Design Optimization Using NEF (Nastran...

MSC Software Confidential

Design Optimization Using

NEF (Nastran Embedded Fatigue) SimAcademy Webinar

Presented by : Shripad Mungi

July 14, 2016


• A Short History of Fatigue Analysis Methods

• Overview of NEF 2016

• New Nastran Bulk Data Entries for Fatigue

• Introduction to Design Optimization using NEF

• Design optimization capabilities of NEF

• Defining fatigue responses – Life, Damage, FOS in NEF

• Demonstration example of Design optimization

Agenda

2


S-N

175 yrs ago

Crack propagation

65 yrs ago

NASTRAN

65 yrs ago

Integration with MBD

25 yrs ago

1st major transportation

disaster Versailles May

11th 1842

Failure

Mechanisms 122

years ago

Use of non-linear

(Marc) FE results

25 yrs ago

MSC Fatigue

35 yrs ago

Fatigue

Dynamics

CAE

Nastran Embedded

Fatigue

2013

A Short History of Fatigue Analysis Methods

3


Overview of NEF 2016

4

MSC Nastran Embedded Fatigue (NEF) 2016 Updates

Three major enhancements to NEF are added to SOLs 101, 103, and

112. These include:

1. Nodal Averaged Stress/strain

2. Surface Resolved Stresses

3. Stress Output


NEF 2016 Updates …

• Default value of LOC field on FTGPARM changed to LOC=NODA

The other options available are NODE or ELEM

• Fatigue analysis can now be performed using nodal averaged stresses or strains.

Nodal averaging is similar to GPSTRESS or GPSTRAIN request in Case Control

This is in addition to the existing element CENTER or ELEMENT NODAL stresses/strains

• Benefits

- Fatigue damage calculated from a more realistic quantity

- Less calculation points, thus increased computation speed

- Greater clarity because only a single fatigue damage result per grid

- Post-processing does not average fatigue life/damage due to multiple values per grid

- Less conservatism in the damage prediction

1. Nodal Average Stress/Strain

5

MSC Software Confidential 6

2. Surface Resolved Stresses

• What is Surface resolved stress?Stress on the surface of a structure which is said to be in a state of plane stress.

The two principal stresses are in the plane of the surface while the third principal which is normal to the

surface is zero.

• How Surface resolved stresses can be obtained ?1. SRESOLVE = YES on FTGPARM

2. Define SET of solid elements on FTGDEF entry

Benefits

• Ensures a 2-D state of stress at the surface

• Results in less calculation points, ignoring interior entities

• Enables multi-axial assessments and correction on 3-D solid models

• Enables use of critical plane stress combination (COMB=CRITICAL) for 3-D solid models



Application of Surface Resolved Stresses

• SRESOLVE=YES

SAE Shaft Modeled with Solid CHEXA & CPENTA Elements

SAE Shaft Skinned Shell Elements with

SRESOLVE=YES (Clipped)SAE Shaft Surface Damage on Notched

Area Only


Total Elements = 22140

Shell Elements = 3852


3. Fatigue stress output

3.1 LAYER


• New output request parameters are available using the LAYER

• Previously, the LAYER option was available on the PFTG entry

• This has been moved to the FTGPARM entry to be a more Global output request setting

• LAYER specifies which layer of results are to be printed to the f06 file

- For shell elements, the output results layer to print to the f06 file.

- Values can be

- 0=Worst, 1=Top(Z2), 2=Bottom(Z1)

• The default is to output the worst case layer

• Top or bottom can also be specified

• Both layers are always output to the results data blocks for post-processing purposes through the

MASTER/DBALL, Output2, or other files for graphical postprocessors


• New parameter to requesting Actual Stresses/Strain used in the standard SN or eN fatigue analysis

Why STROUT ?

- Actual Stresses used in Fatigue analysis can differ than STRESS/STRAIN in case control

- Nastran input file can request more than one fatigue analysis; each possibly requesting different

stress settings

- Standard case control do not allow visualization/printing in this manner

How to consider STROUT ?

• To be specified on FTGPARM entry

• Element set must also be defined on FTGDEF entry

• STROUT has 3 values signifies as

- 0=No Output (Default)

- 1=Print Output

- 2=Plot Output (OESFTG Data block)


3.2 STROUT


Design Optimization Capabilities of NEF

Capabilities of NEF Design Optimization

Capabilities of conventional MSC Nastran SOL 200

SOL 200 has capabilities of optimization with DRESP1, DRESP2, DRESP3

DRESP1: DRESP1 entire define type-1, or first-level responses.

Conventional responses - Structural weight, Displacements, Volume, Frequency, Stress, Strain,

Force, ESE….

10


Fatigue Item Codes (Response Requests)

Rules for Selection of Item Codes

• Selection of appropriate Item Code is very important while doing Design Optimization with Fatigue

Responses

• Selection of Item codes is governed by LOC=ELEM on FTGPARM

• For Element centroid results Element(s), the Fatigue response type, and Layer (top or bottom),

• If both top and bottom, both item codes must be specified on different DRESP1 entries

• For Element nodal results, the Element(s), the fatigue response type, the item code for the particular

node of the element, and the layer, must be specified.

For example for a CQUAD4 element, to request all the nodes of the element for both top and bottom

layers, a total of eight (8) item codes must be specified or eight (8) different DRESP1 entries

11



Selection of proper Item Code is very important while doing Design Optimization with

Fatigue Responses.

Fatigue Item Codes for LOC=ELEM on FTGPARM

Element Group A consists of elements:

CHEXA, CPENTA, CTETRA, CTRIA3,

CSHEAR

Element Group B consists of elements:

CQUAD4, CQUAD8, CQUADR,

CTRIA6, CTRIAR12


• For Spot weld fatigue, there are 3 Layers

End A (top sheet), End B (bottom sheet) and the nugget (center) of the spot weld.

• Responses can be at NANGLE angles around the spot weld perimeter. Default = 18 angles.

Total item codes (all layers at all) 18x3=54 item codes on separate DRESP1 entries.

• This is impractical and cumbersome for optimization purposes

• To over come this limitation, Special Negative item codes are provided

• For example a spot weld LOG of LIFE for element for all angles of element


TOP

NUGGET

BOTTOM

I st angle

of nugget

II nd

angle of

nugget

III rd

angle of

nugget

13


Design Optimization

In NEF

14


Overview:

• Optimize the fatigue life based on

stress/strain inputs from Nastran

Objective

• To obtain the targeted life or

damage or FOS of a particular

component with sizing optimization

Benefit

• Know the minimum thicknesses of

component of interest

Design optimization in NEF- Introduction

15

• Cost, weight saving

• Durability validation

• Once fatigue life objective is achieved the other targets like stress/displacement/stiffness will also

achieved

• Design can be finalize with a single run as Static, Modal, FRA and Fatigue all validation can be obtain

in single run


Design optimization in NEF- Demonstration Example

Objective : To determine the optimum thickness distribution of the cantilever plate such that

structural mass is minimized.

Loading condition : a tip load as shown in the figure above and a uniform pressure.

Design Constraints are placed on

- Max. allowable tip disp.

- the von Mises stresses on upper surface of first row of elements along plate length

- on 1st natural frequency

- 100,000 repeats of a cyclic tip load application

16


Design optimization in NEF- Demonstration Example

Analysis type: Frequency response using a tip load across frequency range of 0.0 to 10.0 Hz.

Fatigue analysis performed with a fatigue life constraint imposed on the same elements as the

stress constraint for the tip load.

*The eight individual plate thicknesses (one for each station along the length of the plate) are

functions of three independent variables α1, α2, and α3.


Set up – Design optimization in NEF

SOL 200

CEND

TITLE: Plate Design Optimization

$ -------- case control section -----------

FATIGUE = 42

SPC = 100

DISP(PLOT) = ALL

STRESS(PLOT) = ALL

DESOBJ(MIN) = 35 $OBJECTIVE FUNCTION DEFINITION

DESGLB = 99 $GLOBAL FATIGUE CONSTRAINT

$ ----------------------------------------------$

SUBCASE 1

ANALYSIS = STATICS

SUBTITLE = LOAD CONDITION 1

LOAD = 300

DESSUB = 10 $ stress design constraint

SUBCASE 2

ANALYSIS = STATICS

SUBTITLE = LOAD CONDITION 2

LOAD = 310

DESSUB = 10 $ design constraint

SUBCASE 3

ANALYSIS = MODES

SUBTITLE = NORMAL MODES ANALYSIS

STRESS(PLOT) = NONE

METHOD = 100

DESSUB = 30 $ DCONSTR 1st frequency

SUBCASE 4

ANALYSIS = MFREQ

METHOD = 100

SET 100 = 9,19,29

DISP(PLOT,PHASE,SORT2) = 100

STRESS(PLOT) = NONE

SUBTITLE = MODAL FREQUENCY ANALYSIS

LOADSET = 2000

DLOAD = 1000

DESSUB = 40

FREQ = 1000

$ FATIGUE SETUP:

SET4 1 PROP PSHELL 1 THRU 8

FTGDEF 42

ELSET 1 999

PFTG 999

FTGPARM 42 SN 1.000 0

STRESS GOODMAN ELEM

DTI UNITS 1

PSI

FTGLOAD 42 98 1 1000.0 7.883

0.0 DB

UNITS 1000. Laps

$$$... Weight response objective function:

DRESP1 35 W WEIGHT

$DCONSTR DCID RID LALLOW UALLOW

$$$... Fatigue Life Constraint (Repeats):

DCONSTR 99 22 1.0+5


Set up NEF Job

$ Fatigue Set Up

SET4 1 PROP PSHELL 1 THRU 8

FTGDEF 42

ELSET 1 999

PFTG 999

$ ------------- Fatigue Parameters --------------------

FTGPARM 42 SN 1 0

STRESS ABSMAXPR GOODMAN ELEM

DTI UNITS 1 PSI

$ ------------- Cyclic Fatigue Loading ----------------

FTGLOAD 42 98 1 1000 7.883 0 DB

UNITS 1000 Laps

$ -------------- Refer External loading file name ----------------------

UDNAME 98

./saetrn .mod

$....... 2....... 3....... 4....... 5....... 6....... 7....... 8....... 9.......

MAT1 51 1.00E+07 0.33 0.1

0 50000 50000 29000

MATFTG MID CNVRT

STATIC YS UTS CODE TYPE RR SE mp STATIC

SRI1 b1 Nc1 b2 Nfc SE BTHRESH SRI1 b1

MATFTG 51 145.0377

STATIC 307.692 400 100 -1 0.1

SN 947 -0.07606 5.00E+08 0 1.00E+30


Set up NEF Job

$ . . . . . . . Define Design Model . . . . . . . .

$ . . . . . . . Define the Design Variable . . . . . . .

$DESVAR ID LABEL XINIT XLB XUB DELXV

DESVAR 10 ALPH1 1

DESVAR 20 ALPH2 1

DESVAR 30 ALPH3 1

$

$ . . . . .EXPRESS ANALYSIS MODEL PROPERTIES LINEARLY IN TERMS OF DESIGN VARIABLE . . . .

$DVPREL1 ID TYPE PID PNAME/FID PMIN PMAX C0

$+ DVID1 COEF1 DVID2 COEF2 ...

DVPREL1 1 PSHELL 1 T

10 1 20 1 30 1


10 1 20 0.875 30 0.7656


10 1 20 0.75 30 0.5625


10 1 20 0.625 30 0.3906


10 1 20 0.5 30 0.25


10 1 20 0.375 30 0.1406


10 1 20 0.25 30 0.0625


10 1 20 0.125 30 0.0156


$ ------------- Cyclic Fatigue Loading ----------------

FTGLOAD 42 98 1 1000 7.883 0 DB

UNITS 1000 Laps

$ IDENTIFY DESIGN RESPONSES

$DRESP1 ID LABEL RTYPE PTYPE REGION ATTA ATTB ATT1

$+ ATT2 ...

$ FATIGUE LIFE RESPONSES

DRESP1 22 LIFE FATIGUE ELEM 4 42 1

2 3 4 5 6 7 8

$ Design constraint screening data

DSCREEN RTYPE TRS NSTR

DSCREEN FATIGUE -100

$$$... Fatigue Life Constraint (Repeats):

$DCONSTR DCID RID LALLOW UALLOW LOWFQ HIGHFQ

DCONSTR 99 22 1.0+5

Set up NEF Job

21


Set up NEF Job

$ Design Responses STATIC VON MISES STRESS

DRESP1 2S12 STRESS ELEM 9 1

2 3 4 5 6 7 8

$ Static displacement at tip

DRESP1 33D1 DISP 3 19

$ 1st Natural frequency

DRESP1 130FFREQ FREQ 1

$$$... Frequency response tip displacement

DRESP1 230TDISP FRDISP 3 19

$DCONSTR DCID RID LALLOW UALLOW

$ Design Constraints

$ Stress constraint

DCONSTR 10 2 -29000 29000

$ Top displacement constraint

DCONSTR 10 33 -2 2

$$$... Normal modes 1st mode frequency constraint:

DCONSTR 30 130 1.5

$ Frequency response top displacement constraint

DCONSTR 40 230 2

$ Optimization Overridden parameters

DOPTPRM DESMAX 20 P1 1 P2 15 IPRINT 7

22


Optimization Results

23



24



25



26



27


Design Optimization Remark:

Fatigue based optimization is powerful features, however user has to be aware of some points

• Zero initial life – no further life change, optimizer cannot progress

• Infinite initial life - design change will be ineffective, optimizer cannot progress

• Initial design – must have at least reasonable damage below yield or UTS.

Should not have either zero or infinite life

• Avoid damage as a constraint – Damage values may below change limit tolerance.

Instead use Life or log of Life

• Type of fatigue responses

For Shell / Surface element use element centroidal fatigue responses are appropriate for optimization

For solid elements, nodal must be used (LOC=NODE or similar element nodal request on FTGPARM

Disadvantages of using nodal results:

1. Currently, averaged grid point stresses due to contributions from each shared element for

fatigue analysis are not used unless LOC=NODA is specified.

2. LOC=NODE results in element nodal fatigue results.

Obviously, LOC=NODE or NODA results in more constraints.

• Correcting the violated constraints: This can be obtain by 2 ways

Adding DSCREEN, FATIGUE,-1.E02, thus the SOL200 will retain the fatigue life constraints

Modify DOPTPRM DELX=0.1. Default=0.5

28


MSC Nastran Embedded Fatigue In A Nutshell

29

“The most important development in CAE

Fatigue since the introduction of MSC Fatigue.”Lots of documentation available

• New Nastran QRG (with NEF cards) Nastran 2016

• NEF User Guide 2016

• MSC Nastran Release Guide 2013 and 2016

• You may find any of these documents from MSC.Software at

www.simcompanion.mscsoftware.com.

• For technical support phone numbers and contact information, please

visit

• http://www.mscsoftware.com/Contents/Services/Technical-

Support/Contact-Technical-Support.aspx

http://www.simcompanion.mscsoftware.com/

http://www.mscsoftware.com/Contents/Services/Technical-Support/Contact-Technical-Support.aspx


Thank You

Date post:	30-Jun-2018
Category:	Documents
Upload:	vuongphuc
View:	257 times
Download:	2 times

Design Optimization Using NEF (Nastran...

Documents