Mechanical Properties Assignment Help

The Procedure Of Design - Mechanical Properties

Mechanical Properties

Properties are quantitative measure of materials behaviour and mechanical properties pertain to material behaviors beneath load. The load itself can be dynamic or static. A gradually applied load is regarded as static. Load applied by a universal testing machine on a specimen is closet illustration of gradually applied load and the results to tension test from that machines are the basis of defining mechanical properties. The dynamic load is not basically a gradually applied load after that how is it applied. Let consider us a load P acting at the center of a beam, such is simply supported at its ends. The reader will feel happy to determine the stress as its maximum value or deflection or both by utilizing a formula from Strength of Materials. However remember that while the formula was derived specific assumptions were made. One of them was that the load P is gradually applied. Such load means that whole of P does not act on the beam at a time although applied in installments. The installment may be, say P/100 and thus after the 100th installment is applied the load P will be said to be acting on the beam. If the complete of P is placed upon the beam, then it comes beneath the category of the dynamic load, often referred to like Suddenly Applied Load. It is a shock load, if the load P falls from a height. A fatigue load is one which changes along with time. Dynamic and Static loads can remain unchanged along with time after first application or may alter along with time reduce or increase in which case, they are fatigue load. A load such remains constantly applied over a long time is named as creep load.

All Strength of Material formulae are derived for static loads. Fortunately the stress caused by a suddenly shock load or applied load can be correlated along with the stress caused by gradually applied load. We will invoke such relationships like and when required. Like stress formulae, the mechanical properties are defined also and determined beneath gradually applied loads since such determination is easy to control and thus economics. The properties so determined are influenced by sample geometry and shape, size and surface situation, testing machines and even operator. Then the properties are likely to vary from one machine to other and from one laboratory to other. Though, the static properties carry much less influence like compared to dynamic particularly fatigue properties. The designer should be fully aware of such influences since most machines are beneath dynamic loading and static loading may simply be a dream.

This is imperative at this stage or level to differentiate between elastic constants and mechanical properties. The elastic constants are dependent upon type of material and not upon the sample. However, strain rate or rate of loading and temperature may affect elastic constants. The materials utilized in machines are basically isotropic or so assumed for which two independent elastic constants exist where three constants are often utilized in correlating strains and stress. The 3 constants are Modules of Elasticity as E, Modulus of Rigidity as G and Poisson's Ratio as v. Any one constant can be expressed in terms of another two.

An isotropic material will have similar value of E and G in all direction however a natural material like wood may contain different values of E and G along fibers and transverse to fiber. Wood is non-isotropic. Most commonly utilized materials as: iron, steel, and aluminum and its alloys, copper, and its alloys, are extremely closely isotropic while plastic and wood are nonisotropic. The strength of material formulae are derived for isotropic materials merely.

The leading mechanical properties utilized in design are ultimate tensile strength. Yield strength, hardness, percent elongation, impact strength and fatigue strength. Before we begin to define them, we will determine that considering tension test is the most suitable beginning.

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