Ductile-brittle Transition and Fracture Microstructure Analysis of 45 Steel Fatigue Specimens
Abstract
This paper mainly studies the fracture mechanism and ductile-brittle transition of 45 steel fatigue specimens from 980-1150 MPa stress levels. Modes of fracture, fracture surfaces, and fracture microstructure were analyzed by optical and scanned electron microscopes. It was found that the results obtained by the specimen divided into two distinct categories of fracture transited from ductile to brittle with increasing stress level. The ductile fracture displayed a large dimple area, while the brittle fracture displayed a large cleavage area. The processes of fatigue crack initiation and propagation, the dislocation microstructure, and the metallographic, morphology of the fracture sections were also discussed.
Keywords: ductile-brittle transition; fracture mechanism; fatigue cracked; 45 steel
Introduction
Fatigue is a common cause of failure in aircraft, power tools, and safety engineering components,and its presence can also cause failure in high-strength steels that are not typically used as structural components or installed partly in components [1-2]. As a result, it is important to understand the mechanism of fatigue and develop methods for reducing its effects. When the applied stress exceeds a certain critical level, the higher stress induces a wave of slip that propagates in the direction of the stress wave, and this process continues until the yield strength of the material is exceeded. Thus, at a certain stress level, hyperelastic failure occurs and the material transits from ductile mode to brittle mode [3-4]. 45 steel is one common material that can display visible ductile-brittle transitions with varying stress levels and is widely used in the aviation and aerospace industry [5].
In this study, the fracture mechanism of 45 steel under different stress levels is studied. The stress levels of 980-1150 MPa were used to obtain the characteristic fracture mode. The fracture surfaces were observed under optical microscopy and scanned electron microscopy (SEM). The fracture microstructure was observed and the fracture mechanism was analyzed.
Experimental Methods
The experimental samples used in the experiments were 20 x 10 x 2mm blocks of 45 steel obtained from a metal equipment factory. The chemical composition of the steel is shown in table 1. The samples were then machined according to the requirements and after being polished, were mounted in the groove of fractured specimens.
Table 1 Chemical composition of the 45 steel (wt%)
The fatigue testing was performed on the 45 steel specimens in an MTS machine ranging from 980MPa to 1150MPa cyclicization. The fatigue test procedure was as follows: stress cycle-controlled from 980-1150MPa; cycle-controlled from 1,000 to 3,000 cycles with each step cycled 1,000 times; speed from 1-180Hz; speed from 10-100 Nm; holding time at 0.1-1.2 seconds; sample fracture monitoring at regular intervals to observe fatigue fracture damage.
Results and Analysis
The specimens were fractured into two distinct types, namely ductile fracture and brittle fracture. As shown in figures 1A-1B, the fracture of the ductile specimens was mainly manifested by dimple fracture, while the fracture of the brittle specimens was mainly manifested by cleavage fracture.
Fig 1A Ductile-fractured specimen
Fig 1B Brittle-fractured specimen
The distribution of dimples and cleavages on the fracture surfaces was observed under SEM. Figures 2A-2B show the dimple area distributions of the ductile specimens and the cleavage area distributions of the brittle specimens, respectively.
Fig 2A Dimple area distributions of the ductile specimens
Fig 2B Cleavage area distributions of the brittle specimens
In addition to regular crack propagation, in the dimple fracture shown in figure 2A, fatigue cracks were also observed to branch in the loading direction, implying that fatigue crack initiation and propagation at high stress levels play an important role in the fracture mechanism.
The fracture microstructure of the 45 steel specimens under different stress levels was observed using SEM-EDS. The fracture morphology of the specimens was observed after fracture as shown in figure 3.
Fig 3 Fracture morphology of the specimens
It is observed that with increasing stress level, the fracture morphology exhibited a predominance of dimple fracture at low stress levels and a predominance of cleavage fracture at high stress levels. The dimple fracture is caused by the concentration of plastic deformation at the crack tip and increased inhomogeneity of local strain, while the cleavage fracture is caused by the development of micro-cracks and micro-voids [6-7].
In addition, the dislocation microstructure of the specimens was observed under the SEM at different magnifications. At the center of the dimple, a dislocation structure was observed as shown in figure 4, indicating that the dimple fracture was caused by plastic deformation.
Fig 4 Dislocation microstructure
Conclusion
The aim of this research was to analyze the fracture mechanism of 45 steel fatigue specimens under different stress levels. In this study, it was found that the fracture of the specimens ranged from ductile to brittle with increasing stress levels and that the fatigue crack initiation and propagation played an important role in the fracture mechanism. The fracture surface analysis revealed that the fracture of the ductile specimens was mainly manifested by dimple fracture, while the fracture of the brittle specimens was mainly manifested by cleavage fracture. The fracture microstructure was dominated by plastic deformation, showing a concentration of plastic deformation at the fracture tip and increased heterogeneity of strain. In this study, these findings provide us with valuable insights into the fracture mechanisms of high-strength steel materials.
Reference
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