MSC Nastran 2014疲劳分析官方教程

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"MSC Nastran 2014 Embedded Fatigue User’s Guide 是一个官方教程，详述了如何在Nastran 2014版本中进行疲劳分析，涵盖了该版本的最新功能。该指南旨在教育用户如何利用Nastran进行疲劳寿命评估，但不保证适用于所有工程问题或设计。" 在Nastran 2014中进行疲劳分析，用户可以利用嵌入式疲劳模块来评估结构在循环载荷下的耐久性。疲劳分析是计算结构或组件因反复应力或应变而可能导致失效的过程。以下是一些关键知识点： 1. **疲劳分析基础**：疲劳分析基于S-N曲线（应力-寿命曲线）或应变-寿命曲线，这些曲线描述了材料在不同应力或应变水平下的寿命。通常，高应力对应短寿命，低应力则对应长寿命。 2. **载荷历史**：在Nastran中，用户需要提供结构承受的动态载荷历史，这可以通过时间历程、频谱或随机振动等方式输入。载荷历史分析用于确定导致疲劳损伤的循环次数。 3. **损伤累积模型**：Nastran可能包含多种损伤累积模型，如雨流计数法（Rainflow Counting），它将复杂的载荷历史分解成一系列独立的循环，便于应用到S-N曲线。 4. **疲劳生命预测**：通过组合各个应力或应变循环的影响，Nastran可以计算出总的损伤分数，并据此预测结构的预期寿命。 5. **安全性评估**：疲劳分析还包括对结构的安全性评估，这涉及到设置安全系数或使用疲劳评定线，以确保结构在设计使用寿命内的可靠性。 6. **材料特性**：正确地输入材料的疲劳特性至关重要，包括疲劳强度、延展性和硬化行为。这些数据可以从材料供应商或实验测试中获得。 7. **结果后处理**：Nastran提供了丰富的后处理工具，帮助用户可视化疲劳损伤分布，识别疲劳热点，并进行结果对比和敏感性研究。 8. **优化设计**：基于疲劳分析的结果，工程师可以优化结构设计，减少高应力区域，提高耐久性。 9. **警告与免责声明**：虽然Nastran提供了强大的疲劳分析工具，但用户应理解，任何软件都不能完全替代工程判断和实验验证。在实际应用中，需要结合具体工程背景和设计要求谨慎使用。 10. **知识产权**：最后，需要注意的是，Nastran可能包含第三方软件的版权，使用时需要遵守相关授权条款。这个官方教程会详细解释这些概念，并提供实例帮助用户理解和应用疲劳分析功能，以确保在实际工程问题中准确、有效地使用Nastran 2014的疲劳分析能力。

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MSC NASTRAN Embedded Fatigue User’s Guide

Overview

MSC Nastran embedded fatigue life estimation, when used early in a development design cycle has the

potential to greatly enhance product life as well as reduce testing and prototype costs, thus ensuring

greater speed to market.

The purpose of this manual is to provide you with typical example problems to demonstrate proper usage

of the program. Each example is designed to show certain aspects and help to convey various principles

of fatigue life estimation. The intent is to get you up to speed as quickly as possible without a steep

learning curve or hours sifting through a thick manual.

Assumptions

This guide makes certain assumptions of the reader. The basic assumptions made are, a good knowledge

of basic computer skills and terminology, and a working knowledge of finite element analysis using MSC

Nastran. This manual does not deal with creation of finite element models or any aspects of actual finite

element analyses except where necessary to achieve proper fatigue life estimations.

This manual assumes that the user has little or no experience with fatigue analysis in general and

therefore makes every effort to explain principles of fatigue life estimation from example to example. It

is not meant to be an exhaustive course on fatigue analysis however. For this we refer you to the many

references sited in the

References and Further Reading (App. E).

Organization of Guide

All chapters starting with Chapter 3, serve as tutorials to learn the basics of fatigue analysis using MSC

Nastran.

First read this chapter in its entirety and then familiarize yourself with the Nastran case control and bulk

data input in the next chapter, after which it is highly suggested that you start at the first example and

work your way sequentially. Each exercise introduces concepts that build on each other from exercise to

exercise.

Main Index

CHAPTER 1

Fundamentals of Fatigue Analysis

What is Fatigue?

The first concept to understand before embarking on this tutorial is the definition of the term fatigue

within the confines of this guide. Very often the terms Fatigue, Fracture, and Durability are used

interchangeably. Each does, however, convey a specific meaning.

Although many definitions can be applied to the word, for the purposes of this manual, fatigue is failure

under a repeated or otherwise varying load which never reaches a level sufficient to cause failure in a

single application.

It can also be thought of as the initiation and growth of a crack, or growth from a pre-existing defect,

until it reaches a critical size, such as separation into two or more parts.

Fatigue analysis itself usually refers to one of two methodologies: either the Stress-Life (S-N) or S-N

method, commonly referred to as Total Life since it makes no distinction between initiating or growing

a crack, or the Local Strain or Strain-Life (e-N) method, commonly referred to as the Crack Initiation

method which concerns itself only with the initiation of a crack.

Fracture specifically concerns itself with the growth or propagation of a crack once it has initiated. MSC

Nastran fatigue analysis concerns itself only with the prior two types of fatigue analysis and is not

applicable for crack growth or propagation. For this capability you are referred to MSC Nastran’s

cohesive zone modeling and/or virtual crack closure technique (VCCT) or MSC Fatigue, which uses a

LEFM method for crack growth prediction.

Durability is then the conglomeration of all aspects that affect the life of a product and usually involves

much more than just fatigue and fracture, but also loading conditions, environmental concerns, material

characterizations, and testing simulations, to name a few. A true product durability program in an

organization takes all of these aspects (and more) into consideration.

Note: Throughout this manual terms in blue italics mean there is a definition provided in the

Glossary of Terms (App. D).

Note: Fatigue cracks initiate and grow as a result of cyclic plastic deformation. Without plasticity

there can be no fatigue failure. All attempts are made in this guide to explain how plasticity

is taken into account when determining fatigue life from linear elastic finite element analysis.

Main Index

MSC NASTRAN Embedded Fatigue User’s Guide

The Fatigue Analysis “Five Box Trick”

These fatigue analysis example exercises are constructed around the concept of the fatigue “five-box

trick.” The illustration below depicts this well. For any life analysis whether it be fatigue or fracture there

are always three inputs. The first three boxes are the inputs; box four the analysis; and box five the results.

1. Cyclic Material Information: Materials behave differently when they are subject to cyclic as

opposed to monotonic loading. Monotonic Properties are the result of material tests where the

load is steadily increased until the test coupon breaks. Cyclic Properties are obtained from

material tests where the loading is reversed and cycled until failure at various load levels. These

parameters differ depending on the fatigue analysis type involved.

2. Service Loading Information: The proper specification of the variation of the loading is

extremely important to achieve an accurate fatigue life prediction. The loading can be defined in

various manners. Whether it be time based, frequency based, or in the form of some sort of spectra

depends on the fatigue analysis type to be used. When working with finite element models the

loading can be force, pressure, temperature, displacement, or a number of other types. Loading in

the test world usually refers to the acquisition of a response measurement, usually from a strain

gauge.

3. Geometry Information: Geometry has different meanings depending on whether you are

working from a finite element model or from a test specimen. In the testing world, the geometry

input is the K

(stress concentration factor) since the point of failure is usually away from the

actual point of measurement. Therefore a geometry Stress Concentration Factor, Kt, is defined

to relate the measured response to that at the failure location. You can think of this as a fudge

factor. The corresponding role of finite elements is to produce fields of K

over the entire model,

with the additional complication that these local stress concentration factors are in the form of

stress tensors.

4. Analysis: The correctness and accuracy of each of the above inputs is important in that any error

in any of these will be magnified through the fatigue analysis procedure, the fourth box, since this

process is logarithmic. A ten percent error in loading magnitude could result in a 100% error in

the predicted fatigue life. In a conventional finite element based fatigue analysis, the 4th box often

contains both the stress prediction and fatigue life calculation.

Main Index

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