DEGRADATION OF MECHANICAL PROPERTIES OF CARBON FIBRE REINFORCED COMPOSITES UNDER DYNAMIC LOADS

Increasing application of the CFRP in the high-loaded constructions is observed during the last period. The high- performance competition gliders can be an example. In such application even small damage of the composite structure may excessively decrease a safety margin. It was the reason for undertaking the works aimed on investigation the CFRP composites vulnerability on impact loads. The interest was focused on the composites with uni-directional (UD) structure. Such composites are used in the wing spar flanges (cups). They are the most loaded elements of the gliders structure. The specimens for compression tests (Fig. 2) were applied in investigations, as the compressive strength is the critical parameter, having an important influence on the wing spar dimensions. The ultrasonic structural tests were performed on the immersion stand, equipped with ultrasonic flaw detector Panametrics 9100 connected to the PC (Fig. 1). The Charpy method was used for impact tests. The impact transverse loads had caused significant differences of the ultrasonic A-scans taken in the initial and the final stadium of the tests (Fig. 4). Relation of the residual strength to the impact energy has non-linear characteristics, with existence of the threshold value (Fig. 5). Beyond this limit a quick residual strength degradation of the CFRP can be observed. The biggest drop of the residual strength was observed in the range of energy density from 0.01 up to 0.04 J/mm(2) (Fig. 6) (at the end of this range the residual strength was 2-times lower than the static compression strength of CFRP). Optimal volume fraction of the CFRP has been estimated (at around 0.5) - both for post- impact residual strength and for static compression strength of the CFRP composites. The influence of pressure applied during CFRP beams forming on the static strength and the residual post-impact strength has also been investigated. The results are shown in Figure 10. Quasi-layered structure of obtained composites, caused by the fabrication method, has no significant effect on impact sensitivity of material on parallel and perpendicular direction relative to impact direction (Fig. 11).