Other Issues with Mechanical Testing of Specimens

The other issues associated with the mechanical testing include:

1. Stress concentrations due to material discontinuities at free edges, ply-drop off regions, which results into early failures.
2. In case of compression testing, there is a susceptibility to buckling for thin specimens. This type of testing demands for additional fixtures.
3. Flat specimen requires special geometry for purpose of gripping. For this reason, the flat specimens with end-tabs are favourably used.
4. The composite is heterogeneous and the volume fractions are the essential data required with the mechanical properties. Hence, additional tests are required to determine the volume fractions and void content, if any.
5. The mechanical properties determined are affected by moisture content. Hence, in some applications the amount of moisture present in the composite is required. Therefore, additional tests are required to determine the moisture content in composite.
6. Further, nondestructive evaluation (NDE) methods are required to assess the quality of the fabricated material and damage development during the loading.

The end-tabs are a special requirement in case of flat specimens. Therefore, it needs additional information. The end-tabs are used almost universally to reduce the probability of failure initiating at the grips during a tensile test. End-tabs can also facilitate accurate alignment of the specimen in the test machine, provided that they are symmetrical and properly positioned on the specimen, but if they are deficient in these respects they can cause misalignment and introduce stress concentrations.