Modified SDOF Models for Improved Representation of the Impact Response of Composite Plates

Abstract

Previous research has shown that simple single degree of freedom (SDOF) models can be used successfully to reproduce the force-time response of composite plates subjected to low-velocity impacts (LVI). In particular, a simple spring-mass (SM) model yields accurate estimates of the peak contact force as a function of impact energy. The major limitation of this model is that it applies only to elastic impact events, and ceases to be valid after the onset of damage. The effort of this investigation is to modify the SM model, while retaining the simplicity of its formulation, and to extend its range of validity to the super-critical regime. Three models are derived, namely the damaged stiffness, the dissipated energy, and the spring-mass-dashpot models, and their validity is examined through comparisons with experimental data. Impact force-time traces for carbon/epoxy laminates exhibiting various combinations of laminate thickness and support span are collected, and it is found that the additional unknowns introduced for each of the models, although determined empirically, are strongly related to the structural parameters for each configuration investigated.

Energy absorption characteristics of sandwich structures subjected to low-velocity impact

“…Alternatively, investigators have used the recorded load history curves to characterise elastic behaviour, failure initiation and failure propagation for the structure in terms of impact force and energy [9,16,18,19,[22][23][24][25]. When the impact energy of the projectile is high enough to exceed the damage threshold of the plate, the load-time history increases up to a critical value before a sudden drop occurs at the onset of damage (Fig.…”

“…In the literature, there is evidence to suggest that low-velocity impact tests strongly correlate with static indentation tests for carbon-epoxy laminates [22,26,[29][30][31] and Nomex honeycomb-cored sandwich panels with thin carbon-epoxy facesheets [9,13,15,24], in terms of load-displacement responses and damage characteristics. On the other hand, several studies have also indicated that the quasi-static equivalence for low-velocity tests is limited; for instance, slight differences in values of damage initiation loads have been attributed to inertial resistance for thick facesheets [9], harmonic oscillations due to the dynamic event [26], and material strain-rate effects [32].…”

“…2.5. However, several concerns have been raised regarding the overall applicability of such Figure 2.7: Peak impact load plotted against impact energy for carbon/epoxy laminates, from [22]. Beyond the elastic regime, reported experimental values were lower than theoretical ones which were predicted using a simple spring-mass (SM) model prediction.…”

“…However, one serious drawback of these elastic models is that they cease to be valid after the onset of damage and thus they are unable to model damage initiation and propagation. Feraboli [22,26] published load-energy plots for carbon-epoxy laminates subjected to low-velocity impacts, and used these plots to show that the spring-mass model overestimates the peak impact load for the plates after the onset of damage (Figure 2.7). Similarly, the energybalance model is inaccurate when damage initiates at higher impact energies [8,71].…”

“…Modified spring-mass models have been proposed to account for damage [22,66,68], but most are heavily based on empirical correlation [22,66]; hence they introduce additional unknowns that have to be determined experimentally.…”