来介绍
Fatigue Fracture Analysis of Globular Cast Iron
Abstract
Globular cast iron (GCI) is a form of cast iron, distinguished from others by the presence of globular graphite particles. GCI is widely used in low-speed, low-load and static applications due to its higher fatigue strength and wear resistance than other types of cast iron. It has proved to be very useful for a wide range of applications, from automotive components to watermills. This paper provides an overview of fatigue fracture analysis of GCI from a microstructure viewpoint, including the mechanical behavior, tools used in fatigue analysis, and different GCI microstructures.
1 Introduction
Globular cast iron (GCI) is a type of cast iron containing distinctive, globular graphite particles in the material. It is used in a wide range of engineering applications, including watermills, agriculture machinery, and automotive components. GCI is typically produced by pouring molten iron into a mould at a lower temperature than other cast iron products, and several processes are used to produce various types of GCI. In comparison with other forms of cast iron, GCI has higher fatigue strength and wear resistance, and a better machinability, making it suitable for applications that require static and low speed, low load performance. GCI has the following advantages:
2 Fatigue Fracture Analysis
Fatigue fracture analysis is the process of understanding a materials response to cyclic loading. It is important to investigate fatigue fracture when designing and manufacturing components made of materials prone to fatigue failure. Fatigue fracture analysis is also carried out to assess fatigue damage to already existing components.
In fatigue fracture analysis of GCI, two of the most common techniques used are optical microscopy and scanning electron microscopy (SEM). This allows for detailed morphological investigations of the fatigue fractures. The microstructures of the fatigue fractures in GCI materials can give valuable information about the fatigue behaviour of the material. The common microstructural features of fatigue fractures in GCI can be classified into four types: equiaxed, globular, flaked, and dendrite structures.
3 Mechanics of GCI
The fatigue behaviour of GCI is strongly influenced by its microstructure, which is related to the cooling rate and other process parameters, as well as its mechanical properties. It is important to characterize the mechanical properties of GCI, such as its yield point, ultimate tensile strength, elongation, and fatigue strength. This helps to understand the behaviour of components made from GCI under fatigue loading.
The elastic modulus and fatigue strength of GCI are also important mechanical properties, which are determined using tensile and fatigue tests. In fatigue tests, repeated loads of a specific amplitude and frequency are applied to the test specimen until failure occurs. The frequency and amplitude of the load, as well as the number of cycles to failure, are used to determine the fatigue properties of the material.
4 Tools used in Fatigue Analysis
When conducting a fatigue analysis of GCI, several tools are typically used, such as hardness testers, tensile tester, and acoustic emission detectors. Hardness tests are carried out to determine the tensile strength, compressive strength, and shock resistance of the material. The tensile tester is used to measure the tensile strength, elongation, and yield point of the material.
In addition, tools such as acoustic emission detectors can be used to detect any sign of fatigue or fatigue damage to the material. Acoustic emission detectors can detect any changes in the structure of the material due to the action of the cyclic stress. This helps to prevent unexpected fracture of components under fatigue loads.
5 GCI Microstructures
The microstructures of GCI are quite complex, and depend on the cooling rate, the mould size, the type of graphite particles, and other factors. The microstructure of GCI consists of ferrite, cementite, and primary, secondary, and tertiary graphite.
The primary graphite particles are globular in shape, and are distributed in a fairly homogenous manner throughout the material. The secondary and tertiary particles are more elongated and irregular in shape than the primary particles.
The percentage of ferrite and cementite phases in the microstructure are also affected by the cooling rate, the grain size, and the composition of the material. A higher cooling rate results in a higher percentage of ferrite, while a lower cooling rate results in a higher percentage of cementite.
The size of the graphite particles also influences the microstructure of the material, as well a its mechanical properties. The fatigue strength and wear resistance of the material are enhanced by increasing the size of the graphite particles.
6 Conclusion
In conclusion, fatigue fracture analysis of GCI is a complex process, which requires detailed observation and characterization of the microstructure of the material. Various tools can be used to assess the fatigue behaviour of the material, including hardness tester, tensile tester, and acoustic emission detectors. The microstructure of the material is affected by the cooling rate, the mould size, the type of graphite particles, and other factors. A higher cooling rate results in a higher percentage of ferrite, while a lower cooling rate results in a higher percentage of cementite. Increasing the size of the graphite particles can improve the fatigue strength and wear resistance of the material.
