The correlation between tensile and compressive strength of wood is weak because of different failure phenomena in tension and compression. Therefore, it is desirable to have information about tensile strength. The following technique can be used to extract specimens of small cross sections from the timbers such that the cross-sectional area of specimens is significantly smaller than the area of the members. This is achieved by a small-diameter thin kerf saw inclined 45˚with respect to the surface of the beam (Figure 9). This means that two cuts are required to obtain a prismatic specimen of a triangular cross-section. The side of the triangle can be adjusted from 3-8-mm in length depending on the depth of the cut. The typical test specimen is shown in Figure 10. The ends of specimens are attached with the epoxy adhesive to grooved wooden blocks so that potential end effect due clamping is minimized. Tensile tests are performed in standard testing equipment using a special set of grips (Figure 11). A displacement transducer is used to measure deformation so that the modulus of elasticity in tension along fibers can be obtained. A typical stress-strain curve for a specimen is shown in Figure 12. The cross-sectional area of the specimens is comparable toFigure 9 Fixture to extract prismatic specimens from large timbers
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NSF/RILEM Workshop In-Situ Evaluation of Historic Wood and Masonry Structures (July 10-14, 2006 – Prague, Czech Republic) 63Figure 10 Tension micro specimen mounted on the test blocks Section narrowedGlued blockFigure 11 Testing of the tensile micro specimen. The central portion defining the gauge length is machined to a smaller cross section.the cross-sectional area of the ASTM tension specimen (about 8 mm²) that is required for small-clear specimens of wood . This means that the values obtained from this test are directly comparable with the standard tests, and no correlation is needed. To induce failure in the central portion of the triangular specimen, it is recommended to reduce the cross-section of the mid-span. Such reduction can be done by sanding, but caution must be exercised to maintain a smooth plane to avoid variation on cross-sections. Experiments were performed in the laboratory with several different species that included Red Oak, Western Cedar, Hard Maple, and Yellow Pine. The goal of the test was to observe failure modes and investigate the feasibility of the technique in strength evaluation of in-situ timbers. Figure 12 shows typical test results. The modulus of elasticity in tension and tensile strength can be easily obtained. One of the drawbacks of the above methods is the local character of measured quantities and the effect of position on the experimental data.
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