Enhanced Evaluation of the Low-Velocity Impact Response of Composite Plates

Abstract

Recent research programs conducted on low-velocity impact events on composite structures have used force as the sole governing parameter and based their damage resistance and tolerance considerations on the peak recorded value. Understanding of other available parameters, such as contact duration and coefficient of restitution, which are related to the effective structural stiffness of the target, is fundamental in the design of a methodology for assessing impact performance and can offer greater insight in the interpretation of future research programs. An experimental database is gathered through drop tower impact testing by means of a rigid striker on clamped circular plates, for a particular polymer composite system. Several researchers have presented data showing that a critical value of the impact force for the onset of damage exists. Structural properties are hereby studied in both the sub- and supercritical regimes, which means for impact energy values below and above the damage threshold. A modified approach to the classic spring‐mass model, which employs the notions of damaged stiffness and dissipated energy, leads to the derivation of approximate formulas that describe the peak force-energy curve. In particular, the introduction of a dashpot to simulate the effect of damage greatly improves the accuracy of the model in the regime beyond the structural integrity threshold. A novel method to assess the residual performance of the damaged plate is developed, and it consists in low-energy, nondestructive impact testing, the results from which bear a striking resemblance with the curves obtained by compression after impact. MPACT tests can prove inherently difficult to understand because of the large number of parameters that play a key role in such events, particularly in the case of laminated composite structures due to their heterogeneous anisotropic nature and the complex failure modes that can occur. The current experimental investigation is conducted on square plates, supported over circular openings, having a quasi-isotropic [0/90/±45]4s stacking sequence. Such a configuration benefits from the axial symmetry of the circular geometry and the low degree of anisotropy of the laminate and thus facilitates the concentration on the mechanics of the impact event and the complex failure mechanisms. An extensive literature review has indicated that many questions still surround the impact response of composite plates. In particular, an ongoing debate exists on whether force or energy should be used to compare impact test results on different configurations, as well as whether a force- or energy-based criterion should be employed to predict the structural integrity threshold or uniquely and satisfactorily assess the state of damage in the plate. The aim of this paper is to show how governing parameters, such as force, energy, and structural stiffness vary between the subcritical and supercritical regimes and that peak force, although extremely valuable for predicting the damage threshold, cannot be used independently for investigating the impact performance of a composite structure. The use of an instrumented drop tower such as the General Re