Improved fatigue performance for wood-based structural panels using slot and tab construction
Introduction
Structural composites have been widely used in many applications such as shipping, aerospace, transportation, and building construction due to its high strength-to-weight ratio [1], [2], [3], [4]. Various materials, designs, and manufacturing, methods for structural composites attempting to improve performance have been developed and investigated in recent years. Isogrid core designs made from aluminum to fabricate structural composites for aerospace applications have shown that the isogrid configurations were more structurally efficiency than either foam or honeycomb structures [5], [6]. The simple manufacturing process using interlocked grid structures with improved mechanical performance by unidirectional pultruded glass fiber rib was presented by Han and Tsai [7]. It was easier and more efficient than the traditional pultruded method. Fan et al. [8] fabricated interlocked kagome structural panels using carbon fiber composites and evaluated the mechanical behaviors using static compression and bending tests. The results indicated that debonding was one of the more significant failure modes for the mechanical tests. For fatigue tests, Belingardi et al. investigated the fatigue behavior of a sandwich beam using the four point bending test, two different failure mechanisms of face compression and core debonding were observed using both undamaged and initially damaged panels [9]. Jen et al. analyzed the effect of the amount of adhesive on the bending fatigue strength of bonded aluminum structural beams, the results showed the fatigue strength of structural beams was improved as the amount of adhesive increased [10]. Their research also demonstrated that the thickness of the face sheets showed no evidence of effect on the fatigue strength [11]. From the researches cited above, there was a common behavior observed that the core:face interface strength had a significant effect on the mechanical behavior of the structural composites, especially for fatigue.
Forest Products Laboratory (FPL) is working to develop engineered structural materials made from wood-fiber-based composites that have enhanced performance for some engineered applications such as air pallet, tactical shelter, transportation, or building construction materials [12], [13], [14], [15], [16], [17], [18], [19]. In an initial study, phenolic laminated paper was used for a tri-axial core configuration within a structural composite panel. The mechanical behavior for these tri-axial core panels were obtained using static bending and compression tests [12], [13], [14], [15], [16], [17]. The results showed these wood-based structural panels had good mechanical performance. However, debonding at the interface between the structural core and face caused premature failure during the mechanical tests. Failure was due to insufficient epoxy resin bond strength at the rib and face interface. For our configuration, the epoxy could not provide enough capacity to resist core:face interface shear failure. One method to strengthen the interface and avoid premature debonding failure was to develop a slot and tab (S/T) construction technique at the core:face interface. The purpose of slot and tab construction technique was to improve the load transfer between the core and faces utilizing both increased surface area and mechanical interlock load transfer. In this paper, the S/T construction was used to compare the static bending and fatigue bending behavior for the tri-axial core wood-based structural panels.
Section snippets
Materials properties
The main material used for both the core and face components to then fabricate the panels was NP610 phenolic impregnated laminated paper obtained from Norplex-Micarta Inc. (Postville, Iowa, USA). Its mechanical properties were obtained using in-house ASTM D638-10 [20] and D695-10 [21] standard coupon tests. Epoxy resin was used to bond the laminated paper faces and core components of the panels. It was obtained from U.S. Composites (West Palm Beach, Florida, USA). The ratio of epoxy to hardener
Configuration of wood-fiber-based structural panels
The structural composite panels were fabricated by the tri-axial core configuration using the laminate paper as linear ribs in each of three axes with an interlocked structure (Fig. 1). The core rib height was 33.0 mm. The slots in the linear ribs were cut slightly oversized to accommodate the 60° angular orientation between the ribs when assembled. For this study, the slot spacing for all pieces was 117.3 mm, thus creating an equilateral triangle. The thickness of the ribs was 2.4 mm. Two layer
Static bending testing
Four point bending test, ASTM C393 [23], was used to investigate the static bending behavior and the results used as reference for load control for the fatigue tests. Six panels were fabricated and statically tested to failure; three panels without S/T configuration were used to compare with the other three panels with the S/T configuration. The span of the simply supported panel was 914 mm (Fig. 2(a)). The width of the panel was 267 mm. A 25 mm LVDT (Linear Variable Differential Transformer) was
Static bending
Fig. 3 shows the typical relationship between load and mid-span deflection for the panels with and without S/T construction. The results showed that the panels with S/T construction have a similar load/deflection curve compared with the panels without S/T construction. The average failure load for the panels with S/T construction was 19.2 kN at 22.2 mm deflection at the mid-span. Similarly, panels without S/T construction had a maximum load of 20.1 kN at 24.9 mm deflection, Table 2. The estimated
Conclusions
In this work, wood-based structural panels made from phenolic impregnated laminated paper were designed and fabricated with and without S/T configuration. Comparisons were made based on the mechanical performances of the static bending and fatigue tests. Static test results showed there were similar strength, stiffness and failure mode for panels with and without S/T configuration. For the fatigue test results, the results showed that the panels with S/T configuration had more fatigue life
Acknowledgements
This work is supported by USDA Forest Service, Forest Products Laboratory and the authors gratefully acknowledge the support of Sara Fishwild, James Bridwell, Marshall Begel, Dave Simpson and Marc Joyal of EMRSL group for the mechanical testing.
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