Load-duration behavior of extruded wood-plastic composites

Christopher Wayne Brandt

One of the important characteristics affecting many materials including wood, plastic, and wood-plastic composites is their time-dependent behavior. This behavior can be divided into two separate, yet related, phenomena known as creep and creep-rupture. Creep is the increase in deformation over time while subjected to a sustained load. Creep-rupture, or load-duration behavior, is the eventual failure to sustain a constant load due to increased deformation over time. Both have a considerable effect on wood-plastic composite members and must be accounted for in design. The load-duration, or creeprupture, behavior of wood and traditional wood composite products has been studied extensively by numerous researchers, and simplified design procedures to account for both effects are in place. However, the load-duration behavior of composite products made out of the combination of wood and plastic is not well understood or documented. Currently, wood-plastic composite products are available on the commercial market, yet no standardized design procedures to account for creep or duration-of-load effects exist. Given the lack of understanding of the load-duration behavior of wood-plastic composites, a research effort was initiated to investigate the load-duration behavior of various wood-plastic composite formulations and to compare the observed behavior to that of solid sawn lumber. An exponential damage rate model commonly used for solid wood was found to similarly describe the load-duration effect for wood-plastic composites. Proposed adjustment factors were developed following existing procedures for wood products. It was also found that the current adjustment factors, as found in the 1997 edition of the National Design Specification for Wood Construction, can be conservatively applied to wood-plastic composite products, although the adjustment is overly conservative for the materials tested. However, applying adjustment factors based on a short-duration loading was found to be non-conservative. Rate-of-load effects on flexural properties were also experimentally investigated. It was found that a rate-of-load effect existed for both modulus of rupture and modulus of elasticity over certain ranges of load-rate values. A rate-of-loading corresponding to 0.01 mm/mm/min was found to produce representative material properties that were slightly conservative

Load-duration behavior of extruded wood-plastic composites

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