Assessment Essay Example
According to Tindell (2014), during a tensile stress, the material undergoes plastic deformation and a neck forms up to a point when it can no longer withstand the force. After the fracture, ductile metals show a cup-cone shaped structure. It looks like a sequential nucleation or even a growth process as shown below in Figure 2.
Figure (2): Ductile Fracture
It occurs in crystallographic planes. It happens instantly unlike in ductile where the crack occurs in a slow process.
Figure (3): Brittle Fracture
Brittle fracture does not undergo plastic deformation. Has three stages; the deformation occurs in slip planes, Micro cracks form nuclear as a result of stress, eventually the crack becomes a fracture. Brittle fractures occur in a transgranular pattern.
Figure (4): Microscopic View of Brittle Fracture
Fatigue of Metals
This happens when stress is exerted on a particular part of a metal repeatedly. In cyclic loading, metals will break at lower stress levels unlike in static loading. During cyclic loading, stress concentration causes the crack to nucleate at that point (Tindell 2014).
Figure (5): Fatigue Fracture
Creep in Metals
It occurs in high temperature conditions. When a metal is under constant stress, plastic deformation occurs progressively. It is categorized into three namely;
Primary creep: creep rate reduces because of strain hardening
Secondary creep: the rate is constant because the hardening by strain and the process of recovery
Tertiary creep: the rate increases hence causing fracture. Figure 6 below show a graphical presentation of creep in metals
Figure (6): Creep in Metals
Shearography Material Testing Method
It is a non-destructive material testing technique. Laser light is used to illuminate the component being tested. The images produced interfere creating a speckle. In case the component being studied is suffering shear deformation, its surface gradient is displayed as a fringe pattern. According to Tindell (2014) this method produces quality images but it takes a lot of time. Shearography is carried out as shown in figure (7) and can be used for testing both metallic and non-metallic materials.
Figure (7) Shearography Testing
Rockwell Hardness Test
This a mechanical test used to determine the hardness of a material by pressing on a specimen an indenter that is cone shaped. First, a minimum load of 10 kg is used and the indentation caused is recorded. After this, maximum load of amount 150 kg is applied on the component and the indenter penetrates past its starting point. This additional penetration is then converted by the machine used for testing to give the Rockwell hardness number, d (Tindell, 2014). This testing can be summarized using figure (8) below.
Figure (8): Rockwell hardness test. (1) Penetration with minimum load (2) Penetration with maximum load
Metal corrosion: The greatest means of metal degradation is corrosion. It is a result of electro chemical attacks that begin on the metal surface. Corrosion deteriorates material properties by reducing their energy bonds when these materials are exposed to moist external environment. The atoms on the metal surface are then oxidized, lose some electrons and leave the material. This leads to a reduction of the material bulk as the material is eaten away (Tindell 2014).
Ceramic deterioration: Ceramics are materials whose properties lie between those of metals and non–metals. They suffer chemical dissolution in environments with high energy of battery systems, heat exchangers and gas turbines.
Polymer degradation: Polymers on the other hand suffer attacks that are physiochemical attacks. One such attack is the bond rapture. The bonds of a polymer are subjected to thermal energy, chemical reactions or radiation effects. Radiation may cause the atoms to be rearranged in the polymer structure or may break its bonds. Thermal energy on the other hand makes polymer bonds weak and this deteriorates their strength and properties. In other instances, presence of oxygen in the ozone layer together with the chemic al reactions that occur on polymers lead to alteration of the rate of chain incision (Tindell, 2014).
Eddy Current Testing Method
This is a non- destructive testing method in which a coil is energized then brought close to a metal surface. The eddy currents induced create a magnetic field. The fields create is the same as the original one and with the coil placed very close to the specimen, its impedance faces the effects of the eddy currents induced in the metal. This field created in the metal is then distorted by the variations and flaws present on its surface. This distortion is measured to give the flaw type and the mechanical condition of the material. Figure (9) below is a diagrammatical representation of eddy current testing technique as applied to test materials.
Figure (9): Set Up for Eddy Current Material Testing
This method is used to identify cracks. It can be also used where flaws, material variation and variations in sizes need to be detected on small component pieces.
Figure (10): progress of a tensile test.
This is the most widely used mechanical testing technique when assessing stress-strain relationship in materials. The material being tested if pulled on both ends by a force. The material diameter diminishes and its length is increased. Tindell (2014) states that the load must be increased progressively until the point where the material fractures. This is as shown in figure (10) above. Elongations and diametrical variations are recorded for each force applied. Stress and strain on the material are recorded and a graphical representation of their relationship obtained when these values are plotted as in figure (11) below.
Figure (11): stress-strain relationship.
Tindell, H. (2014). Engineering materials.
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