Modeling and scaling laws for low-velocity impacts OF COMPOSITE PLATES

The development of the laws that regulate a similarity passes from the certain knowledge of the structural parameters, so that it is possible to build a scaled model that can replicate the response of the "full-scale" system. The similarity theory has found wide use in fluid mechanics thanks for example to the dimensionless Reynolds or Froude numbers. In this case, the similarity laws between model and prototype are well known and the results achieved are particularly valid. The study, on the other hand, of the behavior of a structure subjected to the action of a load impact (dynamics of structures) through the similarity method is a relatively recent activity. In this paper the aim is to apply the theory of scaling to composite materials widely used in the field aerospace and subjected to low-speed impact loads. This work describes the experimental activities about samples with different dimensions tested with a drop weight machine using different impact energy values so to characterize the behavior in and out elastic zone. The idea is to identify a common factor so to show the impact behavior and the evolution of the damage suffered by the specimen examined evaluating the importance of the geometrical scaling on the output results. Results are also discussed in terms of strain rate sensitivity of the specimens in relation of the dissipative interlaminar phenomena and a more marked influence of the dimension on the laminates is investigated. Drop tests results are then compared to numerical ones, obtained by simulating a hemispherical dart impacting a thin composite laminate. Thus, a finite element analysis (FEA) is performed using the explicit finite element code LS-DYNA. Attention is specifically devoted to three composite laminate samples, consisting of polyamide 6 and polyamide 11, both reinforced with basalt fiber fabric, and polypropylene reinforced with a flax fabric, having the same layup but with different scaled dimensions to identify the scaling law for the time histories of the impact load. Low speed impact tests are performed with a drop tower equipped with a movable frame that allows measurements with different falling heights. A supporting plate is used to fix the laminate on the ground and a MEMS accelerometer (X16-1D) is placed on the specimen, recording the local acceleration. Moreover, a sensorized hemispherical dart is used to capture the load time history into the impacting zone. The dimensions and geometric specifications of all the equipment is compliant with ASTM D7136 standard.