In this study, an experimental campaign was conducted to characterize the length effect on the elasticity and tensile strength of clear spruce wood parallel to the grain direction. Four groups of specimens of different lengths, cut from the same log, were tested under the same conditions under longitudinal tensile loading. The cross-sectional area of the specimens was selected as being constant and sufficiently small to exclude the effect of variations of the properties in the transverse direction. Local deformations along the lengths of the specimens were recorded during the tests in order to characterize the spatial variability of the elastic modulus. A connection between the mesostructure of the clear wood and its local elastic modulus was observed. Statistics concerning the elastic modulus, strength and strain to failure and the effect of length change on these properties were extracted. The strength statistics were also used to examine the accuracy of the classical Weibull size effect law. The correlations between the strength, the elasticity and the density were obtained. The results show a variability of approximately 20% in the local elastic modulus. Also, the variability of the effective elastic modulus decreases with increasing length. The mean value of the strength has an upper bound when the length approaches zero, in contrast to the Weibull law, while its variability remains virtually unchanged for different lengths.

No abstract

In the current study, a random field-based size effect model has been proposed for the longitudinal tensile strength of clear timber. Since the failure mode is brittle, the problem is basically an extreme value problem of finding the distribution of minima of strength fields for specimens of different volumes. The stochastic response has been evaluated based on the Monte Carlo method along with the weakest link theory. Within the framework of the spectral representation method, the Weibull distribution has been considered as the marginal distribution to generate realizations of the strength as a 3D random field. The squared exponential autocorrelation function has been used for the description of spatial variability. The error resulting from this model, as compared to existing experimental data in the literature, is much lower than that of the classical Weibull law. The results show that when one of the specimen's dimensions decreases to less than 10 times the correlation length of the strength field, the size effect starts to deviate from the classical size effect law. Moreover, a simple analytical approximation, which includes the correlation length as length scale, has been presented that facilitates the application of the proposed model.

No abstract

No abstract