New ultra lightweight and extreme stiff sandwich composite structure for multiple applications.

Teodorescu-Draghicescu, Horatiu ; Vlase, Sorin ; Motoc Luca, Dana 等

1. INTRODUCTION

Sandwich composite structures have been used increasingly in applications in aeronautics, transportations, in automotive industry, in machine-tools construction, robotics, etc., where high rigidity parts are needed. In most cases, these structures exhibit failures consisting in the connection detachment between core and skin due to impact applied loads. The main objective of the authors' researches is to obtain a sandwich composite structure that can replace the classic stiffening solution with ribs of a panel with imposed weight and dimensions.

2. CRITICAL OVERVIEW

There are a lot of improvements in the construction of sandwich composites especially to improve their impact resistance. For instance, foam filled 3D integrated core sandwich composite laminates with and without additional face sheets have been manufactured using vacuum assisted resin infusion moulding process in multiple steps (Hosur et al., 2007). In order to handle large LCD panels, robot structures have been produced using carbon fibers/epoxy composite material with a polyurethane foam core to increase the stiffness of the wrist blocks (Lee et al., 2002). Developments of sandwich structures have been made reinforcing cores by way of three-dimensional Z-pins embedded into foam, honeycomb cells filled with foam and hollow/space accessible Z-pins acting as core reinforcement (Vaidya et al., 2001). Tooling and manufacturing methods, designing, stress, loads and load testing, vacuum bagging, autoclaves, etc., are presented in three basic references (Marshall, 2006; Smith, 2005; Wiedemann, 1989). Materials described are fibers and fabrics based on E-glass, S-glass, pan based carbon, pitch based carbon, and several Kevlars.

3. THE SANDWICH STRUCTURE

The sandwich composite structure taken into account presents two skins based on epoxy resin reinforced with a 0.3 kg/m2 twill weave fabric and an expanded polystyrene (EPS) 9 mm thick core with a density of 30 kg/[m.sup.3]. The final thickness of the structure is 10 mm (fig. 1). The carbon fibers fabric used in this structure is a high stiffness one, that presents so called twill weave. The main feature of this weave is that the warp and the weft threads are crossed in a programmed order and frequency, to obtain a flat appearance. In order to accomplish a mechanical analysis, an equivalence model of the twill weave fabric is presented in fig. 2. The skins have been impregnated under vacuum with epoxy resin and glued to the core. Data regarding the sandwich architecture are presented in table 1.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The core stiffness's are (Wiedemann, 1986; Jones, 1999):

[r.sub.cole 11] = [r.sub.core 22] = [E.sub.core]/ 1 - [[upsilon].sup.2.sub.core] (1)

[r.sub.cole 12] = [E.sub.core] x [[upsilon].sub.core]/1 - [[upsilon].sup.2.sub.core]; [r.sub.core 33] = [G.sub.core] (2)

Then, sandwich structure stiffness's can be computed:

[[r.bar].sub.ij] = [N.summation over (K = 1)]([r.sub.ijK] x [t'.sub.K]/[t.sub.skin]) + [r.sub.core ij] x h/[t.sub.s]. (3)

Sandwich structure compliances are:

[[c.bar].sub.ij] = 1/[[r.bar].sub.ij]. (4)

4. RESULTS

A comparison between sandwich structure's stiffness's with carbon- and glass fibers skins plies as well as bending strengths are presented in figs. 3 and 4.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

5. CONCLUSIONS AND FURTHER RESEARCH

The sandwich structure with two carbon/epoxy skins reinforced with a 0.3 kg/[m.sup.2] twill weave fabric and an expanded polystyrene (EPS) 9 mm thick core with a density of 30 kg/[m.sup.3] fulfils following special requirements: panel dimensions 10 x 2350 x 4070 mm and overall weight, maximum 10 kg. This sandwich structure is currently used to sustain large solar cells panels but applications can be in the aeronautic and automotive fields, transportations, naval industry and military domain. Sandwich structure's strains with skins based on twill weave carbon fabric reinforced epoxy resin are comparable with those of the structure with skins based on EWR-300 glass fabric/epoxy resin. Stresses in fibers direction in case of the sandwich structure with carbon fabric/epoxy resin reinforced skins, are up to six times higher than those existent in EWR-300 glass fabric/epoxy resin skins. Stresses transverse to the fibers direction in case of the sandwich structure with carbon fabric/epoxy resin reinforced skins are 20% lower than those existent in EWR-300 glass fabric/epoxy resin skins.

The shear stresses in carbon fabric/epoxy resin reinforced skins plies are almost identical with those existent in EWR-300 glass fabric/epoxy resin skins plies. Both sandwich structures core stresses can be neglected, the loading is taken-over exclusively by the two structures skins. Using a 9 mm thick expanded polystyrene core (EPS), according to fig. 3, the stiffness of the sandwich structure with carbon fibers reinforced epoxy resin skins is more than ten times higher than the skins plies stiffness, which is an outstanding achievement.

Further researches will be accomplished in the following directions:

* Determination of the influence of the adhesive type on the sandwich structure stiffness and bending strength.

* Determination of extensions field at bending using optical methods.

* Structure damping analysis computing the most important damping features starting from the dampings, dynamic Young moduli and Poisson ratio for every lamina using the so called correspondence principle of linear viscoelasticity theory.

* Structure sound damping analysis using various sound pressure waves.

6. REFERENCES

Chang Sup Lee; Dai Gil Lee; Je Hoon Oh & Hyun Surk Kim (2002). Composite wrist blocks for double arm type robots for handling large LCD glass panels. Composite Structures, Vol. 57, Issues 1-4, pp. 345-355, ISSN 0263-8223

Hosur, M.V.; Abdullah, M. & Jeelani, S. (2007). Dynamic compression behavior of integrated core sandwich composites. Materials Science and Engineering:A, Vol. 445-446, pp. 54-64, ISSN 0921-5093

Jones, R.M. (1999). Mechanics of Composite Materials, Taylor & Francis, Inc., ISBN 1-56032-712-X, Philadelphia

Marshall, A.C. (2006). Composite Basics, Aircraft Technical Book Co., ISBN 978-0966454048, Tabernash, Colorado

Smith, Z. (2005). Advanced Composite Techniques, Aeronaut Press, ISBN 978-0964282841, Napa, California

Vaidya, U.K.; Nelson, S.; Sinn, B. & Mathew, B. (2001). Processing and high strain rate impact response of multi-functional sandwich composites. Composite Structures, Vol. 52, Issues 3-4, pp. 429-440, ISSN 0263-8223

Wiedemann, J. (1986). Leichtbau. Band 1: Elemente, (Lightweight constructions. Vol. 1: Elements), Springer, ISBN 3-540-16404-9, Berlin, Heidelberg, New York, Tokyo

Wiedemann, J. (1989). Leichtbau. Band 2: Konstruktion, (Lightweight constructions. Vol. 2: Construction), Springer, ISBN 3-540-50027-8, Berlin, Heidelberg, New York

Tab. 1. Sandwich architecture

Structure thickness [t.sub.s] = 10 mm
Thickness of each ply [t'.sub.1-4] = 0.175 mm
Skins thickness [t.sub.skin] = 0.35 mm
Core thickness h = 9 mm
Plies fibers [alpha].sub.1, 3] =
 disposal angle 90[degrees],
 [alpha].sub.2,4] =
 0[degrees]
Plies fibers [[phi].sub.1...4] = 56%
 volume fraction