A systematic approach to failure mode characterisation in a complex sociotechnical context

  • Peter J Bonnington

Student thesis: Doctoral ThesisEngineering Doctorate (EngD)


Product Validation is an integral element of the New Product Development process: through identifying and correcting product failure modes, and demonstrating product reliability in line with customers’ requirements, it drives many of the design and development activities of a business. If a products usage or failure mechanisms aren’t fully characterised, the effectiveness of the Product
Validation process is reduced. Failure Mode Characterisation is dependent on the knowledge held within a business and its capability to create and combine new knowledge in a timely, effective, and resource-sensitive manner, influencing the performance on the New Product Development process.

In the context of high-pressure diesel fuel injection systems, this thesis presents a case study of failure mode characterisation of seat wear of a hydraulic control valve, demonstrated to be a complex sociotechnical problem, which previously represented a source of partial uncertainty. This thesis asks whether a suitable method can be identified to characterise the usage variables that influence the
seat wear, employing a multi-methodological approach. Expert Elicitation through the Delphi Method, and an expert panel with a combined 140 years of product knowledge, was used to capture and combine the existing knowledge of the failure mode. The resultant failure mode definition was then codified in a Causal Loop Diagram, with a total of 55 nodes, representing the complexity of the problem. Using Experimental Design methodology, a total of 60 samples were then tested for 1000 hours, identifying a usage variable as having a significant location effect on the failure mode, and demonstrating an injector performance metric that represents a suitable inference of degradation
over time. Finally, regression modelling was used to generate a generalised model of the failure mode that can be used to predict the response over time for given usage conditions. This thesis concludes that the system operating pressure of the fuel system, that can vary with application, has a significant
location effect on the wear of the hydraulic control valve, and can be inferred through injector performance degradation.

The main contribution to knowledge of this thesis is the characterisation of diesel fuel injector control valve seat wear with respect to the products usage, enabling accelerated tests to be designed, and robust designs solutions to be validated. In addition, a system model is presented that captures the
variables associated with the design, manufacture, and usage of the fuel injector that influence the failure mode, ranked by perceived significance by the expert panel. Furthermore, the method utilised is presented as a generalised method for Failure Mode Characterisation in complex sociotechnical contexts. The method proposed represents a simple and useful model for practitioners, with emphasis on purposeful planning, knowledge creation and combination, reflection, and iteration.
Date of Award25 Jun 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorChris A Mcmahon (Supervisor) & Stuart C Burgess (Supervisor)

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