52 ISE Magazine | www.iise.org/ISEmagazine
Inside IISE Journals
Missiles and airbags: The need to test
reliability is their common denominator
What do missiles and airbags have in common? Missiles are
stored in a dormant environment for decades until there
is a military need, while airbags are installed as a standby
throughout a vehicle’s life cycle unless a collision happens.
Both belong to a special category of one-shot products that
satisfy two unique characteristics – those with a long storage
or standby period and can be used only once.
Failures of these products have happened in real-life in-
cidents. On Feb. 25, 1991, during the Gulf War, an Ameri-
can Patriot Missile battery in Saudi Arabia failed to intercept
an incoming Iraqi Scud missile. Since 2018, there have been
massive recalls related to faulty Takata airbags. The inator
and propellant devices of the airbags improperly deployed
shot-metal fragments into vehicle occupants; as a result, doz-
ens of people have died.
A major issue associated with these products’ operational
use failure is that their reliability can only be verified or deter-
mined upon deployment. As such, engineers need to take ef-
fective actions to ensure their reliability using adequate meth-
odologies for assessing and maintaining them throughout the
long storage/standby period before actual use.
Yao Cheng, an assistant professor from the University of
Hong Kong, and Elsayed A. Elsayed, a distinguished profes-
sor from Rutgers University, interacted with a major defense
company regarding approaches to assess missiles’ status in
terms of their ability to properly function when deployed.
This was the main motivation to investigate generic ap-
proaches to estimate the reliability of such one-shot products.
Cheng and Elsayed developed effective physics-and-statistics
models to characterize the failure modes and proposed ran-
dom inspection approaches in order to ensure products’ reli-
ability throughout the entire storage/standby periods.
In their work “Design of Optimal Sequential Hybrid Test-
ing Plans,” Cheng and Elsayed developed a novel inspection
approach that hybridizes nondestructive test (NDT) and de-
structive test (DT) where in each inspection, an NDT is fol-
lowed by a DT (if needed). By designing such a sequential
inspection plan in a dynamic pattern, they successfully ex-
plored one-shot products’ potential operational-use failures
and optimally assessed products’ storage and operational use
The proposed inspection approach is generic and can be
implemented in industry to accurately assess one-shot prod-
ucts’ operational use reliability with an acceptable amount of
products tested destructively. The optimality and effective-
This month we highlight two articles from IISE Transactions on sequential decision-making. The first article
investigates a special category known as one-shot products that can be used only once but usually have a long
storage or standby period. The most commonly known one-shot products are ammunitions in weaponry systems.
The authors worked with a major defense company and developed a sequential inspection strategy in which both
nondestructive tests and destructive tests are involved but a nondestructive test may be followed by a destructive
test only when deemed necessary. The second paper discusses a decision problem involving running a simulation.
Due to the practical need to make decisions quickly, simulations may not be run in a long stretch of time to
guarantee the reach of its steady state. To safeguard the quality of decisions, the authors devised a sequential
sampling strategy for running the simulation, conducting ranking and selection accordingly, and deciding when the
decision is credible enough so that one can stop the simulation. These articles will appear in the July 2021 issue of
IISE Transactions (Volume 53, No. 7).
Yao Cheng Elsayed A. Elsayed
June 2021 | ISE Magazine 53
ness of the proposed optimal plan have been verified by field
CONTACT: Yao Cheng; yaocheng@hku.hk; + (852) 92966029; Department
of Industrial and Manufacturing Systems, The University of Hong Kong; 8/F,
Haking Wong Building, The University of Hong Kong, Pokfulam Road, Hong
What can we do if the simulation output
does not reach the steady state?
Because of the increasing complexity of real-world systems,
it’s not always possible to derive an analytical expression to
evaluate a systems performance. Discrete event simulation
becomes a widely used tool for evaluating the performance of
stochastic systems since any level of detail of the system can be
modeled via simulation.
When simulation is applied to real-world complex systems,
there are two major challenges. The first is the efficiency of
simulation since the decision-makers tend to favor rapid feed-
back. The second challenge arises with the aspect that many
systems do not reach the steady state when the simulation ter-
minates. Examples of such systems include some healthcare
clinics that have opening and closing times each day and a
berth allocation system for incoming vessels at a large indus-
trial seaport. To broaden the application of simulation, it is
important to both improve simulation efficiency and find a
way to improve its accuracy when the simulation terminates
before it reaches steady state.
After analyzing historical data from a berth allocation sys-
tem in an international port, the authors – professor Hui Xiao
from Southwestern University of Finance and Economics,
professor Loo Hay Lee and graduate student Xiang Hu from
National University of Singapore, professor Douglas Morrice
from the University of Texas at Austin and professor Chun-
Hung Chen from George Mason University – conrmed that
the simulation in use, mimicking the operation of the berth
allocation system, does not reach the steady state, suggesting
that an important assumption for many subsequent analysis is
violated. This problem is made tougher due to the practical
need for quick outcomes from the simulation, foreclosing the
possibility of running the simulation long enough to reach
the steady state.
In their paper “Ranking and Selection for Terminating
Simulation Under Sequential Sampling,” the authors build a
general regression model to estimate the system performance
before it reaches the steady state. To improve the efficiency of
simulation, the authors further derive an asymptotically op-
timal rule for simulation resources allocation. The proposed
model fits the historical data accurately and makes good pre-
diction based on the simulation outputs.
The case study conrms the efficiency of the proposed pro-
cedure relative to extant approaches. The resulting simulation
optimization procedure was applied to the berth allocation for
improving its simulation accuracy and efciency. Its applica-
tion could be extended to other systems in which a system
simulation terminates before it reaches the steady state.
CONTACT: Hui Xiao; msxh@swufe.edu.cn; + 86-28-87092206; Department
of Management Science and Engineering, School of Statistics, Southwestern
University of Finance and Economics, 555 Liutai Avenue, Wenjiang District,
Chengdu 61130, Sichuan Province, China.
Testing the effectiveness of wearing
an exoskeleton during military training
Soldiers carry heavy equipment essential for operational ef-
fectiveness, but doing so also increases the risk of musculo-
skeletal injuries and performance decrements. When work
demands are not modiable during military load carriage,
novel interventions such as passive exoskeletons may reduce
musculoskeletal loading, thereby reducing injury risk.
The Canadian Load Effects Assessment Program (Can-
LEAP), an obstacle course that consists of simulated military
tasks, was used to test the usability of a customized, passive
military exoskeleton. Obstacles within Can-LEAP consist of
a variety of simulated military tasks, including “Hatch and
Tunnel,” “Sprint,” “Stairs and Ladder,” “Agility Run,” “Ca-
sualty Drag,” “Windows,” “Bounding Rushes,” “Balance
Beam,” “Crawls” and “Courtyard Walls.
The aim of the study, “A Pilot Investigation of the Influ-
ence of a Passive Military Exoskeleton on the Performance
of Lab-Simulated Operational Tasks,” was to explore the
This month, we highlight two articles from IISE
Transactions on Occupational Ergonomics and Human
Factors (Volume 8, No. 4). In the first, a team led by
Thomas Karakolis found mixed effects of a customized,
passive exoskeleton on the performance of simulated
military tasks. In the second paper, W. Patrick Neumann
and colleagues assessed predictions of muscle fatigue and
recovery using several existing models, concluding that
these models can give quite different outcomes and should
be used with some caution.
The authors, from left to right, Hui Xiao, Douglas Morrice, Loo
Hay Lee and Chun-Hung Chen. Not pictured: Xiang Hu.
54 ISE Magazine | www.iise.org/ISEmagazine
potential effects of the customized passive exoskeleton on
obstacle completion time and acceptability during Can-
LEAP. The study team included Thomas Karakolis, Kristina
Gruevski, Ian Cameron, Cerys McGuinness and Adrienne
Sy from Defence Research and Development Canada; Ryan
Graham from the University of Ottawa; and Krista Best, Ga-
briel Diamond-Oullette and Laurent Bouyer from Univer-
sité Laval.
Results from this preliminary study suggest that the total
time to complete an operationally relevant military obstacle
course was not improved by wearing the passive exoskel-
eton evaluated. Acceptability ratings of equipment weight
and torso stiffness were similar across conditions, while ac-
ceptability ratings of overall performance during Can-LEAP
were rated lowest while wearing the exoskeleton. The exo-
skeleton increased completion time of obstacles with con-
ned spaces compared to a mass-matched control condition,
whereas carrying and running tasks had similar completion
times across conditions, suggesting that the usability of the
military exoskeleton tested may be task dependent.
CONTACT: Thomas Karakolis; Defence Research and Development Canada
An analysis of modeling data
measuring work-related fatigue
Physical fatigue at work is associated with increases in both
errors and injuries, both critical concerns for industrial en-
gineers. It would be helpful if there were good engineering
models that could help quantify fatigue levels prospectively in
designing a production system. To do this, reliable fatigue and
recovery models are needed that predict when fatigue begins
to accumulate at work.
In pursuing this goal, Ryerson University professor Patrick
Neumann and senior undergraduate student Mufaddal Moti-
wala partnered with Swedish KTH Royal Institute of Tech-
nology researcher Linda Rose to examine the consistency of
available models to predict when operator fatigue would start
to accumulate.
The team considered a standard 60-second cyclic assembly
job. They then systematically changed the muscular workload
level to identify the duty cycle threshold where fatigue accu-
mulation exceeded the ability of simulated workers to recov-
er. Using several existing models, this approach let the team
map the operational envelope in which fatigue is balanced
with recovery and does not
accumulate between cycles.
Their results were recently
published in the paper, “A
Comparison of Work-Rest
Models using a Breakpoint
Analysis Raises Questions.
The authors found that ex-
isting models give very dif-
ferent results. Models based
on the same underlying
A participant completes a military obstacle course while wearing a customized passive exoskeleton.
Photo courtesy of J. Clark (© 2019), Her Majesty the Queen in Right of Canada, Minister of National Defence
Patrick Neumann
June 2021 | ISE Magazine 55
IIISE Transactions (link.iise.org/iisetransactions) is IISE’s flagship
research journal and is published monthly. It aims to foster exchange
among researchers and practitioners in the industrial engineering
community by publishing papers that are grounded in science and
mathematics and motivated by engineering applications.
IISE Transactions on Occupational Ergonomics and Human Factors
(link.iise.org/iisetransactions_ergonomics) is devoted to compiling and
disseminating knowledge on occupational ergonomics and human
factors theory, technology, application and practice across diverse
areas. You can follow on Twitter at twitter.com/iisetoehf or @iisetoehf.
To subscribe, call (800) 494-0460 or (770) 449-0460.
About the journals
methodology tended to predict the same response pattern.
But there were still important differences, which may be re-
lated to the research protocol used to create these models: task
type, muscles investigated, modeling approach and different
participants used in modeling can all affect model outputs.
Though the models gave varying predictions, they can still
be used to evaluate different design scenarios in terms of A
versus B comparisons.
Practitioners seeking more precise estimates of potential
fatigue at a given workload level, though, should be cautious
about which model they choose. The team notes that further
research is needed to create better and more broadly appli-
cable fatigue-recovery simulation tools.
CONTACT: Patrick Neumann, Ryerson University, Toronto, Ontario, Canada
Yu Ding is the Mike and Sugar Barnes Professor of industrial and
systems engineering at Texas A&M University and associate director
for research engagement at the Texas A&M Institute of Data Science.
He is editor-in-chief of IISE Transactions and a fellow of IISE.
Maury Nussbaum is HG Prillaman professor at Virginia Tech in the
Department of Industrial and Systems Engineering, editor-in-chief of
IISE Transactions on Occupational Ergonomics and Human
Factors, and a fellow of IISE.
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Mufaddal Motiwala Linda Rose