Aspects of structural degradation in old bridge steels by means of fatigue crack propagation
The paper presents conclusions drawn from studies related to old steel structures, especially those erected on the turn of the 19th century. The objects of interest of the authors were Wroclaw Pomorskie Bridges: the Central Pomorski Bridge and the North Pomorski Bridge (1885, 1930 respectively), as...
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Lesiuk, G. Szata, M. 2018-06-18T08:27:48Z 2018-06-18T08:27:48Z 2011 Aspects of structural degradation in old bridge steels by means of fatigue crack propagation / G. Lesiuk, M. Szata // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 1. — С. 76-81. — Бібліогр.: 11 назв. — англ. 0430-6252 https://nasplib.isofts.kiev.ua/handle/123456789/138127 The paper presents conclusions drawn from studies related to old steel structures, especially those erected on the turn of the 19th century. The objects of interest of the authors were Wroclaw Pomorskie Bridges: the Central Pomorski Bridge and the North Pomorski Bridge (1885, 1930 respectively), as well as the Sand Bridge (1861). The material used for their construction was puddled steel or cast steel. In the course of long operation the steels (especially the puddled one) show susceptibility to degradation processes. In this paper the results of metallographic tests (light microscopy, SEM) and mechanical properties tests (hardness measurement, static tensile test) presenting the state of structural degradation have been presented. Also, the initial study results for the puddled steel coming from the Sand Bridge and concerning development of a fatigue crack have been presented. Basic quantities describing the kinetics of fatigue crack growth have been determined. Досліджено сталі мостів біля Вроцлава (Польща): дві пудлингові, експлуатовані з 1861 і 1885 рр., а також ливарну (1930 р.). Сталі, особливо пудлингові, чутливі до деградаційних процесів, що проявилось у зміні структури і механічних властивостей, найвідчутніше – у зниженні опору втомному росту тріщини. Исследованы стали мостов в районе Вроцлава (Польша): две пудлинговые, эксплуатируемые с 1861 и 1885 гг., а также литейную (1930 г.). Стали, особенно пудлинговые, чувствительны к деградационным процессам, что проявилось как в изменении структуры, так и механических свойств, наиболее значительно – в понижении сопротивления усталостному росту трещин. en Фізико-механічний інститут ім. Г.В. Карпенка НАН України Фізико-хімічна механіка матеріалів Aspects of structural degradation in old bridge steels by means of fatigue crack propagation Аспекты структурной деградации сталей старых мостов, вызванной распространением усталостной трещины Аспекти структурної деградації сталей старих мостів, спричиненої поширенням втомної тріщини Article published earlier |
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Aspects of structural degradation in old bridge steels by means of fatigue crack propagation |
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Aspects of structural degradation in old bridge steels by means of fatigue crack propagation Lesiuk, G. Szata, M. |
| title_short |
Aspects of structural degradation in old bridge steels by means of fatigue crack propagation |
| title_full |
Aspects of structural degradation in old bridge steels by means of fatigue crack propagation |
| title_fullStr |
Aspects of structural degradation in old bridge steels by means of fatigue crack propagation |
| title_full_unstemmed |
Aspects of structural degradation in old bridge steels by means of fatigue crack propagation |
| title_sort |
aspects of structural degradation in old bridge steels by means of fatigue crack propagation |
| author |
Lesiuk, G. Szata, M. |
| author_facet |
Lesiuk, G. Szata, M. |
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2011 |
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English |
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Фізико-хімічна механіка матеріалів |
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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Article |
| title_alt |
Аспекты структурной деградации сталей старых мостов, вызванной распространением усталостной трещины Аспекти структурної деградації сталей старих мостів, спричиненої поширенням втомної тріщини |
| description |
The paper presents conclusions drawn from studies related to old steel structures, especially those erected on the turn of the 19th century. The objects of interest of the authors were Wroclaw Pomorskie Bridges: the Central Pomorski Bridge and the North Pomorski Bridge (1885, 1930 respectively), as well as the Sand Bridge (1861). The material used for their construction was puddled steel or cast steel. In the course of long operation the steels (especially the puddled one) show susceptibility to degradation processes. In this paper the results of metallographic tests (light microscopy, SEM) and mechanical properties tests (hardness measurement, static tensile test) presenting the state of structural degradation have been presented. Also, the initial study results for the puddled steel coming from the Sand Bridge and concerning development of a fatigue crack have been presented. Basic quantities describing the kinetics of fatigue crack growth have been determined.
Досліджено сталі мостів біля Вроцлава (Польща): дві пудлингові, експлуатовані з 1861 і 1885 рр., а також ливарну (1930 р.). Сталі, особливо пудлингові, чутливі до деградаційних процесів, що проявилось у зміні структури і механічних властивостей, найвідчутніше – у зниженні опору втомному росту тріщини.
Исследованы стали мостов в районе Вроцлава (Польша): две пудлинговые, эксплуатируемые с 1861 и 1885 гг., а также литейную (1930 г.). Стали, особенно пудлинговые, чувствительны к деградационным процессам, что проявилось как в изменении структуры, так и механических свойств, наиболее значительно – в понижении сопротивления усталостному росту трещин.
|
| issn |
0430-6252 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/138127 |
| citation_txt |
Aspects of structural degradation in old bridge steels by means of fatigue crack propagation / G. Lesiuk, M. Szata // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 1. — С. 76-81. — Бібліогр.: 11 назв. — англ. |
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76
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2011. – ¹ 1. – Physicochemical Mechanics of Materials
ASPECTS OF STRUCTURAL DEGRADATION IN OLD BRIDGE STEELS
BY MEANS OF FATIGUE CRACK PROPAGATION
G. LESIUK, M. SZATA
Institute of Materials Science and Applied Mechanics, Wroclaw University of Technology, Poland
The paper presents conclusions drawn from studies related to old steel structures, espe-
cially those erected on the turn of the 19th century. The objects of interest of the authors
were Wroclaw Pomorskie Bridges: the Central Pomorski Bridge and the North Pomorski
Bridge (1885, 1930 respectively), as well as the Sand Bridge (1861). The material used for
their construction was puddled steel or cast steel. In the course of long operation the steels
(especially the puddled one) show susceptibility to degradation processes. In this paper the
results of metallographic tests (light microscopy, SEM) and mechanical properties tests
(hardness measurement, static tensile test) presenting the state of structural degradation
have been presented. Also, the initial study results for the puddled steel coming from the
Sand Bridge and concerning development of a fatigue crack have been presented. Basic
quantities describing the kinetics of fatigue crack growth have been determined.
Keywords: fatigue crack growth rate; puddled steel; microstructural degradation.
Using published data [1–7] and own research of the old steel structures the problem
of worsening the usefulness of such structures is appearing. The Degradation Theory
initiated at the Wrocław University of Technology in the nineties of the last century [1]
was dealing initially with problems related to surface mining machines. Its deve-
lopment has led to systematic analysis of the machine (structure) – environment sys-
tem. In the course of works several problems involved in steel structural degradation
have been documented [2, 3]. The problem is becoming more serious when referred to
bridge structures from the turn of the 19th century. In particular, it concerns the puddled
steel which, as the studies show, is more susceptible to structural degradation processes
than the cast steel. Material aspects of the Degradation Theory have been expressed in
[2, 6]. The essential issue in the old structure studies is evaluation of a steel part wear
degree. Lack of comparison material – the modern steel production technologies are far
different from those in the centuries past – makes the evaluation of a structural part
wear difficult. The only solution elaborated within the Degradation Theory and its
material aspects [2, 7] is assuming the normalised state as the level for comparison.
The steel from Wroclaw Pomorskie Bridges: puddle steels of the Sand Pomorski
Bridge (year 1861), the Central Pomorski Bridge (1885) and cast steel of the North
Pomorski Bridge (1930), all after-operation state, were the objects of the study. The
goal of this work is investigation of the influence the structural degradation processes
on the kinetics of fatigue cracking. According to the above the basic mechanical pro-
perties; yield stress σ0.2, ultimate tensile stress σB, Young Modulus E, uniform elonga-
tion δu, reduction in area ψ (for Sand Bridge) have been obtained from static tensile
test. The threshold stress intensity factor values ∆Kth were obtained from the fatigue
crack growth for experimental data. All investigations were performed in two states;
P – after operating state and the normalised (N) one.
Examples of structural degradation. In order to understand the further part of
this work the puddled steels and their basic mechanical properties will be described. The
Corresponding author: G. LESIUK, e-mail: Grzegorz.Lesiuk@pwr.wroc.pl
77
issue is focused on this kind of the old metallic material because many old riveted
structures (still operating) are erected from puddled steel. So firstly, it was the wrought
iron (later called a puddled steel – as at that time the concept of steel was not clearly
defined yet) was produced with the puddle method. It was produced in small batches
(of some 200…500 kg in weight) in the puddling furnace, in which the pig iron in a
solid state was partially melted with hot gas created in the process of burning hard coal
or coke. Due to the oxidizing influence of combustion gases, a carbon was burned out
from the pig iron in the first place. That way the process was rising the alloy solidifica-
tion temperature. The slag at the alloy surface required permanent mixing, in order to
provide access of the oxidizing gases to melted metal. The steel production process
was called puddling (Eng. puddled steel). At a relatively low temperature obtained at a
puddling furnace (about 1400°C), a solidification of decarbonised pig iron took place,
where it created a lump in the doughy state with low carbon contents but plenty of slag
contaminations [8, 9]. The essential property of structures created from puddled steel
was high heterogeneity of their chemical composition and its laminar structure (Fig. 1).
Fig. 1. Puddled steel – characteristic fracture of this steel after static tensile test, a flat sample frac-
ture view to the left, a magnified fragment of the area marked with the frame to the right – visible
numerous de-laminations and characteristic lamellar fracture structure (a); fracture after fatigue
crack growth tests – on the left the fatigue crack growth area, on the right the static fracture (b).
Most probably, it resulted from small batches placed in a puddling furnace. In ad-
dition, the technology itself caused high contamination of the steel. All that influenced
creation of local material flaws and extremely wide distribution of statistic results.
Such situation was not neutral to the macroscopic properties of the steel – the puddled
steel was characterised with diversification of elongation and yield and ultimate tensile
strength both in the rolling direction, as well as in the direction perpendicular to it.
The level of σ0.2 of puddled steel is about 280…310 MPa, total elongation δ is
between 7…25%. Papers [9] indicate the significant fluctuations related to E ∼ 170…
200 GPa, though results such as E = 132.8 GPa were also recorded.
From [8] it results that puddled iron (the puddled steels) produced in the second
half of 19th century had the following parameters: density g = (7.6…7.8) Mg/m3; carbon
contents C = (0.05…0.1)%; phosphorus contents P ∼ 0.4%; σ0.2 = 210…290 MPa, elastic
limit σH = 150…160 MPa, σB = 300…400 MPa, elongation δ5 = 8…25%, all values for
the rolling direction; E = 200…215 GPa. In the rolling direction the values are signifi-
cantly higher than in the direction perpendicular to it. Degradation processes taking
place in old steels consist, among others, on decomposition of pearlite to ferrite and
carbides, nitrides and carbides separations inside grains and at grain boundaries (in this
case we talk of tertiary cementite).
In order to illustrate the structure types and features indicating the degradation
progress, the following figures present exemplary microscopic observation results
(light microscopy and SEM) for parts coming from Wroclaw bridges. A banded struc-
ture typical of puddled steel (ferritic-pearlitic) in the rolling direction and non- homo-
geneous grain size are shown in Fig. 2.
The frame marks plastically strained chain of non-metallic inclusions. An example
of structure with extreme intensity of contamination with non-metallic inclusions is
78
shown in Fig. 3a, coming from tests of the Central Pomorski Bridge. Fig. 3b presents
also the mentioned structures changed by degradation. Also in case of the cast steel,
degradation separations could be observed – such as in Fig. 4b.
Fig. 2. Microphotographs (light microscopy) of the structure of steel from the North Sand
Bridge: a – ferritic-pearlitic structure of steel with chains of non-metallic inclusions (A),
and the banded structure in the rolling direction, marked with frames; b – magnified area A [10].
The views of Figs. 2a and 4a have to be compared in order to find difference in
contamination of puddled steels with non-metallic inclusions (Fig. 2a), with that of the
cast steels (Fig. 4a).
Fig. 3. Microphotographs (light microscopy) of the structure of steel from the Central Pomorski
Bridge: a – an area with extreme intensity of non-metallic inclusions visible;
b – area with degradation symptoms (A) – nitrides and carbides separations inside grains,
separations of Fe3CIII at grain boundaries marked with arrows [10].
Fig. 4. Microphotographs (light microscopy) of the steel structures from the North Pomorski:
a – fine-grained ferritic-pearlitic steel structure; b – ferrite-pearlite grain structure
of the fine-lamellar structure and seldom separations (nitrides) inside ferrite grains.
Local Fe3C envelope at grain boundaries [3].
Basic mechanical properties of puddled steel (the Sand Bridge). Processes of
structural degradation presented in the previous subsection considerably influence ba-
sic mechanical characteristics i.e. hardness and impact resistance. Based on the litera-
79
ture studies and own research of the authors it could be stated that, in general, the
development and intensification of degradation processes cause distinct increase in
hardness (it drops after normalising), which took place in all the tested steel grades. In
case of impact resistance the effect of structural degradation changes is even more dis-
tinct and is observed in the form of its fall in the after-operation state and subsequent rise
in the normalized state. Physical explanation of such behavior is related to the presence
of numerous separations (especially of those at the grain boundaries) of a brittle phase
such as cementite, which causes a drop in ductility and, in consequence, favors the clea-
vage fracture. In relation to that the impact resistance (KCV) drops drastically at low
temperature. Moreover, as a result of many-year studies it could be stated that in a
significant number of case the brittle-ductile transition temperature stabilizes in the post-
operation state within the range of positive temperatures. We can observe the mentioned
effect in Fig. 5. The black horizontal line means the value of impact resistance – 35 J/cm2
as a minimal value for the contemporary bridge steel. The KCV values are higher in each
case in the post operating state – it is caused by degrading processes.
In the static tensile test the ratio
σ0.2/σB is frequently adapted as a useful
indicator for a wear degree (structural
degradation). However, as the other
study results show [2] it is not always
the case. Development of degradation
processes is also reflected in elongation
(δ), as well as reduction in area (ψ) of
the specimen which is naturally the
measure of material ductility. In the pre-
sented study results for the Sand Bridge
collected in Table 1, the distinct growth
effect in both mentioned characteristics
in the normalized state may be
observed. A hypothesis could be made
that this becomes a rule, but as known
from [8, 9] the puddled steel shows a
significant scatter in static tensile test results, which sometimes upsets observation of
the effect (see [3], and test results for puddled steel from segment I) and may lead to
rejecting the mentioned hypothesis. The yield point and tensile strength behavior
generally show the tendency to assume higher values in the post-operation state than in
the normalized state, but sometimes they could be different (see Table 1).
Table 1. Results of the static tensile tests for puddled steel (the Sand Bridge)
σ0.2, MPa σB, MPa E, GPa ψ, % δu, %
State P 263.3 ± 5.8 410 ± 21.1 197 ± 4.9 18.26 ± 2.9 12.87 ± 2.3
State N 294 ± 20.9 442 ± 18.5 202 ± 10.7 23 ± 0.45 16.7 ± 0.1
The ductility indicators (ψ and δu) have shown the significant difference between
the two states: post operating (P) and normalised (N). A small comment should be re-
quired in the case of quantity δu (see Table 1). The effect of degradation processes
should be stronger visible in hardness than in the results of tensile test. In structural
degraded materials the hardness values are always higher [2, 3, 6] than in the normalised
state N. In our case, the hardness was measured using Vickers Method (in agreement
with normative document PN-EN ISO 6507-1:1999 load: F = 10 kG (98.070 N)). The
results of hardness test are the following: in post operating state HV10 = 145 ± 9; in
normalized state HV10 = 115 ± 15.
Fig. 5. Impact resistance of puddled and cast
steels from the Sand Bridge in the after-opera-
tion (P) and normalised (N) states [11]:
1 – puddled steel (P state); 2 – puddled steel
(N state); 3 – cast steel (P state);
4 – cast steel (N state).
80
Fatigue crack growth study in the puddled steel after 149-year operation period.
In face of the degradation processes influence pattern on structure part behavior in the
structural degradation conditions as presented in the above subsection the cyclic tests
seem to be justified. In particular, it concerns the fatigue cracking propogation tests.
Fatigue and fatigue crack development is the most dangerous pair of hazards for that
structure types. Therefore, the knowledge of that subject, both the direction and character
of the fatigue crack development, enables either the evaluation of the existing cracks
development stability or determination of time between subsequent inspections, or
finally, to make the decision on a bridge load capacity. This subsection presents fatigue
crack growth test results for puddled steel samples coming from the Sand Bridge.
Considering the limited thickness of
samples and technical capabilities of the
dynamics laboratory in the Institute of
Materials Science and Applied Mecha-
nics WUT, it has been decided to use the
M(T) samples according to the ASTM
E647 Standard (Middle Cracked Tension
Specimen type, where t = 5 mm, W =
= 40 mm, L = 4 W), fixed in the testing
machine by hydraulic clamps. The samp-
les were collected from steel parts of the
Sand Bridge. The tests were made on the
MTS 810 testing machine using the
constant amplitude method, and ∆K dec-
reasing test. A sample was being loaded
while maintaining the constant stress
ratio of R = 0.1 with sinusoidal load in the 1.8…18 kN range applied with 10 Hz fre-
quency. The mechanical crack was made using electric discharging machine. Before
the proper test the notch was “sharpened” according to the ASTM E647 Standard,
while strictly keeping up to all the test validity rigors (Fig. 6).
The crack length was measured using compliance method and was also checked
and compared with the length obtained from manual, optical method using traveling
microscope. Test results are presented in Table 2, where ∆Kth – threshold value of the
stress intensity factor, C, m are Paris model constants (characteristic for a given steel
group). Similar values of the C and m constants in the after-operation and normalized
state indicate a similarity in the way of crack propagation in this range.
Table 2. Summary of characteristics related to fatigue crack growth
after-operation state normalized state
∆Kth, MPa· m 10.8 14.4
Exponent m 5.34 5.11
Constant C 10–11.52 10–11.74
A difference in the threshold quantities of ∆Kth has been observed instead, which
could be related to the presence of ageing processes in the tested steel. It has to be
noticed that the exponent m in the Paris formula directly responsible for the rate of
fatigue crack development in the tested steel (low-carbon grade) is high, i.e. much
higher than for the modern low-carbon steels (where m = 3). Also the expected ∆Kfc
quantity will be relatively low in comparison with the modern low-carbon steels.
CONCLUSIONS
The problems related to operating old steel structures from the end of 19th and
beginning of 20th century are cobsidered. The contemporary construction materials
have been discussed [8, 9]. The processes of structural degradation have also been
Fig. 6. Fatigue crack growth diagrams
for bridge steel after-operation (P, ) and
normalised (N, ) states, stress ratio R = 0.1.
81
presented on the example of samples taken from Wroclaw bridges, such as the Central
Pomorski Bridge, North Pomorski Bridge and Sand Bridge. In each case it has been
shown that there are degradation changes on the microstructure level. Such situation is
not neutral for the basic strength characteristics. A drop in steel ductility and increase
in its hardness favour brittle cracking. In such circumstances the cracking mechanics is
a very valuable tool. The research results for fatigue cracking gap development in the
puddled steel coming from the Sand Bridge operated for 149 years are presented. The
results obtained for the post-operating state are comparable with those achieved by
independent research teams [1, 8]. What differs the results of the authors’ study from
those presented in the papers mentioned here is the performance of the tests in two
states: the after-operation and the normalized. The differences in fatigue cracking
kinetics observed in areas I and III (Fig. 6), may result from the development of
degradation processes (threshold range of ∆Kth and fatigue fracture toughness ∆Kfc).
However, since a small number of samples in the tests were used the statistical
verification is not reliable. That is why, while making a hypothesis that such kinetics
changes are a rule, one have to be cautious. In the rectilinear part instead, the relatively
high m exponent value in the Paris law is noticeable, while the lack of differences in
the fatigue cracking kinetics in that area may indicate the insensitivity of the crack
propagation way to structure. That seems to be supported by the experimental facts
known in the cracking mechanics. Independently of the results obtained (here, the
notice related to difference in the fatigue cracking kinetics) the cracking mechanics
enables rational planning of the cracking inspection periods in the structural components.
РЕЗЮМЕ. Досліджено сталі мостів біля Вроцлава (Польща): дві пудлингові, експлуа-
товані з 1861 і 1885 рр., а також ливарну (1930 р.). Сталі, особливо пудлингові, чутливі до де-
градаційних процесів, що проявилось у зміні структури і механічних властивостей, найвід-
чутніше – у зниженні опору втомному росту тріщини.
РЕЗЮМЕ. Исследованы стали мостов в районе Вроцлава (Польша): две пудлинго-
вые, эксплуатируемые с 1861 и 1885 гг., а также литейную (1930 г.). Стали, особенно пуд-
линговые, чувствительны к деградационным процессам, что проявилось как в изменении
структуры, так и механических свойств, наиболее значительно – в понижении сопротив-
ления усталостному росту трещин.
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Received 23.11.2010
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