Abstract
The determination of plywood bonding quality by shear testing is one of the most commonly used and informative tests, alongside bending strength, to characterise this compound material. Two types of birch plywood were studied: indoor use (class 1), thickness 18 mm, urea-formaldehyde glue; and exposed outdoor use (class 3), thickness 15 mm, phenolic formaldehyde glue. The corresponding pre-treatments indicated in standards UNE-EN 314-1 and -2 were applied to each type. To test the suitability of the method, the technique of digital image correlation (DIC) was used to obtain information about the complex modes of fracture present in compound materials such as plywood. Assessing the results based on the statistical extreme value theory provided more reliable and robust information about the failure strength values for low failure probabilities. The results confirm that DIC can be used to determine the true distribution of the deformations during the fracture process and, therefore, it provides greater knowledge of the details of the failure process. Statistical evaluation of the experimental data obtained with DIC indicates that test results exceeding the time limit of 30 ± 10 s should be evaluated, contrary to the operating procedure in standard UNE-EN 314-1, which states that they are invalid.
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