The unified methods of wear-fatigue tests of models of active systems, which are based on a combination of the known mechanical fatigue, friction and wear test methods, are offered. A bending fatigue test method for a uniform cylindrical specimen with a test portion diameter of 10 mm is adopted as a basic one.Предложены единые методики износоуста­лостных испытаний моделей активных систем, основанные на известных методах испытания материалов на механическую усталость, трение и износ. В качестве базо­вой предложена методика испытания на циклический изгиб гладкого цилиндричес­кого образца с рабочей частью диаметром 10 мм

Andriyashin, V.A.

Bahdanovich, A.V.

Elavyi, A.M.

Tyurin, S.A.

Наукова електронна бібліотека періодичних видань НАН України

UDC 539.4Wear-Fatigue Test Methods and Their SignificanceA. V. B ahdanovich ,a S. A. T y u rin ,b V. A. A n d riy ash in ,b and  A. M . E lavyiba Yanka Kupala State University of Grodno, Grodno, Belarusb The Joint Institute of Mechanical Engineering, National Academy of Sciences of Belarus, Minsk, BelarusThe unified methods o f  wear-fatigue tests o f  models o f  active systems, which are based on a combination o f  the known mechanical fatigue, friction and wear test methods, are offered. A bending fatigue test method fo r  a uniform cylindrical specimen with a test portion diameter o f  10 mm is adopted as a basic one.K e y w o r d s : fatigue, friction, wear-fatigue tests, specimen, counterspecimen.In tro d u c tio n . Special wear-fatigue test methods have been elaborated for experimental assessment o f mutual and jo in t influence o f friction and fatigue processes on the working capacity o f  materials and models o f active systems under complex loading conditions [1-5].Under laboratory conditions the wear-fatigue damage resistance is usually studied by testing small models o f active systems. The tests are perform ed on special wear-fatigue test machines o f a SI series [5-6].One o f the ways o f developing complex wear-fatigue test methods is to combine the known m echanical fatigue test methods with the friction and wear test methods. Figure 1 illustrates, as an example, the principle o f such combination whereby a basic fatigue test m ethod incorporates bending w ith rotation. Note that rotary m otion is m ost typical o f  m odern machines; therefore, the methods as shown in Fig. 1 are o f  practical importance.A  similar approach enables the m achines intended for wear-fatigue tests to be used for conventional tests or m echanical fatigue tests or for friction and wear testing under preset conditions.The Basic Test Schem es. A  test object for m echanical fatigue tests is a structural element, for example, a cylindrical one o f a given geom etry (Fig. 2c). If the tests are perform ed in the sliding or rolling friction modes, the test object is a friction pair (Fig. 2b, c) consisting o f specimen 1 and counterspecimen 2; they are also called the body and the counterbody. N ote that here the specimen is always referred to as the cylindrical structural element and the counterspecimen (counter­body) as the bushing or the roller. Finally, in wear-fatigue tests the test objects are the models o f active systems o f two elements -  1 and 2  (Fig. 2a, d).It should be mentioned that all the methods o f wear-fatigue testing (Fig. 2) im plemented on SI series machines are based on using a uniform  smooth cylindrical specimen with a test portion diameter d  =  2 r =  10 mm. It is identical to a standard fatigue test specimen. This provides both the consistency o f tests as well as comparability o f test results.L et’s address the m echano-sliding fatigue test scheme (Fig. 2e). The cylindrical specimen 1 is fixed in a spindle 2  and rotates with angular speed « 1.© A. V. BAHDANOVICH, S. A. TYURIN, V. A. ANDRIYASHIN, A. M. ELAVYI, 2009ISSN 0556-171X. Проблемы прочности, 2009, №  1 121A. V. Bahdanovich, S. A. Tyurin, V. A. Andriyashin, and A. M. ElavyiFriction and wear tests methodsFig. 1. Development of wear-fatigue test methods: MRF -  mechano-rolling fatigue, MSF -  mechano-sliding fatigue, FF -  fretting fatigue.A  vertical bending load Q  (upwards or downwards) is applied to its free end. Also, a nonrotating counterspecimen 3, for example, a plate or a partial bushing, is in contact w ith the specimen test portion zone o f diam eter d  =  10 m m  under a contact load F N . Thus, the m aximum contact and bending stresses arise sim ultaneously in the specimen test portion zone.Im plementation o f the test scheme as shown in Fig. 2e makes it possible to perform  the following tests:-  wear-fatigue tests for m echano-sliding fatigue (Fig. 2e) w ith variables F N , Q , and w 1;-  m echanical fatigue tests in bending w ith rotation (Fig. 2c) w ith variables Q  and w 1. In this case, the counterspecimen 3  is removed, so F N =  0;-  sliding friction and w ear tests (Fig. 2c) with variables F n  and m. In this case, no bending loading is applied (Q =  0), and specimen 1 is made shorter for the sake o f m aterial saving.In m echanical fatigue tests (Fig. 2c) the bending load Q  can be constant (invariable in time t), but the operating norm al stresses at every point o f  the working section o f specimen 1 change during a symmetric cycle (Fig. 3) with period T  due to rotation o f the specimen.I f  the greatest bending m om ent in the specimen working section is M  =  Q l, where l  is the distance from the w eakest section to a load action line Q; the highest norm al stresses in the same section are given bya  =  M / W , (1)where W  is the m om ent o f resistance.In sliding friction tests (Fig. 2d), the contact load F n  can be static, i.e., constant, but the operating contact stresses are cyclic too. Therefore, these tests are essentially the sliding fatigue tests (under asymmetric tension-compression conditions).M echanical fatigue tests methods122 ISSN 0556-171X. npoôneMbi npoHHoemu, 2009, № 1Wear-Fatigue Test Methods and Their SignificanceFig. 2. Typical wear-fatigue test methods: 1, 1a, 1b -  specimens; 2 -  test apparatus spindle; 3, 4 -  counterspecimena; Q is bending load, FN is contact load, and and w2 are rotational speeds of a specimen and counterspecimen, respectively.The conditions whereby sliding fatigue is realized can be described integrally by either contact loading F N  or an average (nominal) contact pressure (2), or a specific sliding friction force called also the frictional stresses (3):P a  =  F N / A a > (2)r  Ws =  f  sP a  =  F s / A a =  f s F N  / A a , (3)where A a is the nominal area o f contact, F s is the sliding friction force, and f s is the coefficient o f  sliding friction.ISSN 0556-171X. npoôëeMbi nponnocmu, 2009, № 1 123A. V. Bahdanovich, S. A. Tyurin, V. A. Andriyashin, and A. M. ElavyiLet us consider the m echano-rolling fatigue test scheme (Fig. 2a). It differs from the mechano-sliding fatigue test scheme (Fig. 2e) in that the fixed bushing is replaced w ith a rotating roller 3. Thus, the specimen and the roller can rotate generally w ith different angular speeds W j and m 2 and in different directions.Realization o f the test scheme as shown Fig. 2a enables one to carry out the following tests:-  wear-fatigue tests for m echano-rolling fatigue (Fig. 2a) w ith variables F N , Q , W j, and m2 ;-  m echanical fatigue tests in bending w ith rotation (Fig. 2c) w ith variables Q  and m j . In this case, roller 3  is removed, so F N =  0 and m2 =  0;-  tests in rolling friction or sliding-and-rolling friction (Fig. 2b) with variables F N , m 1, and m2. In this case, no bending load is applied (Q =  0), and specimen 1 is made shorter for the purpose o f m aterial saving.The conditions whereby rolling friction is realized (see Fig. 2b) can be described by either a contact load F n  , or the highest pressure in the center o f  a contact area (4) which is defined by the Hertz formula (for a case o f elastic deformation), or a specific rolling friction force (5) called also the frictional stress'Wrp 0 n p F N / A p  ,=  f r P 0 =  F r l A a =  f r F N  / A a(4)(5)where A p  is the area o f contact (A p  =  a  for a circular contact area o f  radius a,A p  =  lb  for a band-shaped contact zone m easuring l  X  b , and A p  =  a b  for an elliptic contact area o f dimensions a  X  b), n p  is the factor (n p  =  0.478 for circular and elliptic contact areas and n p  =  0.637 for a band-shaped contact zone), F r is the rolling friction force, and f r is the coefficient o f  rolling friction.Fig. 3 Fig. 4Fig. 3. A symmetrical stress cycle in mechanical fatigue tests. Fig. 4. Cycle of stresses in rolling fatigue tests.In rolling friction tests (see Fig. 2b) the contact load F n  , as in sliding friction, can be static, i.e., constant in time, but the operating contact pressure (for example, p 0 =  a zmax) is cyclic (Fig. 4). Thus, the rolling friction tests by the scheme in Fig. 2, are essentially the rolling fatigue tests o f m aterial surface layer.124 ISSN 0556-171X. npoôëeMbi npounocmu, 2009, N9 1Wear-Fatigue Test Methods and Their SignificanceThe fretting fatigue test scheme is shown in Fig. 5a. In this case, two counterspecimens 3  called the fretting bridges are pressed w ith a contact load F N to a test portion o f  the rotating cylindrical specimen 1 subjected to a bending load Q. It can be given circumferential (with a speed v  j) or axial (with a speed v  2) oscillatory m ovem ent o f small amplitude or to raise both simultaneously to the last.abcFig. 5. Test schemes for fretting fatigue (a), mechanical fatigue (b), and fretting (c).Implementation o f the test scheme as shown in Fig. 5 permits the following types o f tests:-  wear-fatigue tests for fretting fatigue (see Fig. 5a) w ith variables F n  , Q, w, v and v 2;-  m echanical bending fatigue tests with rotation (see Fig. 5b) with variables Q  and w. In this case, no fretting bridges are used, so F N =  0, v  2 =  v  2 =  0;-  fretting tests with axial and/or circumferential sliding (see Fig. 5c) with variables F N , v and v 2 . In this case, no bending load is applied (Q =  0), and specimen 1 is m ade shorter for the purpose o f m aterial saving.The conditions o f force interaction between the specimen and the counter­specimen in fretting fatigue can be represented by cyclic stresses (1), frictional stresses (3) or nominal contact pressure,q  =  f n / a o ,  (6)where A 0 is the initial (nominal) area o f  contact.ISSN 0556-171X. Проблемы прочности, 2009, № 1 125A. V. Bahdanovich, S. A. Tyurin, V. A. Andriyashin, and A. M. ElavyiT he Basic C harac te ris tic s  o f R esistance to  W ear-Fatigue D am ages. Thebasic characteristics o f  resistance to wear-fatigue damages are determined by wear-fatigue testing o f appropriate objects.The basic quantitative characteristics o f fracture strength are assessed by test results and by plotting a corresponding fatigue curve.By w ay o f example, Fig. 6 shows four experimental fatigue curves: a m echanical fatigue curve N ( a  a ) plotted by test results for a specimen o f 0.45% carbon steel (normalized); a rolling fatigue curve N ( p 0 ) constructed by the rolling friction tests results for the pair o f  0.45 carbon steel specimen/25KhGT steel roller (after improvement), and two m echano-rolling fatigue curves plotted by wear-fatigue test results for the active system o f 0.45% carbon steel/25KhGT steel.Steel 45 specimen.Curve of mechanical fatigue N  (oa )Steel 45/steel 25KhGT friction pair. Curve of rolling fatigue N  (p0)Mechano-rolling fatigue curves Direct effect N  (aa , p0 = const) Inverse effect N  (p0, a a =  const)Fig. 6. For determination of basic characteristics of wear-fatigue damages (the point number indicates the sequence of tests).In the m echanical fatigue tests, disintegration o f a specimen serves as a limit state criterion. In rolling fatigue tests, a critical density o f pittings on a specim en’s test surface is taken as a limit state criterion. The limit states based on damage126 ISSN 0556-171X. npoôneMu npounocmu, 2009, № 1Wear-Fatigue Test Methods and Their Significanceand fracture criteria for m echanical and rolling fatigue tests take place in tests for mechano-rolling fatigue.In all o f  the four cases, the fatigue limits ( o _ 1, p  j , o _ 1p , p  f o ), parameters o f slope o f the left-hand branch o f fatigue curves (mo , M p , m op , Mpa ), and the abscissas o f critical points o f  fatigue curves ( N Ga, N Gp , N Gop , N Gp a ) are determined. Note that the fatigue limits at m echanical ( o _ 1) and rolling fatigue ( p  f  ) are unequivocal and unique characteristics o f  the test objects, while those in mechano-rolling fatigue tests ( o _  p , p  f o ) are not. Similar fatigue curves to be plotted can be as m any as the num ber o f preset values o f parameters p 0 =  const or o  a =  const in wear-fatigue tests when the mechanisms o f direct and back effects are studied.The influence o f friction and w ear processes on the variation o f mechanical fatigue resistance characteristics can be represented by the direct effectK D = ° _ 1 p / ° _ 1 .  (7)In this case, the K d  index is a characteristic o f  strength. For the conditions for which the results are given in Fig. 6 we have K d  = 256/165 = 1.62.The influence o f m echanical fatigue processes on the variation of characteristics o f a friction and w ear process can be represented by the back effect indexK b  =  p  fo  I  p f  ■ (8)In this case, the K b  index is a tribological characteristic. For the conditions for w hich the test results are presented in Fig. 6 we have K b  = 2200/1760 = 1.25.Table 1 provides notations and summarizes num erical values o f  all the parameters determined by fatigue curves as shown in Fig. 6. A  study o f these experimental data enables us to make the following conclusions:(i) the lim it stresses in m echano-rolling fatigue are essentially higher than those in m echanical and rolling fatigue ( K d  >  1, K b  >  1);(ii) the fatigue curve exponent increases in passing from the mechanical fatigue curve to the corresponding m echano-rolling fatigue curve (m 0p  > >  m o ) and from the rolling fatigue curve to the corresponding m echano-rolling fatigue curve (m po > >  mp  ).T a b l e  1System of Notation and Numerical Values of Basic CharacteristicsCharacteristics Mechanical fatigue curvesRolling fatigue curvesMechano-rolling fatigue curvesN  (a a ) N  (?0) N  (aa , p0 = const) N  (p0, oa =  const)Fatigue limit, MPa 56w_1a p f  =  1760 a_1 p =  256 pfo =  2200Abscissas of critical points of fatigue curves, cyclesN g q =  9-106 Nap =  2.6 • 107 N gqp =  5 • 106 Napo =  2 • 107Fatigue curve exponent ma w 7.5 mp =  14.5 map w 11.6 mpo =  24-6ISSN 0556-171X. Проблемы прочности, 2009, № 1 127A. V. Bahdanovich, S. A. Tyurin, V. A. Andriyashin, and A. M. ElavyiDifferently, under the given experim ental conditions the wear-fatigue resistance to damage has turned out to be higher than the m echanical or rolling fatigue resistance.C o n c l u s i o n s1. Unified methods for complex wear-fatigue testing o f models o f active systems have been developed, which can be im plemented using m odern machines o f a SI series and ensure assessment o f fracture strength under preset conditions.2. N ew  characteristics o f resistance to wear-fatigue damages, w hich are determined from mechano-sliding, m echano-rolling fatigue, and fretting fatigue tests, are proposed.1. L. A. Sosnovskii, “The m ethod o f wear fatigue tests o f  power systems and their m odels,” In t. J . F r ic t . W ear , 14, No. 5, 937-952 (1993).2. K. V. Frolov and N. A. M akhutov, “N ew  test machines and m ethods,” Z a v o d . L a b ., No. 5, 32-33 (1995).3. M. S. Vysotskii, N. A  M akhutov, V. N. Koreshkov, et al., “On development o f standard methods for wear-fatigue tests,” I b id , 35-38 (1995).4. N. A. Mahutov, A. V. Bogdanovich, P. V. Andronov, et al., “M ethods of wear-fatigue tests and their realisation on the SI m achines,” Ib id , No. 6, 17-42 (1995).5. “SI series machines for wear-fatigue tests,” in: L. A. Sosnovskii and M. S.Vysotskii (Eds.), T r ib o -F a tig u e -9 5 :  A n n u a l  [in Russian], Tribo-Fatigue Ltd., Gomel (1996).Received 11. 06. 2008128 ISSN 0556-171X. Проблемы прочности, 2009, №  1

Методики износоусталостных испытаний и их значимость

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Методики износоусталостных испытаний и их значимость

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Наукова електронна бібліотека періодичних видань НАН України