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-
Background Presentations
This section of the report summarizes the seven invited presentations that
were given during the course of the workshop. Five presentations were given
during the introductory session on the first morning. Two were given prior to the
subgroup discussion sessions on life-prediction issues (on the first afternoon of
the workshop) and accelerated-testing (on the second morning of the workshop).
Materials used during the presentations are found at the end of this report.
FIVE OVERVIEW BRIEFINGS
The first morning of the workshop was devoted to five overview
presentations that were intended to convey the breadth of applications for
accelerated-testing and life-prediction modeling in predicting systems
performance. The first presentation was by John M. Hanson of North Carolina
State University, who reviewed the aging and deterioration issues associated with
civil infrastructure in the United States and current life-prediction methodologies
and accelerated-testing methods applied to these systems. To encourage
discussion between members of the various application communities present and
to review the state of practice in other fields that could potentially be applied to
civil infrastructure, the next four presentations focused on materials durability,
reliability, and degradation issues in other fields. Jack E. Lemons of the
University of Alabama and John Anderson of Case Western Reserve University
spoke on the durability, reliability, degradation, and life-prediction issues
associated with surgical implant devices. Richard Wachnik of IBM
Microelectronics Division gave a presentation on the reliability and testing of
high-performance integrated circuits. Carol M. Jantzen of the Savannah River
Technology Center discussed vitreous materials for the long-term storage of
hazardous and radioactive waste. The final presentation of the morning session,
given by John Stringer of the Electric Power Research institute, focused on life-
cycle performance in the electric utility industry.
12
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BACKGROUND PRESENTATIONS
Infrastructure Aging and Deterioration
John M. Hanson, North Carolina State University
13
Professor Hanson began his presentation by stating that many people
currently believe that practicing engineers and the construction industry in the
United States have been slow to change and have failed to take advantage of
innovations in technology that could improve the nation's infrastructure. He
believes that this view should be put into proper perspective by acknowledging
the significant advances that have been made in construction in the past two or
three decades. For example, a review of the record height of concrete buildings in
the past two decades shows how advances in technology have been translated into
the construction of taller buildings. Professor Hanson pointed out that the
buildings, bridges, and tunnels constructed in the United States are comparable to
similar structures around the world and that U.S. engineers and contractors are
frequently sought as consultants in large construction projects around the world.
Professor Hanson also reminded the group that problems have often
resulted from infrastructure materials and products being introduced before
adequate experience with them had been gained and before their responses to the
environment were fully understood:
.
Field welding led to many of the fatigue and fracture problems that plague
steel bridges.
The introduction of Sarabond, an additive in masonry mortar, caused so many
problems that it was withdrawn from the market, and hundreds of buildings
required recladding.
The epoxy coating of embedded reinforcements for concrete was supposed to
prevent corrosion, but extensive use revealed that corrosion still occurred at
pinholes or where the coating had been damaged during construction.
Glass-fiber reinforced concrete for wall panels was subject to the
unanticipated problem of bowing with exposure to sunlight.
The marble panels used as cladding on the Amoco Building in Chicago had to
be replaced with granite because of excessive bowing and degradation of
strength from aging.
The widespread use of timber treated with fire retardent in roofs has required
major repairs because of unanticipated fractures after the materials had been in
· ~
service tor many years.
Stucco applied over insulation on thousands of homes and buildings has had
to be removed and replaced because the wood framing underneath has rotted;
thousands of other buildings will probably have to be repaired for the same
reason.
The defining conditions for a good structure, Hanson said, are adequate
strength, acceptable serviceability, and long-term durability. Mechanical response
factors, (i.e. load resistance, stability, fatigue resistance, and fracture resistance)
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RESEARCH AGENDA FOR TEST METHODS AND MODELS
must be addressed to ensure structural integrity.
According to Professor Hanson, the mechanisms of deterioration of the
primary construction materials (i.e., concrete, steel, masonry, and timber) are
quite well known, at least at the level required of an engineer. Concrete materials,
as well as masonry materials, may be subject to scaling due to freezing and
thawing, chemical attack, alkali-silica reaction, and corrosion of embedded
reinforcements. The deterioration of structural steels is mainly due to corrosion,
and the deterioration of timber is due mainly to decay. The rate of deterioration
(i.e., durability) is greatly affected by environmental factors, as well as the details
of construction. However, our understanding of these mechanisms is currently not
sufficient to enable us to make quantitative life predictions. Additional research to
enhance understanding of the basic mechanisms is critical to improving the
durability of materials and structures.
Professor Hanson emphasized however, that reviews of many
infrastructure failures have shown that very few occurred because of deterioration,
except when the structure or system had an underlying design or construction
defect. Some forms of deterioration seem to slow down or even stop after a period
of time. Thus, the deterioration of a material or structure does not necessarily
affect safety (See, for example, NRC, 1997 and Levy and Salvadori, 1992~.
Proper maintenance can prolong the life of materials by slowing their rate of
deterioration.
Professor Hanson concluded his presentation by stating that, although the
development of a concrete that does not shrink or creep or a steed that does not
corrode would be of great benefit to the construction industry, the likelihood of
such a material being developed is considered to be very small. Of course,
advancements have been, and continue to be, made, but accurate tests for
assessing the effects of environmental conditions on their lifetimes will require a
much better fundamental understanding of the damage process before they can
test results could be used for making life predictions. The adoption of these
materials will also depend on their economic advantage in a highly competitive
market. Thus, Hanson believes that the lifetimes of structures are more likely to
be extended by improvements in the quality of construction, than the use of new
materials.
References
Levy, M.P., and M. Salvadori. ~ 992. Why Buildings Fall Down. New York:
W.W. Norton and Company.
NRC (National Research Council), ~ 997. Nonconventional Concrete
Technologies: Renewal ofthe Highway Infrastructure. Washington, D.C.:
National Academy Press.
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BACKGROUND PRESENTATIONS
Infrastructure Considerations: Surgical Implant Devices
Jack E. Lemons, University of Alabama, and John Anderson, Case-Western
Reserve University
15
Dr. Lemons began by stating that the quality of life continues to be an
important aspect of human welfare. esnecialiv as the nonulation ares Thins two
, ~ , ]7 ~ ~ , ,, _
~ , · ~ · . · . . ~
important issues tor surgical implants and reconstructions are longevity and
quality of function. The primary issues related to the longevity of implant devices
include time of implantation, patient age and level of activity, and systemic health
and level of function. Many devices constituted from synthetic materials
(biomaterials) can function for decades, affording complete freedom from chronic
pain and the continuation of normal activities. Some surgical implant
reconstructive systems are known to have shorter lifetimes, however, and, in some
situations, successive revisions may have even shorter lifetimes.
Dr. Lemons explained that devices for surgical reconstruction of the
musculoskeletal system are subjected to high-magnitude forces (up to seven times
body weight) and, because of the dimensions of the anatomical sites, device
components can be subjected to mechanical stresses approaching the strength
limits of the biomaterials. The biochemical environment is a harsh organic and
salt-containing solution (saline), which can cause corrosion and reduce the
stability of interfaces for attachments between devices and supporting tissues.
Using joint and tooth-root replacement systems as examples, Lemons and
Anderson then reviewed the research, development, and application experience
required for the introduction of a new materials design. Both speakers stressed the
need for basic research and development, an understanding of the biomechanical
and biochemical properties of the biological host, and the types and sources of
degradation of biomaterials. They also explained why standardized methoclolo~ie~
would be helpful for monitoring device-related outcomes.
_ ~
As High Performance Integrated Circuits Enter the National (and
International) Infrastructure, How Do We Know They Are Reliable?
Richard Wachnik, IBM Microelectronics Division
Integrated circuits are critical parts of the data and telecommunications
of effort has been exceeded in trying to
infrastructure. Although a good deal , , ~
understand the detailed phenomena underlying the degradation of integrated
circuits, determining their stability or reliability is often largely empirical. This is
partly a reflection, Wachnik said, of the pace of change demanded by the
economics of the industry. For many products, conservative design practices are
used to minimize the risk of being left behind in performance or function. The
rapid obsolescence of new products makes many aggressive practices
conservative in hindsight. Thus, there is tremendous leverage in building
reliability into the process and ground rules, and less in accurately describing
degradation phenomena. Wachnik believes that this paradigm will change as
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16
RESEARCHAGENDA FOR TESTMETHODSAND MODELS
integrated circuit technology matures. Barring unforeseen innovations, future
competitive performance and density will depend on pushing reliability to its
limits and providing product designers with easy access to the tools required to
implement the most aggressive possible designs.
Integrated circuit technology can be divided into three subdisciplines: (~)
devices; (2) chip-level interconnections; and (3) higher level interconnections and
product reliability. The assessment and understanding of the reliability of
integrated-circuits draws on many subdisciplines, including physics, chemistry,
and materials science.
The most critical degradation mechanisms of silicon devices are hot
carriers; dielectric degradation and breakdown; and radiation effects. Dielectric
degradation
includes polarization, which affects performance, and leakage, which affects
dynamic circuits and potentially affects static circuits. Radiation effects include
single-event upsets as well as radiation damage. Assessing the reliability of
silicon-based devices depends on knowing the locations of the energy carriers and
the kinetics of defect formation by energetic carriers.
Accelerated testing can be done by increasing the applied voltage and
internal fields to heat the carrier distributions. Temperature accelerates dielectric
breakdown and polarization, and increased radiation flux accelerates radiation
effects. The details of operation can then be used to scale the actual stress time for
actual products. The understanding of hot-carrier degradation has progressed the
most rapidly because of its close relationship with device design and the ease with
which degradation kinetics can be examined using electrical characteristics of the
device. Advancing the understanding of radiation effects, and especially dielectric
breakdown, has lagged behind, partly because of difficulties in studying the
mechanisms in detail.
The reliability or stability of the chip-level interconnections, or wires
between the transistors, depends on assessing the kinetics of diffusion that are
driven by stress gradients or momentum transfer from the carriers in the circuits.
Electromigration failure can be accelerated by increasing both temperature and
current. The acceleration of stress migration is problematic, however, because the
increases in temperature that speed diffusion also tend to reduce the stress, which
is the driving force of failure. The reliability of higher level interconnections or
packaging is governed by mechanical factors (e.g., fatigue in particular) and
electrochemical factors (e.g., moisture entering the circuit or its physical
interface). Accelerated fatigue testing is done by increasing the amplitude during
thermal cycling. Increasing humidity, temperature, and voltage can accelerate
corrosion, as well as mechanical degradation from swelling or crack formation.
Wachnik explained that much of the focus on wiring, or interconnections,
has been on describing semi-empirical kinetic models of time to failure.
Successful, robust processes degrade gracefully, not catastrophically. Thus,
making accurate parametric descriptions of graceful degradation available to
product designers will be important in the long term. Wachnik also explained that
the role of the dielectric separating the wires becomes critical when new materials
· 11 ~· ~. · ~a~ ~
OCR for page 17
BACKGROUND PRESENTATIONS
17
are introduced. Although this has not yet become a problem, it is a significant and
practical issue in defect-controlled reliability.
According to Wachnik, high-performance first-levl] and second-level
packaging depends on providing high-density area connections between integrated
circuits and the substrates and boards to which they are attached. The reliability of
the interconnection requires that careful attention be paid to the mechanical
properties of the material and solders, as well as their sensitivity to moisture.
Because of trade-offs between cost and complexity, a careful evaluation of the
role of moisture in the overall reliability of the product should be carefully
evaluated.
Wachnik noted that his presentation was focused on concerns about the
product wearing out when the lifetime of the product is limited by the material
and/or its application. Another critical problem is finding. analyzing. and
controlling defects that can cause early failures.
Wachnik concluded his presentation with a reminder that many
opportunities remain for improving our understanding of the degradation of
integrated circuits and improving predictive capabilities. Some areas related to
process integration should be investigated in cooperation with industrial
development and fabrication organizations, but some (e.g., physical and chemical
investigations of degradation mechanisms) should be pursued by academic and
research-oriented organizations.
=7 ~- ~---= ~_
Durable Glass for Thousands of Years? That Is the Question.
Carol M. Jantzen, Savannah River Technology Center
Dr. Jantzen began by emphasizing that durable glasses used to stabilize a
wide variety of hazardous, mixed (i.e., radioactive and hazardous), and radioactive
wastes require modeling and assessments of the long-term stability of glass under
a variety of environmental conditions. Because disposal scenarios vary greatly,
the effects of kinetic parameters must also be modeled. Because the wastes being
stabilized vary greatly, the effect of the composition of the glass on long-term
stability must also be modeled. This is especially important for glasses used to
stabilize highly radioactive waste. In these cases, the glass must be durable and
retain radioactive species for thousands of years until they decay.
The chemical durability of glass is a complex phenomenon that depends
on both kinetics (e.g., temperature, length of time the class contacts a solution
exposed surface area, volume of the solution,
.. . . . . ... . . .. ..
and glass surface) and
mell~locynamlcs E.g., glass composition, 1ncluctlng me concentration of oxidized
and reduced species; and glass homogeneity). Long-term durability modeling is
usually based on acceleration of the dissolution process by the acceleration of one
or more of the kinetic test protocol parameters. Extreme caution must be used to
maintain the dissolution mechanism being modeled, to verify the long-term
durability using natural analogs, and to perform service-life tests in actual
disposal environments. The mechanisms modeled for glass durability are
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18
RESEARCH A GENDA FOR TEST METHODS AND MODELS
complex. Dissolution occurs when individual ions diffuse out of, condense in, or
precipitate on the leached layer via one of four operative mechanisms: ion
exchange; matrix dissolution; accelerated matrix dissolution; or surface layer
(possibly of a protective or passivating nature) formation. These mechanisms
control the overall durability of glass.
According to Dr. Jantzen, the modeling for the past 70 years of the
durability of glass as a simple function of composition has shown that the
durability response is nonlinear. Consequently, most durability models are either
empirical or kinetic. Early kinetic models treated glass dissolution as a simple
diffusion process. More recent models mathematically describe the glass
dissolution mechanisms in the form of time-dependent master equations rather
than as simple diffusion processes. Although the kinetic models describe the
leaching behavior of a given glass, they cannot predict which of a given group of
glasses will be most durable or whether a waste glass of composition A will be as
durable as a given natural analog of composition B.
Dr. Jantzen concluded her presentation with a discussion of the
thermodynamic hydration energy reaction mode] THERMOS. Thermodynamic
energy additivity was first proposed as a mode] for understanding the mechanistic
relationships between glass structure, composition, and physical properties as
early as 1945. Thermodynamic modeling was applied to medieval and Roman
window glass in 1977. Modeling of complex waste glasses, initiated in 1982,
resulted in the development of an improved model, THERMO_, in ~ 995.
THERMO_ lineariv predicts the d~,rnhilitv of AL {mm its romnn~itinn
~ r J ~ r
by mechanistically modeling both general glass dissolution and accelerated glass
dissolution. These mechanisms are modeled as a function of solution pH and
weak acid-strong base equilibria. THERMO_ discriminates between the
durability response of homogeneous or phase-separated glasses by a
compositionally dependent, phase-separation discriminator. The mode! can
predict the durability of glass in environmentally specific (e.g., pH-Eh)
environments. Predictions of thermodynamic reaction products derived from
THERMO_ can be used as input to computer codes used for reaction-path
durability assessments in a variety of disposal
modeling and long-term
(repository) environments.
Life-Cycle Performance in the Electric Utility In(lustr~r
John Stringer, Electric Power Research Institute
Dr. Stringer began his presentation by stating that the electric power
system in the United States is a single, very large, interconnected entity. For the
purposes of this presentation, however, he divided the system into three
distinguishable components: generation, transmission, and distribution. The
failure of a single component in any part of the electric power system, however,
can result in serious and expensive consequences, and concerns about potential
failures are increasing as the system ages.
OCR for page 19
BACKGROUND PRESENTATIONS
I
19
Forty percent of U.S. power generating capacity will be more than 30
years old by the year 2000, and many parts will be considerably older. The
generation component involves: coal-fired thermal systems, which are responsible
for approximately 55 percent of the electricity generated in the United States;
natural-gas-fired systems, which include Rankine and Brayton-cycle based
systems; oil-fired systems; nuclear-power thermal generators, which currently
represent about 20 percent of generating capacity; hydroelectric generators; and a
very small percentage of other sources, including biomass-fired systems, wind
turbines, and solar photovoltaic systems. The major lifetime issues are related to
the significant fraction of thermal generating plants that are more than 25 years
old, which was their notional lifetime. Many hydroelectric plants are even older,
some having been built before the Hoover Dam, which is more than 60 years old.
Dr. Stringer said the power transmission and distribution components are
also old and that the determination of their remaining life is complicated by their
inaccessibility. For example, New York City has an extensive underground
distribution system, some of which dates to the time of Edison. A significant part
of the transmission component was put in place in the 1950s, and a second part
was laid in the 1970s as the demand for power grew. Much of the transmission
system consists of"cable in pipe." Examinations have shown that the cable itself
may have a lifetime of more than 100 years, in the right circumstances, but that
the conduit can deteriorate even if the casing (pipe) is unaffected. For example,
the major failure that blacked out Auckland, New Zealand, appears to have
resulted from a rise in the temperature of the cable, which caused the dielectric to
fail; this failure appeared to be the result of a decrease in the thermal conductivity
of the surrounding earth (the "root cause") related to weather conditions.
The main technique for avoiding failure is to attempt to determine the
remaining lifetime of critical components to guide so-called "runJrepair/replace"
maintenance strategies. This approach involves (~) identifying the critical failure,
or life-limiting, processes; (2) identifying the root causes of these failures; (3)
determining whether a damage accumulation process can be identified that would
allow a life fraction to be measured; and (4) developing instrumentation and
inspection procedures for making predictions. EPR] has been developing models
for several key components (e.g., combustion turbine blades, thick-section
pressure components in steam systems, and boiler tubes) for a number of years.
These are computer-based systems, in some cases resembling expert systems, to
assist operators. In addition to deciding on remedial actions, these systems can
also advise an operator of the effect on component lifetime of operations outside
the nominal range. This approach has been advocated for dealing with
infrastructure lifetime issues, Stringer concluded, and opportunities for
advancement include the development of smart materials and systems.
TWO FOCUS BRIEFINGS
Two additional invited presentations were given during the course of the
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RESEARCH A GENDA FOR TEST METHODS AND MODELS
workshop. The first focus briefing, by Professor Kenneth Reifsnider of Virginia
Polytechnic Tnstitute and State University, was a discussion of life-prediction
issues associated with infrastructure applications. The talk was given before the
subgroup discussions, which took place on the first afternoon of the workshop.
The second briefing, by Dr. Jonathan W. Martin of the National Tnstitute of
Standards and Technology, focused on accelerated testing. This talk was given on
the second morning of the workshop, prior to the second subgroup deliberations.
Life-Prediction Approaches for Infrastructure Applications
Kenneth Reifsnider, Virginia Polytechnic Tnstitute and State University
For the purpose of life prediction, Professor Reifsnider described "life" as
a function of time, cycles, or history to the "failure" of a "component." Failure
was defined as unsuitability of service based on measurements of stiffness,
strength, properties, appearance, and other factors. He defined a component as a
structure, element, joint, bond, or sub-element. Life prediction for infrastructure is
complicated by complex environments, long service lives (often exceeding 100
years), dynamic and stochastic applied conditions of load, strain, temperature, and
moisture, and the quasi-brittle, reinforced materials of which infrastructure is
constructed.
The basic issues in life prediction are understanding physical degradation
processes at the basic or constituent level; modeling physical rate processes and
the evolution of material states; establishing independent physical observables
that track the processes; modeling the effects of combined processes; and
validating models on "real" structures. He then described the four elements of life
prediction:
the need to describe the physical behavior, i.e., damage and failure modes
modeling the behavior, including discrete events and multiple processes
identifying measurable independent observables as inputs to the models
actual life predictions, which are extensions, generalizations, and accelerations
of laboratory experience
Professor Reifsnider explained durability and the damage-tolerance
approach to life prediction, described relevant environmental factors, and
described the specific mechanism of polymer degradation. He then presented data
on a number of physical processes affecting service life and showed how these
relationships could be incorporated into models of physical processes, and,
ultimately, into more complex, predictive models. He described a damage
accumulation approach for modeling the combined interactive effects of fatigue,
creep, stress rupture, environment, and microdamage. This mode! has been
applied to life prediction of a buried multilayer composite pipe and a highway
bridge incorporating composite elements.
OCR for page 21
BACKGROUND PRESENTATIONS
21
He then offered some observations on life prediction of composites.
Changes in composite properties may not follow changes in matrix properties;
property evolution may be substantial; and mechanical/thermal/chemical coupling
may be significant. He noted the need for analysis and experiments on changes in
material states; a materials science base for dependence relationships with time,
temperature, and environments; and data with which to populate models.
Accelerated-Testing Approaches for Infrastructure Applications
Jonathan W. Martin, National Institute of Standards and Technology
Dr. Martin described a reliability-based methodology for predicting the
service life of infrastructure materials. The current durability methodology has
been used in the construction and other industries for at least SO years to simulate
natural outdoor weathering factors in the laboratory. Improving the predictive
capabilities of this methodology, however, has proved to be difficult. The
problems have generally been ascribed to inadequacies in laboratory-based aging
tests, especially the difficulty of isolating the ideal "balance of weathering
factors." According to Dr. Martin, however, the failure of the current
methodology can be attributed to faulty premises, inadequacies in experimental
design, and the difficulty of replicating weather conditions realistically over time.
Dr. Martin then described an alternative reliability-based methodology that
has a strong mathematical and scientific basis and a long history of successful
applications in the electronics, medical, aeronautical, and nuclear industries. A
number of experiments with coatings and other construction materials have
already been conducted using this method, and the results indicate that this
methodology will be generally applicable to a wide range of infrastructure
materials, components, and systems (Martin et al., ~ 996~.
He noted that implementation of a reliability-based methodology will
require substantial changes in the current experimental procedures including: (~)
the design of improved exposure equipment; (2) the systematic characterization of
the initial properties of coating systems, (3) the quantitative characterization of
each weathering variable in the in-service environment; (4) the quantification of
macroscopic degradation and relating submacroscopic to macroscopic measures
of degradation: (51 the use of experimental design techniques in Knin ~nr1
_=,__ ~- of-- --- r~-~~~~~= A
.. . . . ~ . . ~ _. _
executing short-term, laboratory-based experiments, and (6) the development of
computerized techniques for storing, retrieving, and analyzing collected data. Dr.
Martin believes that these changes will be justified by greater reliability of the
models and the speed of obtaining results.
Reference
Martin, I.W., S.C. Saunders, F.~. Floyd, and I.P. Winburg. 1996. Methodologies
for Predicting the Service Lives of Coating Systems. Blue Bell, Pa.:
Federation of Societies for Coatings Technology.
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Appendixes
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Representative terms from entire chapter:
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