Structural Analysis Made ‘NESSUSary’
Computer Technology
Originating Technology/ NASA Contribution
Everywhere you look, chances are something that
was designed and tested by a computer will be in
plain view. Computers are now utilized to design
and test just about everything imaginable, from
automobiles and airplanes to bridges and boats,
and elevators and escalators to streets and skyscrapers.
Computer-design engineering first emerged in the
1970s, in the automobile and aerospace industries.
Since computers were in their infancy, however,
architects and engineers during the time were limited
to producing only designs similar to hand-drafted
drawings. (At the end
of 1970s, a typical computer-aided design system
was a 16-bit minicomputer with a price tag of $125,000.)
Eventually, computers became more affordable and
related software became more sophisticated, offering
designers the “bells and whistles” to go beyond
the limits of basic drafting and rendering, and
venture into more skillful applications. One of
the major advancements was the ability to test
the objects being designed for the probability
of failure.
This advancement was especially important for the
aerospace industry, where complicated and expensive
structures are designed. The ability to perform
reliability and risk assessment without using extensive
hardware testing is critical to design and certification.
In 1984, NASA initiated the Probabilistic Structural
Analysis Methods (PSAM) project at Glenn Research
Center to develop analysis methods and computer
programs for the probabilistic structural analysis
of select engine components for current Space Shuttle
and future space propulsion systems. NASA envisioned
that these methods and computational tools would
play a critical role in establishing increased
system performance and durability, and assist in
structural system qualification
and certification. Not only was the PSAM project
beneficial to aerospace, it paved the way for a
commercial risk-
probability tool that is evaluating risks in diverse,
down-to-Earth applications.
Partnership
Southwest
Research Institute (SwRI), headquartered
in San Antonio, Texas, is a multidisciplinary,
independent, nonprofit, applied engineering and
physical sciences research and development organization.
As the prime contractor on the PSAM project, the
company developed a sophisticated computer program
called NESSUS (numerical evaluation of stochastic
structures under stress). Designed specifically
for predicting structural response caused by uncertain
basic variables such as loads, material properties,
geometry, and boundary conditions, NESSUS is used
by NASA to assist in the evaluation of existing
critical Space Shuttle components, including the
Space Shuttle Main Engines (SSMEs).
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Southwest Research
Institute’s NESSUS software is particularly
useful for large, complex calculations, such
as simulating a small car colliding with a
large sport utility vehicle. |
For example, the software examined a SSME fuel
turbopump blade that was subjected to harsh conditions,
which could ultimately affect its performance abilities.
A finite element model of the turbopump blade was
analyzed
by NESSUS for the effects of high-temperature and
high-cycle mechanical fatigue that could cause
it to crack. The software computed a reliability
value of 0.99978 for the SSME component. Furthermore,
the probabilistic sensitivity factors revealed
the variables for which tighter control would result
in a more reliable blade design.
(In this case, hot gas seal leakage clearly dominated
other considerations.) Conversely, the sensitivity
factors also exposed which variables are relatively
unimportant in determining blade reliability—information
important in establishing design and manufacturing
controls for maximum cost-effectiveness as well
as structural reliability.
The 10-year PSAM project officially ended in February
1995. Version 6.2 of NESSUS—regarded the final
version for NASA’s purposes at that time—was delivered
to NASA in September 1995, along with a final report.
Though in 2002, SwRI was again contracted by NASA,
through Glenn, to further enhance NESSUS for application
with large-scale aero-propulsion systems. Additionally,
another government contract issued through the
Los Alamos National Laboratory led to the utilization
of NESSUS for extremely large and complex weapon-reliability
problems, in support of its Stockpile Stewardship
program.
The conclusion of the NASA projects, along with
the Los Alamos project, marked a new beginning
for SwRI, as it went on to commercialize NESSUS
program for non-
aerospace applications.
Product Outcome
NESSUS is a modular computer software system that
integrates advanced reliability methods with finite
element and boundary element methods and probabilistic
algorithms in order to model uncertainties in loads,
material properties, and geometries with random
variables. Probabilistic performance models implemented
in NESSUS include stress, strain, displacement,
vibration, fatigue, fracture, and creep. NESSUS
can perform reliability analyses for multiple components
and failure modes, and identify critical random
variables and failure modes to support structural
design, certification, and
risk assessment.
Version 8.2, the latest, was released to U.S. government
organizations in December 2004 and to the public
in 2005. While the software continues to serve
the aerospace industry, it is also solving a diverse
range of problems in a variety of other industries,
including health and medicine, automotives, biomechanics,
nuclear waste packaging, munitions (weapon systems),
and offshore pipeline construction. To accomplish
this, NESSUS has been interfaced with many well-known
third-party and commercial deterministic analysis
programs.
Recently, NESSUS provided the U.S. Naval Bio-dynamics
Laboratory/U.S. Naval Air Warfare Center Aircraft
Division with a probabilistic methodology
and computational tool for evaluating the risk
of cervical spine injury.
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NESSUS 8.2 was
used to quantify the risk of cervical spine
injury to aviators during ejection. |
Cervical spine injuries occur as a result of impact
or from large inertial forces such as those experienced
by military pilots during ejections, carrier landings,
and emergency crash-landings on water, also known
as “ditchings.” Other examples include motor vehicle,
diving, and athletic-related accidents. Reducing
the likelihood of occupant injury by identifying
and understanding the primary injury mechanisms
and the important factors leading to injury motivates
most research in this area. Therefore, it is of
broad interest to SwRI to design occupant safety
systems that minimize the probability of injury.
To do this, the company says, the designer must
have quantified knowledge of the probability of
injury, due to different impact scenarios, and
also know which model parameters contribute the
most to the injury probability. Although complex
numerical (e.g., finite element) models are becoming
more widely used as a means of augmenting and extending
laboratory testing, most are deterministic in that
they do not quantify the effect of uncertainties
on the computed model responses.
In developing a probabilistic methodology and computational
tool for performing the spinal evaluation, calculations
were generated using the NESSUS probabilistic analysis
program in conjunction with the commercial finite
element program, ABAQUS. The company intends to
use the resulting models for additional applications,
including the study of spinal behavior under normal
and distressed conditions, the design of implants
utilizing novel materials and/or configurations,
analysis of novel instrumentation systems that
may help avoid costly experimentation, and the
design of anthropomorphic test devices or physical
models that replicate human response such that
injury under simulated dynamic conditions can be
replicated.
In helping NASA return to flight, NESSUS 8.2 was
recently used to solve a critical Space Shuttle
problem for the Agency’s Engineering Safety Center,
which was created in the aftermath of the Columbia
accident to serve as an independent technical resource
for NASA managers and employees. This project involved
developing probabilistic and deterministic fracture
mechanics models to quantify the reliability of
a flowliner in a SSME. In another “Return to Flight”
project for Johnson Space Center, the software
is being utilized on a regular basis to assess
the probability of foam-impact damage to the Space
Shuttle’s reinforced carbon-carbon leading edge,
if such were to happen upon ascent.
In July, SwRI won R&D Magazine’s R&D 100
Award, with NESSUS Version 8.2 being crowned as
one of the 100 most significant technological achievements
of the past year. The company has won 29 R&D
100 Awards since 1971.
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