In engineering design, safety and durability cannot be guaranteed by static strength calculations alone. Most real-world structures — aircraft wings, automotive chassis, pipelines, bridges, gears, or welded constructions — operate under repeated loading conditions. This is where fatigue analysis becomes critical.
At Fetech İleri Mühendislik, with advanced fatigue solutions powered by Ansys, it is possible to predict product life accurately, reduce prototype testing costs, and increase reliability.
Fatigue is the process of material damage and eventual fracture due to cyclic stresses or strains over time. A design that is perfectly safe under a single static load may fail after millions of loading cycles.
Fatigue damage typically occurs in three stages:
Crack initiation: Starts at surface imperfections, weld toes, or geometric notches with stress concentrations.
Crack propagation: The crack grows with each cycle, reducing the load-bearing cross-section.
Final fracture: The remaining section cannot carry the load, resulting in sudden failure.
Fatigue analysis can be performed using three main approaches, depending on the material behavior and loading type.
Suitable for High-Cycle Fatigue (HCF).
Assumes elastic material behavior.
Uses experimental Wöhler curves.
Mean stress effects are corrected with Goodman, Gerber, or Walker methods.
Suitable for Low-Cycle Fatigue (LCF).
Applied when plasticity is significant.
Based on Manson–Coffin–Basquin and Ramberg–Osgood relationships.
Elastic-plastic corrections such as Neuber or Glinka are applied to local stresses.
Predicts the propagation of existing flaws or cracks.
Paris–Erdogan law: da/dN = C·(ΔK)^m
More advanced NASGRO model accounts for crack closure and ΔK_th thresholds.
Widely used in aerospace and defense applications for critical parts.
Accurate load definition is the cornerstone of reliable fatigue analysis.
Time Domain Analysis:
Stress/strain time histories are obtained.
Cycles are extracted using the Rainflow algorithm.
Damage accumulation is calculated using the Miner rule.
Frequency Domain (PSD) Analysis:
Applied for random vibration loads (e.g., automotive, railway, aerospace).
Fatigue damage is estimated from spectral moments.
The Dirlik method is the most commonly used approach.
Reliable fatigue analysis requires precise material characterization.
S-N curves: Experimentally obtained for different load ratios (R-values).
ε-N parameters: K′, n′, σ′f, ε′f, b, and c coefficients.
Mean stress correction models:
Goodman (linear)
Gerber (parabolic)
Morrow & Smith-Watson-Topper (SWT) (effective for LCF conditions)
ASTM E606 / ISO 12106 → Low-cycle fatigue testing
ASTM E1049 → Rainflow counting method
ISO 12111 / ASTM E2368 → Thermo-mechanical fatigue
EN 1993-1-9 (Eurocode 3) and IIW guidelines → Fatigue design of welded joints
Fatigue behavior in welded structures is different due to geometric discontinuities and residual stresses. Evaluation methods include:
Nominal stress approach
Hot-spot stress method (extrapolation near the weld toe)
Structural stress method
Fatigue life prediction is carried out according to IIW recommendations and Eurocode 3 standards.
Real components rarely experience uniaxial loads. For this reason, critical plane methods are applied:
Findley: Considers shear stress plus normal stress on the critical plane.
SWT (Smith-Watson-Topper): σ_max·ε_a parameter.
Brown-Miller / Fatemi-Socie: Effective for shear-dominated low-cycle fatigue.
Dang Van: Micro-scale shear and hydrostatic stress; well-suited for HCF.
Ansys Mechanical Fatigue Tool and Ansys nCode DesignLife provide comprehensive solutions for fatigue assessment.
S-N and ε-N approaches
Mean stress correction methods
Time-domain and PSD-based fatigue analysis
Critical plane evaluation
Extensive material database
Welded joint fatigue analysis according to IIW and Eurocode standards
Multiaxial fatigue criteria
Crack growth and remaining life predictions
Calibration with experimental test data
Using only peak element stresses without structural evaluation
Selecting inappropriate mean stress correction models
Ignoring plasticity effects in low-cycle fatigue
Evaluating welded joints with local notch stresses instead of hot-spot stresses
Neglecting environmental effects such as surface finish, temperature, or corrosion
Fatigue analysis is not just a calculation step — it is a critical engineering process directly affecting safety, cost efficiency, and reliability.
With the advanced fatigue solutions of Fetech İleri Mühendislik powered by Ansys, you can perform:
Uniaxial and multiaxial fatigue assessments
Time-domain and PSD-based vibration fatigue analyses
Welded joint evaluations based on IIW and Eurocode standards
Crack growth simulations and remaining life predictions
all in a standard-compliant, accurate, and efficient way.