Sunshine Coast University student Jake Bundred has recently completed a structural analysis on COEN Composite Wood as part of his PHD?
Here is an overview from Jake’s report:
In recent years wood plastic composites have enjoyed rising popularity in non-structural applications although they have been slow to expand their utility in the structural domain. Wood plastic composites possess many beneficial properties that make them ideal for structural uses that include; high durability, inherent resistance to the elements including termites, pleasing aesthetics and a low carbon footprint as they can be made from recycled plastics and fillers.
This project aims to increase the general understanding of wood plastic composites and to expand the body of knowledge surrounding their performance structurally. This testing focused on wood plastic composite members fabricated by COEN Composite Wood investigating primarily flexural, deflection and compressive performance. Using the results of this testing, the modulus of rupture, modulus of elasticity and compressive stress capacities have been calculated. These values form part of the fundamental structural data required to assess suitability for structural applications. Through increased availability of structural information it is envisaged that level of public awareness should improve and also facilitate wider implementation of the product.
Figure 1—Solid deck member undergoing testing in Universal Test Machine
Compressive testing – The compressive testing was conducted using the Pilot 4 Automatic machine with samples of 100x100x25 hollow section. The length of the samples was limited to 150mm to create squat members that would test the axial section capacity of the wood plastic composites and greatly reducing any type of buckling effects. The load application rate was set as 0.333 MPa per second and the total load applied vs time elapsed were recorded automatically. The trials followed the principles outlined in AS1012.9 for concrete cylinder axial testing and the compressive stress capacity was determined by calculating the load at failure divided by cross sectional area. As the samples displayed a close correlation in the test data only 2 samples were tested.
Flexural and deflection testings – A wide range of COEN Composite Wood samples were tested using the Universal Test Machine, and a custom fabricated bracket, to allow the modulus of rupture and modulus of elasticity to be determined. The standard test length was 600mm while the decking samples were tested at 400 and 500mm to replicate real world spans. The loading was applied at a constant deflection rate of 20mm per minute and the time to failure, crosshead deflection and total load were automatically recorded. For each profile only 2 samples were tested unless there was less than a 95% correlation in test results, in which case successive samples were tested until the correlation of results was achieved. The testing and analysis was conducted following the basic principles of AS 4266.5 and AS4266.1 as a dedicated Australian Standard does not exist for wood plastic composites. To determine the modulus of rupture the ratio of the moment at failure and the section modulus were used and the modulus of elasticity was determine by the gradient of the initial straight line section of the stress strain curve for each profile.
Figure 3—Compressive Test Results
Figure 4 – Stress/strain
Recommendations and Conclusion
The testing showed that COEN Composite Wood members all pos-sess sufficient material properties to be used structurally. Further testing is required to produce load tables. Engineering judgement should be used in the interim to determine an adequate safety factor for design. Wood plastic composites should only be used when the potential design risks have been deemed to be accepta-ble although this may result in an uneconomical design if the cost is only determined on a financial basis without considering the sustainability benefits. Long term creep and deflections, a lack of empirical data and a high variability in the composition of each member are some negative aspects that still need to be ad-dressed. Ongoing research and development and lessons gar-nered from ongoing usage of wood plastic composites should pave the way for a superior generation of products in the near fu-ture that are able to overcome these downsides.