Influence of slope of grain on the mechanical properties of tropical hardwoods and the consequences for grading
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ORIGINAL
Influence of slope of grain on the mechanical properties of tropical hardwoods and the consequences for grading Geert Ravenshorst1 · Wolfgang Gard1 · Jan Willem van de Kuilen1,2 Received: 18 November 2019 © The Author(s) 2020
Abstract The main parameter that influences the bending strength of timber from tropical hardwood is the slope of grain. Although, in the grading rules, a specific threshold value is given, in these hardwoods, the global slope of grain is very difficult to quantify by visual assessment. The slope of grain measured after testing gives a better indication, but still it can only poorly describe the Hankinson relations. By rewriting the Hankinson relations, the slope of grain can be determined from the bending strength test values and from the MOE test value, both in combination with the density values and constants derived by non-linear regression analysis. These two values correlate very well, and the average value is designated as the theoretical slope of grain. With the theoretical slope of grain, five test samples of the tropical wood species okan were evaluated, and slope of grain values of 0.3 were observed, where 0.1 is the limit value. Because all pieces passed the visual grading method as applied in practice, slope of grain values should be incorporated in the strength class assignment test program, when these qualities cannot be ruled out for coming on the market. The (dynamic) modulus of elasticity can be used to evaluate the occurrence of the desired range of slope of grain values in the test samples.
1 Introduction To assign a timber beam to a strength class, this beam has to be graded. With visual grading, the grader assesses the most important strength reducing characteristics and designates the visual grade of the beam. In a previously performed laboratory research, the relationship between the visual characteristics and the strength was determined and based on that the strength class connected to that specific visual grade was established. This is documented in grading reports and for a number of species, there is a European standard (EN 1912 2012) that provides information on strength classes that can be assigned to visual grades of a number of species from different growth areas. In Europe, there is a harmonised standard (EN 14081-1 2016) that provides guidance how strength assignments should be performed, to ensure that the same method is followed all over Europe. The harmonised standard is referring to the standard EN 384 (2016)
* Geert Ravenshorst [email protected] 1
Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands
Holzforschung München, TU München, Munich, Germany
2
that gives guidelines for sampling in connection with visual grading. This paper discusses the influence of the strength reducing characteristic slope of grain in combination with visual grading. EN 384 states how the characteristic value of, for instance, the bending strength can be determined based on the number of samples an
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