Contact force history and dynamic response due to the impact of a soft projectile

SUMMARY A series of ballistic impact tests on several different instrumented tar- gets was performed to characterize the dynamic contact force hi story resulting from the impact of a compliant projectile. The results show that the variation of contact force history with impact velocity does not follow the trends pre- dicted by classical impact models. An empirical model was therefore developed to describe this behavior. This model was then used in a finite-element analy- sis to estimate the force history and calculate the resulting dynamic strain response in a transversely impacted composite laminate. INTRODUCTION As the use of composites becomes more common in modern aircraft, the per- formance of these materials under the full spectrum of loads they may encounter in advanced aerospace structural applications is of prime concern. Composite laminates are susceptible to impact damage, particularly that due to transverse impact. Bird strikes, for example, on aircraft flying at high speed and low altitude have recently proven to be extremely hazardous. Classical impact mechanics have been developed largely under the assumption of relatively stiff impactors which undergo small elastic deformations during the impact event. Energy di ssi pation and 1 arge deformations encountered during the high-speed impact of a more compliant projectile, such as a bird, limits the applicability of these classical approaches. The objectives of this work are, first, to describe a simple experimental technique that can be used to determine the force history resulting from the impact of a soft projectile. Secondly, the data taken from a series of tests will be presented, interpreted, and used to form the basis of an empirical force-history model which will be used to analyze the transverse impact of a 1 ami nated beam. In the first section of this report, a detailed description of the experi- mental apparatus and procedure is given. The next two sections outline an accurate and efficient numerical model that is used to analyze the impact experiment. The results of a simple "calibration test" are then compared with those from the numerical analysis to demonstrate the validity of the test pro- cedure. In the following section, the measured response from the impact of the soft projectile is compared to the simpler classical model, and the funda- mental differences between the two are established. An empirical model describing the contact force history due to the soft projectile impact is then devised and adapted to describe the more complicated transverse impact of the