Prediction of Failure of Finger Joints Using Acoustic Emissions

The article describes how a new nondestructive testing technique, called acoustic emissions, can be used to predict the failure of finger joints in 2- by 6-inch Douglas-fir lamination stock. While the technique was developed using finger joints as study material, the eventual application of the method is directed towards the prediction of failure in proof testing of glulam beams. Ten specimens were prepared for a number of types of finger joints. Prediction of failure was made by first plotting a graph of load versus cumulative total of acoustic emissions detected by a sensor attached to the material. A second graph was next plotted of load versus the rate of change of load with respect to cumulative acoustic emissions. Predicted failure load was then obtained by extrapolating the second curve to the point where the abscissa equals zero. The accuracy with which failure could be predicted varied from 1.8 to 25.0 percent, depending upon 1) load at which prediction was made and 2) the nature of a finger joint. For normal commercial finger joints, it appears that load acoustic emission data to a load level just beyond the proportional limit should permit estimates of failure load accurate to plus or minus10 percent.

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