Finite element analysis is used for stress analysis, but this does not answer the most important questions - How long will the component last in service? What design changes are needed to provide optimum durability? How does elevated temperature service reduce durability? What is the cause of failure – creep, fatigue, or creep-fatigue interaction?

fe-safe/TURBOlife™ provides the answers. It is a powerful, comprehensive and easy-to-use suite of creep-fatigue analysis software for finite element models. It has direct interfaces to leading suites of FEA software.

fe-safe/TURBOlife™ calculates

• Where fatigue cracks will occur
• When fatigue cracks will occur
• How creep mechanisms will influence fatigue life
• The factors of safety on working stresses - for rapid optimisation

The results are presented as contour plots of creep-fatigue lives, and stress safety factors for a specified design life. These can be plotted using standard FEA viewers or other leading graphics software.

fe-safe/TURBOlife™ creep-fatigue algorithms have been successfully applied to nuclear power plant components, power station boilers, gas turbine blades, steam turbine components, automotive exhaust components and turbocharger impellers.

fe-safe/TURBOlife™ calculates the endurance of components in high temperature environments where fatigue damage mechanisms and creep damage mechanisms interact to significantly reduce component life.

fe-safe/TURBOlife™ works from elastic finite element analysis and easily available materials data to construct complex, stress-strain hysteresis loops including stress relaxation due to creep. In this way, component specific operating histories and the order of cycling are comprehensively accounted for.

fe-safe/TURBOlife™ identifies whether fatigue and/or creep are the dominant damaging mechanisms, thus enabling re-design to focus on the relevant damage mechanisms and significantly reduce pre-service component testing.

fe-safe/TURBOlife™ provides comprehensive graphical output in terms of life contours and stress safety factors enabling critical areas to be easily identified.

fe-safe/TURBOlife™ includes comprehensive on-line help and information on materials data preparation.

Component Loading

time histories of stresses and temperatures calculated in an elastic FE analysis

Stress-strain modelling

• transition from monotonic to cyclic behaviour
• multiaxial plastic relaxation rules to estimate the actual stress and strain from the elastic stress
• creep materials data enhanced using the Larson-Miller parameter for interpolation and logarithmic extrapolation

Damage Calculation

• thermo-mechanical fatigue damage, creep damage, and fatigue-creep interaction
• damage for each individual cycle, including damage for uncompleted cycles
• advanced treatment of multiaxial stresses
• creep damage for each time increment is calculated from the creep strain increment and the creep ductility as a function of creep strain rate

Output

• fatigue life and creep life contour plots for the component
• creep damage and fatigue damage for the entire loading history is cross-plotted as a damage trajectory on the creep-fatigue damage envelope to indicate creep-fatigue interaction effects
• stress ranges and strain ranges of complete cycles and part-cycles for selected locations
• cumulative creep damage, cumulative fatigue damage, cumulative creep strain, instantaneous stress, instantaneous strain, instantaneous creep strain rate, plotted as a function of time for selected locations

fe-safe/TURBOlife™ creep-fatigue methodology is based on ductility exhaustion concepts which have been developed over the past 25 years in the United Kingdom. These methods are used extensively throughout the nuclear and fossil fuel power generation industries. fe-safe/TURBOlife™ algorithms have been specifically developed for flexibility in their application. They are also used for continuous monitoring applications on power station boilers and gas turbines. fe-safe/TURBOlife™ is supported by highly experienced assessment engineers and extensive materials testing laboratories at Serco Assurance, Risley, UK. fe-safe/TURBOlife™ is easy to learn and easy to use. Designs can be optimised rapidly, development times shortened, material costs reduced and the final design verified on the computer, giving confidence that it will pass test schedules as right-first-time.

• Assessment point
• Materials Data
• Materials Data Processing
• Plastic Relaxation
• Creep Relaxation
• Cycle Recognition
• Damage Calculation
• Technical Support
• Signal Processing & Analysis
• Out put

Assessment point

• calculations are performed for a single assessment point on a component or for the whole model

• for a range of temperatures: Young’s modulus, Poisson’s ratio, monotonic stress-strain curve and cyclic stress-strain curve
• the damage required to progress fully from the monotonic stress-strain curve to the cyclic stress-strain curve is specified as a percentage of damage
• for a range of temperatures: creep deformation behaviour in terms of creep strain, time and temperature
• for a range of temperatures: the fatigue endurance in terms of strain-life
• for a range of temperatures: creep rupture ductility versus creep strain rate including the two values of the upper and lower creep ductility plateaus
• the creep-fatigue damage envelope in terms of the creep damage: fatigue damage coordinates which define the onset of cracking

Materials Data Processing

• Young’s modulus, Poisson’s ratio, monotonic stress-strain curve, cyclic stress-strain curve and creep ductility are linearly interpolated between data sets supplied at different temperatures to obtain the data for the assessment temperature
• creep data is enhanced to cover the problem range using the Larson-Miller parameter for interpolation and logarithmic extrapolation

Plastic Relaxation

• the monotonic and cyclic stress-plastic strain data are fitted to equations of the Ramberg-Osgood form and these equations are used for plasticity calculations
• plastic relaxation rules (Neuber, Glinka or –E’) are employed to estimate the actual stress and strain from the elastic stress and strain

Creep Relaxation

• the creep deformation data is fitted to polynomial equations and these equations used for the creep calculations
• forward creep, creep relaxation or any creep behaviour between these two extremes is accommodated through the use of the elastic follow-up factor Z, the value of which is specified by the user
• the creep strain and creep strain rate for all time increments are calculated by the software

Cycle Recognition

• from the elastic stress history and the nominated plastic relaxation rule, the elastic-plastic stress history is derived
• creep strain effects are added to the elastic-plastic stress history to produce the stress and total strain histories
• the individual stress-strain hysteresis loops are identified as fatigue cycles
• complete cycles and unmatched half cycles are individually identified
• cycle identification is performed as the stress and total strain histories are derived. No post processing of the histories is required

Damage Calculation

• the fatigue damage is calculated for each individual cycle on the basis of the total strain range for that cycle
• fatigue damage for unmatched half cycles is calculated as half the damage of the equivalent complete cycle
• the creep ductility data is fitted to polynomial equations- these equations are used in the creep damage calculations
• creep damage for each time increment is calculated as the creep strain increment divided by the creep ductility as a function of creep strain rate

Technical Support

• extensive materials data base covering ferritic steels, stainless steels, chromium steels, nickel based alloys, aluminum alloys, cast iron
• materials testing facility from cryogenic temperatures to elevated temperatures including creep
• component testing facility for mechanical and thermo-mechanical test cycles
• corrosion testing facility
• advice on creep and creep fatigue design covering endurance and material selection issues
• advice on corrosion and environmental issues
• advice on fracture mechanics and the consequences of cracking

Signal Processing & Analysis

fe-safe/TURBOlifeTM also includes all the standard fe-safeTM features for conventional fatigue analysis, including:

• advanced multiaxial fatigue analysis with automatic algorithm selection
• analysis of complex assemblies in a single operation
• analysis of complex multi-directional loading
• probability of failure and warranty claim curves
• stress-based factors of strength for a specified design life
• comprehensive signal processing for measured loading histories
• easy-to-use WindowsTM-style interface
• macro recording and batch processing

Output

• fatigue lives at each node or element (3D contour plot)
• stress safety factor at each node or element to achieve the design life (3D contour plot)
• probability of failure or survival at specified lives (3D contour plot)
• load sensitivity shows the effect of each load history on the total fatigue damage
• stress histories, cycle histograms, biaxiality, critical plane orientation, Haigh diagrams, fatigue reliability factors, etc, for the whole model or selected elements
• a list of the most damaged elements is saved - re-analysis can concentrate on these elements if required
• a text file of user inputs, analysis type and a results summary is produced for QA trace-back

Turbine Blade

This gas turbine blade has complex internal cooling channels in order to limit the maximum operating temperature. In the past, very conservative design rules were used to assess the component. Finite element analysis of the blade followed by a durability assessment using fe-safe/TURBOlifeTM demonstrated that by analysing actual operating cycles, very realistic life predictions can be obtained. This allows design optimsation and life extension.

compressor wheel

Aluminium alloy compressor wheels can fail by casing rubbing due to creep dilation at elevated temperature, or by cracking due to the accumulation of cyclic creep-fatigue damage. Tests to destruction of compressor wheels under variable load creep conditions were performed to determine the ultimate failure mechanism. Analysis of the test conditions using fe-safe/TURBOlifeTM established that the probability of creep-fatigue failure was very low, and that the calculated creep dilation correlated well with measurements. It was concluded that creep dilation rather than creep-fatigue cycling was the cause of failure. This allowed the designer to focus on the most effective method of extending the life of the compressor wheel.