52 ISE Magazine | www.iise.org/ISEmagazine

Inside IISE Journals

research

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 inﬂator

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

reliability.

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

data.

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

Kong

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 system’s 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 – conﬁrmed 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 conﬁrms 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 efﬁciency. 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 modiﬁable 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-

ﬁned 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

research

A participant completes a military obstacle course while wearing a customized passive exoskeleton.

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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