Application of dynamic testing to the analysis of impact strength in laminates composites
Izabella Hyla, Artur Lizurek Politechnika Śląska, Wydział Inżynierii Materiałowej, Metalurgii i Transportu, ul. Krasińskiego 8, 40-019 Katowice
Annals 2 No. 5, 2002 pages 374-377
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abstract Fibrous polymer composites in the form of multilayer laminates show high sensitivity to any impact loads. Even if we assume that the loads do not lead to fracture they definitely cause microcracks formation, which in turn result in the loss of laminate strength. The strength values of the material as well as the kinetics of the destruction process undergo significant changes. Therefore the dynamic strength of fibrous composites is not only effected by impact strength value but also by the interrelation between elements of the destructive energy in the period between load initiation and the moment of material destruction. Therefore our studies on composites focused on the above problem. The paper presents test results obtained for the composite: polyester resin-glass fibres, which has been subjected to a three-point dynamic bending test. The tests have been performed with Charpy pendulum machine. The instrumentation of the machine enables to record the bending force and the sample deflection at very small time intervals. This helps obtain force-strain diagram (Fig. 1). The diagram illustrates the amount of energy required for destruction of the sample both within elastic strain range (Ue) and the range of irreversible decohesive changes (UP). The results obtained indicated that the impact strength of the composite and its crack resistance strongly depend on the number of fibres and the form in which they are introduced into the matrix. Continuous filament introduced in the form of a woven fabric ensured better impact strength than short fibres introduced in the form of a mat (Fig. 2). This remains in close dependence with the kinetics of composite destruction process (Fig. 3). Such conclusions may be drawn from the image of the fracture. The analysis of the crack length (Figs 4 and 5) showed that the deeper the crack was the smaller the impact strength of the composite became but at the same time different kinetics of its destruction. In composites reinforced with a fabric the crack length effect is visible mainly in the second element of the total destructive energy (i.e. UP energy value). In composites reinforced with a mat both elements of the total destructive energy (i.e. Ue and UP) undergo similar change. This might prove the existence of different environment and different possibilities for microcrack development in both types of reinforcements. In case of short fibre reinforcement the crack can develop only in the matrix and avoid fibres. However at continuous fibre reinforcement the macrodefect has to overcome both resistance of the matrix and that of the fibres.