Finite Element Based Fatigue Analysis

7/28/2019 Finite Element Based Fatigue Analysis

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Paper to be presented at Americas User Conference, Oct 5-9,

Sheraton Universal Hotel, Universal City, California

Title: Finite Element Based Fatigue Analysis

Authors: Dr NWM Bishop, MSC Frimley and Alan Caserio. MSC Costa Mesa

Abstract

Fatigue analysis procedures for the design of modern structures rely on techniques, which have been developed over

the last 100 years or so. The first accepted technique was the S-N or stress-life method generally given credit to the

German August Woehler for his systematic tests done on railway axles in the 1870s. Initially these techniques were

relatively simple procedures, which compared measured constant amplitude stresses (from prototype tests) withmaterial data from test coupons. These techniques have become progressively more sophisticated with the

introduction of strain based techniques to deal with local plasticity effects. Nowadays, variable stress responses can

be dealt with. Furthermore, techniques exist to predict how fast a crack will grow through a component, instead ofthe more limited capability to simply predict the time to failure. Even more recently techniques have been

introduced to deal with the occurrence of stresses in more than one principal direction (multi-axial fatigue) and to

deal with vibrating structures where responses are predicted as PSDs (Power Spectral Densitys) of stress. Evenmore recently researchers have addressed the requirements for the design of specific components such as spot welds.All of these techniques were developed outside of the Finite Element environment. However, they have now been

implanted into many FE based analysis programs, the best known of which is MSCFATIGUE. The FE environment

introduces additional considerations relating to how input data is processed and how fatigue life, or damage, results

are post processed. This paper will deal with the issues associated with how fatigue techniques can be incorporatedinto the FE environment. Modern examples of FE based fatigue design will be included.

Wohlers Fatigue Test Machine (approx 1870)

Examples of typical fatigue failures


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Introduction and Background

MSC/FATIGUE is an advanced fatigue life estimation program for use with finite element analysis. When used

early in a development design cycle it is possible to greatly enhance product life as well as reduce testing and

prototype costs thus ensuring greater speed to market.

However, before describing the features of the product in detail it isuseful to define the termfatigue. Very often the terms fatigue, fracture,

anddurability are used interchangeably. Each does however convey a

specific meaning. Although many definitions can be applied to the word,

for the purposes of this paper, fatigue is failure under a repeated orotherwise 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. Thestress-life (or S-N method), is commonly referred to as the total life

method since it makes no distinction between initiating or growing a

crack. This was the first fatigue analysis method to be developed over100 years ago. The local-strain orstrain-life (_-N) method, commonly

referred to as the crack initiation method, was more recently developedand 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 and this has given rise to many so-calledcrack

growth methodologies.

Figure 1. The FEA fatigue

environment

Figure 2. Fatigue or crack

propagation?

Durability is then the conglomeration of all aspects that effect the life of a product and usually involves much more

than just fatigue and fracture, but also loading conditions, environmental concerns, material characterizations, andtesting simulations to name a few. A true product durability program in an organization takes all of these aspects

(and more) into consideration.

Why FEA based fatigue analysis?

All fatigue analysis calculations are performed within theconstraints of the so-called five-box trick. The illustration

below shows how this concept can be visualized. For any life

analysis, whether it be fatigue or fracture, there are always three

inputs. The first three boxes are these inputs:

Materials

Loading

Geometry

Analysis Results

Figure 3. The fatigue 5 box trick


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Fatigue analysis has traditionally been a test-based activity. Components or models are tested with service loads,

which are as close to the in-service signals as possible. In a test situation loading is usually a stress signal

measured remotely from a critical location. Geometry is usually a stress concentration factor to account for the

separation of the critical location and measurement point and materials are the cyclic fatigue properties.

The biggest drawback with testing is that it can not be undertaken until a prototype exists. If a design problem then

occurs it is usually very difficult to rectify. It is also very expensive to perform fatigue tests. For these reasons FEAbased fatigue analysis has been perceived as an excellent enhancement to the testing process. The FEA model

effectively replaces the geometry box in Figure 3. Loading signals are now forces, displacements or some other

driving function. Material properties still have to be obtained through test, however empirical approximations can be

made based solely on the UTS and Youngs Modulus of the material.

The correctness and accuracy of each of these inputs is important in that an error with any of these will be magnified

through the fatigue analysis procedure (the fourth box,) since this process is logarithmic. A 10% error in loading

magnitude could result in a 100%, or more, error in the predicted fatigue life. The fifth box is the post-processing orresults evaluation. This can take on the form of color contours on a finite element model or a tabular listing, but also

quite often leads back into the three inputs to see what effect variations of these inputs will have on the life

prediction. This is referred to as a sensitivity or a what if study. This is extremely useful at times when you are not

quite sure about the accuracy of one of the inputs.

Figure 4. An overview of an FEA based fatigue analysis

Life Prediction Methods

MSC/FATIGUE uses three life prediction methods. These are total life,, crack initiation, and crack propagation.

Total life is aptly named in that only the total life of the component is of concern. This is in contrast to when a crackwill initiate or how quickly it will grow. The three methods can be related to each other by assuming that the total

number of cycles to failure, Nf, equals the number of cycles to initiate a crack, Ni, plus the number of cycles to

propagate that crack, Np. The three methods have grown out of different needs over the decades using different

techniques and having different degrees of accuracy. So in theory this relationship is true, but in practice, whenapplying the three methods to the same problem, rarely, if ever does it add up.

Geometry & FEA

Loading Histories

Materials

InforDamageDistributions

Life Contours

SensitivityAnalysis

StrainLifePlot

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MSC/FATIGUE

Stress (total) Life Strain (initiation) Life Crack Propagation Vibration Fatigue

Multi-axial Fatigue Spot Weld Analyzer

Software Strain Gauge Utilities


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Total Life Crack Initiation Crack Growt

= +

NiNf NpFigure 4. An idealisation of the fatigue design process

FEA based stress analysis options

There are several FEA based methods for obtaining the stress information that is required to perform a fatigue life

calculation

[1] Static structural (and fatigue) analysis can be undertaken utilising MSC/FATIGUEs superposition capabilitiesfor combining multi load application inputs. Unit inputs of load are applied to all desired load application points.

The resultant stresses (caused by the unit load cases) are then factored by the actual time history of loading for that

load application point. This process is repeated for all load application points and the results are linearlysuperimposed. Fatigue life calculations are then performed using these combined stress histories. This method

ignores dynamic influences such as mass effects.

[2].Dynamic transient analysis. If this approach is used, the stress histories are produced at each point of interestusing a FE transient analysis method. These stress histories are also superimposed to obtain the required combined

stress histories, but the FE solver handles this. Fatigue life calculations are then performed on these stress time

histories. This method accounts for all dynamic effects but is less versatile in that all loads must be combined in a

single FE analysis.

[3]. Frequency Response analysis. In this approach the transfer functions are produced using the desired solver.These transfer functions are then resolved onto the desired stress axis system (usually principal stress). The response

caused by multiple random loading inputs is then obtained using standard random process techniques. The effect ofcorrelation between inputs can be dealt with by including Cross Power Spectral Density functions in the input

loading data. This method accounts for all dynamic effects and is quite versatile.

[4]. Random Vibration analysis. In this approach the response Power Spectral Density function is determineddirectly from the FE solver. Effects due to multiple load inputs must be dealt with in the FE analysis as with a

transient analysis approach. All dynamic effects are accounted for but this method has the limitation that fatigue life

can only be computed for a single component direction. Stress response results are not resolved onto a desired stress

axis system by the FE analysis.

Design Philosophies

There are three main fatigue design philosophies. Each centers around one of the fatigue life estimationmethodologies. To illustrate the three consider the design of a stool.


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Safe Life.The safe life philosophy is a philosophy adopted by many. Products are

designed to survive a specific design life. Full scale tests are usually carried out

with margins of safety applied. In general, this philosophy results in fairlyoptimized structures such as a stool with three legs. Any less than three legs and it

would fall over.

Fail Safe. On the other end of the spectrum of design philosophies is that offailsafe. This is where a failure must be avoided at all costs. And if the structure were

to fail it would fall into a state such that it would survive until repairs could be

made. This is illustrated with our stool now having six legs. If one were to fail thestool would remain standing until repairs could be made. This philosophy is

heavily used in safety critical items such as in the aerospace or offshore industries.

Damage Tolerant. The middle ground philosophy is that of damage tolerance.This philosophy, adopted heavily in the aerospace community and nuclear power

generation, relies on the assumption that a flaw already exists and that a periodic

inspection schedule will be set up to ensure that the crack does not propagate to a

critical state between inspection periods. As implied, this philosophy adopts thecrack growth method. This is illustrated using our stool (now with four legs) but

with someone inspecting it. This particular design philosophy is generally used in

conjunction with the fail safe philosophy first to try and design such that no failureis expected but then to assume that, for whatever reason, a flaw does exist and

must be monitored.

Integrated Durability Management

Durability Management is the control and organization of design, test, and production, to ensure products are

developed to meet the required life within cost and on time. The process has evolved over the last 150 years since

fatigue failures were first recognized. While there are many technologies that have contributed to the understanding

of fatigue and to the solution of fatigue problems, two major procedures are used in durability management: fatiguestesting and fatigue modeling.

Fatigue TestingThe first fatigue tests were carried out on full-scale components to establish their safe working stress. Later, themore complete relationship between cyclic stress or strain and fatigue life was established. Small-scale specimens

were tested to study component life and also fatigue mechanisms. In more recent times, as tests had to becomeincreasingly realistic, special test techniques were developed such as Remote Parameter Control. Today, testing is

still the most common way of confirming the fatigue life of a product prior to releasing it onto the market. However,testing often reveals weaknesses, which necessitate re-design. Assessing the suitability of particular design

modifications using fatigue testing alone can be time consuming and cost far more than just a delayed product.


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Fatigue ModelingThe estimation of fatigue life using mathematical modeling techniques was developed to assist the engineer insolving fatigue problems without always having to physically test all the options. For this reason, techniques such as

local strain or crack initiation modeling have become widely used. Improvements in the power of computers have

enabled the effective use of these techniques. Today, most major companies designing mechanical structures will

use a fatigue life estimation tool such as MSC/FATIGUE in conjunction with testing. The late 1980s had establish

the use of finite element analysis (FEA) as a tool for stress analysis. At the same time the integration of FEA andfatigue life estimation through the MSC/FATIGUE product began to provide new benefits by assessing fatigue

earlier in the development process.

Integrated Durability ManagementUnderstanding and effective implementation of durability management strategies requires a partnership between test

and design analysis. It can reduce product lead-time by focussing the use of fatigue testing to the essential

correlation and sign-off tests. The use of fatigue modeling, at the design analysis stage, allows more options to beassessed for little incremental cost. Integrated durability management can produce better products more quickly and

cheaply.

Components of MSC/FATIGUE. Stress Life and Strain Life Analysis

The stresslife and strainlife methods are the most common forms of fatigue life prediction. They both involvestress cycle counting, using, for instance rainflow cycle counting. Many materials data curve corrections are

available to deal with surface finish, surface treatment, mean stresses and elastic plastic corrections (for strain

life approach). Damage is assessed through a materials look up curve, either S-N or E-N and damage is summedusing the conventional Palmgren Miner cumulative damage hypothesis.

The total life method, more commonly known as the stress-life or S-N method, typically makes no distinction

between initiating or growing a crack, but rather, predicts the total life to catastrophic failure. Total life specific

features include:

Goodman or Gerber mean stress correction Welded structure analysis to BS7608 Material and component S-N curves

Sophisticated crack initiation or strain-life (-N) modeling provides a method for estimating life to the initiation of

an engineering crack. Crack initiation specific features include:

Neuber elastic-plastic correction. Advanced elastic-plastic correction based on Mertens-Dittman or Seeger-Beste methods

Cyclic stress-strain tracking using Massings hypothesis and material memory modeling Smith-Watson-Topper and Morrow mean stress correction Advanced biaxial corrections (proportional loading) based on Parameter Modification or Hoffman-Seeger

Components of MSC/FATIGUE - Crack Propagation Analysis

For all fatigue and fracture analyses we require the three major inputs: geometry, materials, and loading. This is nodifferent for a Crack Growth analysis except that geometry definition takes on a different form. The only

information necessary for this approach is the remote stress used in the Paris Equation and a description of the

stress intensity.


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When a notch becomes a crack, the stress field becomes a singularity (in theoretical elastic terms) and the stress

concentration, Kt, is no longer a useful way of describing the feature. Rather we need something that describes the

intensity of the stress field around the singularity. This concept is well illustrated by the diagram below where a hole

is introduced into an infinite plate. As the hole becomes an ellipse and the ratio of the length to width of the ellipsebecomes greater and greater, tending towards infinity, so does the stress concentration.

Figure 5. The concept of stress intensity

In practice of course, there must be some yielding around a

crack tip, because the material cannot support infinite stress.

There will be some redistribution of stress and strain comparedto the elastic solution. However, if the extent of plasticity islimited to a very small zone around the crack tip, some force

will still control everything in that process zone. If this force is

increased until the fracture toughness of the material K1C is

exceeded, fast fracture will occur.

The driving force behind a crack, that causes it to propagate, is

not stress or strain but the stress intensity factor, K. (This is not

to be confused with stress concentration Kt.) The stress intensityfactor accounts for both the stress and the crack size and is a

way of describing the stress field around a crack tip independent

of the overall geometry. The relationship between stress

intensity, stress, and crack length is known as thefracturemechanics triangle. If you know two of the corners you can

derive the other.

The rate at which a crack grows is governed by the cyclic rangeof stress intensity, K. Crack growth rate and stress intensity are

related by a crack growth law, the most well known and mostwidely adopted (and that used by MSC/FATIGUE ) being the

Paris Equation derived by Paul Paris in 1960 (equation 1):

Now the relationship between the driving force, K, the applied

stress, , and the crack size, a, in the fracture mechanics triangle

is given by equation 2.

Fracture

Zone

PlasticZone

Controlling ForceAround the Crack Tip

Figure 6. Crack tip yieldingStress Intensity

StressCrack Size

FractureMechanics

Triangle

Figure 7. Fracture mechanics

triangle

d a

d N------- C K( )

m=

Equation 1. Paris Law

K Y a( )=

Equation 2. Stress IntensityK

versus compliance factor Y


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Y is known as the compliance function and describes the geometry in which the crack exists. It relates crack length

to geometric features of the part or component. Perhaps one way to describe a compliance function in physical terms

is the change in stiffness or flexibility (compliance) as the crack grows, i.e., the structure becomes more compliant

as the crack gets longer. The dictionary defines compliant as ready or disposed to comply, and compliance as the actor process of complying to a desire, demand, or proposal or to coercion. In engineering terms it is the ability of an

object to yield when a force is applied.

Material response is modeled by measuring crack growth rates versus stress intensity (K) in constant amplitude

tests. From these tests are derived the da/dN curve and the threshold characteristics and fracture toughness of the

material. In fatigue we are concerned with stable crack growth occurring below a catastrophic level. When you plot

crack growth rates against K on log scales, you get sigmoidal shaped curves like these, which have three distinct

regions. There is a linear region in the middle of these curves, which is described by the Paris Equation. At thebottom end of the curves there is a threshold below which no crack growth occurs (very similar to a fatigue limit).

This is caused by crack closure and the interaction of the crack with the micro-structure. If the mean stress is raised

the threshold decreases because the cracks are held open for more of the time. At the other end of the curve, crack

growth rates increase as the maximum stress of each cycle gets close to the fracture toughness of the material.

This curve is called the apparent Kcurve. However there

are many effects that this equation does not take into

account, such as crack closure, corrosive environments, theinfluences of a notch, and static fracture mode contributions

to name a few. MSC/FATIGUE models these by using an

Effective K curve which has the effect of linearizing the

entire Apparent K curve through all three of its distinct

regions. It is this Effective K that is the actual (effective)

driving force that is then used in the Paris Equation to

determine crack growth.

As discussed earlier, the Fracture Mechanics Triangle relates

stress intensity, stress, and crack length. When speaking in

terms of crack growth and overall life, a rectangular rather

than a triangular representation is used. In Crack Growth

there is a relationship between stress range and life just aswith the Total Life (S-N) method except it is extended to

include the initial and final crack lengths (and all crack sizes

in-between these two limits). So in a similar way to solvingthe triangle, the fatigue crack propagation rectangle can be

solved by knowing any three of the four corners to derive the

fourth.

Threshold

Effects

Fast

FractureEffects

Paris Equation

Region

K

da/dN

Figure 8.apparent Kcurve

Final Crack

Size

Initial

Crack Size

Cycles to

Failur

Stress

Range

Figure 9. Fatigue crack propagation

rectangle

Components of MSC/FATIGUE - Vibration Fatigue

MSC/FATIGUE Vibration is an option of the MSC/FATIGUE software package that predicts the fatigue life ofstructures or components subjected to random or vibration load inputs. It is mainly, although not exclusively,

intended for dynamically sensitive systems. Either transfer functions or Power Spectral Densities (PSDs) of stress

can be read in from the FE solver database. From these, subsequent fatigue life estimates can be obtained. Therefore,the functionality of this tool can be separated into the stress analysis procedure and the subsequent fatigue analysis.

Many dynamic systems are subjected to fatigue damage.


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Such systems are currently designed, and analyzed, predominantly through the use of expensive and time consuming

test based procedures. MSC/FATIGUE vibration allows designers to identify and deal with such damage at a much

earlier stage in the design process, thus reducing or eliminating the need for expensive prototype tests.

As well as a fatigue analyzer this module also contains a state of the art analysis tool which provides a complete

solution path for multiple load case frequency domain based analysis. It includes new advances in stress tensor

mobility and biaxiality checking.

The simplest method of obtaining stress based PSD

information is to read the PSDs directly from the FE

solver and this is supported. A more flexible and

sophisticated approach is to read transfer functions fromthe solver database. These transfer functions are rotated

onto any one of a number of user defined stress systems

(maximum principal being the most relevant). Results

for these axis systems are computed for each incomingload case (each frequency and each load application

point).

Blade structural transfer

functions h()1andh()2

G11() G12()

G22()G21()

PSD

For c

e

frequency

PS

D

Forc

e

frequency

PS

D

Forc

e

frequency

PS

D

For c

e

frequencyReaction

Figure 10. Multiple random load inputs

A stress tensor mobility check is performed to assess the spread of axis orientation for each load case. Once the

desired axis system has been obtained, PSD responses are computed for multiple load application points with

correlated, uncorrelated or partially correlated load inputs. This procedure is state of the art and the most advanced

random analysis capability available.

The so-called PSD moments are used to determine the characteristics of the PSD stress response. These moments

provide (all of) the information required to perform a fatigue life calculation. In the last 10 years new techniques

have enabled the fatigue life of a wide range of engineering structures subjected to random vibration to be assessedin the frequency domain with a far higher accuracy than ever before. In particular, the conservatism associated with

the so-called Narrow Band method has been overcome with new methods such as the Dirlik approach. This means

that a fatigue life check could, and should, be included at the structural FEA stage of analysis. This could,potentially, highlight deficiencies at an earlier stage in the design cycle than is currently possible. MSC/FATIGUE

now includes a wide range of tools for such analysis.

Components of MSC/FATIGUE - Software Strain Gauges

MSC/FATIGUE Software Strain Gauge is an option of the MSC/FATIGUE software package which allows thecreation of virtual strain gauges within a finite element model. These gauges can be used to produce theoretical

result time histories from the finite element model under the effect of multiple time varying applied loads. Stress &

strain time histories may be extracted at any point on the finite element model surface, based on either standard oruser defined strain gauge definitions. The results obtained from the Software Strain Gauge may be based on

transient or quasi static finite element loading. Use of the MSC/FATIGUE software strain gauge allows the finite

element analyst to correlate theoretical structural integrity calculations with experimentally determined results. Thistool permits the engineering analyst greater confidence in the finite element model of the real world structure.


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The software strain gauges are defined as finite element groups, each containing between 1 to 3 elements. All

standard strain gauge definitions are supported in both planar and stacked formulations. User defined gauges may

also be created, with definitions stored in a gauge definition file.

The virtual strain gauges are positioned on the finite element model surface, with the gauge aligned in any

orientation, and the gauge covering multiple finite elements. The results obtained from the Software Strain Gauge

are averaged results from the underlying finite elements, modeling the same geometric averaging obtained withactual instrumentation. Results are transformed to the coordinate system and alignment of the software strain gauge.

The Software Strain Gauge has the following features:

Multiple Gauge Geometries Uniaxial Gauges

T Gauges Delta & Rectangular Gauges Stacked & Planar Gauges User Specified Gauge Definitions

Gauge Definition Files (user definable gauges) Up to 200 simultaneous Software Strain Gauges

The Software Strain Gauge is also of benefit to the analyst performing MSC/FATIGUE weld durability calculations

in accordance with British Standard 7608. The Gauge tool allows ready access to strain time histories at the weldtoe, providing important information for weld durability calculations.

Components of MSC/FATIGUE - Spot Weld Analyser

MSC/FATIGUE SpotWeld is an option of the

MSC/FATIGUE software package, which allows theprediction of fatigue life for spot welds joining two steel

sheets, based on finite element analyses. The calculation

requires the spot welds in a structure to be modeled as

stiff bar between two sheets of thin shell elements. Themethod uses the bar element cross sectional forces and

moments to calculate weld stresses. These are then used

for a total life fatigue analysis based on the S-N

technique. Analysis using MSC/FATIGUE Spot Weldallows the user to accurately predict fatigue life using

loading histories and component geometrys. The

number, size and location of spot welds may be readily

optimized to reduce manufacturing costs and increase

durability.

Figure 11. A typical spot weld fatigue

analysis

Spot Welds are defined by the weld nugget and parent sheet dimensions. Groups of similarly defined spot welds are

allocated to finite elements groups, with the capability to analyze multiple spot weld groups simultaneously. Thestructural stress methodology uses rigid beam elements to transmit forces and moments between the thin shellelements sheets. The calculated forces and moments together with the geometric definition of the spotweld allows

structural stresses to be calculated in both sheets and the weld nugget. These structural stresses are calculated at 36

intervals around each weld, in both the sheets and the weld nugget. These stresses are then used to make fatigue life

predictions on the spot weld using the S-N (total life) method. The spot weld analyzer has the following features:


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Analysis of welds joining two metal sheets S-N (total life) technology The number of Spot Welds within the model is limited only by the FEA analyzer Up to 20 different groups of similarly defined Spot Welds may be simultaneously analyzed in the FEA

model Unlimited number of Spot Welds per definition group

The analyzer simultaneously calculates weld nugget and sheet fatigue life

108 sets of fatigue calculations are performed for each spot weld

Components of MSC/FATIGUE - Multiaxial fatigue

MSC/FATIGUE Multiaxial has a number of tools

suitable for handling proportional loadings. It has notch

correction procedures (the Hoffmann-Seeger andParameter modification methods) and a variety of

combined stress-strain parameters available. A new

stress/strain combination option, Critical Plane has also

been included. This works for both S-N and E-N

calculations and makes calculations on 18 planes at 10degree intervals. The stresses have to be surface

resolved.

In addition, a new multiaxial option is available which

has the following functionality.

The Dang-Van and MacDiarmid safety factor

methods. These are high cycle fatigue or multiaxial

fatigue limit approaches. The output from these

methods is either a fringe plot of safety factors, or agraphical plot giving details of the stress variations

at individual calculation points.

6 local strain critical plane methods. These are"Normal Strain", "Shear Strain","SWT-Bannantine","Fatemi-Socie" and "Wang-Brown" with and

without mean stress correction.

The local strain methods allow global and singlenode/element fatigue calculations with a variety of post-

processing options including fringe plots, histogram

plots, time correlated damage plots and polar damage

plots. The method includes a new multiaxial non-proportional notch correction procedure, incorporating

an energy-based notch rule based on Neuber's rule and a

Mroz-Garud cyclic plasticity model.

Figure 12. Forces and moments applied to a

steering knuckle

Figure 13. Fatigue life contour plot for

steering knuckle under the application of 12

correlated loading inputs


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Components of MSC/FATIGUE - Utilities Module

Sometimes the test and CAE functions are separated. However, many CAE departments also have a strong need to

do test type analyses, and many test groups often want to interface with the CAE world. Sometimes this might

include the pre-processing of data for a subsequent FE analysis. In order to cater for this customer need thefollowing modules have been included as fully integrated solutions in MSC/FATIGUE Utilities as described below.

Fatigue analysis based on measured structural response time histories (e.g. stress-time or strain-time).

Fatigue analysis based on LEFM crack propagation techniques using measured time histories.

Multi-file display of time histories to compare concurrent events.

Comprehensive frequency analysis, including FFTs, filtering, cross PSDs etc.

Mathematical manipulations of single and multiple time histories.

Multi-channel graphical editing for time history manipulation and cleaning.

Integration and differentiation of time histories - e.g. conversion of acceleration to displacement.

Single and joint probability distribution analysis.

Statistical analysis of time series.

Running statistics analysis for trend analysis, e.g. stationarity, drift, pattern recognition.

ASCII file import and export for links to other analysis tools.

Stress concentration factor library and calculator.

Stress-strain hysteresis loop analysis for crack initiation.

Cycles and damage analysis and display.

General Features of MSC/FATIGUE

A number of general features exist in MSC/FATIGUE which include

Rainflow cycle counting Various matrix (bin) sizes (32, 64, 128) Statistical confidence parameters Surface finish/treatment corrections Palmgren-Miner linear damage summation Flexible Miners sum (>0, default=1.0)

User-defined life units Multiaxial stress state assessments Factor of safety analysis


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Result access features include FE results data from:

MSC/PATRAN database in the following form: Linear static (stresses & strains)

Linear transient (stresses & strains)

Includes results from MSC/NASTRAN, ANSYS,ABAQUS, MARC & any other PATRAN supportedanalysis codes

MSC/ACCESS (MSC/NASTRAN xdb files) Linear static & transient stresses SDRC Universal files Linear static stresses & strains

External result files PATRAN nodal & elemental result files MSC/PATRAN FEA result files

Figure 14. How MSC/FATIGUE relates to

other solvers and MSC/PATRAN

Multiple FE load cases with associated time variations can be defined and applied simultaneously. A time history

database is supplied to facilitate creation and storage of these files. Up to 100 simultaneously applied load cases Load magnitude definition for normalizing FE results Scale Factors (stress concentration definitions) Offsets (constant residual stress definition) Static load cases (variable residual stress definitions across model) Results transformations Global system

Surface resolved

Time histories can be stored centrally or in a local database. Time history creation & modifications is possible from: ASCII file import (XY & rainflow matrix data) XY point specification

Graphical interaction Wave form definitions (sine, triangular, square) Block definitions Rainflow matrix creation from time series data

Graphical plots & hardcopies of time history and matrix data includes multiple file display, crossplots & overlays Data transformations Polynomial transforms

Lookup tables

Unit conversions Sample rate adjustments Peak valley slicing extraction

Time Histor

Materials

MSC / NASTRAN

ABAQUSANSYS

MARC

PATRAN

MSC / FATIGUEModel Definition Analysis Setup

Post-Processing

USER


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A materials database manager stores and manipulates a library of cyclic material properties. Features include:

Approximately 200 materials (steels) supplied Add, create or modify your own or supplied materials data (Imperial & SI units supported) Generate materials data from UTS & E

Weld classifier based on BS7608 Graphical display of: Component & material S-N curves Cyclic & monotonic stress/strain curves Strain-life curves

Elastic-plastic lines Fatigue limits (endurance limits) Graphical display, hardcopies & tabular comparison of materials

Figure 15. The 3 alternative material curves used in fatigue analysis

A variety of results data is reported both in tabular form and graphically. A single location analyzer can be used for

what-if studies after a global analysis has identified hot spots. Results data include:

Damage/life (reported in linear & log form) Multiaxial assessment parameters:

Factor of safety Contour Plotting of:

Life EstimatesLog of Life

Damage Component Specific Life Units (Flights, Miles,

etc.) X-Y Plots of Sensitivity Studies

Histogram plotting

Figure 16. Fatigue damage contour plots

on an FE model

S-N E-N_ LEFM

S-N DataPlot

MANTEN_SN

SRI1:3162 b1:-0.2b2:0 E:2.034E5 UTS:600

1E1

1E2

1E3

1E4

1E0 1E1 1E2 1E3 1E4 1E5 1E6 1E7 1E8 1E9

Life(Cycles)

nCodenSoft

StrainLifePlotBS4360-50D

Sf':1036b:-0.123Ef': 0.622 c:-0.618

BS4360-43C

Sf':930b: -0.103 Ef':0.173c:-0.437

1E-4

1E-3

1E-2

1E-1

1E0 1E1 1E2 1E3 1E4 1E5 1E6 1E7 1E8

Life(Reversals)

nCodenSoft

DeltaK ApparentPlot

2024-T3:Ratio0 Environment:AIR

C:1.86E-11 m:4.05Kc:31 D0:3.6 D1:0.7 Rc:0.8

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

da

/dN(m/cycle)

1E0 1E1 1E2

DeltaK Apparent(MPam1/2)


15/15

15

A number of what if scenarios can be investigated including back calculations based on design life of:

Scale factor (stress concentration) Residual stress Probability of failure (design criterion) Sensitivity studies of:

Multiple scale factors (stress concentrations) Multiple residual stress values Multiple probabilities of failures (design criteria) Surface finish/treatment Mean stress correction methods Graphical display and hardcopy of sensitivity plots Change materials or surface finish/treatment

Material searches based design life

Figure 17. The sensitivity of fatigue life to

applied loading

Future Development

Future plans for development include the following

Thermal-Mechanical Fatigue. This involves a strategic relationship with a major US car company andautomobile producer.

Fatigue of Cast Iron. This involves a strategic relationship with a major US agricultural equipment company.

Fatigue Editing.This involves a major ground vehicle company in the US.

Fatigue Analysis of Rotating Structures (wheels). This involves a major aerospace company.

Other MSC/FATIGUE future technology projects.Elastic-plastic non-linear FE results, load step analysis,

families of S-N curves and an open architecture

Documents

Finite Element Based Fatigue Analysis