EXPERIMENTAL ANALYSIS OF SOME CHARACTERISTICS OF THE [600748]

EXPERIMENTAL ANALYSIS OF SOME CHARACTERISTICS OF THE
SUPERFICIAL LAYER DURING LOW CYCLES FATIGUE PROCESS
Silviu MACUTA*, Dumitru DIMA *
ABSTRACT
Using laminate samples, the investigations have been realized at variable solicitation of pure bending on a
patented machine. The machine has two working places and it can be adjusted for different deformations,frequencies and number of cycles. The tests have developed in a symmetrical alternative regime at twofrequencies. In this paper some results concerning the evolution of the crystalline lattice parameter, level of thetexture were realized in steps of each two thousand cycles until a limit of 10
4 cycles. The X-ray diffraction
method was used and information on the ferrite phase were obtained. The crystalline lattice parameter presents a
decreasing tendency when the number of the fatigue cycles increase. This decreasing in jumps occur. During
fatigue tests at high testing frequencies an increasing texture process is displayed, but at small fatigue testingfrequencies an increasing and decreasing texture processes occur. Experimental data allow estimate the historyof sample and its behavior in fatigue process.
1.GENERAL CONSIDERATIONS
In order to perform a complete study of the behaviour of the superficial layer during fatigue
test and to evince the main factors, which determine the damage process, the structural
cybernetic model was introduced. In fig.1 this model is presented and it allows to a systematic
study of the input parameter changes under action of the commanding ones. The input/out
parameters are: superficial layer parameters (S s-S’s) [X 1- macro and micro-geometry, X 2 –
microhardness and hardness, X 3 – tension state, X 4 – chemical composition, X 5 – structure, X 6
– purity] and tribosystem parameters (C s) [noise, debris]. Some of the mentioned parameters,
as X 2, X 3, X 5, can be changed from exterior such the durability of the material to be in a
certain interval. The commanding parameters (U) [U 1 – nature of material, U 2 – shape of the
sample, U 3 – dimension of the sample, U 4 – working medium, U 5 – kinematics,
University ,,Dunarea de Jos’’ of Galati, Romania.

U6 – energetic called external factors, by their action can change some superficial layer
parameters X i, i=1 – 6. In our experimental program the evolution of the X 2, X3
Fig.1 Used cybernetic model
and X 5 was showed by changing U 1 (type of steel: OL 52) and U 5 (testing frequency: ν1=20
cycles/min, ν2=40 cycles/min., testing deformation: ε1=2000 µm/m, ε2=2500 µm/m, ε3=3500
µm/).
2.EXPERIMENTAL.
We have conducted the examination of material features in fatigue process for two steels used
in pressure vessel engineering: OL 52 and 10TiNiCr180. Using laminate samples, the
investigations have been realized at variable solicitations of pure bending on a patented
machine [1]. The tests have developed in a symmetrical alternative regime at two frequencies:
f1=20 Hz, f 2=40 Hz.
The deformation domain was established in an experimental program of tests using the
resistive tensometry. The deformations of samples were imposed at the superior limit of
elastic domain, evaluated from characteristic curves. The deformations imposed in case of
OL52 were: ε1=2000 µm/m, ε2=2500 µm/m, ε3=3500 µm/m and in case of 10TiNiCr180:
ε1=1500 µm/m, ε2=2000 µm/m, ε3=2500 µm/m, respectively.
In the present paper we presented the experimental results only for OL52 steel. The
experiments were performed in steps of two thousand cycles until a limit of 104 cycles. In
every step, we investigated the lattice parameter and texture level [2][3]. For this investigation
we used an X rays diffractometer, DRON-3.

3.RESULTS AND DISCUSSIONS
In figure 2 and 3 the evolution of the lattice parameter for OL 52 steel tested to fatigue
process for three imposed strains, ε1, ε2 and ε3 at two frequencies are respectively presented.
Analyzing those two figures, a general tendency of decreasing of the ferrite phase lattice
parameter, is evinced, when the number of the testing fatigue cycles increases. This occurs
for the all imposed strains. A strong decreasing is revealed for small strains and frequencies,
respectively for high strains and frequencies. The change of the lattice parameter can be
explained by supposing of a migration process of some alloyed elements during fatigue tests.
This fact presents validity by a soft increasing of the lattice parameter during the fatigue test
for an established testing cycle number.
Fig.2 Evolution of lattice parameter for f = 20 cycles/min
This increase appears more early when the strain is bigger. The migration process of atoms in
and from elementary cell of the ferrite phase shows the existence of a high kinetic of atoms in
the superficial layer during fatigue tests. This kinetic can have negative effects if the material
is tested in corroding media. The migration process is more slowly in case of the small strain
and more intensive at high frequencies and big strains; at small strain the process is more
intensive at small frequencies. The presented data show that there is the possibility to
command from exterior the durability of tested material by changes of the imposed strain and
testing frequencies in a certain range.

Fig. 3 Evolution of the lattice parameter for f = 40 cycles/min
In figure 4 and 5, for 20 cycles/min and 40 cycles/min, the evolution of the texture parameter,
measured by ratio I max/I0, where I max and I 0 are, respectively, the maximum intensities of the
X-ray diffraction line (220) of the ferritic phase, for tested and non-tested samples, are
presented. From figures, an inverse texture (IT) process of material is evinced. The higher
Fig.4 Evolution of texture level for f = 20 cycles/min

degree of the IT occurs in the case of the bigger strain, ε3. This IT can be associated to some
mechanical micro-processes that lead to a preferential orientation of the crystalline planes in
[220] direction in relation to laminated state of material, when I max/I0 =1. From the view of
presented histograms, can be analyzed the
Fig.5 Evolution of texture level for f = 40 cycles/min
distribution of the texture degree of steel during fatigue tests and a relationship with plasticity
properties can be established. At small frequencies the material presents a normal hysteresis,
but at high value the inertial properties of material are smaller.
4.CONCLUSION
1. During fatigue tests, the lattice parameter presents a general tendency of decreasing when
the number of the cycles increases, for those three strains and those two frequencies. The
decreasing occurs in jumps and can be associated to a migration of alloying elements from
steel. The migration process is strongly influenced by number of fatigue cycles and level of
strain.

2. During fatigue tests, a texture process of ferrite phase occurs and it is influenced by level of
strain and frequencies. The texture process, evinced by X-ray diffraction method, shows data
about inertial properties of fatigue tested material. The steel looses the elastic properties when
it is tested at high frequencies, the influence of strain being not very important.
5.REFERENCES
[1] I. CRUDU, S. MACUTA, L. PALAGHIAN, L. FAZEKAS, Patent brevet, nr.102714 (1991), Romania
[2] C. GHEORGHIES, Control of fine structure of metal by X-rays , Ed. Tehnica, Bucuresti (1990), p.128
[3] S. MACUTA, C. GHEORGHIES, The evolution of some parameters of superficial layer in fatigue process ,
15-th Symposium “Danubia-Adria” Sept. 30-Oct. 3,Bertinoro, Italy, (1998), p. 59[4] S.MACUTA, I. CRUDU, Experimental researches regarding the evolution of some parameters of the
superficial layer in low cycles fatigue process for some steels used in machines construction, The Annual
Symposium Of The Institute Of Solid Mechanics, SISOM 2000, Bucharest, October 26-27, 2000.