1.5 HOOKE'S LAW AND MODULUS OF ELASTICITY

1.5   HOOKE'S LAW AND MODULUS OF ELASTICITY

We can learn some of the mechanical properties by performing tensile tests using a testing machine that can impose a pure axial load. From these tests, we have some data and data of which are stress and strain occur on the specimen being studied. Examples of stress-strain diagram is as figure 5.3.

In the elastic limit (0-1), the material will return to its original condition after its release. It meets one law called Hooke's Law, which states that stress is proportional to the strain of an elastic material so long as does not exceed the elastic limit. Thus, a constant that can show this Hooke's law, the Young's Modulus (Modulus of Elasticity), where it also is the slope straight line in the elastic limit.

                                         Units for young modulus is Pascal (Pa).

... From the basic equation:

 

Apart from the stress-strain data, several other results can be known from tensile tests carried out, including

From the results obtained, several new properties of materials are available, namely:

1.4 STRAIN

1.4   NORMAL STRAIN (e)

When a force P applied to a material, deformation will occur on the material. The ratio of the material deformation strain is called Normal. Normal strain is defined as an extension or shottening had happened to the size of the material per unit length.

There are no units for strain because it is a ratio, while the shortening of the unit for extension @ original length is m. Strain can also be broken down into two types: Axial Strain and Strain side. Axial strain is also referred to as longitudinal strain, occurs in the direction of the applied force, while the lateral strain occurs in the direction perpendicular to it. For rods, the definition of axial and lateral strain can be simplified as follows.

 For elastic materials, these strains have a relationship, where a constant can be formed, called the Poisson ratio, (V). Poisson's ratio can be defined as the ratio of lateral strain to axial strain.





1.3 STRESS

1.3 Normal stress  (s)

When the force P applied at a bar, it just causes elongation at section x-x. A force of similar value to the force P will be created / exists in a bar in the opposite direction to prevent them getting it will be in equilibrium. The forces that exist at section x-x is said to be uniformly distributed over the entire cross-section and it is known as stress.

Units for stress is N/m² @ Pascal (Pa), where the unit force is Newton (N) and unit area is m².

The normal stresses can be broken down into two types of tensile stress and compressive stress, which distinguish both direction of the internal forces. Tensile stress (Figure 5.1) occurs when the internal forces are in tension or in the direction out of the material. Conventions of the tensile stress is positive (+ ve). Meanwhile, the compressive stresses (Figure 5.2) is the opposite, that the internal forces in compression or in the direction into the material. Signs of the compressive stress is negative (-ve).

1.1

1.2 TYPE OF FORCE
The force can be broken down into two main categories, namely normal force and shear force. The normal force means the force applied is perpendicular to the surface action, while the mean shear force acting parallel to the surface action. The normal force can be broken down into two types of tensile forces and compressive forces.

1. Tension force

    - If a bar @ the material applied force P is shown in the diagram, the material will change shape, the longitudinal and cross-sectional  area will be narrowed. Materials are said to be in tension.  The force P is called a tension force. (+ ve).

                

2. Compression force

    - If a bar @ the material applied force P is shown in the diagram,   the material will change shape, which shorten and cross-sectional   area will increase. Materials are said to be in compression. The force P is called the compression force. (-ve.)

            

3. Shear force

    - When subjected to tensile force P, the plate will be segregated   from each other. Forces that are at the connection of opposing tensile force called shear forces.