Intro to ANSYS NCode DL 14 5 L02 CAE Fatigue

Introduction to ANSYS nCode DesignLife

Customer Training Material

Lecture 2: Introduction to CAE Fatigue


L2-2 ANSYS, Inc. Proprietary

2013 ANSYS, Inc. All rights reserved. Release 14.5

April 20133

Customer Training Material Agenda

Review basics of fatigue

what is fatigue

physics of fatigue cracks

fatigue history

fatigue testing

Introduce basics of CAE durability

fatigue approach

stress-life and strain-life

crack-growth

Introduce basics of fatigue analysis

stress cycle

cycle counting

damage accumulation




April 20133

Customer Training Material Introduction

Fatigue damage is the initiation and/or growth of a crack under fluctuating loading

progressive damage

component seems to lose strength and get tired after multiple load

applications, hence the name fatigue

almost all structural components are subjected to cyclic service

Time

Str

es

s

y

Fatigue failure occurs after

repeated loadings even

though the stress is low




April 20133

Customer Training Material Fatigue Life

Fatigue failure is typically viewed as a 3-stage phenomena

stage 1: crack initiation

stage 2: stable crack growth

stage 3: fast fracture

Fatigue life = crack initiation + crack growth

no precise transition from crack initiation to crack growth




April 20133


Surface flaws initiate at tiny dislocations in the materials microstructure

very localized stress concentration

practically undetectable and impossible to model using FE

These tiny surface flaws create persistent slip bands that propagate along the maximum shear plane under alternating stress

bands slip back and forth like a shifting deck of cards

Crack Initiation

Alternating Stress

Crystal surface

Slip bands form

along planes of

maximum shear

giving rise to

surface extrusions

and intrusions




April 20133


Slip bands grow into micro-cracks (Stage I cracks)

small compared to materials microstructure

difficult to detect, invisible to the naked eye

grow in direction of maximum shear stress

450 to the direction of the applied load

Stage I Cracks




April 20133


After traversing 2-3 grain boundaries, the micro-cracks grow into fatigue cracks (Stage II cracks)

large compared to materials microstructure

crack itself physically interrupts the flow of stress

stress concentration causes plastic stresses at the crack tip

local plastic stress causes the crack to change direction

growth is now propagated by cyclic plastic stress at the crack tip

oriented perpendicular to maximum principal stress

Stage II Cracks




April 20133

Customer Training Material Fatigue Failure

If sufficient energy exist, Stage II cracks continue to grow until tensile failure occurs

most lack sufficient energy to propagate across grain barriers

crack arrests

fatigue damage

producing striations

(aka beach marks)

fast final fracture due

to tensile failure - no

striations

initial crack

slip bands micro-cracks fatigue cracks tensile failure




April 20133

Customer Training Material History of Fatigue Analysis

Fatigue failures has been investigated for more than 150 years

initially applied to the study axel failures in the railroad industry

first industrial components to be subjected to millions of cycles

failures continue




April 20133


1837 - Wilhelm Albert publishes the first article on fatigue

1860 August Whler devises a test for fatigue

1901 - O.H. Basquin proposes a log-log relationship for S-N curves

1945 - A.M. Miner introduces a linear damage hypothesis

1958 - Fatigue crack growth explained in terms of plastic strain

1968 - Tatsuo Endo introduces the rainflow cycle count algorithm

Key Events in Fatigue Analysis




April 20133

Customer Training Material Fatigue Analysis

Initially fatigue analysis was done after an in-service failure, if at all

build > break > redesign

Customer

Usage Life

Re-Design




April 20133

Customer Training Material Incorporating Fatigue Testing

As customer usage became better understood, it was applied to fatigue testing

build > test > redesign

Customer

Usage

Test Life

Re-Design




April 20133

Customer Training Material Fatigue Testing Limitations

Fatigue testing has many limitations

requires many physical prototypes

realistic tests are difficult or impossible to achieve

slow and expensive

fails to deal with over-design

difficult to handle late changes and design variations

test results may differ significantly

requiring statistical interpretation




April 20133

Customer Training Material CAE Fatigue Analysis

CAE can predict fatigue damage in a virtual environment

evaluations can be performed during design phase

can assess cumulative damage caused by multiple loads




April 20133

Customer Training Material 5 Box Trick

CAE fatigue analysis are built around five basic actions

so called 5 box trick

inputs > pre-processing

solver post-processing




April 20133

Customer Training Material CAE Durability Approaches

Two basic approaches for performing CAE durability analysis

fatigue approach

fracture mechanics

also known as crack-growth or crack-life method

Fatigue approach

use calculated stresses/strains and material fatigue curve to predict

cycles to failure (life)

two basic fatigue analysis approaches

stress-life (SN)

uses elastic stresses

strain-life (EN)

uses elastic-plastic strains




April 20133

Customer Training Material CAE Fatigue Methods

Stress-life (SN) uses elastically calculated stress and stress vs. cycle fatigue failure curves (S-N curves)

assumes stress drives fatigue

only applicable high cycle fatigue

greater than 100,000 cycles for ductile metals

Strain-life (EN) uses elastic-plastic strains and Strain Life Relationship Equation

assumes local plastic strains drives fatigue

applicable to low and high cycle fatigue




April 20133

Customer Training Material Stress-Life Approach

Stress-life (SN) fatigue method assumes that fatigue damage is produced by fluctuating stresses

only applicable to elastic stress, so limited to limited to low stress/high

cycle applications

usually more than 100,000 cycles to failure for ductile metals

fatigue life is based on alternating stress range and Whler fatigue curve

(S-N curves)

original numerical fatigue approach

vast amount of industry experience

simple technique

often can be checked with hand calculations




April 20133

Customer Training Material S-N Curves

S-N curves are plots of elastic stress versus total cycles to failure

usually log-log plots

S (y-axis): either full stress amplitude or alternating stress range

N (x-axis): total cycles to failure (initiation and propagation)




April 20133

Customer Training Material Strain-Life Fatigue

Strain-life fatigue (EN) assess fatigue damage using cyclic strain ranges and the Strain Life Relationship equation

applicable to both low cycle and high cycle applications

stresses less than or greater than yield

uses local elastic-plastic strains

either directly calculated or adjusted from elastic results

predicts crack initiation

Relatively new fatigue analysis technique

usage began approximately 30 years ago

difficult to implement with hand calculations

limited to CAE




April 20133

Customer Training Material Strain-Life Fatigue

EN method is based on assumption that material behavior at notch root is analogous to small test specimen under strain controlled

conditions

F

F

overall body remains elastic

local deformation is strain controlled

behavior of notched component with localized plasticity

can be simulated using smooth test specimen




April 20133

Customer Training Material Strain Life Relationship

be ff

NE

22

cp ff N22

cb fff

f

NNE

22 2

Basquins equation

Coffin-Mansons equation




April 20133

Customer Training Material Cyclic Loading Terms

Fatigue damage is caused a changing stress/strain state

sine curve is a simple representation of cyclic loading

stress ratio (R) = Smin/Smax

amplitude ratio (A) = Sa/Sm

Sa

Sa

S

Smax

Smin

Sm

Smax = maximum stress

Smin = minimum stress

Sm = mean stress

Sa = alternating stress

S = stress range

Common loadings:

fully reversed: R = -1, A = infinity

zero to maximum: R = 0, A = 1




April 20133

Customer Training Material CAE Cycle Counting

If multiple cyclic loadings occur, magnitude and number of each stress/strain reversal must be determined

minimum stress, maximum stress, mean stress, and number of cycles

associated with each stress reversal must be determined

Rainflow is the most common cycle counting algorithm

applicable to stress-life (SN) and strain-life (EN) methods

automated in DesignLife

Rainflow counts number of closed strain loops




April 20133

Customer Training Material Rainflow Cycle Counting

Rotate strain history 90 degrees and plot vs time

Cyclic behavior can be visualized as rain flowing off of a pagoda roof

fatigue cycles are:

a-d, b-c, e-f, and g-h




April 20133


Rainflow cycle counting can also be visualized as water draining from valleys

determine peaks and valleys of stress/strain during cycling

invert stress/stain history and imagine it is filled with water

drain water - start at deepest valley and repeat until all valleys are drained

total depth drained = stress range

mean depth = mean stress

time

100

300

200

400

500

45

0

22

5

time

100

300

200

400

500

time

100

300

200

400

500

drain water starting

at lowest valley

imagine filled with

water

time

100

300

200

400

500

invert and reorder to

start with absolute max




April 20133


Rainflow approach divides any arbitrary load history into groups (bins or blocks) of similar loading

rainflow matrix can be visualized using a histogram

Each group represents a number of loading cycles for a particular stress range and mean stress




April 20133

Customer Training Material Damage Accumulation

Multiple cyclic loading conditions have a cumulative fatigue effect

Rainflow groups the loading history into blocks loading blocks

each block causes a fraction of the total damage (partial damage or damage fraction")

i

ii

N

nD fraction damage

Di = damage fraction caused by loading block (i)

ni = number of applied cycles of loading block (i)

Ni = available fatigue life for loading block (i)




April 20133

Customer Training Material Damage Accumulation

Damage fraction is the amount of life that has been used by a block of loading

If ni = 1e5 and Ni = 1e6, the damage fraction is 0.10

10% of the total fatigue life has been used up by loading block i

allowable

actual

N

n

i

i fraction damage




April 20133

Customer Training Material Miners Rule

Miners Rule assumes that the total damage is simply the linear summation of the partial damages

first proposed by Palmgren in 1924 and further refined by Miner in 1945

also referred to as Linear Damage Rule

applicable to both SN and EN

widely used

simple to implement and as accurate as more complicated methods

load sequence is not considered

failure occurs when sum of damage fractions equals the fatigue life

D is equal to or greater than 1.0

automated in DesignLife

n

i i

i

n

n

N

n

N

n

N

n

N

n

N

nD

13

3

2

2

1

1 damage total




April 20133

Customer Training Material Accuracy

Fatigue calculations are much less precise than strength calculations

statistical, not deterministic, phenomenon

empirical rules

order of magnitude errors in life estimates are common

large factor of safety (FS) typically assumed to ensure safe design

FSs of 10 are common

CAE fatigue can enable smaller FSs




April 20133

Customer Training Material Crack-Life Approach

Crack-life method uses fracture mechanics parameters (e.g., J-integral, stress intensity, etc.) to predict flaw propagation under

cyclic service

typically uses Pariss Law (da/dN) to relate fracture mechanics parameters to crack growth rate

often used to establish inspection intervals

for an inspection technique the smallest detectable flaw size is known

number of cycles required to grow a crack from smallest detectable size to critical size is determined

inspection intervals are based on crack growth rate

ANSYS Mechanical APDL and DesignLife both have some fracture mechanics capabilities




April 20133

Customer Training Material Flowchart of CAE Durability Analysis

CAE Durability

Fatigue Approach Crack Life Approach

Strain-Life (EN) Stress-Life (SN)

Paris Law

crack growth

rate

rainflow cycle counting

Miners rule damage accumulation

number of cycles

to failure (life)




April 20133

Customer Training Material Summary

Fatigue cracks start at surface dislocations and are driven by reversing local plastic flow

Fatigue failure occurs at stress levels insufficient to cause failure in a single application

damage is cumulative and unrecoverable

difficult to detect progressive deterioration during fatigue process

catastrophic failures can occur without warning

Fatigue failure is typically viewed as a 3-stage phenomena

stage 1: crack initiation

stage 2: stable crack growth

stage 3: fast fracture

Fatigue life = crack initiation + crack growth




April 20133

Customer Training Material Summary

Fatigue testing is slow and expensive

CAE durability analysis predict fatigue in a virtual environment

characterize capability of a component to survive cyclic service

known as fatigue analysis

fatigue life calculations are less precise than strength calculations

Two basic CAE durability analysis approaches

fatigue approach

stress-life (SN) or strain-life (EN)

fracture mechanics

crack life

Rainflow method commonly used to count cycles for both SN and EN

Miners Rule commonly used to accumulate damage for SN and EN




April 20133

Customer Training Material Workshop 1

Workshop 1: WB - DesignLife interface

restore archive of existing WB project

review WB project page

review Engineering Data

open Mechanical and solve

review DesignLife results from project schematic

modify design points in Parameter Manager

Intro to ANSYS NCode DL 14 5 L02 CAE Fatigue

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