Graduate Thesis Or Dissertation
 

Nondestructive detection of proportional limit and prediction of destructive parameters

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  • The conventional method for determining lumber strength depends on visual evaluation by lumber graders which often results in undergrading of lumber. Nondestructive proof testing is used less often and provides for only estimates of elastic moduli of elasticity and rupture. The evaluation of nondestructive variables such as proportional limit (PL) and acoustic emissions (AE), offers a possibility of not only improved predictions for elastic but also nonelastic moduli. A microcomputer-controlled testing machine was used to pretest three machine-stress-rated grades of Douglas-fir lumber up to PL under an accelerated deflection rate. These specimens were then tested to failure. The load, deflection, and AE were continuously monitored throughout the testing. The observations from nondestructive testing were chosen for independent variables in regression models for predicting the destructive parameters. It is found that PL can be determined in a microcomputer-controlled test, with the computer-detected PL highly correlated with PL determined from destructive testing (r = 0.92). Although this computer-detected PL, in combination with modulus of elasticity, is a good estimator of lumber strength (r = 0.83), it results in a poor prediction of ultimate deflection (r = 0.54). However, not only is a combination of AE variables below the PL and physical properties strongly correlated with PL Cr = 0.76), but the same combination is also strongly correlated to strength Cr = 0.93) and ultimate deflection Cr = 0.83).
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