Bending analyses for 3D engineered structural panels made from laminated paper and carbon fabric
Introduction
Sandwich panels are used for a variety of applications including those for building, transportation, furniture, decking, packaging, marine, and aerospace. Marine and aerospace applications have the most demanding performance requirements and use the highest strength materials. Many of these sandwich panels are fabricated using honeycomb construction for the core structure. However, over the past 15 years, researchers have shown a new interlocked composite grid arrangement that could offer improved performance for some applications [1], [2]. The interlocking grid uses linear ribs that are double-slotted for 1/3 the width or single-slotted for 2/3 the width of the rib. The double-notched ribs were used as the main rib orientation and the 2/3 notched ribs are inserted from either the top or bottom side to create a triangular core design (Fig. 1). According to Fan et al, this core configuration has been shown to be stiffer and stronger than foams and honeycombs [3]. The size of the equilateral triangle shown could be modified by adjusting the distance between slots or changing the design to an isosceles triangle by adjusting the distances between slots for the double-slotted rib, thus creating a core with performance options that can be engineered.
This study was initiated to investigate the performance characteristics for this core structure as it relates to wood-fiber-based composite materials. An integrated equivalent stiffness model was developed to describe the grid structure with consideration of multiple loads, multiple failure mechanisms, and design variations as outlined by Chen and Tsai [1] but using wood-based composite materials.
The Forest Products Laboratory (FPL) is working to develop 3-dimensional engineered structural panels (3DESPs) made from a significant portion of wood-based composite materials yet that have enhanced performance capabilities. For some applications, high-performance and water resistance are critical requirements but do not have the same high performance or weight requirements as marine or aerospace panels. It may be possible that a phenolic impregnated laminated paper might be sufficient to fill some niche applications at slightly reduced costs. The laminated paper was used to fabricate the tri-axial rib core components and the initial top and bottom faces. The lower cost laminated paper faces were also used to support a thinner yet higher MOE carbon-fabric material. The thin single layer of carbon fabric would have easily buckled across the widely spaced ribs during bending, but by laminating the paper laminate and carbon fabric together achieved a high stiffness panel structure.
This work is a preliminary experiment to evaluate the bending performance of 3DESP. For the experiment, the bending failure, deflection, and bending stress were evaluated using numerical and finite element analyses (FEA). Also, the 3DESP experimental and FEA results were compared with traditional I-beam theory.
Section snippets
I-beam equations
Conventional I-beam equations were used to estimate the quarter-point beam bending performance for the tri-axial core 3DESP. The face and rib tension/compression stress, shear stress, and deflection were determined using standard strength of materials for bending [4].
Finite element method for plane elements
ANSYS FEA software (ANSYS, Inc., Canonsburg, Pennsylvania, USA) [5], [6], was used to model the core and face using their eight-node quadrilateral shell element, Shell 99. The virtual work equation is given by the following
Material properties
The material properties used for this study were obtained from either the manufacturer or by material testing according to ASTM test methods D229, D638, and D695 [8], [9], [10]. The material properties for the individual components are provided in Table 1. Phenolic impregnated laminated paper NP610, Norplex-Micarta Inc. (Postville, Iowa, USA), was used for the core and faces. The laminated paper had orthogonal properties designated MD and CD. Carbon fiber fabric, a tri-axial woven material,
The experimental design and finite element models for bending test
The 3DESPs were constructed with the tri-axial core configuration using laminated paper for its linear ribs in each of the three axes (Fig. 1). The core or linear rib height was 33.0 mm. The slots in the core pieces were cut slightly oversized to account for the 60° angular orientation between parts when assembled. The distance between slots for all pieces was 117.3 mm, thus creating an equilateral triangle. The double-slotted linear ribs aligned with the MD of the laminated paper. The laminated
Panel failure mode
Fig. 4 shows two types of bending failure observed for the 3DESP. For the laminated paper panels, panels 1 and 2, both failures were on the compression side between the mid-span of the loading bars. Under test load but prior to failure, the compression face showed signs of out-of-plane displacement. It is unknown what failure mode initiated the brittle failure of the compression side face, but it could have been either compression buckling of the core ribs or out-of-plane buckling of the face.
Conclusions
The design of the 3DESP structure can be conservatively analyzed with I-beam equations, but FEA evaluation provides better analyses of the potential performance or potential failures as exhibited by the experimental tests. The effect of adding carbon-fiber fabric showed significant improvement in overall stiffness and shifting of buckling failure stresses from the faces between the load span to shear between loading and reaction points. By knowing where maximum stresses occur in the panel, it
Acknowledgment
This work is supported by USDA Forest Products Laboratory and Plan Projects of Introducing Advanced International Forestry Technology (2010-4-14).
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