lecture 2

A static load is a stationary force or couple applied to a member . To be stationary, the force or couple must be unchanging in magnitude, point or points of application, and direction. A static load can produce axial tension or compression, a shear load, abending load, a torsional load, or any combination of these. To be considered static, the load cannot change in any manner.
The engineering materials are classified into two groups- ductile and brittle materials. A ductile materials is one which has a relatively large tensile strain before fracture takes place. On the other hand, a brittle material has a relatively small tensile strain before the fracture. Structure steel and aluminum are ductile materials, while cast iron is an example of a brittle material.
Q/ How can we distinguish between brittle and ductile material in practice?

A mechanical components may fail, that is, may be unable to perform its function suitably, as a result of any one of the following three modes of failure:
a) Failure by elastic deflection;
b) Failure by general yielding; and
c) Failure by fracture.
In application like transmission shaft supporting gears, the maximum force acting on the shaft, without affecting its performance, is limited by the permissible elastic deflection.
The mechanical components made of ductile material loses its engineering usefulness due to a large amount of plastic deformation after the yielding point stress is reached. Since the yield strength of a material is an important property when a component is designed against failure due to general yielding.
Components made of brittle material terminate to function because of sudden fracture without any plastic deformation. The failure in this case is sudden. In such cases, ultimate tensile strength of the material is an important property to determine the dimensions of these components. Figures 1 to 5 are photographs of several failed parts. The photographs exemplify the need of the designer to be well-versed in failure prevention.