Dog-bone Samples may not Provide Direct Access to the Longitudinal Tensile Strength of Clear-wood

Abstract

Background: Testing standards prescribe dog-bone samples for the determination of clear-wood longitudinal tensile strength. However, the literature reports a high number of invalid tests due to the unexpected failure of the sample outside the gauge length. Motivation: The paper aims at understanding the reason for the premature failure of dog-bone samples and suggesting possible strategies for improving testing protocols. Methods: The paper starts with a comparative review of standards for different orthotropic materials. Thereafter, it analyzes the stress distribution in a clear-wood dog-bone sample using a recently proposed stress-recovery procedure and Finite Elements. Finally, the sample failure is considered applying Tsai-Wu and SIA criteria. Results: Comparative review highlights the controversy on the choice of the sample geometry. Both analytical and numerical results confirm the presence of shear and transversal stresses in necking regions, overlapping with axial stress greater (up to 2%) than the one in the gauge region. As a consequence, clear-wood dog-bone samples fail not due to a pure axial stress state in the gauge region (as expected), but due to complex stress state in necking region, where failure index is 4 ~ 5% greater than the one in gauge region. Conclusion: Assuming that dog-bone samples fail in the gauge region due to pure axial stress is simplistic, as demonstrated by analytical and numerical evidence. As a consequence, interpretations of experimental results based on this belief are misleading and testing protocols should be refined. Indeed, the presence of spurious stresses interfering with expected pure axial stress seems unavoidable. Therefore, clear-wood testing standards should allow to use prismatic samples or, alternatively, to consider as valid also tests on samples breaking outside the gauge region. Both the proposed solutions apparently reduce the accuracy of the experiments, while in contrast, they provide the best achievable results, speeding up the testing procedure and reducing the testing costs.