EXPERIMENTAL RESEARCHES REGA RDING THE EVOLUTION OF SOME [601469]

EXPERIMENTAL RESEARCHES REGA RDING THE EVOLUTION OF SOME
PARAMETERS OF THE SUPERFICIAL LAYER IN LOW CYCLE FATIGUE PROCESS
Silviu MACUTA*, Ion CRUDU*
*University "Dun ărea de Jos", Gala ți, România
The development of some mechanic engineering systems in, feasible to pressure vessels, aeronautics,
ship building technology, requires a full investigation of the material features th at are to be examined
under a low cycles fatigue to strains close to the limit of the material elastic ity. In this paper some
results concerning the evolution of inner second order tensions, microhardness in superficial layer
and its microscopic images during low cycles fa tigue process are presented. The studies are
performed on two steels used in pressure vesse l engineering. Using laminate samples, the
investigations have been realized at variable solicitations of pure bending on a patented machine. A
relationship between mentioned si zes is evinced and can be used in establishment of damage
mechanism in fatigue process.
Keywords: fatigue, low frequency, structure
1. A SYSTEMIC STUDY CONCERNING THE BEHAVIOUR OF THE SUPERFICIAL IN
FATIGUE PROCESS
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 pr esented and it allows to a systematic study of the input parameter changes
under action of the commanding ones. The input/out pa rameters are: superficial layer parameters (S s-S’ s)
[X1- 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
(Cs) [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,
U3 – dimension of the sample, U 4 –
working medium, U 5 – kinematics,
U6 – energetics] 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, X 3
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/).In order to estimate the dislocation density the X-ray
diffraction method was used.

Figure 1 A cybernetic model used in study of friction process
adapted in study of the low cycle fatigue process

Silviu MACUTA, Ion CRUDU 287
2. DISLOCATION DENSITY
In figure 2 the evolution of the ()max/I If 220 ratio against of the testing cycles number is presented.
This evolution is done for: OL52 steel,
three imposed deformations, ε1, ε2, ε3,
and a frequency ν1=20 cycles /min. As
general aspect, it can see a decreasing of the dislocation density, ρ [( I
f /Imax) ∼ ρ,
where I f, I max are respectively the
background and maximum intensities of
the (220) X-ray diffraction line provided of the ferrito-perlitic phase from steel] in
case of small deformations ( ε
1, ε2) and an
increasing in case of the big deformations (ε
3). These evaluations are made in
relation with the initial state of the
dislocation density obtained before
fatigue tests. For deformation ε2=2500
μm/m, there is an increasing tendency of
dislocation density, which occurs in
jumps. This evolution can lead to their
crowd and, finally, to microcracks
generation. For deformation ε1=2000
μm/m there is an opposite tendency; this
aspect can be connected with a bigger durability of the tested sample at this
deformation. For ε
3=3500 μm/m there is
a general increasing tendency of the
dislocation density which will hurry the damage process in the fatigue tests. The
ε
3 deformation can be called as a
“critical” value. 00,050,10,150,2
2000
3000
4000
5000
6000
7000
8000
9000
10000N cyclesIf/Imax
deformatia 3 deformatia 2 deformatia 1
nivel initial nivel initial nivel initial

Figure 2

In figure 3 the same evolutions are
presented but for the testing frequency of
40 cycles /min. It can see that when the testing frequency growths the “critical”
value for ε decreases. The slopes of the
increasing and decreasing tendencies of
the dislocation density can be considered as a evaluation criterion of the passing deformation from the elastic to elasto-
plastic deformation. Variation of the
dislocation density with number of cycles presents a minimum. It appears lately when the testing
deformation degree decreases. As a conclusion: in orde r to reduce the damage effect during tests performed
to high deformation it is necessary to use big frequency. 00.020.040.060.080.10.120.140.160.18
2000
3000
4000
50006000
7000
8000
9000
10000N cyclesIf/Imax
deformatia 1 deformatia 2 deformatia 3
nivel initial nivel initial nivel initial

Figure 3

3. ANALYSES OF THE SUPERFICIAL LAYER MICROHARDNESS
In figure 4 and 5 the variation of the, HV, microhardness on number of testing cycles, for ε3=3500
μm/m and the two testing frequencies are presented. There is a jumps decreasing of the HV by hardening and
softening processes. At low frequency this decreasing is lesser like in case of high frequency for the same

Experimental Researches regarding the evolution of some parameters of the superficial layer in low cycle fatigue process
288
imposed deformation. This shows that the hardening and softening process occur with different speeds.
168160178170 166
050100150200250
0
2000
4000
6000
8000
10000
N cycles
Figure 5 Variation of the microhaedness on
number of testing cycles, ν=40 c/min 205
160193
165200
050100150200250
0
2000
4000
6000
8000
10000
N cycles

Figure 4 Variation of the microhaedness on
number of testing cycles, ν=20 c/min
4. ANALYSES OF THE MICROSTRUCTURE
In figure 6 and 7 the microstructure of the superficial layer at deformation ε3=3500 μm/m for the two
frequencies are presented. It can see that when number of the testing cycles increases, as well as when the
testing frequency is big, the sliding bands density inside of a grain is highly. This result can be connected
with the damage process.

Number of cycles
Initial state 2000 4000 6000 10000

Figure 6
x500, ν1=20 cycles/min , ε=3500μm/m

Figure 7,
x500, ν 1=40 cycles/min, ε=3500μm/m
5.CONCLUSIONS
a. By extension of the tribolayer and tribosystem c oncepts to the study low cycle fatigue process of
the steel the structural changes in the superficial layer are shown. This allows to establish a
relationship between structural parameters of s uperficial layer and damage degree during fatigue

Silviu MACUTA, Ion CRUDU 289
tests. It was evinced a microfatigue process wh ich is strong influenced of: frequency testing,
deformation level, and number of the fatigue tests.
b. Our results can be used in order to explain the damage mechanism of the tested samples subjected
to low frequency fatigue test and high tensions.
REFERENCES
[1] Crudu, I., Mă cuță, S. D. Patent no.102714/1991, Romania
[2] Măcuță, D., C. Gheorghies, Proc. of the 15-th Symp. Danubia-Adria, Sept.30-Oct.3, 1998, Bertinoro,
Italy
[3] Măcuță, S. D. Doctoral Thesis, University "Dunărea de Jos" of Gala ți, Romania, 1998.
[4] Măcuță S.,Ghiorghie ș C., The evolution of some parameters of superfici al layer in fatigue process-15-th
Symposium"DANUBIA-ADRIA" on experimental me thods in solid mecanics ,Bertinoro 30sept.-
3oct.1998,ITALY.
[5] Macuta S.-The Evolution of Some Parameters of the Superficial Layer in Low Cycle Fatigue Process.
Proceedings of the 2nd ESAFORM Conference on Material Forming April 13 – 17, 1999, Guimares –
Portugal
[6] Macuta S.,-Extension of the Tribomodel Concept in Study of Fatigue Process.The 3rd International
Conference of Tribology “BALCANTRIB-99”,June 2 – 4, 1999, Sinaia, Romania