Background to Mechanical Testing of Composites:

Objectives of Mechanical Testing:

The development of the mechanical testing of the materials depends upon other scientific factors. These factors help in better understanding and facilitate the progress in evaluating the various processes. These processes include:

1. quality control of a process
2. quality assurance for the material developed and structure fabricated from thereof
3. better material selection
4. comparisons between available materials
5. can be used as indicators in materials development programmes
6. design analysis
7. predictions of performance under conditions other than test conditions
8. starting points in the formulation of new theories

It should be noted that these processes are dependent upon each other. However, if they are considered individually then the data required can be different for the evaluation. For example, some tests are carried out as multipurpose tests using various processes. A conventional tensile test carried out under fixed conditions may serve quality control function whereas one carried out varying factors like temperature, strain rate, humidity etc. may provide information on load bearing capacity of the material.   
The properties evaluated for materials like composite is very sensitive to various internal structure factors. However, these factors depend mainly upon the fabrication process or other factors. The internal structure factors that affect the properties are, in general, at atomic or molecular level. These factors mostly affect the matrix and fibre-matrix interface structure.

The mechanical properties of the fibrous composite depend on several factors of the composition. These factors are listed below again for the sake of completeness.

1. properties of the fibre
2. surface character of the fibre
3. properties of the matrix material
4. properties of any other phase
5. volume fraction of the second phase (and of any other phase)
6. spatial distribution and alignment of the second phase (including fabric weave)
7. nature of the interfaces

Another important factor is processing of the composites. There are many parameters that control the processing of composites that access the quality of adhesion between fibre and matrix, physical integrity and the overall quality of the final structure.

In case of composite the spatial distribution and alignment of fibres are the most dominating factor which causes the variation of properties. The spatial distribution and alignment of the fibres can change during the same fabrication process. Thus, for a given fabrication process the property evaluated from the composite material may show a large variation.