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Failure analysis of military systems

Trikas Alexandros

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Summary

Although today’s weapon systems are becoming more sophisticated and able to perform complicated functions, they are also more complex, consisted of numerous components and thus, usually more susceptible to failures. In order to avoid potential failures, quality control and assurance activities are being carried out during the stage of design and manufacturing. But, these predictive and preventive measures cannot eliminate the presence of systems’ failures. This thesis addresses the failures of weapon systems that may occur during their life-cycle and deals with the set of techniques and methods that must be systematically carried out by an engineer in order to confirm “HOW and WHY” a system does not perform its intended function. The objective of failure analysis is to identify the causes and the factors of the failure by analyzing failure mechanisms and modes like overstress, fracture, fatigue, corrosion, wear, deformation, etc and in addition to suggest corrective actions.System failures can be induced by component failures or they can occur as a result of complex component and subsystem interactions, without any parts failing. As far as early failures is concerned, they are usually caused due to manufacturing defects, assembly or implementation faults, design errors, etc, whereas the chance and wear-out failures are generally caused by chance factors (overstresses, inadequate use) and due to wear and lack of maintenance respectively. Moreover, a failure can be occurred in the hardware or in the software part of the system. With regard to hardware failures, they can be observed in the mechanical, electrical, electromechanical or electronic parts of it. This thesis focuses on the hardware failures and especially on mechanical failures. In addition, components can be manufactured by metallic or non metallic materials (composites, polymeric) which perform different failure modes. This thesis deals mainly with metallic materials failures, while it also provides a brief discussion of composites’ failure modes and a respective case study. Finding the root causes of systems failures is far more difficult. Several techniques and methods are used in failure analysis investigation to find out the cause of failure, depending on the severity and type of problem. These techniques range from simple optical macroscopic observations (visual and obvious manifestations) to examinations of samples under a microscope or carrying out other mechanical or chemical analysis. This thesis came to the conclusion that each failure is different and it has to be addressed as an unique incident. Only with a systematic failure analysis which deploys all the available techniques and methods (NDT –DT) depending on the type of component failure and the prevailing circumstances, the analyst can identify the causes and the factors that made the system incompetent to function properly. Failure analysis can provide valuable feedback to the product manufacturers and operators for improving design, manufacturing or even maintenance faults that might have been overlooked. The thesis is divided in five chapters, as follows: Chapter 1: “Introduction”: outlines this thesis and provides some historical examples of military systems’ failures as well as a brief look at criteria of failure throughout the years.Chapter 2: “Development of a system”: discusses the concept of “system”, the stages of its life-cycle, the types of the materials that are used in military systems and the respective treatments that are used in order to obtain desirable properties. Chapter 3: “Basic concepts of Engineering Fracture Mechanisms”: provides a basic background of fracture mechanics and failure criteria. Chapter 4: “Failure Analysis of Systems”: examines the failure analysis purpose and procedure, the fundamental causes and factors of failures, the basic failure modes and mechanisms and finally the basic investigation techniques.Chapter 5: “Case Studies of Military Systems’ Failures”: examines and discusses eight (8) examples of real-world failures of interest and their respective failure analysis.Finally, the Epilogue summarizes the thesis conclusion followed by References and some useful appendices, whereas lists of images and matrices included.

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