Impact damage processes in composite sheet and sandwich honeycomb materials

doi:10.1016/j.ijimpeng.2005.02.005

International Journal of Impact Engineering

Volume 32, Issues 1–4, December 2005, Pages 130-154

https://doi.org/10.1016/j.ijimpeng.2005.02.005 Get rights and content

Abstract

The research interest of this study is the impact toughness of different lightweight sandwich panels and composite sheet materials. This is as to the degrees of damage inflicted on the contact surface, through-thickness and rear surface of the materials when subjected to different impacts. For this study, specimens were prepared from sheet moulding compound (SMC), glass mat thermoplastic (GMT) and honeycomb sandwich panels employing different skin and core materials. For all experiments, drop-weight impact facilities were used and for some experiments a servo-hydraulic impact machine was also employed. The reason for this study is that very much needed for the next generation of more advanced aircraft, marine-craft, road and rail vehicles are improved lightweight stiff materials that can absorb higher impact energy and retain a higher degree of structural integrity.

Introduction

Increasingly, lightweight composite and sandwich materials are used in structures and other products that have to withstand demanding working and environmental stresses. Often, this is when the risk of some impact damage is unavoidable. The need in these cases is to determine the impact damage threshold for different failure processes in the materials to be used. Of particular interest in this study are the threshold conditions and the extent of damage that can be inflicted on the contact surface, through thickness and rear surface of the materials by the impacts and also the extent the materials have been deprived of their structural integrity. Needed for some of these studies are data from experiments using both drop-weight and servo-hydraulic machines. The latter provides for recording impact damage stage by stage as the impactor deforms the specimen. Related drop-weight and controlled servo-hydraulic impact experiments are helpful in obtaining data revealing the development of damage throughout the impact event. This is for minor to full penetration damage of the different materials studied. Obtaining these sets of impact damage data, is helpful, for example, in matching and combining several types of material in one product to achieve the overall required integrity and structural performance.

The choice of materials for this study is for the following reasons. Glass-fibre composites, for example, are now increasingly used in many kinds of transport vehicles. An advantage of SMC and GMT materials for many applications is that the available sheets of these materials can, with ease and low cost, be formed into products of many different shapes. An advantage of honeycomb sandwich structures is that the materials have a very high stiffness to weight ratio. As the quality, toughness and cost competitiveness is improved, so these materials are gaining new markets. For some applications, impact toughness is particularly important, for example, when the materials are handled a great deal in the assembly of large structures and if the end product is likely to be subjected to impact damage. Not to be understated is that the safe and functional effectiveness of stressed structures can often depend on the retention of integrity of each of the different materials used in its manufacture.

There has been considerable research on the impact performance and damage development in carbon-fibre composite materials—see for example Refs. [1], [2], [3], [4], [5], [6], [7]. Corbett et al. [8] have reviewed comprehensively the impact loading of plates by projectiles with particular reference to damage induced in metallic plates. Many of the failure and damage processes identified are also evident in composite and laminate materials. Harrigan et al. [9] have studied crush processes in aluminium honeycomb showing the good energy absorbing properties of these lightweight materials. Langdon et al. [10] have shown that combining glass-fibre reinforced polypropylene composites and aluminium layers to produce fibre–metal laminates can give very high impact energy absorption ability. This ability to absorb impact energy is linked to the presence of many interfaces e.g. delamination in the composite, debonding between aluminium and composite as well as spalling and petalling of the aluminium. Such failure mechanisms are also observed in laminated glass and other composite structures [11], [12]. Herup and Palazotto [13] have studied the low velocity impact performance of laminates with a composite skin and Nomex honeycomb and found the c-scan technique most useful in observing development of damage. Mines et al. [14], Abrate [15] and Olsson [16] have independently compared for sandwich panels of both aluminium and Nomex honeycomb the impact conditions for upper skin failure, core crush, lower skin failure and perforation and have linked the different stages of failure to the force–time traces. The objective in this paper was to relate for SMC, GMT and a variety of honeycomb sandwich materials the types of damage observed to force–time–displacement traces. Both c-scan and optical techniques are employed to observe the development of damage.

Section snippets

Material specification

Table 1(a) characterises the constituents of sheet moulding compound (SMC) as well as a glass mat thermoplastic (GMT) material. The SMC sheet materials were cured at 140 °C with an applied pressure of 7 MPa for 2 min. The GMT sheet material is consolidated at 210 °C with an applied pressure of 17 MPa for 40 s.

Table 1(b) shows the constituents and the dimensions of the components for the sandwich structures, which have composite or metallic skins and aramid or metallic honeycomb cores. This selection

Results

Fig. 2 shows, for SMC material, a comparison between drop-weight impact experiments using a mass of 5 kg at an initial velocity of 4.4 m s⁻¹ and servo-hydraulic impact experiments at a constant velocity of 1 m s⁻¹. Force–time, force–displacement, displacement–time and energy–displacement data are shown in Fig. 2 for the drop-weight and the servo-hydraulic experiments.

Fig. 3 compares, for SMC, front face damage and rear face damage with the associated ultrasonic c-scan image for the drop-weight

Discussion

Table 2(a) compares the drop-weight results for SMC and GMT sheet materials as determined from force–time and displacement–time data. There is a good correlation between the peak load (F_peak), the displacement at peak load (x_peak), total energy absorbed (E_Total) and the in-plane modulus (E₁₁) for SMC and GMT materials. The in-plane tensile modulus ( $E_{11}$ ) and Poisson's ratio ( $ν_{12}$ ) for a uniaxial tensile test were obtained at a displacement rate of 2 mm min⁻¹ [17]. The SMC material has a fibre

Conclusions

SMC, GMT, Fibrelam and Hexlite (H220, H620 and H640) materials have been studied in the dimensions that they are frequently used in products. There is an increase in the wider use of these lightweight honeycomb sandwich materials and there is now considerable interest in the ability of these materials to survive impact damage. In many structures, both solid composites and sandwich honeycomb material are used in the same products and in some cases aluminium skins and honeycomb core are used.

Acknowledgements

The authors thank the Engineering and Physical Sciences Research Council (EPSRC) for providing studentships and HEXCEL composites for providing samples.

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Impact damage processes in composite sheet and sandwich honeycomb materials

Abstract

Introduction

Section snippets

Material specification

Results

Discussion

Conclusions

Acknowledgements

Int J Impact Eng

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Int J Impact Eng

Compos: Part B

Int J Impact Eng

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