Introduction
There are different perspectives that are being peddled concerning fracture processes, this has led to confusion on what actually this term means. In this review, a deeper understanding is sort by analyzing different aspects of the nomenclature of the fracture process. In order to gain a deeper understanding of the failure processes several aspects are put in consideration key among them will be discussed below and include; the energy of fracture, macroscopic structure path and microscopic fracture mechanisms.
Energy to Break
The toughness of a material is the measure of the absorbed energy during and before fracture. The toughness of the material can, therefore, be determined analytically by calculating the area under the tensile stress-strain curve. The higher the energy the tougher the material, the lower the energy the brittle the material. The extent of plasticity surrounding the crack tip may be used in the determination of relative toughness and brittleness of a material. The toughness of notched material increases with increase in the volume of the crack tip, this is due to the dissipation of more energy during plastic flow than during the elastic deformation.
The notched to the smooth bar tensile strength ratio is also used in the determination of toughness of a notched sample. The lower the ratio the higher the sensitivity thus lower toughness levels, on the other hand, the higher the ratio means lower sensitivity levels and thus higher toughness behavior.
Macroscopic fracture mode and texture
The macroscopic crack path is used in the determination of useful information about the toughness of a material before failure more especially in cases of sheet and plate type components. Thin Flat and slant sample structures exhibit higher values of toughness when compared to thick flat sample fracture mode. Intermediate toughness levels can be achieved by application of mixed models that is partly slant and partly flat structures. Engineering processes tend to result in larger fracture surfaces. Analysis of given failure and failure identification s dependent on the mechanism of failure identification. Identification of the fractures can be done through the use of electron microscope. From micrographs generated from electron microscopes, it is easy to study the chevrons. Generally, chevrons grow outwardly radially from a relative direction. The study of the various cracks resulting from the micrographs taken is fundamental in providing precise information of the crack and the materials micro-structure.
Microscopic fracture mechanisms: metals
Over the last few years, the light microscope has been employed in the microscopic determination of most fracture processes. It has however presented enormous challenges owing to the fact that only shallow depths of focus can be achieved thus making it difficult to obtain the required information about the fracture. By comparison of the path of fracture to the metallurgic grain structure, the transcrystalline and intercrystalline nature of the failure can be determined.
The information about the fracture profile is important more especially in cases where secondary cracks exist ad can be used in the determination of the profiles of the mating fracture surfaces. The introduction of the electron microscope has enabled much more advances I understanding of the fracture mechanisms because of its superior depth of field and resolution when compared to that of the light microscope. A scanning electron microscope has also been used to further the advancement of failure analysis.
Microvoid Coalescence (MVC)
It occurs by nucleation of microvoids in most engineering plastics and eventual growth and coalescence. The study of the ancient computations indicates that much of the MVC consumption occurs during the growth of microvoids. There are two main growth mechanisms in place they include; the plastic flow of the matrix involving nucleation site and the plastic flow influenced by decohesion of tiny particles found in the matrix. The stress applied on the material influences the appearance of the fracture surface of microvoids. The fractured structure is also dependent on the heat treatment temperature.
Cleavage
It occurs in FCC metals, the cleavage facets are normally flat. It appears as river pattern, where finer steps merge to form larger patterns. Metals like ferrite steel alloys result in cleavage formation. Cleavage mechanism can be due to a set of external conditions, and the presence of cleavages, therefore, may be indicative of the temperatures, for example, low or high strain temperaturs.
Intergranular fracture
One of the most easily recognizable forms of fracture mechanism, this is because intergranular failure follows the grain surface. Intergranular fractures result from a number of processes for example mirovoid nucleation, coalescence at inclusions and grin boundary crack. It is mainly as a result of the presence of impurities for example hydrogen and liquid metals.
Fracture mechanisms: Polymers
The formation of polymers is as a result of thin crazes contained in interconnected microvoids and polymer fibrils which are an extension of craze thickness direction. The existence of different crazes can be indicated by different colors having uniform thicknesses. Through the pre crazed matter, there is a progression in various ways at the mirror region. The view of the pores under the transmitted light results in the appearance of the series as concentric rings.
Fracture Surfaces of Ceramics
The surfaces of brittle solids for example glass, crystal ceramics results in fracture of surfaces that appear unique. The mirror region of the fracture surface of ceramics is highly reflective fracture surface. Several microcracks exist that have small radial ridges associated with them. Low strength glass leads to mirror zone extension in the fracture surface.
Quantitative fractography
It is important to analyze different engineering solids depending on the fractographic features, the size of the fragments dependent upon the appropriate parameters that are appropriate. Observation of such. In order to have a better understanding of the specific failure mechanism happening during a fracture. It is important to have a visit to the scene of the crime of fracture surface.
A scanning electron microscope is the best choice for use in the determination of the depth of the sharpness, chemical element analysis and the contrast of the material. The reasons of damage can therefore be inferred by the use of fractography. The patterns formed are as a result of the different failure mechanisms can be used in reference to the fractographic atlases.
Conclusion
As already discussed above, there are several aspects of mechanical fracture. There are different classifications of fracture processes that are based on different individual aspects. The different classifications groups are based on a variety of fracture processes that occur resulting in failure. The fracture experienced in a material is dependent on properties of the material under consideration, further, the characteristic behavior of a material depends on the micro-structural level of the material that determines the materials characteristic behavior. Most engineering materials have different microscopic structures and diverse failure mechanisms that are key in determining the various engineering materials properties.
The type and form of the external loading determine the fracture behavior making it possible to differentiate the different forms of fractures. The fractures occurring in different materials depend on various factors including among other factors the static, dynamic and or cyclic loading. Other factors like temperature, multiaxiality of the load, deformation rate, chemical and or chemical conditions.
There are three main microscopic appearances of a crack that have already been identified; they include, the cleavage fracture that is mainly identified by the appearance of plane fractures faces and minor deformations. It is mainly as a result of brittle cracking occurring along crystallographic that occur due to high normal stresses. The main materials experiencing this kind of structure include body-centered cubic metals normally at low temperatures and ceramic materials which also tend towards cleavage structures.
Works Cited
Hertzberg, Richard W. Deformation And Fracture Mechanics Of Engineering Materials. New York: Wiley, 1976. Print.
