Effect of type of reinforcement and stacking sequence on low velocity impact damage tolerance for hybrid woven carbon, glass, kevlar-epoxy laminates
Krystyna Imielińska, Rafał Wojtyra Politechnika Gdańska, ul. Narutowicza 11/12, 80-952 Gdańsk
Annals 2 No. 5, 2002 pages 364-368
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abstract Interest in composite materials for structural components has increased rapidly in the last decade. Proper selection of fibre and matrix may be expected to provide weight savings up to 50 percent over structural metals as well as improvements in fatigue endurance and corrosion resistance, reduced manufacturing costs and precise tailoring of material properties to service loads. In spite of these and other advantages which can be achieved with composite materials their use in many applications has been limited by concerns over their susceptibility to impact damage during the manufacturing process and in service as well as the corresponding reductions in mechanical properties resulting from such damage [1-3]. Consequently damage resistance and damage tolerance under impact loading can be regarded as most important characteristics of fibre reinforced composites [4]. Depending on the characteristics of composite constituents the choice of fibre and matrix type, laminate configuration and loading geometry the damage process can be a very complex combination of energy absorption mechanisms. In particular the selection of the types of fibres, form of reinforcement (woven or unidirectional) as well as stacking sequence play an important role in determining failure mechanisms, the extent of damage and the threshold energy of the composite. Often this damage may not be visible from the laminate surface, but its presence substantially reduces the mechanical properties of the composite, especially the in-plane stiffness and strength [7]. The results presented in this paper are a part of a broader study of new laminates for marine structures. The purpose of the project was to characterise the impact response and impact damage tolerance of four different laminates considered for use in construction of small ships. Woven carbon fabric and glass fabric laminates have been compared with the behaviour of laminates with different stacking sequence of carbon and glass layers (E/C/E/C/E/C/E, C/E/C/C/E/C, C/C/E/E/E/E/C/C) as well as Kevlar-glass and Kevlar-carbon fabric laminates. Impact behaviour was assessed on (100x100 mm) square laminate specimens using a dropweight facility. Weights with a hemispherical nose of 12 mm in diameter were dropped through 1 m along the rails to hit the laminate at the centre of the span of the specimen freely supported on the steel plate with a central, circular opening 38 mm in diameter. The weights were adjusted to give the incident energies 10÷64J. Three specimens have been tested for each impact energy level. The impact resistance was assesed in terms of internal delamination surface area resulting from impact of a given incident energy. The extent of impact damage has been estimated by ultrasonic C-scan technique or by visual inspection. The first method has been applied to the specimens containing carbon fibres which were opaque to light. The second approach was used in the case of glass and Kevlar-glass laminates for which visual observation of internal damage against the strong source of light was possible. Compression after impact tests were performed on impacted square specimens (100x100 mm) using abtibuckling device. The graphs of impact damage area vs. normalised impact energy have been plotted and compression strength after impact has been related to the area of impact damage. For comparison, the same plots have been analysed for Kevlar-carbon fabric and Kevlar-glass fabric laminates. Tensile and bending strengths have also been compared for different laminates. It has been concluded that all carbon-glass laminates had similar post-impact compression strength for the same area of impact damage regardless the stacking sequence. Kevlar-glass fabric laminate showed similar damage tolerance and mechanical properties as carbon-glass laminates. Despite the high price laminates reinforced with Kevlar-carbon fabric were importantly inferiour than other materials. Regarding low velocity impact damage tolerance, in the case when the extreme strength and stiffness are not required, expensive carbon laminates tested in the present work can potentially be replaced by cheaper carbon glass-epoxy laminates of diverse stacking sequence as well as by Kevlar-glass fabric laminates.