Some Problems only get worse
It was recognised at the start of the industrial revolution that mechanical components can fail with cyclically repeated loads much lower than their static strength. Early research on mine shaft hoist chains and railway axles led to the notion that metals appear to become 'tired' with repeated use and the concept of metal 'fatigue' was born. The tenacity with which components fail by fatigue has never abated and every new engineering age has spurned a new twist to the problem and a new dimension to the solution. The problem is common to all mechanical engineering disciplines. With the wide-spread adoption of power stations in the early 20th century, materials were required to operate hotter for longer. This brought into focus the phenomena of creep in metals and highlighted a step reduction in component endurance due to interactions between the fatigue and creep mechanisms. Recent changes in the operating rational of the power industry worldwide has led to many power stations operating on two shifts per day rather than as base load generation for which they were designed, further exacerbating the problem.
Total quality management - six sigma
In the automotive industry, increasingly demanding emissions legislation is leading to engine and exhaust components operating hotter. International competitiveness driven by globalisation continues to demand higher performance and higher reliability from automotive components. To meet these challenges, many manufacturers have adopted the six sigma approach to total quality management whereby all aspects of component design and manufacture are examined to reduce failure rates to as near zero as sensibly possible. In practical terms, this translates into less than 3.4 failures per million population which is six standard deviations from the mean.
Modern methods respond to the six sigma challenge
It is clearly more desirable for quality to be designed into a component rather than for failures to be managed away during manufacturing. For a typical automotive component such as an exhaust manifold experiencing thermal and mechanical loads with the possibility of cracking by fatigue damage and creep damage mechanisms, this is a complex problem. Aspects to be considered cover metallurgy, thermodynamics, stress analysis, cycle recognition, damage mechanics and others. Building on the well established finite element method for stress analysis and modern methods for creep-fatigue endurance assessment based on developments in the nuclear industry, post processing software is now available* for easily and accurately determining the expected failure rate for a particular component design.
Rapid prototyping comes of age
"For the first time," says Prof. John Draper of Safe Technology Ltd. "engineers can now design high performance components to achieve six sigma reliability, before any component testing has taken place." Certainly for mechanical components operating hot and cyclically, rapid prototyping has come of age. Very significant shortening of the design phase and consequential cost reductions on component testing are already being achieved by this technology. An excellent correlation is demonstrated between simple materials test data and complex, thermal/mechanical component endurance tests to failure. It is now simply a matter of assigning acceptable levels of creep damage and fatigue damage calculated at the design stage, in order to achieve six sigma quality standards during component service.
*fe-safe/TURBOlifeTM has been jointly developed by Safe Technology Ltd and Serco Assurance.