Analysis software offers route to six sigma

t 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 mineshaft 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 widespread 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.

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 fewer than 3.4 failures per million population which is six standard deviations from the mean.

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, postprocessing software is now available for easily and accurately determining the expected failure rate for a particular component design.

"For the first time", says Prof John Draper of Safe Technology, "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/Turbolife is a new product resulting from the incorporation of Serco Assurance's Turbolife creep fatigue damage software into fe-safe, Safe Technology's leading edge durability analysis software for FE models.

fe-safe/Turbolife can be used for new design or life extension analyses.

Residual life analyses play an essential part in the maintenance and development of components in a number of industries.

Recent changes in the operating rationale of power stations from base load to demand following means that gas turbines and boilers are now operating beyond their design basis.

Increasingly demanding emission legislation in the automotive industry means that engine components are operating hotter.

fe-safe/Turbolife provides a unique and validated tool to meet these technical challenges.

Turbolife assesses creep and fatigue damage individually and the known effects of their interaction on component durability.

fe-safe/Turbolife is able to differentiate between crack initiation hot-spots by the mechanisms of creep, fatigue, or creep fatigue interaction and produce creep-fatigue contour maps for component design and failure analysis.

Turbolife has already been used in the power generation, gas turbine and automotive industries.