I need help understanding these topics in my Engineering class:
- A description and explanation of Engineering Stress and Engineering Strain, and true stress and true strain.
- A plot of the stress strain curve and explanation of the properties that can be obtained from the curve.
- Description of the three strengthening methods.
Material Science and Engineering
When force is applied to an object, it tries to change its shape. The object opposes this deformation, and internal opposing forces are developed inside the object. The internal resistance created in the body per unit area is known as stress. It is also the distribution of pressure inside a material that either reacts or balances to its loads. Now let`s consider a scenario in which an individual applies pressure on a body, and it causes it to change in shape. The ratio of the change in the object`s size to the original size of the body before the application of force is known as strain.
True stress is the stress calculated by the load acting upon the area that instantly changes. On the other hand, true strain is equivalent to the common logarithm of the quotient of current size over the actual size. In material science and engineering, a stress-strain curve for a grain provides the relation between strain and stress. To obtain this, one should gradually add a load to a control coupon and measure the displacements from tensile testing. An individual should elongate the test while noting the stress changes with strain up to the time the material fractures to plot the curve. The strain is plotted on the X-axis while stress on the Y-axis. An assumption that should be made is that the cross-sectional area of the substance remains unchanged during the entire process of deformation. Notwithstanding, that is not practical because the exact size decreases in the deformation process.
Below is an illustration of a stress-strain curve:
Some of the properties obtained from the curve include the yield strength, which indicates the upper limit on which a material can develop without creating plasticity. It is incredibly significant for controlling production techniques of materials such as rolling, forging or pressing. The second property is the elastic modulus, which explains how easily a material is stretched or bended. It may also mean determining the capacity of a material to resist stretching in length when put under compression. The third property is the ultimate tensile strength, the point in a fabric that starts to break down and tear. When a malleable material attains its utmost strength, necking occurs, in which the rate of work hardening is lower than the flow stress.
The Three Strengthening Methods
There are different mechanisms of strengthening materials. One of the methods is grain size reduction. When a grain size is decreased, the number of borders increases that should give way before movement can occur. Slowing the movement of grain dislocations strengthens the material, and the smaller the grain size, the stronger the fabric. Secondly, the solid solution method is an alloying that can be applied to boost the hardness of the native metal. This method works by applying the alloying material to the original metal, forming solid solutions. The third method is cold working, whereby a material is strengthened by changing its shape without heat. When a piece of metal is twisted, dislocations are produced, and as they increase, they will get tangled and unable to stretch. This will harden a material, making it hard to change in shape. The dislocations get pinned against one another and can become intertwined. The entanglement inhibits any further permanent deformation of that specific grain without the use of more incredible energy.
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