There are currently several trends in strength-of-materials research. The most important of them concern the relationship between the fatigue characteristics and structure of the material [1][2][3][4] and the dependence of these characteristics on the duration of cyclic loading. According to [1], the dependence of the fatigue limit of structurally simple condensed microlayer materials in bending on the thickness of each layer h is described by an equation of the Mott-Stroh type:where a i and K i are coefficients. However, Eq. (1) does not contain the characteristic N --the number of loading cycles. A value of I" 106 cycles was taken for N in [2], while the authors of [3, 4] took 5.107 cycles. The loading base is chosen on the basis of the behavior of the material under cyclic loading and the potential for conducting tests of sufficient duration. If the fatigue Curve of-the material in the coordinates o-log N has a point of inflection and changes to a horizontal line, then the stress corresponding to this point is called the physical fatigue limit. Empirical relations of this type are usually characteristic of steels. The fatigue curves of copper and aluminum alloys have the form of a monotonically decreasing function for bases of more than 5.108 loading cycles. The concept of fatigue strength is used in this case, and the loading base on which the estimate was obtained is indicated.As regards the second important direction in research on fatigue strength, we must point out that the dependence of fatigue strength on the test base can be described by equations of the Weibull type [5]: rr_ I (N) = a_ I + C(N + B}--",where o_ l is the fatigue limit of the material at N = co on the horizontal segment of the curve; the coefficients B, C, and a are parameters that have no definite physical meaning, which is a shortcoming of the given equation. We did not find any studies that addressed both problems.The goal of the present investigation is to study the effect of structure and the duration of cyclic loading on the fatigue strength of condensed microlayer materials with allowance for the relationship between the above-indicated characteristics? (.a_l., h, N): We should point out that the complete fatigue cur,~e of the material may have several points of inflection [6]. In the present investigation, we are examining the behavior of the material in the high-cycle region, i.e. within the range N = 1.106-5 .107 cycles. One of the main features of the approach we will take is the use of data on fatigue strength at several points on the fatigue curve tlu'oughou! the given range of N.We will examine (Fig. 1) the fatigue curves of some of the condensed microlayer materials studied in [3, 4]. One feature of these relations is that they all decrease monotonically with an increase in N. Thus, the concept of fatigue strength applies to each point of the curve. A second feature of the results is that all of them were obtained within the range N = 1.106-5 . 107 cycles by bending with the frequency 1-2 kHz, i.e. they were all obtained b...
