Mechanics is a very wide field that includes various kinds of sub-fields inside it. But all these sub-fields emerged from classical mechanics and classical physics. Once you understand the general philosophy of these two disciplines, sub-disciplines will not be hard for you.
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One of these sub-disciplines of classical mechanics is the fracture mechanics. Fracture mechanics is a very important field where possible failures of the engineering systems are calculated. A fracture of machinery or a part of the machinery can cause very big material and moral damage to human civilization. So, fracture mechanics is a very important field in design engineering.
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In this article, we would like to elaborate on the fracture mechanics, its application areas, and their importance.
What is the Fracture Mechanics?
As we stated above, fracture mechanics is a sub-field of classical mechanics that deals with the crack development mechanisms on materials.
Fractures take place because of the minimal defects on materials or parts. These minimal defects originated from the manufacturing of that part. In different manufacturing fields, there are always studies to minimize these defects. But due to the nature of the material, defects will always be in the structure of manufactured parts.
With the application of loads and stresses, these cracks will grow as time progresses. After a while, rapid fractures take place. But ductile materials, the material will give some physical alerts before the fracture which makes them preferred materials in most engineering design studies.
In the design phase of parts that are undergoing critical stresses, fracture control must be made. These studies are also made in the name of fatigue design. The fracture control of the parts is made according to the biggest possible defect that can occur from the manufacturing of the part and the maximum cyclic stress that can occur on that crack which is at the most critical point of the part.
Think about a shaft that transmits a value of power from a power source. Because of the transmission, some combined stress values would occur. At which part that maximum stress is occurring is calculated on the shaft, which is generally the sharp corners of keyways or the steps of stepped shafts. And in the manufacturing of that shaft, which size of the maximum defect that would occur is determined.
According to these variables, possible crack formation and failure are calculated with the fracture mechanics technique.
Most Encountered Crack Modes
In fracture mechanics, there are different modes of crack formations encountered in most engineering systems.
Tearing of the material takes place where the opposite forces make the defect tear apart from opposite sides. This kind of crack and fracture mechanism may be seen on mechanical pins that connect different mechanical elements such as two wagons of trains.
The opening is the most classical fracture mode in which crack takes place because of the tensile stress application. In tensile stress fracture tests, this phenomenon is generally observed. The possible defect is opened and propagated because of the application of tensile stress on it. The opening is the easiest crack propagation mechanism that can be observed and calculated.
Sliding takes place because of the shear stress application on the possible defect. Again in the applications where shear stresses are in the stage, sliding crack propagation may take place.
Fracture Toughness of Materials
As you remember from the strength of materials and materials science, toughness is the total energy that is absorbed by the material up to fracture occurs. Fracture toughness is a mechanical value that depicts the total energy required to crack a defect on a material. According to the defect type, loading conditions, and materials, the fracture toughness of materials changes.
For ductile materials, yield strength is used as design criteria where plastic deformation starts. Designers are willing the stay in the designed shape from the parts. So, no plastic deformation is intended from any mechanical design.
For brittle materials, nearly no plastic deformation occurs. After an amount of load, an abrupt fracture occurs. So, fracture mechanics must be considered in the design of parts from brittle materials. So, for the worst-case scenario of the fracture, fracture toughness is a very important parameter for brittle materials.
The general explanation of the fracture toughness can be made like above.
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