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User Guide
This manual provides detailed information regarding the features, functionality, and simulation methods available in OptiStruct.
Fatigue Analysis
Fatigue Analysis is intended for solutions to structures in a large number of loading cycles.
Equivalent Stress Amplitude
Overview of Equivalent Stress Amplitude (EQSTSAMP).

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Overview
OptiStruct is a proven, modern structural solver with comprehensive, accurate and scalable solutions for linear and nonlinear analyses across statics and dynamics, vibrations, acoustics, fatigue, heat transfer, and multiphysics disciplines.
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Discover OptiStruct functionality with interactive tutorials.
User Guide
This manual provides detailed information regarding the features, functionality, and simulation methods available in OptiStruct.
- Run OptiStruct
  The execution of OptiStruct is described here.
- Elements
  Elements are a fundamental part of any finite element analysis, since they completely represent (to an acceptable approximation), the geometry and variation in displacement based on the deformation of the structure.
- Materials
  The different material types provided by OptiStruct are: isotropic, orthotropic, and anisotropic materials. The material property definition cards are used to define the properties for each of the materials used in a structural model.
- High Performance Computing
  High Performance Computing leverages computing power, in standalone or cluster form, with highly efficient software, message passing interfaces, memory handling capabilities to allow solutions to improve scalability and minimize run times.
- Solvers
  OptiStruct includes a variety of in-house and third-party solvers for applications in different engineering and technology fields.
- Contact
  Contact is an integral aspect of the analysis and optimization techniques that is utilized to understand, model, predict, and optimize the behavior of physical structures and processes.
- Structural Analysis
  The Structural Analysis section provides an overview of the following analyses.
- Thermal Analysis
  The Thermal Analysis section provides an overview of the following analyses.
- Electrical Analysis
  An electrical analysis involves calculation of electric potential in structures subject to electrical loads.
- Acoustic Analysis
  The Acoustic Analysis section provides an overview of the following analyses.
- Fluid-Structure Interaction
  OptiStruct and AcuSolve are fully-integrated to perform a Direct Coupled Fluid-Structure Interaction (DC-FSI) Analysis based on a partitioned staggered approach.
- Fatigue Analysis
  Fatigue Analysis is intended for solutions to structures in a large number of loading cycles.
  - Rainflow Cycle Counting
    Cycle counting is used to extract discrete simple "equivalent" constant amplitude cycles from a random loading sequence.
  - Uniaxial Fatigue Analysis
    Uniaxial Fatigue Analysis, using S-N (stress-life) and E-N (strain-life) approaches for predicting the life (number of loading cycles) of a structure under cyclical loading may be performed by using OptiStruct.
  - Multiaxial Fatigue Analysis
    Multiaxial Fatigue Analysis, using S-N (stress-life), E-N (strain-life), and Dang Van Criterion (Factor of Safety) approaches for predicting the life (number of loading cycles) of a structure under cyclical loading may be performed by using OptiStruct.
  - Spot Weld Fatigue Analysis
    Allows for the study of fatigue performance of spot welds in structures.
  - Seam Weld Fatigue Analysis
    Seam Weld Fatigue analysis is available to facilitate Fatigue analysis for seam welded structures. It allows you to simulate the Fatigue failure at the seam weld joints to assess the corresponding fatigue failure characteristics like Damage and Life.
  - Random Response Fatigue Analysis
    The study of fatigue life of structures under Random Loading.
  - Sine Sweep Fatigue Analysis
    The study of fatigue life of structures under Sinusoidal Loading.
  - Sine-on-Random Fatigue Analysis
    Sinusoidal vibration superimposed on random vibration and is a standard solution used in military applications.
  - Multiple Sine Tones Fatigue Analysis
    When there is no underlying random vibration but there are a sufficient number of simultaneously occurring sine tones, it can be considered random vibration.
  - Sine-Sweep on Random Fatigue Analysis
    Sine-sweep on random vibration is a superposition of swept sinusoidal vibration on random vibration. It is considered as a series of single sine tones on top of random vibration.
  - Nonlinear Fatigue Analysis
    Small Displacement Nonlinear Static Analysis results can be used to run fatigue analysis.
  - Solder Fatigue Analysis
    Solder fatigue is available to analyze and predict fatigue damage of the solder joint between a component and the base Printed Circuit Board (PCB).
  - Other Factors Affecting Fatigue
  - Surface Damage
    Since fatigue is a surface phenomenon, it is a common practice to assess damage only at the surface of a structure.
  - Stress Gradient Effect
    Stress gradient effect can be taken into consideration through either FKM guideline method or Critical Distance method.
  - Equivalent Stress Amplitude
    Overview of Equivalent Stress Amplitude (EQSTSAMP).
  - Fatigue Analysis Setup
  - Restart for Fatigue Analysis
    Restart for fatigue analysis is available from linear static analysis results, local model results, and random response results.
- Aeroelastic Analysis
  Aeroelastic Analysis is the study of the deflection of flexible aircraft structures under aerodynamic loads, wherein the deformation of aircraft structures in turn affect the airflow.
- Multibody Dynamics Simulation
  A multibody system is defined to be an assembly of sub-systems (bodies, components, or sub-structures).
- Rotor Dynamics
  Rotor Dynamics is the analysis of structures containing rotating components.
- Piezoelectric Analysis
  Piezoelectric materials are a class of materials in which structural deformation triggers electrical potential and vice versa.
- NVH Applications and Techniques
  The NVH Applications and Techniques section provides an overview of the following:
- Powertrain Applications
  OptiStruct provides industry-leading capabilities and solutions for Powertrain applications. This section aims to highlight OptiStruct features for various applications in the Powertrain industry. Each section consists of a short introduction, followed by the typical Objectives in the field for the corresponding analysis type.
- Hi-tech Electronic Applications
  This section provides an overview of the capabilities of OptiStruct for the electronics industry. Example problems pertaining to the electronics industry are covered and common solution sequences (analysis techniques) are demonstrated.
- Aerospace Applications
  Provides an overview of Finite Element Analysis (FEA) for aerospace applications using OptiStruct.
- Analysis Techniques
  The Analysis Techniques section provides an overview of the following:
- Loads and Boundary Conditions
  The following boundary conditions are outlined here.
- Output
  OptiStruct generates output depending on various default settings and options. Additionally, the output variables are available in a variety of output formats, ranging from ASCII (for example, PCH) to binary files (for example, H3D).
- Create Output for Third Party Software
  How to create output from OptiStruct for third party programs.
- Design Optimization
  This section provides an overview of the following optimization types.
- Design Interpretation - OSSmooth
  A semi-automated design interpretation software, facilitating the recovery of a modified geometry resulting from a structural optimization, for further use in the design process and FEA reanalysis.
- Debugging Guide
  Guides in identifying and solving commonly encountered errors during the OptiStruct run.
- Error Message Database
  This section is comprised of error messages in ascending numerical order.
- Warning Message Database
  This section is comprised of warning messages in ascending numerical order.
- Build External Libraries
- Legacy Data
- References
Reference Guide
This manual provides a detailed list and usage information regarding input entries, output entries, and parameters available in OptiStruct.
Example Guide
The OptiStruct Example Guide is a collection of solved examples for various solution sequences and optimization types and provides you with examples of the real-world applications and capabilities of OptiStruct.
Verification Problems
This manual presents solved verification models including NAFEMS problems.
Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding OptiStruct.

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Equivalent Stress Amplitude

Overview of Equivalent Stress Amplitude (EQSTSAMP).

In S-N fatigue analysis, equivalent stress amplitude ( $S_{e q}$ ) is a stress amplitude that corresponds to reported life ( $N_{f}$ ) in an adjusted stress-life curve:

S_{e q} = S R I 1 {(N_{f})}^{b}

In E-N fatigue analysis, equivalent strain amplitude is first obtained from an adjusted strain-life curve:

ε_{e q} = \frac{S_{f}^{'}}{E} {(2 N_{f})}^{b} + ε_{f}^{'} {(2 N_{f})}^{c}

The corresponding elasto-plastic stress amplitude can be obtained by solving the following equation for elasto-plastic stress amplitude ( $σ_{e q}$ ):

ε_{e q} = \frac{σ_{e q}}{E} + {(\frac{σ_{e q}}{K'})}^{\frac{1}{n'}}

Finally, Neuber correction gives the elastic equivalent stress amplitude:

S_{e q} = \sqrt{E σ_{e q} ε_{e q}}

Output Request

EQSTSAMP = SET ID gives the equivalent stress amplitude result in supported format. If multiaxial fatigue is carried out, critical plane angle is also available in the output files. The angle is measured from the local X-axis of a plane where plane stress was calculated.