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1999 safe technolog y ltd

fesafe-brochure1/rev5 Page 1

INTRODUCTION

FE-SAFE is an advanced ea sy-to-use suite o f durab ility analysis softwa re which inte rfac es

to finite element mode ls.

FE-SAFE com bines com ponent loading, FEA stresses, and m aterials data , and performs

advanced multiaxial fatigue analysis. Fatigue hot-spots are automatically identified. 3-D

contour plots can be displayed for fatigue life and for allowable stress factors for a

spec ified design life.

FE-SAFE can be used for re-design and wha t -if ana lysis, for the w hole m od el or for

selected area s, to investiga te the effec t o f removing m eta l from non-critical reg ions and

to increa se the life at ho t -spot loc a tions.

Effects of temperature, surface finish, notch sensitivity, geometry changes, and changes

in ma terial prope rties and service duty ca n b e investigated q uickly.

Designs can be optimised rapidly, material costs are reduced, and the final design can

be verified on the computer, giving more confidence that the design will pass test

sched ules as right-first -time.

FE-SAFE was developed unde r a $1million projec t in collab ora tion with Rover Group .Extensive tests on rea l c om ponents we re used to d evelop the software. FE-SAFE

interfaces to ma ny FEA suites and post -p roc essors, inc luding ABAQUS, ANSYS, FEMSYS, I-

DEAS, Pro/Engineer, Pro/Mechanica , Hypermesh and FAM.

The ABAQUS interfac e rea ds and writes to the .fil file a nd the new ABAQUS/ CAE

data ba se a va ilab le with ABAQUS 5.8.

A CATIA a nd a NASTRAN interface w ill be released late r this yea r.

FE-SAFE is supported on Silicon Graphics, Hewlett Packard and Sun UNIX worksta tions,

and will be ava ilable on PCs running Window s NT.

Continuing research projects and customer-specified developments are being used to

ensure tha t FE-SAFE rema ins at the forefront o f enginee ring durab ility by de sign.

FATIGUE HOTSPOT IN AN SAE NOTCHED SHAFT MODELLED IN IDEAS

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AN OVERVIEW OF FE-SAFE

FE-SAFE consists of a user interfac e, a m ateria ls database managem ent system, and

fatigue analysis programs.

FEA stresses are usua lly ela stic stresses, so tha t the results can b e scaled and supe rimposed to

prod uc e service stress time histories

ela stic-plastic stresses can be a na lysed for ce rtain types of load ing.

component loading

a time history of com po nent load can b e a pp lied to a set o f FEA stresses

time histories of multi-axis loading can be superimposed to produce a time history ofthe stress tensor at e ac h loca tion o n the m od el

a seq uenc e of FEA stresses for exam ple, the results of a transient a na lysis, or theanalysis of several rotations of an engine crankshaft, or models of several discreteloading c onditions, ca n be a nalysed

a c omb ination of the ab ove 2 items

block loading programs, consisting of blocks of constant amplitude cycles, can beproduced and analysed.

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materials data

a c omp rehensive d ata ba se o f materials prope rties is provided the database c an be extended and modified b y the customer test reports and ba ckground da ta c an b e accessed using in-bu ilt Netscap e links

the databa se is accessed by FE-SAFE during the fatigue ana lysis set -up andma terials data is transferred into the ana lysis programs. The d atabase may also beaccessed direc tly through the FE-SAFE user interfac e, to ente r new data fo r

example.

additional factors

noda l temperatures can be used to mod ify ma terials fatigue properties

effec ts of surfac e finish ca n be inc luded for all or part of the c ompo nent a llow ing

mac hined and a s-forged surfac es to b e d ifferentiated notch sensitivity effects can be included important for cast irons, and some

aluminium a lloys and lower streng th steels (in Relea se 3.1)

a de sign life m ay b e spec ified

the fatigue a nalysis can b e for the comp lete mod el, or for an e lement group .

Different materials data or stress concentration factors can be used for eachelement group (to allow for machined and as-forged surfaces on the same

component, for example)

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analysis

uniaxia l ana lysis using stress-life c urve s - Goodman, Gerber or no mean stresscorrection.

uniaxial a na lysis using strain-life curve s Morrow, Smith-Wa tson-Topper or no mean

stress correction.

biaxial fatigue analysis using local stress-strain analysis (maximum shear strain,maximum direct strain, Brown-Miller combined shear and normal strain)

imp lemented as c ritical plane p roc ed ures.

vo n Mises stress.

ana lysis of we lded structures using the stress-life d a ta from BS7608.

output

FE-SAFE writes outp ut files of nod al fat igue lives. If a design life has been spec ified ,FE-SAFE calculates the stress fac tor which c ould b e a pp lied at ea ch nod e to

ac hieve this life. Both of the se files can be displayed as 3-D cont our plo ts.

a list of the most damaged elements is saved, and re-analysis can be concentratedon these elements if req uired .

a text file of user inputs, analysis type, program version numbers and a resultssumma ry, is produced .

ANALYSIS OF AN EXHAUST MANIFOLD MODELLED IN ABAQUS

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re-analysis

the user may chang e a ny of the inputs and re-run the analysis.

FE-SAFE reload s all the p revious input p aramete rs when the program is re-run.

utilities

plots of ma terials data a nd loa d h istories. impo rting ASCII and other stand ard format da ta files.

preparation of single and multi-channel load histories scaling, peak/valley withcycle omission.

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FATIGUE ANALYSIS ALGORITHMS

The following fa tigue da mage algorithms are included .

Uniaxial stress-life

2

= 'f, (2Nf )b

Stra in-life for uniaxia l stress

2

='fE

(2Nf )b

+ 'f (2Nf)c

Direc t strain

12

='fE

(2Nf )b

+'f (2Nf)c

Ma ximum shea r strain

max2

= 1.3'fE

(2Nf )b

+1.5'f (2Nf)c

vo n Mises stra in

eff2

='fE

(2Nf )b +'f (2Nf )

c

Brown-Miller

max

2+n

2= 1.65

'fE

(2Nf)b

+ 1.75 'f (2Nf)c

BS7608 The S-N curves for weld ed joints are built in to FE-SAFE for the we ld c lassifica tion

ta ble in BS7608.

Where the direction of the principal stresses at a node changes during the loading

history, critica l plane proc ed ures are used .

For the shear strain and Brown -Miller c rite ria , p lanes a t = 900 and =450 to the surfaceare used . Each p lane is rotated from = 0 to = 1800 in steps of 100.

Su r face

For the direct strain criteria, a plane at = 900 to the surface is used, rotated from = 0to= 1800 in steps of 100.

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EXAMPLES

A single load history on a component

The FEA file w ould b e an e lastic ana lysis for a unit loa d. FE-SAFE allows a sca le fac to r tobe a pp lied to the loa ding if the FEA stresses are for a non-unit loa d.

For each nod e, FE-SAFE calculates a time history of the 6-stress tensor by multiplying the unit loa dstress tensor by the time history of the load .

If the load ca se for the FE da ta set is a load PFE, and for this load the e lastic stress at the no de isSFE.If the load ing to b e a nalysed is a loa d time -history P(t), and one d ata point in P(t) is a value PK

then the elastic stress at the n od e SK = SFE

PK

PFE

and the t ime history of e lastic strains S(t ) = SFE

P(t)

PFE

A time history of the p rinc ipa l stresses in the plane of the element is calculated . The

multiaxial Neubers rule is used to calculate the elastic-plastic stresses and strains whichresult from any c yc lic yielding, using a ma teria l mem ory a lgorithm . The c yc lic stress-stra in

c urve is calculate d for the biaxial stress cond ition a t the no de .

If the user has specified an additional stress concentration factor, its effect is included at

this stage. For a single load history the principal stresses at the node do not change

direction, so a single fatigue analysis is performed. In this, the shear or direct strains are

rainflow cycle counted and the fatigue damage for each cycle is calculated. Miners

rule is used to c alculate the fat igue life a t the nod e.

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If a design life has been specified, the program uses an iteration procedure to calculate

the fac tor which c ould b e a pp lied to the stresses in order to ac hieve the d esign life.

Multiple load direc tions on a component

For ea c h loa ding d irec tion, the FEA file w ould conta in the results of a n e lastic a nalysis for

a unit load.

FE-SAFE ta kes the 6-stress tensor for one unit loa d, and multiplies it by the t ime h isto ry of

this loa d . FE-SAFE the n ta kes the 6-stress tensor for the second unit loa d, multiplies it by

the time history of this load to form a time history of the stress tensor, and adds the time

history ob ta ined from the first load . This is repeated for each load , to c reate a time

histo ry of the stress tensor for all the loa ds.

For a stress data set, each of the elastic stresses at the node will be given by an equation of the

form

S = (SFE)P(P

PFE) + (SFE)Q(

Q

QFE) + .......

where S is the instanta neo us va lue of one of the six stresses Sij

PFE is the load used for the FE da ta set (the loa d a pp lied to the mod el)

(SFE)P is the corresponding elastic stress at the node for load PFE

P is the instantaneous value of load P(t) in the load- time history

QFE is the loa d used for the 2nd FE da ta set

(SFE)Q is the corresponding elastic stress at the node for load QFE

Q is the instantaneous value of load Q(t) n the load- time history

etc.

The m ultiaxial Neub er s rule is used to c a lculate the elastic-p lastic stresses and stra ins

which result from any cyc lic yielding. The cyc lic stress-strain c urve is re-ca lculate d for the

biaxial stress condition for each event in the stress history, using a material memory

algorithm.

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If the user has spec ified an add itiona l stress conc entra tion fac tor, its effec t is include d a t

this stage.

For multiple load directions, the principal stresses at the node may change direction

during the load history, so a critical plane fatigue analysis is performed. On each plane,

the shear or direct strains are rainflow cycle counted and the fatigue damage for each

cyc le is calculated . Miners rule is used to c alculate the fatigue life a t the node . The

shortest fatigue life on a ny plane is taken as the fatigue life a t the node.

If a design life has been specified, the program uses an iteration procedure to calculate

the fac tor which c ould b e a pp lied to the stresses in order to ac hieve the d esign life.

Data set sequence

A data set sequence may be the result of a transient analysis. It may also be created by

modelling a series of discrete events, for example the stresses in an engine crankshaft

calculated at each 50 of rotation of the crankshaft through (say) four crankshaft

revolutions. The c alculated stresses at e ac h ang le are termed a da ta set.

The da ta set sequenc e is spe c ified in a d a ta set seq uenc e file, which allows thesequence of the data sets to be specified, and allows a scale factor to be applied to

any data set. A different scale factor may be applied to each data set (positive or

nega tive), and d ata sets may a ppear more than once in the sequence.

At eac h node FE-SAFE creates a time-history of the stress tensor from the sequenc e o f

da ta sets. This is converted into e lastic -plastic stress/ strain and a c ritica l p lane fa tigue

ana lysis is used to ca lcula te the fa tigue life. Aga in, a de sign life ma y be spe c ified.

Block load ing ana lysis

Bloc k loa ding fa tigue tests are com mo nly used . FE-SAFE can simulate b lock-loa ding

sequenc es. The user crea tes a unit loa d FEA d ata set for ea ch load cond ition.

For each data set the user may specify two scale factors and a number of cycles. For

example, the stresses in data set 1 may be scaled by +1 and +0.1 to create a stress

cyc le, and the user ma y spe c ify 1000 cyc les of th is load ing. This forms one block in the

block load ing sequence.

At present up to 64 blocks may be specified. Data sets may appear more than once in

the sequence .

FE-SAFE calculates the fa tigue da mage for eac h bloc k, and the fatigue da mage

caused by the transition from block to block. Critical plane analysis is used. Again, adesign life ma y be spe c ified.

Combined transient and multi-axis time history analysis

This analysis allows da ta set sequences and multi-axis load h isto ries to be superimposed.

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TECHNICAL DETAILS

FEA data

There is no prac tica l limit to the num ber of elements in the FEA data file. Up to 50 nod esmay be present in any e lement. Beam e lements are not ana lysed .

Up to 1024 da ta sets ma y be p resent.

Element groups

Up to 32 element groups may be p resent in the FEA d ata file.

Loading

Loa ding ca n be defined a s:

Single c hanne l .da c format b inary files

Multichanne l .am c format b inary files

Single c olum n ASCII files

Multi column ASSCII files

Utilities to save load ing files to a ny of the suppo rted formats is provided .

Materials data

The ma teria ls da ta base

management system is supplied.

A database of commonly used

steels and aluminium alloys is

included.

The user ma y sea rch the

database, for example to list all

aluminium alloys, and may enter

new m ate rials into the data ba se.A master database, and a users

local database, may both be

accessed if they are available.

The system ma nager may set

read-only access to the master

database to prevent editing of

the data. Netscape links are

provided, so that reports of

materials test procedures may be

accessed.

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The Seege r materials app roximation a lgorithm is included with the da taba se. This allows

app roximate ma terials data to b e generated , using the ma teria l UTS and elastic

mod ulus. These generated da ta may b e g iven a da ta set nam e and saved in the

database.

The mate rials da ta in the d ata ba se must include:

the p arame ters for the uniaxial strain-life c urve2

='fE

(2Nf )b

+ 'f (2Nf )c

i.e. the fatigue strength co efficient 'f,the fatigue duc tility c oefficient 'fthe fa tigue strength exponent b

the fatigue d uctility exponent c

the e lastic modulusE

and parameters for the uniaxial stable cyc lic stress-strain curve =E

+

K'

1

n'

i.e. the strain harde ning co efficient K

the strain ha rdening expone nt n

The ma teria l UTS must a lso b e d efined.

Kt

A stress concentration factor may be entered. Guidance is supplied on the factors

which rep resent different surfac e finish c ond itions.

Scale fac tor

A linea r scale fac tor may b e a pp lied to the loa ds in the load ing history.

Design life

The user may enter a design life for the ana lysis. FE-SAFE calcula tes the fa c tor to be

app lied to the stresses a t ea ch node in order to achieve the d esign life. The user may

limit the magnitude of the factors to be considered, for example only factors in therange 0.5 to 5.0. These fa c tors will be c alculated to a resolution o f 0.1. The user ma y a lso

set an inner range of factors which will be calculated to a resolution of approximately

0.01, for exam ple fac tors in the rang e 0.8 to 1.5.

Temperature effec ts

The user may c hoose to c orrec t the mate rials data for the effec ts of tem pe rature. In thecurrent release, for each nod e FE-SAFE read s the maximum tem perature at the node

and adjusts the materials fatigue properties by interpolating between the materials

data . Materials data for at lea st three te mperatures must be supp lied in the data ba se.

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FUTURE RESEARCH

Safe Tec hno logy is taking part in a resea rch p rojec t, sponsored by the Europea n

Community, to ad d further intelligenc e into CAD procedures. This will inc lude further

extensions to FE-SAFE, and its interfac ing to auto ma tic compo nent modification

proced ures for rap id re-design. Rover Ca rs (for whom Safe Tec hnolog y are preferred

supp liers of fa tigue software), and Porsche a re am ong the p artners in this projec t.

Safe Tec hnology ha s also p rop osed a joint p rojec t with Rover Cars and bo th the

universities in Sheffield to extend and va lida te o ut -of-phase multiaxial ela stic-p lastic

c orrec tion a lgorithms. This p rojec t is expe c ted to sta rt during 2000.

Customer contracts are being negotiated to include spot-welded joints, seam-welds,

and to further enhance the high tem perature fatigue c ap ab ility.

Other customer-reque sted deve lopm ents will be w illingly c onsidered .

Safe Tec hnolog y Limited

1 The South We st C entre

Archer Roa d

She ffie ld S8 0JR

England

Tel (+)44 (0) 1142 555 919Fax (+)44 (0) 1142 555 910

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e-mail [email protected]

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