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Fatigue
Fatigue analysis is intended for predicting life of structures under cyclic loading.
Fatigue Theory
Background information on Fatigue analysis in SimSolid.
Stress-life (SN) Approach
The stress-life method works well in predicting fatigue life when the stress level in the structure falls mostly in the elastic range.

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- Dynamics
  Evaluate your design in real-time using SimSolid dynamics analysis.
- Thermal
  Thermal analysis predicts how thermal effects propagate through the model. Steady state and thermal transient analyses are supported.
- Fatigue
  Fatigue analysis is intended for predicting life of structures under cyclic loading.
  - Fatigue Theory
    Background information on Fatigue analysis in SimSolid.
    - Fatigue Assessment Methods
      In SimSolid, uniaxial and multiaxial fatigue analysis using SN (stress-life) and EN (strain-life) approach is supported.
    - Stress-life (SN) Approach
      The stress-life method works well in predicting fatigue life when the stress level in the structure falls mostly in the elastic range.
    - Strain-life (EN) Approach
      The EN Approach uses plastic-elastic strain results to perform strain-life analysis.
    - Spot Weld Fatigue Analysis
      Allows for the study of fatigue performance of spot welds in structures.
    - Seam Weld Fatigue
      The seam weld fatigue implemented in SimSolid is based on linearization of stress in the conjunction with Volvo method and following that it predicts fatigue damage and life. Linearized stress decomposes a through-thickness elastic stress field into equivalent membrane and bending.
    - Random Response Fatigue Analysis
      The study of fatigue life of structures under random loading.
    - Rainflow Cycle Counting
      Use cycle counting to extract discrete simple "equivalent" constant amplitude cycles from a random loading sequence.
    - Mean Stress Correction
      Use mean stress correction to account for the effect of non-zero mean stresses.
    - Damage Accumulation Model
      Palmgren-Miner's linear damage summation rule is used.
    - Endurance Limit Modifying Factors
      Account for factors that can affect fatigue strength of a structure.
    - Scatter in Fatigue Material Data
      Handle scatter in fatigue test results.
  - Import Loadcase-channel Map
    Import a loadcase-channel map file to import all the channels defined in the file and create appropriate events.
  - Import Channels
    Import channels which define load amplitude changes in time.
  - Review Channels
    Review channels in a group.
  - Create Event
    Create an event consisting of channels and loadcases.
  - Import Sequences
    Import a file to automatically create load case sequences.
  - Create Sequence
    Create a sequence with load cases for the fatigue analysis.
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Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding

Stress-life (SN) Approach

The stress-life method works well in predicting fatigue life when the stress level in the structure falls mostly in the elastic range.

Under such cyclical loading conditions, the structure typically can withstand a large number of loading cycles; this is known as high-cycle fatigue.

When the cyclical strains extend into plastic strain range, the fatigue endurance of the structure typically decreases significantly; this is characterized as low-cycle fatigue, more details about it can be found in the next section.

The generally accepted transition point between high-cycle and low-cycle fatigue is around 10,000 loading cycles.

SN Curve

The SN curve, first developed by Wöhler, defines a relationship between stress and number of cycles to failure.

Typically, the SN curve (and other fatigue properties) of a material is obtained from experiment through fully reversed rotating bending tests. Due to the large amount of scatter that usually accompanies test results, statistical characterization of the data should also be provided (certainty of survival is used to modify the SN curve according to the standard error of the curve and a higher reliability level requires a larger certainty of survival).

Figure 1. SN data from testing

When SN testing data is presented in a log-log plot of alternating nominal stress amplitude S_a versus cycles to failure N, the relationship between S and N can be described by straight line segments. Normally, a one or two segment idealization is used.

Note: Some SN curves use stress range instead of amplitude. In the material database, you can set the material property Stress Definition as Amplitude or Range. Amplitude is used by default.

Figure 2. One segment SN curves in log-log scale

(1)

S = S 1 {(N_{f})}^{b 1}

for segment 1 (1)

Where:

S is the nominal stress amplitude
N_f are the fatigue cycles to failure
b₁ is the first fatigue strength exponent
S1 is the fatigue strength coefficient
Nc1 is cyclic limit of endurance

The SN approach is based on elastic cyclic loading, inferring that the SN curve should be confined, on the life axis, to numbers greater than 1000 cycles. This ensures that no significant plasticity is occurring. This is commonly referred to as high-cycle fatigue.