1 Introduction

Fibre-reinforced polymer laminates (FRP) have been increasingly used for manufacturing components in several industrial sectors. Their tailorable properties and excellent specific mechanical properties make them competitive against more traditional materials for those applications in which the weight is a very important factor.

Fibre-reinforced polymers are, however, highly susceptible to impact loading. Even low-energy impacts may seriously damage these materials because of delamination issues, reducing their mechanical properties and, therefore, generating disadvantages for the extension of their use (Abrate 1998). Hence, the damage of composites under low-velocity impact has been widely studied for all kinds of fibre-reinforced polymers at different temperatures and environmental conditions (Lopresto et al. 2017; Gómez del Río et al. 2005). On the contrary, high-speed impact performance of composites has been analysed almost exclusively for those fibres used for producing armours, such as Kevlar, Dyneenma, Twaron or Spectra. Published information about the performance of fibre-reinforced polymers made of other fibres under high-speed impact is much scarcer (Syngellakis 2014; Rupert and Blethen 2010; Cunniff 1992, 1996).

In the civil naval sector, composites have been widely used for producing hulls of small-length ships (usually, up to about 20 m long), being glass fibre-reinforced polyester or vinyl ester, the most common configurations used for this purpose. In the last decades, there has been an increasing interest to analyse the utilisation of fibre-reinforced polymers to manufacture components for the military naval sector. So far, composites have been extensively used for manufacturing hulls of small-length navy ships like minehunters or disembark boats. Nevertheless, nowadays, composites are being considering to produce some components of the superstructure of battleships and even to produce hulls of warships and submarines (Department of Defense USA 2000; Shenoi et al. 2011; Grabovac 2005). In such applications, the naval structural components are subjected to multiple actions, not only mechanical (i.e., impact, fatigue, etc.), but also environmental (thermal cycles, saline ambient, humidity, etc.).

Although many efforts have been performed in recent years to study the effect of each of these actions on the performance on composite materials, the research activities considering the coupled effect of several factors acting simultaneously are scarce. Therefore, analysing the dynamic response and failure of composite materials of naval relevance under impulsive loads coupled with environmental effects is worth to be carried out. Some preliminary results of such wide research programme are presented in this paper. In particular, the response and failure of six different configurations fibre-reinforced polymers under high-speed impact in several environments are covered.

2 Experimental details

2.1 Materials

Two different fibres have been considered, S2 glass and carbon. With respect to the matrix, only vinyl ester resin has been chosen, being the most widely used for naval applications.

Experimental tests (firing tests) have been carried out on six different target configurations:

  • Plain carbon-reinforced vinyl ester (20 plies) CFRC.

  • Plain S2 glass-reinforced vinyl ester (40 plies) GFRC.

  • Mixed hybrid S2 glass/carbon-reinforced vinyl ester.

  • Non-mixed hybrid S2 glass-backed-by-carbon vinyl ester.

  • Non-mixed carbon-backed-by-S2 glass vinyl ester.

  • Mate glass (short E-glass fibres with vinyl ester matrix).

All plates were produced by infusion of the resin through the reinforcement material by a vacuum bag (see Fig. 1). After manufacturing, the plates were cut to obtain specimens with dimensions of 100 × 100 mm and 6 mm thickness, approximately. Except for mate glass, all plates were woven fabric reinforced. Hybrid-reinforced laminates had 10 plies of carbon and 20 plies of S2 glass.

Fig. 1
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Resin infusion process

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On the other hand, mate glass is a widely used composite material for shipbuilding that is manufactured by stacking several layers of E-glass reinforcement and that may be used when light and flexible structural components, manufactured by resin infusion, are needed. A single layer of mate glass contains (see Fig. 2): Mat + 45° Uniaxial layer + − 45° Uniaxial layer + Woven Roving + Mat. This configuration was duplicated to achieve a thickness of about 6 mm, similar to those of the other composites.

Fig. 2
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Schematic representation of the mate-glass structure

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2.2 Experimental set-up

High-speed impact tests were carried out by means of shooting 6.49-mm-diameter steel balls in a compressed-gas gun with 7.62-mm calibre barrel; so the use of 7.62-mm-diameter plastic sabots was needed. Both the impact velocity and the residual velocity after perforation of the target were measured by recording the tests and determining the time needed for the projectile to travel a certain distance, using a Phantom high-speed camera with an 80,000 fps set-up. Figure 3 illustrates the experimental set-up that was used.

Fig. 3
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Compressed-gas gun with the high-speed camera system

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The experimental campaign covered tests on the six different configurations (see Sect. 2.1) at room 25 °C, low − 50 °C, and high 50 °C temperatures. With the aim of studying the effect of the environment on the performance of the composites, as-received specimens and seawater-saturated specimens, named conditioned specimens, were studied. Seawater saturation was achieved by immersing the composite plates into a bath maintained at 50 °C, inside a climatic chamber, to speed up the water entrance into the material. Seawater saturation was checked according to EN2823 AECMA standard by measuring dimensions and weight of plates every week. According to the aforementioned standard, the process finished when plates weight did not increase more than one percent in a week as indication of saturation and this happened after 1 month of immersion.

The specimens tested at 50 °C and − 50 °C were heated up or cooled down using a furnace and pellets of solid CO2, respectively. To control the temperature during this process, each specimen was instrumented on the surface with a type-K thermocouple. Once the target temperature was reached, the samples were kept in the same thermal condition for 1 h to ensure temperature homogenization. Finally, a polystyrene box was used to put inside the sample during testing to keep the temperature constant (see Fig. 4). Moreover, the conditioned specimens were wrapped with polyvinyl chloride film (PVC) to avoid the loss of water in the conditioned specimens.

Fig. 4
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On the left, the polystyrene box with CO2 pellets that was used to obtain a controlled temperature, in this case the low temperature − 50 °C, and on the right, the thermocouple to monitor the testing temperature

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3 Experimental and modelling results

Experimental results in the form of graphics of residual-velocity vs. impact-velocity curves were obtained for all materials, temperatures and environmental conditions (see Figs. 5, 6, 7, 8, 9, 10). An easier comparison of the performance of the different configuration is allowed by arranging the ballistic limit V50, which is defined as the impact velocity for 50% probability of full perforation of the target, for each configuration and environmental condition, as can be checked in Fig. 11 for non-conditioned samples (on the left) and for the conditioned ones (on the right).

Fig. 5
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Experimental data and analytical residual- vs. impact-velocity curves at room temperature for non-conditioned specimens. On the left curve, the GFRC (solid line) and CFRC (dashed line) analytical curves are identical. The critical strain for the S-2 glass fibre was set equal to 3.22% and for the carbon fibre was set equal to 1.5%

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Fig. 6
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Experimental data and analytical residual- vs. impact-velocity curves at low temperature (− 50 °C) for non-conditioned specimens. The critical strain for the S-2 glass fibre was set equal to 3.43% and for the carbon fibre was set equal to 1.5%

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Fig. 7
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Experimental data and analytical residual- vs. impact-velocity curves at high temperature (50 °C) for non-conditioned specimens. On the left curve, the glass (solid line) and carbon (dashed line) analytical curves are identical. The critical strain for the S-2 glass fibre was set equal to 3.02% and for the carbon fibre was set equal to 1.42%

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Fig. 8
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Experimental data and analytical residual- vs. impact-velocity curves at room temperature for conditioned specimens. The critical strain for the S-2 glass fibre was set equal to 3.55% and for the carbon fibre was set equal to 1.58%

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Fig. 9
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Experimental data and analytical residual- vs. impact-velocity curves at low temperature (− 50 °C) for conditioned specimens. The critical strain for the S-2 glass fibre was set equal to 3.43% and for the carbon fibre was set equal to 1.5%

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Fig. 10
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Experimental data and analytical residual- vs. impact-velocity curves at high temperature (50 °C) for conditioned specimens. The critical strain for the S-2 glass fibre was set equal to 3.35% and for the carbon fibre was set equal to 1.52%

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Fig. 11
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Graphical representation of the ballistic limit V50 for non-conditioned specimens (left bar diagram) and for conditioned specimens (right bar diagram)

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The adverse effect of high temperatures on the strength of fibres and matrix is widely known. For example, glass fibre loses strength above 200 °C and carbon fibre above 500 °C (Fitzer 1988). However, the properties of polymeric matrix degrade from lower temperatures, i.e. 125 °C in the case of vinyl ester (Stone et al. 2000). In this study, because the maximum testing temperature 50 °C is much lower than these “critical” temperatures, the mechanical-performance degradation is almost negligible.

Usually, the effect of low temperature is connected with an increase of the strength and stiffness of the matrix and fibres, and with the appearance of compressive stresses in the matrix around the fibres due to the matrix–fibre coefficient expansion mismatch (Cormier and Joncas 2010; Schutz 1998). It is important to mention that the improvement of glass and carbon fibres is just significant at cryogenic temperatures (− 196 °C) (Schutz 1998). Nevertheless, the main conclusion that can be obtained from literature is that the mechanical performance of the composite at low temperatures is dependent on the nature of the constituents and on the stress configuration (Cormier 2017).

On the one hand, the tests performed in this work show an improvement of the efficiency of the non-conditioned S2, carbon and mate-glass samples (see left chart of Fig. 11) at − 50 °C, as expected, but at the same time, quite similar to the results obtained at room temperature. The reason of that behavior (the performance at −50ºC is just slightly better than at room temperature) is twofold: (i) the temperature is not low enough to improve the strength of the fibers and (ii) the matrix, which may improve its performance at −50 ºC, does not play an important role in the performance of the composite under high-speed impacts (Paradela and Sánchez-Gálvez 2013).

On the other hand, the low temperature on the conditioned specimens decreases the ballistic limit with respect to both: the samples tested at room temperature and the low-temperature non-conditioned values. The expansion of the seawater inside the samples at − 50 °C may lead to internal stresses in the sample that lessen the interface between the matrix and the fibres, although it seems that the effect of seawater saturation does not affect negatively to the performance of the composites studied at room and high temperature.

Moreover, few years ago, the authors developed an analytical model able to simulate high-speed impact response and failure of thin fibre-reinforced laminates. The model was recently updated to simulate impact onto hybrid-reinforced polymers (Paradela and Sánchez-Gálvez 2013; Sánchez-Gálvez et al. 2014, 2017). This model is fed up with the next laminate properties: number of layers, Young’s modulus of the fibre, distance between cross-overs, volumetric density of the target, yarn critical strain and sound velocity of the material. One of the strengths of these analytical models is that all these properties are obtained from measurements before testing or from suppliers’ technical sheet.

The model has been used to simulate the penetration process into the materials studied with the aim of determining its capacity to predict the high-speed impact performance of navy-relevant composites. All configurations were analysed except for mate glass because it does not have a “traditional” structure (woven fabric + resin) but a more complex one (see Fig. 2) and therefore, some assumptions of the model are not fulfilled.

In this work, we had the yarn critical strain at room temperature, and as it can be seen in Fig. 5, the model is able to predict with accuracy the residual-velocity curve. However, the mechanical properties of the fibers at +50 ºC and −50 ºC were unknown. On the basis of previous work (Sánchez-Gálvez et al. 2017) it was assumed that just the dependence of the critical strain of the fibers with the temperature is needed to simulate the performance of the structure and its value were estimated by fitting the analytical and experimental results at that corresponding temperatures.

As it was mentioned before, the complex configuration of mate glass involves a great difficulty to be easily modelled by a simple analytical model. Therefore, high-speed impact simulations of mate glass were performed by the commercial hydrocode LS DYNA. Each layer of woven fabric was modelled with shell elements, while matrix layers were modelled with solid elements. Figure 12 summarises both experimental and numerical results. Due to the abovementioned difficulties associated with this material, only the tests at room temperature were addressed by these numerical simulations.

Fig. 12
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Impact (Vi) vs residual (Vr) velocity curve of mate glass for non-conditioned samples and room temperature (25 °C)

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4 Conclusions

Concluding that there are no significant differences from one material to another neither for different temperatures nor for conditioned materials was allowed by the experimental programme. Even though the behaviour of the samples is not too much different, it can be pointed out that the worst performance is that of mate glass, while the best behaviour is that of the non-mixed hybrid glass/carbon (glass fibres in the front face of the target and carbon fibres in the rear one).

Moreover, it was observed that saturation with seawater improved the high-speed performance of composites except for low temperatures. This could be a surprising result and a feasible explanation could be that seawater does not degrade the mechanical performance of the fibres significantly but increase the weight of the matrix because of the absorption, so the energy needed to move a certain area of matrix and break the fibres is higher.

With respect to the analytical modelling, it may be pointed out that the model is able to predict the behaviour of the different configurations satisfactorily. Therefore, the analytical model is a reliable tool for the prediction of the response of fibre-reinforced composites under high-speed impact.