The hardness of wood. (1980)
AuthorsDoyle, Johnshow all
This investigation covers the development of hardness testing in general, including reference to the considerable work done in this field with metals. Hardness testing of wood is reviewed from the early 19th century to its culmination in the development of a number of standard methods now in use for testing hardness. The effects of factors such as density, structure and environmental conditions are considered with reference to the literature. Wood is tested on three faces using wedges and the Janka test tool to determine the influence of sample orientation on hardness results. The effect of loading rate is investigated and comment is made on loading rates and penetration depths used in the different methods. Investigations into the hardness properties of rigid cellular polyurethane foams are also reported. Evidence is submitted showing that woods should be considered as a range of materials - low density cellular and almost rigid-solid at the two extremes. Correlations for Wedge Hardness with other strength properties and density of timber in the air dry state are given for New Zealand timbers and some overseas timbers covering a density range from 140 to 1270 kg/m³. The Wedge test is shown to be capable of good prediction of many properties and is closely dependent on density. Correlations with green timbers were less good, but still show general agreement with trends in other strength properties. Results indicate an application for the use of sharp wedges as an alternative method of determining cleavage strength parallel to the grain. Tests on particle board and fibreboard are not encouraging. A 136° wedge appears a useful guide to hardness on the board face, especially for use with thinner board, but does not reliably detect density gradient across the edge of the board.