Predicting bending strength of fire-retardant-treated plywood from screw-withdrawal tests
This report describes the development of a test method and predictive model to estimate the residual bending strength of fire-retardant-treated plywood roof sheathing from measurement of screw-withdrawal force. The preferred test methodology is described in detail. Models were developed to predict loss in mean and lower prediction bounds for plywood bending strength as a function of a screw-withdrawal force. Our analysis of fire-retardant-treated plywood from three different studies, each with various fire-retardant-treatment, processing, plywood thickness, and exposure temperature groupings, clearly indicated that different fire-retardant-treatments and plywood thicknesses could not be grouped into a single “universal” model. Nevertheless, some grouping was possible; parameter estimates for several grouped fire-retardant formulations and plywood thicknesses are reported for mean trends and lower prediction boundaries. Although the models were shown to acceptably predict plywood bending strength, additional work is needed to expand these models to address the effects of plywood quality, wood temperature, and moisture content at time of test.
Mechanical properties of fire-retardant-treated plywood after cyclic temperature exposure
Some fire-retardant-treated (FRT) plywood used as roof sheathing has demonstrated loss of strength in service conditions. Research at the Forest Products Laboratory is aimed toward identifying the failure mechanisms of FRT wood, the chemicals that contribute to wood degradation, the temperature levels at which degradation occurs, the influence of moisture content (MC), and the correlation between these factors and the rate of degradation. To date, these efforts have led to the development of constant temperature test regimes for both plywood and lumber (ASTM D 5516-1994, and D 5664-1995, respectively). The objective of the research addressed here is the determination of how well constant temperature cycles relate to actual cyclic in-service temperature fluctuations. In this study, a special grade of southern pine plywood was treated with two fire-retardant chemicals and subjected to a cyclic temperature variation from room temperature to 65°C at two targeted MC levels, 6 and 12 percent. Results indicate that modulus of elasticity (MOE) and modulus of rupture (MOR) were relatively unchanged. The MOR values indicated a slight negative trend at both targeted MC levels, although the MOR values for the 12 percent specimens were slightly less than the MOR values for the 6 percent specimens. The most affected strength property was work to maximum load. The treated material showed a greater tendency for degrade than did the untreated material. In comparison with data from a plywood study using 65°C and 12 percent MC at constant temperature exposure, cyclic exposure appears to be less severe than constant temperature exposure. However, uncertainty regarding the internal temperature of the plywood could account for the difference in severity.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME