A method for prediction of in-situ strength of timber beams based on higher order elastic coefficients

A number of investigators have used the velocity of an ultrasonic pulse to predict the in-situ strength of timber elements. However, the correlation of strength to pulse velocity remains weak and may not be acceptable for strength classification of timber. In spite of the limitations, pulse velocity methods remain the primary tools available for in-situ evaluation of timber elements. This paper explores an alternative in-situ method of predicting the strength of timber beams using a measurement of the higher order elastic modulus. The proposed inspection method is suitable for timber elements that can be loaded to a degree in which measurable material non-linearities occur. A practical application, under which such non-linearities are evident, is shown. A field application test of a timber railroad bridge has been performed that shows that the load of a standard diesel locomotive on the bridge results in measurable changes in the ultrasonic-pulse velocity. This operational level of loading is shown to result in a measurable change in the velocity of the ultrasonic pulse. Experiments were then performed under laboratory conditions to determine if the change in wave speed associated with the higher order modulus is a more accurate predictor of the strength of standard wood timbers compared to a pulse velocity measurement. The correlation between the strength of the wood beams and the ultrasonic pulse velocity in this work is consistent with other results. The proposed use of non-linear pulse velocity is shown to give similar correlation results to those obtained with pulse-velocity measurements, thus demonstrating an absence of a measurable performance improvement under the conditions shown.