Evaluation of the threshold for the development of fatigue cracks in a railway steel under harmonic and operational loading

The results of experimental studies of the fatigue crack development in 20GFL steel specimens cut from a cast bolster of a freight car are presented. The ratio of the threshold stress intensity coefficient Kth determined from the kinetic diagram of fatigue fracture and from the average parameters of the operational loading process is considered using the experimental results with a simulation of operational loading. Tests were carried out upon the development of permanent blocks of crack opening in the specimen (in a rigid loading mode). The operational process is presented in the form of a block of consecutive loading cycles recorded during the test of the car frame in conditions typical for a straight section of the railway track. The threshold operational level is determined by the algorithm of gradual reduction of the loading similar to the original process. The regularities in a decrease of the rate of crack development and corresponding decrease in the load were determined. Subsequent extrapolation of the obtained experimental regularities to zero value of the crack propagation rate provided estimation of the threshold loading level, similar to the initially specified value. It is shown that the value of the threshold level of the fatigue crack development in low-alloy steel 20GFL obtained from the fatigue fracture diagram (i.e., under harmonic loading) is significantly higher than that obtained from the estimate based on the average values of the operational loading process. The considered model of operational loading gives greater damage compared to harmonic loading, on the basis of which the survivability of structural elements is usually assessed.

1. Makhutov N. A., Moskvitin G. V., Lebedinsky S. G., et al. Problems of strength, technogenic safety and structural materials science. — Moscow: Lenand, 2018. — 720 p. [in Russian].

2. Romanov A. N. Propagation of fatigue cracks and a single curve of cyclic crack resistance of structural materials / Probl. Mashinostr. Nadezhn. Mash. 2013. N 5. P. 47 – 57 [in Russian].

3. Kogaev V. P., Makhutov N. A., Gusenkov A. P. Calculations of machine parts and structures for strength and durability: handbook. — Moscow: Mashinostroenie, 1985. — 224 p. [in Russian].

4. Elber W. The significance of fatigue crack closure. — Philadelphia: ASTM STP 486, 1971. P. 230 – 422. DOI: 10.1520/STP28334S

5. Willenborg J., Engle R. H., Wood H. A. A crack growth retardation model based on effective stress concepts, AFFDL-TM-71-1 FBR, WPAFB, OH, 1971. DOI: 10.21236/ada956517

6. Newman J. C. A crack-closure model for predicting fatigue crack growth under aircraft spectrum loading. Methods and models for predicting fatigue crack growth under random loading / Chang J. B., Hudson C. M., eds. ASTM STP 748, 1981. P. 53 – 84. DOI: 10.1520/STP748-EB

7. Petit J., Henaff G., Sarrazin-Baudoux C. Mechanisms and Modeling of NearThreshold Fatigue Crack Propagation, Fatigue Crack Growth Thresholds, Endurance Limits and Design / ASTM Spec. Tech. Publ., Newman J. C., Jr. and Piascik R. S., Eds. Vol. 1372. American Society for Testing and Materials, West Conshohocken, PA, 2000. — 429 p. ISBN 0-8031-2624-7.

8. Ling M. R. and Schijve J. Fractographic analysis of crack growth and shear Lip development under simple variable-amplitude loading / Fatigue Fract. Engg Mater. Struct. 1990. Vol. 13. P. 443 – 456.

9. Sunder R., Porter W. J., Ashbaugh N. E. The Role of Air in Fatigue Load Interaction / Fatigue Fract. Eng. Mater. Struct. 2003. Vol. 26. P. 1 – 16. DOI: 10.15593/perm.mech/2018.4.22

10. Sunder R. Characterization of Threshold Stress Intensity as a Function of Near-Tip Residual Stress: Theory, Experiment, and Applications / Materials Performance and Characterization. 2015. Vol. 4. N 2. P. 105 – 130. DOI: 10.1520/MPC20140037

11. Sunder R., Unraveling the Science of Variable-Amplitude Fatigue / J. ASTM Int. 2012. Vol. 9. N 1. P. 20 – 64. Paper ID JAI103940.

12. Sunder R., Andronik A., Biakov A., Eremin E., Panin S., Savkin A. Combined action of crack closure and residual stress under periodic overloads: A fractographic Analysis / Int. J. Fatigue. 2016. N 82b. P. 667 – 675.

13. Lebedinsky S. G., Moskvitin G. V., Pugachev M. S., Polyakov A. N. Regularities in the development of fatigue cracks in steel at a low level of operational loading / Probl. Mashinostr. Nadezhn. Mash. 2020. N 2. P. 73 – 79 [in Russian].

14. RD 50-345-82. Calculations and strength tests. Methods of mechanical testing of metals. Determination of crack resistance (fracture toughness) characteristics under cyclic loading. Methodological guidelines. — Moscow: Izd. standartov, 1983 [in Russian].

15. Lebedinsky S. G., Zmeeva V. N. Regularities of development of fatigue cracks in cast steels of railway structures / Probl. Mashinostr. Nadezhn. Mash. 2000. N 3. P. 98 – 103 [in Russian].

16. Moskvitin G. V., Lebedinsky S. G. Determination of the threshold for the development of fatigue cracks in railway construction steels / Probl. Mashinostr. Nadezhn. Mash. 2007. N 6. P. 63 – 68 [in Russian].