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Material assignment 3 Figure 1

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Material assignment 3

Figure 1

Josh Ayres

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Task 1 Failure modesA- CreepA creep occurs when a material is put under mechanical stress that it cant handle. A creep is when materials grow longer overtime because there is a constant tensile stress applied to it. The stress permanently deforms the material after the yield point. A creep is a time dependent which deforms at elevated temperatures and constant stress. There are three stages of a creep which are primary secondary and tertiary. The primary stage is fast to happen but is not always present. The second stage is constant as the material extends and is slower than the primary stage. At the tertiary stage the extension quickens again and leads to a failure. Most materials will not creep until a certain amount of force is applied and this is called as creep limiting. When a material is at a high temperature the creep limit will lower and result in a failure with less stress applied to it. When the materials experience creeps the grains will also become stretched which affects the mechanical properties of the material at the creep point.

Figure 2

B - FatigueFatigue is the weakening of a material that is repeatedly put under applied loads. Cyclic stress is a force that is being repeatedly applied and the force changes throughout the process. Fatigue failure happens because microscopic cracks start to form and makes the stress concentrate at that point. Eventually it will crack because of the stress that is being applied to a weak point. Material surfaces will have cracks, valleys or imperfections that can be seen under a microscope. By having smooth surfaces with no cracks fatigue failure is less likely. The materials can have different surface finishes depending on the application. After a material has failed from fatigue the surface will be very rough and the cracks will be visible to the eye. By looking at the surface you can usually see where the crack started. Things that can have an impact on fatigue life are; Temperature Corrosion Surface finish Residual surface finish Size of the component The way that the stress fluctuates How the stress is concentrated

Figure 3

C- Ductile and brittle fractureWhen a brittle material fractures it will do so without warning unlike a ductile material which will plastically deform before the fracture. Brittle materials can fail suddenly and do not show a neck before fracturing. When a brittle material fractures a crack will rapidly spread. The crack can spread further without any additional forces being applied to the material. Gradually loading will cause a brittle fracture to rapidly crack or shatter. Impact loading a brittle material will cause it to shatter. There are two types of brittle fractures which are transgranular and intergranular. In transgranular the fracture will travel through the grain of the material. The fracture will change direction because of the orientation of atoms in each grain. The cracks will basically choose a path with the least resistance. You can see that the fracture changed direction because in the surface you can see that the crack is slightly bumpy on the surface. An intergranular crack is when the fracture travels along the grain boundaries. Polymer materials have a low yield point but can withstand a lot of stretching before breaking. Grain size has an impact on brittle fractures. Larger grain sizes will result in loss of the materials strength. Grain size will also determine the control of cracks spreading. At higher temperatures the yield point of a material is lowered which can cause the fracture to become more ductile. Temperature can determine the amount of ductile or brittle fracture in a material. When a materials temperature increases the atoms vibrate which causes the properties of the material to change slightly. Brittle materials can become more ductile as they become hotter. A bittle fracture will have a clean and grainy surface which can also be flat. A ductile fracture will become more fibrous and sort of elongated because its tearing (pulling the material apart).

Figure 4

Task 2.1 Percentage of elongationCarbon steel = (10.7mm/25mm) x 100 = 42%Brass = (8.4mm/25mm) x 100 = 33.6%Copper = (15.15mm/25mm) x 100 = 60.6%

Task 2.2 Non-destructive testWe tested the hardness of 4 different materials by using a Rockwell hardness test. We used a colonial indenter.

This is the indenter

This is where the material is placed

Figure 5

Figure 6Hardness results Aluminium - 36.0 HRAMild Steel 50.6 HRANylon 2.7 HRAAcrylic 15.1 HRA

Task 4 - DegradationMetals - corrosion.Corrosion of metals is a natural process and is a gradual destruction. Metals are corroded through a chemical reaction associated with the environment. Corrosion is commonly used to explain an electrochemical oxidation of metal which has reacted to an oxidant such as oxygen. Corrosion degrades the properties of the metals which include strength, appearance and permeability to liquids and gasses. Most metal alloys can corrode by having exposure to moisture in the atmosphere but can be amplified depending on the environment. Corrosion does not happen to all metals and some alloys are created specifically to have corrosion resisting properties such as stainless steel. Corrosion which starts to form on a metal can extend across a wide area and cause cracks and lead to the material failing. Polymer UV lightMost of the natural and synthetic polymers are affected by UV radiation. If the polymer is not UV stable it can crack or disintegrate. Polymers can wear over time by simply being left in sunlight. Prolonged exposure is a more serious problem because it all depends on the extent and degree of exposure Ceramics - heatCeramics that are exposed to high temperatures can suddenly fail or have cracks gradually form on the surface. When the material is heated it tried to expand which will cause microscopic cracks to form which will increase over time. If it is exposed to ridiculously high temperatures it will fail immediately because it is trying to expand more than what it would be doing at a lower temperature. If it is exposed to the same temperatures that are making the small cracks form continuously it will eventually fail even though it has been able to function at that temperature for a while. The cracks will basically get bigger over time and can greatly impact the properties of the ceramics.

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