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MSC.Fatigue


MSC.Fatigue / FE-Fatigue
Deepak Deshpande

OUTLINE
? ? ? ? ?
Definition of Fatigue How does Fatigue occur? Basic Methods of Fatigue analysis Inputs for Fatigue analysis Fatigue analysis process

Definition of fatigue
? Fatigue is basically a type of failure…,
? Weakening of metal due to microscopic changes in molecular structures caused by vibration or exposure. ? In materials science, fatigue is a process by which a material is weakened by cyclic loading. The resulting stress may be below the ultimate tensile stress, or even the yield stress of the material, yet still can cause catastrophic failure

History

Expert’s Opinion

Wohler

How does Fatigue occur?
? Occurs when a material experiences
lengthy periods of cyclic or repeated loads ? Fluctuating Loads ? Multi-axial Loading ? Vibrations / Excitations

? The fatigue life of a component is governed
by the loading environment to which it is subject, the distribution of stresses and strains arising from that environment, and the response of the material from which it is manufactured.

? As a result, the major inputs to any fatigue
analysis are component geometry, service loading, and cyclic material properties.

Fatigue – Black Box

Opening the black box….
S-N (Total Life)

Basic Fatigue methodologies

E-N (Crack Initiation)

Crack Growth (LEFM)

Fatigue technology in MSC.Fatigue
Deterministic Load History In-Deterministic Load History

Pseudo-Static Fatigue analysis

Dynamic Fatigue analysis

Vibration Fatigue Analysis

Transient Fatigue Analysis

Modal Super Position Method

Low Cycle Fatigue and High Cycle fatigue

Basic Methods for Fatigue analysis
Several methods may be used, but according to the problem considered: S-N analysis (Total Life Method) Relates nominal or local elastic stress local elastic stress to total life

2. E-N analysis (Crack Initiation Method) Relates local strain local strain to crack initiation life crack initiation life
3. LEFM (Crack Propagation Method)
Relates stress intensity stress intensity to crack propagation rate crack propagation rate

S-N and E-N Fatigue Analysis Process
Load Time Histories

L1
Local Stress Histories

Repeat for each node/element

s1A* L1(t) = sA(t)
L1=1

sA
Fatigue life of Node

Stress for Unit Load Case

S-N Analysis

S-N Analysis (contd..)

S-N Analysis (contd..)

S-N Analysis (contd..)

Rain flow counting

Mean stress Correction

Mean stress Correction…
sRange =100 s

sMean = 50

S

sRange =100

s

100

sMean = 0
5000

s

N

sRange =100

sMean = -50

Mean stress Correction… s
s
sRange =200

sRange =100

sMean = 50

S
sMean = 0

200 100

sRange =100

s
sMean = 0

500

5000 7000

3000

N

sRange =50

s

s
sMean = 0

sRange =100

sMean = -50

Mean stress Correction (contd…)

Palmgreen-Miner’s Linear damage rule

This states that damage can be summed by determining the ratio of the number of cycles experienced to the number of cycles to failure for a given stress range or level and then summing all the ratios for every stress range.

Summary of “variations on a simple approach”
1
Constant amplitude fully reversed cycles in to S-N diagram
Log S

Log N

2
3

Block loading

Palmgren Miner Eg, Goodman

1

Mean stresses

=
Rainflow Cycle Counting

2

1

4

Irregular sequences

3

2

Palmgren Miner

1

5

Notches, etc, etc, etc..

E N Analysis

E N Analysis

E N Analysis (Contd..)

Neuber’s correction

When to use which
S-N (Total Life) ? Long life fatigue problems where there is little plasticity since the S-N method is based on nominal stress ? Components where crack initiation or crack growth modeling is not appropriate, e.g., composites, welds, plastics, and other non-ferrous materials ? Situations where large amounts of pre-existing S-N data exist ? Components which are required by a control body to be designed for fatigue using standard data such as MIL handbook data. ? Spot weld analysis and random vibration induced fatigue problems Crack Initiation ? Mostly defect free, metallic structures or components ? Components where crack initiation is the important failure criterion - safety critical components ? Locating the point(s) where cracks may initiate, and hence the growth of a crack should be considered ? Evaluating the effect of alternative materials and different surface conditions ? Components which are made from metallic, isotropic ductile materials which have symmetric cyclic stress-strain behavior ? Components that experience short lives - low cycle fatigue - where plasticity is dominant

When to use which (Contd..)

Opening the black box….
S-N (Total Life)

Basic Fatigue methodologies

E-N (Crack Initiation)

Crack Growth (LEFM)

Fatigue technology in MSC.Fatigue
Deterministic Load History In-Deterministic Load History

Quasi-Static Fatigue analysis

Dynamic Fatigue analysis

Vibration Fatigue Analysis

Transient Fatigue Analysis

Modal SuperPosition Method

Quasi-Static Fatigue analysis

Quasi-static method (linear superposition)
Load Time Histories

L1

L2

- repeat for each node/element

s1A* L1(t) + s2A* L2(t) + ... = sA(t)
L1=1 L2=1
Stress for Unit Load Cases
Local Stress Histories

sA

Transient Fatigue

Modal Super-Position Method

Modal Super-Position Method

Advantages of Quasi Static Analysis

Advantages of Transient Analysis

Summary - I

Summary - II

GEOMETRY
? ?
In the context of fatigue analysis the term geometry is often used to describe how loads are transformed into stresses and strains at a particular point in a component. The geometry is the function between the externally applied load(s) and the local stress. The effect of geometry may be determined in either one of two ways. 1) by means of an elastic stress concentration factor, Kt 2) by means of finite element analysis. Stress concentration factors are used to calculate local stresses and strains at specific locations from their nominal counterparts or from the applied loading. Stress concentration factors for specific geometries are usually obtained from handbooks, experimental stress analyses, or finite element methods.

?

LOADING ? Loading information can be obtained using a number of different methods.

?

Local or nominal strains can be measured by means of strain gages. Nominal loads can be measured through the use of load cells or, more recently, they can be derived externally by multi body dynamic analysis. It is important to note that, in this context, loading environment is defined as the set of phase-related loading sequences (time histories) that uniquely map the cyclic loads to each external input location on the component.

? MATERIAL
? Another major input to fatigue analysis is a definition of how a material ?
behaves under cyclic loading conditions. Cyclic material properties are used to calculate elastic-plastic stress-strain response and the rate at which fatigue damage accrues due to each fatigue cycle. The material parameters required depend on the analysis methodology being used.

?
?

Characteristics of Fatigue failure
? Failure is essentially Probabilistic. The number of cycles
required for failure varies between homogeneous material samples.

? The greater the applied stress, the shorter the life.
? Damage is cumulative. Materials do not recover when
rested.

? Fatigue life is influenced by a variety of factors, such as
Temperature and surface finish, in complicated ways.

? Some materials, for example Steel and Titanium, exhibit a

fatigue limit, a limit below which repeated stress has no effect. Most others, for example Aluminium, exhibit no such limit and even infinitesimally small stresses will eventually cause failure.

Questions ?

Thank You


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