Disc Low-Cycle Fatigue
Low-cycle fatigue of turbomachinery discs from start-stop centrifugal and thermal strain ranges, including bore and slot hotspots, strain-life, plasticity, mission usage and verification.
Disc LCF mechanism
Turbomachinery discs experience a large cyclic change in centrifugal stress between stopped and operating conditions. Thermal gradients add further cyclic strain, especially at the bore, web transitions and blade slots. The number of start-stop cycles may be modest compared with HCF vibration, but each cycle can have sufficiently large strain to drive low-cycle fatigue. The assessment therefore focuses on strain range and mission count rather than one maximum steady stress.
Mission-cycle definition
Define speed and temperature histories for start-up, steady operation, shutdown and any intermediate duty changes. Different mission types can produce different bore and rim ranges. Proof spins and overspeed tests may also consume life. The LCF calculation should use the actual or design-basis event count over service life and should preserve the timing between centrifugal and thermal load.
Critical locations
The bore is often critical because of high hoop stress and assembly effects, but web fillets, bolt holes, cooling holes and blade-slot roots can govern. Local geometry must be represented with real radii where notch strain is calculated. A nominal axisymmetric model can identify the global hot region; local submodels then resolve the geometric concentration without making the entire rotor model unnecessarily fine.
Strain-life methods
When local cyclic strain includes appreciable plasticity, strain-life methods are generally more appropriate than high-cycle S-N methods. The analysis requires the local elastic-plastic strain amplitude, mean stress treatment and temperature-appropriate material fatigue data. If a purely elastic FE solution is used with a notch correction, the correction method should be validated for the stress state and geometry.
Assembly mean stress
Interference fits, bolt preload and residual stress can shift the mean stress on which the centrifugal cycle operates. The LCF cycle may therefore not run from zero stress to the hot operating value. Establish the assembled cold state first where these effects are significant. Mean-stress treatment should be consistent with the fatigue method and should not double-count conservatism already built into design curves.
Thermo-mechanical cycling
Thermal stress can reverse during startup and shutdown and may be partly self-limiting through local plasticity. The governing strain range should be extracted from the actual time history at each hotspot. Independent peak thermal and centrifugal stresses should not be added as if simultaneous. Nonlinear cyclic analysis can be useful where local yielding is substantial or where shakedown behaviour is uncertain.
Usage accumulation
Calculate damage for each defined mission or event family and accumulate according to the approved fatigue rule. Keep cycle counts traceable to service requirements. A rare severe overspeed event can consume disproportionate LCF usage even when most operation is benign. Report the contribution of each event so future duty changes can be assessed without rebuilding the full life calculation.
Verification
Check centrifugal stress range against speed-squared scaling, thermal strain against free-expansion estimates and local strain convergence against mesh refinement. Compare predicted critical locations with spin-test strain gauges or teardown evidence where available. Report material curve, temperature, surface condition, mean-stress method and design factors clearly.
Disc LCF is governed by the cyclic path through speed and temperature. A single maximum-stress contour cannot define the fatigue range.
Surface condition and bore treatment
Disc LCF capability depends on surface finish, machining marks, broached slot quality, peening and protective treatments. The FE model usually predicts structural strain, while the fatigue methodology must account for the controlled surface condition and process. A design change that preserves nominal stress but changes manufacturing route can therefore alter life. Critical fillets and bores should have drawing and inspection requirements consistent with the fatigue basis.
Mission changes and life tracking
An uprate, revised start schedule or additional overspeed testing can change consumed disc life even if maximum steady stress remains unchanged. Maintain a mission-based usage ledger so the effect of extra cycles can be assessed directly. Reporting damage by event type makes future life updates transparent and avoids rerunning the entire assessment merely to understand how a changed duty profile affects remaining margin.
Engineering judgement — governing sensitivities
For Disc Low-Cycle Fatigue, the most useful review question is not simply whether the solver has produced a plausible contour or scalar result, but whether the model preserves the interaction between temperature, mean stress, cyclic amplitude, dwell time and material state. Life cannot be separated from the thermo-mechanical load history because stress relaxation, cyclic plasticity, creep strain and environmental degradation can redistribute the local field. This is where apparently small modelling choices can change the engineering conclusion. The analyst should identify the variables that can move the governing response, separate physical uncertainty from deliberate conservatism, and show that the selected modelling fidelity is proportionate to the decision being supported. Where the response is close to an acceptance boundary, sensitivity cases should bracket credible changes rather than apply arbitrary percentage perturbations.
Verification evidence for the engineering record
A defensible Disc Low-Cycle Fatigue assessment should leave an evidence trail that another engineer can independently interrogate. At minimum, review temperature-dependent material data, cycle definition, mean/alternating stress extraction, dwell representation, local notch treatment, life-model validity and sensitivity to mission mix or operating-hour assumptions. Numerical convergence should be demonstrated on the response quantity that drives the decision, not only on generic mesh or solver metrics. The report should distinguish verified numerical behaviour from validation against test or service evidence, record any extrapolation beyond the supporting data, and state which assumption would most likely change the conclusion. This turns the analysis from a plausible calculation into an auditable engineering substantiation.