Reliability Edge Newsletter

Volume 4, Issue 2

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Physics of Failure for Making High Reliability a Reality

Guest Submission

Thomas Stadterman, Ph.D.
David Mortin, Ph.D.
U.S. Army Materiel Systems Analysis Activity (AMSAA)

The Army's Aberdeen Proving Ground (APG), located in Harford County, Maryland, is home to a diverse array of laboratories and centers that spearhead military research and development, test and evaluation, and science and engineering. In addition to military endeavors, these centers are actively involved with helping solve complex problems in engineering design, tests and analyses for the commercial and industrial sectors.

The U.S. Army Materiel Systems Analysis Activity (AMSAA) is one of these APG centers. AMSAA and a team of leading engineering and analysis organizations continue to expand physics-of-failure technology applications to Department of Defense weapon systems to improve reliability and reduce costs. Physics of failure is an engineering-based approach to reliability that uses modeling and simulation to eliminate failures early in the design process by addressing root-cause failure mechanisms in a Computer-Aided- Engineering environment. This approach is proving essential for meeting Army and Department of Defense (DoD) transformation objectives.

Figure 1: Stryker

Figure 1: Mechanical Simulations International developed the dynamic simulation of the Stryker, which produces the force and acceleration time history of the vehicle traversing terrain. This information will be used in the suspension component fatigue analysis

The physics-of-failure approach proactively incorporates reliability into the design process by establishing a scientific basis for evaluating new materials, structures and electronics technologies. Information to plan tests and screens and to determine electrical and thermo-mechanical stress margins are identified by the approach. Physics of failure encourages innovative, cost-effective design through the use of realistic reliability assessment. Generic failure models are used by physics of failure, which are as effective for new materials and structures as they are for existing designs. This approach involves the following (Cushing et al, 1993):

  • Identifying potential failure mechanisms (chemical, electrical, physical, mechanical, structural or thermal processes leading to failure); failure sites; and failure modes.
     
  • Identifying the appropriate failure models and their input parameters, including those associated with material characteristics, damage properties, relevant geometry at failure sites, manufacturing flaws and defects, and environmental and operating loads.
     
  • Determining the variability for each design parameter when possible.
     
  • Computing the effective reliability function.
     
  • Accepting the design, if the estimated time-dependent reliability function meets or exceeds the required value over the required time period.

A central feature of the physics-of-failure approach is that reliability modeling, which is used for the detailed design of electronic equipment, is based on root-cause failure processes or mechanisms. These failure-mechanism models explicitly address the design parameters which have been found to influence hardware reliability strongly, including material properties, defects and electrical, chemical, thermal and mechanical stresses. The goal is to keep the modeling in a particular application as simple as possible without losing the cause-effect relationships, which benefits corrective action. Research into physical failure mechanisms is subjected to scholarly peer review and published in the open literature. The failure mechanism models are validated through experimentation and replication by multiple researchers (Cushing et al, 1993).

Physics-of-failure analyses have been successfully performed for many DoD systems resulting in improved reliability and millions of dollars in savings. Electronics have been analyzed in numerous systems including radar ground stations, hand-held monitors, helicopters, tracked vehicles, wheeled vehicles, power supplies and missiles. Mechanical structures have been analyzed on Army trailers, floating bridges, dry bridges and wheeled vehicles. Reliability improvements were applied to many of these systems, which resulted in fewer field failures and significant cost benefits.

One of the many examples of physics-of-failure success was for commercial-off-the-shelf (COTS) circuit cards used in a wheeled-vehicle system. The AMSAA analysis showed that the COTS circuit cards could be used in the system without reducing system reliability, which decreased the acquisition costs of the electronics by $12,000 per circuit card and over $1 million during initial production. Another analysis was performed on a tri-service radio, which identified a failure mechanism that would cause the radio to fail repeatedly (Osterman and Stadterman, 1999). Different components were used to remove this failure mechanism. The estimated operating and support cost avoidance was $27 million due to elimination of transportation, repair and spare parts associated with the failures. AMSAA is currently performing analysis on electronics used in missile, helicopter and tracked-vehicle systems.

Figure 2: University of Maryland CALCE software

Figure 2: University of Maryland CALCE software was used to calculate the displacement of circuit cards during vibration. This information was used to estimate component life

In addition to electronics, the tools have been applied to Army bridging systems. AMSAA has served as the Accreditation Agent for physical simulation undertaken to address bridge durability. In this capacity, AMSAA monitored the physical simulation and validation activities, reviewed the data, and performed independent physics-of-failure analyses to quantify the levels of fatigue imposed in the bridge structures from both the real world and simulated events. For a floating bridge, results of the analysis were used to avoid a lengthy retest and to support an Urgent Materiel Release for fielding to Iraq. Estimated savings from this analysis was $2 million.

An Army team including representatives from the Universities of Iowa and Tennessee performed a comprehensive physics-of-failure analysis on an Army trailer experiencing fatigue failures (Stadterman et al, 2003). The analysis represented the first documented use of an integrated process of testing for model inputs, dynamic simulation to produce loads, extensive validation testing and fatigue analysis. Results from this analysis indicated that the trailer drawbar would experience fatigue failures after use on cross-country terrain. Using University of Iowa software, an optimized design based on weight and fatigue life was developed for the drawbar. If this analysis had been performed during design, millions of dollars of redesign and test cost could have been avoided.

Figure 3: Trailer

Figure 3: The solid model of the trailer was used to develop the dynamic simulation model and the finite element model used in the physics-of-failure analysis

Physics of failure continues to be an award-winning initiative that will allow the DoD to reduce logistics footprint and produce military platforms with the high levels of reliability that our soldiers deserve. Additional information is available on the Web at https://stb.apg.army.mil/BDO/.

References
Cushing, M., D. Mortin, T. Stadterman and A. Malhotra (1993). "Comparison of Electronics-
     Reliability Assessment Approaches," IEEE Transactions on Reliability, Vol. 42, No. 1,
     December.

Stadterman, T., W. Connon, K. Choi, J. Freeman and A. Peltz (2003). "Dynamic Modeling and
     Durability Analyses from the Ground Up," Journal of the IEST, Vol. 46.

Osterman, M. and T. Stadterman (1999). "Failure Assessment Software for Circuit Card
     Assemblies," Proceedings for the Annual Reliability and Maintainability Symposium.

End Article

 

About the Authors
Thomas Stadterman is the Physics-of-Failure Development Team Leader for the U.S. Army Materiel Systems Analysis Activity. He conducts and manages research on applying physics-of-failure and prognostic concepts to military systems. He has a Ph.D. and M.S. in Reliability Engineering from the University of Maryland, an M.S. in Administration from Central Michigan University and a B.S. in Electrical Engineering from the University of Pittsburgh.

Dr. David E. Mortin is Chief of the Reliability Branch at the U.S. Army Materiel Systems Analysis Activity. He has a Ph.D. in Reliability Engineering from the University of Maryland, an M.S. in Statistics from the University of Delaware and a B.S. in Aerospace Engineering from the State University of New York at Buffalo.

The authors can be reached by e-mail at stad@amsaa.army.mil.