Study Of Fatigue Behavior And Mechanism Of Fatigue Failure In The Ultra-High-Cycle Regime In 40Cr And 50 Axles Steels

The concept of a fatigue limit has been the basis of design against fatigue failure since the late 19th century and is still in use today. Ferrite steels, in particular, were considered to possess a well-defined fatigue limit corresponding to threshold stress amplitude below which fatigue life was believed to be infinite. However, in conventional fatigue testing, the range of numbers of cycles investigated is usually limited to about 107 or, at best, 108 cycles, since performing experiments in the very high cycle regime is extremely time consuming and expensive. Hence, the assertion that fatigue life would be infinite at stress amplitudes below the classical fatigue limit was actually an unproved assumption. With technical development in modern industry, in many applications, the lifetime of the mechanical components have to endure up to 108 cycles of loading without failure. A number of recent studies, extending into the ultra-high-cycle fatigue (UHCF) or gigacycle fatigue range, showed clearly for different steels and other metallic materials that, even at stress amplitudes below the classical fatigue limit, fatigue life was finite. Moreover, it was reported that, whereas fatigue failure in the conventional high-cycle fatigue (HCF) range (<106-107 cycles) occurred at the surface, fatigue failures in the UHCF range generally originated from internal defects. As a result, the endurance limit determined by the conventional fatigue tests cannot provide the safety design data of the mechanical structures. Thus the fatigue property of metallic materials in the UHCF range tends to be an important subject in the mechanical design to ensure the long term safety of the mechanical structures.Since performing experiments of the ultra-high-cycle regime in the range of 108~1010 cycles using a conventional fatigue testing method is very time consuming and expensive, in this paper, a kind of acceleration fatigue test technique, called the ultrasonic fatigue testing, was developed and studied. In the study, based on the symmetrical pull-push ultrasonic fatigue testing machines, the methods of unsymmetrical pull-push ultrasonic fatigue load and the three points bending ultrasonic fatigue load are been developed, and the factor influencing testing process and accuracy is analyzed. These studies will give direction to the method of the ultrasonic fatigue testing.In this study, fatigue behavior of the 40Cr and 50 axles steels including smoothand notched specimens was studied with the ultrasonic fatigue testing technique, with a loading frequency of 20kHz. The fracture surface of specimens was examined by scanning electron microscopy (SEM).Experimental results showed that the S-N curve of 40Cr steel quenched at 850 and tempered at 560, displays the characteristic of "continually decreasing type" up to 1010 cycles and exhibit no traditional horizontal plateau beyond 106 cycles. And the S-N curve of 50 axles steel displays the characteristic of "the multi-stage type", and two decreasing curve and separated by a horizontal step in the region of 106~10 cycles. Over 107 cycles and up to 1010 cycles, fatigue failure in 40Cr and 50 steels do occurs.The observation of the fracture surface showed that fatigue cracks initiate from surface of specimen for short lives (<107 cycles) at high stress levels, and from internal inclusion for very long lives at low stress levels. This indicates that crack initiates mechanism of 40Cr steel and 50 axles steel are of two kinds, and each mechanism corresponds to the respective S-N curves. The S-N curve can be described by the relative position of these two curves and is called "double S-N curve model", in this paper.Study of fatigue behavior on the notched specimen of 40Cr steel and 50 axles steel showed that the influence of notch stress concentration on fatigue properties exhibits characteristic of "different stage", and near 107 cycles, exist a critical cycle Nc (or a critical range of cycle) to correspond to maximum fatigue-notch factor or maximum fatigue-notch sensitivity factor. As number of cycles to f...