Although, over the years, impressive progress has been made in maintaining engineering equipment in the eld environment, maintenance of equipment or systems is still a challenging issue due to factors such as cost, complexity, size, and competition. Each year billions of dollars are spent on engineering equipment maintenance around the globe. For example, as per Ref. [1], each year in the United States alone, over $300 billion is spent on plant maintenance by the U.S. industrial sector and about 80% of this amount is spent to rectify the chronic failure of systems, machines, and people.