- Email: [email protected]
On the influence of fatigue on machine part wear
Wear, I70 (1993)
167
167-172
On the influence of fatigue on machine part wear Vladimir P. Bulatov The Institute for Mechanical
and Dmitri
N. Vedernikov*
Engineering Problems, The Russian Academy of Sciences, 5-l-500 Bajkonurskaja Street, 197348 St. Petersburg (Bus&n Federation) (Received October 28, 1992; revised and accepted April 1, 1993)
Abstract The
main aspects of the relationship between fatigue and machine part wear are considered and the relevant problem is formulated. The results of an investigation of the influence of the fatigue process on machine part wear are given for diesel engine piston rings. As the results show, the solution of the problem could open up new prospects in the development of the theory of machine part cyclic wear and the details of the technology of cyclic straining treatments for increasing fatigue wear resistance.
1. Introduction
The relationship
As tribologists we are doomed to be on the brink, in the sense that tribology is a multidisciplinary and contiguous field [l] on the borders of which a common phenomenon, called wear by everybody occurs. It is difficult to avoid the impression that fatigue problems are present in most tribological investigations as a visible or invisible shadow. Also, it is difficult to ignore the impression that the large amount of knowledge accumulated in the area of fatigue approaches a critical level when a qualitative leap would be expected to occur. The words (synonyms) “periodicity” and “cyclicity” should be emphasized owing to their special significance in nature. It is now becoming evident that further tribological developments will be determined to a considerable extent by knowledge of the problems of relations between fatigue and machine part wear. The purpose of this paper is to attempt to systematize knowledge in the area of relations between fatigue and machine part wear. In addition, experimental data analysis [2, 31 and determination of suitable directions of future research into the above-mentioned problems are considered. In general the relations between fatigue and machine part wear can be considered from the following main points of view (Fig. 1). 1.1. The relationship between fatigue strength limit and wear This direction involves the traditionally studied questions of the influence of detailed micro- and macro*Author to whom correspondence
0043-1648l93/$6.00
should be addressed.
between
fatigue.
The associated problem mechanical operatmnal
of cyclic stramlng
Fig. 1. Problems of the relationship between fatigue and machine part wear.
geometric changes on the fatigue strength limit. Usually an investigation reaches an experimental evaluation of the change in fatigue strength limit during detailed work with a machine as a result of information on the decrease in cross-sectional area during the wear process, the appearance of scratches as stresses concentrators on sliding surfaces, changes in the residual stress field and other factors [7, 81. 1.2. The relationship between details of the fatigue changes in structural material elements and wear This aspect involves the problems of the influence of fatigue changes in detail of structural material ele-
0 1993 - Elsevier Sequoia. All rights reserved
168
VP. Bulatov, D.N. Vedemikov I Infruence of fatigue on machine part wear
ments (which may be seen at any level, usually beginning from the crystal lattice [9]) on the details of the wear. Certainly, the changes in the surface layer structure are most significant in their consequences. Therefore it is expedient to classify this relationship, considering in this respect the kind of fatigue factor (cyclic mechanical straining, thermal, frictional and other possibfe kinds of strain) and the time during which these fatigue factors act (before work or during particular work in a machine). At present the problems of cyclic frictional straining are studied to a very high degree. The fatigue theory of wear has been evolved and formulae for calculation have been obtained [4, 51. Development of the fatigue theory of wear was one of the main trends in tribological research in the former USSR [6]. It is not expedient to consider at present cyclic thermal straining together with cyclic straining of other possible kinds as starting problems owing to their particular difficulties. There are many frictional machine parts which are subjected to a cyclic load in operation. The processes take place in parallel during operational straining, so a complicated detailed picture of fatigue changes is created. Therefore, it is necessary to limit our intentions to consideration of the wear change processes that depend on the accumulation of fatigue changes which can take place as a result of so-called cyclic mechanical pre-straining before specific work (operation) in a machine (the initial problem, Fig. 1). The solution of this problem will permit us to pass to the next study: that of the influence of cyclic mechanical operational straining on wear in detail. Further, the analysis of results of theoretical and experimental investigations of the influence of cyclic straining on wear would certainly lead to an expedited development of a new theory: this may be called the theory of machine part cyclic wear. The future theory would help to make up a deficiency of knowledge in the area of the relationship between fatigue and machine part wear.
2. Experimental procedure The experimental determination of the influence of cyclic mechanical pre-straining on wear was carried out using standard cast iron 150 mm diameter piston rings of a diesel engine. The method involved the following stages: cyclic pre-straining of the ,piston rings on the fatigue machine, making (cutting out) the differently loaded specimens from a ring, and testing the specimens on the reciprocating frictional machine. Piston ring cyclic straining was carried out in accordance with the scheme shown in Fig. 2 in the following manner. A piston ring (1) with cut-off ends was fastened to the moving (2) and to the non-moving (3) fatigue machine
Fig. 2. Schematic
’
‘\
diagram of piston ring cyclic straining.
(bf IP
22% Fig. 3. (a) Schematic diagram of cut-out piston ring specimens. (b) Schematic diagram of specimen engagement with a cylinder liner segment: 1, specimen; 2, liner segment. TABLE 1. The dependence section Cross-section
Nominal (“ro)
4 5 6 7
22 62 90 100
of nominal stress level on ring cross-
stress a,
Nominal stress a, @@a) 40 110 160 180
holders and compressed in accordance with the calculated nominal stress level in the critical ring crosssection. Then the ring was driven to oscillate at 2840 min-’ in its own plane by the vibration of one holder, connected with the electromotor through an eccentric gear (4). In this investigation rings were loaded for N= 3 X 106, 10 x 106 and 16 X lo6 cycles. After cyclic straining seven specimens (specimens 4-10) were cut out of each ring as shown in Fig. 3(a), so that specimen 7 corresponded to the most loaded ring cross-section. It is evident that the process of a~umulation of fatigue changes as a result of cyclic straining will differ from specimen to specimen for the same number of cycles, because there are differences in a nominal stress level in these crosssections of the ring (Table 1). Further, specimens were tested jointly with the diesel engine cylinder liner segment (stainless steel) in the reciprocating frictional machine with dry friction (Fig. 3(b)). The test duration for each specimen was 1 h at a 1.5 MPa nominal
169
VP. Bulatov, D.N. Vedemikov / Influence of fatigue on machine part wear
pressure. Specimen and liner segment wear values were determined with an analytical balance ( zt 0.1 mg error), To obtain basic data, necessary for the next comparison, non-strained ,specimens were tested in the friction’ai majiine. Wear rates, obtained from test results, were.averaged in the following ring groups: nonstrained ring group (N-O, basic group) and cyclically strained ring groups with N= 3 x lo6 cycles (first group), N= 10X 106 cycles (second group) and N=16X lo6 cycles (third group). The wear rate averages were obtained from
e:
0.5
\
-
2 z & e s t 8
/
0.4
-
J cf
where V,, (mg min-‘) is the piston ring average wear rate in a particular specimens group, V, (mg min-l) is the wear rate of the ith specimen, and II is the number of tested specimens (n = 33 in the basic group, )2=28 in the first group, n =26 in the second group and II = 21 in the third group). In addition, a ring circuit average wear rate spread was calculated for each ring. The results obtained were averaged according to the above-mentioned ring groups:
I 0.35
V,, (mg mine’)=
M,,-M’ 7
where Mb-M’ (mg) is the liner segment wear and n is the number of tested specimens in the group (as described above).
3. Results The analysis of the obtained data (Fig. 4) shows that the piston ring average wear rate V,, decreases from 0.612 to 0.355 mg rnin-‘, that is by a factor of 1.72, with the increase in the number N of piston ring prestraining cycles from 0 to 10 X 106. At the same time the cylinder liner (segments) average wear rate increases from 0.178 to 0.226 mg mm-l, that is by a factor of 1.27. This character of the change in piston ring and cylinder liner wear rates can be explained by the phenomenon of external surface hardening of piston rings subjected to cyclic straining. The observed increase in ring circuit wear rate spread from 13.1% to 32.1% (on average by a factor of 2.45) bears witness to an
I
I
I
/
/
/ \
/
,
/
,
,
, ’
9
‘\
\
\
.
\ \
.
I
\
;
.
\/ .’
\
\
\
ae od f! :: 2 e \
\-\
20
f
-,I d f
o.22~
10~10”
3406
0
Cycles
16*106
number N
Fig. 4. Influence of piston ring cyclic straining on the ring average wear rate V,., on the liner segment average wear rate V, and on the ring circuit average wear rate spread 8,: - . -, V,,; -, V,; ---,
where S, (%) is ring circuit average wear rate spread from the mean. The liner segment average wear rate was calculated from
s
8,.
unequal influence of the cyclic straining process on ring circuit wear. Thus there are hardening sectors and sectors with increasing wear rate but on the whole the ring circuit average wear rate is decreasing. Further, when the number N of straining cycles rises to 16 x lo6 (which corresponds to about 100 h of cyclic straining on the fatigue machine), an increase in the specimens’ average wear rate to 0.523 mg min- ’ is observed, which indicates the non-linear character of the wear resistance change process under cyclic straining. Simultaneously, the liner segment wear rate decreases to 0.202 mg min-‘, which indicates the socalled dishardening (weakening) of the rings external surface in comparison with that observed after N = 10 x lo6 cycles and the ring average wear rate spread decreases to 18.7%. Figure 5 shows the results of an additional investigation of the influence of cyclic straining on specimen wear rates for various nominal stresses in the ring crosssection (every point in Fig. 5 is an averaged result of tests on three specimens). The rise in number of prestraining cycles from 0 to 10X lo6 exerts different influences on the specimens’ wear rates in cross-sections 4, 5, 6 and 7. Thus straining for N=3X lo6 cycles has almost no effect on the initial (with N=O) wear rate of specimens cut off in cross-section 4, decreases sharply the wear rate of specimens in cross-section 5, increases, again insignificantly, the wear rate of specimens in
170
V2. Bulatov, D.N. Vedemikzov I Influence of fatigue on machine part wear
\Ii-0
&
0.6
0.5 \E E” P e b $ 0.4 E B 2 0.3
0.2
t 0
3.106
IO-IO6
woe
Cycles number N
Fig. 5. Influence of piston ring cyclic straining on the average wear rate of the ith specimen (average values from nine rings): - . -, cross-section 4 (u” = 40 MPa); -, cross-section 5 (an = 110 MPa); - . * -, cross-section 6 (o;l = 160 MPa); - - -, cross-section 7 (a, = 180 MPa).
cross-section 6 and decreases sharply the wear rate of specimens in cross-section 7. With the rise in the number N of cycles to 10 x lo6 a tendency of the wear rate to decrease is observed for cross-sections 4, 5 and 6 but a tendency of the wear rate to increase to a level somewhat lower than the initial level is observed for cross-section 7. With a further increase in the number N of cycles to 16 X lo6 an increase in wear rate in all cross-sections is observed. Thus, the plot of the specimens’ wear rate change in the most heavily loaded cross-section 7 (a, = 180 MPa) has a character which may be conventionally called periodical. A specimen wear rate decrease, with a subsequent increase in the wear rate of one specimen to a level somewhat higher than the initial level, takes place during a conventional period of 16 x lo6 cycles. An analogous character of the wear rate change is found in all the other ring cross-sections (4, 5 and 6). However, the periodic&y of the wear rate change in these cross-sections is not as complete in form as in cross-section 7, probably as a consequence of the lower stress a,. In this connection the representation of the given data in the form of the dependence of the specimens’ average wear rate on nominal stress level at N= 10X lo6 cycles is of special interest. As shown
in Fig. 6, it can be seen that the so-called pe~~ci~ of specimen wear rate change with a particular number of straining cycles is related to the dependence on nominal stress level. It must be noted that the investigated character of the change in wear rate of cast iron subjected to cyclic straining should be examined in the context of the known complex fatigue-induced changes in the physical-mechanical properties of Fe-C alloys [lo]. Therefore the evaluation of the influence of cyclic straining on the residual stress value and sign is of special interest. For the purpose of estimating the residual stress in the external surface layer of the piston rings the specimens were investigated with an X-ray diffractometer (DRON3M) and the so-called sin?Pmethod for comparison of cyclically strained (N= 10 x 106) and unstrained (N = 0) rings. A level of N = 10 X lo6 cycles was used because the specimens exhibited the lowest average wear rate at this value of N (Fig. 4). As shown in Table 2 the average residual stress level in unstrained specimens equals 242 MPa, that is a tensile stress was found. The average residual stress level in cyclically strained specimens was -331 MPa, that is compressive stress was found. Thus, in this case the cyclic straining has caused the reversal of the sign of the residual stress from positive to negative. Figure 7 shows a schematic diagram and the preliminary results of average change in wear rate V,
0.5
0.3
0.2 80
40 Nomi~l
120
:,
stress, MPa on
Fig. 6. Dependence of specimens’ average wear rate on nominal stress level (at N= 10X 106 cycles).
VP. Bulatov,LkN. Vedem&m f Incise TABLE 2. The effect of cyclic straining stress
on specimens’
residual
Number of straining cycles
Specimen
0
261 262 263
275 f 9.8 216iQ.g 258k9.8
i-242
161 162 163
-3QZi9.8 -323~9.8 - 275 f 9.8
-331
1OXlod
(bf
v
.c
0.6
Residual stress (MPa)
Average residual stress (ma)
E
ii!
OS 0.5 B ij L 0.4 ; 5
0.3
0.2
l,I
I
2
3
Series test number (number of worn-out layer)
Fig. 7. (a) Schematic diagram and (b) results of specimen wear rate determination in the direction of specimen radial thickness.
estimates in the direction of the specimen’s radial thickness (that is, in the “depth” of the specimen’s external surface layer). Three series of 1 h tests of seven specimens were carried out. After every test a specimen was taken out and weighed. An average wear rate was calculated from the results of every test series. As the preliminary results reveal, the wear rate gradient in the radial thickness direction has a complicated variable character in the general case. Both more and less wear-resistance layers could lie under the specimen surface layer (more wear resistance at N= 16 X 106 cycles and less wear resistance at N= 10X lo6 cycles). The calculated thicknesses of worn-out layers after a 1 h test were found to cover a wide range and averaged about 0.05 mm. It can be seen that the distinct wear rate change for specimens subjected to cyclic straining
of fatigueon machinepart wear
171
reaches a “depth” not less than 0.15 mm from the initial specimen’s external surface.
4, Discussion The results in Table 2 are of considerable interest in connection with the fact that many real technological methods for increasing the wear resistance of machine parts are based on residual compressive (negative) stress formation in a particular surface layer. The results obtained for the example of diesel engine piston rings show that there is possibility of developing a new method of increasing the wear resistance of machine parts by cyclic mechanical pre-straining. This method is conventionally called the technology of specific cyclic straining treatment for increasing the fatigue wear resistance and it differs from the traditional technology by, in particular, the operation of specific cyclic straining in an additional special unit, introduced into the concluding stage of the manufac~~g process. Considering this aspect more widely and taking into account the data of Fig. 7, it can be said that there is a possibility of controlling a material’s frictional behaviour by corresponding stimulation or delay of the processes taking place at a frictional contact [5]. It is of practical interest, for example, in the special case when the external surface layer turns out to be less wear resistant than layers in the “depth” as a result of cyclic straining (from the viewpoint of improving the details of heavy duty running-in). Considering on the whole the experimental results shown in Figs. 4-7, we can suppose that there are cyclic dependences, reflecting details of the oscillation in the wear rate I’, about the initial value V, subjected to a change in the number N of straining cycles (Fig. 8(a)), the nominal stress level at N=constant (Fig. 8(b)) and the distance a from an external surface at N-constant (Fig. 8(c)). Thus, we can suppose that specific cyclic mechanical straining cases a specific cyclic wear change. This supposition reflects the essence of the above-mentioned theory of machine part cyclic wear (which has already been noted in Section 1) for which Fig. 8 can serve as a typical i~us~ation. The im~~ance of developing the previously mentioned theory now becomes evident. It must be noted that the curves’ form shown in Fig. 8 has a conventional character and defines the main peculiarity only as an initial wear rate change (increase or decrease) and next a return to a level close to the initial value. Questions concerning the number of periods and their duration (whether for more 16X lo6 cycles or less), direction of oscillation and character (damping or increasing), location of extrema etc. are left open. All these questions are closely associated with fatigue problems, so the large
KP. Bulatov, D.N. Vedendkm I inlftuence of fatigue on machine part wear
172 (a)
vd
,
,I--\ I
vo . e 8 i% 2
I t \
:
\\
\
I
‘_’
J
i
Cycles number N(,) (b)
Stress v”nfTt
(c) s!
e
P Distance a CT1
Fig. 8. Supposed cyclic character of specific wear rate change as a function of (a) number of straining cycies, (b) nominal stress level and (c) distance from the extemai surface V, the initial value of the wear rate V,.
amount of knowledge available in the area of fatigue must be taken into account in subsequent investigations in any case.
5. Conclusions (1) Aspects of the relationship between fatigue and machine part wear are considered. (2) Results from an investigation of the ir&uence of the fatigue process on machine part wear are given for the example of diesel engine piston rings.
(3) The problem’s solution could open a prospect for the development of a new technology for increasing the fatigue wear resistance of piston rings by a cyclic straining treatment. (4) The supposition that a particular cyclic mechanical straining process causes a corresponding cyclic wear change is put forward on the basis of the analysis of the results.
References 1 H.P. Jost, Tribology - origin and future, Wear, 136 (1990) 1-17. 2 V.P. Buiatov and D.N. Vedemikov, Correlation between piston ring fatigue properties and wear resistance, Dvigatelestrojeni (2) (1989) 12-14. 3 V.P. Buiatov and D.N. Vedernikov, Effect of cyclic straining on the piston ring wear resistance, Tmzie fmos, 12 (1991) 515-520. 4 I.V. Kragelsky, M.N. Dobychin and V.S. Kombalov, Friction and Wear Calculation deter, Pergamon, Oxford, 1982, pp. 464. M. Hebda and A.V. Chichinadze (eds.), Tribology Handbook, Vol. 1, Mashinostrojenije, Moscow-VI& Warsaw, 1989. VS. Avduevsky and M.A. Bronovets, Main trends in tribologicai development in the USSR, Wear, 136 (1990) 47-60. N. Yahata, T. Hirata, T. Kate and M. Watanabe, Effect of sliding friction on the fatigue strength of a medium carbon steel, Wear, 121 (1988) 197-209. Part: a I.M. Fedorchenko (ed.),Dictionary-HandbookofMachine Friction, Wear and L.&ication, Naukova Dumka, Kiev, 1990, pp. 224. 9 V.V. Bolotin, Service Life of Machines and Constructions, Mashinostrojenije, Moscow, 1990, pp. 447. Mos10 S. Kocanda, Fatigue Cracks in Metals, Mashinostrojenije, cow, 1990, pp. 622.
167
167-172
On the influence of fatigue on machine part wear Vladimir P. Bulatov The Institute for Mechanical
and Dmitri
N. Vedernikov*
Engineering Problems, The Russian Academy of Sciences, 5-l-500 Bajkonurskaja Street, 197348 St. Petersburg (Bus&n Federation) (Received October 28, 1992; revised and accepted April 1, 1993)
Abstract The
main aspects of the relationship between fatigue and machine part wear are considered and the relevant problem is formulated. The results of an investigation of the influence of the fatigue process on machine part wear are given for diesel engine piston rings. As the results show, the solution of the problem could open up new prospects in the development of the theory of machine part cyclic wear and the details of the technology of cyclic straining treatments for increasing fatigue wear resistance.
1. Introduction
The relationship
As tribologists we are doomed to be on the brink, in the sense that tribology is a multidisciplinary and contiguous field [l] on the borders of which a common phenomenon, called wear by everybody occurs. It is difficult to avoid the impression that fatigue problems are present in most tribological investigations as a visible or invisible shadow. Also, it is difficult to ignore the impression that the large amount of knowledge accumulated in the area of fatigue approaches a critical level when a qualitative leap would be expected to occur. The words (synonyms) “periodicity” and “cyclicity” should be emphasized owing to their special significance in nature. It is now becoming evident that further tribological developments will be determined to a considerable extent by knowledge of the problems of relations between fatigue and machine part wear. The purpose of this paper is to attempt to systematize knowledge in the area of relations between fatigue and machine part wear. In addition, experimental data analysis [2, 31 and determination of suitable directions of future research into the above-mentioned problems are considered. In general the relations between fatigue and machine part wear can be considered from the following main points of view (Fig. 1). 1.1. The relationship between fatigue strength limit and wear This direction involves the traditionally studied questions of the influence of detailed micro- and macro*Author to whom correspondence
0043-1648l93/$6.00
should be addressed.
between
fatigue.
The associated problem mechanical operatmnal
of cyclic stramlng
Fig. 1. Problems of the relationship between fatigue and machine part wear.
geometric changes on the fatigue strength limit. Usually an investigation reaches an experimental evaluation of the change in fatigue strength limit during detailed work with a machine as a result of information on the decrease in cross-sectional area during the wear process, the appearance of scratches as stresses concentrators on sliding surfaces, changes in the residual stress field and other factors [7, 81. 1.2. The relationship between details of the fatigue changes in structural material elements and wear This aspect involves the problems of the influence of fatigue changes in detail of structural material ele-
0 1993 - Elsevier Sequoia. All rights reserved
168
VP. Bulatov, D.N. Vedemikov I Infruence of fatigue on machine part wear
ments (which may be seen at any level, usually beginning from the crystal lattice [9]) on the details of the wear. Certainly, the changes in the surface layer structure are most significant in their consequences. Therefore it is expedient to classify this relationship, considering in this respect the kind of fatigue factor (cyclic mechanical straining, thermal, frictional and other possibfe kinds of strain) and the time during which these fatigue factors act (before work or during particular work in a machine). At present the problems of cyclic frictional straining are studied to a very high degree. The fatigue theory of wear has been evolved and formulae for calculation have been obtained [4, 51. Development of the fatigue theory of wear was one of the main trends in tribological research in the former USSR [6]. It is not expedient to consider at present cyclic thermal straining together with cyclic straining of other possible kinds as starting problems owing to their particular difficulties. There are many frictional machine parts which are subjected to a cyclic load in operation. The processes take place in parallel during operational straining, so a complicated detailed picture of fatigue changes is created. Therefore, it is necessary to limit our intentions to consideration of the wear change processes that depend on the accumulation of fatigue changes which can take place as a result of so-called cyclic mechanical pre-straining before specific work (operation) in a machine (the initial problem, Fig. 1). The solution of this problem will permit us to pass to the next study: that of the influence of cyclic mechanical operational straining on wear in detail. Further, the analysis of results of theoretical and experimental investigations of the influence of cyclic straining on wear would certainly lead to an expedited development of a new theory: this may be called the theory of machine part cyclic wear. The future theory would help to make up a deficiency of knowledge in the area of the relationship between fatigue and machine part wear.
2. Experimental procedure The experimental determination of the influence of cyclic mechanical pre-straining on wear was carried out using standard cast iron 150 mm diameter piston rings of a diesel engine. The method involved the following stages: cyclic pre-straining of the ,piston rings on the fatigue machine, making (cutting out) the differently loaded specimens from a ring, and testing the specimens on the reciprocating frictional machine. Piston ring cyclic straining was carried out in accordance with the scheme shown in Fig. 2 in the following manner. A piston ring (1) with cut-off ends was fastened to the moving (2) and to the non-moving (3) fatigue machine
Fig. 2. Schematic
’
‘\
diagram of piston ring cyclic straining.
(bf IP
22% Fig. 3. (a) Schematic diagram of cut-out piston ring specimens. (b) Schematic diagram of specimen engagement with a cylinder liner segment: 1, specimen; 2, liner segment. TABLE 1. The dependence section Cross-section
Nominal (“ro)
4 5 6 7
22 62 90 100
of nominal stress level on ring cross-
stress a,
Nominal stress a, @@a) 40 110 160 180
holders and compressed in accordance with the calculated nominal stress level in the critical ring crosssection. Then the ring was driven to oscillate at 2840 min-’ in its own plane by the vibration of one holder, connected with the electromotor through an eccentric gear (4). In this investigation rings were loaded for N= 3 X 106, 10 x 106 and 16 X lo6 cycles. After cyclic straining seven specimens (specimens 4-10) were cut out of each ring as shown in Fig. 3(a), so that specimen 7 corresponded to the most loaded ring cross-section. It is evident that the process of a~umulation of fatigue changes as a result of cyclic straining will differ from specimen to specimen for the same number of cycles, because there are differences in a nominal stress level in these crosssections of the ring (Table 1). Further, specimens were tested jointly with the diesel engine cylinder liner segment (stainless steel) in the reciprocating frictional machine with dry friction (Fig. 3(b)). The test duration for each specimen was 1 h at a 1.5 MPa nominal
169
VP. Bulatov, D.N. Vedemikov / Influence of fatigue on machine part wear
pressure. Specimen and liner segment wear values were determined with an analytical balance ( zt 0.1 mg error), To obtain basic data, necessary for the next comparison, non-strained ,specimens were tested in the friction’ai majiine. Wear rates, obtained from test results, were.averaged in the following ring groups: nonstrained ring group (N-O, basic group) and cyclically strained ring groups with N= 3 x lo6 cycles (first group), N= 10X 106 cycles (second group) and N=16X lo6 cycles (third group). The wear rate averages were obtained from
e:
0.5
\
-
2 z & e s t 8
/
0.4
-
J cf
where V,, (mg min-‘) is the piston ring average wear rate in a particular specimens group, V, (mg min-l) is the wear rate of the ith specimen, and II is the number of tested specimens (n = 33 in the basic group, )2=28 in the first group, n =26 in the second group and II = 21 in the third group). In addition, a ring circuit average wear rate spread was calculated for each ring. The results obtained were averaged according to the above-mentioned ring groups:
I 0.35
V,, (mg mine’)=
M,,-M’ 7
where Mb-M’ (mg) is the liner segment wear and n is the number of tested specimens in the group (as described above).
3. Results The analysis of the obtained data (Fig. 4) shows that the piston ring average wear rate V,, decreases from 0.612 to 0.355 mg rnin-‘, that is by a factor of 1.72, with the increase in the number N of piston ring prestraining cycles from 0 to 10 X 106. At the same time the cylinder liner (segments) average wear rate increases from 0.178 to 0.226 mg mm-l, that is by a factor of 1.27. This character of the change in piston ring and cylinder liner wear rates can be explained by the phenomenon of external surface hardening of piston rings subjected to cyclic straining. The observed increase in ring circuit wear rate spread from 13.1% to 32.1% (on average by a factor of 2.45) bears witness to an
I
I
I
/
/
/ \
/
,
/
,
,
, ’
9
‘\
\
\
.
\ \
.
I
\
;
.
\/ .’
\
\
\
ae od f! :: 2 e \
\-\
20
f
-,I d f
o.22~
10~10”
3406
0
Cycles
16*106
number N
Fig. 4. Influence of piston ring cyclic straining on the ring average wear rate V,., on the liner segment average wear rate V, and on the ring circuit average wear rate spread 8,: - . -, V,,; -, V,; ---,
where S, (%) is ring circuit average wear rate spread from the mean. The liner segment average wear rate was calculated from
s
8,.
unequal influence of the cyclic straining process on ring circuit wear. Thus there are hardening sectors and sectors with increasing wear rate but on the whole the ring circuit average wear rate is decreasing. Further, when the number N of straining cycles rises to 16 x lo6 (which corresponds to about 100 h of cyclic straining on the fatigue machine), an increase in the specimens’ average wear rate to 0.523 mg min- ’ is observed, which indicates the non-linear character of the wear resistance change process under cyclic straining. Simultaneously, the liner segment wear rate decreases to 0.202 mg min-‘, which indicates the socalled dishardening (weakening) of the rings external surface in comparison with that observed after N = 10 x lo6 cycles and the ring average wear rate spread decreases to 18.7%. Figure 5 shows the results of an additional investigation of the influence of cyclic straining on specimen wear rates for various nominal stresses in the ring crosssection (every point in Fig. 5 is an averaged result of tests on three specimens). The rise in number of prestraining cycles from 0 to 10X lo6 exerts different influences on the specimens’ wear rates in cross-sections 4, 5, 6 and 7. Thus straining for N=3X lo6 cycles has almost no effect on the initial (with N=O) wear rate of specimens cut off in cross-section 4, decreases sharply the wear rate of specimens in cross-section 5, increases, again insignificantly, the wear rate of specimens in
170
V2. Bulatov, D.N. Vedemikzov I Influence of fatigue on machine part wear
\Ii-0
&
0.6
0.5 \E E” P e b $ 0.4 E B 2 0.3
0.2
t 0
3.106
IO-IO6
woe
Cycles number N
Fig. 5. Influence of piston ring cyclic straining on the average wear rate of the ith specimen (average values from nine rings): - . -, cross-section 4 (u” = 40 MPa); -, cross-section 5 (an = 110 MPa); - . * -, cross-section 6 (o;l = 160 MPa); - - -, cross-section 7 (a, = 180 MPa).
cross-section 6 and decreases sharply the wear rate of specimens in cross-section 7. With the rise in the number N of cycles to 10 x lo6 a tendency of the wear rate to decrease is observed for cross-sections 4, 5 and 6 but a tendency of the wear rate to increase to a level somewhat lower than the initial level is observed for cross-section 7. With a further increase in the number N of cycles to 16 X lo6 an increase in wear rate in all cross-sections is observed. Thus, the plot of the specimens’ wear rate change in the most heavily loaded cross-section 7 (a, = 180 MPa) has a character which may be conventionally called periodical. A specimen wear rate decrease, with a subsequent increase in the wear rate of one specimen to a level somewhat higher than the initial level, takes place during a conventional period of 16 x lo6 cycles. An analogous character of the wear rate change is found in all the other ring cross-sections (4, 5 and 6). However, the periodic&y of the wear rate change in these cross-sections is not as complete in form as in cross-section 7, probably as a consequence of the lower stress a,. In this connection the representation of the given data in the form of the dependence of the specimens’ average wear rate on nominal stress level at N= 10X lo6 cycles is of special interest. As shown
in Fig. 6, it can be seen that the so-called pe~~ci~ of specimen wear rate change with a particular number of straining cycles is related to the dependence on nominal stress level. It must be noted that the investigated character of the change in wear rate of cast iron subjected to cyclic straining should be examined in the context of the known complex fatigue-induced changes in the physical-mechanical properties of Fe-C alloys [lo]. Therefore the evaluation of the influence of cyclic straining on the residual stress value and sign is of special interest. For the purpose of estimating the residual stress in the external surface layer of the piston rings the specimens were investigated with an X-ray diffractometer (DRON3M) and the so-called sin?Pmethod for comparison of cyclically strained (N= 10 x 106) and unstrained (N = 0) rings. A level of N = 10 X lo6 cycles was used because the specimens exhibited the lowest average wear rate at this value of N (Fig. 4). As shown in Table 2 the average residual stress level in unstrained specimens equals 242 MPa, that is a tensile stress was found. The average residual stress level in cyclically strained specimens was -331 MPa, that is compressive stress was found. Thus, in this case the cyclic straining has caused the reversal of the sign of the residual stress from positive to negative. Figure 7 shows a schematic diagram and the preliminary results of average change in wear rate V,
0.5
0.3
0.2 80
40 Nomi~l
120
:,
stress, MPa on
Fig. 6. Dependence of specimens’ average wear rate on nominal stress level (at N= 10X 106 cycles).
VP. Bulatov,LkN. Vedem&m f Incise TABLE 2. The effect of cyclic straining stress
on specimens’
residual
Number of straining cycles
Specimen
0
261 262 263
275 f 9.8 216iQ.g 258k9.8
i-242
161 162 163
-3QZi9.8 -323~9.8 - 275 f 9.8
-331
1OXlod
(bf
v
.c
0.6
Residual stress (MPa)
Average residual stress (ma)
E
ii!
OS 0.5 B ij L 0.4 ; 5
0.3
0.2
l,I
I
2
3
Series test number (number of worn-out layer)
Fig. 7. (a) Schematic diagram and (b) results of specimen wear rate determination in the direction of specimen radial thickness.
estimates in the direction of the specimen’s radial thickness (that is, in the “depth” of the specimen’s external surface layer). Three series of 1 h tests of seven specimens were carried out. After every test a specimen was taken out and weighed. An average wear rate was calculated from the results of every test series. As the preliminary results reveal, the wear rate gradient in the radial thickness direction has a complicated variable character in the general case. Both more and less wear-resistance layers could lie under the specimen surface layer (more wear resistance at N= 16 X 106 cycles and less wear resistance at N= 10X lo6 cycles). The calculated thicknesses of worn-out layers after a 1 h test were found to cover a wide range and averaged about 0.05 mm. It can be seen that the distinct wear rate change for specimens subjected to cyclic straining
of fatigueon machinepart wear
171
reaches a “depth” not less than 0.15 mm from the initial specimen’s external surface.
4, Discussion The results in Table 2 are of considerable interest in connection with the fact that many real technological methods for increasing the wear resistance of machine parts are based on residual compressive (negative) stress formation in a particular surface layer. The results obtained for the example of diesel engine piston rings show that there is possibility of developing a new method of increasing the wear resistance of machine parts by cyclic mechanical pre-straining. This method is conventionally called the technology of specific cyclic straining treatment for increasing the fatigue wear resistance and it differs from the traditional technology by, in particular, the operation of specific cyclic straining in an additional special unit, introduced into the concluding stage of the manufac~~g process. Considering this aspect more widely and taking into account the data of Fig. 7, it can be said that there is a possibility of controlling a material’s frictional behaviour by corresponding stimulation or delay of the processes taking place at a frictional contact [5]. It is of practical interest, for example, in the special case when the external surface layer turns out to be less wear resistant than layers in the “depth” as a result of cyclic straining (from the viewpoint of improving the details of heavy duty running-in). Considering on the whole the experimental results shown in Figs. 4-7, we can suppose that there are cyclic dependences, reflecting details of the oscillation in the wear rate I’, about the initial value V, subjected to a change in the number N of straining cycles (Fig. 8(a)), the nominal stress level at N=constant (Fig. 8(b)) and the distance a from an external surface at N-constant (Fig. 8(c)). Thus, we can suppose that specific cyclic mechanical straining cases a specific cyclic wear change. This supposition reflects the essence of the above-mentioned theory of machine part cyclic wear (which has already been noted in Section 1) for which Fig. 8 can serve as a typical i~us~ation. The im~~ance of developing the previously mentioned theory now becomes evident. It must be noted that the curves’ form shown in Fig. 8 has a conventional character and defines the main peculiarity only as an initial wear rate change (increase or decrease) and next a return to a level close to the initial value. Questions concerning the number of periods and their duration (whether for more 16X lo6 cycles or less), direction of oscillation and character (damping or increasing), location of extrema etc. are left open. All these questions are closely associated with fatigue problems, so the large
KP. Bulatov, D.N. Vedendkm I inlftuence of fatigue on machine part wear
172 (a)
vd
,
,I--\ I
vo . e 8 i% 2
I t \
:
\\
\
I
‘_’
J
i
Cycles number N(,) (b)
Stress v”nfTt
(c) s!
e
P Distance a CT1
Fig. 8. Supposed cyclic character of specific wear rate change as a function of (a) number of straining cycies, (b) nominal stress level and (c) distance from the extemai surface V, the initial value of the wear rate V,.
amount of knowledge available in the area of fatigue must be taken into account in subsequent investigations in any case.
5. Conclusions (1) Aspects of the relationship between fatigue and machine part wear are considered. (2) Results from an investigation of the ir&uence of the fatigue process on machine part wear are given for the example of diesel engine piston rings.
(3) The problem’s solution could open a prospect for the development of a new technology for increasing the fatigue wear resistance of piston rings by a cyclic straining treatment. (4) The supposition that a particular cyclic mechanical straining process causes a corresponding cyclic wear change is put forward on the basis of the analysis of the results.
References 1 H.P. Jost, Tribology - origin and future, Wear, 136 (1990) 1-17. 2 V.P. Buiatov and D.N. Vedemikov, Correlation between piston ring fatigue properties and wear resistance, Dvigatelestrojeni (2) (1989) 12-14. 3 V.P. Buiatov and D.N. Vedernikov, Effect of cyclic straining on the piston ring wear resistance, Tmzie fmos, 12 (1991) 515-520. 4 I.V. Kragelsky, M.N. Dobychin and V.S. Kombalov, Friction and Wear Calculation deter, Pergamon, Oxford, 1982, pp. 464. M. Hebda and A.V. Chichinadze (eds.), Tribology Handbook, Vol. 1, Mashinostrojenije, Moscow-VI& Warsaw, 1989. VS. Avduevsky and M.A. Bronovets, Main trends in tribologicai development in the USSR, Wear, 136 (1990) 47-60. N. Yahata, T. Hirata, T. Kate and M. Watanabe, Effect of sliding friction on the fatigue strength of a medium carbon steel, Wear, 121 (1988) 197-209. Part: a I.M. Fedorchenko (ed.),Dictionary-HandbookofMachine Friction, Wear and L.&ication, Naukova Dumka, Kiev, 1990, pp. 224. 9 V.V. Bolotin, Service Life of Machines and Constructions, Mashinostrojenije, Moscow, 1990, pp. 447. Mos10 S. Kocanda, Fatigue Cracks in Metals, Mashinostrojenije, cow, 1990, pp. 622.