MSC Nastran疲劳分析用户指南

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"mscfatigue疲劳分析" 在工程领域，疲劳分析是一项至关重要的技术，用于预测和评估结构或组件在反复荷载作用下发生疲劳破坏的可能性。本资源是MSC Nastran疲劳分析的原版手册，对于进行精确的疲劳寿命预测具有极高的价值。MSC Nastran是一款强大的有限元分析软件，它集成了疲劳分析功能，能够帮助工程师们深入理解材料和设计的疲劳特性。 MSC Nastran嵌入式疲劳用户指南涵盖了2016版本的详细信息，提供了一套完整的工具集，用于模拟和评估复杂工程结构的疲劳性能。该指南包含的主要内容可能包括以下几点： 1. **疲劳分析基础**：介绍疲劳的基本概念，如S-N曲线、应力集中、疲劳裂纹的形成与扩展等，以及这些因素如何影响结构的疲劳寿命。 2. **数据输入与设置**：详细说明如何在MSC Nastran中设置疲劳分析，包括定义荷载历史、选择疲劳材料属性、设定疲劳模型等。 3. **载荷谱分析**：讲解如何处理实际工况下的非平稳荷载，通过载荷谱来模拟多种不同工况组合对结构的影响。 4. **损伤积累与寿命预测**：介绍如何计算每个元素或节点的疲劳损伤，并根据损伤分布预测整个结构的疲劳寿命。 5. **疲劳裂纹分析**：涵盖裂纹起始、扩展和最终断裂的分析方法，可能包括线性弹性断裂力学（LEFM）和基于塑性变形的裂纹扩展模型。 6. **结果后处理**：指导用户如何解读疲劳分析的结果，包括应力应变分布、损伤分布图、寿命预测图表等，以便进行设计优化。 7. **案例研究**：通过具体的工程实例，展示如何应用MSC Nastran进行疲劳分析，帮助用户理解和掌握软件的实际操作。请注意，尽管此手册提供了丰富的信息和示例，但其目的是教育和说明性的，而非详尽无遗。在实际应用中，工程师需要结合具体工程问题和设计进行调整。此外，由于软件可能会持续更新和改进，因此，使用时应确保参考的是最新版本的文档，以获取最准确的信息。最后，该文档的版权属于MSC Software Corporation，未经许可不得复制或分发。任何未经授权的使用都可能违反版权法。同时， MSC Software Corporation明确声明，对因使用手册中的信息而造成的任何直接或间接损害不承担任何责任。软件中可能还包含由第三方提供的受版权保护的软件，这些软件已获得许可并集成到MSC Nastran中。

MSC NASTRAN Embedded Fatigue User’s Guide

Overview

Main Index

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.

CHAPTER 1

Fundamentals of Fatigue Analysis

Main Index

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.

MSC NASTRAN Embedded Fatigue User’s Guide

The Fatigue Analysis “Five Box Trick”

Main Index

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 is the analysis; and box five contains

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.

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