2011 CAE Associates

Fatigue Analysis in ANSYS

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Fatigue Analysis in ANSYS

There are two general categories of fatigue analysis: Fatigue based on crack formation. Fatigue based on crack growth.

The choice of analysis type is based on the given application. When in the design phase, or for components requiring only a few cycles of

life, crack formation may be sufficient. For highly engineered parts, for components that are manufactured in bulk

such as automotive parts, or for in-service life prediction, crack growth may be required.

Crack Growth Crack Formation

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Methods of Fatigue/Fracture Analysis

Fatigue crack formation analysis. Cracks/flaws are not explicitly considered. Use ANSYS to predict far-field (un-cracked body) conditions. ANSYS results are fed to a fatigue analysis tool.

Basic approach could be a S-N table look-up, performed by hand or via macros. More comprehensive fatigue analysis using nCode.

To be discussed/demonstrated in this presentation.

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Methods of Fatigue/Fracture Analysis

Fatigue crack growth analysis, including the following techniques: Crack is not included in ANSYS analysis, but stress field in crack region is fed

to separate fatigue crack propagation tool, such as nCode. Crack is included in ANSYS model.

Need to continually update mesh to account for crack growth. May or may not know crack path. Various methods used to obtain K, which is then used with a crack growth law to

determine remaining life. Additional energy approaches include virtual crack closure technique (VCCT) and

cohesive zone modeling, typically used in delamination prediction.

To be discussed/demonstrated after lunch.

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Methods of Fatigue/Fracture Analysis

Typical procedures for performing fatigue crack formation analysis:

Simple cyclic loading, R=-1 Model minimum and maximum loadings. Obtain stress range () at critical location. Find cycles to failure using S-N data.

Simple cyclic loading, R-1 Model minimum and maximum loadings. Obtain mean stress and stress range at all

points. Find cycles to failure by:

Obtaining test data for varying R. Plot mean stress vs. alternating stress in

Goodman Diagram.

Complex loading: Perform a comprehensive fatigue analysis

using nCode.

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Goodman Diagram

To include the effect of mean stress, approaches have been developed to take the mean stress into account.

The alternating stress is plotted against the mean stress. The straight line representation is called the Goodman diagram.

The Goodman diagram is typically preferred in engineering design. Actual data can vary around lines by being either concave or convex.

Points below Goodman line are considered safe, points above line are considered failed.

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Goodman Diagram

Sample Goodman Diagram application: Finite element analysis of stent inserted in artery, minimum and maximum

cyclic loading due to blood pressure. Mean and alternating stress at every point in the model is found and plotted in

ANSYS using APDL to create Goodman diagram.

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Goodman Diagram

Sample Goodman diagram: 2D and 3D response, indicating magnitude and location of critical

regions.

Goodman line

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ANSYS nCode DesignLife

ANSYS now offers an advanced set of fatigue analysis capabilities built on nCodes DesignLife technology.

Integrated with the ANSYS Workbench platform. Can run stand-alone using ANSYS RST, DYNA D3PLOT, ABAQUS,

NASTRAN, etc. Provides ease of use and powerful fatigue analysis.

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nCode DesignLife

nCode is the leading developer of fatigue analysis software in the world. DesignLife is the easiest-to-use, most powerful, and most flexible

CAE/FEA fatigue and durability software in the industry. nCode provides their DesignLife Material Library for distribution to all

customers.

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nCode Main Features

Some of the main features of nCode: Low cycle and high cycle fatigue analysis. General loading capability uses rainflow cycle counting to define fatigue

cycles. Damage accumulation based on Miners rule to account for different amplitude

loading. Plasticity corrections. Mean stress corrections. Time-history corrections. Multiaxial fatigue.

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Fatigue Curves

Stress-life curve: Makes no attempt at modeling plasticity. Appropriate for high cycle fatigue.

Strain-life curve: Models material yielding and plasticity. Appropriate for low cycle fatigue.

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Fatigue Loading

Material fatigue curves are created with constant amplitude sinusoidal loading.

Typically with a zero mean stress, i.e. fully-reversed loading. Real loading can be constant amplitude, blocks of different amplitudes, or

variable amplitude, and can have non-zero mean stress.

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Rainflow Cycle Counting

To account for variable amplitude loading in fatigue calculations, must have a method to define fatigue cycles in a varying waveform.

Rainflow cycle counting is a method of finding fatigue cycles in a varying waveform.

Peak valley extraction Reorder to start from absolute max

Imagine the signal is filled with water

Drain water starting at lowest valley, measure total & mean depth drained

Continue by draining next lowest, etc.

Range Mean No. 450 225 1 50 150 1

100 300 2

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Miners Rule of Damage Accumulation

To include the effect of multiple blocks of different amplitude cyclic loading, Miners rule of damage accumulation is used.

Calculates partial damage for each amplitude loading. Failure is predicted when the sum of all partial damage equals one.

Miners rule is very simplistic (for example, the order of loading has no effect),

but it is widely used.

1 damage Partial3

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Elastic-Plastic Corrections

For low-cycle fatigue analyses, where plasticity plays a role, two approaches are available:

Calculate the nonlinear material response and use in fatigue calculations. Use linear material response with a correction to account for material yielding,

typically used for localized regions of yielding in notches.

The Neuber approach is an example of a plasticity correction. Assumes elastic strain energy = plastic strain energy. Can therefore calculate plastic response directly from elastic solution.

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Mean Stress Corrections

Fatigue damage is influenced by the mean stress of each cycle.

Mean stress correction methods allow the effect of mean stress to be modeled and taken into account.

Supported methods: Goodman Gerber Interpolate FKM Goodman Tension Only Gerber Tension Only

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Additional Fatigue Analysis Features

Time-history compression. Time series analysis can be accelerated with various methods. Peak-valley method uses only the load reversals. Limits method uses only the max and min of each load channel.

Addressing multiaxial fatigue. Condense stress tensor down to a single stress, such as a signed Von Mises

or absolute maximum principal value. Critical plane approach finds plane with the most damage.

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nCode Architecture

nCode creates an analysis process using glyphs. Each glyph represents an engineering function that can be used to graphically

create an analysis process. Double-click the glyph header bar (or right-click) to access properties. Common processes are available for use, can create custom processes. Can save the process as a .flo file.

Working folder is searched and available data populates list.

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nCode Fatigue Analysis Steps

Drag in results file from list. Can plot original FE model and results here. Also access and create components for display or analysis purposes.

Define cyclic loading based on combination of FE loads and multipliers.

Define type of fatigue analysis and settings. Also, perform load and material mapping.

Display results such as fatigue life or damage.

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Load and Material Mapping

Must define which cyclic loading to associate with which FE load, and which fatigue material properties go with which component.

Set fatigue analysis properties.

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Types of Fatigue Loading

Supported cyclic loading in nCode: Constant amplitude.

Apply loading in FE analysis. Loading is scaled assuming sine wave.

Time series. Apply loading in FE analysis. Define load time history factors to scale FE loading.

Time step transient dynamic. Apply transient loading in FE analysis. Loading is defined from each time step read in.

Duty cycle. Combination of various loadings.

Vibration frequency domain. Run harmonic FE solution. Define PSD loading in nCode.

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nCode Demonstration

Demonstration problem: Fatigue analysis of a shaft under combined bending and torsion.

Fatigue Analysis in ANSYSFatigue Analysis in ANSYSMethods of Fatigue/Fracture AnalysisMethods of Fatigue/Fracture AnalysisMethods of Fatigue/Fracture AnalysisGoodman DiagramGoodman DiagramGoodman DiagramANSYS nCode DesignLifenCode DesignLifenCode Main FeaturesFatigue CurvesFatigue LoadingRainflow Cycle CountingMiners Rule of Damage AccumulationElastic-Plastic CorrectionsMean Stress CorrectionsAdditional Fatigue Analysis Features nCode ArchitecturenCode Fatigue Analysis StepsLoad and Material MappingTypes of Fatigue LoadingnCode Demonstration