’ Int. J. Fracture 48, R19 (1991).
16. M. F. Kanninen and C. H. Popelar, Advanced Fracture Mechanics, Oxford University Press, New
York, 1985.
17. A. F. Madayag, Metal Fatigue, Theory and Design, Wiley, New York, 1969.
18. Proceedings of the Conference on Welded Structures, Vols. I and II, The Welding Institute,
Cambridge, England, 1971.
19. H. J. Grover, S. A. Gordon, and L. R. Jackson, Fatigue of Metals and Structures, GPO, Wash-
ington, DC, 1954.
20. H. F. Moore, ‘‘A Study of Size Effect and Notch Sensitivity in Fatigue Tests of Steel,’’ ASTM
Proceed., 45, 507 (1945).
21. W. N. Thomas, ‘‘Effect of Scratches and Various Workshop Finishes Upon the Fatigue Strength
of Steel,’’ Engineering 116, 449ff (1923).
22. G. V. Smith, Properties of Metals at Elevated Temperatures, McGraw-Hill, New York, 1950.
23. N. E. Frost and K. Denton, ‘‘The Fatigue Strength of Butt Welded Joints in Low Alloy Structural
Steels,’’ Brit. Welding J., 14(4), (1967).
24. H. J. Grover, Fatigue of Aircraft Structures, GPO, Washington, DC, 1966.
25. E 647-88a, ‘‘Standard Test Method for Measurement of Fatigue Crack Growth Rates,’’ ASTM,
Philadelphia, 1988.
26. J. Schijve, ‘‘Four Lectures on Fatigue Crack Growth,’’ Eng. Fracture Mech. 11, 167–221 (1979).
27. P. C. Paris and F. Erdogan, ‘‘A Critical Analysis of Crack Propagation Laws,’’ Trans. ASME, J.
Basic Eng., 85, 528–534 (1963).
28. S. J. Maddox, Fatigue Strength of Welded Structures, 2nd ed., Abington, Cambridge, England,
1991.
29. W. G. Reuter, J. H. Underwood, and J. C. Newman (Eds.), Surface-Crack Growth: Models,
Experiments and Structures, STP-1060, ASTM, Philadelphia, 1990.
30. W. Elber, Engr. Fracture Mech., 2, 37–45 (1970).
31. J. C. Newman and W. Elber (Eds.), Mechanics of Fatigue Crack Closure, STP-982, ASTM,
Philadelphia, 1988.
32. L. P. Pook, The Role of Crack Growth in Metal Fatigue, The Metals Society, London, 1983.
33. H. O. Fuchs and R. I. Stephens, Metal Fatigue in Engineering, Wiley, New York, 1980.
34. D. Broek, Elementary Engineering Fracture Mechanics, 4th ed., Kluwer, London, 1986.
35. A. P. Parker, The Mechanics of Fracture and Fatigue, E. & F. N. Spon Ltd., New York, 1981.
36. R. D. Ritchie and J. Lankford (Eds.), Small Fatigue Cracks, The Metallurgical Society, Warren-
dale, PA, 1986.
37. W. G. Clark, Jr., ‘‘Fracture Mechanics in Fatigue,’’ Experimental Mechanics, Sept. (1971).
38. R. C. Juvinall, Engineering Considerations of Stress, Strain, and Strength, McGraw-Hill, New
York, 1967.
772 FAILURE MODES
39. F. R. Larson and J. Miller, ‘‘Time-Temperature Relationships for Rupture and Creep Stresses,’’
ASME Trans. 74, 765 (1952).
40. R. G. Sturm, C. Dumont, and F. M. Howell, ‘‘A Method of Analyzing Creep Data,’’ ASME Trans.
58, A62 (1936).
41. N. H. Polakowski and E. J. Ripling, Strength and Structure of Engineering Materials. Prentice-
Hall, Englewood Cliffs, NJ, 1966.
42. J. A. Collins, ‘‘Fretting-Fatigue Damage-Factor Determination,’’ J. Eng. Industry, 87(8), 298–
302 (1965).
43. D. Godfrey, ‘‘Investigation of Fretting by Microscopic Observation,’’ NACA Report 1009, Cleve-
land, OH, 1951 (formerly TN-2039, February 1950).
44. F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Oxford University Press,
Amen House, London, 1950.
45. D. Godfrey and J. M. Baily, ‘‘Coefficient of Friction and Damage to Contact Area During the
Early Stages of Fretting; I-Glass, Copper, or Steel Against Copper,’’ NACA TN-3011, Cleveland,
OH, September 1953.
46. M. E. Merchant, ‘‘The Mechanism of Static Friction,’’ J. Appl. Phys., 11(3), 232 (1940).
47. E. E. Bisson, R. L. Johnson, M. A. Swikert, and D. Godfrey, ‘‘Friction, Wear, and Surface
Damage of Metals as Affected by Solid Surface Films,’’ NACA TN-3444, Cleveland, OH, May
1955.
48. H. H. Uhlig, ‘‘Mechanisms of Fretting Corrosion,’’ J. Appl. Mech. 76, 401–407 (1954).
49. I. M. Feng and B. G. Rightmire, ‘‘The Mechanism of Fretting,’’ Lubrication Eng., 9, 134ff (June
1953).
50. I. M. Feng, ‘‘Fundamental Study of the Mechanism of Fretting,’’ Final Report, Lubrication Lab-
oratory, Massachusetts Institute of Technology, Cambridge, MA, 1955.
51. H. T. Corten, ‘‘Factors Influencing Fretting Fatigue Strength,’’ T. & A. M. Report No. 88, De-
partment of Theoretical and Applied Mechanics, University of Illinois, Urbana, IL, June 1955.
52. W. L. Starkey, S. M. Marco, and J. A. Collins, ‘‘Effects of Fretting on Fatigue Characteristics
of Titanium-Steel and Steel-Steel Joints,’’ ASME Paper 57-A-113, New York, 1957.
53. W. D. Milestone, ‘‘Fretting and Fretting-Fatigue in Metal-to-Metal Contacts,’’ ASME Paper 71-
DE-38, New York, 1971.
54. G. P. Wright and J. J. O’Connor, ‘‘The Influence of Fretting and Geometric Stress Concentrations
on the Fatigue Strength of Clamped Joints,’’ Proceedings, Institution of Mechanical Engineers,
186 (1972).
55. R. B. Waterhouse, Fretting Corrosion, Pergamon, New York, 1972.
56. ‘‘Fretting in Aircraft Systems,’’ AGARD Conference Proceedings CP161, distributed through
NASA, Langley Field, VA, 1974.
57. ‘‘Control of Fretting Fatigue,’’ Report No. NMAB-333, National Academy of Sciences, National
Materials Advisory Board, Washington, DC, 1977.
58. N. P. Suh, S. Jahanmir, J. Fleming, and E. P. Abrahamson, ‘‘The Delamination Theory of Wear-
II,’’ Progress Report, Materials Processing Lab, Mechanical Engineering Dept., MIT Press, Cam-
bridge, MA, September 1975.
59. J. T. Burwell, Jr., ‘‘Survey of Possible Wear Mechanisms,’’ Wear, 1, 119–141 (1957).
60. M. B. Peterson, M. K. Gabel, and M. J. Derine, ‘‘Understanding Wear’’; K. C. Ludema, ‘‘A
Perspective on Wear Models’’; E. Rabinowicz, ‘‘The Physics and Chemistry of Surfaces’’; J.
McGrew, ‘‘Design for Wear of Sliding Bearings’’; R. G. Bayer, ‘‘Design for Wear of Lightly
Loaded Surfaces,’’ ASTM Standardization News 2(9), 9–32 (1974).
61. P. A. Engel, ‘‘Predicting Impact Wear,’’ Machine Design, May, 100–105 (1977).
62. P. A. Engel, Impact Wear of Materials, Elsevier, New York, 1976.
63. J. A. Collins, A Study of the Phenomenon of Fretting-Fatigue with Emphasis on StressField
Effects, Dissertation, Ohio State University, Columbus, 1963.
64. J. A. Collins and F. M. Tovey, ‘‘Fretting Fatigue Mechanisms and the Effect of Direction of
Fretting Motion on Fatigue Strength,’’ J. Materials, 7(4), (December 1972).
65. W. D. Milestone, An Investigation of the Basic Mechanism of Mechanical Fretting and Fretting-
Fatigue at Metal-to-Metal Joints, with Emphasis on the Effects of Friction and Friction-Induced
Stresses, Dissertation, Ohio State University, Columbus, 1966.
66. J. A. Collins and S. M. Marco, ‘‘The Effect of Stress Direction During Fretting on Subsequent
Fatigue Life,’’ ASTM Proceed., 64, 547 (1964).
REFERENCES 773
67. H. Lyons, An Investigation of the Phenomenon of Fretting-Wear and Attendant Parametric Effects
Towards Development of Failure Prediction Criteria, Ph.D. Dissertation, Ohio State University,
Columbus, 1978.
68. P. L. Ko, ‘‘Experimental Studies of Tube Fretting in Steam Generators and Heat Exchangers,’’
ASME/ CSME Pressure Vessels and Piping Conference, Nuclear and Materials Division, Mon-
treal, Canada, June 1978.
69. R. B. Heywood, Designing Against Fatigue of Metals, Reinhold, New York, 1962.
70. F. P. Bowden and D. Tabor, Friction and Lubrication, Methuen, London, 1967.
71. E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1966.
72. C. Lipson, Wear Considerations in Design, Prentice-Hall, Englewood Cliffs, NJ, 1967.
73. N. P. Suh, ‘‘The Delamination Theory of Wear,’’ Wear 25, 111–124 (1973).
74. J. E. Shigley and C. R. Mischke, Mechanical Engineering Design, 5th ed., McGraw-Hill, New
York, 1989.
75. M. G. Fontana and N. D. Greene, Corrosion Engineering, 2nd ed. McGraw-Hill, New York,
1978.
76. L. S. Seabright and R. J. Fabian, ‘‘The Many Faces of Corrosion,’’ Materials in Design Eng.,
57(1) (1963).
77. G. Nelson, Corrosion Data Survey, National Association of Corrosion Engineers, Houston, TX,
1972.
78. H. H. Uhlig (Ed.), Corrosion Handbook, Wiley, New York, 1948.
79. E. Rabald, Corrosion Guide, Elsevier, New York, 1951.
775
CHAPTER 26
FAILURE ANALYSIS OF PLASTICS
Vishu Shah
Consultek
Brea, California
1 INTRODUCTION 775
1.1 Material Selection 775
1.2 Design 776
1.3 Process 776
1.4 Service Conditions 776
2 TYPES OF FAILURE 776
2.1 Mechanical Failure 776
2.2 Thermal Failure 778
2.3 Chemical Failure 778
2.4 Environmental Failure 778
3 ANALYZING FAILURES 778
3.1 Visual Examination 778
3.2 Identification Analysis 779
3.3 Stress Analysis 780
3.4 Heat Reversion Technique
(ASTM F1057) 782
3.5 Microtoming 783
3.6 Mechanical Testing 784
3.7 Thermal Analysis 784
3.8 Nondestructive Testing
Techniques 784
BIBLIOGRAPHY 785
1 INTRODUCTION
Part failure is generally related to one of four key factors: material selection,
design, process, and service conditions.
1.1 Material Selection
Failures arising from hasty material selection are not uncommon in plastics or
any other industry. In an application that demands high-impact resistance, a high-
impact material must be specified. If the material is to be used outdoors for a
long period, an ultraviolet resistant (UV) material must be specified. For proper
material selection, careful planning, a thorough understanding of plastic mate-
rials, and reasonable prototype testing are required. The material selection should
not be solely based on cost. A systematic approach to the material selection
process is necessary to select the best material for any application. The proper
material selection technique involves carefully defining the application require-
ment in terms of mechanical, thermal, environmental, electrical, and chemical
properties. In many instances, it makes sense to design a thinner wall part taking
advantage of the stiffness-to-weight ratio offered by higher-priced, fast cycling
engineering materials. Many companies, including material suppliers, have de-
veloped software to assist in material selection simply by selecting application
requirement in order of importance.
HandbookofMaterialsSelection.EditedbyMyerKutz
Copyright Ó 2002 John Wiley & Sons, Inc., NewYork.
776 FAILURE ANALYSIS OF PLASTICS
1.2 Design
Proper material selection alone will not prevent a product from failing. While
designing a plastic product, the designer must use the basic rules and guidelines
provided by the material supplier for designing a particular part in that material.
One must remember that with the exception of a few basic rules in designing
plastic parts, the design criteria changes from material to material as well as
from application to application. Today, design-related failures are by far the most
common type of failure. Design guides for proper plastic part design are readily
available from material suppliers. Table 1 shows a typical part design checklist.
1.3 Process
After proper material selection and design, the responsibility shifts from the
designer to the plastic processor. The most innovative design and a very careful
material selection cannot make up for poor processing practices. Molded-in
stresses, voids, weak weld lines, and moisture in the material are some of the
most common causes for premature product failures.
The latest advancement in process control technology allows the processors
to not only control the process with a high degree of reliability but also helps
in record keeping should a product fail at a later date. Such records of processing
parameters are invaluable to a person conducting failure analysis. Any assembly
or secondary operation on processed part must be evaluated carefully to avoid
premature failures. Failures arising from stress cracking around metal inserts,
drilled holes, and welded joints are quite common.
1.4 Service Conditions
In spite of the built-in safety factor warning labels, and user’s instructions, fail-
ures arising from service conditions is quite common in the plastics industry.
Five categories of unintentional service conditions are as follows.
1. Reasonable misuse
2. Use of product beyond its intended lifetime
3. Failure of product due to unstable service conditions
4. Failure due to service condition beyond reasonable misuse
5. Simultaneous application of two stresses operating synergistically
Most of the stresses imposed on plastics products in service can be grouped
under the headings of thermal, chemical, physical, biological, mechanical, and
electrical.
2 TYPES OF FAILURES
2.1 Mechanical Failure
Mechanical failure arises from the applied external forces that, when they exceed
the yield strength of the material, cause the product to deform, crack, or break
into pieces. The force may have been applied in tension, compression, and im-
pact for a short or a long period of time at varying temperatures and humidity
conditions.
2 TYPES OF FAILURES 777
Table 1 Part Design Checklist
778 FAILURE ANALYSIS OF PLASTICS
2.2 Thermal Failure
Thermal failures occur from exposing products to an extremely hot or extremely
cold environment. At abnormally high temperatures the product may warp, twist,
melt, or even burn. Plastics tend to get brittle at low temperatures. Even the
slightest amount of load may cause the product to crack or even shatter.
2.3 Chemical Failure
Very few plastics are totally impervious to all chemicals. Failure occurring from
exposing the products to certain chemicals is quite common. Residual or molded-
in stress, high temperatures, and external loading tend to aggravate the problem.
2.4 Environmental Failure
Plastics exposed to outdoor environments are susceptible to many types of det-
rimental factors. Ultraviolet rays, humidity, microorganisms, ozone, heat, and
pollution are major environmental factors that seriously affect plastics. The effect
can be anywhere from a mere loss of color, slight crazing and cracking to a
complete breakdown of the polymer structure.
3 ANALYZING FAILURES
The first step in analyzing any type of failure is to determine the cause of the
failure. Before proceeding with any elaborate tests, some basic information re-
garding the product must be gathered. If the product is returned from the field,
have the district manager or consumer give you basic information, such as the
date of purchase, date of installation, date when the first failure encountered,
geographic location, types of chemicals used with or around the product, and
whether the product was used indoors or outdoors. All this information is very
vital if one is to analyze the defective product proficiently. For example, if the
report from the field along with the defective product indicates a certain type of
chemical was used with the product, one can easily check the chemical com-
patibility of the product or go one step further and simulate the actual-use con-
dition using that same chemical. Record keeping also simplifies the task of
failure analysis. A simple date code or cavity identification number will certainly
enhance the traceability. Many types and styles of checklists to help analyze the
failures have been developed. Seven basic methods are employed to analyze
product failure: (1) visual examination, (2) identification analysis, (3) stress anal-
ysis, (4) microtoming, (5) mechanical testing, (6) thermal analysis, and (7) non-
destructive testing (NDT) techniques. By zeroing in on the type of failure, one
can easily select the appropriate method of failure analysis.
3.1 Visual Examination
A careful visual examination of the returned part can reveal many things. Ex-
cessive splay marks indicate that the materials were not adequately dried before
processing. The failure to remove moisture from hydroscopic materials can lower
the overall physical properties of the molded article and in some cases even
cause them to become brittle. The presence of foreign material and other con-
taminants is also detrimental and could have caused the part to fail. Burn marks
on molded articles are easy to detect. They are usually brown streaks and black
3 ANALYZING FAILURES 779
spots. These marks indicate the possibility of material degradation during proc-
essing, causing the breakdown of molecular structure leading to overall reduction
in the physical properties. Sink marks and weak weld lines, readily visible on
molded parts, represent poor processing practices and may contribute to part
failure.
A careful visual examination will also reveal the extent of consumer abuse.
The presence of unusual chemicals, grease, pipe dope, and other substances may
give some clues. Heavy marks and gouges could be the sign of excessively
applied external force.
The defective part should also be cut in half using a sharp saw blade. The
object here is to look for voids caused by trapped gas and excessive shrinkage,
especially in thick sections during molding. A reduction in wall thickness caused
by such voids could be less than adequate for supporting compressive or tensile
force or withstanding impact load and may cause part to fail. Lastly, if the
product has failed because of exposure to UV rays and other environmental
factors, a slight chalking, microscopic cracks, large readily visible cracks, or loss
of color will be evident.
3.2 Identification Analysis
One of the main reasons for product failure is simply use of the wrong material.
When a defective product is returned from the field, material identification tests
must be carried out to verify that the material used in the defective product is,
in fact, the material specified on the product drawing. Chapter 21 discusses
simple material identification techniques. However, identifying the type of ma-
terial is simply not enough. Since all plastic materials are supplied in a variety
of grades with a broad range of properties, the grade of material must also be
determined. A simple technique such as the melt index test can be carried out
to confirm the grade of a particular type of material. The percentage of regrind
material mixed with virgin material has a significant effect on the physical prop-
erties. Generally, the higher the level of regrind material mixed with virgin, the
lower the physical properties. If during processing higher than recommended
temperature and long residence time are used, chances are that the material is
degraded. This degraded material, when reground and mixed with virgin mate-
rial, can cause a significant reduction in overall properties. Unfortunately, the
percentage of regrind used with virgin is almost impossible to determine by
performing tests on the molded parts. However, a correlation between the melt
index value and the part failure rate can be established by conducting a series
of tests to determine the minimum or maximum acceptable melt index value.
Part failures due to impurities and contamination of virgin material are quite
common. Material contamination usually occurs during processing. A variety of
purging materials are used to purge the previous material from the extruder barrel
before using the new material. Not all of these purging materials are compatible.
Such incompatibility can cause the loss of properties, brittleness, and delami-
nation. In the vinyl compounding operation, failure to add key ingredients, such
as an impact modifier, can result in premature part failure. Simple laboratory
techniques cannot identify such impurities, contamination, or the absence of a
key ingredient. More sophisticated techniques; such as Fourier transform infrared