The overall goal of achieving improved life cycle management of aircraft engine, gas turbine components is a major industry thrust. Low cycle fatigue (LCF) crack initiation prediction, an important element of life cycle management as traditionally applied, may be overly conservative in estimating total cyclic life capability. Consequently, there is increasing pressure to improve predictive methods both for crack initiation and for subsequent crack propagation. The utility of equivalent damage concepts for application to hot section components of aircraft engines was studied. Specifically, the topics examined were mean stress, cumulative damage, and multiaxiality. Other factors inherently linked to this study were the basic formulation of damage parameters at elevated temperatures and the fact that hot section components experience severe temperature fluctuations throughout their service lifetime

Cook, T. S.

Laflen, J. H.

English

Concepts in equivalent damage were evaluated to determine their applicability to the life prediction of hot path components of aircraft gas turbine engines. Equivalent damage was defined as being those effects which influence the crack initiation life-time beyond the damage that is measured in uniaxial, fully-reversed sinusoidal and isothermal experiments at low homologous temperatures. Three areas of equivalent damage were examined: mean stress, cumulative damage, and multiaxiality. For each area, a literature survey was conducted to aid in selecting the most appropriate theories. Where possible, data correlations were also used in the evaluation process. A set of criteria was developed for ranking the theories in each equivalent damage regime. These criteria considered aspects of engine utilization as well as the theoretical basis and correlative ability of each theory. In addition, consideration was given to the complex nature of the loading cycle at fatigue critical locations of hot path components; this loading includes non-proportional multiaxial stressing, combined temperature and strain fluctuations, and general creep-fatigue interactions. Through applications of selected equivalent damage theories to some suitable data sets it was found that there is insufficient data to allow specific recommendations of preferred theories for general applications. A series of experiments and areas of further investigations were identified

Laflen, J. R.

NASA Technical Reports Server

Equivalent damage:  A critical assessment

Equivalent Damage - A Critical Assessment 
J. H. Laflen and T. S. Cook 
General Electric Co. 
Cincinnati, Ohio 
ABSTRACT 
. The overall goal of achieving improved life cycle management of aircraft 
engine, gas turbine components is a major industry thrust. Low Cycle 
Fatigue (LCF) crack initiation prediction, an important element of life 
cycle management as traditionally applied, may be overly conservative in 
estimating total cyclic life capability. Consequently, there is increasing 
pressure to improve predictive methods both for crack initiation and for 
subsequent crack propagation. This increased emphasis is the result of 
significantly higher component replacement costs as a consequence of more 
complex designs coupled with advanced materials and processing techniques. 
Moreover, despite added strength, the increased performance demands placed 
on engine components to achieve higher engine thrust-to-weight ratios 
have resulted in decreased cyclic lives. It is apparent, therefore, that 
significant cost savings can be realized through improved accuracy in high 
temperature, LCF crack initiation prediction. 
In practical applications, engine components generally undergo very 
complex cycles of multiaxial strain, temperature, and dwell time, all of 
which add uncertainty to the problem of life prediction. During the process 
of designing and analytically evaluatirig the lifetime of gas turbine engine 
components, it is necessary to simplify many of these complexities to make the 
problem tractable. Nevertheless, there remain several important questions 
which can be clarified through the study of life prediction models: among 
".t:-
these are how to address the problems of multiaxial loading, cumulative damage, 
and mean stress effects, and how they influence fatigue crack initiation life. 
Consequently, an lS-month study was undertaken to determine the utili~y 
of equivalent damage concepts for application to hot section components of air-
craft engines. Specifically, the topics studied were mean stress, cumulative 
damage, and multiaxiality. Other factors inherently linked to this study 
were the basic formulation of damage parameters at elevated temperatures and 
the fact that hot section components experience severe temperature fluctuations 
throughout their service lifetime. Both of these latter considerations placed 
contraints on the level of confidence with which recommendations regarding 
11 
https://ntrs.nasa.gov/search.jsp?R=19850002644 2020-03-20T21:17:19+00:00Z
specific equivalent damage criteria could be made since most such criteria 
were developed for use at lower temperatures. Despite this, the study yielded 
useful results, both from the. point of view of data consolidation techniques 
under isothermal conditions and in producing concepts that will be useful in 
future studies • 
. Through a literature review of the three areas of interest, the most 
promising techniques were extracted for further study. In the case of mean 
stress techniques, statistical evaluations of suggested approaches were made 
by comparing each technique to various isothermal data sets. Similiarly, a 
combined literature review and isothermal data· analysis technique was used to 
determine the most appropriate cumulative damage approach. In the case of 
multiaxiality, this decision process rested solely on the basis of the litera-
ture review. Following the ~nitial screening, both the mean stress techniques 
and the cumulative damage concepts were tested ag~inst data sets involving 
either time dependent aspects of damage and/or varying temperature. The 
following conclusions were suggested by this study: 
1. The equivalent strain relationship is the best mean stress criteria 
for low homologous temperatures and aircraft gas·turbine engine alloys 
if the appropriate isothermal data are available. The Leis tech-
nique appears to be the best predictive mean stress parameter when 
data are not available to determine the exponent in the equivalent 
strain technique. 
2. Thermal mechanical fatigue (TMF) experiments are required to 
verify the mean stress criteria. However, an elevated temperature 
mean stress criterion should be more conservative than normal 
approaches when applied to the out-of-phase TMF cycles which are 
normally encountered in hot path components. 
3. Isothermal mean stress criteria should be verified in the longer life 
(design) regime. Most experimental results are obtained in the 
shorter cycle life range where more inelastic strain is present. A 
specific series of experiments was described. 
4. The double linear damage rule is the best isothermal cumulative 
damage technique curr~ntly available. However, the technique was 
not consistent (it was conservative) in predicting a series of 
two-step tests where a temperature change was introduced into the 
second block of loading. 
12 
5. A test series was described for developing a consistent damage 
methodology. These tests used a so-called equal life technique 
to aid in the formulation of uniaxial equivalent damage criteria 
. , 
and involved changing temperature during the experiments. Such a 
test series was viewed as a first step in developing a damage methodo-
logy for TMF. 
6. A multiaxial equivalence criterion was developed based on a litera~ 
ture review. The criteria deemed important were the use of a tri-
axiality factor function and a consistent mean stress formulation. 
7. A need exists fo~ multiaxial test data on aircraft engine industry 
alloys. These experiments should concentrate on positive biaxial 
stress ratios, and should study the effect of mean stress. In 
general, multiaxial relationships for elevated temperature applica-
tions will remain an open research area for quite a while. Current 
research should concentrate on lower temperature phenomena which 
would suggest criteria at elevated temperatures. Uniaxial damage 
considerations at elevated temperatures appear complicated enough 
for the present. 
13 
l Theorlts I l Theone. I l Theorle. I 
~ , , 
MultlAIII.llty Cumulativ. Oam.ge Mean Stre .. EIf'ects , , , 
Theoretical Theoretical Theoretical 
and/or Empirical andlor Emplrlc.1 and/or Emplrlc.1 
Sasls Salll S.III 
l ., ., 
Testing Tntlng Testing 
and/or Field andlor Field and/or Field 
Correl.tion Correlation Correlation 
l Consblency I. Consllten~ I , , , 
Applies 10 Applle.lo AppUe. to 
AGTE Probleml AGTE Problem. AGTE Problem. 
, , , 
Material Response M.terlal Rllpon .. Materl.1 Re'l'onse 
D.la Required Data Required Oala Required 
t ~ ~ 
Required Anllysis Required Analysis Required Analysis 
! ~ ~ 
T,st Olla Required Ttst Oala Required Test Oala Required 
to Define Initiation to Deline InlliaJlon to O,fln, Inltlalion 
~ ~ ~ 
Ease of IncorporOltion Ease of Incorporallon Ea .. of Incorporation 
Inlo Ouign Procell Inlo De.lgn Proctss Inlo Design Process 
, 
~ , , 
Ease of Incorl)orallon Ea .. 0' Incorporation Ease of Incorporalion 
Into Life Management Inlo Llle Managemenl Inlo Llle Manlgemenl 
Process Process Process 
L I I , 
Ov,raU Synlhesls 
~. 
Conclusions 
and 
Recommendallon. 
Criteria Diagram. 
14 
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15 . 
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Total Strain Range. t:£. percent 
Mean Stress as a Function of Total Strain Range for Inconel 718 at 566 0 C. 20 CPM. 
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Cycles To Initiation, Ni 
Baseline Data at 343 0 C Analyzed by the Equivalent Strain and Leis Parameter Methods. 
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1.21 AI 14 . 0 
Inco 718. 5660 C 
1.1 I I~ 
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Cycles to Initiation, Ni 
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Results of Two-Step Load/Temperature Tests: Re: Variable in Second Step. 
19 
Low Temperature Damage Par~met~r, PI 
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Cycle Summation, First Block, 
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21 
~ 
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Tangential Stress. cree' MFa 
Example of the Complex States of Stress That Can Evolve 
During a Simulated Mission in a Disk Bore. 
22 
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Suggested Multiaxial Formulation 
d fe 
2 
Ef. (2Nd- a + (Of - k*Igm) (2Nf)-f3 
'2 (TF) E* 
Where: 
dfe - / i delj dell 
elj - - 1 6 f lj "3 fkk IJ 
TF - 01 + 02 + 03 J- Ie CJ1 - CJ2 )2 + (CJ2 - CJ3 )2 + (U1 - CJ3)2 
10m - °1m + °2m + °3m 
..... 
E* 3E - 2(1 + Il) 
f f' ex. , Of' k* , fJ are material parameters 
Nf = Cycles to Failure 


Equivalent damage: A critical assessment

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