Influence of structure and mechanical properties for cyclic fracture rates of cast iron/Strukturos ir mechaniniu savybiu itaka ketaus ciklinio irimo rodikliams.
Leonavicius, M.K. ; Krenevicius, A. ; Bacevicius, J. 等
1. Introduction
There is a lot of information about the resistance of cast iron to
cyclic loads when a number of cycles exceeds [10.sup.8] [1-7]. Some
consistent patterns and peculiarities were observed during experiment
data analysis to be used for the purpose of applying the gained results
for the calculation of designed durability. Current paper presents the
already accomplished achievements as well as the analysis of further
investigations by determining the limits of the fatigue crack growth
threshold at very low speeds.
The durability of such elements exceeds 25 years, and the number N
of loading cycles enters the giga-cycle range N > [10.sup.8] cycles.
The casting of hull details from grade iron and gears from spherical
graphite iron and the following heat treatment are connected with
certain pronounced problems. To achieve the required microstructure, it
is necessary to apply the heat treatment normalization, which has to
guarantee a microstructural change throughout all the large volume.
During the normalization process it is necessary to obtain
microstructures with a pearlite matrix, but usually there remains some
part of ferrite, which is mild and undesirable. The ferrite matrix
around graphite nodules produces a negative influence on the mechanical
properties: it reduces hardness and strength of the structure.
Austempered ductile iron (ADI) is a spheroid graphite iron produced
by using special stages of heat treatment, which comprise the production
of a ductile cast iron, austenisation (at about 800-1000[degrees]C),
followed by quenching of the temperature (at about 250-450[degrees]C),
suitable for the final stage, and the isothermal transformation
(austempering) of some of the austenitic matrix to other phases before
subsequent cooling till it reaches 20[degrees]C. By varying the heat
treatment parameters and changing the obtained microstructure, the
mechanical properties can alter significantly ([[sigma].sub.u] =
800-1600 MPa). Using both the metallurgical and technological means,
composition of the cast iron is optimised, the intergrain surfaces are
cleaned from impurities. While the casting is held at the austempering
temperature, the nucleation and growth of accrular ferrite occurs,
accompanied by the transition of carbon into austenite. The resulting
microstructure, known as "Ausferrite", gives ADI its special
attributes. Ausferrite exhibits twice the strength of a given level of
ductility, if compared to the pearlitic, feritic or martensitic
structures formed by conventional heat treatments.
2. Resistance to cyclic fracture
Stress intensity factor K and load cycle asymmetric factor R =
[K.sub.min]/[K.sub.max] are the parameters defining the state of stress
and strain in the top crack of the elastic body and determining the
crack's propagation consistent pattern in cyclic fracture
mechanics. Parameter K places the state and size of load, shape and size
of the crack and the body. Therefore, during consistent patterns
research of the fatigue fracture, it is enough to use the fatigue crack
growth rate da/dN dependence either on stress-intensity factor range
[DELTA]K = (1 -R) [K.sub.max] either on maximum stress-intensity factor
[K.sub.max]
da/dN = V = f ([K.sub.max], [K.sub.min]/[K.sub.max], C m) (1)
where a is the propagating fatigue crack depth, N is the number of
load cycles, C and m are constants.
[FIGURE 1 OMITTED]
The typical kinetic fatigue diagram, presented in logarithmic
scale, is shown in Fig. 1. The diagram is limited by the stress
intensity factor value AKth (fatigue crack growth threshold) on the
left. The crack does not propagate below this factor or its very slow
propagation is taking to be acceptable. Critical cyclic stress intensity
factor Kfc limits the diagram on the right side. Rude fracture of
specimen takes place after above mentioned value has been reached.
Usually kinetic fatigue diagram is charted by testing specimen either
through the special eccentrically-loaded tension or bending. It is
possible to test constructive elements. The experiment begins by making
crack growth as shown by pointer A in Fig. 1. The microstructure of the
material, stress and indicators of ambient impact make a big influence
in sector 1, the so-called low speed sector. It is accepted that the
crack does not grow when the speed is below than [10.sup.-10] m/cycle in
standard, regulating cyclic strength tests. The influence of
microstructure decreases in sector 2 (medium speeds sector), but the
load and ambient indicators gain strength. The steepness the line in the
sector 2 (connected with different m values in well-known Paris
equation) the faster the crack's growth becomes in current
material. There is a big influence of the body thickness, microstructure
and the main stress value in the final fracture (high fatigue speeds)
sector 3 [2,8,9].
The big quantity of microscopic points of plastic strain appears in
spheroidal cast iron before the main crack formation. The following
points become the focuses of microscopic fatigue cracks. This phenomenon
is caused by graphite cores, microstructure defects and internal voids.
Under the cyclic load, when the maximum stress is lower than cyclic
durability, the fatigue cracks originate, but do not propagate beyond
the particular length. Such nonpropagatable cracks show that the cyclic
strength is not a limit of stress, under which the fatigue cracks do not
form, but indicate the material's ability to resist the propagation
of the originated cracks. When a crack originates in the graphite core
environment of spheroidal graphite cast iron it is possible to consider
that the stress in the core is small in comparison with the stress in
the matrix. The stress and the limit stress under which fatigue cracks
originate but do not propagate, is 2-3% less than cyclic durability
limit [1, 2].
Various graphs present the propagation of short cracks in the
typically kinetic fatigue diagram (Fig. 1). The experimental analysis is
insufficient for the determination of further crack propagation taking
place in this range.
Such a limit may not be sufficient if the durability of the design
equipment exceeds 25 years. Therefore the fatigue strength research is
relevant to the limit approaching to [10.sup.-12] m/cycle which
corresponds to its longer service life (25-50 years) [3].
If parameter AKth is used for cyclic strength and durability
calculations, then the stress intensity factor and cracks growth rate
diagrams are drawn, but the kinetic fatigue diagram not. In the latter
case the crack growth is suspended as shown by the marker B in Fig. 1.
The data about the resistance to cyclic load of different cast
irons when AKth dimensions are at v = 5 x [10.sup.-12] m /cycle are
presented in the works [3-5]. Larger results dispersal than at v = 1 x
[10.sup.-10] m /cycle have been observed as shown in Fig. 1 (lined
sector of diagram).
3. Structure and mechanical properties
For the experiment four as cast iron and four normalized cast iron
plates of all kinds have been prepared. The chemical compositions of the
investigated irons are presented in Table 1.
The chemical compositions, produced the setting and the casting
procedures were carried out in the foundry, and differ from the well
known similar cast irons used in the USA, Germany, Australia and
Finland.
During heat treatment, the investigated plates were settled at
various sites of the furnace beside large sized details. Microstructure
of the cast iron 1 is shown in Fig. 2, a. Graphite is present in the
shape of flakes of different size; the structural matrix is represented
by ferrite-pearlite. At some points the spherical graphite gets formed.
Microstructure of the normalized cast iron 1 is shown in Fig. 2, b. The
microstructure is fine-grained, the graphite is spherical, and the base
of the microstructure shows pearlite with a small amount of ferrite.
The microstructure of the cast iron 2 is shown in Fig. 3, a. It
consists of flakes of different size and spherical graphite. The matrix
is pearlite with irregularly situated ferrite, although sometimes
bainite were observed. The microstructure after normalizing is shown in
Fig. 3, b. It consists of spherical graphite, small-grained pearlite,
bainite and ferrite. As we can see, normalizing makes the microstructure
uniform and fine. The obtained nodular graph ite is of a similar size.
Microstructure of the cast iron 3 and 4 is shown in Fig. 4. The
matrix is bainite. Graphite is spheroidal and of various sizes,
anomalous shape (shape of flakes) also can be noticed.
From all the plates, tension specimens are fabricated. Mechanical
properties are defined; they are presented in Table 2.
CT specimens have been produced from all the
plate-materials-altogether six types of specimens characterised by
different mechanical properties (Table 2). Cylindrical tensile specimens
taken from some broken specimens for the specification of mechanical
properties have been produced.
Hardness of the cast iron 1 is 250-277 BHN, of the normalized is
228-256 BHN, and hardness of the cast iron 2 is 289-311 BHN, and of the
normalized one is 290311 BHN. Hardness of the cast iron 3 is 318-323 BHN
and 299-311 BHN for cast iron 4. As we can see, in the process of heat
treatment, the hardness changes slightly. The heat treatment process
consists of normalization and annealing, performed when following the
requirements set technological processes for of the real construction.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
4. Analysis of experimental results
In order to obtain the cracking threshold, standard test method
(ASTM E-647-00) has been modified and applied for the experimental
research of eccentrically loaded specimens (CT). The proposed experiment
implementation and the postprocessing modification of the resulting data
enables researchers to estimate the void's influence on the
cracking threshold.
The total number of the cycles for austempered ductile iron
specimens varies from 11.4 to 20.3 million cycles, for pearlitic cast
iron--from 10.0 to 19.6 million cycles.
One of the basic quantitative crack resistance indices is the
cracking threshold limit stress intensity factor range
[DELTA][K.sub.th]. The performed research is remarkable due to the fact
that the obtained values of the limit stress intensity factor range are
obtained at lower crack propagation rates than the standard test method
requires. The propagation rates are decreased to v [approximately equal
to] 5 x [10.sup.-12] m/cycle in order to obtain the cracking threshold
corresponding to a longer lifetime of the parts. To be able to calculate
the stress intensity factor, the following formula is applied
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
K = F/B[square root of W] f (a/W) (2)
where f (x) = [(2+[lambda])/[(1-/[lambda]).sup.2/3] 3 (p(l),X=
a/W],
[phi]([lambda]) = 0.866 + 4.64[lambda]- 13.32[[lambda].sup.2]
+14.72[[lambda].sup.3] -5.6[[lambda].sup.4]
and F is tension force, B is specimen width, W is specimen base
length, a is crack length.
The front of the crack propagation is different in each specimen
because of different reasons (mainly due to the inhomogenity of the
structure and its porosity).
The crack growth rate versus stress intensity factor range diagrams
have been following the experimental research results of all 6 sorts CT
specimens made of cast iron. Fig. 5 presents the cast iron 1 diagram: as
cast iron 8 specimens, normalized -8 specimens. There are data of the
cast iron 2 in Fig. 6: as cast iron -4 specimens, normalized -4
specimens. Fig. 7 presents data of the cast iron 3 and 4 iron: ADI-1 -5
specimens, ADI-2 -4 specimens.
[FIGURE 7 OMITTED]
During the experimental research, it has been observed that
[DELTA]Kth rely in crack's depth for some specimens. Additional
tests have been performed for CT specimen of cast iron 2.
[DELTA][K.sub.th] versus v diagram is presented in Fig. 8. Fracture of
CT specimen with fixed crack's depth 16.2; 18.5; 21.1; 25.2; 29.2
mm is shown in Fig. 9. If [DELTA][K.sub.th] [approximately equal to]
[approximately equal to] 9.8 MPa [square root of m] at the speed v =
[10.sup.-10] m /cycle, when approaching to speed v = 5 x [10.sup.-12] m
/cycle--[DELTA][K.sub.th] [approximately equal to] 8.79.9 MPa [square
root of m]. There we observe different [DELTA][K.sub.th] values at
different crack's depth in the diagram.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Additional tests have been performed with CT specimens of ADI-2
cast iron. Width of the specimen was reduced to 12.5 mm in order to
stabilise the crack's front e. g. to reduce its obliquity.
It is observable that [DELTA][K.sub.th] values practically coincide
at different depth of crack 15.4 mm; 22.5 mm and 24.7 mm for specimen
ADI2-a1. Measured [DELTA][K.sub.th] values at 18 mm, 26.4 mm and 33.6 mm
depth of coincide also with specimen ADI2-a2. Dispersal of
[DELTA][K.sub.th] results is small--8.28.4 MPa [square root of m]. The
obtained values are close to minimal [DELTA][K.sub.th] presented in Fig.
7.
5. Static fracture of compact specimens
After crack growth rates versus stress intensity factor range have
been designed (Figs. 5-8), compact specimens with cracks were used to
measure the plane strain fracture toughness. For this purpose, the
static fracture tests have been performed.
The measurement of plane strain fracture toughness [K.sub.1C] value
is not always possible, but the measured data are noteworthy and could
be used to evaluate the material's suitability for production of
structural elements (fracture strength). It is auxiliary information for
the calculation of structural elements following the main criterion
threshold stress intensity range [DELTA][K.sub.th].
[FIGURE 10 OMITTED]
When performing static fracture of CT specimen's (Fig. 10),
the measuring system records the value of alternate force F and
crack's opening. Hardness of each CT specimen has been additionally
measured (at points 2). Tensile specimens have been produced from cut
part 1 and indicators of mechanical properties have been revised. After
the analysis of crack's opening diagram, the value of force
[F.sub.Q] received becomes possible to use when calculating [K.sub.1C]
or [K.sub.C]. Works [1,9] present three types of crack's opening
diagrams and introduce the appropriate calculating technique. Fig. 11
presents the static fracture diagram of cast iron 1 specimen. The
diagram is specific because of the topical force maximum which is
considered [F.sub.Q] value. This force is suitable for the calculation
of plane strain fracture toughness K1 c value if there is an adequate
chart point between the lines 0-a and 0-b and [F.sub.Q] > [F.sub.5],
the same as [F.sub.max]/[F.sub.Q] [greater than or equal to] 1.1. If the
topical force maximum is on the right side of the line 0-b, it is
possible to calculate KC value only when using the measured [F.sub.Q]
value.
The calculating value of crack's length [L.sub.cr], is de
termined by three measurements of fracture (at centre and sides of
specimen). The average of these three measurements is [L.sub.cr]
calculating value. [K.sub.1C] can be calculated only if the following
conditions are met
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)
where [L.sub.cr,max] and [L.sub.cr,min] are maximum and minimum
values of measured crack's length.
[FIGURE 11 OMITTED]
When calculating value [L.sub.cr] the determined following
condition should be verified
0,45 [less than or equal to] a [less than or equal to] 0,55W (4)
This condition indicates the limits where the formula for the
calculating of stress intensity rate K1 for compact specimen may be used
[K.sub.1] F/B[square root of W]] f (a/W) (5)
where
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)
The above-mentioned conditions should be verified when using
calculated value and other known or de termined indicators of the
specimen and material. If the conditions are satisfied--the calculated
[K.sub.C] value is plane strain fracture toughness indicator [K.sub.1C].
If one of the conditions does not match, the calculated [K.sub.C] is the
critical stress intensity factor only (in literature it is sometimes
mentioned as the conditional critical stress intensity factor [K.sub.C])
[8-12].
The critical stress intensity factor Kq is determined for CT
specimens of cast and normalized iron 1, 2 and 3, 4. When one of the
mentioned conditions does not match the requirements the fracture
toughness is determined if all the conditions correspond. The results
are presented in Table 3.
According to experimental research the crack growth rate versus
stress-intensity factor range has been designed (Fig. 12).
[FIGURE 12 OMITTED]
The picture presents critical stress intensity factor [K.sub.C].
The current part of the diagram includes all the static break results of
CT specimens ([K.sub.1C]--plane strain fracture toughness,
[K.sub.C]--critical stress intensity for static failure).
5. Conclusions
1. The analysis of the experimental analytical research of critical
stress intensity factors (cracking threshold and fracture toughness) for
6 types of cast iron has been performed. Mechanical properties and the
structure have been measured.
2. The regulated methodology for critical stress intensity factors
(cracking threshold) measurement has been changed in so that the results
would be used for the increase of the durability of big size cast iron
parts.
3. The experimental research analysis shows that the cracking
threshold of the same cast iron differs in quite a wide range while
decreasing the cracking speed from 1 x [10.sup.-10] m /cycle to 5 x
[10.sup.-12] m /cycle.
4. The conditional critical stress intensity factor which was
determined for CT specimens with crack could be used as auxiliary
criteria when calculating cyclic strength and confirming the material
suitability for the structure elements if the calculation methodology is
based on the main criteria--cyclic cracking threshold.
Received February 15, 2010 Accepted April 10, 2010
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M. K. Leonavicius *, A. Krenevicius **, J. Bacevicius ***
* Vilnius Gediminas Technical University, Sauletekio 11, 10223
Vilnius, Lithuania, E-mail: mindaugas.leonavicius@vgtu.lt
** Vilnius Gediminas Technical University, Sauletekio. 11, 10223
Vilnius, Lithuania, E-mail: kron@vgtu.lt
*** Vilnius Gediminas Technical University, Sauletekio 11, 10223
Vilnius, Lithuania, E-mail: jonas.bacevicius@gmail.com
Table 1
Chemical composition
Cast iron C Si Mn Ni Mo Cu Cr
%
1 3.64 1.75 0.870 0.497 0.520 0.518 0.062
2 4.96 1.17 0.083 0.680 0.190 1.010 --
3 3.50 2.24 0.353 2.920 0.407 0.800 0.407
4 3.61 2.22 0.351 1.968 0.395 0.805 0.395
Table 2
Mechanical properties
Ultimate
Yield strength tensile strength Elongation
Cast iron [R.sub.p0.2], MPa [R.sub.m], Mpa [A.sub.5], %
1 As cast 443 to 503 622 to 671 1.80 to 2.43
Normalized 454 to 467 684 to 727 6.13 to 10.30
2 As cast 630 to 634 863 to 933 3.70 to 6.70
Normalized 607 to 621 862 to 882 3.60 to 4.00
3 ADI-1 834 to 845 1034 to 1077 8.90 to 11.20
4 ADI-2 699 to 705 918 to 952 4.30 to 5.70
Table 3
Critical stress intensity factors of different types of cast iron
Cast iron [K.sub.C], MPa [square root of m]
1 as cast 28.7-38.8
normalized 38.1-40.3
2 as cast 2.0-30.9
normalized 28.4-32.6
3 ADI-1 60.8-78
4 ADI-2 59.5-67
