Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2002-04-12
2004-01-27
Lefkowitz, Edward (Department: 3747)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06681639
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the technical field of thermally highly stressed members, such as for example components (pistons or the like) of internal combustion engines, gas turbine blades, combustion chambers etc.
PRIOR ART
Thermally highly stressed components from the high-temperature region of gas turbines, such as for example blades or vanes, are coated for two reasons:
to protect the blade or vane material from corrosive attacks, and
to reduce the temperatures of the metal to a level, which can be withstood.
Usually, two coatings are applied to the base material. The first one is known as the “overlay” coating, which protects against corrosion. The second coating, which is usually referred to as the thermal barrier coating (TBC) and is only applied if need be, serves as the aforementioned thermal isolation.
However, components coated in such a way may also become defective for various reasons: mismatches with respect to the coefficients of thermal expansion between the various materials of which the member consists can cause thermal stresses and deformations in the system. Furthermore, instances of scaling due to oxidation and growth thereof at relatively high temperatures can produce additional stress loads.
These stresses and deformations can finally lead to cracking and spalling of the coating layers. According to
FIG. 5
, the coating system
10
is therefore usually made up of four different layers:
the base material
11
, which is usually several mm thick,
the bond coat
12
, which has a thickness of about 0.1 . . . 0.3 mm,
a thermally grown oxide (TGO) layer
13
, which grows to a thickness of 0.02 mm, and
the thermal barrier coating
14
, which is approximately 0.2 . . . 0.4 mm thick.
In-depth investigations of the stress/deformation behavior have already been carried out on the basis of finite-element networks, in which all four elements of the coating system were modeled in all details, including a nonlinear material behavior. It has been found in these investigations that both the thermal growth of the oxide layers and the creep behavior of the bond coats play a major part in the formation of defects (see in this respect, for example, the article by Freborg, A. M., Ferguson, B. L.: ‘Modelling oxidation induced stresses in thermal barrier coatings’. Material Science and Engineering A245, 1998, pages 182-190). However, the results of these investigations cannot be used for predicting lifetimes.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to specify a simplified method of estimating the lifetime of a thermal barrier coating, which also takes into account the part played by the changing oxide layer.
The object is achieved by the entirety of the features of claim 1. The essence of the invention is to use in the calculation of the number N
i
of cycles to failure material parameters C(&dgr;
ox
) and m(&dgr;
ox
) which depend on the thickness (&dgr;
ox
) of an oxide layer, which is located between the thermal barrier coating
14
and the bond coat
12
and grows with cyclical loading.
The calculation is particularly simple here if, according to a preferred refinement of the method according to the invention, the dependence of the material parameters C(&dgr;
ox
) and m(&dgr;
ox
) on the thickness (&dgr;
ox
) of the oxide layer is assumed to be linear, if furthermore a growth law of the form
&dgr;
ox
=k
p
t″
with a growth constant k
p
and an exponent n is used for the increase in the thickness (&dgr;
ox
) of the oxide layer with time t, if a damage increment &Dgr;D, which satisfies the approximation formula
Δ
D
(
N
)
≈
1
C
(
N
)
(
Δσ
n
)
-
m
(
N
)
is calculated, N giving the number of loading cycles, and C(N) and m(N) being parameters which satisfy the equations
C
(
N
)=&agr;
c
(
NT
)″+&bgr;
c
and
m
(
N
)=&agr;
m
(
NT
)″+&bgr;
m
with the exponent n, the constants &agr;
c
, &agr;
m
, &bgr;
c
, &bgr;
m
, and the holding time T at high temperature per loading cycle, and if the number of loading cycles to failure N
i
of the member is determined by the damage increment being summed up in accordance with the formula
D
=
∑
N
=
1
N
i
Δ
D
(
N
)
until D has reached the value 1.
Further embodiments are disclosed in the dependent claims.
REFERENCES:
patent: 4169742 (1979-10-01), Wukusick et al.
patent: 4388124 (1983-06-01), Henry
patent: 4643782 (1987-02-01), Harris et al.
patent: 5270123 (1993-12-01), Walston et al.
patent: 5366695 (1994-11-01), Erickson
patent: 5470371 (1995-11-01), Darolia
patent: 5540790 (1996-07-01), Erickson
patent: 5625153 (1997-04-01), Sawai et al.
patent: 5888451 (1999-03-01), Konter et al.
patent: 6007645 (1999-12-01), Cetel et al.
patent: 0937786 (1999-08-01), None
patent: 1031637 (2000-08-01), None
Robert A. Miller, “Oxidation-Based Model for Thermal Barrier Coating Life”, Journal of the American Ceramic Society, vol. 67, No. 8, pp. 517-521, Aug. 1984.
Bernhardi Otto
Muecke Roland
Schmutzler Hans Joachim
Sommer Christoph
Sommer Marianne
Alstom (Switzerland) Ltd.
Cermak Adam J.
Lefkowitz Edward
Mack Corey D.
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