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Based on the experimental idea of reverse simulation, a quantitative area of hole was excavated at the sectional center of a wood specimen. The excavation area was 1/32S, 1/16S, 1/8S, 1/4S, and 1/2S (where S represents cross-sectional area of the complete specimen) and stress wave nondestructive testing of six sensors was performed. The stress wave propagation paths were statistically summarized to obtain the stress wave propagation velocity (Va) for two adjacent sensors, the stress wave propagation velocity (Vb) for two separated sensors, and the stress wave propagation velocity (Vc) for two opposite sensors. Furthermore, by analyzing the advantages and disadvantages of grey relation and stepwise discriminant model when both of them were used alone, a coupling model generated from them was established to dispose the test data. The attenuation ratios Ψa, Ψb, and Ψc of stress wave under three propagation paths and their relation ratios Va/Vb, Vb/Vc, and Va/Vc, a total of six groups of measured data, were selected as discriminant factors for the hole area grade of the wood specimen. The verification results showed that the discriminant accuracy of the coupling model was 100%, and it was concluded that the attenuation ratio (Ψb) of the stress wave propagation velocity for two separated sensors had the strongest discriminant ability against cross-sectional area of the specimen.

A coupling model based on grey relational analysis and stepwise discriminant analysis for wood defect area identification by stress wave