Evaluation of the damaged area of glass-fiber-reinforced epoxy-matrix composite materials submitted to ballistic impacts

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Abstract

Fiber-reinforced composites submitted to ballistic impact show large, and sometimes irregular, areas of delamination. The evaluation of these damaged areas is important to assess the effects of the laminate microstructure on the response of the material to the imposed loading. In this paper bulletproof laminates were obtained using 30 plies of glass-fiber reinforced-epoxy matrix composites. The ballistic performance of these laminates was varied by modifying the properties of the constituent laminate or by changing the laminate assemblage. An automated digital analysis procedure was developed in order to determine size and shape parameters that characterize the damaged areas. The procedure developed proved to be robust and sensible enough to cope with subtle variations on the damaged areas, which are due to the differences on the mechanical response of the composites caused by modifications on their microstructure. The results obtained show that not only the area of delamination is important to quantify the ballistic performance. The isotropy of the ballistic response, determined measuring the Feret Ratio or the Circular Shape Factor, is also important.

Introduction

Nowadays, lightweight armors are widely used on ordinary daily applications like the protection of passenger vehicles against handgun shots. Aramid reinforced composites have one of the best protection to weight ratios for these applications [1], [2]. Nevertheless, glass-fiber reinforced composites could also be used due, mainly, to their lower cost. Common to all these composite materials used for ballistic protection is the problem of how to evaluate the damaged area after a non-penetrating shot. In fact, this problem is common to composite materials whenever submitted to an impact event, and is one of the major problems posed to the use of these materials in the aerospace industry [3], [4], [5].

Many test methods are being employed to evaluate the damaged area in composites, such as liquid penetrant [6], C-scan ultrasonic [7], [8], [9] or radiographic analysis [8], [9], [10]. For glass-fiber reinforced composites one could also take advantage of the transparency of this material to white light [9], since the defects, like the ones produced by a ballistic impact, will produce dark areas that can be mapped and measured. These methods, however, are not able to discriminate between the main failure modes of a composite material, i.e., fiber breakage, fiber debonding, delamination and matrix cracking [11], [12]. Nevertheless, the size and geometry of the damaged area can give reliable information about the ballistic performance of these materials.

In this work a digital image analysis procedure is used to measure damaged areas and their shapes in glass fiber epoxy composites submitted to ballistic impact. The results obtained were used to evaluate the performance of the tested materials as a function of the ply stacking sequence and modifications on the formulation of the epoxy matrix material.

Section snippets

Experimental procedure

Eight glass fiber epoxy laminates were fabricated using the hand-lay up procedure. Four of the laminates were fabricated with 20 plies and the other four with 30 plies. The weight fraction of fibers was kept constant at 60%, and a woven glass cloth with plain weave configuration and with 223 g/m2 was used throughout this work. The plies were laminated so that the warp and fill directions were perpendicular to the edges of the obtained plates. Therefore, the laminates obtained were said to have

Damaged region evaluation

The traditional manual method for evaluating areas involves square sampling and counting. It can provide precise results if the square size is small enough but is prone to human errors and is extremely slow. Moreover, it does not easily provide other measurements like Perimeter, major axes or shape.

Image Analysis (IA) offers an accurate, fast, operator independent method, and allows the measurement of many sophisticated parameters that are virtually impossible to obtain manually. However,

Laminates No. 1 (20 plies)

The results obtained for laminates 1a–1d are shown in Table 2, Table 3, Table 4 and in Fig. 6, Fig. 7. For laminate 1a only the rear side delaminated area was measured. This laminate was cut for microprobe X-rays analysis and for acid digestion, and is not available any more. Also, since a slightly different procedure than the one here described was used for the area evaluation, the other parameters obtained by image analysis are not given here to avoid any biased result. The result obtained

Conclusions

Bulletproof laminates were obtained using 30 plies of glass-fiber reinforced-epoxy matrix composites. The ballistic performance of these laminates was varied by modifying the properties of the constituent laminate or by changing the laminate assemblage.

The laminate fabricated with a tougher resin matrix showed the smallest delaminated area. This behavior was attributed to the increase of the force to produce delamination with the increase of GIIc.

The largest delaminations were obtained for the

Acknowledgements

The authors acknowledge the financial support of the Brazilian funding agencies CNPq and FAPERJ.

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