The reason for increase in frequency of laminates with nano clay is that the clay increases the stiffness of the specimen.
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It is observed that as the velocity of impact increases the delamination area also increases and correspondingly there is decrease in natural frequency. Percentage of decrease in natural frequency at higher mode is higher than first mode. The percentage of decrease in natural frequency is high in laminates without clay. Hence it is clear that addition of clay controls reduction in delamination area which intern controls the reduction of natural frequency. Clay addition in thicker laminates improves reduction in natural frequency, this is because the control of delamination area by nano clay is high in thicker plates at high velocities.
In all the laminates an increased delaminated area was observed on the rear side of the target than the front side which is due to bending of the target during impact. The damping factors of the impacted specimens are obtained by half power band width technique from FRF plot.
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The damping factor is obtained for the first five modes of the impacted laminates. The same trend follows in other modes. The improvement in damping factor for mode 1 is noticed due to clay dispersion in matrix. Similar improvement is observed in mode 2 to mode 5 of vibration. The post impact damping factors for 8 layer laminates are shown in Fig. Comparing 3, 5 and 8 layer laminates, the improvement percentage in damping factor is in high 3 layer laminates.
This is due to higher delamination area of 3 layer laminates than 5 and 8 layer laminates, when subjected to impact loading. The improvement in damping factor of laminates with clay is due to clay dispersion in matrix and degree of delamination. Figure 16 shows the fracture surface of epoxy-fiber composite without clay. It shows failure of bundle of fibers without any damage in the surface area as it is held by the matrix.
There is no resistance from the matrix and hence the interfacial property of composite is poor and shows low strength. Failure of fibers are seen with non uniform length. It is also observed shear failure in fiber cross section as the failed cross section shows irregular shape. In this case, matrix at the surface of the laminates is completely damaged and fibers are not failed at impacted zone.
The impact energy is absorbed by the matrix and crack in matrix is observed in the region away from the point of impact. This is because the energy is obsorbed by the matrix with nano clay and the clay has participated in the load sharing. Complete brittle failure of both fiber and matrix is observed. This is due to the aggolerimation of clay particles leading to weak bonding between the matrix and clay. The experimental natural frequencies for pre and post impacted laminates are compared and the results for Mode I of three layer laminates are shown in Fig. Similarly for mode V, the decrease in natural frequency after the impact is between Similarly for mode V, the decrease in natural frequency is between 9.
It is to be understood that as the impact velocity increases the delamination area increases which intern reduces the natural frequency of the laminate. Addition of clay improves the natural frequency and also it controls the delamination area in laminates subjected to impact loading. Hence the decrease in frequency of laminates with clay subjected to impact loading is much less than that in laminates without clay. Figures correspond to Mode I damping factor values for 3, 5 and 8 layer laminates with and without clay respectively. It is observed in laminates with clay that as the velocity of impact increases the increase in damping factor is almost double.
This is because, the damping factor is improved by nano clay as well as by the delamination area. The reason being that the clay controls the delamination due to which the increase in damping factor is less for the velocity of impact increases.
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But in laminates without clay the increase in damping factor with increase of impact velocity is high as the delamination area increases with increase of impact velocity. The natural frequency and damping factor of the laminates are obtained before and after impact. The following conclusions are made. Natural frequency and damping factor in laminates are improved due to the presence of nano clay.
The natural frequency for the impacted specimen is less that in pre impacted specimen which is due to the damage caused by impact. The damping factor and delamination area increase with increase of impact velocity. In composites with clay the delamination area decreases with the increase of clay content and is seen for all velocity ranges.
Due to this the increase in damping factor is less than that in laminates without clay. Dispersion of clay improves the energy absorption capacity of matrix and protects failure of fiber. The authors gratefully acknowledge the support of the Aeronautical Research and Development Board, Structures panel, India. Adams and M. Dynamic flexural properties of anisotropic fibrous composite beams.
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Vaidya, S. The intuitive interface will guide you through the different steps of a complete modal analysis. Associating expertise and easy-to-use methods, Modal guarantees pertinent results in a minimum of time. All our software solutions are customizable to be integrated in your environment. Dedicated acquisition interface Define the test setup in few clicks whatever the excitation: sequence, transducer definition, measurement and trigger parameters, interactive display, control panel.
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