Electric resistance tomography and stress wave tomography for decay detection in trees-a comparison study

Background. To ensure the safety of trees, two NDT (nondestructive testing) techniques, electric resistance tomography and stress wave tomography, were employed to quantitatively detect and characterize the internal decay of standing trees. Comparisons between those two techniques were done to make full use of the individual capability for decay detection. Methods. Eighty trees (40 Manchurian ash and 40 Populus simonii) were detected, then wood increment cores were obtained from each cross disc trial. The D-t, which was defined as the value determined by the mass loss ratio of each wood core, was regarded as the true severity of decay. Using ordinary least-squares regression to analyze the relationship between D-t and D-e (D-e was defined as the severity of decay determined by electric resistance tomography) and between D-t and D-s (D-s was defined as the severity of decay determined by stress wave tomography). Results. The results showed that both methods could estimate the severity of decay in trees. In terms of different stages of decay, when D-t < 30%, D-e had a strong positive correlation with D-t (R-2 = 0.677, P < 0.01), while, when D-t >= 30%, D-s had a significant positive correlation relationship with D-t (R-2 = 0.645, P < 0.01). Conclusion. Electric resistance tomography was better than stress wave tomography for testing in the early stages of decay, while stress wave tomography can be used effectively in the late stage of decay. It is suggested that each technique can be used in the practice of internal decay testing of standing trees based on decay stages and operating conditions.