Machining of composites and specimen manufacture

An important issue not yet covered satisfactorily in the standards is the machining of composites and specimen preparation. Some minimal instructions are included in the mechanical test methods as a common annex. Some further information is given in the ASTM standard (D 5687) for test panel manufacture. Recent research under the CPD programme by NPL and QinetiQ lead to the publication NPL Measurement Good Practice Guide No. 38 on "Machining of Composites and Specimen Preparation". An annex to the Guide provides a working draft that can be used as the basis of a future standard equivalent to ISO 2818 - "Machining of Plastics". Several aspects were researched through experimental round-robins involving UK companies. The work was conducted under the DTI "Composite Performance and Design" programme.

The quality of specimens is very important to the quality of the resultant data. Important features are, for example,

alignment and symmetry of tabbing, and the resultant specimen, are particularly important for compression loading both because of the need to avoid buckling failures, also important for tension loading,
accurate thickness control is needed also for the Celanese fixture (BS EN ISO 14126) to avoid jamming of the loading jig,
accurate notching of shear specimens