Influence of material thickness on the response of carbon-fabric/epoxy panels to low velocity impact

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Abstract

Low velocity impact tests were carried out on carbon-fabric/epoxy laminates of different thicknesses, by means of a hemispherical impactor. The force and absorbed energy at the point of delamination initiation, the maximum force and related energy, and penetration energy were evaluated. From the experimental results, all these quantities, except the energy for delamination initiation, followed the same trend, increasing to the power of approximately 1.5 with increasing plate thickness. For what concerns the force at delamination initiation, it is shown that its trend agrees with the assumption of a Hertzian contact law, coupled with the hypothesis that only the shear stress is responsible for delamination. It is also demonstrated that the force/displacement curves recorded for the different thicknesses sensibly superpose with each other when the forces are scaled to the power 1.5 and the displacements are held unchanged. This explains the observed dependence of the maximum force, energy at maximum force, and penetration energy on the thickness. Finally, the energy at delamination initiation is calculated by an analytical model, assuming that the total energy is shared in two parts, one of which is stored in flexure, and the other in the material volume close to the contact zone.

Introduction

Low-velocity impact is one of the most subtle threats to composite materials in aeronautics: owing to the weak bonds between the plies, even small energies imparted by out-of-plane loads can result in hardly detectable damages, causing considerable strength losses in tension and, especially, in compression.

Generally, the earliest observable damage affecting a laminate subjected to low-velocity impact is delamination, mainly responsible for compression strength impairment. For this reason, much research work has been devoted to the mechanisms of delamination initiation and growth [1–5]. Since the residual material properties after impact are of primary concern in applying damage-tolerance concepts, many authors have also tried to correlate analytically or experimentally the residual tension and compression strength with impact energy and damage mechanisms [3], [6], [7], [8], [9].

When application fields other than from aeronautics are considered, other parameters, such as the maximum force achievable for an assigned energy level, or the energy required to penetrate the body, are of major interest. For instance, in the case of motorcycle helmets, the maximum acceleration experienced by the human brain must be appropriately reduced in order to avoid fatal injuries, so that the maximum force is of paramount importance. On the other hand, the body panels of car, truck and rail vehicles must be designed in such a way as to prevent penetration from foreign objects of known mass and velocity.

In the literature, the problem of the calculation of maximum contact force has also been addressed extensively, by the use of spring-mass models [10], [11] or the principle of conservation of energy [12], [13]. Fewer data are available on the penetration process and related energy at low velocity [14], [15], although this phenomenon has received considerable attention in the domain of moderately high velocity or ballistic conditions [16], [17].

Looking at the topic of low-velocity impact, it appears that, despite the availability of many studies published in the last two decades, the wide range of specimen geometries and impactor parameters used render difficult a comparison of the experimental results and the assessment of general laws. This situation highlights the importance of scaling rules [18], [19], which would allow us to extend the data derived from given geometries, velocities and masses to other cases of practical interest.

In this work, low-velocity impact tests were performed on graphite-fabric/epoxy specimens of various thicknesses, with an instrumented drop-weight apparatus. From the load/displacement curves recorded during impact, the influence of material thickness on the main parameters involved in the impact phenomenon was evaluated. It was found that the force at delamination initiation can be accurately predicted by assuming a Hertzian contact law, coupled with a simple strength criterion based on the interlaminar shear stresses evaluated by strength-of-materials considerations. All the force/displacement curves pertaining to the different thicknesses effectively converge to a single master curve, if a scaling parameter varying according to the power 1.5 is adopted for forces, whereas the displacements are held unchanged. Consequently, the dependence of the maximum force, energy at maximum force, and penetration energy on the thickness can easily be established. Special considerations are needed for the energy in correspondence of delamination initiation: it is shown that, in the case examined, the latter can be correctly calculated only by taking into account the energy stored in the material volume near the contact surface, which can predominate when large specimen thicknesses are involved.

Section snippets

Materials and experimental methods

The basic layer employed for the fabrication of the laminates examined in this work was made of T400 fibres and HMF 934 epoxy resin. The fibres were in the form of plain-weave fabric 193 g/m2 in areal weight. The fibre content was Vf=0.55, resulting in a cured lamina of nominal thickness 0.19 mm.

Four quasi-isotropic panels having a {[(0, 90)/(±45)]s}n stacking sequence, with n=1 to 4, were fabricated by hand lay-up and autoclave cured at 177°C under 0.7 MPa pressure. From the panels, square

Overall load/displacement curve

Despite the differences in thickness, some features common to all the laminates emerged from the analysis of the load/displacement curves recorded during impact. They are discussed in the following with reference to Fig. 1, where a load/displacement curve is schematically drawn for illustration purposes.

When the contact force is sufficiently low, the material response is approximately linear. This behaviour is preserved up to point a (Fig. 1), where delamination starts propagating, resulting in

Conclusions

Low-velocity impact tests were carried out on graphite fabric/epoxy laminates having different thicknesses. From the results obtained, the main conclusions are as follows.

  • 1.

    The force required for delamination initiation increases following a power law whose exponent is very close to 1.5. Together with the observation of the failure modes, this suggests that delamination is mainly due to shear stresses, which can be calculated using the Hertzian contact law. This renders straightforward the

Acknowledgements

Alenia Company, Foggia (Italy), and in particular Dr. A. Marcone, are gratefully acknowledged for providing materials and performing impact tests whose results are discussed in this work.

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