The non-destructive testing (NDT) of timber using the longitudinal vibration method is based on the natural frequency of wood which is in relation to its quality. In the present paper, the suitability of this tool is investigated and the results of grading 395 pieces are presented. Structural timber of Radiata pine ( Pinus radiata D. Don.), Scots pine ( Pinus sylvestris L.), and Laricio pine [ Pinus nigra ssp. salzmannii (Dunal) Franco] from Spanish sources were investigated. The specimens were tested for bending according to the European standard EN 408 (2003) and the values of strength and stiffness were compared with the results estimated by means of NDT. The vibration equipment applied permits the measurement of the longitudinal natural frequency and mass of the specimen, and then the density and the dynamic modulus of elasticity can be calculated. There is a strong relationship between the static modulus of elasticity obtained from the bending test and the dynamic modulus of elasticity obtained by the NDT technique. There is an acceptable relationship between modulus of rupture and dynamic modulus of elasticity if the visual defects (knot sizes) are taken into account. Acoustic measurements have become widely acceptable, and they have great potential for stress grading of coniferous timber.

The use of wood in construction requires knowledge of the mechanical properties and the particularities that wood presents in comparison with other materials used for structural purposes such as steel, concrete, brick, or stone. The introduction mentions the environmental advantages that justify the use of wood today. The orthotropy of wood is one of the differentiating characteristics that must be taken into account when studying its behaviour. The determination of the properties of wood is then addressed from a historical perspective and the differentiation is made between the properties of small clear wood (defect-free timber) and structural timber. The timber grading systems (visual and mechanical grading) and the non-destructive techniques that currently prevail are explained. Finally, the factors that influence the mechanical properties, such as duration of the load, moisture content, quality, temperature, and the effect of size are explained. The objective of this work is to provide an overview of the current knowledge on the mechanical properties of wood, based mainly on published articles and European and North American standards, including historical references to the beginnings and current trends in this field.

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The aim of this research is to evaluate the influence of the moisture content of wood on stress wave velocity, as a nondestructive technique for estimating the mechanical properties of gross cross-section Spanish Scots pine (Pinus sylvestris) lumber for structural use. 26 100 x 150 x 3000 mm pieces from Segovia, Spain, were tested with different moisture content values, from 35,5 to 9%. Measurements of longitudinal stress wave velocity were carried out during the natural drying process by using three commercial portable devices based on ultrasonic, acoustic and vibrational techniques. The results can be summarized as three percentage points of decreasing velocity per percentage point of increased wood moisture content in the range from 11,8 to 21,4%: 0,48% for ultrasonic, 0,50% for acoustic wave and 0,65% for longitudinal vibration.

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RESUMENSe han realizado mediciones detalladas de las fendas de secado en 28 piezas de madera de pino silvestre de gran escuadría. Se han propuesto diferentes parámetros para medir su magnitud en longitud, profundidad, grosor e inclinación, así como un índice de áreas en el plano medio de rasante (superficie eficaz respecto a superficie total). Se han relacionado los parámetros de fendas con resistencia a flexión, resistencia a cortante, módulo de elasticidad y energía de rotura obtenidos mediante ensayos mecánicos, teniendo en cuenta si la rotura se produce por flexión o por cortante. Todas las fendas observadas permiten clasificar las piezas como MEG según la norma UNE 56544. Se propone cuantificar las fendas mediante el índice de áreas y las propiedades mecánicas mediante la energía de rotura. El análisis realizado no muestra una relación significativa entre las fendas analizadas, los modos de rotura o las propiedades mecánicas.Palabras clave: madera, propiedades mecánicas, caracterización, fendas de secado. INTRODUCCIÓNLa calidad de la madera para uso estructural se evalúa en función de la cantidad y tamaño de los defectos (nudos, fendas, desviación de la fibra, gemas, etc.) de SUMMARY Detailed measurements of drying fissures were carried out on 28 pieces of gross cross-section structural Scots pine timber. Different parameters have been proposed to measure their magnitude: length, depth, thickness and slope of grain, as well as an index of area in the neutral axis plane (ratio of effective to total surface). The parameters of fissures have been related to bending strength, shear strength, modulus of elasticity and rupture energy obtained from mechanical tests, considering whether rupture is due to bending or shear. All of the fissures observed allows to classify the pieces as MEG according to the UNE 56544 standard. To quantify the magnitude of fissures the index of areas is proposed, while mechanical properties can be quantified by rupture energy. Analysis of the results does not show any relation between the fissures analyzed and the mode of failure, their magnitude and mechanical properties.