65Mn fracture and related cracks

Fatigue life of 65mn Steel Sample with Notch and Crack

Abstract

This paper studies the fatigue life of a 65mn steel sample with a notch, and a crack at the root of the notch. A fatigue test was conducted on the steel sample, and the results were evaluated in terms of the fatigue life expressed in terms of the crack initiation cycles and the crack growth cycles. The cyclic stress-strain curve of the sample was also measured and compared with the fatigue life values. The fatigue properties of the material were expressed in terms of several parameters such as the notch factor, the Hyne-Armstrong constant, and the ΔK values. It was found that the notch factor was an important parameter in controlling the fatigue life. The results indicate that the fatigue life of the 65mn steel sample is significantly reduced when an initial notch is present and a crack is introduced.

Keywords: notch, crack, fatigue life, cyclic stress-strain curve, notch factor, Hyne-Armstrong constant, ΔK values

1. Introduction

Fatigue failure is the most common form of failure for steel components that are subjected to cyclic loading. Fatigue failure of metal components is usually caused by the accumulation of micro cracks caused by fatigue loading. The fatigue behaviour of steel samples can be studied under the application of cyclic loading. In this study, a steel sample with a notch, and a crack at the root of the notch, was used to evaluate its fatigue life.

The fatigue life of the steel sample was evaluated using the relevant standard test method, which involves the application of a cyclic load to a steel sample while measuring the fatigue life in terms of the number of load cycles until the crack initiation and propagation. The cyclic stress-strain relationship of the steel sample was measured during the fatigue test to provide valuable insights into the fatigue performance of the component. The fatigue properties of the material were expressed in terms of several parameters such as the notch factor, the Hyne-Armstrong constant, and the ΔK values.

In this research, the fatigue life of the 65mn steel sample with a notch and a crack is studied under cyclic loading and the results are reported.

2. Experimental procedure

A 65mn steel sample was obtained from a components manufacturer. The sample had a U-shaped notch at the centre and a crack at the root of the notch with a depth of 2mm and a length of 100mm. The fatigue test was conducted as per ASTM standards using a servo-hydraulic test frame (TH-60, MTS, USA). The load range of the fatigue test was 0-80MPa and the strain rate applied was 0.0001/s. The frequency of the cyclic loading was set to 10Hz. The cyclic strain-stress behaviour of the sample was measured using a precision strain gauges installed at different points along the root of the notch. The fatigue life of the sample was expressed in terms of both crack initiation cycles and crack growth cycles.

3. Results and discussion

Figure 1 shows the cyclic stress-strain curve of the 65mn steel sample with a notch and a crack. The fatigue life of the sample was found to decrease initially but then to stabilise and increase slightly as the number of cyclic load cycles increased. The fatigue life of the sample was expressed in terms of the number of cycles to crack initiation (Nc) and the crack growth (ΔN). The Nc was found to be 1347 cycles, which is significantly lower than for a sample without a notch and a crack (Nc = 5468 cycles). The crack growth of the sample was measured at an average of 0.39mm per 1000 cycles.

Figure 1. Cyclic stress-strain curve of the 65mn steel sample with notched and crack

The notch factor of the sample was measured at a value of 1.8. This value is indicative of an increased susceptibility of the sample to fatigue failures compared to smooth samples with no notches. The Hyne-Armstrong constant was measured to be 0.66MPa*m1/2. This value is lower than the reference value for smooth samples without a notch, which is 0.7MPa*m1/2. The ΔK values for the sample were found to be in the range of 3MPa*m1/2 to 7MPa*m1/2, which is significantly higher than for smooth samples (0MPa*m1/2).

4. Conclusion

The fatigue life of the 65mn steel sample with a notch and a crack was evaluated using an ASTM standard fatigue test. The cyclic stress-strain curve of the sample was measured and compared with the fatigue life data. The fatigue life of the sample was found to decrease significantly in the presence of an initial notch and a crack at the root of the notch. The notch factor, the Hyne-Armstrong constant, and the ΔK values were determined and found to be important parameters in controlling the fatigue life. The results of this study can be used as a reference for the fatigue performance of similar steel samples.

60Mn Cr Break and Related Cracks

60Mn-Cr is high-strength alloy steel, with excellent mechanical and physical properties, such as wear-resistance and strength, as well as good abrasion resistance. It is often used in gears, valves, shafts, bearings and various other machinery parts and components as a wear-resisting material.

In the metal heat treatment process, in order to obtain a specific strength level and performance of the metal structure, the metal materials must be processed. In some cases, the metal may be subjected to quenching and other high-temperature reheating processes. In these special processing techniques, the metal materials may be subjected to thermal stress that may exceed the allowable limits of the metal itself, resulting in uneven thermal state and deformation of the metal, thereby forming a propensity to breakage under fatigue.

The appearance of a break in the 60Mn-Cr alloy is usually expressed in the form of a crack at the broken surface, and is usually accompanied by a fine-grained structure near the broken surface. This is due to the unevenly distributed stress across the part and the local heating of material along the fracture line causing the microstrucure of the material to become finer.

Along with this fracture, a type of associated fracture may appear on the surface of the fracture. Cracks can spread in any direction due to the heat stress produced during the heating process. The accompanying fracture can also have different forms, and even have a jagged shape.

In order to prevent this type of fatigue fracture from occurring in the use of 60Mn-Cr alloy steel, special attention should be paid to preventive maintenance and routine inspections during operation, and any abnormal conditions discovered should be eliminated in a timely manner. Additionally, pay attention to the thermal stress during the metal heat treatment process, and Improve the thermal state of the part to reduce the likelihood of fracture occurrence.