Dynamic behaviour of composite sandwich beams and plates with debonds

Dynamic behaviour of composite sandwich beams and plates with debonds

Fibre Reinforced Polymer (FRP) composites are continuing to gain prominence in structural as well as non-structural applications all over the world due to their outstanding properties such as high strength to weight ratio, corrosion resistance, good thermal performance, anti-fire performance and reduction of carbon dioxide emissions both through its method of production and their effective thermal insulation qualities. The increased popularity and demand for FRP composites have spurred research efforts in both academia and civil construction industry.

A composite sandwich structural element can be made-up by attaching two thin and stiff skins to a lightweight and thick core, which serves as a building block for constructing laminated structural sandwich composites for civil engineering applications. A structural composite multilayer beam or plate can be manufactured by gluing two or more composite sandwiches together to form a laminated composite. An Australian manufacturer has fabricated a new generation structural Glass Fibre Reinforced Polymer (GFRP) sandwich panel made from E-glass fibre skin and a high strength modified phenolic core for civil engineering applications, the outstanding features of the sandwich material being high strength to weight ratio, good thermal insulation and termite resistance. These features offer the composite panel a wide range of applications in Australian construction industry as structural elements such as beams, slabs, bridge decks and railway sleepers.

While sandwich composite construction has some great benefits, the behaviour of sandwich structures containing damage is much more complex and one of the major factors limiting the optimum usage of the same. Although perfect bond between the skin and the core is a common assumption, an important issue that needs to be considered in using a composite beam or slab is the development of debonding between the skin and the core, which is a predominant damage mode of these sandwiches. Interlayer debonding or delamination is also a predominant form of damage phenomenon in laminated composites, which can often be pre-existing or can take place under service conditions. Debonding and delamination cause significant changes in the vibration parameters, such as natural frequencies and mode shapes of structures leading to serviceability issues related to deflection limits. During the design stages of FRP composite structures, it is vital to identify how the global response of these structures will be affected by skin-core debonding and interlayer delamination.

Even though the dynamic behaviour of undamaged sandwich panels is the subject of extensive research, papers reported on the dynamic behaviour of sandwich panels with debonding are less presented in the literature. Specifically, knowledge on seismic behaviour of composites with debonds is severely limited. Further research is therefore needed into investigation of the dynamic behaviour of debonded composite structural elements to gain wider acceptance of composites by the structural composite field around the globe. Finite element method is particularly versatile and effective in the analysis of complex structural behaviour of the composite structures. The use of dynamic analysis methods helps the engineer to better understand the behaviour of a structure subjected to an earthquake.

This research deals with the investigation of the influence of debonding on the dynamic characteristics of novel GFRP beams and plates by finite element based numerical simulations and analyses using STRAND7 finite element (FE) software package. The research approach is to develop a three dimensional computer model and conduct numerical simulations to assess the dynamic behaviour. The FE model developed has been validated with published experimental, analytical and numerical results for fully bonded as well as debonded beams and slabs. Dynamic seismic response investigation of structures containing GFRP slab panels with debonds subjected to a probable earthquake loading is also incorporated. The influence of various factors such as debonding size, location of debonding, boundary condition of the structural member and the effect of multiple debonding has been delineated with the aid of an extensive parametric investigation and comparative analyses.

Generally it was evident from all the analyses that debonding and interlayer delamination cause reduction in magnitudes of natural frequency. Moreover, some vibration modes and accordingly the mode shapes are also noticeably changed. It is generally observed that higher natural frequencies and mode shapes are more influenced by the presence of debonding. Yet there are exceptions to this trend depending on how severely the local modes are affected by debonding. It is observed that the associated mode shapes explain the causes of these inconsistencies. Furthermore, the results show that the presence of relatively small debonding or delamination has an insignificant effect on the natural frequencies and associated mode shapes. The results also suggest that fastening the delamination region is an effective corrective measure in decreasing the natural frequency variation, hence improving its dynamic performance compared to the delaminated panel.

To sum up, the results suggest that debonding and delamination predominantly leads to reduction of the natural frequencies and modifying the modes of vibrations thus altering the mode shapes as well, resulting in dramatic changes in dynamic characteristics when extents of debonding are large. The more the supports are restrained, the greater the influence on free vibration characteristics. Most importantly, the findings demonstrate the feasibility of non-destructive methods to detect debonding and delamination damage in practical composite structures. The results of the seismic study show that the seismic performance of the considered buildings is unresponsive to small percentages of debonding of the GFRP slab panels. An existence of extensive percentage of debonding causes a slight increase in the maximum vertical displacement and reduction of natural frequencies of the buildings due to loss of stiffness occurring due to debonding.

The results of this study will offer engineers and designers a better understanding of the influence of debonding and delamination on the dynamic performance of FRP composites in general, in addition to its direct application to Australian composite industry. Finally, the study provides valuable insights into the seismic behaviour of composite slabs with debonding thus facilitating the actual application of these findings in worldwide composite industry.