Fatigue life consists of fatigue crack nucleation and propagation periods. In order to predict fatigue life accurately, a methodology for the quantitative assessment of these two fatigue damage processes had to be devised. Grain boundary oxidation penetrates faster than does oxidation within a grain. This faster oxidation penetration causes intergranular fatigue failures at elevated temperatures. Grain boundary oxidation accelerates both crack nucleation and propagation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were measured. Quantitative applications of the grain boundary oxidation kinetics to fatigue crack nucleation and propagation were analyzed. A method, based on the Weibull distribution, of extrapolating the laboratory oxidation data measured with small samples to large engineering structures is presented

Liu, H. W.

Oshida, Y.

English

NASA Technical Reports Server

N88-22420
GRAIN BOUNDARY OXIDATION AND LOW-CYCLE FATIGUE
AT ELEVATED TEMPERATURES*
H.W. Liu and Y. Oshida
Syracuse University
Syracuse, NY
ABSTRACT
Fatigue life consists of fatigue crack nucleation and propagation periods. In
order to predict fatigue life accurately, a methodology for the quantitative
assessment of these two fatigue damage processes had to be devised.
Grain boundary oxidation penetrates faster than does oxidation within a grain.
This faster oxidation penetration causes intergranular fatigue failures at ele-
vated temperatures. Grain boundary oxidation accelerates both crack nucleation
and propagation.
Grain boundary oxidation kinetics and the statistical distribution of grain
boundary oxide penetration depth were measured. Quantitative applications of
the grain boundary oxidation kinetics to fatigue crack nucleation and propaga-
tion were analyzed. A method, based on the Weibull distribution, of extrapola-
ting the laboratory oxidation data measured with small samples to large
engineering structures is presented.
Work performed for Fatigue and Fracture Branch and funded under NASA
Grant NAG3-348 (monitor: Jack Telesman).
3-173
https://ntrs.nasa.gov/search.jsp?R=19880013036 2020-03-20T06:31:44+00:00Z
OVERV I EW 
DAMAGE MECHANISMS OF HIGH-TEMPERATURE, LOW-CYCLE FATIGUE 
At high temperature both creep and oxidation are distinctive LCF damage mech- 
anisms. Need exists to identify the regions at which each mechanism is domi- 
nant. A quantitative study of grain boundary oxidation and its effects on 
fatigue life was conducted. 
In RATE 
RECIPROCAL TEMPERATURE 
(ABSOLUTE) 
CD-88-31859 
I 
3-174 
STATISTICAL DISTRIBUTION OF GRAIN BOUNDARY OXIDE PENETRATION
Grain boundary oxide penetration depth was measured as a function of oxidation
temperature T and exposure time t. The penetration depth exhibited a wide
statistical scatter which followed the Weibull distribution function as shown
below. The distribution function allowed the extrapolation of the small labo-
ratory samples to the large engineering components in service.
6O
PROBABILITY 40
OF FINDING
< _i, 20
PERCENT 10---
4 1-
2-
1
.1
Zf
(LOCATION PARAMETER)--
.2
i I i I I I_I I i I t l JI,I I
.4 .6 1 2 4 6 10 20×10 -4
VALUE OF THE CONSTANT o_i FOR A GIVEN OXIDE DEPTH ami
CD-88-31865
3-175
FREQUENCYEFFECTSONFATIGUECRACKGROWTHAT HIGHTEMPERATURE
A model is proposed that predicts fatigue crack propagation behavior of high
temperatures. The model is based on an intermittent microrupture of oxide par-
ticles which is controlled by grain boundary oxidation kinetics. The model
predicts an inverse relationship between da/dN and cyclic frequency. This
relationship agrees with observed behavior of a number of materials.
100
10 -1
10 -2
da/dN,
mm/cycle
lO -3
10 -4
MATERIAL TEMP., ,_K, WAVE FORM
°C MPa{m
O INCONEL718 649 28
• INCONELX-750 650 30 NVVVV
_ INCONELX-750 650 30
A ASTROLOY 760 50 /-v-'v-
• ASTAOLOY 6SO 50
",%_. _ ,_ ASTROLOY 700 10 /vVvvv
'Z_(_ "_ [] 304 S.S. 538 30 /vvvvv
. _ \ • WASPALOY 649 30
I__,_ 2V4Cr-MoSTEEL 565 10 /_
[] zx
- _• _
V V
10 -5
10 -4
I I I I I I
10 -3 10 -2 10 -1 100 101 102
FREQUENCY, (cycle/sec)
CD-88-31866
3-176
POSTER PRESENTATION
DAMAGE MECHANISMS OF HIGH-TEMPERATURE, LOW-CYCLE FATIGUE
Two primary damage mechanisms of high-temperature, low-cycle fatigue are creep
and oxidation. Both mechanisms are thermally activated as illustrated schemat-
ically below.
The question is not which of these two mechanisms is dominant. The pertinent
question is rather which one is dominant in the high-temperature region and
which in the low-temperature region. Quantitative studies on creep and oxida-
tion damage mechanisms are necessary in order to answer this question.
A quantitative study on grain boundary oxidation and its effects on fatigue
life was conducted.
In RATE
E
RECIPROCALTEMPERATURE(ABSOLUTE)
CD-88-31860
3-177
A grain boundary is a path for rapid diffusion. Grain boundary oxidation is 
controlled by the diffusion of oxygen along the boundary. 
diffusion causes a deep grain boundary oxidation penetration, and the fast oxi- 
dation penetration causes the accelerated intergranular fatigue fractures at 
high temperatures. 
The rapid oxygen 
TAZ-8A AT 1000 "C FOR 500 HOURS 
CD-00-31861 
3-1 78 
GRAINBOUNDARYOXIDEMORPHOLOGIES
Twodifferent grain boundary oxide morphologies were found: the pancake type
and the cone type. The larger and deeper pancake type is more damaging.
300
200
XORY,
_m
100
PANCAKE TYPE
Y
-- 150
100
XORZ,
/_m
50
20 40
DEPTH FROM SURFACE, _m
CONE TYPE
Z \
X
I I
20 40
DEPTH FROM SURFACE,/_m
CD-88-31862
3-179
RELATIONSHIPSMEASURINGOXIDEPENETRATIONOFGRAINBOUNDARY
Grain boundary oxide penetration am in the cobalt-base superalloy, TAZ-8A,
was measuredby Liu and Oshida (1984 and 1985) and Oshida and Liu (1985 and
1988) as a function of oxidation temperature T and exposure time t.
The coefficient of correlation is 0.96 for 144 data points. This quantitative
relation was used to study the accelerated fatigue crack nucleation and
fatigue crack growth rate at elevated temperatures.
WHERE
Q = APPARENTACTIVATIONENERGY
R = GAS CONSTANT
CD-88-31863
3-[90
RADIOACTIVENICKELPENETRATIONAT GRAINBOUNDARY
Grain boundary morphology and the chemical elements in a grain boundary may
also cause the wide variation in the grain boundary oxide penetration. This
wide variation maycause wide variations in the rates of crack nucleation and
growth and fatigue life. Grain boundary penetration by impurities (radioac-
tive nickel) is a function of the orientations of neighboring grains as shown
below.
PENETRATION,
#m
200
100t 0
] f _ LATTICE PENETRATION
I-_- : .......... "--
I I I I I I I
0 15 30 45 60 75 90
BOUNDARYANGLE, deg
CD-88-31864
3-181
STATISTICALDISTRIBUTIONOFGRAINBOUNDARYOXIDEPENETRATION
The statistical scatter of the grain boundary oxide penetration depth follows
Weibull's distribution law as shownbelow. The variation of the calculated
values reflects the statistical scatter of the measuredoxide values. The
empirical distribution law can be used to extrapolate the data obtained in a
laboratory by using small samples to a much larger structural component in
service.
99.9
99
90
6O
PROBABILITY 40
OF FINDING
o_< ozi, 20
PERCENT 10
4
2-
.1
Zf
(LOCATION PARAMETER)-----,-
I
.2
, I ,I,l,J I , I , IIIII I
.4 .6 1 2 4 6 10 20x10 -4
VALUE OF THE CONSTANT _i FOR A GIVEN OXIDE DEPTH ami
CD-88-31865
3-18 o
EFFECTOFPRECRACKONLOW-CYCLEFATIGUE
Whenan oxide reaches a critical size under given loading conditions, it will
fracture. A grain boundary oxide crack becomesa fatigue crack nucleus, or a
precrack. The precrack will shorten the crack nucleation period and the total
fatigue life. This reduction in nucleation life will be significant if the cy-
clic frequency is very low, the temperature is very high, and the oxidation ex-
posure time is very long.
The ratio between the fatigue life of a precracked specimen and the fatigue
life of a smooth specimen is a function of the precrack size:
Nfi/Nfo = f(precrack size)
where Nfi is the fatigue life of a precracked specimen and Nfo is the fa-
tigue life of a smooth specimen. The functional relationship between the
ratio and the precrack size can be found empirically.
The wide variations in oxidation penetration and oxide crack size may cause a
wide variation in Nfi and the total fatigue life. A large structural compo-
nent has a high probability of having a large oxide crack and a short fatigue
life. The empirical relation of the equation, together with the grain bound-
ary oxide penetration kinetics and the Weibull distribution, would be able to
predict the effect of the oxide crack on the nucleation life of a structural
component.
In conclusion, quantitative studies based on the physical damagemechanisms
will lead to an improved life-prediction methodology.
DEFORMATION
WORK DENSITY
RANGE,
_,o-2 + o.yc,A_p
8E
101L _..,,.,,,___ A PRECRACKED SPECIMEN
100
10 -1 J J I t l,llJ I I I I I I Ill I I I I lalll
101 102 103 104
FATIGUE LIFE, Nf, CYCLES
CD-88-31867
3-18L_
FREQUENCY EFFECTS ON FATIGUE CRACK GROWTH RATE AT HIGH TEMPERATURE
The basic concept of grain boundary oxidation kinetics was used to analyze
fatigue crack growth rate at the elevated temperatures. A high-temperature
fatigue crack growth model based on intermittent microruptures of grain bound-
ary oxides was proposed. The model is consistent with the observed intergranu-
lar fracture and the observed inverse relationship between crack growth rate
and the cyclic frequency in the low-frequency region as shown in the figure
(0shida and Liu, 1984 and 1985), Liu and Oshida (1985 and 1986). In the high-
frequency region fatigue failure couiJ be mixed intergranular and transgranular
or transgranular entirely.
10o
10 -1
10 -2
da/dN,
mmlcycle 10 3
10 -4
MATERIAL TEMP., ,_K, WAVEFORM
°C MPa\/m
O INCONEL718 649 28
• INCONELX-750 650 30
__ _ INCONELX-750 650 30
A ASTROLOY 760 50
'_ • ASTROLOY 650 50
,,%_._ /k ASTROLOY 700 10
1_t_._"_ [] 304 S.S. 538 30
\ • WASPALOY 649 30
\ 10
__,_ 21/4Cr-MoSTEEL 565 10
[] zx
v V
lo-5 I I I I I I
10 -4 10-3 10 -2 10 -1 100 101 102
FREQUENCY, (cyclelsec)
CD-88-31866
3-183
REFERENCES
Liu, H.W., and Oshida, Y., 1984, "Literature Survey on Oxidation and Fatigue
Lives at Elevated Temperatures," NASACR-174639.
Liu, H.W., and Oshida, Y., 1985, "Grain Boundary Oxidation and Oxidation
Accelerated Fatigue Crack Nucleation and Propagation," Proceedings of the
MinnowbrookConference on Life Prediction for High Temperature Gas Turbine
Materials, Blue Mountain Lake, NY.
Liu, H.W., and Oshida, Y., 1986, "Grain Boundary Oxidation and Fatigue Crack
Growth at Elevated Temperatures," Theoretical and Applied Fracture Mechan-
ics, vol. 6, no. 2, pp. 85-94.
Oshida, Y., and Liu, H.W., 1984, "Oxidation and Low Cycle Fatigue Life Predic-
tion," Turbine Engine Hot Section Technology - 1984, NASA CP-2339,
pp. 321-329.
Oshida, Y., and Liu, H.W., 1985, "Grain Boundary Oxide Crack, Oxidation Accel-
erated Fatigue Crack Growth and Fatigue Life," Proceedings of the 40th
Meeting of the Mechanical Failures Prevention Group, Symposium on the Use
of New Technology to Improve Mechanical Readiness, Reliability and Main-
tainability, National Bureau of Standards, Gaithersburg, MD, Cambridge Uni-
versity Press, pp. 287-295.
0shida, Y. and Liu, H.W., 1988, "Grain Boundary Oxidation and an Analysis of
the Effects of Pre-oxidation on Subsequent Fatigue Life," ASTM Symposium on
Low Cycle Fatigue - Direction for the Future, ASTM STP 942, pp. 1199-1217.
Q _ _ 3-185


Grain boundary oxidation and low-cycle fatigue at elevated temperatures

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