Fatigue properties of notched specimens made of FeP04 steel
Fatigue properties of the specimens with different notches made of FeP04 steel are presented. The specimens ware characterized by double symmetric lateral notches with a notch root radius ranging from ρ = 0.2 to 10 mm. The MTS 809 servo-hydraulic device was used for tests. All fatigue tests were per...
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України
2011
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| Цитувати: | Fatigue properties of notched specimens made of FeP04 steel / D. Rozumek, Z. Marciniak // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 4. — С. 41-46. — Бібліогр.: 9 назв. — англ. |
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Rozumek, D. Marciniak, Z. 2018-06-18T11:23:33Z 2018-06-18T11:23:33Z 2011 Fatigue properties of notched specimens made of FeP04 steel / D. Rozumek, Z. Marciniak // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 4. — С. 41-46. — Бібліогр.: 9 назв. — англ. 0430-6252 https://nasplib.isofts.kiev.ua/handle/123456789/138216 Fatigue properties of the specimens with different notches made of FeP04 steel are presented. The specimens ware characterized by double symmetric lateral notches with a notch root radius ranging from ρ = 0.2 to 10 mm. The MTS 809 servo-hydraulic device was used for tests. All fatigue tests were performed under force control, by imposing a constant value of the nominal load ratio (R = 0) and a load amplitude Pa = 6 kN for the notch root ρ = 0.2 mm and 7 kN for the notch root ρ = 1.25; 2.5 and 10 mm. The test frequency varied from 13 and 15 Hz. During the tests under constant load fatigue weakening of the material and an increase in strain were observed. Подані втомні властивості зразків з подвійними симетричними боковими вирізами (радіуси концентратора від ρ = 0,2 до 10 mm) з сталі FeP04. Для випробувань використовували сервогідравлічний пристрій МТС 809. Втомні випроби виконані за постійного номінального коефіцієнта навантаження (R = 0), амплітуди навантаження 6 kN для радіуса концентратора ρ = 0,2 mm і 7 kN для радіусів ρ = 1,25; 2,5 і 10 mm. Частота навантаження 13...15 Hz. Під час випробувань з постійним втомним навантаженням спостерігали знеміцнювання матеріалу зі збільшенням деформованості. Представлены усталостные свойства образцов с двойными симметричными боковыми вырезами (радиусы концентратора от ρ = 0,2 до 10 mm) из стали FeP04. Для испытаний использовали сервогидравлическое устройство МТС 809. Усталостные испытания выполнены при постоянном значении номинального коэффициента нагружения (R = 0), амплитуде нагружения 6 kN для радиуса концентратора ρ = 0,2 mm и 7 kN для радиусов ρ = 1,25; 2,5 и 10 mm. Частота нагружения изменялась в пределах от 13 до 15 Hz. Во время испытаний с постоянным усталостным нагружением наблюдалось разупрочнение материала и увеличения деформируемости. en Фізико-механічний інститут ім. Г.В. Карпенка НАН України Фізико-хімічна механіка матеріалів Fatigue properties of notched specimens made of FeP04 steel Усталостные свойства образцов с надрезом из стали FeР04 Втомні властивості зразків з надрізом із сталі FeР04 Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
Fatigue properties of notched specimens made of FeP04 steel |
| spellingShingle |
Fatigue properties of notched specimens made of FeP04 steel Rozumek, D. Marciniak, Z. |
| title_short |
Fatigue properties of notched specimens made of FeP04 steel |
| title_full |
Fatigue properties of notched specimens made of FeP04 steel |
| title_fullStr |
Fatigue properties of notched specimens made of FeP04 steel |
| title_full_unstemmed |
Fatigue properties of notched specimens made of FeP04 steel |
| title_sort |
fatigue properties of notched specimens made of fep04 steel |
| author |
Rozumek, D. Marciniak, Z. |
| author_facet |
Rozumek, D. Marciniak, Z. |
| publishDate |
2011 |
| language |
English |
| container_title |
Фізико-хімічна механіка матеріалів |
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| format |
Article |
| title_alt |
Усталостные свойства образцов с надрезом из стали FeР04 Втомні властивості зразків з надрізом із сталі FeР04 |
| description |
Fatigue properties of the specimens with different notches made of FeP04 steel are presented. The specimens ware characterized by double symmetric lateral notches with a notch root radius ranging from ρ = 0.2 to 10 mm. The MTS 809 servo-hydraulic device was used for tests. All fatigue tests were performed under force control, by imposing a constant value of the nominal load ratio (R = 0) and a load amplitude Pa = 6 kN for the notch root ρ = 0.2 mm and 7 kN for the notch root ρ = 1.25; 2.5 and 10 mm. The test frequency varied from 13 and 15 Hz. During the tests under constant load fatigue weakening of the material and an increase in strain were observed.
Подані втомні властивості зразків з подвійними симетричними боковими вирізами (радіуси концентратора від ρ = 0,2 до 10 mm) з сталі FeP04. Для випробувань використовували сервогідравлічний пристрій МТС 809. Втомні випроби виконані за постійного номінального коефіцієнта навантаження (R = 0), амплітуди навантаження 6 kN для радіуса концентратора ρ = 0,2 mm і 7 kN для радіусів ρ = 1,25; 2,5 і 10 mm. Частота навантаження 13...15 Hz. Під час випробувань з постійним втомним навантаженням спостерігали знеміцнювання матеріалу зі збільшенням деформованості.
Представлены усталостные свойства образцов с двойными симметричными боковыми вырезами (радиусы концентратора от ρ = 0,2 до 10 mm) из стали FeP04. Для испытаний использовали сервогидравлическое устройство МТС 809. Усталостные испытания выполнены при постоянном значении номинального коэффициента нагружения (R = 0), амплитуде нагружения 6 kN для радиуса концентратора ρ = 0,2 mm и 7 kN для радиусов ρ = 1,25; 2,5 и 10 mm. Частота нагружения изменялась в пределах от 13 до 15 Hz. Во время испытаний с постоянным усталостным нагружением наблюдалось разупрочнение материала и увеличения деформируемости.
|
| issn |
0430-6252 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/138216 |
| citation_txt |
Fatigue properties of notched specimens made of FeP04 steel / D. Rozumek, Z. Marciniak // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 4. — С. 41-46. — Бібліогр.: 9 назв. — англ. |
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| fulltext |
41
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2011. – ¹ 4. – Physicochemical Mechanics of Materials
FATIGUE PROPERTIES OF NOTCHED SPECIMENS MADE
OF FeP04 STEEL
D. ROZUMEK, Z. MARCINIAK
Opole University of Technology, Poland
Fatigue properties of the specimens with different notches made of FeP04 steel are presen-
ted. The specimens ware characterized by double symmetric lateral notches with a notch
root radius ranging from ρ = 0.2 to 10 mm. The MTS 809 servo-hydraulic device was used
for tests. All fatigue tests were performed under force control, by imposing a constant
value of the nominal load ratio (R = 0) and a load amplitude Pa = 6 kN for the notch root
ρ = 0.2 mm and 7 kN for the notch root ρ = 1.25; 2.5 and 10 mm. The test frequency
varied from 13 and 15 Hz. During the tests under constant load fatigue weakening of the
material and an increase in strain were observed.
Keywords: fatigue, notch, strain, hysteresis loops.
In the case of ductile cracking the specimen surface separation for pure metals
occurs as a result of successive slip bands, and for technical alloys the cracking process
starts with the harder components (non-metallic inclusions), creating voids thus causing
the increasing damage to the process. In paper [1] ductile cast irons are characterized
by high fatigue crack propagation resistance, although this property is still not widely
investigated. In work [1], three different ferritic–pearlitic ductile cast irons, characte-
rized by different ferrite/pearlite volume fractions and an austempered ductile cast iron
were considered. Their fatigue crack propagation resistance was investigated in air by
means of fatigue crack propagation tests according to ASTM E647 standard, conside-
ring three different stress ratios (R = 0.1; 0.5; 0.75). When fatigue crack is nucleated
and propagates in the notch vicinity [2], the crack growth rate is generally higher than
that expected by using the stress intensity factor concept. The current study attemptes
to describe the crack growth at notches quantitatively with a detailed consideration of
the material cyclic plasticity. The combined effect of notch plasticity and possible
contact of cracked surface was responsible for the observed crack growth phenomenon
near the notch. In paper [3], the influence of the notch radius on the crack growth rate
under low- and high-cycle fatigue is discussed. Tests were carried out on the plates
made of FeP04-UNI 8092 deep-drawing steel, weakened by symmetric lateral notches
of varying acuity. It has been shown that the notch strongly influences the variation of
∆J parameter values both globally and locally.
The aim of the paper is the determination of the fatigue properties of notched
specimens made of FeP04 steel.
Material, properties and test stand. Static properties and fatigue tests of speci-
mens. Tests were carried out on the plane specimens made of FeP04-UNI 8092 deep-
drawing steel. Steel FeP04 was used for the load-bearing elements of construction
vehicles. For static and fatigue tests, a Schenck PSA100 servo-hydraulic device was
used. Coefficients of the Ramberg–Osgood equation describing the cyclic strain curve
under tension-compression with Rε = –1 (a Schenck extensometer was used with a
gauge length equal to 25 mm) for FeP04 steel are the following [4]: the cyclic strength
coefficient K′ = 838 MPa, the cyclic strain hardening exponent n ′ = 0.220.
Corresponding author: D. ROZUMEK, e-mail: d.rozumek@po.opole.pl
42
The specimens had double sym-
metric lateral notches with the notch
root radii ranging from 0.2 to 10 mm
(Fig. 1). The theoretical stress concen-
tration factor in the specimen under
tension Kt = 9.61; 4.30; 3.23 and 1.85
was estimated with use of the model
presented in [5].
The surfaces of the specimens, w = 50 mm wide, had been accurately polished in
order to make the cracks originated from the notch tip easily distinguishable. Test ma-
terial is FeP04 steel (0.05 wt.% C, 0.30 Mn, 0.05 Si, 0.032 P, 0.02 S, 0.043 Al, 0.07 Cu,
balance – Fe). The monotonic quasi-static tension properties of FeP04 steel are the
following: yield stress σy = 210 MPa, ultimate stress σu = 330 MPa, Young’s modulus
E = 191 GPa, Poisson’s ratio ν = 0.30. The MTS 809 servo-hydraulic device was used
for tests. Both testing systems were equipped with a 100 kN load-cell. The stand MTS
809 contains a modulus which allows to perform tests under a given crack tip opening
displacement δ. The modulus is included into the control panel. The displacement of a
gage length was controlled. During fatigue crack growth, displacement was measured
with an extensometer. For measurements the extensometer MTS was applied (gage
length – 10 mm, model 632, 13F-20 manufactured by Systems Corporation Eden
Prairie, USA). The test frequency ranged from 13 and 15 Hz. Crack initiation and pro-
pagation phases were observed on the specimen surface by means of a portable micro-
scope having a magnification factor 20 times.
All fatigue tests were performed under force control, by imposing a constant value
of the nominal load ratio (R = σmin /σmax = 0) and a load amplitude Pa = 6 kN (Pmax =
= 12 kN) for the notch root ρ = 0.2 mm and 7 kN (Pmax = 14 kN) for the notch root
ρ = 1.25, 2.5 and 10 mm (which corresponded to the nominal amplitude of normal net
stresses σa = 100; 117 MPa before the crack initiation).
Results and discussion. Microstructure and fatigue crack path in FeP04 steel.
Steel FeP04 can be easily subjected to cold working, it belongs to ferritic steels. Since
the amount of carbon in ferrite is low, ferrite properties are very similar to the proper-
ties of pure iron α. The considered steel is applied for deep drawing. The material
structures were tested on the metallographic specimens with the metallographic micro-
scope magnifying from 50 till 2000 times. Fig. 2 shows microstructure of FeP04 steel,
containing the ferrite (light) and numerous non-metallic inclusions. The structure exhi-
bits a distinct rolling texture. Against a background of long ferrite grains there are nu-
merous non-metallic inclusions visible, mainly chains of oxides about 1 µm (black). On
the ferrite grain boundaries coalesced cementite can be seen in Fig. 2. In the material
transcrystalline cracks through the grains of phase α are dominating, but cracks along
the grain boundaries are also observed. The main cracks propagate in the direction per-
pendicular to the loading action, but secondary cracks are also visible. Fig. 3 presents
the surface of a specimen tested under loading Pa = 7 kN and with the radius of the
notch root ρ = 10 mm after Nf = 128700 cycles to failure. The development of cracks
was observed under microscope at a magnification of 17 times. The initiation and growth
of many cracks were observed at the notch tip. Due to high plasticity of the material
ductile cracking is observed which is characterized by the presence of voids (black
field around the notch – Fig. 3) after stratification of the material found on the crack
paths. Stress concentration and intensification of the plastic flow occur around the voids.
The tests of fatigue crack growth in FeP04 steel subjected to tension were perfor-
med in the low and high cycles fatigue. During tests a number of cycles to the crack
initiation Ni (i.e. to the moment of occurrence of a visible crack) was recorded, and the
Fig. 1. Specimen for tests (dimensions in mm).
43
fatigue crack lengths were measured. The cracks initiated [6] (minimal observable
crack length about 0.1 to 0.2 mm) at the same time on the left and on the right sides of
the notched specimen.
Fig. 2. Fig. 3.
Fig. 2. The FeP04 steel microstructure (grains phase α).
Fig. 3. The paths of cracks in the specimen with a notch radius ρ = 10 mm.
Fatigue characteristics of strain. Examples of the strain and force histories for
the initial period (Fig. 4) and for the entire lifetime of the specimens (Fig. 5). Fig. 4
presents the sample characteristics of the strain and load for four types of notches ρ =
= 0.2, 1.25; 2.5 and 10 mm. On the basis of Fig. 4b some anomalies can be stated in the
strain history for the initial period of researches in comparison with other charts. This
behavior could be caused by the material ratcheting.
Fig. 4. Characteristics of strain, ε, and force, F, versus time, τ, for the initial period
of researches and different notches: a – ρ = 0.2 mm; b – 1.25; c – 2.5; d – 10 mm.
The presented strain histories shown in Fig. 5 relate to different life obtained for
different notches. For specimens with a notch ρ = 2.5 and 10 mm a slight increase in
strain by about 85 and 95% life of the specimens can be seen. In contrast, for specimens
with a notch ρ = 0.2 mm fluctuations were observed, and with a notch ρ = 1.25 mm the
decrease in strain for about 120 s was observed. For the results shown in Fig. 5a in
44
low-cycles fatigue the initiation of cracks was observed in the initial period of resear-
ches. Fig. 5b shows that fatigue cracks initiation occurred after 8300 cycles due to the
blunt notch radius ρ = 1.25 mm, although the crack growth took place under low-cycle
fatigue. For the specimen in Fig. 5d the initiation of cracks occurred at the end of the
specimen life. This is typical of high-cycles fatigue research. From the curves “strain
vs time” reported in Fig. 5, it appears that after changing the notch root radii ρ from 0.2
to 10 mm fatigue life increases. It is evident that with the highest radius the initiation
phase, which depends on the stress conditions at the notch tip, prevails.
Fig. 5. Characteristics of strain, ε, versus time, τ, for the entire lifetime and different notches:
a – ρ = 0.2 mm; b – 1.25; c – 2.5; d – 10 mm (A – crack initiation).
Fig. 6. Selected hysteresis loops at different times of tests for notches:
a – ρ = 0.2 mm; b – 2.5; c – 10 mm.
Analysis. Fig. 6 shows the examples of hysteresis loops at different life times of the
specimens with notches, where; 1 – the initial period of a specimen study (from Fig. 5),
45
2 – half of the specimen life and 3 – the final test period. Based on Fig. 6 it can be con-
cluded that during the tests a marked increase in the total strain was noticed. Under test
conditions for Pa = const the weakening of the material occurs when the strain amplitude
increases with the increase of the hysteresis loop width. The fourth graph presented in
Fig. 6 for different notches radii gives the hysteresis loops for periods 1, 2 and 3. From
the graphs it can be concluded that for ρ = 0.2 and 10 mm increase in strain occurs
evenly. However, for ρ = 2.5 mm it can be observed that in the late specimen life a sud-
den increase in strain takes place and the scope of these strains have the highest values.
Stresses at the notch root. In order to calculate the maximum stresses at the root
notch the strain energy density models of Neuber [7], Molski–Glinka [8], Łagoda–
Macha [9] were used. These models are based on the cyclic stress-strain curve and are
presented by the following formulas:
12
max max
max
n
NW
E K
′σ σ⎛ ⎞= + σ ⎜ ⎟′⎝ ⎠
(for Neuber), (1)
12
max max max
2 1
n
MGW
E n K
′σ σ σ⎛ ⎞= + ⎜ ⎟′ ′+ ⎝ ⎠
(for Molski–Glinka), (2)
12
max max
max
1
2 1
n
LM
nW
E n K
′′σ σ− ⎛ ⎞= + σ ⎜ ⎟′ ′+ ⎝ ⎠
(for Macha–Łagoda). (3)
Above models are compared to the energy calculated using the nominal value of
stresses and strains. The strain energy density value corresponding to the nominal
stress was calculated by the equations:
2( )t n
n
KW
E
σ
= ,
2( )f n
n
K
W
E
σ
= (for Neuber ), (4)
2( )
2
t n
n
KW
E
σ
= ,
2( )
2
f n
n
K
W
E
σ
= (for Molski–Glinka and
Macha–Łagoda), (5)
where Kf is fatigue stress concentration factor.
The distribution of stresses in a notched specimen subjected to tension is shown in
Fig. 7.
Fig. 7. Gradient stress in a notched tensile specimen.
Comparison of models (1)–(3) with equations (4) and (5) allowed to calculate the
maximum stresses at the notch root. The results are shown in Tables 1 and 2.
Table 1. Dependences of σmax versus Kt
and strain energy density models
Model
Neuber Molski–
Glinka
Macha–
Łagoda Kt
σmax, MPa
9.61 421 386 405
4.30 329 302 316
3.23 294 270 282
Table 2. Dependences of σmax versus Kf
and strain energy density models
Model
Neuber Molski–
Glinka
Macha–
Łagoda Kf
σmax, MPa
4.62 319 293 305
3.05 287 265 276
2.52 266 245 255
Based on these results it can be concluded that the Neuber model gives the highest
stresses and Molski–Glinka model – the lowest values of stresses at the notch root.
46
Comparing Tables (3) and (4) we may note that the Kt factor is higher than Kf. In the
case of a sharp notch ρ = 0.2 mm this difference is more than twice. Therefore, using
the analysis of Kt calculated for the sharp notches one should be aware that the value is
significantly different from the actual results obtained from the experiment.
CONCLUSIONS
The following conclusions can be drawn, based on the experimental fatigue lives
for the tested material: during testing a strong influence of stress concentration on
fatigue life was noticed; for different notches different periods of the fatigue cracks
were observed; during constant load fatigue testing the weakening of the material and
initiation increase in strain was observed; stress values at the notch root for the tested
material derived from the Macha–Łagoda model are intermediate compared to other
analyzed models.
РЕЗЮМЕ. Подані втомні властивості зразків з подвійними симетричними боковими
вирізами (радіуси концентратора від ρ = 0,2 до 10 mm) з сталі FeP04. Для випробувань ви-
користовували сервогідравлічний пристрій МТС 809. Втомні випроби виконані за постій-
ного номінального коефіцієнта навантаження (R = 0), амплітуди навантаження 6 kN для
радіуса концентратора ρ = 0,2 mm і 7 kN для радіусів ρ = 1,25; 2,5 і 10 mm. Частота наван-
таження 13...15 Hz. Під час випробувань з постійним втомним навантаженням спостеріга-
ли знеміцнювання матеріалу зі збільшенням деформованості.
РЕЗЮМЕ. Представлены усталостные свойства образцов с двойными симметричны-
ми боковыми вырезами (радиусы концентратора от ρ = 0,2 до 10 mm) из стали FeP04. Для
испытаний использовали сервогидравлическое устройство МТС 809. Усталостные испы-
тания выполнены при постоянном значении номинального коэффициента нагружения
(R = 0), амплитуде нагружения 6 kN для радиуса концентратора ρ = 0,2 mm и 7 kN для ра-
диусов ρ = 1,25; 2,5 и 10 mm. Частота нагружения изменялась в пределах от 13 до 15 Hz.
Во время испытаний с постоянным усталостным нагружением наблюдалось разупрочне-
ние материала и увеличения деформируемости.
Acknowledgements. Authors are grateful to the scientists Filippo Berto, University
of Padova, Department of Management and Engineering, Vicenza, Italy, for the
conducting common resarches.
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Plasticity – 2007. – 23. – P. 1167–1188.
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Received 17.01.2011
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