Volume 224

1976

RESISTANCE TO FATIGUE FRACTURE OF Kh12 STEEL UNDER IMPACT AND SMOOTH CYCLIC LOADING

V. V. MOROZOVA, G. V. TOPOROV

(Presented by the Department of Applied Mechanics)

Since the question of the resistance of materials to impact-fatigue fracture is at present insufficiently studied, data obtained in testing a given material for ordinary fatigue are used to assess the quality of the material and its suitability for parts that, in service, undergo repeated impact loads.

However, data on the influence on fatigue of periodically smoothly increasing and decreasing variable loads do not reflect the action of repeated impact loading.

Table 1

In order to clarify the question of the resistance to impact-fatigue and ordinary fatigue fracture of Kh12 die steel, we carried out comparative tests for impact fatigue and ordinary fatigue of smooth and notched specimens made of the indicated steel.

Cylindrical specimens 8 mm in diameter and 80 mm long were tested. The notched specimens had a diameter at the notch likewise of 8 mm. The notch had a V-shaped form with an angle of 60°, with a rounding radius at the bottom of the notch of 0.1 mm; the notch depth was 0.25 mm.

The carbide heterogeneity of specimens of Kh12 steel, made from rolled stock 18 mm in diameter, may be characterized as grade 4 on the scale adopted for high-speed tool steel (GOST 5952–51).

Testing of specimens for impact fatigue was carried out on a machine designed by Prof. A. I. Lampsi and Eng. V. N. Polyubin. Testing for ordinary fatigue was carried out on a pulsator. In both cases the specimens were tested according to the scheme of one-sided bending (pulsating cycle). To avoid the influence of the scale factor, the specimens during test-

in the impact-fatigue and ordinary-fatigue tests had the same shape and dimensions.

The stresses under smooth cyclic loading were determined by the usual method of calculation from the maximum deflection. The stresses arising in the specimens during impact tests were determined from strength-of-materials formulas and experimentally with the aid of resistance strain gauges.

Fig. 1. Fatigue curves of smooth specimens of Kh12 steel (tempered at 160–180°):
—— impact fatigue,
— — — ordinary fatigue

As a characteristic for evaluating the test results, the value of limited life was adopted, obtained by testing 4–5 specimens at each stress level. The results of testing the specimens, in the form of a graphical dependence of the logarithm of the number of impacts to fracture on the stress magnitude, are shown in Figs. 1–4.

It is known that, in impact-fatigue tests of smooth specimens made of structural steels, the resistance to fracture is higher than under

Fig. 2. Fatigue curves of smooth specimens of Kh12 steel (tempered at 300°):
—— impact fatigue,
— — — ordinary fatigue

under static cyclic loading [1, 2], which can be explained by the favorable effect of increasing the strain rate on the reduction of microplastic deformations responsible for the initiation of fatigue cracks [3].

For specimens with a notch, at a sufficiently high stress level the opposite relationship is observed: the resistance to fracture under impact-cyclic loading is lower than under smooth cyclic loading [4].

Fig. 3. Fatigue curves of notched specimens of Kh12 steel (tempering at 160–180°):
— impact fatigue,
— — — ordinary fatigue

Fig. 4. Fatigue curves of notched specimens of Kh12 steel (tempering at 300°):
— impact fatigue,
— — — ordinary fatigue

The tests we performed on smooth and notched specimens of Kh12 steel under impact and smooth cyclic loading, the results of which are given in Figs. 1–4, show that for specimens with a notch the limited lives under impact cyclic loading are significantly lower than in the case of smooth cyclic loading. The resistance of smooth specimens to impact-fatigue fracture differs only slightly from the resistance to fracture under static cyclic loading,

The results obtained can be explained by the distinctive structure of Kh12 die steel. In the case of metals with a homogeneous structure, their properties in the most highly stressed regions can be judged from the data of tests in a macrovolume. In metals with heterogeneous structures, these properties are determined by the properties of the individual structural constituents and their aggregate. The fact that the resistance of smooth specimens of Kh12 steel to impact-fatigue fracture differs only slightly from the resistance under smooth cyclic loading can be explained by the distinctive structure and the tendency of the structural constituents toward brittle fracture under impact cyclic loading. In notched specimens, however, the tendency of the structural constituents toward brittle fracture is manifested to a greater degree than in smooth specimens; therefore, the limited lifetimes under impact-fatigue loading are lower than under ordinary fatigue loading.

On the basis of the test results, it may be concluded that the notch sensitivity of Kh12-type steels in low-cycle fatigue is higher under impact loading than under smooth cyclic loading.

References

1. N. N. Davidenkov, E. I. Belyaeva. Metallovedenie i obrabotka metallov, p. 75, No. 11, 1956.

2. E. A. Silkin, A. F. Zasova. Zavodskaya laboratoriya, No. 11, p. 98, 1960.

3. S. F. Medvedev. Cyclic Strength of Metals. Mashgiz, Moscow, 1961.

4. S. V. Tolkanchik. Izv. AN SSSR, OTN, No. 5, p. 112, 1958.