Fatigue calculations in ANSYS Workbench. Martin Eerme

Transcription

1 Fatigue calculations in ANSYS Workbench Martin Eerme

2 What is fatigue? In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading (material is stressed repeatedly). Clients tous différents Routes de qualités variables Fatigue Design in Automotive Industry PSA (Peugeot Citroën) Contraintes Conception fiable Résistances 3s 3s Dispersion matériau Dispersion de production

3 Fatigue Fracture mechanics can be divided into three stages: 1C 1. Crack nucleation 2. Crack-growth 3. Ultimate ductile failure

4 Methods for Fatigue Life calculation Physical testing is clearly impractical for every product design. In most cases, designers need the ability to predict component fatigue life accounting for anticipated service loads and materials. The fracture portion of the fatigue process essentially occurs instantaneously, so the total component life can be defined as: Total life = life to initiation i i i + life taken to propagate crack to failure CAE programs uses 3 main methods to estimate fatigue effect: The stress life approach (SN) based on stresses and uses only Wöhler s methods. Simplest to implement and gives good results in case of high-cycle fatigue. Not useful for components with plastic effects. Not exact in case of low-cycle fatigue. Strain life (EN) Strain Life typically deals with a relatively low number of cycles and therefore addresses Low Cycle Fatigue (LCF), but works with high numbers of cycles as well.. Linear Elastic Fracture Mechanics (LEFM) Fracture Mechanics starts with an assumed flaw of known size and determines the crack s growth as is therefore sometimes referred to as Crack Life. Facture Mechanics is widely used to determine inspection intervals. For a given inspection technique, the smallest detectable flaw size is know.

5 Loading Constant Amplitude, Proportional Loading Non-Constant Amplitude, Proportional Loading Constant Amplitude, Non-Proportional Loading Non-Constant Amplitude, Non-Proportional Loading

6 Constant Amplitude, Proportional Loading Principal stress axes do not change over time Only 1 set of finite element stress results is needed

7 Non-Constant Amplitude, Proportional Loading Principal stress axes do not change over time Only 1 set of finite element stress results is needed Think of this as coupling an FEM analysis with strain-gauge results collected over a given time interval

8 Constant Amplitude, Non-Proportional Loading principal stress axes are free to change between these 2 load sets there are exactly 2 load cases that need not be related by a scale factor Since the loading is of constant amplitude, no cycle counting needs done Mean Stress Correction

9 Non-Constant Amplitude, Non-Proportional Loading Most general case, multiple (>2) load cases are involved that have no relation to one another ANSYS program does not support this type of fatigue loading.

10 Gerber theory, Mean Stress Correction Goodman theory, Soderberg theory. Use static material properties (yield stress, tensile strength) along with S-N data to account for any mean stress. In general, most experimental data fall between the Goodman and Gerber theories, the Soderberg theory usually being over conservative, The Goodman theory can be a good choice for brittle materials, The Gerber theory is usually a good choice for ductile materials.

11 Mean Stress Correction Goodman and Soderberg do not applies correction to negative mean stresses. Negative mean stresses (compression) lower the crack propagation. Gerber deals with positive and negative mean stresses in the same way

12 Fatigue Strenght Factor K f This is the fatigue strength reduction factor. The stress-life or strain-life curve(s) are adjusted by this factor when the fatigue analysis is run. This setting is used to account for a "real world" environment that may be harsher than a rigidlycontrolled laboratory environment in which the data was collected. Common fatigue strength reduction factors to account for such things as surface finish can be found in design handbooks

13 Stress biaxiality indicator Principal stress 1 2 axial stress state 0 1 axial stress state -1 pure shear stress The most dangerous places are areas with pure shear stress

14 Fatigue Workshop 1

15 Boundary conditions Rdildi Radial direction fix

16 Case1 Add Fatigue tool Choose fully reversed loading. Define stress-life fatigue analysis. Choose K f =0,8 Analyse safety factor Find Stress sensitivity when force is changing from 50% until 200% from nominal. Find multiaxial stress state ( biaxiality indicator )