Module 8: Composite Testing
  Lecture 35: Background to Mechanical Testing
 


Samples and Specimens for Mechanical Testing

The samples from which the specimens are made for mechanical testing can be in the form of:

  1. pultrusions,
  2. filament wound tubes and
  3. flat sheets

The former two types are used because they represent the most important fabrication processes and their ease of fabrication process. As we know from our earlier studies, in pultrusion the fibres are aligned along the pultrusion axis. In case of filament wound tubes, the fibres are aligned either spirally or circumferentially. However, the alignment can be optimally chosen in case of other filament shapes. In these two fabrication processes, the degree of void content is less and there is better consolidation of structure.
The advantage of using the tubular specimens is the ease with which specimen can be subjected to axial tension or compression, internal pressure, torsion and multi-axial loading. However, the limitations with these specimens include the high cost of fabrication and testing, fabrication may result in different micro-structure and hence different equivalent properties.

The flat sheets available for commercial use come in following four categories:

  1. layers of unidirectional fibres aligned with reference to an axis
  2. sheets of randomly oriented fibres in a plane
  3. layers of woven fibres aligned with reference to an axis
  4. sandwich structure

The flat sheets, depending upon the nature of alignment of the fibres can result into various behaviours like orthotropic, transversely isotropic or even isotropic. When one uses the mechanical properties of the composite, it is essential to quote the volume fractions and spatial arrangement of fibres. The flat specimen is an obvious choice because of economic reason.

The limitations with flat specimens can be:

    1. Specific states of stress can not be developed. For example, the state of pure shear is difficult to develop in such specimens.
    2. The axial compression is also a difficult issue due to buckling
    3. Further, developing a combined state of stress in such specimens is also a difficult task.