BEHAVIOUR OF INNOVATIVE FIBRE COMPOSITE SANDWICH PANELS UNDER

POINT LOADING

M.M. Islam 1, T. Aravinthan 1 and G. Van Erp 2 1 Centre of Excellence in Engineered Fibre Composites,

Faculty of Engineering and Surveying, University of Southern Queensland Toowoomba, QLD 4350, Australia. Email: islamm@usq.edu.au

2 LOC Composites Pty Ltd, Darling Heights, Toowoomba, QLD 4350, Australia.

ABSTRACT Sandwich composites are widely used in various industries such as aeronautical, mechanical, automotive, marine and others. However, most of the existing sandwiches are not suitable and cost effective for applications in civil infrastructure. An innovative fibre composite sandwich panel made of glass fibre reinforced skins and a high strength core material has been developed for building and other structural applications. The behaviour of this new generation sandwich panel has been studied to assist the technical persons and the construction crews in better understanding its properties for an efficient use. If a fibre composite sandwich panel is placed not in the main fibre direction, an adverse effect on its performance may occur. Also, the point load is critical because the load creates stress concentration, which can easily damage the sandwich panel. This paper discusses the initial experimental investigation on this innovative sandwich for its one and two-way spanning floor applications under point loading. Initial investigation has shown that the stiffness of the sandwich is affected by the fibre orientation of the sandwich skins. No much effect has been observed for variation of fixity between slab and joist. KEYWORDS Sandwich panels, one and two-way spanning, slab, joist, flooring system, core cracking load, experimental. INTRODUCTION Composite materials have been utilised in a variety of engineering fields such as marine, aeronautical and automotive industries (Corigliano et al. 2000; Davalos et al. 2001; Khan 2006). Composites have only just recently been utilised in civil engineering practices (Karlsson and Astrom 1997). The use of sandwich panels as a civil construction material has been overlooked to traditional materials such as concrete and steel. These traditional materials are relatively cheap and readily available. The advantages of sandwich panels over traditional building materials though are starting to become apparent (Karbhari 1997; Burgueno et al. 2001; Keller 2006). Sandwich panels are light weight, strong, water resistant and fire resistant making them a very viable alternative for civil construction (Reis and Rizkalla 2008; Van Erp and Rogers 2008). A major area where sandwich panels are beneficial is flooring systems. Due to their light weight and strength properties, the use of sandwich panels proves a much better alternative to traditional wooden or concrete flooring (Karbhari 1997). The reduced dead weight of the floor results in reduced overall load and hence smaller supporting members. An innovative fibre composite structural sandwich panel has recently been developed for various civil applications (Van Erp and Rogers 2008). This new generation sandwich panel has potentiality for applications in floors, bride decks, walls, roofs, etc. The behaviour of sandwich panels in flooring systems and one and two-way slabs have not yet been fully researched. This paper presents the preliminary experimental results on one and two-way spanning sandwich panels applying point loads and varying fibre orientation and panel fixity with the joists. The behaviour of one and two-way slabs under different loading conditions was investigated to provide more knowledge into the behaviour of sandwich panels as flooring systems.

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MATERIALS AND METHODOLOGY Test Specimens The fibre composite sandwich panels under study are made of a modified phenolic core material and bi-axial glass fibre reinforced skins with a plant based resin. In the sandwich skin (1.9 mm thick), the 0° fibres have a content of 400 gsm with the 90° fibres comprising of 300 gsm content. The core comprises a density of 850 kg/m3. The combined density of the overall sandwich panel is around 990 kg/m3 with the same panels being used for initial four-point bending test and the test with one and two-way spanning except the sandwich thickness. The prototype slabs were designed and constructed to replicate a one and two-way slab systems in a typical structure adopted from the Particleboard Structural Flooring Design Manual published by the Australian Wood Panels Association Incorporated. The design was based on a deck type system with a 15 mm thick sandwich panel of 950 mm x 950 mm with the timber joists underneath the panel. The joists were 45 mm x 145 mm hardwood timber and support the panel such that the centre to centre spacing of the joists was 450 mm for one-way slab and 900 mm centre to centre for the two-way slab. Figures 1 and 2 show the design layout of the slab specimens. A list of variables for specimen preparation is given in Table 1. The specimen preparation and test for small scale four-point bending were conducted for both 0° and 90° fibre orientations in accordance with the standard ASTM C393-00. Load-deflection behaviour will be discussed later.

Figure 1. One-way slab design (dimensions are in mm)

Figure 2. Two-way slab design (dimensions are in mm)

Table 1. List of variables for preparing slab specimens

Edge support type Slab fixity with joists

Main fibre orientation at 0°

Main fibre orientation at 90°

One-way slab Screw only S1 S2 One-way slab Screw and glue S3 S4 Two-way slab Screw and glue S5 S6

Test set-up and procedure The static testing of one and two-way spanning sandwich slabs was conducted applying a flexural point load onto an area of 100 mm x 100 mm at the centre (mid-point of each direction) of one of the spans of the slab. The set-up and instrumentation of the sandwich testing is shown in Figure 3. The load was applied to the specimen using a hydraulic jack and measured through a load cell. The load was applied to the slab until the failure occurs and the deflection was measured on both spans through a data logging system.

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(a) One-way slab testing (b) Two-way slab testing

Figure 3. Instrumentation and set-up for one and two-way spanning fibre composite sandwich slab testing RESULTS AND DISCUSSION Failure mode The experimental investigation showed that the sandwich panels exhibited almost a common failure mechanism for all the slab specimens regardless of their skin fibre orientations, fixity between panels and joists and their edge support (one and two-way spanning). Figure 4 shows the typical failure phenomena occurred during the test. In general, a core cracking was observed first near the loading point of the specimen at the applied load of around 18 to 23 kN. However, the nearest (outer) edge of the sandwich from the loading point started to separate from the joist before the first crack occurred in the sandwich core, due to wrinkling of the panel at the vicinity of the loading point (Figure 4a and 4b). The delamination between sandwich core and skins was also easily noticed at the edge of the wrinkled part (Figure 4a). The separation of the sandwich from the joist became extensive with the continuation of applying the load (Figure 4c). Some simultaneous cracking and skin delamination were observed before the final failure of the specimen. The core cracking was not found to be necessarily diagonal from the corners for all the specimens although some diagonal cracking were also observed.

(a) Separation of the sandwich (screw only) (b) Separation of the sandwich (screw and glue)

(c) Extensive separation before failure (screw and glue) (d) Final failure of the specimen

Figure 4. Failure phenomena of the sandwich panels

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Load-deflection behaviour The relationship between load and load point deflection for different test variables obtained from one and two-way spanning sandwich slab testing is shown in Figures 5, 7 and 8; while a summarised test result on load and deflection is listed in Table 2. As per the Particleboard Structural Flooring Design Manual, a 2.1 kN load was considered as service load and the deflection was recorded during the test of each specimen. The deflections at the point of loading at 2.1 kN were found to be ranging from 1.81 to 2.38 mm depending on the test variables, whereas the allowable deflection as per design criteria of the sandwich for flooring application was ‘Span/360’, i.e. 450/360 or 1.25 mm, that shows the experimental deflection exceeded the allowable deflection at service condition. This may be due to the difference in structure for testing and that for original floor application with continuous spanning. It should be noted that no damage was observed in the slabs at 2.1 kN load. In general, the maximum deflection (at midspan) of adjacent unloaded span was less than 10% of that of the loaded span. The core cracking load ranged from 17.84 to 22.88 kN for all the specimens tested depending on the test variables, whereas the deflection of point of loading at core cracking load ranged from 18.78 to 22.57 mm. Moreover, a linear relationship was found. The final failure of the slabs generally occurred at a load of 32 to 50 kN.

Table 2. Test results on load and deflection for all the specimens

Edge support

type

Slab fixity with joists

Main fibre

orient-tation

Speci-men

number

Deflection at point of loading at

2.1 kN (mm)

Unloaded span’s

midspan deflection at 2.1 kN (mm)

Core cracking

load (kN)

Deflection at point of loading at

core cracking

load (mm)

Unloaded span’s

midspan deflection

at core cracking

load (mm) One-way

slab Screw only

0° S1 2.38 - 0.06 22.88 22.57 - 2.86 90° S2 2.31 - 0.13 18.72 20.70 - 1.92

One-way slab

Screw and glue

0° S3 1.96 - 0.05 20.61 19.19 - 1.67 90° S4 2.19 - 0.40 21.05 22.56 - 2.05

Two-way slab

Screw and glue

0° S5 1.81 - 0.04 22.44 21.36 - 1.30 90° S6 2.34 - 0.26 17.84 18.78 - 1.19

Effect of variation of fibre orientation The sandwich slabs of both one and two-way spanning systems were tested in 0° and 90° main fibre directions of the sandwich skins. The load-deflection relationships for variation of fibre orientation under each unique form of slab edge support and fixity of joists are shown in Figure 5. The figure indicates that the stiffness of the slabs with 0° fibre orientation is comparatively higher than 90° fibre orientation, as the fibre fraction for 0° direction is relatively higher. The core cracking load for 0° fibre orientation is also higher, except for one-way slab system fixed with joist by screw and glue (Figure 5a), where the core cracking load and deflection are higher for 90° fibre orientation. This result has similarity with that obtained from small scale 4-point bending test as shown in Figure 6. For 90° fibre orientation, as the bending stiffness of sandwich is less, the deflection is comparatively more. In general, a linear behaviour of load-deflection was observed until the first core cracking for both fibre orientations.

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)

1-way, 0-degree, screw only1-way, 90-degree, screw only1-way, 0-degree, screw and glue1-way, 90-degree, screw and glue

0

10

20

30

40

50

60

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)

2-way, 0-degree, screw and glue2-way, 90-degree, screw and glue

(a) S1 & S3 (0°); and S2 & S4 (90°) (b) S5 (0°) and S6 (90°)

Figure 5. Load-deflection relationships for different skin fibre orientation

Core cracking

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0

1000

2000

3000

4000

5000

6000

7000

0 5 10 15 20Deflection (mm)

Load

(N)

4-point bending, 90-degree4-point bending, 0-degree

Figure 6. Load-deflection (at load points) relationships of sandwich specimens for different fibre orientation obtained from 4-point bending tests

Effect of variation of fixity between slab and joist The one-way spanning sandwich panels were tested by varying the fixity between panel and joist. The variable was fixing the slab with screw only or screw and glue to the joist. The load-deflection relationships for this variation with 0° and 90° fibre directions are shown in Figure 7. The figure indicates that there is no noticeable effect of variation on stiffness and load capacity of the slabs for adding the glue with screw. However, the screw and glue fixity option increased the core cracking load approximately by 12% for 90° fibre orientation (Figure 7b).

0

10

20

30

40

50

60

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)

1-way, 0-degree, screw only1-way, 0-degree, screw and glue

0

10

20

30

40

50

60

0 10 20 30 40 50Deflection (mm)

Load

(kN

)

1-way, 90-degree, screw only1-way, 90-degree, screw and glue

(a) S1 and S3 (b) S2 and S4

Figure 7. Load-deflection relationships for different fixity between slab and joist

Effect of slab edge support as one and two-way The sandwich composite slabs with 0° and 90° main fibre directions were tested under both one and two-way spanning systems. It should be noted that the slabs were fixed with the joists with screw and glue in this case. The load-deflection behaviour for this variation of slab edge support with both fibre orientations are shown in Figure 8. The figure indicates that there is no change in slab stiffness for the change from one-way to two-way slab systems. This may be due to slab properties relevant to its aspect ratio. That means a change in properties may be noticed in the case of lower aspect ratio. However, an increase in core cracking load and deflection was observed for two-way slab in the case of 0° fibre orientation (Figure 8a), whereas a decrease in those was observed for 90° fibre orientation (Figure 8b). In general, a linear load-deflection relationship was observed until the core cracking load for both one and two-way slab systems.

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0

10

20

30

40

50

60

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)1-way, 0-degree, screw and glue2-way, 0-degree, screw and glue

0

10

20

30

40

50

60

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)

1-way, 90-degree, screw and glue2-way, 90-degree, screw and glue

(a) S3 and S5 (b) S4 and S6

Figure 8. Load-deflection relationships for different way of edge support (one and two-way)

CONCLUSIONS The behaviour of the innovative structural fibre composite sandwich panels was investigated experimentally by developing prototype one and two-way spanning slab systems. Various test variables were considered to determine effects on the slab load-deflection properties due to the variation of the sandwich skin fibre orientation, the fixity between slab and joist and the slab edge support as one and two-way systems. Experimental results suggest that using the same type of fixity between slab and joist, the sandwich slabs in 0° fibre orientation have higher stiffness than in 90° fibre orientation. There is no noticeable change in stiffness for variation of the fixity of slab with joist and the slab edge support as one and two-way systems. However, the core cracking load usually does not show a unique trend for a test variable. In general, a linear relationship between load and deflection was observed until the first core cracking. The sandwich core failed first during loading the specimens. Currently, research is being conducted to determine the sandwich behaviour both experimentally and numerically for floor application considering both point load and uniformly distributed load with an objective to provide some useful recommendations for a simplified design tool for the new generation structural sandwich panels. ACKNOWLEDGMENTS The first author gratefully acknowledges the financial support provided by University of Southern Queensland through an Early Career Researcher Program (ECRP) Grant for 2008-09. The authors thankfully acknowledge the support of Mr Matthew Plain and the staff of LOC Composites Pty Ltd in the preparation of test specimens and the support of Mr Wayne Crowell and Mr Atul Sakhiya in testing the sandwich panels. REFERENCES Burgueno, R., Karbhari, V.M., Seible, F. and Kolozs, R.T. (2001). “Experimental dynamic characterization of an

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